Study On Formation And High Temperature Stability Mechanism Of Micro/Nano Ni-based Alloys

An inherent relation between the non-equilibrium property and the subsequent solid-state grain growth is becoming increasingly important for material preparation.Based on this,two types of non-equilibrium methods: deep undercooling rapid solidification method and ball milling method were used respectively to prepare singlephase supersaturated Ni-B,Ni-Si micro/nano-crystalline solid solution.In this paper,the essential relationship between non-equilibrium forming process and high temperature stability of grain was studied by combining non-equilibrium solidification model and thermodynamic model through high-speed camera,XRD,SEM,TEM after annealing treatment.The main conclusion are as follows:(1)The high-undercooled(>200K)Ni-B alloy,Ni-Si alloy were prepared by the integration of cyclical superheating method and fusion purification method.It can be proved that the stress accumulation originating from organization leads to the recrystallization of grain by using high speed camera real-time monitoring method,non-equilibrium dendrite growth theory and stress accumulation model.(2)Based on the heat treatment of non-equilibrium samples,the dynamic segregation of two grain growth and stable stages in grain boundary were found.The calculation of thermodynamics model for grain growth proves that the segregation of solute to grain boundary leads to the primary stabilization,which results in the decrease of grain boundary energy,and the subsequent second growth results from the formation of the second phase,and GB energy increases as a result.A decreasing trend between the final grain size and the initial melt undercooling was detected,and an inverse “V” relation between the final grain size and solute concentration was deduced at the same time.The above analyses indicate that the twice grain growth and stabilization were controlled by the competition of the decrease of grain growth thermodynamics(rain boundary energy)and kinetics(drag of precipitations and second phase).(3)Ni-based single-phase supersaturated nano-powder with different composition was prepared by mechanical alloying.The certificate relationship between grain size,annealing time and temperature was studied by XRD,TEM and HRTEM after annealing treatment with different annealing time and temperature.It can be found that the addition of solid solute elements can improve the thermal stability of the powder,which keeps the grain size under 100 nm at a temperature of 900 degrees.