Relationships between tensile deformation behavior and microstructure in Ti-Nb-Ta-Zr system alloys

The behaviors of loading-unloading stress-strain curves of Ti-20Nb-10Ta-5Zr and Ti-25Nb-10Ta-5Zr air-cooled after final heating of the manufacturing process are similar to that obtained in metastable β type titanium alloys that have the shape memory effect. Therefore, the shape memory effect was expected in Ti-20Nb-10Ta-5Zr and Ti-25Nb-10Ta-5Zr alloys. The elastic deformation of Ti-30Nb-10Ta-5Zr disobeyed Hooke's law. However, stress or strain-induced martensite (SIM) is not observed on the loading-unloading stress-strain curve. The deformation mechanism of Ti-25Nb-10Ta-5Zr changes with varying its microstructure. In Ti-25Nb-10Ta-5Zr air-cooled after final heating, the microstructure consisted of an ω phase in a β phase. The stress for inducing martensite in a β phase, σM, was nearly equal to the yielding stress, σy. Therefore, stress-induced martensitic transformation and movement of dislocations occurred together. In Ti-25Nb-10Ta-5Zr water-quenched after final heating of the manufacturing process, the microstructure consisted of a single β phase, where σM is lower than σy. Therefore, stress-induced martensitic transformation occurred before yielding.