Microstructures and tensile properties of Mg-4Al-4La-0.4Mn-xB (x = 0, 0.01, 0.02, 0.03) alloy

The influences of trace boron on microstructures and tensile properties of Mg-4Al-4La-based alloys prepared by cold-chamber high-pressure die-casting method were thoroughly investigated. The results indicated that adding trace boron to Mg-4Al-4La-based alloy can refine the dendrite arm spacing of primary α-Mg phases, which are mainly due to the little inoculating AlB2 particles. In addition, we found that adding 0.01-0.02 wt.% boron can drastically changes the eutectic morphology, with secondary particle dispersion becoming irregular and eutectic volume fraction being reduced. These phenomena can be attributed to the competitive nucleation between α-Mg and AlB2 particles for Al11La3 phases, and to the fact that more Al and La atoms saturate into the α-Mg matrix. Considering the tensile properties, although adding 0.01-0.02 wt.% boron decreased the strength of Mg-4Al-4La-based alloy, adding 0.03 wt.% boron significantly improved the tensile properties due to dispersion strengthening and, to a certain extent, solid-solution strengthening.