Growth Mechanism of Strongly Anisotropic Intermetallic Compounds and Thermophysical Properties of Al-Ni Alloy Solidified under High Pressure

Author:Wang Xiao Hong

Supervisor:wang hong wei


Degree Year:2019





The effects of pressure,superheat degrees,and Ni mass fraction on the solidification behaviour and phase composition of binary Al-Ni alloys were studied in this thesis.Meanwhile,the constitutional undercooling criterion,the dendrite tip radius and the interface stability growth equation of the nil solubility intermetallic compounds Al3Ni were established.Furthermore,the phase selection and competitive growth behavior between dendrite and eutectic were calculated with the consideration of pressure.Finally,the effect of high pressure on the heat capacity,Debye temperature,potential energy,and linear thermal expansion coefficient of Al-Ni alloy was analyzed by testing the specific heat capacity curve in the temperature range 2-300K.According to the solute segregation characteristics of Al3Ni,there is no constitutional undercooling at the frontier of nil solubility intermetallic compound Al3Ni growth interface during the solidification of Al-Ni alloy.Meantime,the classical theory of dendrite tip radius which deduction based on solid solution is no longer applicable to Al3Ni.Therefore,a new dendritic tip radius equation for nil solubility intermetallic is obtained by considering the effect of kinetic undercooling on the disturbed interface temperature.Furthermore,based on the steady state equation and considering the effect of kinetic undercooling on the disturbed interface temperature,the interface stability growth equation which suitable for the nil solubility intermetallic compound Al3Ni was derived.Theα-Al,bulkβ-Al3Ni,and eutectic phase exist simultaneously after the eutectic Al-5.69%Ni alloy solidified under high pressure.And the high pressure solidification microstructure of hypo-eutectic Al-5%Ni alloy and hyper-eutectic Al-6%Ni alloy proves that the eutectic point of Al-Ni alloy has no change.Combined with the established dendrite tip radius model,theα-Al andβ-Al3Ni phase takes precedence over the eutectic phase nucleate when the solidification pressure is greater than 0.66GPa.The microstructure of the hypo-peritectic Al-38%Ni alloy which solidified at atmospheric pressure is composed of bulk Al3Ni phase,inter-phase eutectic phase,and Al3Ni2 phase which embedded in the Al3Ni phase.However,the microstructure of Al-38%Ni alloy solidified at 2GPa and 4GPa,which is a typical hypereutectic microstructure with the Al3Ni2 phase disappears.The microstructure of peritectic Al-43%Ni alloy solidified at different pressures is all single-phase Al3Ni intermetallic compound.The microstructure and phase composition of hyper-peritectic Al-47%Ni alloy solidified under different pressures were investigated.The results show that Al3Ni2 and Al3Ni phases coexist at atmospheric pressure.While another new phaseα-Al exists simultaneously when solidified at high pressure.Combined with the established dendrite tip radius model,the interfacial temperature of primary Al3Ni2 phase in hypo-peritectic Al-38%Ni alloy is less than that of Al3Ni phase,and the undercooling degree of each phase during the high pressure solidification of hyper-peritectic Al-47%Ni alloy isΔTAlNi>ΔTβ-Al3Ni>ΔTAl3Ni2.The interface stability of Al3Ni during the solidification of hypo-peritectic Al-30%Ni alloy under the effect of high pressure and combined action of high pressure and superheating was calculated.The results show that the Al3Ni phase grows in a dendrite behaviour after solidification under the condition of 2GPa with the superheat degree of 600K.While the whole sample can be divided into four regions(high pressure solid solution region I,mushy zone II,and fully melted region which consists of planar growth single phase Al3Ni area III and hypo-eutectic zone IV)under the condition of3GPa/600 K.Region I contains a small portion of Al3Ni2 phase,region II is a typical hyper-eutectic microstructure without Al3Ni2 phase residue.Meanwhile,the sample is composed of bulk single phase Al3Ni region and hyper-eutectic microstructure area after solidified at 4GPa with a superheat degree of 800K.Furthermore,the equation for the interfacial stability growth pressure transition point and superheat degree of Al3Ni phase is obtained.The specific heat capacity of Al-Ni alloys with different compositions increases with the increase of temperature in the range of 2-300K.For Al-Ni alloys with the same composition,the specific heat capacity decreases with the increase of pressure.The change of Debye temperature of Al-Ni alloys with pressure has the same trend as the specific heat capacity,and the Debye temperature of Al-Ni alloys is proportional to melting point.According to the newly developed potential function,the potential curves of Al-Ni alloys with different compositions under different pressures and the linear thermal expansion coefficient curves in the temperature range of 2-300K were investigated.The results show that the minimum potential of Al-Ni alloys with the same composition decreases with the increase of pressure.The linear thermal expansion coefficient curve of Al-Ni alloy with the same composition descends with the increase of pressure.