Study On Precipitates And Phase Transformation Mechanism Of Mg-Sm-Gd System Alloys

When two RE elements belonging to different lanthanide subgroups(Ce and Y ones), are added into magnesium alloys together, the precipitation strengthening effect will be promoted in the alloys. This is also the most effective way to improve mechanical properties of high strength magnesium alloys developed to date. The strengthening effect of Mg-LRE(Ce subgroups) alloys is obviously different from those of Mg-HRE(Y subgroups) alloys. In order to develop high strength Mg-RE alloys and their corresponding heat-treatment crafts, clear studies on precipitates and phase transformation mechanism of Mg-LRE, Mg-HRE and Mg-HRE-LRE alloys are remarkably significant.Element Sm and Gd with close atom numbers and radiums were choosed in our research to design Mg-Sm-Gd ternary alloys on the basis of the research on Mg-Sm binary and Mg-Gd binary alloys. The microstructure evolution of Mg-Sm and Mg-Sm-Gd alloys under different heat-treatment conditions were observed using optical microscope(OM), scanning electron microscope(SEM) and transmission electron microscope(TEM). The age-hardening behaviour and mechanical properties were tested by Vickers Hardness and tensile tests, respectively. Fracture observations of Mg-Sm alloys during isothermal ageing were carried on using SEM. Precipitates and phase transformation in Mg-Sm and Mg-Sm-Gd alloys during isothermal ageing were systematicly studied in details. Morphologies, crystal structures and orientation relationships of precipitates were obeserved and analysed with TEM. A better experimental method of HAADF-STEM with Z2 contrasts was used to observe the atomic arrangements amd phase interfaces of β″(D019) and β′(cbco) precipitates. The formation enthalpies of β″, β′ and β1(fcc) calculated by first principle were used to estimate the thermodynamic stabilities of these three phases. Transformation directions among the these phases were also deduced from the formation enthalpies.Some β′(cbco) phases similar to those in Mg-Gd alloys were found in our peak-aged Mg-Sm alloys, which confirms the common existense of β′(cbco) in Mg-RE alloys. β′ and β″ were coherently connected together and there are certain trasformations between them. The crystal structures of β″(D019), β′(cbco) and β1(fcc) were established according to the analysis of precipitatesThe formation enthalpies of β″, β′ and β1 are all negative values calculated by the first principle, indicating the stabilities of these three phases. The chemical reaction heat of β″→β′ and β′→β1 imply the direction of the reaction. The phase transformation directions are from β″ to β′ and β′ to β1.