Study On Heat Treatment Process Of Ni-based High Entropy Alloy

The high entropy alloy has a wide application prospect due to its unique properties.Most of the equal atomic ratio high entropy alloys are BCC or FCC single phase structures,and it is difficult to achieve both strength and toughness.Therefore,researchers break the limit of equiatomic ratio or near-equiatomic ratio to obtain a large number of high entropy alloys with two-phase structure and excellent mechanical properties.A series of Ni_x（FeCoCrAl）_100-x alloys and Ni_x（Fe VCrAl）_100-x alloys were prepared by enhanced the atomic percentage of Ni element gradually on the basis of equiatomic ratio Ni FeCoCrAl and Ni Fe VCrAl.The heat treatment process of the alloy was studied by different time and temperature.The microstructure and mechanical properties of the alloy were investigated by OM,SEM,XRD,EDS,hardness and tensile properties tests.The phase composition of as-cast Ni_x（FeCoCrAl）_100-x alloy changes from BCC（x=20）to BCC+FCC（x=30,40）and finally to FCC（x=50～80）with the increase of x value.With the increase of Ni content,the microstructure changes from fine dendritic to dendritic+eutectic to coarse dendritic.The BCC phase is rich in Ni Al in the eutectic structure.The strength and hardness of the alloy decrease,the elongation increases gradually,and the fracture morphology changes from cleavage fracture to dimple.The transformation of Ni₄₀（FeCoCrAl）₆₀ and Ni₅₀（FeCoCrAl）₅₀ alloys does not occur after cold rolling and annealing.With the increase of heat treatment temperature,the yield and tensile strength of the alloys decrease,and the elongation increases.Ni₅₀（FeCoCrAl）₅₀ alloy is completely recrystallized after annealing at 700℃for 1h.With the further increase of temperature,the recrystallized grains grow and annealing twins appear.However,there are two phase eutectic structures in Ni₄₀（FeCoCrAl）₆₀ alloy,which limit the occurrence of recrystallization.The recrystallization structures appear when the annealing temperature reaches 1200℃.With the increase of Ni content,the enthalpy of mixing,entropy of mixing and atomic radius difference of Ni_x（Fe VCrAl）_100-x alloy decrease gradually,and the valence electron concentration increases.Ni₂₀（Fe VCrAl）₈₀ is BCC+B2 phase,Ni₃₀（Fe VCrAl）₇₀ and Ni₄₀（Fe VCrAl）₆₀ alloy is BCC single phase,Ni₅₀（Fe VCrAl）₅₀ is BCC+FCC double phase,Ni₆₀（Fe VCrAl）₄₀ is FCC single phase.The microstructure changes from fine dendrite+large interdendrite to long straight dendrite+irregular linear and granular interdendrite.The energy spectrum scanning shows that the interdendritic phase is rich in Cr V,and the dendritic phase is rich in Ni Al,and the distribution of Fe element is relatively uniform.The tensile property of the alloy increases first and then decreases.The Ni₅₀（Fe VCrAl）₅₀ alloy has the best tensile property with the yield strength of 875MPa,the tensile strength of 1237MPa,the elongation of 32.4%,and the hardness of the alloy decreases gradually with the increase of the Ni content.After annealing,σphase and FCC phase appear in Ni₄₀（Fe VCrAl）₆₀ alloy,and the microstructure is black matrix and white short rod structure in dendrite.With the increase of annealing temperature or the extension of annealing time,the hardness of the alloy increases first and then decreases,and the maximum hardness is reached at 1100℃for 30min.There is no phase transition in Ni₅₀（Fe VCrAl）₅₀ alloy after heat treatment.With the increase of annealing temperature or the extension of annealing time,precipitates on the dendrite increase,and the hardness value decreases gradually.Ni₆₀（Fe VCrAl）₄₀ alloy is FCC single phase structure after heat treatment.Recrystallization occurs at 1000℃,and the alloy completely recrystallizes at 1000℃for 60min.The hardness of the alloy decreases with the increase of recrystallization degree.