Crack initiation and mechanical properties of TC21 titanium alloy with equiaxed microstructure

Deformation and crack initiation of equiaxed microstructure were investigated using in-situ scanning electron microscopy (SEM). With α_s sizes of 0μm and 0.1μm, the cracks initiated along α_p/β and α_p/β_tr boundaries (α_p, primary alpha phase; β_tr, transformation microstructure), respectively. When the size of coarsened α_s (α_s, secondary alpha phase) was about 0.5μm, the crack initiated along α_p/β and α_s/β boundaries. The results show that the crack growth was put off by continuous slip across the coarsened α_s phase. The coarsening of α_s leads to increase in ductility of the alloy, while it has a slight effect on the strength. Based on the in-situ observations, the morphology of coarsened α_s was further optimized by heat treatment procedure. According to mechanical tests performed, it was seen that the best property combination could be obtained by adjusting the sizes and morphologies of α_p and α_s phases.