Study On Microstructure Evolution Behavior And Mechanical Properties Of Mg-(Y)-Nd Alloys Refined By Alumimum

Mg-RE casting alloy has a broad application prospect in aerospace and military industry due to its high strength and high heat-resistance.Grain refinement is the basis to ensure excellent properties of Mg-RE casting alloy.In this paper,with the aim of developing new low-cost and high-performance casting mg-re alloy,Mg-?Y?-Nd-x Al alloy was prepared by using cheap Al instead of Zr as grain refiner.Optical microscope?OM?,scanning electron microscope?SEM?,transmission electron microscope?TEM?,X-Ray diffraction?XRD?,universal electronic tensile test machine,vickers hardness tester are used to investigate the effect of Al on grain refining behavior,microstructure and mechanical properties of Mg-3Nd and Mg-4Y-3Nd alloy;On this basis,the effect of cooling rate on grain refining of Al-refining Mg-?Y?-Nd alloy is studied,the mechanism of cooling rate on grain refinement and coarsening is revealed;Heat-treatment process of Al-refining Mg-4Y-3Nd alloy is optimized,the rule of microstructure evolution during heat-treatment and strengthening mechanism are clarified.The main conclusions are follows:The microstructure and mechanical properties of Mg-3Nd-x Al?x=0,1,2,3,4?alloy are studied.Microstructure of Mg-3Nd is refined significantly by Al,mechanical properties are also improved markedly.When the content of Al is 3%,the best grain refinement effect and highest mechanical properties are achieved?grain size is 48±4?m?,ultimate tensile strength,yield strength and elongation are respectively 222MPa,71MPa,16.8%.The microstructure of as-cast Mg-3Nd-Al alloy is consist of?-Mg matrix,acicular phase Al₁₁Nd₃ near the grain boundries and granular phase Al₂Nd in the grains.Orientation relationship between preferential precipitated Al₂Nd particles and?-Mg is heterogeneous nucleation.Al₁₁Nd₃ is decomposed to Al₂Nd and Al after solution treatment,Al₂RE has not changed,the effect of aging strengthening is weak.Comparing to Mg-3Nd-0.5Zr,as-cast Mg-3Nd-3Al alloy possesses higher ultimate tensile strength,yield strength,elongation and good grain stability.Effect of Al on microstructure and mechanical properties of Mg-4Y-3Nd-x Al?x=0,0.6,1.2,1.5,2.0?are studied.Adding 4%Y into Mg-3Nd alloy,the best refinement effect is achieved when the addition of Al is 1.5%,and the grain size is 46±4?m.The microstructure of as-cast Mg-4Y-3Nd-x Al is consisted of?-Mg matrix,network Mg₅RE phase and Al₂RE?Al₂YNd?phase.Thereinto,granulate Al₂RE phase can play a role of heterogeneous nucleation,which has refined the alloy.Mg₅RE phase is dissolved during solution treatment,while Al₂RE cannot be dissolved.The number density and size of Al₂Nd or Al₂RE nucleation particles are influenced by cooling rate,and finally influences the refinement effect of alloy.As cooling rate increasing,the number of Al₂RE nucleation particles increases,refinement effect is improved.Generally,grain size is decreased as cooling rate increasing.However,when the cooling rate is high,the size of Al₂Nd or Al₂RE nucleation particles are decreased,which result in less effective nucleation particles.Therefore,grains are coarsened?such as Mg-3Nd-2Al,Mg-4Y-3Nd-0.6Al and Mg-4Y-3Nd-1.2Al alloy?.Lath-like 18R-type LPSO phase and fine acicular Al₂RE phase are precipitated in Mg-4Y-3Nd-1.5Al alloy during solution treatment.These precipitations can be dissolved at higher solution temperature.Aging precipitation sequence is same to Zr-refining alloy.Ultimate tensile strength,yield strength and elongation of Mg-4Y-3Nd-1.5Al alloy after optimal heat treatment process are respectively 285MPa,197MPa and 5.7%.Mechanical properties of Mg-4Y-3Nd-1.5Al are almost same to Mg-4Y-3Nd-0.5Zr.