Superplasticity of the Mg-Ca eutectic alloy

Abstract

Superplastic flow is often associated with eutectic alloys having a microduplex structure, i.e. a two deformable phase structure with the grains of the two phases present on a micrometric scale and equiaxial in shape. This particular morphology may be achieved by heavy hot deformation if the volume fraction of the eutectic second phase is sufficiently high to stabilize a fine grain size at elevated temperature [1-3] . This is the case for the Pb-Sn [4-6] , A1-Cu [7-9] , A1-Ca [10, 11] and A1-Pd [12] eutectics. However, in the A1-Cu eutectic alloy the superplastic flow can be observed also starting with cast material ifa large amount of tensile straining occurs [7, 13]. Initially the material is not superplastic and the deformation is localized to a neck. As a result the cast structure is converted by straining and dynamic recrystallization to a microduplex structure and the necked zone is able to stretch 500% or more [7, 13]. This singular necking behaviour was also observed by the authors on some cast specimens of eutectic Mg-Ca (Mg-rich) deformed at high temperature ( T = 773K) at medium strain rates. It then appeared evident that a prior ingot hot working would produce an initially superplastic material and this occurred in practice. The preliminary results of experiments with both cast and hot worked Mg-Ca eutectic structures are reported here. According to Hansen [14] the Mg-Ca reaction occurs at a nominal composition of 16.2wt% (10.5 at%) Ca and at a temperature of 790K. The phase in equilibrium with magnesium is the hexagonal Mg2Ca compound [14], which melts at 990K [14]. This intermetallic compound is brittle at room temperature and it is harder (Vickers hardness 2000MPA [15]) than pure magnesium. The solubility of calcium is limited: maximum value l w t % (0.6 at %) Ca at 790K [14]. The second phase volume fraction at the eutectic temperature as calculated by the authors is 0.54. A master alloy of ~ 4 5 w t % Ca was first prepared by induction furnace melting of the constituent elements, magnesium of 99.9% and calcium of 99.5% purity, in an amorphous carbon crucible under a positive pressure of argon. An ingot of nominally eutectic composition (16.2wt% Ca) was then obtained by remelting the master alloy, diluting with a suitable amount of magnesium and chill casting into a cast iron mould with argon atmosphere. The ingot cast morphology of the Mg-Ca eutectic alloy is shown at different magnification in Figs. l a to b. The second phase, identified by X-ray diffraction analysis as being intermetallic Mg2Ca as envisaged on the Mg-Ca diagram [14], is dispersed in e-Mg matr ix as lamellae which are sharply bent. The lamellae (0.5-1/2m thick) tend to be aligned with respect to each other and are dispersed in groups of different discrete orientations (cellular macromorphology). According to the eutectic growth literature [16], this microstructure can be classified as "Chinese script" structure, that is a degenerate form of the usual lamellar morphology, depending on the systems and on the solidification conditions. In fact, when the alloy was unidirectionally solidified, a much more regular morphological arrangement of the lamellae was obtained [15]. Tension specimens of 14.5mm gauge length and 4 mm diameter were machined from the ingot. Tensile tests were performed on an Instron electromechanical machine at two constant extension rates (initial strain rate, ~ = 2.4 x 10 -4 and 2.4 x 10 -3 sec-1). An argon atmosphere furnace controlled + 2 K was employed. Fig. 2 shows the typical appearance of some cast tensile specimens after straining at 773 K. It is clear that, as cast material, the Mg-Ca eutectic did not extend uniformly and that it deformed superplastically in the necked zones which could stretch 1000% or more and pull down to the very point of fracture (reduction of area ~> 99%). This type of intrinsic plastic fracture is considered as ideal for