Effect Of Al₂Ca And Al₄Ce On The Grain Refinement Of Mg-Al Magnesium Alloys

The coarse grains in as-cast magnesium alloys have great effect on the improvement of their performance and applications. Therefore, this article has been researched on generating the metallic compounds through in situ formation by adding alloy elements to refine grains through heterogeneous nucleation which depends on the crystallographic relationship between the compounds and magnesium matrix, thus optimizing their performance.In this paper, the investigation on the effects of the value of Al/Ca or Al/Ce on the as-cast microstructure of magnesium alloys has been carried out by means of optical microscope, electrical resistivity measurements, X-ray diffraction technique, etc. Thermodynamic and crystallographic calculation are also been used to explore the formation mechanism of the compounds preliminarily. Following research results were obtained.Grains refined obviously when the Al content changed in 0.5 2wt% in Mg-Al-0.5Ca alloys, with the increase of Al content, the grain size decreased from 420μm to 287μm. When Ca content was higher, the Al₂Ca dimension showed an increasing trend and weak refinement. With the adding 0.2 2.5 wt% Al to Mg-Al-1Ce alloy, the grain size decreased at first, but with the Al content increasing the grain size increased in a short range and decreased finally. When Ce content was higher, the Al₄Ce size showed an increasing trend and weak refinement.From thermodynamic calculation it is clear that: the formation enthalpy of Al-Ca is smaller than that of Mg-Ca or Mg-Al, which can promote the formation of Al-Ca metallic compounds; Al-Ce metallic compounds have smaller the formation enthalpy than Mg-Ce and Mg-Al, in Mg-Al-Ce system, and Al and Ce would react firstly and generate metallic compound Al₄Ce. It was confirmed that Al₂Ca and Al₄Ce presented in the liquid by liquid quenching experiments.In Mg-Al-Ca alloys, with the increase of Al content, the type of second phase transfromed from Al₂Ca and Mg2Ca to Al₂Ca and Mg₁₇Al₁₂, and the morphology evolved from granular and nod to net-like. In Mg-Al-Ce alloys, with the Al content increased, the second phases changed from Mg₁₂Ce, Mg₁₇Ce)2 and Al₄Ce to Mg₁₇Al₁₂ and Al₄Ce, and the morphology evolved from granular and nod to net-like bone-shape. Crystallographic calculation results showed that: there are two ORs between Al₂Ca andα-Mg matrix which misfits are less than 10%, Mg Al₂Ca{1 010}/{220}2[1120]_Mg /[111]Al Ca and Mg Al₂Ca{1011}/{311}2[1120]_Mg /[112]Al Ca , i.e. Al₂Ca could play as the effective nucleant for Mg alloys. The two ORs 4{1010}M g /{112}_Al4Ce 4[1120]_Mg /[111]_Al4Ce and 4{1010}M g /{112}_Al4Ce 4[1120]_Mg /[110]_Al4Ce between Al₄Ce andα-Mg matrix which is got by crystallographic calculation also proved the role of heterogeneous nucleant of Al₄Ce. Therefore, Al₂Ca and Al₄Ce could play the role of heterogeneous nucleant for Mg matrix and refine grains of the as-cast Mg alloys.