Ecaped Heat-resistant Mg-Zn Based Alloys Reinforced By Mg₂Si Phase

In recent years, increasing attention is paid to energy and environmentalissues, and vehicles with light weight are of great demand. Therefore,magnesium alloys, as the lightest structural metal material, attract researchers’increasing attention. However, magnesium alloys show poor mechanicalproperties at elevated temperature, especially over393K, and the creepproperties declines rapidly. So developing a new heat-resistant magnesium alloyis of need and urgency.One of the main ways to fabricate heat-resistant magnesium alloy is to addcheap element of silicon in magnesium alloys. The ZS32alloy was fabricated byadding2wt.%Si to the Mg-Zn alloy. Equal channel angular pressing (ECAP)deformation was carried out to refine the coarse fishbone-like Mg2Si phasewhich had high thermal stability and the microstructure of the alloy. Opticalmicroscope (OM), X-ray diffraction (XRD), energy dispersive spectroscopy(EDS), scanning electron microscope (SEM) and transmitted electronmicroscope (TEM) techniques were employed to analyse the microstructure, andtensile test at room temperature and creep test at elevated temperature wereundertaken to examine the mechanical properties of the alloy before and afterECAP deformation. The refinement mechanism of ECAP deformation on thegrains and Mg2Si phase, and the fracture behaviours of the alloys were explored.The present study provides reliable theoretical basis for developing highperformance heat-resistant magnesium alloys. The results are as followed:(1) The microstructure and second phases (bulk and fishbone-like Mg2Siphase, MgZn phase and MgZn2phase) of the as-cast ZS32magnesium alloy were notably refined by ECAP deformation. After4passes ECAP deformation,the grain size reduced from50μm to25μm, and the Mg2Si phase was refinedremarkably. After8passes, almost all the grains were refined to about10μm.Mg2Si phase was gradually broken into small particles and it uniformlydistributed in grains and at the grain boundares. The plastic deformation inducedby the pure shear stress attributed to the dynamic recrystallization, so themicrostructure of ZS32magnesium alloy was refined after8passes ECAPdeformation.(2) As the pass of ECAP increases, the grain size reduced continuously,which improved the mechanical properties at room temperature and the creepproperties. The yield strength and ultimate tensile strength of the alloy processedby4passes ECAP improved by200%and218%, respectively, compared withthe as-cast alloy. Through the creep test at elevated temperature, it can beobtained that the creep properties of the samples subjected to the same ECAPpassees declined as the creep temperature increased when the samples weresubjected to a constant stress. Moreover, creep life of samples increased and thesteady creep rate decreased as ECAP passes increased when the samples weresubjected to the same temperature and the same stress. The rupture regime ofZS32magnesium alloy changed from brittle fracture to ductile fracture in boththe tensile test at room temperature and the creep test at elevated temperature.(3) The source of cracks formed at the second phase particles. ECAPdeformation refined the thermally stable phase, and the refined particles pinnedthe dislocation and retarded the grain boundary gliding, so stress concentrationwas caused and the cracks formed. According to the calculation of creepexponent (4<n<6) and activation energy (80KJ/mol<Q<135KJ/mol), the creepmechanism of the alloy after4passes ECAP was confirmed as dislocationclimb.