Coupling Control Of High Temperature Stability And Strengthening Of Nanocrystalline Alloys Decorated By High Entropy Grain Boundary

Nanocrystalline alloys(<100 nm)have excellent mechanical properties.However,due to the large grain boundary proportion,high energy and poor thermal stability,the grain is easily unstable under the influence of external environment.Solute segregation is an effective means to improve the thermal stability of nanocrystalline alloys.The high entropy grain boundary decoration is based on the segregation of solute elements and a variety of solute elements are introduced into the nanocrystalline alloy to further enhance the thermal stability of nanocrystalline alloys.In this paper,Fe-ZrNbMoTa and Fe-Zr Nb Hf Ta nanocrystalline alloys were prepared by mechanical alloying with the introduction of a new concept of high entropy grain boundary decoration,and their high temperature thermal stability was studied.Nanocrystalline alloy powder with excellent stability was prepared into bulk alloy by high temperature and high pressure sintering technology.The microstructure and mechanical properties of nanocrystalline alloy were studied by X-ray diffractometer,scanning electron microscope,transmission electron microscope and material testing machine.The thermal stability mechanism and mechanical properties of nanocrystalline alloy were analyzed with the thermal stability model of nanocrystalline alloy.The following conclusions are drawn:(1)When the nanocrystalline iron-based alloy is prepared by the mechanical alloying method,the grain size of the powder gradually decreases with the extension of the milling time,and a single-phase supersaturated solid solution is formed.The grain size of nanocrystalline Fe-Zr_0.2Nb_0.2Mo_0.2Ta_0.2 alloy can be refined to 17 nm after ball milling for 50 h.When the single-phase nanocrystalline solid solution is subjected to isothermal annealing treatment,the multi-component solute continuously segregates to the grain boundary,which increases the thermal stability of the alloy.Nanocrystalline Fe-Zr_1.0Nb_1.0Mo_1.0Ta_1.0 alloy,annealed at 900 ? for 10 h,has an average grain size of55 nm.Fe-Zr_0.2Nb_0.2Hf_0.2Ta_0.2 with Hf element shows better thermal stability than Fe-Zr_0.2Nb_0.2Mo_0.2Ta_0.2 alloy.(2)According to the study of thermal stability of ball milling nanocrystalline alloys,Fe-based nanocrystalline alloy bulk was prepared by high pressure sintering at 800 ?.The hardness of nanocrystalline Fe-Zr_0.5Nb_0.5Mo_0.5Ta_0.5 alloy with high thermal stability can reach 738.6 HV(7.238 GPa),and the yield strength and compressive strength are 3107 MPa and 3596 MPa,respectively.The solute element Hf is used instead of Mo,the nanocrystalline Fe-Zr_0.2Nb_0.2Hf_0.2Ta_0.2 alloy exhibits a smaller grain size(79 nm),higher strength(compressive strength 3523 MPa)and hardness value(729.5 HV).(3)Based on the principle of thermodynamic extreme value,a multi-element nanocrystalline alloy grain growth model was derived,and compared with the binary simple nanocrystalline alloy system,the grain boundary energy and the temperature-size relationship during annealing were analyzed.Thermodynamically,the multi-element solute segregation significantly reduces the grain boundary energy compared to a single element,and improves the thermal stability of nanocrystals.Dynamically,the solute drag effect caused by solute segregation and the delayed diffusion caused by high-entropy grain boundaries have a synergistic effect that hinders grain boundary migration and inhibits grain growth.At lower temperatures,the thermal stability of nanocrystalline alloys is mainly affected by the reduction of grain boundary energy caused by solute segregation and the solute drag effect.At higher temperatures,the second phase precipitates along the grain boundary region,and the thermal stability of the nanocrystalline alloy is mainly due to the combined effects of high entropy decoration and second phase pinning.