Effects of processing conditions on microstructure and ...

Materials Science and Technology

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Effects of processing conditions on microstructure and mechanical properties of equal-channelangular-pressed titanium M. Shaat To cite this article: M. Shaat (2018): Effects of processing conditions on microstructure and mechanical properties of equal-channel-angular-pressed titanium, Materials Science and Technology, DOI: 10.1080/02670836.2018.1478481 To link to this article: https://doi.org/10.1080/02670836.2018.1478481

Published online: 05 Jun 2018.

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MATERIALS SCIENCE AND TECHNOLOGY https://doi.org/10.1080/02670836.2018.1478481

Effects of processing conditions on microstructure and mechanical properties of equal-channel-angular-pressed titanium M. Shaat

a,b

a Engineering and Manufacturing Technologies Department, DACC, New Mexico State University, Las Cruces, NM, USA; b Mechanical Engineering Department, Zagazig University, Zagazig, Egypt

ABSTRACT

ARTICLE HISTORY

This review presents an investigation on effects of the processing conditions on the microstructural evolution and mechanical properties of commercial pure titanium processed by Equal Channel Angular Pressing (ECAP). An overview of ECAP processing is presented. A discussion on the microstructure evolution of ECAPed titanium emphasising effects of the ECAP-route type, processing temperature, number of ECAP passes, and mechanical/thermal treatments is presented. Moreover, the variations of the mechanical properties (yield strength, tensile strength, and ductility) of titanium as functions of the grain size are reported for the different conditions of ECAP processing. In addition, the best estimates of the Hall–Petch parameters for titanium processed by ECAP, ECAP followed by mechanical and/or thermal annealing are reported.

Received 16 March 2018 Revised 13 May 2018 Accepted 15 May 2018

Introduction Severe plastic deformation (SPD) is a metal forming process under an extensive hydrostatic pressure that imposes a very high strain on a bulk material in order to refine the grains inside its microstructure [1,2]. In most of the SPD methods, materials are kept with no significant changes in their overall dimensions. A SPD method shows significant merit if it produces ultrafine-grained structures with a high fraction of high-angle grain boundaries, uniform nanostructures within the whole material volume, and the material has no damages or cracks. Thus, according to Valiev’s definition, a SPD processing of a material is impossible without exposing the material to special mechanical schemes of plastic deformation at low temperatures, for example, torsional straining under high pressure and Equal Channel Angular Pressing (ECAP) [3]. Thus, the traditional methods of metal forming (e.g. rolling, forging, drawing, and extrusion) cannot satisfy this idealised definition of SPD. However, recently, the combination of the conventional techniques of SPD with these traditional metal-forming processes led to ultrafine-grained/nanostructured materials with enhanced microstructure and mechanical properties [4–17]. Biocompatibility, corrosion resistance to body fluids, mechanical static and fatigue strengths, and mechanical/thermal stability are essential requirements for a material to be used for medical applications. Commercial pure (CP) titanium is one of the bioactive materials

KEYWORDS

Severe plastic deformation; ECAP; titanium; Hall–Petch; microstructure; mechanical properties

that has shown a great success in various medical applications due to its high biocompatibility [18]. However, conventional CP titanium is relatively weak (low mechanical strength) compared to other metallic alloys such as 316L stainless steel. This limits its applicability for orthopaedic implants that bears high forces such as Locking Compression Plates (LCP). LCP implants are needed to bear loads as high as ∼ 2.7 times human’s body weight [19]. For LCPs, this exceeds the strength limits of CP titanium. Therefore, a technique to increase the strength of CP titanium with no degradations in its biocompatibility is SPD processing [4]. Several methods have been proposed to provide ultrafine-grained/nanostructured materials with enhanced mechanical properties. Some of these methods (or techniques) were harnessed to produce ultrafinegrained/nanostructured titanium. These techniques of processing titanium can be classified into Basic SPD Processing that includes ECAP [5,8–17,20–32], Accumulative Roll-Bonding [33–37], High Pressure Torsion (HPT) [28,38–58], and Torsion Extrusion [59–61], Conventional Metal Forming-Based Processing including Hydrostatic Extrusion [27,62–71], Asymmetric rolling [72–77], Cryorolling [78–83], and MultiDirection Forging [84], and SPD with post-processing such as SPD followed by mechanical treatment (MT) [4–17] and/or annealing [7–9,28,39,41,85]. Early methods of producing ultrafine-grained materials depended on ECAP and HPT [1–3]. ECAP processing is very common where it has been utilised in an uncountable

CONTACT M. Shaat Engineering and Manufacturing Technologies Department, DACC, New Mexico [email protected], [email protected] State University, Las Cruces, NM 88003, USA; Mechanical Engineering Department, Zagazig University, Zagazig 44511, Egypt © 2018 Institute of Materials, Minerals and Mining.

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list of studies. Although HPT may give preferable microstructures than ECAP, the merit that ECAP gives over HPT is that it can be scaled up to find a potential implementation in industry. Therefore, in this review, studies on CP titanium processing using ECAP are collected and discussed. Titanium is a good example of Hexagonal Close Packed (HCP) metals; thus, the presented discussions and analyses in this review explain effects of processing conditions on microstructures and mechanical properties of ECAPed HCP metals. Several reviews are available in the literature in which the principles of SPD processing of metals were presented and discussed [2,3,86–95]. Moreover, in a few studies, ultrafine-grained titanium was introduced as a material candidate for medical applications [4,51,65,86,87,96,97]. However, to the author’s knowledge, there has not yet been a review of the influence of processing conditions of SPD processes on the microstructure evolution and the mechanical properties of titanium. Therefore, this review is devoted to fulfil this gap. Thus, the impact of ECAP processing conditions including the processing temperature, number of passes, and post-processing on the microstructural and mechanical characteristics of titanium is

compared from various investigations, revealing how variations of the yield strength, tensile strength, and ductility of ECAPed titanium as functions of the grain size depend on the different conditions of ECAP processing. These characteristics are needed for the design of titanium for several medical applications. In this review, an overview of ECAP processing is first presented showing its scalability for industrial use. Then, the grain boundaries of ultrafinegrained/nanocrystalline materials and the twinning of titanium due to ECAP are investigated. Afterwards, a discussion on the microstructure of titanium after ECAP emphasising the effects of the ECAP-route type, processing temperature, number of ECAP passes, and mechanical/thermal treatments on the microstructure evolution is presented. Finally, the mechanical properties of ECAPed titanium under the different processing conditions are reported.

ECAP: an overview ECAP is the process of developing a simple shear strain deformation pattern in a material by pressing it to extrude through a die with a channel that involves a

Figure 1. (a) A schematic of ECAP processing; (b) The induced strain εN , as a function of the channel angle , for different values of the curvature angle , for a material processed by one-pass of ECAP (N = 1); (c) The principle of the simple shear using ECAP processing.

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bend with a confined angle , as shown in Figure 1(a). Segal [98,99] was the first to use this process as a conventional extrusion of materials with high strains. Then, Valiev et al. [100–102] were the first to utilise this technology for producing ultrafine-grained materials. The induced strain in a material processed by N passes of ECAP, εN can be estimated, neglecting the friction at the material-die interface, as follows [88,103]:        N   εN = √ 2 cot + + cosec + 2 2 2 2 3 (1) where  is the channel angle and  is the curvature angle as shown in Figure 1(a). The principle of ECAP processing is shown in Figure 1(c). When the material is pressed through the channel, at the bend, a simple shear is generated through the cross section resulting in an intensive refinement of the grain size. The intensity of this shear deformation depends on the angles of the channel ( and ). Figure 1(b) shows the variation of the induced strain in a material processed by one-pass of ECAP (N = 1) as a function of the channel angle , and the curvature angle , according to Equation (1). It is clear that a decrease in the channel angle  and/or the curvature angle  is accompanied with an increase in the material shear strain due to ECAP. Indeed, many factors affect the deformation regime by ECAP processing, for example, material-die friction [104,105], channel angles [104–107], and pressing velocity [105]. Accounting for these factors, a more accurate relation for the induced strain in an ECAPed material than Equation (1) can be proposed. There are four different routes of ECAP processing. When a multi-pass ECAP processing is applied for a material, different slip systems may be activated by rotating the material-specimen between the consecutive passes [89,108]. In ECAP-route A (ECAP-A), the specimen is transferred between passes without

Figure 2. The four routes of ECAP processing [110].

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rotation (Figure 2(a)). In ECAP-route BA (ECAP-BA ), in each two successive passes, the specimen is, respectively, rotated by +90◦ and −90◦ (i.e. alternative directions), as shown in Figure 2(b). As for ECAP-route BC (ECAP-BC ) which is shown in Figure 2(c), the specimen is rotated by 90◦ in the same direction between passes. Finally, in ECAP-route C (ECAP-C) (Figure 2(d)), the specimen is rotated by 180◦ between passes [89,109,110].

Scalability of ECAP processing There is a current interest to scale up ECAP for industrial applications. Most of the research on ECAP was confined to lab-scale material samples. Therefore, a few studies have reported the possibility to scale up ECAP for commercial and industrial benefits [111–122]. For instance, Horita et al. [111] investigated the scalability of ECAP by measuring the influence of the increase of the billet size on the microstructural and mechanical properties of commercial 1100 aluminium alloy. Samples with different sizes (from 6 to 40 mm diameters) were ECAPed and tested. They demonstrated that ECAP can be scaled up for industrial utilisation where the microstructure and mechanical properties were obtained independent of the initial size of the sample. Chaudhury et al. [112] and Srinivasan et al. [113] studied the effects of scaling up ECAP on the mechanical properties, microstructure, and hot workability of aluminium 6061 alloy. They processed samples with different cross sections (12.5, 50, and 100 mm square billets) using ECAP-BC at Room Temperature (RT). Similar microstructures and mechanical properties were obtained for the different samples. The workability of the ECAPed alloys for forging was enhanced when the billet size was increased. Lefstad [114] performed an ECAP processing for aluminium 6060 alloy at RT and 150◦ C with different pressing speeds. They succeeded to process 50 mm square samples up to 6

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passes. Medvedev et al. [115] showed the merit of ECAP-Conform to process and scale up ECAP for industrial use (for more details about ECAP-Conform, readers can refer to [1]). They processed 12 mm diameter titanium rods by ECAP-Conform followed by a thermomechanical processing. They demonstrated that ECAP-Conform outweighs conventional ECAP dies where exceptional tensile properties and fatigue strength were obtained. Frint et al. [116] showed that there are no scaling effects regarding the mechanical properties and the microstructures when scaling up ECAP. They processed square copper billets with different sizes up to 50 mm. All the aforementioned efforts demonstrated the high scalability of ECAP for industrial use.

Titanium processed by ECAP: review In various studies, ECAP processing was utilised to produce ultrafine-grained/nanocrystalline titanium [5,8–17,20–30]. Titanium was processed by a singlepass ECAP [5,17,21,24,25,27] and multiple-pass ECAPBA [30], ECAP-BC [7,8,10,12,14,15,22–25,29], and ECAP-C [9,28,30]. In studies such as [7,8,14,17,29,30], titanium was ECAPed up to 8 passes while in [5,10] it was processed up to 10 passes. Stolyarov et al. [9] succeeded to process titanium up to 12 passes recording the highest number of passes of ECAP. A few studies were carried out to investigate the effects of the number of passes [5,12,17,22] and the route type [30] on the microstructural and mechanical characteristics of nanocrystalline titanium processed by ECAP.

Table 1. Grain size and mechanical properties of ultrafine-grained titanium produced by ECAP processing (with no subsequent mechanical/thermal treatments). Mat.

Type/route

G4a G4 – – – – – – – – G4 G4 G4 G4 G4 G4 G2 – G2

CONFORMb – – – – BC BC BC BC BC CONFORM – A CONFORM – A CONFORM – A CONFORM – A CONFORM – A CONFORM – A – BC BC BC BC BC BC BC BC C – – BC A–C C C – BC BC BC BC BC BA C BC BC CONFORM – BC CONFORM – BC CONFORM – BC CONFORM – BC CONFORM – BC

G2 – G2 G2 – – – – G3 – – – G2 G2 G4 G2 G2 – – – – G2 G4 G4 G4 G4 G4

N

temp

dc (nm)

σy c (MPa)

σt c (MPa)

δ c (%)

Ref

1 1 2 2 4 6 8 1 2 4 6 8 10 1 1 2 2 4 4 4 6 6 4–8 1 4 4 5 7 8 4 8 8 8 8 8 8 8 8 10 1 2 4 6 8

– – – RT RT RT RT RT RT RT 200 200 200 200 200 200 300 300 300 300 300 300 300 300 300 350 450 450 400 450 450–500 450 450 400–450 400–450 450 400–500 450 450 450 450 400–450 < 800 < 800 < 800 < 800 < 800

200–300 250 200 < 28,000 500 – 350 250 200 200 300 – 230 220 230 – 100–200 6560 150–200 3530 150–201 2880 100–200 150–202 890 – < 100 < 100 < 500 960 300 350 500 450 450 350 200–500 260 320 240 100 450 300 300 300 250 250

985 1190 1020 680 520 680 565 620 690 710 853 869 921 973 966 950 530 489 510 502 555 511 636 615 558 655 338 603 830 535 520 640 545 549 750 750 652 640 720 640 640 582 – – – – 990

1085 1240 1050 780 590 750 645 655 735 790 875 884 962 1020 1030 1020 573 601 680 648 740 657 678 790 685 680 651 663 850 670 540 710 630 633 815 882 705 710 760 720 710 645 950 960 1050 1080 1100

11 11.5 10 14 16 6 16 17 17 19 16 14 16 14 13 14 25 24 29 23 32 21 23 32 20 12 29 21.3 18 26 16 27 22 27 19 14 – 17 10 14 14 21 17 12 11 11 11

[16] [20] [123] [21] [22] [23] [22] [22] [22] [22] [5] [5] [5] [5] [5] [5] [24] [25] [12] [25] [12] [25] [24] [12] [25] [26] [27] [27] [15] [13] [28] [9] [11] [10] [10] [29] [14] [30] [30] [30] [8] [10] [17] [17] [17] [17] [17]

a G ≡ titanium grade. b CONFORM ≡ Continuous Forming. For more details about ECAP-Conform, readers can refer to [1]. c d is the grain average size. σ and σ denote, respectively, the yield strength and tensile strength. δ y t

is the elongation to failure.

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annealing, which demonstrates the potential thermal stability of ECAPed titanium [8]. A summary of the results available in the literature for ECAPed titanium is presented in Tables 1–3. Table 1 presents the average grain size along with the mechanical properties of titanium processed by ECAP where neither mechanical nor thermal treatments were involved. Tables 2 and 3, on the other hand, summarise the grain size and mechanical properties of titanium processed by ECAP followed by MT (Table 2) and/or annealing (Table 3). By tracking the literature on ECAP processing of titanium and using the results presented in Tables 1–3, the effects of the ECAP processing conditions (e.g. processing temperature, number of passes, route type, and post-processing) on the microstructural and mechanical characteristics of titanium are

In the majority of the studies, titanium specimens were ECAPed at 400–500◦ C [8–11,13–15,27–30]. However, enhanced mechanical and microstructure properties were obtained when ECAP was performed at lower temperature levels such as 300◦ C [12,24], 200◦ C [5], and RT [22,23]. The microstructural and mechanical properties of titanium processed by ECAP followed by drawing [5,16,17], extrusion [8], forging [14], HPT [28], or rolling [9,10,12,15,24,29] have also been investigated. The incorporation of a MT process after ECAP results in an additional reduction in the grain size, providing an enhancement in the mechanical properties. On the other hand, in some studies, titanium was annealed after ECAP processing and/or the MT [7–9,85], with no significant changes in the mechanical properties after

Table 2. Grain size and mechanical properties of ultrafine-grained titanium produced by ECAP processing followed by mechanical treatments (MT). Mat. – G2 G2 G4 G4 G4 – – – – G2 G2 G2 G2 G2 G2 G2 G2 G3 G3 G2 G4 G4 G2 G2 G2 G4 – – – – – G4

Type/route

N

Temp.

d (nm)

σy (MPa)

σt (MPa)

δ (%)

MT

Ref

– BC BC CONFORM – A – BC A–C C C C – – BC BC BC BC BC BC BC BC BC CONFORM CONFORM-BC BC BC BC BC BC BC – C C BC

– 8 8 6 – 4 5 8 8 12 4 4 2 4 6 2 4 6 8 8 4 – 8 8 10 10 8 8 8

– 400–450 400–450 200 – 450 450 450 450 450 450 450 300 300 300 300 300 300 400 400 300

150 100–400 50–300 150 200 120 960 350 350 350 250 250 150–203 150–204 150–205 150–206 150–207 150–208 < 500 < 500 < 100 200–300 200 190 190 190 190 – – 200 300 300 105–120

1200 800 1000 1190 1220 1300 710 940 1020 920 845 1100 740 730 785 970 880 915 1040 1150 796 1300 1220 665 736 736 1006 910 970 625 530 625 1400

1240 900 1080 1230 1280 1310 797 1040 1050 995 1030 1150 880 900 910 1080 1040 1030 1100 1218 876 1350 1300 938 945 928 1135 930 1050 730 640 730 1600

12 – – 11 7 12 19 7 6 15 12 11 32 32 34 32 34 32 13 13 21 9 11 18 15 18 11 – 8 25 30 25 6

Forging and drawing Forging at RT Forging and drawing at RT Drawing (200◦ C, ε = 6) Swaging and drawing Rolling at 350◦ C Cold extrusion Cold rolling (35%) Cold rolling (55%) Cold rolling (35%) TMTa (ε = 0.6) TMT (ε = 0.8) Rolling at RT Rolling at RT Rolling at RT Rolling at sub-zero Temp. Rolling at sub-zero Temp. Rolling at sub-zero Temp. Rolling at RT Rolling at LNTb Cold rolling (R = 70%) Drawing (ε = 1.9) Drawing (ε = 1.4) Cold rolling (77%) Cold rolling (77%) Hot rolling (81%) at 350◦ C Cold rolling (83%) Cold extrusion (47%) Cold extrusion (75%) HPTc (1.5 GPa) HPT (at 450◦ C) HPT (at RT) Drawing (300◦ C, ε = 0.81) and HPT (6 GPa, RT)

[4] [14] [14] [5] [6] [7] [13] [9] [9] [9] [11] [11] [12] [12] [12] [12] [12] [12] [15] [15] [24] [16] [17] [10] [10] [10] [10] [8] [8] [39] [28] [28] [41]

7 7 8

< 800 400–450 400–450 400–450 400–450 450 450 – 450–500 450–500 450

a TMT ≡ Thermomechanical treatment. b LNT ≡ Liquid nitrogen temperature. c HPT ≡ High-pressure torsion.

Table 3. Grain size and mechanical properties of ultrafine-grained titanium produced by ECAP followed by mechanical treatment (MT) and annealing. Mat – G4 – – – UFa

Route BC BC C C C –

N 8 4 8 12 8 8

Temp 450 450 450 450 450 450

a UF ≡ Ultrafine-grained titanium.

d (nm) 120 150 350 350 350 < 260 nm

σy (MPa) 970 1200 985 920 942 800

σt (MPa) 1050 1210 990 1000 1037 1000

δ (%) 8 11 8 14 13 10

MT

Annealing

Ref

Cold extrusion (75%) Cold drawing (ε = 0.6) Cold rolling (35%) Cold rolling (35%) Cold rolling (73%) Cold rolling (73%)

200◦ C 30 min

[8] [7] [9] [9] [9] [85]

350°C for 1.5 h 200°C for 0.5 h 300°C for 0.5 h 300°C for 0.5 h 300°C for 30 min in Ar

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compared and analysed in Sections ‘Microstructure of titanium processed by ECAP’ and ‘Mechanical properties of titanium produced by ECAP’.

Microstructure of titanium processed by ECAP Many studies were carried out to examine the microstructure of titanium produced by ECAP processing. Depending on the processing conditions of ECAP (e.g. number of passes, processing temperature, ECAProute type, and post-processing) titanium was obtained with different microstructural characteristics. In this section, these studies are reviewed, summarising the influence of the ECAP processing conditions on microstructure evolution of titanium.

Grain boundaries of ultrafine-grained/nanocrystalline materials In the conventional form of a polycrystalline material, the microstructure consists of many crystals (or grains) with low-angle misorientations and small dislocation densities [3]. The majority of these conventional materials possess equilibrium grain boundaries [124]. Materials processed by SPD, in contrast, are distinguished by non-equilibrium grain boundaries. After SPD, the material microstructure converts from a cellular structure with wavy grain boundaries (i.e. non-distinct grain boundaries) to a granular structure with distinct grain boundaries (i.e. grain boundaries can be clearly seen in the microstructure). When the material is subjected to a plastic deformation strain, in the beginning, twins are formed (see Section ‘Twinning of titanium due to ECAP’), and dislocations of different signs accumulate at the cell boundary. When more strains are applied, the grain size is decreased and excess dislocations of only single sign exist [3]. These excess dislocations lead to non-equilibrium grain boundaries which are featured with increased misorientations, excess grain boundary energy, and long-range stress fields [3]. For more details about the grain boundaries of ultrafine-grained materials processed by SPD, readers can refer to [3,124].

When intensive plastic deformation is imposed, the grain size is decreased to a few nanometres (generally < 100 nm) giving a nanocrystalline material, and the microstructure has many influential defects. In order to reveal these defects and their impact on the microstructure and mechanical properties, the atomic structure of a nanocrystalline material should be considered. The atomic structure can be seen as shown in the schematic diagram of Figure 3. As shown in this figure, the atomic structure of nanostructured materials has many imperfections including vacancies, interstitials, high-density dislocations, porosities, and impurities residing in grain boundaries [125,126]. Indeed, the role of these imperfections increases as the grain size decreases to a few nanometres. Both experiment and theory have demonstrated that the elastic and plastic properties of nanostructured materials strongly depend on these imperfections as well as the atomic structure of the grain core and the grain boundaries [125–140]. Recently, the impact of these imperfections on the mechanics of nanocrystalline materials in mechanical [127,141], electromechanical [126,142,143], and medical applications [128] have been revealed.

Twinning of titanium due to ECAP HCP metals, for example, titanium, have limited slip systems; therefore, twinning is one of their main plastic deformation mechanisms [144]. Studies revealed that, at the early stages of ECAP processing (i.e. after one-to-two passes of ECAP), twinning has a significant role on the plastic deformation and strengthening of titanium [21,22,144–149]. For instance, Kim et al. [144] observed high density of {101¯ 1} twinning in CP titanium processed by one-pass of ECAP; in contrast, low dislocation densities were seen in the obtained TEM micrographs. Later they demonstrated that the twin density increases with an increase in the processing temperature up to 350◦ C, and then it decreases at higher temperatures [145]. Shin et al. [146] revealed by inspecting the TEM micrographs and SEM images of CP titanium processed by ECAP at 350◦ C that, the deformation mechanism is {101¯ 1} twinning during the

Figure 3. A schematic for the atomic structure of nanocrystalline materials with grain average size of a few nanometres [126].

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first pass while, during the second pass, the deformation mechanism is changed to a dislocation slip. Zhao et al. [21] revealed that the microstructure of CP titanium after one-pass of ECAP processing at RT consists of a mixture of shear bands and {101¯ 1} twins. On the other hand, Zhao et al. [22] revealed that the microstructure of titanium processed by one-pass of ECAP at RT is characterised by a high density of {101¯ 2} twinning. After the first pass of ECAP, Chen et al. [147] observed both {101¯ 1} and {101¯ 2} twins in the microstructure of CP titanium processed at 450◦ C. Gu et al. [148] ECAPed titanium at 420◦ C and detected {101¯ 2} twins after the first pass and {101¯ 1} twins after the second pass. Moreover, they revealed that the dislocation slip is the main deformation mechanism when processing titanium with third and fourth passes of ECAP. Next, the microstructure of titanium as obtained by ECAP under different processing conditions is examined.

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Titanium microstructure after ECAP Effects of ECAP-route type Stolyarov et al. [28,30] investigated the influence of the ECAP-route type on the microstructure of titanium, using 8 passes of ECAP-BC , ECAP-BA , and ECAPC at 450 − 500◦ C. In these two studies, TEM micrographs along with Selected Area Electron Diffraction (SAED) Patterns are obtained from different sections of the processed sample to examine the microstructure of titanium, as shown in Figure 4. For all samples obtained by the different routes, SAED patterns were obtained containing large number of diffraction spots arranged in circles. This indicated that after eight passes of ECAP, a homogeneous microstructure with high fraction of high-angle grain boundaries was obtained. Some diffraction spots were split demonstrating the existence of a small fraction of low-angle grain boundaries. Moreover, elongated diffraction spots were observed in SAED patterns indicating a microstructure with high internal stresses. From TEM micrographs,

Figure 4. TEM micrographs and SAED patterns from the transverse and longitudinal sections of titanium processed by 8 passes (at 450◦ C) of (a,b) ECAP-BC , (c,d) ECAP-BA , (e,f) ECAP-C [30]. (Reprinted with a permission).

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multiple extinction contours were seen at the boundaries of some grains which refers to equilibrium grain boundaries (see Figure 4(d)). Moreover, the grains were clearly seen with different resolutions due to the high dislocation density (e.g. the dislocation density was estimated by 1012 m−2 in [28]). Inspecting images in Figure 4, it is clear that titanium with a granular microstructure was obtained after eight passes of ECAP. When seen from the transverse section, the grains are equiaxed when titanium is processed by any of the three routes (BC , BA , or C). However, seen from the longitudinal section, EACP-BA and ECAP-C processing resulted in elongated grains (Figure 4(d,f)). As for EACP-BC processing (Figure 4(b)), no significant differences between the microstructures from the transverse and longitudinal sections can be observed. In contrast, the TEM micrographs, from another study [9], of titanium processed by ECAP-BC under the same conditions showed somewhat elongated grains in the longitudinal direction. The dependence of the microstructure on the ECAProute type can be attributed to the orientation of the shearing path with respect to the texture plane of the grain elongation and the crystal structure [3,30]. To explain effects of the ECAP route on the microstructure, the shearing paths due to the different ECAProute types are illustrated in Figure 5. As shown in Figure 5(a,d), in ECAP routes A and C, shearing occurs at only one plane of the material through the successive passes of ECAP. It is clear that, in ECAP-A, the grains are elongated in the direction of ECAP processing. Although ECAP-C recovers the elongation in the direction of ECAP processing by switching the direction of shearing (as shown in Figure 5(d)), the deformation of the grains in the transverse plane is not controlled. Thus, depending on the crystal structure and its anisotropy, grains may experience different

deformations in the longitudinal and transverse directions. Because titanium has a HCP crystal structure with a slip system of {0001}112¯ 0 at a temperature range of 400 − 450◦ C, the grains of titanium processed by ECAP-C are elongated in the direction of ECAP processing, as shown in Figure 5(d). In ECAP-BA and ECAP-BC (Figure 5(b,c)), shearing takes place at two orthogonal planes of the material through the different passes of ECAP. In ECAP-BA , the direction of shearing is the same over the orthogonal planes for all the ECAP passes, as shown in Figure 5(b). Thus, for the case of titanium and because of the crystal anisotropy, the deformation of the grains in the longitudinal and transverse directions is not the same (as can be observed in Figure 4(c,d)). However, ECAP-BC gives equiaxed grains because, for each four passes, the shearing directions are switched over the orthogonal planes, as shown in Figure 5(c). This explains why ECAP-BC is preferred over the other ECAP routes. Another aspect that should be considered when investigating effects of ECAP-route type on the microstructure of the processed material is the geometry and the angles of the channel. Thus, the shearing path depends on the channel angle , as well as its curvature angle . For instance, using an ECAP-die with a channel angle  = 120◦ , Prangnell et al. [150] examined effects of ECAP-route type on the microstructure of an Al alloy. They concluded that ECAP-A is the most effective in refining grains and generating microstructures with high-angle grain boundaries while the least effective route was ECAP-C. Slight differences were observed between microstructures obtained using ECAP-BC and ECAP-BA . In other studies, when Al was processed using an ECAP-die with a channel angle  = 120◦ , Iwahashi et al and Nemoto et al [151–153] observed that ECAP-BC is the most effective route while ECAP-A is the least effective one.

Figure 5. Shear strain paths for ECAP (a) route A, (b) route BA, (c) route BC, and (d) route C for a 90◦ ECAP die.

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Titanium was processed using dies with different channel angles such as  = 90◦ in [8–11,15,30],  = 120◦ in [12,21,22] and  = 135◦ in [23]. Although investigations on effects of ECAP-route type and number of ECAP passes were conducted in different studies, the literature lacks explanations of effects of the channel geometry and angles on the microstructure of ECAPed HCP metals (e.g. titanium). Effects of ECAP passes and processing temperature The microstructure of titanium after a number of ECAP passes and different processing temperatures can be examined by comparing results from the available literature. Titanium was processed by multiple-pass ECAP at temperatures below recrystallisation (1 pass–8 passes) [17], at 300◦ C (1 pass–6 passes) [12,25], at 200◦ C (1 pass–10 passes) [5], and at RT (1 pass–8 passes) [22]. Polyakov et al. [17] performed 1–8 passes of ECAPBC (ECAP-Conform) processing at a temperature below the recrystallisation temperature of titanium (< 800◦ C). For the ECAPed samples, they provided TEM micrographs and SAED patterns. After two passes of ECAP processing, an inhomogeneous-cellular microstructure was obtained with fragmented grains and wavy (non-distinct) grain boundaries with low dislocation density and low-angle misorientations. Elongated grains were seen in both the transverse and longitudinal sections. After 4 passes, more diffraction spots forming circles were seen in the SAED patterns which indicates the increase in the microstructure’s homogeneity with an increase in the number of grains in a similar volume of the foil with high-angle grain boundaries. After 6 passes, a homogeneous-granular microstructure with equiaxed grains (250 nm average grain size) and high-angle grain boundaries was obtained. Moreover, very similar transverse and longitudinal microstructures were obtained after 6 passes of ECAP-BC processing, which matches the observations in [28]. Effects of the number of passes of ECAP-BC processing at 300◦ C on the microstructure of titanium can be explained by considering studies by Sordi et al. [12] and Qarni et al. [25]. TEM micrographs of titanium microstructure after 1 and 4 passes of ECAP were presented in [12]. Electron Backscattering Diffraction (EBSD) analyses of titanium after 1, 2, 4, and 6 passes of ECAP were described in [25]. From the TEM micrographs, elongated grains with some twins were observed within the microstructure of titanium after one pass. Moreover, the EBSD analyses showed clusters of refined grains within a mixture of elongated and equiaxed grains inside the microstructure of titanium after the first pass of ECAP. As the number of passes increased up to 6 passes, more refined grains were obtained, and more microstructural homogeneity was observed. Moreover, the number of twinned

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grains decreased and the grains became equiaxed after 6 passes of ECAP. Gunderov et al. [5] examined the microstructure evolution after 1, 2, 4, 6, 8, and 10 passes of ECAP (ECAP-Conform) processing at 200◦ C. The TEM micrographs and the SAED patterns were obtained for the processed samples after the different number of passes. After up to 4 passes, the microstructure was inhomogeneous with a cellular character. Further passes of ECAP led to a granular microstructure that featured with high-angle grain boundaries. Moreover, high dislocation densities were obtained (e.g. 6 × 1014 m−2 after one pass and 9 × 1014 m−2 after four passes). After 8 passes, equiaxed grains/sub-grains were observed in the TEM micrographs with a dislocation density of 11.7 × 1014 m−2 . This study shows that processing titanium at a low temperature of 200◦ C results in an increase in the dislocation density which increases with the increase in the number of passes. The ECAP processing of titanium at RT gives similar microstructures as processing it at elevated temperatures. When titanium was ECAPed at RT, a cellular microstructure was seen in the TEM micrographs up until the first 4 passes [22]. With a low number of passes, heterogeneous microstructures were obtained with shear bands, some twins, and a few grain boundaries. After 4 passes, the microstructure started to convert to a granular structure. With further processing, a homogeneous microstructure with equiaxed grains and distinct grain boundaries was observed. It follows from the aforementioned studies and the results presented in Table 1 that the microstructure of an ECAPed titanium depends on the processing temperature. The increase in the processing temperature increases the granularity of the microstructure. Thus, when processing titanium at low temperatures, a wavy microstructure may be obtained with twins are formed. The density of the twins decreases when processing titanium at higher temperatures. Moreover, processing titanium at low temperatures allows for more dislocations where the dislocation density decreases with an increase in the processing temperature. Furthermore, when processing titanium at low temperatures, the grain size decreases while the fraction of the high-angle grain boundaries increases. Conclusions Inspection of the aforementioned studies and their results lead to the following conclusions: • Titanium with a homogeneous-granular microstructure can be obtained by multiple-pass of ECAP processing (6–8 passes). • At high number of passes (6–8 passes), microstructures of titanium can be obtained with distinctrefined grains and high-angle (probably equilibrium) grain boundaries.

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• ECAP-BC processing can provide a microstructures with equiaxed grains in all directions. • Other ECAP routes provide titanium microstructures with grains elongated in one direction and equiaxed in other directions. • Titanium can be obtained with a granular microstructure (with distinct grains boundaries) when processed by multiple-pass ECAP at an elevated temperature. • A wavy microstructure (i.e. grain boundaries are indistinct) of titanium may be obtained when performing ECAP at low temperatures. • The dislocation density of ECAPed titanium increases with the decrease in the processing temperature and increasing number of passes. • Twins may be formed when processing titanium by ECAP at low temperatures. These twins decrease with the increase in the number of ECAP passes. Titanium microstructure after ECAP followed by MT Recent studies revealed that, when ECAP is followed by further MTs, titanium can be produced with enhanced microstructural and mechanical properties [4–7,9,10,12–17,23,39]. In various studies, titanium was processed by ECAP followed by drawing [5–7,16,17], rolling [7,9,10,12,15,23,39], Extrusion [13], or forging [4,14] as further mechanical processing. In this section, the impacts of these MTs on the microstructure of ECAPed titanium are discussed. Drawing after ECAP Raab et al. [16], Polyakov et al. [17], and Gunderov et al. [5] examined effects of drawing after ECAP (ECAPConform) on the microstructure of titanium. When titanium was drawn after ECAP, the microstructure gained more homogeneity, with a higher fraction of high-angle grain boundaries, and more refined grains were obtained. Moreover, the grain boundaries became more sharply defined and multiple extinction contours were observed in the TEM micrographs indicating equilibrium grain boundaries. Grains from the transverse section were equiaxed while grains were elongated in the drawing direction. Rolling after ECAP Stolyarov et al. [9], Stolyarov et al. [10], and Fan et al. [15] investigated the impact of cold rolling after ECAP on the microstructure of titanium. Cold rolling after ECAP led to intensive reductions in the grain size of titanium (e.g. the grain size was decreased from 280 to 170 nm after 35% of cold rolling [9]). Moreover, the dislocation densities inside the grains and at grain boundaries were increased after rolling. When rolling was performed after ECAP-BC , equiaxed grains became elongated in the rolling direction in proportion to the

rolling strain. Furthermore, after rolling, microstructures with wavy grain boundaries were obtained. The significant change due to the increase in the rolling strain was the intensive decrease in the average grain size. In some studies [12,15] rolling was carried out at Liquid Nitrogen Temperature (LNT) (−196◦ C) [15] and a sub-zero temperature (−100◦ C) [12] after ECAP processing. When rolling was performed at sub-zero temperatures, titanium was obtained without twins. No other significant changes were observed when rolling was performed at these temperatures. In another study, Semenovo et al. [7] compared the impact of drawing and rolling after ECAP processing on the microstructure of titanium. Samples processed by cold drawing at RT (60% strain) with annealing at 350◦ C for 1.5 h after ECAP-BC (4 passes at 450◦ C) were compared to samples obtained by ECAP-BC (4 passes at 450◦ C) followed by warm rolling at 350◦ C (70% elongation ratio). The warm rolled titanium specimens had more refined microstructures than the cold drawn specimens. For both cases, when seen from the transverse section, grains were equiaxed with high-angle grain boundaries and high dislocation density. On the other hand, elongated grains/sub-grains with low-angle grain boundaries were seen in the longitudinal section. Forging after ECAP Gubicza et al. [14] examined the microstructure after 8 passes of ECAP-BC (at 400 − 450◦ C) followed by radial forging and drawing (both at RT). The subsequent forging processing increased the dislocation density form 6 × 1014 to 12 × 1014 m−2 . The dislocation density was doubled (24 × 1014 m−2 ) by drawing after ECAP and forging. After drawing, the grain size was decreased from ∼ 200 to 50 nm, but the microstructure was obtained with wavy grain boundaries. Conclusions It follows from the aforementioned studies that titanium gains new microstructural features when processed by ECAP followed by mechanical deformation processing (MTs). Thus, the following observations are summarised from these studies: • When titanium is rolled/drawn after ECAP processing, grains are obtained elongated in the rolling/ drawing direction with low-angle grain boundaries. • In the transverse direction of rolling/drawing, titanium is obtained with more uniform microstructure with equiaxed grains and high-angle-wavy grain boundaries. • The average grain size of titanium is intensively decreased (see Section ‘Average grain size of titanium produced by ECAP’) when following ECAP by drawing/rolling/forging.

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• Performing MTs at sub-zero temperatures gives titanium with equilibrium grain boundaries without forming twins. • The dislocation density inside the microstructure is increased by as much as a factor of two when ECAP is followed by a mechanical deformation processing (e.g. drawing, rolling, or forging). Titanium microstructure after ECAP followed by annealing A few studies have been carried out to examine the microstructure of titanium produced by ECAP followed by annealing [7,8,9,85]. Stolyarov et al. [8] investigated the thermal stability of titanium processed by ECAP when annealed at 300 and 450◦ C for 30 min (Figure 6). When compared to Figure 4, Figure 6 does not show significant changes in the microstructure of ECAPed titanium after annealing which demonstrates its thermal stability. However, annealing led to equilibrium grain boundaries and resulted in a reduction of internal strains and dislocation densities. These observations can be easy observed from Figure 6 where, after annealing, grains became more distinct and extinction fringes can be recognised at the grain boundaries (Figure 6(b)). Moreover, after annealing, titanium was obtained with equiaxed grains [9].

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from 22 to ∼ 3.5 µm after 2 passes and to ∼ 890 nm after 6 passes [25]. A reduction in the grain size from 30 µm to 450 nm was obtained by Stolyarov et al. [10] after 8 passes of ECAP. In studies such as [5,8,22,24], the grain size was decreased by a factor of ∼ 100 from ∼ 10 − 25 µm to ∼ 100 − 200 nm after 4–8 passes of ECAP. When ECAP is followed by a MT, titanium experiences more grain size reduction (compare Table 1 with Table 2). For instance, the grain size of ECAPed titanium with 6 passes was decreased from 220 to 150 nm after a further drawing process at 200◦ C with a strain ε = 6 [5]. By following the 10 passes-ECAP by rolling, Stolyarov et al. [10] succeeded to decrease the grain size from 450 to 190 nm. Annealing after ECAP, on the other hand, does not involve significant changes in the grain size although a small increase in the grain size might be observed (see Table 3). For example, the grain size was increased from 120 to 150 nm after annealing at 350◦ C for 1.5 h [154]. The average values of the grain size of titanium processed by ECAP, ECAP followed by MTs, and ECAP followed by MTs and annealing are plotted in Figure 7 using Tables 1–3, respectively. It follows from the figure that titanium processed by ECAP experiences an intensive decrease in the grain size to ∼ 280 nm in average.

Average grain size of titanium produced by ECAP The feature that SPD processes provide is the production of materials with ultrafine-grained microstructures. The reduction in the grain size results in an enhancement of the mechanical properties of these materials. Therefore, reporting the grain size reduction that each one of the SPD processes can provide is a significant outcome. In this review, the average grain sizes of titanium processed by ECAP, ECAP followed by a MT, and ECAP followed by a MT and annealing are reported. As reported in Table 1, titanium is obtained with a grain size ranging from a few micrometres to ∼100 nm. Using ECAP, the grain size of titanium was decreased

Figure 7. Average grain size of titanium processed by ECAP with/without mechanical treatments (MT) and/or annealing.

Figure 6. TEM micrographs and SAED patterns of Ti processed by 8 passes ECAP-BC followed by annealing at (a) 300◦ C for 30 min and (b) 450◦ C for 30 min [39]. (Reprinted with a permission).

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When ECAP is followed by a MT, the grain size of titanium is further decreased to ∼ 230 nm in average. Annealing, in contrast, increases the grain size to ∼ 265 nm in average.

Mechanical properties of titanium produced by ECAP In this study, the effects of ECAP working conditions (i.e. number of passes, processing temperature, postprocessing, and annealing) on the mechanical properties of ultrafine-grained titanium are reported. Moreover, the grain size-strength relations are determined deriving the best estimates of the Hall–Petch parameters for nanocrystalline titanium processed by ECAP. Effects of ECAP passes and processing temperature To reveal the effects of number of passes and processing temperature on the mechanical properties of titanium, the available studies on titanium processed by ECAP are reported in Tables 1–3. Moreover, the averages of the reported data in Table 1 are calculated and used to depict, in Figure 8, the average property change in the yield strength (σy /σy0 ), tensile strength (σt /σt0 ), and ductility (δ/δ0 ) for different processing temperatures (i.e. σy0 , σt0 , and δ0 denote, respectively, the yield strength, tensile strength, and elongation to failure of the coarse-grained titanium). It should be mentioned that the quantities (σy /σy0 ), (σt /σt0 ), and (δ/δ0 ) are the ratios of the properties of the processed titanium to the properties of its underlying raw-coarse-grained titanium as reported in each study. In different studies, titanium was processed by ECAP at temperatures from 400to500◦ C [8,10,13,14,29,30].

Figure 8. Effects of ECAP processing temperature on mechanical properties of titanium. (σ refers to the yield strength or tensile strength while δ denotes the elongation to failure as a measure of ductility. σ0 and δ0 denote to the mechanical properties of the raw-coarse-grained titanium as reported in each study).

Working at this temperature allowed for an increase in the yield strength by a factor of ∼ 1.6 while the tensile strength was increased by a factor of ∼ 1.4, as shown in Figure 8. The ductility after ECAP processing at this elevated temperature retained ∼ 50 − 70% of its original value of the coarse-grained titanium of 14 − 21%. On the other hand, when ECAP-BC was performed at RT [22], yield and tensile strengths increased by factors of ∼ 2.6 and ∼ 2, respectively. The ductility was decreased to ∼ 50% of its original value of 19%. Ko et al. [26] obtained an increase in the yield strength by ∼ 2 factor and an increase in the tensile strength by ∼ 1.8 factor when titanium was processed by ECAP-C for 6 passes at 350◦ C. The ductility, however, decreased to 35% of its original value. Filho et al. [24] processed titanium by ECAP-BC at 300◦ C for 4 passes resulting in an increase in each of the yield and tensile strengths by factors of ∼ 2 and ∼ 1.6, respectively. In this study, ECAP-BC at 300◦ C did not affect the ductility, as the elongation to failure was maintained at ∼ 21% after processing [24]. In another study [12], similar results were obtained by performing ECAP-BC for 6 passes at the same working temperature. It follows from the aforementioned studies that performing ECAP at low temperatures gives titanium with enhanced strength properties. As shown in Figure 8, yield and tensile strengths, respectively, increase by ∼ 2.2 and ∼ 1.6 factors when titanium is ECAPed at RT. Warm processing of titanium (300 − 500◦ C), on the other hand, gives an increase in the yield and tensile strengths only by a factor of ∼ 1.7 and ∼ 1.4, respectively. In contrast, the ductility decreases after ECAP processing to ∼ 50 and ∼ 60% of its original value when titanium is processed at RT and 400 − 500◦ C, respectively. However, a good ductility (∼ 90% of the original ductility) can be obtained when processing titanium at a moderate temperature of 300◦ C. To explain the reasons behind the different trends in Figure 8, effects of the processing temperature on the microstructure of titanium are considered. The increase in the processing temperature is accompanied with an increase in the grain size and a decrease in the fraction of high-angle grain boundaries [88]. Moreover, as demonstrated in Section ‘Microstructure of titanium processed by ECAP’ and according to Gunderov et al. [5], the dislocation density and twins increase as the processing temperature decreases. Thus, performing ECAP at low temperatures leads to a reduction in the grain size, an increase in the twins and dislocation density, and an increase in grains’ misorientations which, as consequence, enhance titanium’s strength. This indicates that titanium processed by ECAP obeys a grain size-strengthening mechanism. In various studies, titanium was ECAPed with different processing passes. In these studies, as reported in Table 1, the strength of titanium sharply increases after the first two passes. Afterwards, titanium shows a slow

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increase in the strength for the further ECAP passes higher than 2. Although the further ECAP passes might add a small strength to titanium, these extra passes are needed to enhance ductility. Studies revealed that the ductility may be enhanced when titanium is ECAPed with more than 2 passes. For instance, Zhao et al. [22] obtained an intensive decrease in the ductility to ∼ 45% of its original value after the first 2 passes of ECAP. However, they obtained an increase in the ductility by 6% when titanium was further ECAPed. Sordi et al. [12] obtained an increase in the ductility from 29 (after 2 passes) to 32% when titanium was further ECAPed for 6 passes. These observations demonstrate that titanium with enhanced mechanical properties can be obtained with multiple-pass ECAP processing. Effects of post-processing In some studies, titanium was mechanically processed and/or thermally annealed after ECAP in order to enhance its mechanical properties [5,7–10,12–17,24,28, 30,85]. For instance, yield and tensile strengths of titanium reached 1006; and 1135MPa when ECAPed titanium was further processed by cold rolling (83% reduction) [10]. When titanium was further processed by cold rolling (70% reduction) post-processing, yield and tensile strengths were increased to ∼ 2.5 and ∼ 2.15 times their original values [24], respectively. In another study [12], ECAPed titanium was rolled at a sub-zero temperature which resulted in a high ductility of 32% and yield and tensile strengths of 915 and 1030MPa, respectively. In this section, effects of thermal/mechanical treatments after ECAP on the mechanical properties of titanium are presented in Figure 9. The reported data in Tables 1–3 are used to present the changes in the yield strength, tensile strength, and ductility of ECAPed

Figure 9. Effects of the mechanical treatments and thermal annealing on the mechanical properties of titanium. (σ refers to the yield strength or tensile strength while δ denotes the elongation to failure as a measure of ductility. σ0 and δ0 denote to the mechanical properties of the raw-coarse-grained titanium as reported in each study).

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titanium with/without mechanical/thermal treatments. As shown in Figure 9, titanium experiences an increase in the yield strength by a factor of ∼ 1.5 and an increase in the tensile strength by a factor of ∼ 1.3 when it is processed by ECAP with no thermal or mechanical treatments. When titanium is mechanically processed after ECAP, yield and tensile strengths are increased by factors of ∼ 1.9 and ∼ 1.65, respectively. No significant changes in the ductility can be observed when ECAPed titanium is mechanically processed. Figure 9 demonstrates the thermal stability of ECAPed titanium where no significant changes in the mechanical properties can be observed when titanium is thermally annealed. This is consistent with the observations on the thermal stability of titanium by Stolyarov et al. [9]. It follows from Figure 9 that titanium can be obtained with enhanced mechanical properties when it is further mechanically processed after ECAP. This can be attributed to the decrease in the grain size and the significant increase in the dislocation density due to the mechanical processing (e.g. drawing, rolling, forging) (refer to Section ‘Microstructure of titanium processed by ECAP’). Hall–Petch (H-P) equations In this section, the variations of the yield strength, tensile strength, and ductility of ECAPed titanium as functions the grain size accounting for the effects of the processing conditions are reported. Using the collected grain size-strengthening data in Table 1, the parameters of the Hall–Petch (H-P) equation are estimated.

Figure 10. The variations of the yield strength (σy ), tensile strength (σt ), and elongation to failure (δ) of titanium processed by ECAP (no mechanical/thermal treatments) as functions of the grain size (d). The inserted figure shows the identification of the Hall–Petch (H-P) equation parameters. The estimated Hall–Petch curves are plotted in parallel to the collected data in Table 1 (these data presented by markers).

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Table 4. Yield strength, tensile strength, and elongation of ECAPed titanium as functions of the grain size, d. (Grain size is in µm). Processing ECAP ECAP and MT ECAP, MT, and Annealing

Yield strength (MPa)   σy = σy0 1 + 0.39 √1 d   σy = σy0 1.25 + 0.4875 √1 d  σy = σy0 1.23 + 0.48 √1 d

First, the ratio of the strength of ECAPed titaniumto-strength of coarse-grained titanium (σ/σ0 ) along with the ratio of the elongation of ECAPed titanium-toelongation of coarse-grained titanium (δ/δ0 ) are plotted against the inverse of the square root of the grain size, as shown in the inset in Figure 10. Then, the average linear trend of the aggregated data is obtained to estimate the Hall–Petch parameters for titanium processed by ECAP (without mechanical/thermal treatments). Table 4 shows the resulting Hall–Petch equations for the yield and tensile strengths and the elongation of ECAPed titanium. Figure 11 shows the variations of the mechanical properties of titanium processed by ECAP (no mechanical/thermal treatments) as functions of the grain size. The curves in Figure 11 are plotted utilizing the obtained Hall–Petch equations (Table 4). It follows from the figure that the estimated Hall–Petch equations perfectly give excellent match with the collected data in Table 1. It is clear that titanium exhibits an increase in the yield strength that is higher than the increase in the tensile strength with the decrease in the grain size. In contrast, the ductility decreases with the decrease in the grain size. To incorporate the effects of post-processing on the variations in the mechanical properties of ECAPed titanium as functions of the grain size, the reported relations in Figure 9 are similarly used. Thus, the changes in the mechanical properties of ECAPed titanium due to the incorporation of a MT and/or

Tensile strength (MPa)   σt = σt0 1 + 0.243 √1 d   σt = σt0 1.25 + 0.304 √1 d  σt = σt0 1.24 + 0.301 √1 d

Elongation (%)   δ = δ0 1 − 0.195 √1 d   δ = δ0 1.021 − 0.2 √1 d   δ = δ0 0.88 − 0.172 √1 d

annealing can be derived from Figure 9. Accordingly, the variations of the yield strength, tensile strength, and elongation of titanium processed by ECAP, ECAP followed by a MT, and ECAP followed by a MT and annealing are reported in Table 4 and depicted in Figure 11. As shown in Figure 11, titanium can be obtained with enhanced mechanical properties (i.e. high strength and good ductility) by ECAP processing followed by a mechanical post-processing treatment. Moreover, it can be observed that titanium is thermally stable where no significant changes in the strength are obtained for titanium produced by ECAP followed by a MT. However, the ductility is affected by the thermal treatment after ECAP; therefore, special considerations for ductility should be given when titanium is annealed.

Conclusions In this review, the microstructural evolution and mechanical properties of titanium processed by ECAP under the various working conditions were compared and analysed. In the foregoing sections, discussion of the microstructure of titanium after ECAP emphasising the effects of the ECAP-route type, processing temperature, number of ECAP passes, and mechanical/thermal treatments on the microstructure evolution were reviewed and depicted. For the first time, the variations of the yield strength, tensile strength, and ductility of ECAPed titanium as functions of the grain size were reported for the different conditions of

Figure 11. Yield strength, tensile strength, and elongation of titanium obtained by ECAP, ECAP followed by a mechanical treatment (MT), and ECAP followed by mechanical treatment (MT) and annealing as functions of the grain size. (Plotted using the reported equations in Table 4).

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ECAP. Based on the presented discussions, some recommendations can be derived to produce titanium with enhanced mechanical properties. It was demonstrated that multiple-pass ECAP processing (6–8 passes) at a moderate processing temperature followed by a MT (e.g. drawing, rolling, and forging) gives titanium with high strength and good ductility. Indeed, nanomaterials are unique due to their non-traditional physical phenomena that they exhibit. Owing to these non-traditional phenomena, the classical mechanics of materials are inapplicable for nanomaterials. For instance, classical mechanics cannot account for the dependence of the mechanical properties on the grain size. This, indeed, limits the use of nanomaterials in various applications. In fact, there is a significant lack of studies on the mechanics of nanomaterials. When compared to conventional mechanics of materials, mechanics of nanomaterials is behind by a 100 factor (based on Google Scholar). Thus, in order to fill the gap, studies should be conducted to relate the mechanical properties of nanomaterials to their microstructures accounting for their non-traditional phenomena. This study presented an initiative effort towards filling the gap between conventional mechanics and mechanics of nanomaterials. In fact, the reported characteristics of titanium are crucially needed for its design for several applications. For example, locking compression plate implants for orthopaedic surgeries should satisfy the requirements of the mechanical stability along with the mechanical strength [19]. Therefore, to design these implants from titanium, the variations of its mechanical properties with the grain size reported in Figure 11 can be utilised.

Disclosure statement No potential conflict of interest was reported by the author.

ORCID M. Shaat

http://orcid.org/0000-0002-7594-1652

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[5] Gunderov DV, Polyakov AV, Semenova IP, et al. Evolution of microstructure, macrotexture and mechanical properties of commercially pure Ti during ECAPconform processing and drawing. Mater Sci Eng A. 2013;562:128–136. [6] Sabirov I, Perez-Prado MT, Molina-Aldareguia JM, et al. Anisotropy of mechanical properties in highstrength ultra-fine-grained pure Ti processed via a complex severe plastic deformation route. Scr Mater. 2011;64:69–72. [7] Semenova IP, Yakushina EB, Nurgaleeva VV, et al. Nanostructuring of Ti-alloys by SPD processing to achieve superior fatigue properties. Int J Mat Res. 2009;100:1691–1696. [8] Stolyarov VV, Zhu YT, Lowe TC, et al. Microstructure and properties of pure Ti processed by ECAP and cold extrusion. Mater Sci Eng A. 2001;303:82–89. [9] Stolyarov VV, Zhu YT, Alexandrov IV, et al. Valiev grain refinement and properties of pure Ti processed by warm ECAP and cold rolling. Mater Sci Eng A. 2003;343:43–50. [10] Stolyarov VV, Zeipper L, Mingler B, et al. Influence of post-deformation on CP-Ti processed by equal channel angular pressing. Mater Sci Eng A. 2008;476:98–105. [11] Latysh V, Krallics G, Alexandrov I, et al. Application of bulk nanostructured materials in medicine. Curr Appl Phys. 2006;6:262–266. [12] Sordi VL, Ferrante M, Kawasaki M, et al. Microstructure and tensile strength of grade 2 titanium processed by equal-channel angular pressing and by rolling. J Mater Sci. 2012;47:7870–7876. [13] Kang DH, Kim TW. Mechanical behavior and microstructural evolution of commercially pure titanium in enhanced multi-pass equal channel angular pressing and cold extrusion. Mater Des. 2010;31:S54–S60. [14] Gubicza J, Fogarassy Z, Krállics G, et al. Microstructure and mechanical behavior of ultrafine-grained titanium. Mater Sci Forum. 2008;589:99–104. [15] Fan Z, Jiang H, Sun X, et al. Microstructures and mechanical deformation behaviors of ultrafinegrained commercial pure (grade 3) Ti processed by two-step severe plastic deformation. Mater Sci Eng A. 2009;527:45–51. [16] Raab GI, Valiev RZ, Gunderov DV, et al. Longlength ultrafine-grained titanium rods produced by ECAP-conform. Mater Sci Forum. 2008;584–586: 80–85. [17] Polyakov AV, Gunderov DV, Raa GI. Evolution of microstructure and mechanical properties of titanium grade 4 with the increase of the ECAP-conform passes. Mater Sci Forum. 2010;667–669:1165–1170. [18] Brunette DM, Tengvall P, Textor M, et al. Titanium in med. Berlin: Springer-Verlag; 2003. [19] Shaat M. Reporting the fatigue life of 316L stainless steel locking compression plate implants: the role of the femoral and tibial biomechanics during the gait. J Biomech Eng. 2017;139(10):104502. [20] Nie FL, Zheng YF, Wei SC, et al. In vitro and in vivo studies on nanocrystalline Ti fabricated by equal channel angular pressing with microcrystalline CP Ti as control. J Biomed Mater Res Part A. 2013;101A:1694–1707. [21] Zhao X, Fu W, Yang X, et al. Microstructure and properties of pure titanium processed by equal-channel angular pressing at room temperature. Scr Mater. 2008;59:542–545.

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