Effect Of Al-Nd Compound On The Microstructure And Mechanical Properties Of Mg-Al Magnesium Alloys

As-cast Mg-Al alloys have disadvantages of coarse grain structure and the poor mechanical properties,which are not conducive to their subsequent industrial application.Adding alloy elements to refine grain is considered as an effective method to improve these disadvantages of Mg-Al alloys.Previous researches found that the addition of rare earth Nd element can obviously refine the microstructure of Mg-Al alloys and improve their mechanical properties.However,there is no systematic study about the effect of Al-Nd compounds on magnesium alloys.Therefore,this article has researched the effect of in situ generated Al-Nd compounds on the microstructure and mechanical properties of Mg-Al alloys by adding rare earth Nd element to Mg-Al alloys,and the grain refinement mechanism was analyzed.The grain thermal stability and high temperature performance of magnesium alloys refined by Al-Nd compounds and Zr element were comparative investigated.The extruded Mg-Al-Nd master alloys were prepared and added to AZ31 alloy to study the effect on the microstructure of AZ31alloy.The results in experiment show:The grain of as-cast pure magnesium can be refined by the single addition of Nd element,attributed to the solute effect of Nd element during solidification process.The composite addition of Al and Nd elements can refine grains of as-cast Mg-Al alloys,and the refinement mechanism is related to Al-Nd compounds acted as heterogeneous nucleation sites.Moreover,the effect of grain refinement on as-cast Mg-Al alloys is closely related to the contents of Al and Nd elements.In as-cast Mg-xAl-0.5Nd alloys,the grain size gradually decreases with the increase of Al content.However,in Mg-xAl-2.5Nd alloys and Mg-xAl-5.0Nd alloys,the grain size firstly increases and then decreases with the increase of Al content.In Mg-xAl-0.5Nd alloys,the yield strength,ultimate tensile strength and elongation of the alloys are gradually enhanced with the increase of Al content.When Al content is1.2wt.%,the alloy has the optimum comprehensive mechanical properties.However,in Mg-xAl-5.0Nd alloys,compared to Mg-5Nd alloy,the yield strength,ultimate tensile strength and elongation of Mg-3.0Al-5.0Nd alloy are increased by 45.3%,72.4%and264.0%,respectively.The grains of Mg-2.5Nd alloy refined by Al-Nd compounds and Zr element have good thermal stability on high temperature,which is mainly due to the high temperature stable Al₁₁Nd₃ and Mg₁₂Nd phase distributed at grain boundaries which can inhibit the grain growth.The compression properties of Mg-2.5Nd alloy with Zr addition are comparable to those of Mg-2.5Nd alloy refined by Al-Nd compounds at room temperature,while the latter is obviously superior to the former for compression properties at high temperature.The type of second phase of Mg-Al-Nd master alloy does not change by hot extrusion process,while the second phase is crushed and shows dispersive distribution.With increasing the addition of Mg-Al-Nd master alloy,the grain size of AZ31 alloy shows a trend of decrease and then increase after firstly increases.When the content of Mg-Al-Nd master alloy is 15.38wt.%,the alloy has the most significant grain refinement.