Studies On Microstructure Tailoring Of Primary Mn-rich Phases And Its Effect On Strength Of Heat-resistant Aluminum Alloy

In this thesis,the microstructure of coarse primary Mn-phases in Al-12 wt.%Si-4 wt.%Cu-2 wt.%Mn heat-resistant aluminum was tailored by controlling solidification cooling rate,adding Al-2La-1B master alloy and Cr-alloying.The structure characters of the primary Mn-rich phases and the effect of microstructure trailoring on strength were investigated by tension test at different temperature and microstructure characterization by OM,XRD,SEM-EDS,TEM-EDS/SAD,XRD and DSC.The crystallography characters and the crystallizing order of two primary Mn-rich phases during solidification of Al-12wt.%Si-4wt.%Cu-2wt.%Mn alloy were studied though controlling the cooling rate of solidification.Results indicate that theses primary Mn-rich phases observed are two quite different.One is Al₁₃Mn₄Si₈?simple tetragonal,a=b=1.240nm,c=0.489nm?with 2D lath-shaped morphology,and the other is Al₁₅Mn₃Si₂?simple cube,a=b=c=1.253nm?with highly developed 3D dendritic morphology.Al₁₃Mn₄Si₈ is first found and identified.A peritectic reaction:L+Al₁₃Mn₄Si₈?Al_l5Mn₃Si₂ is put forward to describe the transformation relation between them,which has been demonstrated by microstructure observation of liquid quenching samples during solidification.The cooling rate plays an important role in this reaction:a fast cooling rate can suppress it completely and a slow rate will promote it.By E2EM model,it is found that there is a good orientation-match relationship between LaB₆ and Al₁₃Mn₄Si phases,thus it is thought that the former can act as efficient nucleation substrate for the latter during solidification.1.0wt.%addition of Al-2La-1B master alloy in Al-12 wt.%Si-4 wt.%Cu-2 wt.%Mn results in the formation of a great number of fine short-rod-like or particulate Al₁₃Mn₄Si₈ phase.This considerable refinement of primary Mn-rich phase demonstrates the above prediction by E2EM model.At the same time,the peritectic reaction from Al₁₃Mn₄Si₈ to Al_l5Mn₃Si₂ is completely suppressed by adding Al-2La-1B master alloy.However,this microstructure evolution does not improve the strength of the alloy both at room temperature and high temperature.Cr-alloying?with 0.5 wt.%,1.0 wt.%and 1.5 wt.%Cr addition?can promote the transformation of primary Mn-rich phase from Al₁₃Mn₄Si₈ into Al_l5Mn₃Si₂,and also refine Al_l5Mn₃Si₂ phase considerably.When Cr addition level is at 1.0 wt.%,the tension strengths at room temperature and at 350^oC reach to 279MPa and 106MPa respectively.However,when 1.5wt.%Cr is added,the strength of the alloy is conversely decreased because the casting defects become severe.