Microstructure And Mechanical Properties Of Mg-9Al-2Sm(wt.%)Cast Alloy

Mg-Al alloy is one of the most widely used commercial Mg alloys owing to it’s good castibility and low cost. Grain refiners are badly needed since coarse grain size limits the further improvement of its mechanical property. Mg-Al alloy is usually casted under high pressure thus its mechanical property is limited and can’t be used as load-bearing structure. To improve the elongation and reduce the cost of production, people try to refine the grain size and modify precipitation by alloying.E2EM model and recently research showed that Al2 RE particles can significantly refine the grain size of Mg-RE alloy. But new rearch should be done to find whether Al2 RE particles can refine the grain size of Mg-9Al alloy. This study adds Sm and Sm with Si/Sr/Ca to Mg-9Al, trying to get refined grain and modified precipitation, thus improving the mechanical property.Studies have shown that:(1) The addition of Sm(0.2wt.%) to Mg-9Al has coarsened the grain.(172μmâ†'396μm). The addition of more Sm(2wt.%) reduced the grain size to 300μm, still larger than Mg-9Al.The addition of Si/Sr/Ca has little effect on the grain size of Mg-9Al-2Sm alloy.(2) The addition of Sm introduced Al-Sm phase to Mg-9Al. Some Al-Sm phase has a function of heterogeneous nucleation with coarsen grain. Porous eutectic Mg17Al12 phase turned into simgle bulk. The addition of Sm reduced the concentration difference of Al between grain boundary and internl in as-cast alloy. The particle size of Al2 Sm phase was exponential distribution under different heat-treatment condition.(3) Compared to Mg-9Al alloy, Mg-9Al-2Sm have the same 200℃peak aging hardness, but longer aging time to get peak hardness. The addition of Sm reduced the number of discontinuous precipitation at grain boundary significantly and increased the number of continuous precipitation inside the grain particle, leading to higher yield strength. The coarsen grain size and Al2 Sm resulted in a lower elongation.(4) First-principline calculation showed that the introduction of rare earth element Y to Mg17Al12 can reduce the interface energy, thus can accelerate the nucleation and growth of continuous Mg17Al12 and then reduce the number of discontinuous Mg17Al12. Sm addition can reduce the concentration of Al at grain boundary and reduce micro segregation of Al, thus inhibit the nucleation of discontinuous precipitation.