An Investigation Into Microstructure And Mechanical Properties Of Cold-rolled Ti-24Nb-4Zr-7.9Sn Alloy

Ti-24Nb-4Zr-7.9Sn (Ti2448, abbreviated from its chemical composition in weight percent) alloy has excellent comprehensive performance, and is one of the most promising Ti alloys for the application of bone implants due to the low elasticity modulus and remarkable superelasticity. At room temperature, orthorhombic martensite a" phase can be induced in Ti2448alloy by applied stress and transform back to P phase after unloading. It is reported that the reversible martensite transformation has significant influence on both the superelasticity and the elastic anisotropy in Ti-Nb based alloy. However, there are only a few studies about the micro structure feature of martensite transformation during cold working in Ti-Nb based alloy, especially few reports about the orientation relationships between a" martensite and (3phase in the cold deformed Ti2448alloy.With the aid of X-ray diffraction, transmission electron microscopy, optical microscopy, tensile machine and micro-hardness tester, the microstructure and mechanical properties of Ti2448alloy in hot-rolled state and after different cold rolling reductions are investigated. Furthermore, the microstructure feature of a" martensite transformation and Young’s modulus anisotropy of the cold rolled sheets are discussed. The conclusions can be summarized as follows:(1) There is only β phase with bcc structure in hot-rolled Ti2448alloy. The orthorhombic a" martensite with multiple morphologies is induced by cold rolling and the orientation relationships between a" martensite and β phase are various. The amount of a" phase in40%cold-rolled specimen is the highest and decreases with the increase of deformation.(2) During cold-rolling, the grains in the Ti2448alloy are gradually refined and nanocrystalline areas appear in80%cold-rolled specimen. Both the dislocations in matrix phase and the sub-structures including twin and lamellar tissue in a" phase are beneficial to the grain refinement. With the increase of cold deformation, both the yield strength and ultimate tensile strength are improved, while Young’s modulus and the elongation at break show decline.(3) The main matrix β phase textures are{001}<110> and {112}<110> in40%cold-rolled sheet, and{111}<112> texture is dominating in80%cold-rolled sheet. The largest difference between Young’s modulus in rolling direction and that in transverse direction is about26GPa, which has great relationship with the β phase textures and the existence of a" phase.