Specific features of two phase alloys response to cyclic deformation

The fatigue short crack propagation in polycrystalline metals and alloys is strongly dependent on microstructure because of the presence of interfaces (grain, twin or phase boundaries) which cannot be crossed without additional energy supply. The conventional approach to the problem of microstructure exclusively considered as interfaces appears insufficient to explain the cyclic response and the fatigue resistance of alloys with more complex microstructures. In the present paper, a generalised concept of microstructural barriers, valid both for single phase and multiphase materials, is proposed. It emphasises the role of physical properties of the grain (particle) adjacent to the element of microstructure in which a short crack has been nucleated and their evolution during cyclic straining. This concept is then applied to explain qualitatively the cyclic behaviour of two different types of two phase materials: austenitic–ferritic stainless steels and eutectic Al–Si alloys. On this basis, some general rules for the design of two phase alloys, taking into account the mechanical properties of both phases and the morphological parameters of microstructures, are drawn.