Heterogeneous structure and mechanical hardness of biomedical -type Ti-29Nb-13Ta-4.6Zr subjected to high-pressure torsion

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• 作者：H. Yilmazer^a ; M. Niinomi^b ; ^{niinomi@imr.tohoku.ac.jp} ; M. Nakai^b ; J. Hieda^b ; Y. Todaka^c ; T. Akahori^d ; T. Miyazaki^e
• 关键词：Ti&ndash ; 29Nb&ndash ; 13Ta&ndash ; 4.6Zr ; Metallic biomaterial ; High-pressure torsion ; Microstructure ; Mechanical hardness
• 刊名：Journal of the Mechanical Behavior of Biomedical Materials
• 出版年：2012
• 期刊代码：pc168_17516161
• 类别：ms
• 出版时间：June, 2012
• 卷：10
• 期：Complete
• 页码：235-245
• 文件大小：1903 K

摘要

A novel -type titanium alloy, Ti-29Nb-13Ta-4.6Zr (TNTZ), has been developed as a candidate for biomedical applications. TNTZ exhibits non-toxicity and a low Young鈥檚 modulus close to that of bone (10-30 GPa). Such a low Young鈥檚 modulus of this alloy is achieved by comprising a single metastable phase. Greater mechanical biocompatibility, which implies higher mechanical strength and hardness while maintaining a low Young鈥檚 modulus, has been aimed for TNTZ. Therefore, strengthening by grain refinement and increasing dislocation density is expected to provide TNTZ high mechanical strength while keeping a low Young鈥檚 modulus because they keep the original phase. In this case, high-pressure torsion (HPT) processing is one of the effective ways to obtain these properties simultaneously in TNTZ. Thus, in this study, the effect of HPT processing on the microstructure and mechanical hardness of TNTZ was systematically investigated at rotation numbers () of 1 to 20 under a pressure of around 1.25聽GPa at room temperature.

On the cross sections of TNTZ subjected to HPT processing (TNTZ_HPT) after cold rolling (TNTZ_CR) at any rotation number, a heterogeneous microstructure consisting of a matrix and a non-etched band, which is not corroded by etching solution, can be observed. The thickness of non-etched band increases as rotation number and distance from specimen center increase. Both matrix and non-etched band comprise a single phase, but their grain geometries are different each other. Equiaxed grains and elongated grains are observed in the matrix and the non-etched band, respectively. The equiaxed grain diameter, which is ranged from 155聽nm to 44聽nm, in the matrix decreases with increasing rotation number. Contrastingly, the elongated grains with a length of around 300聽nm and a width of 30聽nm, which are nearly constant with rotation number, are observed in the non-etched band.

The mechanical hardness of TNTZ_HPT is consistently much higher than that of TNTZ_CR. The mechanical hardness distribution on the surface of TNTZ_HPT is heterogeneous in the radial and depth directions, while that of TNTZ_CR is homogeneous; the mechanical hardness is higher in the peripheral region than in the central region on the surfaces of TNTZ_HPT at all . Further, the mechanical hardness distribution on the cross sections of TNTZ_HPT at all is also heterogeneous in depth direction; the mechanical hardness is higher in the peripheral region than in the central region. The heterogeneous mechanical hardness distribution depending on the position on the surface and cross section of TNTZ_HPT is considered to be related to grain refinement and imposed strain due to HPT processing.