Phase equilibria in alloys displaying diffusional and displacive characteristics

Abstract

The characteristics of phase equilibria and their representation by means of phase diagrams are examined for simple systems that can undergo both diffusive and displacive phase transitions. The displacive and diffusional transitions are modeled using a simple shear and miscibility gap with a consolute critical point, respectively. Under certain conditions, both the displacive and diffusional transitions can be graphically represented using free energy-concentration diagrams with accompanying common-tangent construction. The displacive transition can lead to tricritical points and critical end points in a temperature-concentration phase diagram. It is also shown that martensite start and finish temperatures can be equilibrium features of a phase diagram under certain mechanical loading conditions and that surface relief effects can also result from a purely diffusional phase transition.