Effect of intermetallic compounds of magnesium on the hot hardness of its alloys

Abstract

The effect of intermetallic compounds on the hardness of magnesium alloys at the temperatures between room temperature and 450°C was investigated on binary alloys of Mg-Ag, Mg-Al, Mg-Bi, Mg-Ca, Mg-Cu, Mg-La, Mg-Ni, Mg-Sb, Mg-Si, Mg-Th, and Mg-Zn, containing various amount of the respetive alloying elements from a small amount up to the composition of the intermetallic compound. Hot strength of two phase alloys is thought to depend on the hot hardness of the matrix solid solution as well as that of the second phase including intermetallic compound. This was examined on binary magnesium alloys named above.The results obtained are as follows:1) Hot hardness-temperature relationships showed that the softening was displaced to higher temperature in the primary solid solution alloys containing La, Th and Ag than the alloys containing Al, Zn, Si and others.2) The definite inflection point on hardness-temperature curve was observed on some alloys at the composition of intermetallic compound and the point a was found to correspond to about 50-70% of the melting temperature (°K) of the compound.3) Most intermetallic compounds were found to be brittle at the temperature below the inflection point but were found to be plastic at the temperature above it. Higher inflection temperature was observed in Mg-La, Mg-Th and Mg-Ni compounds whereas lower inflection temperature was found in Mg-Zn and Mg-Ag compounds. On the compounds of Mg-Bi, Mg-Sb and Mg-Si no clear inflection point could be found, where hardness went down gradually by heating.4) Hardness of compounds at room temperature was found to be well correlated to the size factor of the alloying element against magnesium and the higher hardness was observed on the compound of the element with greater difference in atomic diameter from magnesium.