Effect of annealing temperature on mechanical properties of TC21 titanium alloy with multilevel lamellar microstructure

The purpose of the present work is to reveal the effect of annealing temperature on the microstructure and mechanical properties of multilevel lamellar microstructure of TC21 titanium alloy in triple heat treatments, where the annealing temperatures are 760, 800 and 840 °C, respectively. The multilevel microstructure consists of β grain, α colony (αc), α plates (αp) and aged α plates (αs). The strength, plasticity, impact toughness and fracture toughness of the material were tested. The microstructure was characterized by using optical microscopy (OM), scanning electron microscopy (SEM), laser scanning confocal microscope (LSCM) and electron backscatter diffraction (EBSD). Results show that the size of αc increases and the widths of αp and βt increase gradually with the increase in annealing temperature. The number of αs precipitated on β matrix increases sharply and their size becomes smaller after aging treatment. The strength of TC21 titanium alloy increases as increasing annealing temperature while plasticity decreases. The αs is a critical unit which plays an important role in controlling strength of the alloy through Hall-Petch fitting. Furthermore, increasing the size of αc and decreasing the size of αp and αs are beneficial to increase the strength of the alloy. The impact toughness and fracture toughness of the alloy increase firstly then decrease with the increase in annealing temperature. The synergistic effect originating from different level α phase determine the toughness of the alloy. Massive cleavage and intergranular fractures in the fracture contribute to the increased length of crack propagation path so as to the decrease of toughness of the alloy. Meanwhile, it is confirmed found the toughness is also related to the change of size of αc after annealing treatment. And too large or too small size of αc is not conducive to the improvement of the toughness of the alloy.