Effect of cryogenic treatment and aging treatment on the tensile properties and microstructure of Ti-6Al-4V alloy

The effects of cryogenic treatment and aging treatment on the tensile properties and microstructure of Ti-6Al-4V were experimentally investigated in the present work. Different treatments contained cryogenic treatment, aging treatment and the combination of the two treatments were conducted on the commercial annealed Ti-6Al-4V alloy. The results of tensile test show that both cryogenic treatment alone and cryogenic treatment prior to aging can improve the plasticity of Ti-6Al-4V alloy, the maximum improvement is by the extent of 22.7% which is obtained by the execution of cryogenic treatment prior to aging at 650 °C. However, all the treatments reduce the strength of this alloy except for cryogenic treatment prior to aging at 400 °C, but these variations in strength are no more than 5.5%. Meanwhile, there is no obvious change in microhardness. The methods of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to examine the changes of microstructure after different treatments. The execution of cryogenic treatment reduces the quantities of β phases by transforming the metastable β phases into stable β phases and α phases while the aging treatment at 550 °C and 650 °C increases the β phase particles to a small extent. The examinations of TEM show that dislocation networks generate after cryogenic treatment which resulted from the movement and interaction of dislocations. Aging treatment after cryogenic treatment increases the density of dislocations and promotes the formation of twins in the microstructure of Ti-6Al-4V alloy.