Deformation behavior of as-rolled and strip-cast AZ31 magnesium alloy sheets

Abstract

Deformation behavior of AZ31 Mg alloy has been studied in relation to test temperature and grain size. Tensile specimens were prepared in the rolling direction of an as-rolled as well as a strip-cast sheet with 1 mm thickness. The as-rolled and strip-cast sheets had the mean grain sizes of 75 μm and 5 μm, respectively. Tensile tests were then carried out under the strain rate of 10⁻²/s at room temperature, 100 °C, and 200 °C. Biaxial tests were also performed to obtain flow curves in terms of stress and strain as well as to estimate the biaxial forming limit. Plastic deformation was found due to the usual dislocation slip and dynamic recrystallization (DRX) together with some twinning. A number of twins were observed especially in large grained specimens at low homologous temperatures. The strip-cast sheet exhibited many small grains generated by the DRX at higher temperature. These fine grains due to DRX appeared to lead into an enhanced formability, evidenced by a sudden formability increase of a strip-cast sheet at 200 °C. The double twin mode was observed under biaxial loading, contrary to extension twins observed under uniaxial loading.

Introduction

Magnesium alloys have become one of the promising light weight structural materials due to their low density (1.74 g/cm³), the high specific strength as well as an excellent damping capacity [1]. Demand for magnesium alloys has, therefore, been increasing steadily in an effort for weight saving in transportation as well as electronics industries in recent years. Magnesium alloys exhibit, however, very low ductility near room temperature to severely limit their applications as wrought products [2]. Magnesium is now well known to have less than five independent slip systems required for general plastic deformation of polycrystalline materials. Namely, the basal slip systems with the lowest critical resolved shear stress (CRSS) can only provide the two independent slip systems [3], [4], [5]. Thus, the deformation twinning is an important behavior to accommodate effectively plastic strain due to limited slip systems of polycrystalline magnesium alloys [6]. The hexagonal close-packed (HCP) structure of Mg alloys has also been reported to show a strong crystallographic anisotropy to hinder the room temperature formability [7].

There have been many attempts to increase the formability of magnesium alloys, such as by the addition of alloying elements [8], [9], by developing high temperature forming processes [10], [11], or by controlling microstructures [12], [13], etc. Adding an alloying element such as Li was reported to reduce the c/a ratio to activate additional slip systems resulting into an enhanced workability [14]. Addition of rare earth elements was reported to improve the ductility by forming a weak basal texture [15], [16]. Raising process temperatures is now well known to provide much higher ductility due to the activation of non-basal slip systems at higher temperatures [17]. They can even show superplastic deformation behavior caused by the usual grain boundary sliding under some specific process conditions [18], [19]. The last one is to control microstructures such as refining grain size and/or modifying textures of magnesium alloys. A severe plastic deformation method like an equal channel angular processing (ECAP) has recently been attempted for this purpose [20].

The temperature and grain size dependence on deformation behavior of magnesium alloys have generally been studied through uniaxial tensile tests. The improvement of workability by raising process temperature has actively been studied in recent years to provide additional slip systems in hexagonal close-packed (HCP) crystals. There have, however, been relatively fewer studies in relation to loading modes and temperature dependence of deformation mechanisms up to date, despite of different deformation mechanisms exhibited under uniaxial and biaxial loadings. The usual dislocation slip mode and dominant twinning mode were observed to depend upon the grain size and loading method. It is thus attempted in this study to clarify the effects of test temperature and grain size on the deformation behavior of an as-rolled and a strip-cast AZ31 Mg alloy under a biaxial as well as a uniaxial tensile loading. The twin modes of deformation have also been investigated by using an electron backscattered diffraction (EBSD) analysis under each loading mode. The results obtained from a biaxial forming were then compared with the deformation mechanisms observed in a uniaxial tensile test. These results were analyzed to determine the effect of grain size on the relative contribution to each mechanism with a particular focus on biaxial test results.