Model of grain growth induced by superplastic deformation based on grain switching

Abstract

Deformation induced grain growth is usually observed during structural superplasticity. This paper proposes a new deformation model explaining this grain growth based on grain switching proposed by Ashby and Verrall. To begin with, an irregularity of a pentagon-heptagon pair is introduced into a two-dimensional grain structure constructed with a regular hexagonal array. This irregularity geometrically corresponds to an edge dislocation of the grain structure. The tensile stress oblique to the pair results the grain switching described by dislocation glide. On the other hand, the stress parallel to it results the grain switching which transforms the pentagon into the quadrilateral and then removes it. It can be described by dislocation climb and results the enhancement of grain growth. Therefore superplastic deformation (grain switching) induces grain growth due to the strain. In order to extend this model to three dimensions, an edge dislocation is introduced into a three-dimensional grain structure constructed with Kelvin's tetra-kai-dekahedra (14-faced grains). The grain at the tip of the half grain plane is 13-faced or 11-faced. A 17-faced or 15-faced grain is contacting it, respectively. Since these coincide with the pentagon-hexagon pair in two dimensions, the above discussion in two dimensions can use in three dimensions with little modifications. This model predicts the grain growth-rate equation as d ln D̄/dε = α, α = 0.3. This well agrees with the experimental results in dual phase structure.