Effect Of Al-La Compound On The Grain Refinement Of As-cast Mg-Al Alloys

The microstructure of Mg-Al alloys can be refined effectively by the addition ofRare Earth elements. However, the effect of every single Rare Earth element on theMg-Al alloys has not been revealed as they are always introduced as Misch-metal.Therefore, this article has been researched on generating the metallic compoundsthrough in situ formation by adding single La element into Mg-Al alloys, and the effectof Al-La compounds on the grain refinement was inverstigated, as well as thethermodynamic formation mechanism.Mg-Al-La master was prepared by melting andhot extrusion, then the extruded master alloys wasadded into the AZ31alloy toinvestigate the grain refinement effect.The study concluded as follows:As-cast microstructure of pure magnesium can be refined by single La addition, therefinement mechanism is related to constituent supercooling of La element during thesolidification process.The effect of grain refinement on the Mg-Al alloy is deeplyinfluenced by the contents of Al and La.When Al and La content is relatively low, suchas in Mg-xAl-0.8La alloys, grain refinement effect is obvious; while in Mg-xAl-1.5Laalloys, grain refinement effect is little weakened;when come to Mg-xAl-3.5La alloys,onthe contrary,the grain coarsening was very obvious.In Mg-Al-La alloys of this study, with the increase of Al content, the type ofsecond phase transformed from Mg17La2to Al2La, Al2.12La0.88, Al11La3and Mg17Al12,and their morphology evolved from small granular, small bar, small needle into bones,reticular and thick needle shape.Through thermodynamic calculation it can be conjectured that the formationenthalpy of Al-La compounds is much smaller than that of Mg-La and Mg-Al, whichsuggestedthat the formation of Al-La compounds is preferred in the Mg-Al-Laalloys.With copper quenching experiment, it was confirmed that the Al-La compoundsindeed generated in initial solidification of Mg-Al-La alloys.Through hot extrusion, the second phase of Mg-Al-La master alloy was obviouslyrefined, and the type of second phase was not changed. Moderate amount addition ofextruded master alloy can refine AZ31alloy obviously.The initial small mount ofalloying element leaded to slightly grain coarsening, and then the grain size decreased with further increase of alloying elements, the most significant refining effect wasobtained with the1.10%(Al+La) addition.