Study On The Precipitation During Ageing In Mg-Gd Binary Alloys

Among high strength magnesium alloys, Mg-Gd series alloys have agreat precipitation strengthening effect during ageing and excellentmechanical properties at room and high temperatures. Moreover, theprecipitation process and precipitates in Mg-Gd alloys are typical inmagnesium alloys containing heavy rare-earth elements. Therefore, it’snecessary to study the precipitation during ageing in Mg-Gd series alloys.In this research, Mg-Gd binary alloys were investigated by usingoptical microscope, X-ray diffractometer, scanning electron microscopeand transmission electron microscope etc. The influence of Gd content,heat treatment condition, hot extrusion and ageing precipitation on microstructure, mechanical properties and fracture behavior of Mg-Gd alloyswas studied by hardness and tensile tests at room temperature. Precipitatesmorphology, crystal structures and transformation process wereemphatically studied. The crystal structures, crystal distortion, stability ofprecipitates and precipitation sequence were studied through the firstprinciple calculation. At last, based on experimental study and theoreticalcalculation, precipitation process at early ageing stage was analysed anddiscussed, atomic models of GP zones and β″precipitate were established.At the early stage of ageing in Mg-1.81Gd at225℃, GP zones appearbefore β″phase. β″and β′precipitates may not have a transformationrelation.The first principle calculation shows that β″has a larger crystaldistortion than β′. β″, β′, β1all have a negative formation energy whichproves their stability. The reaction heats of the two transformations β″â†'β′,β′â†'β1are negative as well, the two reactions are exothermic.There are two kinds of GP zones and the smallest configuration containing three Gd atoms at the early stage of ageing. They’re formedbased on the solute-vacancy and vacancy diffusion mechanisms. Thenucleation of the two kinds of GP zones and β″is based on this smallestconfiguration. The nucleation process can be described by atomic-scaledmodels.