Study On The Microstructure And Kinetics Evolution Of Ni-x Al-(20-x)Cr Alloys

Aluminum-enriched??-Ni₃(Al,Cr)phase precipitates from?matrix when the Ni-Al-Cralloys are aged under high temperature conditions.The morphology and distribution of??phase are related to the content of Al and Cr,the aging temperature and applied stress.However,when the alloy composition is not properly designed,or the aging temperature is too high,the excessive coarsening of??phase makes the alloy fail.Therefore,designing proper alloy compositions and studying the effects of temperature,external force and element interdiffusion on the microstructure and kinetics behavior of Ni-x Al-(20-x)Cr(at.%)alloys are valuable for improving the performance of nickel-based superalloys.A phase field model based on Cahn-Hilliard equation,Ginzburg-Landau equation and Khachaturyan elasticity theory is established in this work to study the precipitation kinetics behavior of?and??phase in the Ni-x Al-(20-x)Cr(at.%)(x=7.5-13.7)alloys.As Al content increases,the??phases change from dispersed near-circular particles to matrix phase with separated?phase inside.The coarsening rate of??phase reaches its minimum value when the Al content x=9,and for the??phase in Ni-9 Al-11 Cr(at.%)alloy,the round-cubic transformation occurs,with particles oriented along the elastic soft direction.The effects of aging temperature on microstructure evolution and precipitation kinetics behavior of?and??phase are investigated.With the increasing aging temperature,the coarsening of??phase in Ni-9 Al-11 Cr(at.%)alloy is promoted,whereas the coarsening of?phase in Ni-13.5 Al-6.5 Cr(at.%)alloy is retarded.The evolution of interface and interdiffusion of elements in diffusion couples under temperature gradient are quantitatively analyzed.The interdiffusion of element is promoted by the temperature gradient in Ni-10 Al-15 Cr/Ni-10 Al-10 Cr(at.%)and Ni-12 Al-8.5 Cr/Ni-10Al-8.5 Cr(at.%)diffusion couples.Al atoms diffuse from low temperature to high temperature region by crossing the interface,while Cr atoms diffuse from high temperature to low temperature region.The single-phase regions appear on both sides of the initial interface of the diffusion couple with the increasing aging time,and the interface position does not change with time.Moreover,the uphill diffusion of Al is observed in Ni-12 Al-8.5 Cr/Ni-10Al-8.5 Cr(at.%)diffusion couple,which is driven by the decreasing of its chemical potential.The effects of applied stress on microstructure evolution and precipitation kinetics behavior of??phase in Ni-9 Al-11 Cr(at.%)alloy are studied.The applied stress can promote the coarsening of the??phase.With the increasing stress,the change rates of the average particle radius and particle number density of the??phase increase.Also,the width of the particle size distribution curve gradually increases,and the peak values decrease with moving to the left side.