Control of microstructure and mechanical anisotropy of biomedical superelastic titanium alloys through introducing high density lattice defects.

本研究的目的是通过热机械处理，通过剧烈变形引入高能高密度晶格缺陷，阐明无ni生物医学形状记忆合金，特别是TiNbAl合金的显微组织、缺陷和力学性能之间的关系。为了提高材料的超弹性性能，提高材料滑移变形的临界应力是有效的，这是通过冷轧厚度减少99%的剧烈变形和快速凝固来试验的。首先，采用马氏体相变温度较低的Ti-26Nb-3A1合金，研究了织构的形成和超弹性变形。结果表明，变形织构的发育与真应变密切相关，当真应变大于3时，织构迅速发育。固溶处理后的晶粒尺寸在500μm左右较为粗糙，但随着轧制方向的变化晶粒逐渐变细，呈针状拉长。然后，研究了不同冷轧压下再结晶织构的发展。结果表明，再结晶织构的发育与还原速率密切相关，且发育出不同的织构。当冷轧压下率低于90%时，形成{001}_β<110>_β型织构，进一步增大冷轧压下率，形成的织构变为{112}_β<110>_β型。{001}_β<110>_β型的超弹性各向异性小于{112}_β<110>_β型，因此{001}_β<110>_β型织构在实际应用中更有价值。其次，研究了α“和α”+β型TiNbA1合金的织构形成。结果表明，α”+β合金的形变织构为{112}_β<110>_β型，再结晶织构为{111}_β<110>_β型。α”合金的变形织构为{223}_<α”><302>_<α”>型，再结晶织构为{011}_<α”><311>_<α”>型。因此，织构形成的类型和超弹性在很大程度上取决于TiNbAl合金中稳定相的类型。冷轧线材也得到了类似的结果。最后，对Ti-24Nb-3A1进行快速凝固处理，即使在高能快速凝固过程中也未形成织构和细小晶粒。结果表明，通过适当的热处理和剧烈变形可以控制钛合金的织构和超弹性性能。少

The purpose of this research is to clarify the relationships among microstructures, defects and mechanical properties of Ni-free biomedical shape memory alloys, especially TiNbAl alloys, through thermo-mechanical treatments to introduce high-energy and high-density lattice defects by severe deformations. In order to enhance their superelastic properties, material strengthening to increase critical stress for slip deformation is effective and this was tried by severe deformations of cold rolling up to 99% in reduction thickness and rapid solidification. Firstly, the formation of texture and superelastic deformation were investigated using Ti-26Nb-3A1 alloy with relatively low martensitic transformation temperatures. It was found that the development of deformation texture is strongly related to true strain and that the texture is rapidly grown after when the true strain becomes over 3. Although the grain size after the solution treatment is coarse around 500μm, the grains become fine an … More d needle-like shape elongated with the rolling direction. Then, the development of recrystallization texture was investigated with various cold rolling reductions. It was found that the development of recrystallization texture is strongly dependent on reduction rate, and that different textures are developed. {001}_β<110>_β type texture is formed when cold rolling reduction is lower than 90%, and further increasing cold rolling reduction, the texture formed is changed to {112}_β<110>_β type. {001}_β<110>_β type exhibits less anisotropy in superelasticity than {112}_β<110>_β, resulting that {001}_β<110>_β type texture is more useful for practical applications. Secondly, the texture formation of α" and α"+β type TiNbA1 alloys was investigated. It was found that deformation and recrystallization textures are {112}_β<110>_β type and {111}_β<110>_β for the α"+β alloy, respectively. Besides, deformation and recrystallization textures of α" alloy are {223}_<α"><302>_<α"> type and {011}_<α"><311>_<α"> for the α" alloy, respectively. Therefore, the type of texture formation as well as superelasticity strongly depends on the type of stable phase in the TiNbAl alloys. Similar results were obtained for the cold rolled wires. Finally, the rapid solidification was done for Ti-24Nb-3A1 but neither texture nor fine grains were formed even though the high energy process of the rapid solidification. It is conclude that texture and superelastic properties of the titanium alloys can be controlled through a proper thermomechanical treatment combined with severe deformation. Less