Effects of Aging Treatment on the Microstructures and Mechanical Properties of a TC18 Alloy

In the present work, the effects of aging treatment on the microstructures of a TC18 alloy are studied. The influence of aging treatment on the tensile properties and failure mechanisms is systematically analyzed. It is found that the size and morphology of the primary α (α_p) phases are insensitive to aging temperature and time. Furthermore, the aging temperature and time dramatically influence the precipitation of the secondary α (α_s) phases. Massive α_s phases precipitate and gradually coarsen, and finally weave together by increasing the aging temperature or extending the aging time. The variations in α_p and α_s phases induced by aging parameters also affect the mechanical properties. Both yield strength (YS) and ultimate tensile strength (UTS) first increase and then decrease by increasing the aging temperature and time, while ductility first decreases and then increases. There is an excellent balance between the strengths and ductility. When the aging temperature is changed from 450 to 550 °C, YS varies from 1238.6 to 1381.6 MPa, UTS varies from 1363.2 to 1516.8 MPa, and the moderate elongation ranges from 9.0% to 10.3%. These results reveal that the thickness of α_s phases is responsible for material strengths, while the content of α phases can enhance material ductility. The ductile characteristics of the alloy with coarser α_s phases are more obvious than those with thinner α_s phases. Therefore, the aging treatment is helpful for the precipitation and homogeneous distribution of α_s phases, which are essential for balancing the strengths and ductility of the studied Ti alloy.