Study On The Technology Of Controlling The Pattern And Amount Of β-Mg₁₇Al₁₂ Phase In The Magnesium-Aluminum Alloys

In order to improve the high-temperature creep resistance,toughness and plasticity of alloys,and provide some reference to the study of strengthening and toughening of Mg-Al alloys,an understanding of the solidification mechanism of Mg-Al alloys,especially the cognitions of hard and crispedβ-Mg₁₇Al₁₂phase are essential.The technology on the controlling the amount and pattern of divorced eutecticβphase in the Mg-Al alloys has been studied in this article.The microstructures were analyzed by optical microscope and scanning electron microscopy,compositions and microzone composition of phases were analyzed by energy dispersive spectrometry,phases and the weight fractions ofβphase in the Mg-Al alloys were analyzed,the area fractions ofβphase andα-Mg dendrite size were calculated in the AZ31 and AZ61 magnesium alloys by used the image analyzed software.It indicated that:(1)In the graphite mould microstructure of Mg-Al binary alloy,the amount ofβphase is increased with the increasing of Al addition and the increasing of cooling rate,βphase is precipitated when Al addition is 2%wt;the pattern ofβphase changed with the increasing of Al addition,is from small granule,big granule,to not completely continuous net and continuous net at the grain boundary;theα-Mg dendrite size is decreased with the increasing of Al addition and the increasing of cooling rate,small secondary and tertiary dendritic arms are appeared.(2)βphase is restrained in the AZ31 magnesium alloy with Ce addition and in the AZ61 magnesium alloy with Nd addition,its pattern is changed with the increasing of rare-earth elements addition,is from not completely continuous net to small granule at the grain boundary,βphase is refined by the Ce and Nd addition;The metamorphism time has little effects on the amount and pattern ofβphase;the effects of cooling rate on the amount and pattern ofβphase is more than metamorphism on its amount and pattern;it is obviously that to restrain the amount ofβphase and to improve its pattern in the AZ61 magnesium alloy with Nd addition more than in the AZ31 magnesium alloy with Ce addition;under lower cooling rate conditions,the amount ofβphase is decreased sharply and its pattern is also improved,meanwhile,the amount of precipitatedβphase is increased,the microstructure of perlite form is appeared in the AZ61 magnesium alloy;theα-Mg dendrite size is increased with the increasing of Ce and Nd addition,it became coarsed and disperse,a kind of rod-like compound Al₁₁Ce₃,Al₁₁Nd₃ have been formed respectively in the AZ31 magnesium alloy and AZ61 magnesium alloy.(3)The value of hardness andσ_b is increased in the AZ31 alloy after Ce metamorphism and in the AZ61 alloy after Nd metamorphism,but the elongation value is increased firstly then decreased.