Abstract: TC4 dual-phase titanium alloy with 10%, 20% and 40% compression was subjected to 12 h cryogenic treatment at -196 ℃ by liquid nitrogen and then annealed at 500 ℃ for different time. The grain sizes were characterized by optical microscope, the volume fraction of α and β phases was measured by scanning electron microscope, the hardness and tensile properties were tested by Vickers hardness tester and electronic universal testing machine, respectively. Through the change of grain size and the structure of α phase and β phase, the changes of the hardness and tensile properties were explained. The results show that the grain size decreases first and then increases, and the volume fraction of β phase gradually decreases and transforms into α phase with the annealing time increases. When annealed for 1 h, the grain size decreases slightly, which is related to the smaller grains produced during the annealing process, and the continued cryogenic treatment after rolling makes more deformable, and more small grains are easily produced during the annealing process which is more conducive to promote the transition from β phase to α phase. So the alloy has excellent comprehensive mechanical properties when annealed at 500 ℃ for 1 h. When the annealing time exceeds 1 h, the volume fraction of α and β phases does not change significantly, and with the extension of the annealing time, the grain coarsening phenomenon is more obvious. Therefore, the strength and hardness of the material are lower than that when annealed for 1 h. It is concluded that annealing at 500 ℃ for 1 h is an ideal processing for the cold rolled TC4 titanium alloy after deep cryogenic treatment for 12 h, under which can obtain preferable comprehensive mechanical properties.

Key words: cryogenic treatment, TC4 dual-phase titanium alloy, microstructure, mechanical properties, annealing treatment