Microstructural aspects of spalling for hard metallic materials

Abstract
Study and modeling of physical mechanisms of spalling observed via plate impact experiments for several industrial alloys are the subject of this contribution. Because spalling is a specific kind of fracture, which is loading history dependent, the aspects of the initial microstructure and its evolution during plastic deformation are very important. In order to understand better the physical mechanisms of spall, numerous scanning electron microscopy (SEM) pictures of the free surface created by spalling have been taken for two materials. It has been confirmed that the microstructure has a direct influence on the mechanism of nucleation, growth and coalescence of micro-cavities or micro-crack by means of distribution of nucleation sites and decohesion between the harder particles and the softer lattice. This contribution reports results of measurements in the form of statistical distribution of horizontal micro-segments of fractured surface of targets, corresponding to quasi-brittle fracture, and vertical micro-segments, corresponding to ductile and / or adiabatic shear banding, all along the entire cross-section of a target. As a result a new model has been proposed, based on the mesoscale approach, which is in agreement with physical mechanisms which occurs during spall fracture.