Calculation of internal back stress of different gravity-die cast Mg-alloys during creep deformation

Abstract

The present work confers to the characterization of two as-cast Mg-alloys (Mg8%Sn and Mg-8%Sn-3%Al-1%Si (TAS831)) before and after creep tests in four stress level of 30, 35, 40 and 45 MPa at 150°C. Castings of both Mg-8%Sn and TAS831 alloys are produced using gravity-die casting technique. X-ray diffraction (XRD) is employed to identify the phases present in as-cast Mg-8%Sn and TAS831 alloys. Scanning electron microscopy (SEM) is used to characterise as-cast and crept Mg-8%Sn and TAS831 alloys. XRD analysis and SEM micrographs confirm the presence of α-Mg matrix and Mg2 Sn precipitate in as-cast Mg-8%Sn alloy and occurrence of Mg2 Sn, Mg2 Si, Mg17 Al12 precipitates on the α-Mg matrix in as-cast TAS831 alloy. Image analyses (using Image J software) of the SEM micrographs reveal that both as-cast Mg-8%Sn and TAS831 alloys comprise almost same amount of precipitates (∼6.7 vol. %). Creep strength of as-cast TAS831 alloy is more than that of Mg-8%Sn, due to presence of Mg2 Si as "Chinese Script" morphology in TAS831 alloy. Internal back stress, a resisting force for creep deformation arises from the initial microstructure of precipitation hardened alloy is found same (∼10 MPa at 150°C) for both as-cast Mg-8%Sn and TAS831 alloys. Presence of low melting precipitate, Mg17 Al12 lowers the minimum stress required for creep deformation of as-cast TAS831 alloy at 150°C. True creep exponent (n) differs for as-cast Mg-8%Sn and TAS831 alloys, which signifies different creep deformation mechanism in the matrix of as-cast Mg-8%Sn and TAS831 alloys.