metals Article
The Low Strain Rate Response of As-Cast Ti-6Al-4V Alloy with an Initial Coarse Lamellar Structure Zhixiong Zhang 1 1 2
*
ID
, Shoujiang Qu 1 , Aihan Feng 1 , Xin Hu 2 and Jun Shen 1, *
ID
School of Materials Science and Engineering, Tongji University, Shanghai 201804, China;
[email protected] (Z.Z.);
[email protected] (S.Q.);
[email protected] (A.F.) Sicuan Shifang Future Aerospace Industry LLC, Shifang 618400, China;
[email protected] Correspondence:
[email protected]; Tel.: +86-21-6958-1009
Received: 28 March 2018; Accepted: 12 April 2018; Published: 15 April 2018
Abstract: The microstructure and microtexture evolution of the as-cast Ti-6Al-4V alloy with an initial lamellar microstructure was investigated in the temperature range of 900–1050 ◦ C and the low strain rate range of 10−3 –10−1 s−1 . In the ranges 900–950 ◦ C and 10−3 –10−2 s−1 , globularization of the α lamellar structure took place; the initial α {0001} texture softened, and the {0001} texture rotated to the transverse direction. Additionally, the increasing strain rate led to more randomization. The globularization process was considered to be continuous dynamic recrystallization (CDRX). At strain rates higher than 10−2 s−1 and temperatures between 900 and 950 ◦ C, flow instabilities occurred. In the β range, dynamic recrystallization (DRX) occurred at a temperature of 1050 ◦ C and a strain rate of 10−3 s−1 . Keywords: Ti-6Al-4V; globularization; texture; lamellar structure; DRX
1. Introduction The Ti-6Al-4V is one of the most important and widely used α + β titanium alloys in the world [1–3]. Bulk metalworking of Ti-6Al-4V alloy is generally comprised of a series of mechanical processing steps that are used to transform the as-cast microstructure into an equiaxed α + β two-phase microstructure [4]. Previous studies [5,6] have shown that thermomechanical processing (TMP) of the Ti-6Al-4V alloy above the β transus temperature results in a lamellar microstructure morphology that consists of α platelets and an inter-platelet β phase. When processed below the β transus, the conversion of transformed β microstructure to the equiaxed microstructure needs superior process control to avoid the generation of the microstructure defects like flow localization or wedge cracks [7,8]. Expect for processing temperature, other thermomechanical parameters, such as strain rate and strain, also influence the microstructure [9–12]. The response of the lamellar microstructure during hot deformation will be of great importance in projecting industrial thermomechanical processing schedules to get the desired microstructure without defect occurrence. Besides the microstructure, the texture is more critical to the mechanical properties of the Ti-6Al-4V alloy [13]. In spite of the great importance of crystallographic textures, the information about the microtexture development of Ti-6Al-4V alloy deformed in low strain rate ranges is still limited. The objective of this study is to determine the microstructure and microtexture evolution of the as-cast Ti-6Al-4V with a coarse lamellar structure in low strain rate ranges, and to systematically investigate the hot deformation behavior in both α + β and β phase fields.
Metals 2018, 8, 270; doi:10.3390/met8040270
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transus temperature reported for this grade was about 975 °C [4]. Table 1. Chemical composition of ELI grade Ti-6Al-4V alloy (wt. %).
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Al 6.08
V 4.03
Fe 0.14
Ti Bal.
Si 0.013
C 0.018
O 0.11
N 0.0047
H 0.0008
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2.2. Experimental Procedures 2. Materials and Methods Cylindrical compression samples of 8 mm diameter and 12 mm height were machined from the 2.1. Materials as received cylinder. Hot compression tests were performed in the temperature range of 900–1050 °C −3, 5 × 10−3, 10−2, 5 × 10−2, 10−1 s−1) on a Gleeble-3800 at 50The °C intervals and constantalloy true used straininrates ELI grade Ti-6Al-4V this (10 work is in cylinder form with a diameter of 355 mm simulator by with a height reduction of 60% (Figure 1). The temperature of thein specimens was produced conventional ingot metallurgy. Its chemical composition is shown Table 1. The β ◦ monitored by a chromel-alumel welded of the surface of the specimens. transus temperature reported forthermocouple this grade was about on 975the C middle [4]. Two tantalum foils with a thickness of 0.1 mm were put between the anvil and the specimen to minimize the friction. to isothermal compression, were heated to the designed TablePrior 1. Chemical composition of ELI gradespecimens Ti-6Al-4V alloy (wt. %). temperature with a speed of 10 °C/s and held for 5 min to obtain a homogeneous heat distribution. Al specimen V Fe Ti Si C immediately. O N After compression, each was water quenched HotHdeformed samples were axially sectioned parallel the compression axis and the0.11 longitudinal sections were prepared in 6.08 to4.03 0.14 Bal. 0.013 0.018 0.0047 0.0008 order to determine the microstructure evolution. The microstructures 2.2. Experimental Proceduresof the deformed samples were examined using optical microscopy (OM, Carl Zeiss Axio Observer. 5 m, Jena, Germany), scanning electron microscope (SEM, FEI Quanta 250, Cylindrical compression samples of 8 mm diameter and 12 mm height were machined from ),the as Houston, TX, USA), transmission electron microscopy (TEM, Hitachi H-800, Tokyo, Japan and ◦ C at received cylinder. Hot compression tests were performed in the temperature range of 900–1050 electron backscatter diffraction (EBSD, Apollo 300, Cambridge, UK). The specimens were ◦ C intervals and constant true strain rates (10−3 , 5 × 10−3 , 10−2 , 5 × 10−2 , 10−1 s−1 ) on a 50 electrochemically polished with a solution of 60% carbinol, 34% n-butanol, and 6% perchloric acid Gleeble-3800 simulator 60% 1). The temperature of the specimens using a voltage of 30 Vwith and aa height currentreduction of 80 mAoffor the(Figure OM, SEM, and EBSD characterization. The was monitored by a chromel-alumel thermocouple welded on the middle of the surface ofacid, the TEM foils were twin-jet electrochemical polished in a solution of 60% carbinol, 4% perchloric specimens. Two tantalum foils with a thickness of 0.1 mm were putanbetween thestep anvil the and 36% n-butanol. The EBSD characterizations were performed with increment of and 0.2 μm. specimen to minimize the friction. Prior to isothermal compression, specimens were heated to the In order to remove the orientation noise and uncertainty caused by severe deformation, ◦ designed temperature with speed of 10 and held forangle 5 mingrain to obtain a homogeneous heat misorientation below 2° wasa not taken intoC/s account. Low boundaries (LAGBs) were distribution. After compression, each specimen was water quenched immediately. Hot deformed defined as the grain boundaries with misorientation between 2° and 15° and shown in yellow. High samples were axially sectioned to the compression and the longitudinal sections were angle grain boundaries (HAGBs)parallel were defined as those withaxis misorientation above 15° and shown in prepared in order to determine the microstructure evolution. black.
Figure 1. Schematic of isothermal compression and viewing position (CD and TD represents compressing direction and tranverse direction, respectively): (a) undeformed sample; (b) deformed sample; (c) sample position.
The microstructures of the deformed samples were examined using optical microscopy (OM, Carl Zeiss Axio Observer. 5 m, Jena, Germany), scanning electron microscope (SEM, FEI Quanta 250, Houston, TX, USA), transmission electron microscopy (TEM, Hitachi H-800, Tokyo, Japan ), and electron backscatter diffraction (EBSD, Apollo 300, Cambridge, UK). The specimens were electrochemically
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polished with a solution of 60% carbinol, 34% n-butanol, and 6% perchloric acid using a voltage of 30 V and a current of 80 mA for the OM, SEM, and EBSD characterization. The TEM foils were twin-jet electrochemical polished in a solution of 60% carbinol, 4% perchloric acid, and 36% n-butanol. Metals 2018, 8, x FOR PEER REVIEW 3 of 13 The EBSD characterizations were performed with an increment step of 0.2 µm. In order to remove the orientation noise and of uncertainty by severe misorientation below 2◦ was not Figure 1. Schematic isothermalcaused compression anddeformation, viewing position (CD and TD represents takencompressing into account. Low angle grain boundaries (LAGBs) were as the grain (b) boundaries direction and tranverse direction, respectively): (a) defined undeformed sample, deformed with Metals 2018, 8, x FOR PEER REVIEW 3 of 13 ◦ and 15◦ and shown in yellow. High angle grain boundaries (HAGBs) misorientation between 2 were sample, (c) sample position. ◦ defined asFigure those1.with misorientation above 15 and and shown in black. Schematic of isothermal compression viewing position (CD and TD represents 3. Resultscompressing direction and tranverse direction, respectively): (a) undeformed sample, (b) deformed 3. Resultssample, (c) sample position. The initial microstructure was lamellar and had Widmannstatten morphology (Figures 2 and 3). The initial microstructure was of lamellar and mm had separated Widmannstatten morphology 3). It consisted of large prior β grains about 6–7 by a grain boundary(Figures α layer 2ofand 5 μm 3. Results It consisted large prior β grains of about by a αgrain boundary layer ofon 5 µm thick. The α of lamellar colonies formed inside6–7 the mm priorseparated β grains, and lamellae, whichαformed the The initial microstructure was lamellar and had Widmannstatten morphology (Figures 2 and 3). on thick. The α lamellar colonies formed inside the prior β grains, and α lamellae, which formed prior β grain boundaries, maintained a Burger’s relationship with their parent β matrix [14–16]. The It consisted ofboundaries, large prior β maintained grains of about 6–7 mm separated by a with grain their boundary α layer of 5 μm the prior β grain a Burger’s relationship parent β matrix [14–16]. thickness the α lamellae was formed about 3inside μm, andprior a continuous β αlayer of approximately 1μm thick. of The α lamellar colonies the β grains, and lamellae, which formed on the thick The of thebetween α lamellae was about 3 µm, and a continuous β layer of approximately thick was thickness distributed grain boundary and the Widmannstatten As1µm shown in prior β grainin boundaries,the maintained a Burger’sαrelationship with their parentsides-plates. β matrix [14–16]. The was distributed in between the grain boundary α and the Widmannstatten sides-plates. As shown Figure 3c,d), the texture of thewas scanned of two components. The stronger can be thickness of the α lamellae about 3area μm,consisted and a continuous β layer of approximately 1μmone thick in Figure 3c,d), the in texture of the scanned area consisted of two components. The stronger one can expressed by Euler angles as the (ϕ1grain = 132°, Φ = 75°, ϕ2the =38°), which is represented by in the was distributed between boundary α and Widmannstatten sides-plates. As A shown in pole ◦ , Φ = 75◦ , ϕ2 =38◦ ), which is represented by A in the pole be expressed by Euler angles as (ϕ1 = 132 Figure 3c,d), the texture of the scanned consistedby of B two components. The stronger one can be figures. Additionally, the weaker one is area represented and can be expressed by Euler angles as figures. Additionally, weaker one is represented B and canisbe expressed Euler as as (ϕ1textures = 132°, Φ 75°, ϕ2 by =38°), which represented byby Atoward in the angles pole (ϕ1 =expressed 38°, Φ = by 75°,Euler ϕ2 the =angles 10°). The all= rotated approximately 75° from ND TD (ND Additionally, the◦ ). weaker one is represented B and can be expressed Euler anglesTD as (ND (ϕ1 =figures. 38◦ , Φ = normal 75◦ , ϕ2direction). = 10 The textures all rotatedbyapproximately 75◦ frombyND toward represents the (ϕ1 = 38°, = 75°, direction). ϕ2 = 10°). The textures all rotated approximately 75° from ND toward TD (ND represents the Φ normal represents the normal direction).
Figure 2. Macro structure theundeformed undeformed sample: sample: (a) and (b) (b) transverse section. Figure 2. Macro ofof the (a)top topsurface surface and transverse section. Figure 2. Macro structure structure of the undeformed sample: (a) top surface and (b) transverse section.
Figure 3. Cont.
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Figure 3. Initial microstructure of Ti-6Al-4V used in this study: study: (a)OM, OM, (b) SEM, (c) the orientation 3. Initial microstructure Ti-6Al-4Vused usedin in this this SEM, (c) (c) the orientation FigureFigure 3. Initial microstructure of of Ti-6Al-4V study:(a) (a) OM;(b)(b) SEM; the orientation map, map, and (d) pole figure. andpole (d) pole figure. map; and (d) figure.
Typical macro pictures of of deformed shownininFigure Figure shown in Figure Typical macro pictures deformedspecimen specimen are are shown 4. 4. AsAs shown in Figure 4, in 4, in −3 −2 s−1, of Typical macro pictures deformed specimen are shown in Figure 4. As shown in Figure 4, −3 −2 −1 the range of 10 –10 deformation bands in the samples were not clear, while in the range of of the range of 10 –10 s , deformation bands in the samples were not clear, while in the range −3 –10−2 s−1 , deformation bands in the samples were not clear, while in the range of −2–10 −1 −1 in the range of 10 10 s , adiabatic shear bands were obvious. −2 −1 −1 10 –10 s , adiabatic shear bands were obvious. 10−2 –10−1 s−1 , adiabatic shear bands were obvious.
Figure 4. Cont.
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−3 s−1, (b) 950 °C/10−3 s−1, (c) 900 Figure 4. 4. Macro ◦ C/10− 3 s−1 ; (b) 950 ◦ C/10−3 s−1 ; Figure Macro pictures pictures of of Ti-6Al-4V Ti-6Al-4V sample sample deformed deformed at at (a) (a)900 900°C/10 −3 s−1, (d) 950 °C/5 × 10−3 s−1, (e) 900 °C/10−2 s−1, (f) 950 °C/10−2 s−1, (g) 900 °C/5 × 10−2 s−1, (h) 950 °C/5 × 10 ◦ − 3 − 1 ◦ − 3 − 1 ◦ − 2 (c) 900 C/5 × 10 s ; (d) 950 C/5 × 10 s ; (e) 900 C/10 s−1 ; (f) 950 ◦ C/10−2 s−1 ; −2 s−1, (k) 900 °C/10−1 s−1, and (i) 950 °C/10−1 s−1. °C/5 × 10 (g) 900 ◦ C/5 × 10−2 s−1 ; (h) 950 ◦ C/5 × 10−2 s−1 ; (k) 900 ◦ C/10−1 s−1 ; and (i) 950 ◦ C/10−1 s−1 .
Figure 5a–d,f exhibits different microstructures at various positions of Ti-6Al-4V sample Figure 5a–d,f exhibits different microstructures at various of Ti-6Al-4V sample −2 spositions −1. According compressed at a temperature of 900◦ °C and a strain rate of 10 to Figure 5, three compressed at a temperature of 900 C and a strain rate of 10−2 s−1 . According to Figure 5, three different deformation zones could be defined: “dead” zone at bottom and top areas near the contact different deformation zones could be defined: “dead” zone at bottom and top areas near the contact surfaces, partial plastic deformation zone close to the both sides, and severe plastic deformation surfaces, partial plastic deformation zone close to the both sides, and severe plastic deformation zone zone in the center. In this study, the microstructures of inner severe plastic deformation zone are in the center. In this study, the microstructures of inner severe plastic deformation zone are determined. determined. Metals 2018, 8, x FOR PEER REVIEW 6 of 13 Microstructure of the samples deformed in the α + β phase field are shown in Figures 6 and 7. The studied areas were all taken from the central part of the deformed samples due to the largest amount of deformation. From Figure 6, it is noted that the lower temperature and lower strain rate domain (900–950 °C, 10−3–10−2 s−1) represented spheroidization of lamellar structure. Microstructural and microtexture evolution during the spheroidization process were further elucidated by EBSD and TEM. As shown in Figure 8, the microtexture of the specimen deformed at the temperature of 900 °C and strain rate of 10 −3 s−1 consisted of two components: one was the {0001} transverse texture and the other was the {0001} texture rotated 75° from ND to TD. When deformed at a temperature of 900 °C and strain rate of 10 −2 s−1, the microtexture of the specimen consisted of three components: there was an additional 50° {0001} texture, except for the transverse texture and the 75° texture (Figure 9). TEM images exhibited the detailed information of the globularization of α lamellae (Figures 10 and 11). As shown in Figure 10a,b, shearing of the α lamellae and splitting of α/α boundaries were observed, and formation of subgrains was observed inside the α phase (Figure 10c,d). The microstructure of the samples deformed in the β phase field are shown in Figure 12; the results indicated that DRX occurred when the deformation temperature was in the β phase field. As given in Figure 11, the penetration of the β phase along the α platelets was observed. Figure 5. Macro picture of Ti-6Al-4V sample deformed at 900 °C and a strain rate of 10−2 s−1. (a), (c), (d)
Figure 5. Macro picture Ti-6Al-4V sample deformed at 900 ◦zone, C and a strain rate of 10−2 s−1 . partial plasticof deformation zone, (b) severe plastic deformation (f) “dead” zone. (a,c,d) partial plastic deformation zone; (b) severe plastic deformation zone; (f) “dead” zone.
Microstructure of the samples deformed in the α + β phase field are shown in Figures 6 and 7. The studied areas were all taken from the central part of the deformed samples due to the largest
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Figure 5. Macro picture of Ti-6Al-4V sample deformed at 900 °C and a strain rate of 10−2 s−1. (a), (c), (d)
amount of deformation. From Figure 6, it is noted that the lower temperature and lower strain rate partial plastic deformation zone, (b) severe plastic deformation zone, (f) “dead” zone. domain (900–950 ◦ C, 10−3 –10−2 s−1 ) represented spheroidization of lamellar structure.
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Figure 6. Microstructures of Ti-6Al-4V specimens deformed at (a) 900 °C/10−3 s−1, (b) 950 °C/10−3 s−1, Figure 6. Microstructures of Ti-6Al-4V specimens deformed at (a) 900 ◦ C/10−3 s−1 ; (b) 950 ◦ C/10−3 s−1 ; (c) 900 °C/5 × 10−3 s−1, (d) 950 °C/5 × 10−3 s−1, (e) 900 °C/10−2 s−1, and (f) 950 °C/10−2 s−1. (c) 900 ◦ C/5 × 10−3 s−1 ; (d) 950 ◦ C/5 × 10−3 s−1 ; (e) 900 ◦ C/10−2 s−1 ; and (f) 950 ◦ C/10−2 s−1 .
Figure 6. Microstructures of Ti-6Al-4V specimens deformed at (a) 900 °C/10−3 s−1, (b) 950 °C/10−3 s−1, Metals 2018, 8, 270 (c) 900 °C/5 × 10−3 s−1, (d) 950 °C/5 × 10−3 s−1, (e) 900 °C/10−2 s−1, and (f) 950 °C/10−2 s−1.
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Figure 7. Microstructures of Ti-6Al-4V specimens deformed at (a) 900 °C/5 × 10−2 s−1, (b) 900 °C/10−1 Figure 7. Microstructures of Ti-6Al-4V specimens deformed at (a) 900 ◦ C/5 × 10−2 s−1 ; s−1, and (c) 950 °C/10−1 s−1. (b) 900 ◦ C/10−1 s−1 ; and (c) 950 ◦ C/10−1 s−1 .
Microstructural and microtexture evolution during the spheroidization process were further elucidated by EBSD and TEM. As shown in Figure 8, the microtexture of the specimen deformed at the temperature of 900 ◦ C and strain rate of 10−3 s−1 consisted of two components: one was the {0001} transverse texture and the other was the {0001} texture rotated 75◦ from ND to TD. When deformed at a temperature of 900 ◦ C and strain rate of 10−2 s−1 , the microtexture of the specimen consisted of three components: there was an additional 50◦ {0001} texture, except for the transverse texture and the 75◦ texture (Figure 9). TEM images exhibited the detailed information of the globularization of α lamellae (Figures 10 and 11). As shown in Figure 10a,b, shearing of the α lamellae and splitting of α/α boundaries were observed, and formation of subgrains was observed inside the α phase (Figure 10c,d). The microstructure of the samples deformed in the β phase field are shown in Figure 12; the results indicated that DRX occurred when the deformation temperature was in the β phase field. As given in Figure 11, the penetration of the β phase along the α platelets was observed.
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−3 s−1:−(a) ◦ C/10 3 sthe −1 : Figure 8. 8. Microstructure Ti-6Al-4V specimens deformed at 900 °C/10 Figure Microstructureand andmicrotexture microtextureofof Ti-6Al-4V specimens deformed at 900 Figure 8. Microstructure and microtexture of Ti-6Al-4V specimens deformed at 900 °C/10−3 s−1: (a) the orientation map, (b) pole (c) distribution of grain size of of αα phase, (a) the orientation map; (b) figure, pole figure; (c) distribution of grain size phase;and and(d) (d)grain grain boundary boundary orientation map, (b) pole figure, (c) distribution of grain size of α phase, and (d) grain boundary misorientation of of α α phase. phase. misorientation misorientation of α phase.
Figure 9. Cont.
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−2 −1− ◦ C/10 2 s− 1: Figure 9. 9. Microstructure Microstructure and microtexture ofof Ti-6Al-4V specimens deformed at 900 900 °C/10 (a) the −2 ss−1 −1 : (a) Figure Microstructureand andmicrotexture microtexture Ti-6Al-4V specimens deformed at 900 −2 and microtexture of Ti-6Al-4V specimens deformed at °C/10 the Figure 9. Microstructure of Ti-6Al-4V specimens deformed at 900 °C/10 s :: (a) the orientation map, (b) pole figure, (c) distribution of grain size of α phase, and (d) grain boundary (a) the orientation map; (b) figure, pole figure; (c) distribution of grain phase;and and(d) (d)grain grain boundary orientation map, (b) (b) pole figure, (c) distribution distribution of grain grain sizesize of of αα phase, and (d) grain boundary orientation map, pole (c) of size of α phase, misorientation of of α α phase. phase. misorientation α misorientation of α phase.
−3 −1: (a) breakdown of the Figure 10. 10. TEM TEM images images of of Ti-6Al-4V Ti-6Al-4V specimens specimens deformed deformed at at 900 900 °C/10 °C/10−3 −3 ss−1 −1 (a) breakdown of the Figure Figure 10. TEM images of Ti-6Al-4V specimens deformed at 900 °C/10 s :: (a) breakdown of the ◦ − 3 Figure 10.structure TEM images of Ti-6Al-4V specimens deformed at 900 C/10 s−1 : (a) breakdown of the lamellar and (b) subgrains inside the recrystallized grains. lamellar structure structure and and (b) (b) subgrains subgrains inside inside the the recrystallized recrystallized grains. grains. lamellar lamellar structure and (b) subgrains inside the recrystallized grains.
Figure 11. Cont.
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−2 s−1: (a) bright field image, (b) Figure 11. TEM images of Ti-6Al-4V specimens deformed at 900 °C/10 Figure 11. TEM images of Ti-6Al-4V specimens deformed at 900 ◦ C/10−2 s−1 : (a) bright field image; dark field image, (c) SAED patterns, and (d) DRX grains. (b) dark11. field image; (c) of SAED patterns; and (d) deformed DRX grains. Figure TEM images Ti-6Al-4V specimens at 900 °C/10−2 s−1: (a) bright field image, (b) dark field image, (c) SAED patterns, and (d) DRX grains.
Figure 12. Microstructures of Ti-6Al-4V specimens deformed at 1050 °C/10−3 s−1. Figure 12. Microstructures of Ti-6Al-4V specimens deformed at 1050◦ °C/10−3 s −.1 4. DiscussionFigure 12. Microstructures of Ti-6Al-4V specimens deformed at 1050 C/10 s . −3
−1
4. Discussion 4.1. Globularization 4. Discussion In the ranges 900–950 °C and 10−3–10−2 s−1, the microstructures of deformed samples exhibited 4.1. 4.1. Globularization Globularization the transformation of the lamellar structure of α phase to equiaxed. As seen in Figures 6, 8 and 9, −2 s−1, the microstructures of deformed samples exhibited In the ranges ranges 900–950 900–950◦°C and1010 –10− −−3 3 –10 2 s−1 , the microstructures of deformed samples exhibited In the C and microstructure of specimens compressed at 900–950 °C and different strain rates indicated the transformation of the lamellar structure of α phase to equiaxed. As seen Figures 6, 8 and the transformation lamellar structure α phase to900 equiaxed. As in seen in Figures 6, 9,8 globularization of theofαthe lamellar structure. It is of shown that at °C large deformation broke down microstructure of specimens compressed at 900–950 °C and different strain rates indicated ◦ andα9,lamellae microstructure of specimens compressed at 900–950 C and different strain rates the into segments; then, the small α segments transformed into spherodized αp indicated particles, globularization of the α lamellar structure. It is shown that at 900 ◦°C large deformation broke down globularization thelayers α lamellar structure. the It isαshown thatinata900 C large deformation broke down and the retainedofthin of β separated p particles microstructure called fully equiaxed. the lamellae into segments; then, the small α α segments segments transformed transformed into spherodized α αp particles, the α αgrain lamellae into then, thewas small spherodized The size of thesegments; primary α phase 3.4 μm at a stain rate of 10−3into s−1, and it reduced ptoparticles, 2.1 μm, and the retained thin layers of β separated the αp particles in a microstructure called fully equiaxed. −2 s−1 (Figures and the thin layers ofto β 10 separated the αp8 particles with theretained strain rate increasing and 9). in a microstructure called fully equiaxed. −1 The grain size of the primary α phase was 3.4 μm at stainrate rateofof1010 to −−3 3 ss−1, ,and The grain size ofβ the primary phase was 3.4completely µm at aastain andititreduced reduced to2.1 2.1 μm, µm, The initial grains and ααcolonies were lost during the deformation. The 75° texture −2 −1 with the strain rate increasing to 10−2s −(Figures 8 and 9). withgenerally the strainpreserved rate increasing to 10 s 1 (Figures 8 andsuggesting 9). was from the as-received material, a degree of texture memory. And, The initial β grains and α colonies were completely lost during the deformation. The 75° ◦ texture The initial of β grains and α texture colonies indicated were completely lost during the deformation. The 75 texture the existence transverse the texture randomization resulting from the was generally preserved from the as-received material, suggesting a degree of texture memory. And, was generally preserved from the as-received material, suggesting50° a degree texture memory. globularization process. Furthermore, the appearence of additional texture of suggested that the the existence of transverse texture indicated the texture randomization resulting from the And, the of transverse texture indicated the texture resulting from the strain rateexistence increase led to more randomization. The evolution of randomization pole figure density also confirmed globularization process. Furthermore, the appearence of additional 50°◦ texture suggested that the globularization this conclusion. process. Furthermore, the appearence of additional 50 texture suggested that the strain rate increase led to more randomization. The evolution of pole figure density also confirmed strainInrate increase led9,tothe more randomization. evolution of pole also confirmed Figures 8 and LAGBs and HAGBsThe were represented byfigure solid density yellow lines and black this conclusion. this conclusion. lines, respectively. The fraction of HAGBs was high, and nearly all the new equiaxed grains were In Figures 8 and 9, the LAGBs and HAGBs were represented by solid yellow lines and black In Figures 8 and 9, the LAGBs and were represented solid yellow linesappeared and blackinlines, encircled by high-angle boundaries. OnHAGBs the contrary, low-angle by boundaries mostly the lines, respectively. The fraction of HAGBs was high, and nearly all the new equiaxed grains were respectively. The fraction of HAGBs high, and nearly all the newhad equiaxed grains were encircled retained lamellar α phase. Several was works [8,17–21] on Ti-6Al-4V provided insight into the encircled by high-angle boundaries. On the contrary, low-angle boundaries mostly appeared in the by high-angle boundaries. Onlamellar the contrary, low-angle boundaries mostly appeared in the retained globularization process of the α phase during hot deformation. Weiss et al. [21] invoked the retained lamellar α phase. Several works [8,17–21] on Ti-6Al-4V had provided insight into the lamellarshear α phase. Several works on Ti-6Al-4V haddriving provided insight intoαthe globularization intense bands inside the [8,17–21] α lamellae as the main force of the breakdown, and globularization process of the lamellar α phase during hot deformation. Weiss et al. [21] invoked the boundaries were formed by β penetrating the α platelets by diffusion. Additionally, the observations intense shear bands inside the α lamellae as the main driving force of the α breakdown, and boundaries were formed by β penetrating the α platelets by diffusion. Additionally, the observations
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process of the lamellar α phase during hot deformation. Weiss et al. [21] invoked the intense shear bands inside the α lamellae as the main driving force of the α breakdown, and boundaries were formed by β penetrating the α platelets by diffusion. Additionally, the observations agreed with this (Figure 11). Seshacharyulu [8,22] reported that the shearing of α lamellae was due to imposed shear strain; then, dislocations were generated along the line of shear, and an interface consisting of dislocations with same sign was formed; finally, the process of globularization of α phase was completed by migration of interfaces. Zherebtsov [23] also noted that dislocation walls formed in some of the lamellar α phases after a height strain ε of 0.29 and a microstructure involving a high dislocation density and subgrains were formed after strain ε of 0.69, and a mixed globular/lamellar microstructure was formed when the strain ε increased to 1.2. As given in Figures 10 and 11, the process of globularization could be considered as a type of CDRX. In this process, the low-angle boundaries formed by dislocations were produced at low strains, and then the LAGBs transformed into HAGBs at large strains to complete the globularization of α phase. Additionally, the fraction of the HAGBs increased from 72% to 74%, with the strain rate increasing from 10−3 s−1 to 10−2 s−1 . It was noted that the degree of recrystallization was increased with the strain rate increasing. When the temperature was up to 950 ◦ C, the increasing temperature enabled the transformation of the β phase into new α lamellae in a matrix of retained β. The microstructure consisted of spherodized primary α particles and new lamellae of secondary α in a matrix of retained β phase, which was called a bimodal microstructure. The volume fraction of primary α phase increased with the strain rate increasing. 4.2. Deformation Behavior of the β Phase DRX is an important microstructural mechanism that can occur during hot working of the Ti-6Al-4V alloy in the β phase field. As seen in Figure 12, microstructure of the specimen deformed at 1050 ◦ C/10−3 s−1 in the β phase field revealed that the new prior β grains were formed. Additionally, when deformed in the β phase field, the microstructure was comprised of almost complete martensite due to water quenching. This evidence clearly indicated that β phase underwent DRX. 4.3. Flow Instabilities The microstructural observations in the temperature range 900–950 ◦ C and strain rate range 0.03–0.1 s−1 confirmed the flow instabilities of the coarse lamellar structure. Microstructures of the samples deformed under conditions covered in this regime were given in Figure 7. From Figure 7, it could be seen that, in the lower temperature regime, nonuniform deformation led to regions of intense localized shear in which the lamellae broke up. So, the instability mechanism was mainly flow localization, even cracking. The explanation for this was that during deformation, the time was not sufficient for the heat generated during deformation to conduct away, which caused highly localized flow instability [10]. Otherwise, when deformed at 950 ◦ C/10−1 s−1 , the majority of the α laths became kinked. Generally, kinks were formed inside colonies during deformation at low temperatures (550–700 ◦ C) [24,25]. However, in severe deformation conditions, the adjacent colonies rotated towards one another, causing localized flow in the boundary between the colonies and therefore loss of the (orientation relationship) OR [26]. In details, α laths broke up with increasing strain, resulting in the formation of rows of shorten α grains/subgrains (Figure 7c). The instability domain identified above must be taken into consideration to avoid defect during hot deformation of Ti-6Al-4V in the α + β phase field. 5. Conclusions The microstructure and microtexture evolution of an ELI grade Ti-6Al-4V alloy with a lamellar initial microstructure was investigated by means of isothermal compression tests over the temperature range 900–1050 ◦ C and low strain rate range 10−3 –10−1 s−1 . The following conclusions were obtained from this study.
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In the ranges 900–950 ◦ C and 10−3 –10−2 s−1 , globularization of the lamellar structure occurred. The process of globularization could be considered as a type of CDRX, and grain boundaries were formed by β penetrating the α platelets by diffusion. After globularization, the initial α {0001} texture softened, and the {0001} texture rotated to the TD direction. The globularization process resulted in texture randomization and the increasing strain rate led to more randomization. In the ranges 900–950 ◦ C and 10−2 –10−1 s−1 , flow instabilities occurred; the instability mechanism was mainly flow localization, even cracking. In the β range, DRX occurred at a temperature of 1050 ◦ C and a strain rate of 10−3 s−1 .
Acknowledgments: The authors are grateful for the financial support provided by the National Nature Science Foundation of China. (Grant Nos. U1302275 and 51305304). Author Contributions: Zhixiong Zhang and Shoujiang Qu conceived and designed the experiments; Zhixiong Zhang performed the experiments; Zhixiong Zhang and Xin Hu analyzed the data; Zhixiong Zhang and Aihan Feng and Jun Shen wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
References 1. 2.
3.
4. 5. 6.
7. 8.
9. 10. 11. 12. 13. 14.
Xu, W.; Lui, E.W.; Pateras, A.; Qian, W.; Brandt, M. In situ tailoring microstructure in additively manufactured Ti–6Al–4V for superior mechanical performance. Acta Mater. 2017, 125, 390–400. [CrossRef] Zhang, Z.X.; Qu, S.J.; Feng, A.H.; Shen, J. Achieving grain refinement and enhanced mechanical properties in Ti–6Al–4V alloy produced by multidirectional isothermal forging. Mater. Sci. Eng. A 2017, 692, 127–138. [CrossRef] Li, B.; Shen, Y.; Luo, L.; Hu, W. Effects of processing variables and heat treatments on Al/Ti–6Al–4V interface microstructure of bimetal clad-plate fabricated via a novel route employing friction stir lap welding. J. Alloy. Compd. 2016, 658, 904–913. [CrossRef] Boyer, R.; Welsch, G.; Collings, E.W. (Eds.) Materials Properties Handbook: Titanium Alloys; ASM International: Materials Park, OH, USA, 1994. Prasad, Y.V.; Seshacharyulu, T.; Medeiros, S.C.; Frazier, W.G. A study of beta processing of Ti–6Al–4V: Is it trivial? J. Eng. Mater. Technol. 2001, 123, 355–360. [CrossRef] Prasad, Y.V.; Seshacharyulu, T.; Medeiros, S.C.; Frazier, W.G. Microstructural modeling and process control during hot working of commercial Ti–6Al–4V: Response of lamellar and equiaxed starting microstructures. Mater. Manuf. Process. 2000, 15, 581–604. [CrossRef] Seshacharyulu, T.; Medeiros, S.C.; Frazier, W.G.; Prasad, Y.V. Unstable flow during supratransus working of Ti–6Al–4V. Mater. Lett. 2001, 47, 133–139. [CrossRef] Seshacharyulu, T.; Medeiros, S.C.; Morgan, J.T.; Malas, J.C.; Frazier, W.G.; Prasad, Y.V. Hot deformation and microstructural damage mechanisms in extra-low interstitial (ELI) grade Ti–6Al–4V. Mater. Sci. Eng. A 2000, 279, 289–299. [CrossRef] Ding, R.; Guo, Z.; Wilson, A. Microstructural evolution of a Ti–6Al–4V alloy during thermomechanical processing. Mater. Sci. Eng. A 2002, 327, 233–245. [CrossRef] Seshacharyulu, T.; Medeiros, S.C.; Frazier, W.G.; Prasad, Y.V. Microstructural mechanisms during hot working of commercial grade Ti–6Al–4V with lamellar starting structure. Mater. Sci. Eng. A 2002, 325, 112–125. [CrossRef] Zhang, Z.X.; Qu, S.J.; Feng, A.H.; Shen, J.; Chen, D.L. Hot deformation behavior of Ti–6Al–4V alloy: Effect of initial microstructure. J. Alloy. Compd. 2017, 718, 170–181. [CrossRef] Salem, A.A.; Semiatin, S.L. Anisotropy of the hot plastic deformation of Ti–6Al–4V single-colony samples. Mater. Sci. Eng. A 2009, 508, 114–120. [CrossRef] Lütjering, G. Influence of processing on microstructure and mechanical properties of (α + β) titanium alloys. Mater. Sci. Eng. A 1998, 243, 32–45. [CrossRef] Semiatin, S.L.; Seetharaman, V.; Weiss, I. The thermomechanical processing of alpha/beta titanium alloys. JOM 1997, 49, 33–39. [CrossRef]
Metals 2018, 8, 270
15.
16. 17. 18.
19. 20. 21. 22. 23. 24. 25.
26.
13 of 13
Bhattacharyya, D.; Viswanathan, G.B.; Denkenberger, R.; Furrer, D.; Fraser, H.L. The role of crystallographic and geometrical relationships between α and β phases in an α/β titanium alloy. Acta Mater. 2003, 51, 4679–4691. [CrossRef] Stanford, N.; Bate, P.S. Crystallographic variant selection in Ti–6Al–4V. Acta Mater. 2004, 52, 5215–5224. [CrossRef] Stefansson, N.; Semiatin, S.L.; Eylon, D. The kinetics of static globularization of Ti–6Al–4V. Metall. Mater. Trans. A 2002, 33, 3527–3534. [CrossRef] Semiatin, S.L.; Brown, J.O.; Brown, T.M.; DeLo, D.P.; Bieler, T.R.; Beynon, J.H. Strain-path effects during hot working of Ti–6Al–4V with a colony-alpha microstructure. Metall. Mater. Trans. A 2001, 32, 1556–1559. [CrossRef] Semiatin, S.L.; Stefansson, N.; Doherty, R.D. Prediction of the kinetics of static globularization of Ti–6Al–4V. Metall. Mater. Trans. A 2005, 36, 1372–1376. [CrossRef] Stefansson, N.; Semiatin, S.L. Mechanisms of globularization of Ti–6Al–4V during static heat treatment. Metall. Mater. Trans. A. 2003, 34, 691–698. [CrossRef] Weiss, I.; Froes, F.H.; Eylon, D.; Welsch, G.E. Modification of alpha morphology in Ti–6Al–4V by thermalmechanical processing. Metall. Trans. A. 1986, 17A, 1935–1947. [CrossRef] Seshacharyulu, T.; Medeiros, S.C.; Morgan, J.T.; Malas, J.C.; Frazier, W.G.; Prasad, Y.V. Hot deformation mechanisms in ELI grade Ti–6Al–4V. Scr. Mater. 1999, 41, 283–288. [CrossRef] Zherebtsov, S.; Murzinova, M.; Salishchev, G.; Semiatin, S.L. Spheroidization of the lamellar microstructure in Ti–6Al–4V alloy during warm deformation and annealing. Acta Mater. 2011, 59, 4138–4150. [CrossRef] Zherebtsov, S.; Mazur, A.; Salishchev, G.; Lojkowski, W. Effect of hydrostatic extrusion at 600–700 ◦ C on the structure and properties of Ti–6Al–4V alloy. Mater. Sci. Eng. A 2008, 485, 39–45. [CrossRef] Zherebtsov, S.V.; Salishchev, G.A.; Galeyev, R.M.; Valiakhmetov, O.R.; Mironov, S.Y.; Semiatin, S.L. Production of submicrocrystalline structure in large-scale Ti–6Al–4V billet by warm severe deformation processing. Scr. Mater. 2004, 51, 1147–1151. [CrossRef] Mironov, S.; Murzinova, M.; Zherebtsov, S.; Salishchev, G.A.; Semiatin, S.L. Microstructure evolution during warm working of Ti–6Al–4V with a colony-α microstructure. Acta Mater. 2009, 57, 2470–2481. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
