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 of 0.1-0.5 μm was prepared by hot rolling.The effects of heat treatment on the microstructure and mechanical properties were investigated by SEM and TEM.The results indicate that both the strength and hardness of the alloy increase firstly and then reduce with increasing the heat treatment temperature from 350 ℃ to 650 ℃.The peak strength of 1486.09 MPa appears at 450℃.The strength dramatically reaches 1536 MPa and then becomes stable with prolonging the holding time when heat-treated at 450 ℃.While the maximum elongation increases firstly and then decreases.During the annealing process the dynamic recovery occurs in the alloy and the grain size remains smaller than 1 μm.The dynamic recovery stimulates the grain refinement effect by eliminating the hardening process and improves the stability of the grain boundary/phase boundary.The phase transitions of α→α_2 and β→ω→α enhance the second phase particle dispersion effect during the annealing process.However,the ductility of alloy significantly decreases when the second phase particles grow up to a certain size.The mechanical property evolution during annealing process is mainly related to the effect of strengthening mechanisms.