Study On Hot Compression Deformation And Microstructure Control Of 60NiTi Alloy During Heat Treatment

60NiTi alloy（60 wt.%Ni,40 wt.%Ti）was considered as an excellent aviation bearing material because of its high hardness,high strength,low density,non-magnetism,and good corrosion resistance.The microstructure of 60NiTi alloy was complex,which resulted in difficulty in processing and microstructure regulation in heat treatment.In this paper,metallographic microscope,scanning electron microscope,transmission electron microscope,X-ray diffraction analysis and mechanical property testing methods were used to study the effect of 60NiTi alloy during hot compression deformation and traditional heat treatment and electric pulse treatment on the microstructure and properties of the alloy.The soft-hardening mechanism were revealed,which provided a reference for the formulation of hot processing process and the regulation of microstructure and properties of 60NiTi alloy during heat treatment.The main conclusions are as follows:（1）The hot deformation activation energy Q of the 60NiTi alloy was327.89 kJ/mol,and the peak stress constitutive equation was calculated as:（?）=9.0×10¹⁵×[sinh(2.7×10^-3σ)]^5.7exp（-327887.5/RT）The suitable hot processing parameters were determined as deformation temperature 600-650℃ and strain rate 0.005-0.05 s^-1.During hot compression,dislocation slip,twinning,dynamic recovery and dynamic recrystallization occurred in 60NiTi alloy.（2）Under the solid solution temperature of 950℃and the water cooling condition,the content of Ni₃Ti phases in 60NiTi alloy gradually decreased as the holding time increased from 1 h to 4 h,and a small amount of 20～50 nm Ni₄Ti₃ phase particles were formed.When the solid solution temperature was 1025℃,the Ni₃Ti phase basically dissolved back into the NiTi matrix.After holding at 1075℃ for 4 h and water cooling,a large number of lens-like Ni₄Ti₃ phases with a width of 20～40 nm and a length of 80～200 nm were formed.Besides,the interface between the Ni₄Ti₃phase and the NiTi matrix was the coherent interface.（3）Under the solid solution temperature of 950℃and the furnace cooling condition,a large amount of Ni₃Ti phases with uneven distribution precipitated in NiTi matrix,resulting in many precipitate free zone.When the solid solution temperature increased to 1025℃and furnace cooling,the number of Ni₃Ti phase increased significantly and the distribution was more uniform.Acicular and granular Ni₃Ti phases precipitated in the intra-crystalline,and granular Ni₃Ti phase was formed at the grain boundaries.（4）The cooling rate significantly affected the phase transformation behavior of the 60NiTi alloy.During the cooling process,the phase transformation of the alloy was in the coherent order of B2→Ni₄Ti₃→Ni₃Ti.With the increase of the cooling rate,the transformation process from metastable phase Ni₄Ti₃ to stable phase Ni₃Ti in the alloy was inhibited,which increased Ni₄Ti₃ phase content and decreased the number of Ni₃Ti phase content.The faster the cooling rate,the higher the hardness of the alloy,i.e.,water cooling treatment hardness>oil cooling treatment hardness>liquid nitrogen cooling hardness>air cooling treatment hardness>furnace cooling treatment hardness.The hardness of the alloy increased with the number of Ni₄Ti₃ phases,and decreased with the number of Ni₃Ti phases.（5）Compared with the traditional heat treatment,the electropulsing treatment significantly decreased the solid solution temperature and increased the solution rate of the Ni₃Ti phase.With the current intensity increased,the content of Ni₃Ti phase gradually decreased.After 10 s of367 A electropulsing treatment,the number of Ni₃Ti phase decreased.When the current intensity increased to 396 A,most of Ni₃Ti phases had been dissolved into NiTi matrix.