Loading path dependent distortional hardening of Mg alloys: Experimental investigation and constitutive modeling on cruciform specimens

Abstract

The forming process of Mg alloys is strongly affected by the anisotropic mechanical behavior due to crystallographic texture. Traditionally, the anisotropic behavior is captured by uni-directional loadings along different directions (e.g. Rolling Direction, RD, Transverse Direction, TD and 45∘ from RD directions). However, two more important factors have been ignored. Firstly, the subsequent deformation behavior and plastic flow of metals are determined by the whole evolution of mechanical behavior along all dimensions (i.e. σxx, σyy, σzz, σxy, σyz and σzx) instead of uni-directional mechanical property. Secondly, since the metal forming process is under multi-axial instead of uni-directional loading, the anisotropic plastic flow behavior of metals under multi-axial loading are crucial to the forming technology. Clearly both factors cannot be clarified by traditional uni-directional loading tests. In line with the state of the art progress in Elasticity and Plasticity theory at finite strain, the aforementioned factors can be clarified by the anisotropic evolution of yield surfaces, i.e., distortional hardening. For wrought Mg alloys, the anisotropic evolution of yield surfaces in σxx and σxy space for extruded bar of Mg alloys has been clarified in [1]. As a series work, the evolution of yield surfaces of AZ31 rolled sheet in σxx and σyy space under uniaxial tension was probed experimentally by means of compressive and biaxial tensile tests with specifically designed non-proportional strain paths. The anisotropic evolution of yield surface is observed under uniaxial tension, and the underlying deformation mechanism is discussed based on microstructure and texture analysis. It is found that the yield surface evolution is ascribed to dislocation hardening and texture rotation induced by basal slip with biaxial in-plane tension. A thermodynamically consistent constitutive model at finite strain is employed to capture the anisotropic behavior. The anisotropic evolution of yield surfaces of AZ31 rolled sheet under uniaxial loading along RD and TD directions are predicted and validated after model parameters identification.