Environmental and economical issues have been increasingly demanding reduced fuel-consumption and exhaustemission vehicles. Among the different means to satisfy these demands, reduction of mass remains the most influential and least costly one, provided that large cuts of 20–40% are achieved.[1] Leading automotive manufacturers have shown that more than 50% of fuel consumption is mass dependent, a fact that keeps increasing the interest in lightweight materials over conventional ones.[2–4] Being the lightest constructional metal on earth, it is rather natural and quite expected for magnesium to be receiving such great attention over the last decade.[1, 4, 5] Magnesium’s low density makes it 35% lighter than aluminum and 78% lighter than steel. And with proper design considerations, magnesium could replace these two metals in many areas, promising significant weight reductions. Many examples of magnesium auto parts that have evolved recently proof the initial signs of such promises.[3–5] However, those examples fall mainly into the cast-components category. Unless magnesium usage is expanded to cover other areas, mainly sheet metal forming, feasible weight reductions will be limited. The metal’s inferior ductility at room temperature is a key factor in limiting such an expansion. The AZ31 magnesium alloy is commercially available in sheet form, and possesses good mechanical properties. High strength-to-weight ratio in particular provoked the interest in this alloy for structural components. Warm forming has been carried out to enhance the formability of the alloy, as demonstrated by many investigators who successfully formed various components, some for automotive applications.[6–8] On the other hand, a very attractive attribute of this alloy is its superplastic behavior at elevated temperatures.[9–11] Superplasticity stretches the limits of formability of several magnesium alloys beyond conventional, offering more opportunities for magnesium usage in the automotive sector.

Quite a large number of studies investigating the superplastic behavior of the AZ31 Mg alloy have evolved recently.[11–16] These different studies covered the various mechanical aspects and microstructural changes during superplastic deformation in the alloy. Yet, no available study covers and combines both, over a wide range of temperatures and strain rates. Our goal is to develop a multi-scale constitutive model that can describe the superplastic behavior of this alloy under various forming conditions, taking both anisotropy and microstructural evolution into account. The framework of such a model has been already developed by the investigators, based on the continuum theory of viscoplasticity.[17–18] However, to calibrate the model requires various mechanical tests followed by microstructural examination, covering a wide-range of operating conditions. In this study, a comprehensive investigation of the elevated temperature superplastic behavior in the AZ31-H24 magnesium alloy is presented. Uniaxial tensile tests are carried out at constant strain rates, varying between 2× 10–5 and 10–2 s–1. Each band of strain rates is covered at temperatures between 325 and 450 C, in a 25 C increment. The effects of these two parameters, forming temperature and loading strain rate, on the mechanical behavior of the alloy is characterized by correlating flow stresses to plastic strains up to failure. Moreover, and in order to accurately determine the strain rate sensitivity of the material, its sensitivity index is evaluated through a series of strain rate jump tests. These tests are also carried out over similar ranges of temperature and strain rate, by imposing four jumps at four …