Participation of liquid phase in solidification cracking of cast alloys

Abstract

According to widely accepted W.S, Pellini “liquid -film” theory, hot tears initiate at the stage of solidification, when the last solidifying liquid distributed in a film-like morphology separates solid grains. When shrinkage is restrained, stresses arise, that meet very low the over-all strength of the metal (liquid phase can support only very modest tensile stress, at most a few psi, before cavitation occurs). Assumption concerns such morphology of solidifying alloy raises objections. In general one can distinguish at least two zones in the cross section of the casting wall when solidification process of an alloy takes place: a solid layer and an internal mushy zone (presence of solid and liquid phases). There are crystal boundaries in the solid layer that have contact with liquid phase. Such triple points creates of cracks tips. There are three major factors caused by presence of liquid, when stresses occur: - increasing atomic diffusional flux out of the crack tip, - creating a path for abnormally quick diffusion of atoms from the crack tip, - decreasing the energy needed for crack development. All macroscopic manifestations of liquid phase embrittlement, namely the reduction in the elongation to failure and strength under active deformation, can be described in terms of the time to failure reduction caused by presence liquid phase.