Effect of microporosity on the tensile properties of AZ91 magnesium alloy

The effect of microporosity on the tensile deformation of as-cast AZ91 magnesium alloy was investigated through systematic experimental approaches and theoretical predictions of a constitutive model for tension instability. The strain rate sensitivity was measured at room temperature by the incremental strain rate change test, and the microporosity was quantitatively obtained by fractography analysis on fractured surfaces. The tensile strength and elongation of as-cast AZ91 alloy have a strong inverse parabolic dependence on microporosity variation. The constitutive model can exactly predict the tensile deformation of AZ91 alloy through a practical estimation of strain-related terms and load carrying capacity by quantitative fractography. The overall plastic deformation of AZ91 alloy depends fundamentally upon not only the variation of load carrying capacity, but also on the strain hardening exponent and strain rate sensitivity. The contribution of strain rate sensitivity to plastic deformation becomes increasingly significant with a decreased strain hardening ability. As the strain rate sensitivity of the conventional material is very low but not zero, the constitutive model for the exact prediction of plastic deformation should take the strain rate sensitivity term into account. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.