Effects of the phase interface on initial spallation damage nucleation and evolution in dual phase titanium alloy

The Ti-6Al-4V dual phase alloy samples were dynamic loaded by one-stage light gas gun experiment and samples with initial spallation were softly recovered. During the loading experiment, the velocity of free surface particles was measured by photonic doppler velocimetry (PDV). The effect of α/β phase interface on nucleation, growth, and coalescence of dynamic damage in Ti-6Al-4V were investigated by 2-D or 3-D testing techniques, such as optical microscopy(OM), x-ray computer tomography(XRCT), and electron backscattered diffraction(EBSD). The results showed that the majority of voids were nucleated within α phases, rather than on the α/β phase interface as predicted by quasi-static damage theory. Due to the effects of reflection and transmission of shock wave at the phase interface, a tensile pulse would be formed within α phase when the shock wave transmit from α phase with high impedance to β phase with low impedance. When this tensile pulse was large enough, voids would be formed within α phase. The analyses of OM and XRCT indicated that the voids at the beginning of nucleation were nearly spherical, then grew up along the direction of 45° with the shock loading direction, and finally the rod-shaped voids were formed. Besides, the voids were not randomly nucleated within α phase, and the EBSD analysis showed that the voids were mainly nucleated at grain boundary triple points which composed of grains with large difference of Taylor Factor(TF) value within α phase. This is because the difference of plastic deformation capacity of this grains is larger, and it is easier to produce stress concentration. Thus these sites became the prior nucleation position of voids.