Nanostructured Shape Memory Alloys and Intermetallic Compound by Severe Plastic Deformation
TiNi shape memory alloy exhibits excellent shape memory effect, superelasticity, corrosion resistance and biocompatibility, and is one of the most important engineering materials. Area of application has been rapidly expanding, especially in the field of medical devices, e.g., orthodontic wires, stents, guide wires, etc. Another unique characteristics of this material is that it undergoes crystalline-to-amorphous transformation in solid state. Amorphization can be induced by particle beam irradiation (electron, neutron and ions), and by severe plastic deformation (SPD), such as, cold rolling (CR), shot peening(SP) high pressure torsion (HPT). Recently we have shown that the amorphous TiNi can be obtained by cold drawing. We investigated the effect of HPT deformation and post-deformation aging on the microstructures, martensitic transformation and mechanical properties were described. It was revealed that stress-induced B19' phase reverts to B2 phase by deformation, and local amorphization starts at a relatively early stage of deformation. Post-deformation aging resulted in the formation of nanocrystalline B2 phase. Nano-scale grain size stabilizes the B2 phase and suppress the formation of B19' martensite while it stabilize the R phase to some extent. The nanocrystalline TiNi exhibits very high value of microhardness which can be beneficial to increase shape recovery stress and a wider temperature window for superelasticity. Mehcanical properties of amorphous/nanocrystalline TiNi wires is rather promissing. The 70% drawn TiNi wire exhibits high stregth and high elastic modulus compared to conventional TiNi. These properteies can be useful for medical devices, e.g., stents and guide wires. Amorphization and nanostructure formation in other intermetallic compounds as well as metallic glass by SPD are also under investigation.
Reference
- K. Tsuchiya, Y. Hada, M. Ohnuma, K. Nakajima, T. Koike, Y. Todaka and M. Umemoto, “Production of of TiNi Amorphous/Nanocrystalline Wires with High Strength and Elastic Modulus by Severe Cold Drawing”, Scripta Mater., 60 (2009) 749–752.
- K. Tsuchiya, Q.-F. Cao, A. Hosokawa , M. Katahira, Y. Todaka and M. Umemoto, “Nanostructured Shape Memory Alloys for Biomedical Applications”, Mater. Sci. Forum, 539-547(2007)505-510.
- K. Tsuchiya, M. Inuzuka, D. Tomus, A. Hosokawa, H. Nakayama, K. Morii, Y. Todaka and M. Umemoto, “Martensitic Transformation in Nanostructured TiNi formed via Severe Plastic Deformation”, Mater. Sci. Eng., A438-440 (2006) 643-648.
Fig.TEM micrograph of amorphous TiNi processed by severe plastic deformation.
Fig. Tensile stress-strain curves of Ti-50.9mol%Ni wire. (a) before(SE)and after cold drawing. (b) cyclic stress-strain curve of 70% drawn wire. Inset of (a): Young’s modulus vs. drawing reduction.