Study On Mechanism Of Effect Of Heat Treatment Process On Metastable Phase Ni₃₁Si₁₂

Ni-Si eutectic alloys can be widely used in the manufacture of high-performance aerospace,aerospace,ship and tank engines and other power unit discs,blades,etc.due to their excellent corrosion resistance,wear resistance,and high strength.The primary dendrite spacing of the Ni-Si eutectic alloy decreases,the microstructure refines,and the fracture toughness increases after the directionally solidified Ni-Si eutectic composites.However,due to the undercooling during solidification,the etastable phase Ni₃₁Si₁₂ is generated inside the alloy that makes the toughness and plasticity of the material reduced.This paper aims to study the transformation mechanism of the heat treatment process on the metastable phase and the effect on the overall properties of the alloy.After vacuum consumable electrode arc melting of Ni-Si alloy,the modified Bridgman and high temperature gradient electron beam suspension zone melting directional solidification have been used to obtain Ni-Si eutectic composites at different solidification rates respectively.And then the different heat treatments have been carried out for them.The changes of metastable phase have been analyzed before and after heat treatments by metallographic microscope.The causes of the metastable phase Ni₃₁Si₁₂ and the influence mechanism of the heat treatment process have been clarified.The microhardness under different heat treatment processes has been tested and calculated.The corrosion performance has been compared.At last,the heat treatment process parameters with the best metastable phase transformation have been determined.It is found that with the increase of solidification rate,the degree of undercooling is larger,the phase transition during solidification deviates from equilibrium solidification,and the more metastable phase is generated,the metastable phase under relatively stable state can be changed by external conditions.Since the solidification rate of Bridgman directional solidification selected in this paper is higher than that in the electron beam suspension zone melting directional solidification,the cooling rate is large and the degree of supercooling is large.Therefore,the number of metastable phase in the Ni-Ni₃Si eutectic in-situ composite after Bridgman directional solidification is more than that in the electron beam suspension zone melting directional solidification.The optimum heat treatment process parameters for the metastable phase transformation of the Ni-Si eutectic composites after Bridgman directional solidification are 1050?annealing+4 h insulation.For the Ni-Si eutectic auto-composite composite after electron beam suspension zone melting directional solidification,after annealing at 950?,the metastable phase elimination effect after 2h incubation is the best.The heat treatment process has contributed greatly to the transformation of the metastable phase and the overall performance of the material,so that the Ni-Si eutectic composite can be used in more demanding environments.The Bridgman directional solidification eutectic alloy after different heat treatment processes has been tested for corrosion resistance,and an amorphous SiO₂passivation film is formed on the surface of the sample,which has good corrosion resistance.After annealing at 1000°C,the eutectic alloy after 4 hours of holding has a higher open circuit potential value,a positive self-corrosion potential,a smaller self-corrosion current density and a larger capacitive anti-arc radius.Therefore,the eutectic alloy under the heat treatment process parameters has the best corrosion resistance.