Study On Continuous Precipitation Kinetics Of Ni-Al Alloys

The excellent high temperature creep strength,fatigue resistance,oxidation resistance and hot corrosion resistance of nickel-based superalloys are mainly due to the ?' phase with Ni3X structure,while the morphology,particle size and volume fraction of ?' phase are all related to the composition of the alloy and the aging temperature.Therefore,the research on the ?' precipitation kinetics in nickel-based superalloys can provide a theoretical reference for the material design,heat treatment and microstructure evolution of the alloy.In this paper,the kinetics of continuous precipitation of ?' phase in Ni-Al alloy was analyzed by phase-field method and experiments(TEM).It was demonstrated that the effects of composition,aging temperature and cooling mode on the kinetics of continuous precipitation of ?' phase.According to the law of precipitation dynamics,the continuous precipitation process of Ni-Al alloys can be divided into nucleation and growth,growth and coarsening and steady-state coarsening stage.The volume fraction of ?' phase reached equilibrium at the steady-state coarsening stage and increased with increasing A1 contents.The number density tended to increase at nucleation and growth stage and decrease at the growth and coarsening and steady-state coarsening stage,and the rate of change increased with the increasing Al contents.The exponential relationship between the average radius<r>of ?phase and the aging time tn showed that the value of time index n was closest to the classical LSW theoretical value of 0.33 at the growth and coarsening stage.The coarsening rate of the ?'phase increased with increasing Al contents.The width of particle size distribution broadened at nucleation and growth stage,and got narrowed at growth and coarsening stage,then became wider at the steady-state coarsening stage.The width of particle size distribution had no significant relationship with AI contents.The width of the particle size distribution of y phase in the inverse Ni-Al alloy became narrower with increasing Al contents.Comparing the simulation and experimental results,we summarized that the interface mobility of Ni-15at.%AI alloy at 873K is M?=1.68×10-9m3J-1s-1.The effect of aging temperature and cooling rate on ?' phase precipitation kinetics was studied with three-dimensional phase field model and TEM microstructure.It can be seen in Ni-16at.%Al alloy that the aging temperature increased,the coarsening rate and the average particle radius of ?' phase increased,the ?' phase number density decreased,and the equilibrium volume fraction decreased.The peak value of particle size distribution decreased with the increasing aging time.The larger the cooling rate in Ni-15at.%AI alloy,the more regular the morphology of the precipitated phase particles,the more obvious the array arrangement.The smaller the ?' phase particle radius,the lower the equilibrium volume fraction,the wider the particle size distribution and the smaller the peak value of the particle size distribution.After the alloy aging treatment with water quenching cooling in the Ni-Al alloys lead to secondary ?' phase,and the aging temperature was higher,the longer the aging time,the second ?' phase became less with the higher aging temperature and longer aging time.