Research On Microstructure And Properties Of 90W-7Ni-3Fe Tungsten Heavy Alloy By Vacuum Hot-pressed Sintering

Tungsten heavy alloys have been widely used as core of rod type kinetic energy penetrator due to their high density,high strength,good ductility and so on.Tungsten heavy alloys for projectile need to meet higher requirements,because of the improvement of armor protection and science.Recently,many scholars have made researches on powder modification,enhanced sintering,deformation strengthening and heat treatment to improve properties of tungsten heavy alloys.In this paper,vacuum hot-pressed sintering was used to obtain 90W-7Ni-3Fe with good comprehensive performance,especially high plasticity,which could provide sintered billets with excellent plasticity for subsequent deformation strengthening.The sintering groups were designed by combining sintering temperature,holding time and pressure temperature.The mechanical properties and microstructures at room temperature of each group of 90 W were analyzed to obtain the best sintering schemes.There were two wonderful sintering groups: a)sintering temperature 1470℃/holding time 45min/pressure temperature 1000℃ and b)sintering temperature 1470℃/holding time 60min/pressure temperature 1400℃.These two groups of 90 W had microstructure with good interface bonding and uniform distribution and good strength and plasticity(ultimate strength:869MPa and 723MPa;elongation:23.1% and 18.1%).The homogenization parameters of the two groups of 90 W were 0.85 and 0.93 respectively based on the sintering homogenization model,which are high degree of homogenization.It indicated that the diffusion and migration of powder particles are fully and the sintering process is more complete.Therefore,the properties of the sintered billets are excellent.Obvious strain hardening was shown in tensile tests at room temperature.The strain hardening rate decreased with increasing strain and the plasticity decreased with the increase of strain rate.There were strain hardening effect and thermal softening effect in quasi-static compression tests at room temperature,and thermal softening effect existed in large deformation.The strain hardening rate decreased with the increase of strain.Quasi-static loading is a process of continuous deformation of tungsten particles and matrix phases.The dynamic mechanical behavior of 90 W alloy in the strain rate range of 700~3900s-1 was studied.The deformation under dynamic loading was a competitive process of strain hardening effect and thermal softening effect,expressed in strain hardening rate-plastic strain curve with oscillations.The yield strength of 90 W increased remarkably and the rate of strain hardening rate reached a relatively stable state was faster under dynamic compression with respect to quasi-static compression.With the increase of strain rate,the yield strength,the plasticity and deformation degree of microstructure increased.Adiabatic shear band appeared at the high strain rate of 3900s-1.Dynamic loading was a process of simultaneous deformation of tungsten particles and matrix phases.The 90 W alloys prepared by vacuum hot-pressed sintering were treated by solid solution-quenching in the temperature range of 1000~1300℃.There were pores on the phase interphases and the contiguity of W-W increased after heat treatment.With the increase of heat treatment temperature,the dissolution of W in the matrix phase increased,and the hardness of heat treated 90 W alloy increased but was lower than that of sintered 90 W.The strength and plasticity of heat treated 90 W alloys were worse than that of sintered 90 W alloys.The higher the heat treatment temperature,the worse mechanical properties.Results showed that the morphology,structure and composition of the precipitated phased of matrix will be changed with the increase of heat treatment temperature.Therefore,single and conventional heat treatment can not be used to improve properties of heavy tungsten alloys.