Microstructure and mechanical properties of a biomedical β- type titanium alloy subjected to severe plastic deformation after aging treatment H. Yilmazer1, a, M. Niinomi2, b *, M. Nakai2, c, J. Hieda2, d, T. Akahori3, e and Y. Todaka4, f 1
Graduate Student, Tohoku University, Sendai 980–8579, Japan
2
Institute for Materials Research, Tohoku University, Sendai 980–8577, Japan
3
Department of Materials Science and Engineering, Faculty of Materials Science and Technology, Meijo University, Nagoya 468-8502, Japan
4
Department of Production Systems Engineering, Toyohashi University of Technology, Toyohashi 441–8580, Japan a
[email protected],
[email protected],
[email protected], d
[email protected],
[email protected], f
[email protected]
Keywords: Ti-29Nb-13Ta-4.6Zr; Metallic biomaterial; Severe plastic deformation; Microstructure; Mechanical hardness
Abstract. Strengthening by grain refinement and increasing dislocation density through highpressure torsion (HPT), which is an attractive technique to fabricate ultrafine grained and nanostructured metallic materials, is expected to provide β-type Ti-29Nb-13Ta-4.6Zr (TNTZ) higher mechanical strength while maintaining low Young’s modulus because they keep the original β phase. However, the ductility shows reverse trend. Greater strength with enhanced ductility can be achieved by controlling precipitated phases through HPT processing after aging treatment. Aged TNTZ subjected to HPT processing at high N exhibits a homogeneous microstructure with ultrafine elongated grains having a high dislocation density and consequently non-equilibrium boundaries and distorted subgrains with non-uniform shapes and nanostructured intergranular precipitates of α phases. Therefore, the effect of HPT processing on the microstructure and mechanical hardness of TNTZ after aging treatment was systematically investigated in this study. TNTZ, which was subjected to aging treatment at 723 K for 259.2 ks in vacuum followed by water quenching, subjected to HPT processing at rotation numbers (N) of 1 to 20 under a pressure of around 1.25 GPa at room temperature. The microstructure of TNTZAT consisted of precipitated needle-like α phases in β grains. However, TNTZAHPT at N ≥ 10 comprises very fine α and small amount ω phases in ultrafine β grains. Furthermore, the hardness of every TNTZAHPT was totally much greater than that of TNTZAT. The hardness increased from the center to peripheral region of TNTZAHPT. In addition, the tensile strength of every TNTZAHPT was greater than that of TNTZAT. The tensile strength of TNTZAHPT increased, but the elongation decreased with increasing N and then both of them saturated at N ≥ 10. Introduction β-type titanium alloys are attractive candidates for surgical implants due to its unique combination of favorable mechanical properties including a high tensile strength, good cold workability, enhanced corrosion resistance, and outstanding biocompatibility [1]. A novel β-type titanium alloy, Ti-29Nb-13Ta-4.6Zr (TNTZ), with a low Young’s modulus (~60 GPa), relatively closer to that of bone as compared with the conventional metallic biomaterials such as stainless steel, Co-Cr-Mo alloy and Ti-6Al-4V ELI [2, 3], has been developed by present authors [4]. A high mechanical biocompatibility, which implies excellent mechanical properties such as great strength and hardness with keeping low Young’s modulus in TNTZ, can be achieved by microstructural controlling.
In this alloy, such a low Young’s modulus can be achieved by metastable β structure after solution treatment [5]. However, the mechanical strength of this alloy in solutionized conditions is less than that of conventional titanium alloy, Ti-6Al-4V ELI, which is currently applied for practical biomedical applications. Therefore, various thermomechanical treatments were examined previously to improve the mechanical strength of TNTZ [3] The aging treatment is the simple way to increase mechanical strength because of the precipitation of secondary phase such as α and/or ω phases. In this case, precipitation strengthening due to α and/or ω phase formations during aging occurs so that the mechanical strength can be drastically improved. However, the Young’s modulus increases simultaneously to be relatively high because Young’s moduli of the α and ω phases are much higher than that of the β phase, which is the matrix phase of TNTZ. Therefore, the methods to possess high mechanical biocompatibility have been studied for TNTZ [6-9]. Recently, severe plastic deformation (SPD) techniques such as equal-channel angular pressing (ECAP) [10] and high-pressure torsion (HPT) [11] have been developed to produce bulk nanostructured materials by inducing very intense plastic strain [11, 12]. The materials composed of nanosized grains with high angle grain boundaries exhibit better mechanical properties and higher hardness values as compared to coarse–grained materials. HPT processing developed by Bridgman [11] is also quite effective to produce materials with ultra-fine grains, whose diameters are below 100 nm. For example, it has been reported that the grain diameters of Al-3Mg alloys are 270 nm after ECAP [13] but 90 nm after HPT processing [14]. The grain diameter of Ti-6Al-4V has been also reported to be reduced to 30-50 nm [15] and that of Ni3Al is reduced to 25 nm [16] by HPT processing. The HPT processing is considered to be one of the most effective ways to provide ultra-fine grains and extra-high dislocation density leading to grain refinement strengthening and dislocation strengthening. In our previous study [6], HPT processing has been effective to improve the mechanical strength of TNTZ by grain refinement strengthening and dislocation strengthening. The tensile strength of TNTZ subjected to HPT processing (TNTZHPT) after cold rolling increased with increasing imposed equivalent strain (εeq) and reach to around 1000 MPa, which was greater than those of TNTZ subjected to cold rolling and solution treatment. However, the elongation of TNTZHPT continuously decreased with increasing imposed εeq. HPT processing combined with aging treatment is expected to improve the mechanical properties of TNTZ further. Therefore, the effect of HPT processing on the microstructure and mechanical hardness of Ti-29Nb-13Ta-4.6Zr (TNTZ) subjected to aging treatment was investigated for biomedical applications in this study. Experimental Procedures Material. The material used in this study was a hot-forged TNTZ bar with a diameter of 25 mm and a length of 50 mm. Its chemical composition is listed in Table 1. This hot-forged TNTZ bar was subjected to a solution treatment at 1063 K for 3.6 ks (TNTZST) and then subjected to aging treatment at 723 K for 259.2 ks (TNTZAT) in vacuum, followed by water-quenching. Finally, TNTZAT was machined into coin-shaped specimens with a diameter of 20 mm and a thickness of 0.8 mm for HPT processing. Table 1. Chemical composition of hot-forged TNTZ [mass %]. Element
Nb
Ta
Zr
C
N
O
H
Fe
Ti
28.6
12.3
4.75
0.02
0.01
0.09
0.04
0.22
0.22
bal.
HPT processing. The coin-shaped specimens of TNTZAT were subjected to HPT processing between two anvils opposed vertically by rotating the lower anvil under a pressure of 1.25 GPa at rotation numbers (N) of 1 to 20 at a rotation speed of 0.2 rpm at room temperature. A schematic illustration of the HPT processing is shown in Fig. 1. The applied force was maintained at a
constant value of 40 tons, corresponding to a pressure of 1.25 GPa. The flat bottoms of the holders were roughened to increase the frictional force between the specimen and the anvils. At the same time, a lubricant containing MoS2 was applied around the periphery of the holder for the lower anvil to decrease the frictional force between the anvils. Hereafter, the coin-shaped specimen of TNTZAT subjected to HPT processing is referred to as TNTZAHPT.
Fig. 1. A schematic illustration of HPT processing.
Material Characterization. The microstructure of each specimen was observed using both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM observations were carried out at a half radius, rh, position, r = rh = 5 mm, for each specimen with an accelerating voltage of 200 kV. In this study, the grain size, namely grain diameter for TNTZAHPT was measured quantitatively using TEM bright field images. The phase constitution of each specimen was analyzed at rh position using an X-ray diffractometer (XRD) using a Cu-Kα radiation tube with a voltage of 40 kV and a current of 40 mA. Mechanical hardness measurements. Mechanical hardness measurements were performed using a Vickers hardness tester with a load of 500 g for a dwell time of 15 s on the surface of TNTZAHPT. The measurements on the surface of the coin-shaped specimen were performed from the center to peripheral region at intervals of 1 mm between the measurement positions along the radial. The radial mechanical hardness distribution was then plotted. 1. Results and Discussion Microstructure. Fig. 2 shows the XRD profiles of TNTZAT and TNTZAHPT at N = 1 to 20 obtained at rh position. TNTZAT consists of precipitated α phase and β phase. However, the intensity of the β {110} peak is significantly stronger than those of the other peaks in TNTZAHPT at N = 1, 5, 10, and 20. However, the β and α peaks are sharp in the XRD profile of TNTZAT, and the XRD profiles of TNTZAHPT have broadened diffraction peaks, as shown in Fig. 2. XRD peak broadening has been linked with either a grain diameter of less than 1 µm for a bulk material [17, 18] or residual stress [17-19]. Therefore, the broadened XRD profiles shown in Fig. 2 indicate that TNTZAHPT was composed of very small grains with residual stresses formed as a result of severe εeq imposed by the HPT processing. On the other hand, the XRD peaks of TNTZAHPT also exhibit shifts in the β and α peaks, as shown in Fig. 2. These peak shifts are considered to be related to the different types of residual stresses [20]. Previous studies have shown that the peak shifts are caused by macro-residual stress, whereas peak broadening shows micro-residual stress [20]. It is well known that HPT processing is characterized by very high residual stresses in the processed bulk material [11]. However, the peak positions of TNTZAHPT at all N are almost the same, which means whereas the effect of the macro-residual stress could be negligible with increasing N.
Fig. 2. XRD profiles of TNTZAT at 723K for 259.2 ks and TNTZAHPT at N= 1, 5, 10, and 20 at rh position. Figs. 3 and 4 show the SEM images obtained from the cross sections of TNTZAT and TNTZAHPT at N = 1 and 20 from rh position, respectively. SEM observations show that the microstructure of TNTZAT exhibits needle-like precipitated α phases within β equiaxed grains, as shown in Fig. 3, which has been also reported in previous studies [3, 21]. The needle-like precipitates of α phase, which had a length of
