Characterization Of Eutectic Solidification Microstructure And Phase Composition Of The Al-4Ni-2Mn Alloy

The high-temperature strength of the aluminum alloy can be improved by adding the the transition metal?TM?elements,which has been confirmed by rapid solidification?RS?and powder metallurgy?PM?technologies.However,it is hard to achieve scale production because of the high cost of RS/PM technologies.Therefore,materials scientists have been searching for a novel casting aluminum alloy,in which a large amount of second phases containing transition metal elements can be obtained by traditional casting technology.Al-4Ni-2Mn alloy is proposed under this background.The studies suggest that nickel is added to form a fine?Al?+Al₃Ni eutectic in the process of solidification,manganese is involved in the formation of Al-based solid solution,then Al₆Mn precipitates can be formed in the Al matrix during ageing treatment.However,the analysis shows that it is virtually impossible for2 wt.%Mn to dissolve into the Al matrix of the Al-4Ni-2Mn alloy.In this case,it is necessary to reconsider the phase composition of the alloy.This is very important for optimizing the existing Al-Ni-Mn alloy and designing new alloys.Based on the existing research results about Al-Ni-Mn alloy,the phase composition of the Al-4Ni-2Mn alloy has been questioned in this dissertation.The eutectic solidification microstructure and phase composition of an Al-4Ni-2Mn alloy have been revealed by scanning electron microscope and transmission electron microscope.The phase transformation and morphology evolution of the Al₉?Mn,Ni?₂eutectic phase is also investigated respectively.Finally,the constituent of the alloy has been optimized by adding Sc and Zr,and the mechanical properties of the alloy have been also characterized.In order to reveal the eutectic solidification microstructure of the Al-4Ni-2Mn alloy,the sample is etched in 10 wt.%HCl solution and observed by scanning electron microscope.The results show that non-matrix phase in the alloy has a rod-like morphology.During the solidification,the eutectics nucleate at the the primarya-Al and grow toward the liquid region.In the early stage,the eutectic phase grows with a lamellar morphology,then changes into rod-like morphology and grows toward liquid region until the end of solidification.In contrast to the predicteda-Al and Al₃Ni eutectic solidification microstructure,a novela-A1 and Al₉?Mn,Ni?₂eutectic solidification microstructure has been revealed in the study.The Al₉?Mn,Ni?₂eutectic phase is proved to be of Al₉Co₂-type structure,and the cell parameters can be determined to be a=0.8565?3?nm;b=0.6266?2?nm,c=0.6196?2?nm and?=94.60?3?°by XRD.Besides,damage of electron irradiation to Al₉?Mn,Ni?₂phase has been studied.The results indicate that the lattice structure of the Al₉?Mn,Ni?₂phase has been destroyed during the focusing process,however,the damage region is very small?5?10 nm?,which has no influence on the SAED results.Al₉?Mn,Ni?₂eutectic phase is a metastable phase.The phase transformation has been studied by heat treatment at different temperatures.The results indicate that Al₉?Mn,Ni?₂eutectic phase can transform into Al-Ni-Mn?phase during the heat treatment at350??550?.In the process of phase transformation,the?phase is prior to nucleate at the interface between Al and Al₉?Mn,Ni?₂.This kind of nucleation mainly occurs at the position where the eutectic breaks and closely arranges.After nucleation,the?phase not only grows toward the inner of Al₉?Mn,Ni?₂phase,but also toward the side of Al matrix.However,the migration rate of growth interface to the Al matrix is obviously slower than that to the Al₉?Mn,Ni?₂phase.When the heat-treated temperature rises up to 600?,it can be found that Al₉?Mn,Ni?₂ phase transforms into O phase after long-time heat treatment.The interface between the parent phase and the phase transformation product is flat,which shows a coherent interface.In order to verify the stability of the Al-4Ni-2Mn alloy at 300-350?.The alloy is heat treated at 350?for a long time,and the morphology evolution of the Al₉?Mn,Ni?₂phase has been investigated in the study.The results indicate that partial ribbon-like Al₉?Mn,Ni?₂phase gradually changes into rod-like morphology during the heat treatment,and the morphology evolution is always accompanied by the diffusion of the Al atoms toward the inner of the eutectic phase.As to the Al₉?Mn,Ni?₂phase with a rod-like morphology,when the heat treatment time reaches up to 74 h,the thermal grooves can be seen at the sub-boundary of the Al₉?Mn,Ni?₂phase.The rod-like eutectic phase will break up into several short rods with extension of the heating time.Even the sample is heat treated at 350? for 360 h,the morphology will be quite stable if there is no sub-boundary in the rod-like eutectic.Furthermore,the mechanical property of the eutectic microstructure is monitored by the micro hardness.The results show that the micro hardness does not decrease,but slightly increases after long-time heat treatment.Besides,the second phases in the Al matrix of the Al-4Ni-2Mn alloy have been identified by TEM.In the as-cast Al-4Ni-2Mn alloy,besides the large primary O phase,there are some small second-phase particles dispersed in the Al matrix,such as Al-Ni-Mn?phase,three-dimensional icosahedral quasicrystal and two-dimensional decagonal quasicrystal.Considering the precipitation strengthening effect,Sc and Zr elements have been added to the Al-4Ni-2Mn alloy.It turns out that the addition of Sc and Zr will not change the microstructure of the alloy,but has significant precipitation strengthening effect on Al matrix after ageing treatment.The experimental results show that combination addition of 0.1 wt.%Sc and 0.2 wt.%Zr and heat treatment at 300?for 3 h can produce the optimized strengthening effect.At last,the mechanical properties of the Al-Ni-Mn alloy have been evaluated by tensile test.