Effect of cold rolling and heat-treatment on the microstructure and mechanical properties of β-titanium Ti-25Nb-25Zr alloy

The present work describes a detailed investigation on the effect of cold deformation and subsequent heat treatment on a recently developed commercial β-type Ti-25Nb-25Zr (wt%) alloy. The β-type Ti-25Nb-25Zr alloy exhibited excellent phase stability against severe plastic deformation as well as after subsequent heat-treatments at elevated temperatures as primarily β-phase was retained. However, at low annealing temperatures, i.e. 673 K, a small amounts of metastable HCP-α phase precipitated, which inhibited any recrystallization and grain growth. Subsequent annealing at 873 K and 1073 K resulted in the gradual dissolution of HCP-α phase and recrystallized randomly oriented fine-grained equiaxed structure wherein average grain size increased with increasing temperature. High annealing temperatures, i.e. at 1273 K, resulted in extremely coarse-grained structure with preferential crystallographic orientation due to abnormal rapid grain growth. The average hardness, yield strength, and ultimate tensile strength increased whereas elongation-to-fracture, i.e. ductility, decreased after cold rolling due to strain hardening owing to large amounts of accumulated lattice defects. The retained deformed structure with α phase particles exhibited highest strength but lowest ductility. Increasing annealing temperatures from 873 K to 1273 K exhibited continuously decreasing strength values and increasing ductility due to the recrystallized structure and increasing grain size. The increasing grain size together with decreasing amounts of HCP-α phase also demonstrated a higher uniform plastic deformation and shifting of plastic instability point, i.e. UTS, towards higher strain values in the present beta Ti-25Nb-25Zr alloys.