Effect of Heat Treatment on Microstructure and Mechanical Properties of Ultra-fine Grained Ti-55511 Near β Titanium Alloy

The ultra-fine grained (UFG) Ti-55511 near β titanium alloy with grain size 0.1∼0.5 μm was prepared by hot rolling. The effects of heat treatment on the microstructure and mechanical properties were investigated using SEM and TEM. The results indicate that both strength and hardness increase firstly and then reduce with increasing of heat treatment temperature from 350°C to 650°C. The peak strength (1486 MPa) appears at 450°C. The strength dramatically reaches to 1536 MPa and then is stablized with increasing of the holding time when heat treated at 450°C. While the elongation increases firstly and then decreases. The microstructure analysis shows that the dynamic recovery occurs and the grain sizes remain at smaller than 1 μm during annealing. The recovery stimulates the grain refinement effect by eliminating the hardening process and stimulating the grain boundary/phase boundary to be stable. The phase transformation of α→α2 and β→ω→α enhances the second phase particle dispersion effect during annealing. However, the ductility of the alloy could significant decrease when the second phase particles grow up to a certain size. The mechanical properties evolution during annealing are mainly related to the effect of strengthening mechanisms.