Effect of Ca and Rare Earth Additions on the Texture, Microhardness, Microstructure and Structural Properties of As-Cast Mg–4Al–2Sn Alloys

Abstract

In this work, alloys with nominal composition of Mg–4Al–2Sn–xRE–yCa (where RE = rare earth, x = 0, 1, 3 and y = 0, 1) have been prepared using tilt casting method. Prepared as-cast samples have been investigated by X-ray diffractometry, optical microscopy and scanning electron microscopy techniques as well as hardness measurement. The texture, microstructure and structural parameters of samples were also refined from X-ray diffraction patterns utilizing the Rietveld method and generalized spherical-harmonic model. It was found that with addition of rare earth and calcium elements, intermetallic phases of γ-Mg₁₇Al₁₂ and β-Mg₂Al₃ disappeared in cast alloys while small amount of Al₁₁RE₃ and CaMgSn intermetallics phases are formed. The texture factor of α-Mg as a main phase of samples was decreased with addition of rare earth up to 1 % and increased with more addition of rare earth elements. According to the results, with addition of rare earth elements, texture of Mg phase changes from 〈112〉 direction to 〈100〉 and 〈002〉 directions while Ca addition causes the texture in 〈002〉 direction. The microhardness of Mg–4Al–2Sn alloy was enhanced with addition of rare earth and calcium elements which is in agreement with the expected trend based on computed phase fraction of the samples. Addition of 1 wt% of calcium causes a dramatic change in the morphology and chemical composition of intermetallic phases, from acicular shape with composition of Al₁₁RE₃ into fine feather of CaMgSn intermetallic phase which accumulated in cluster morphologies in interdendritic regions.