Effect Of Trace Element Addition On Microstructure And Properties Of AlCuFe Quasicrystalline Alloy

AlCuFe system quasicrystals are of great interest because of their non-toxic,inexpensive and excellent performance.Although the preparation process about ternary quasicrystal alloys has become mature,the study of quaternary quasicrystal alloys still has problems such as the design component is relatively single and the influence of valence electron concentration on quasicrystals is not clear.In this paper,the classical Al₆₅Cu₂₀Fe₁₅quasicrystal composition alloy was chosen as the substrate.Al₆₅Cu₂₀Fe_15-xSi_x(x=1,5,9),Al₆₅Cu₂₀Fe_15-xMn_x(x=1,5,9),Al₆₅Cu₂₀Fe_15-xCr_x(x=1,5,9)were designed by changing the valence electron concentration of three alloy systems.The alloy systems were prepared by ordinary casting and arc melting methods.The microstructure,mechanical properties and thermal expansion properties of the prepared quaternary quasicrystalline alloys were studied by means of analytical tests such as OM,SEM,TEM,DSC,XRD,nanoindentation and thermomechanical analysis.The main conclusions are as follows.The AlCuFeSi quaternary quasicrystalline alloy prepared by ordinary casting method consist of quasicrystalline I phase and a small amount of?phase,?phase and?phase,when the valence electron concentration is 1.91 and 2.15.The average particle size of the quasicrystalline phase is 0.4?0.5?m.The DSC curves show that the cooling stages of the AlCuFeSi quasicrystalline alloy were mainly?-phase,quasicrystalline phase,and Al₂Cu phase.The maximum quasicrystal formation calculated by the area method is 51.1%,when the valence electron concentration is 2.15.The quasicrystalline phases include I phase and Al₆₅Cu₂₀Fe₁₀Mn₅phase,as well as a small amount of?phase,?phase and?phase,when element additions to AlCuFeMn quaternary quasicrystal alloys prepared by ordinary casting method is 1 and 5 at.%.The addition of 1 at.%and 5 at.%Mn is beneficial to the refinement of the quasicrystalline phase in the quasicrystalline alloy,and the average particle size of the quasicrystalline phase is 0.2?m.The DSC curve shows that the peak of?-phase precipitation appears at 972°C and the peak of quasicrystalline phase precipitation appears when the temperature is reduced to 868°C in the AlCuFeMn quasicrystalline alloy.The highest value of quasicrystalline content of52.46%is obtained with the addition of Mn at 5 at.%calculated by the area method.When valence electron concentrations is 1.76,1.80,and 1.84,quasicrystalline phases were obtained in AlCuFeCr quaternary quasicrystal alloy prepared by arc melting,and the quasicrystalline phases containe both quasicrystalline I phase and Al₆₅Cu₂₀Fe₁₀Cr₅phase.The average particle size of the quasicrystalline phase can reach 0.6?m.DSC cooling curves show that the precipitation peaks of the AlCuFeCr quasicrystalline alloy are in the order of?phase,?phase and quasicrystalline phase,and precipitation temperatures are 1010°C,971°C and806°C,respectively.The maximum quasicrystalline phase content of 87.22%is obtained for AlCuFeCr quasicrystalline alloy when valence electron concentration is 1.76.The comparative study of AlCuFeSi alloy,AlCuFeMn alloy and AlCuFeCr alloy shows that:the addition of semi-metallic element Si in the AlCuFe system alloy can also obtain the generation of higher quasicrystalline amount,but the defects such as shrinkage and shrinkage holes are very easy to appear in the microstructure,results in no significant enhancement of mechanical properties and thermal expansion properties.As the addition of the fourth metal element Cr or Mn increases,the quasicrystalline phase changes from I phase to the quaternary quasicrystalline compounds Al₆₅Cu₂₀Fe₁₀Cr₅and Al₆₅Cu₂₀Fe₁₀Mn₅;the?phase content decreases due to the additional consumption of Fe element in the generation of quasicrystalline phase;the Al and Cu elements released from the decomposition of?phase will combine with Al Cu in the matrix to generate Al₂Cu,Al₂Cu₃phases.AlCuFeMn and AlCuFeCr quasicrystalline alloys with additions of 1 and 5 at.%increase the hardness and elastic modulus while decreasing the coefficient of thermal expansion.Compared with AlCuFe ternary quasicrystalline alloys,AlCuFeMn and AlCuFeCr quasicrystalline alloys have the highest hardness and elastic modulus of 13.99 GPa and 196.04 GPa,which are increased by112.6%and 94.6%,and the thermal expansion coefficient can be reduced by up to 51.26%.The substantial enhancement in mechanical properties of the quasicrystalline alloys with the addition of Mn and Cr elements is attributed to the refinement of the quasicrystalline phase and the increase in quasicrystalline content.The addition of Cr elements with low thermal expansion coefficients leads to the extreme values of low thermal expansion coefficients when Al₆₅Cu₂₀Fe₆Cr₉alloy,which is in agreement with the prediction of the mixing rate law.