Accelerated grain refinement induced by dispersed coarse particles

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A Ni-30 mass% Fe alloy with dispersed coarse oxide particles, ranging from 1 to 3 μm in diameter, was multi-directionally forged (MDF) at temperatures between 773 K and 873 K at a true strain rate of 1 × 10^<-3> s^<-1> in vacuum. For comparison, a particle-free Ni-Fe alloy was also prepared and MDFed. At 873 K, an obvious flow softening occurred during MDF especially in the alloy with dispersed particles, while work hardening followed by steady-state flow appeared at 773 K in both alloys. With increasing cumulative strain, the average grain size became gradually finer. However, the evolved microstructure was quite different depending on temperature and dispersion of particles. At 873 K, in the alloy with particles, equi-axed fine grains of about 1.3 μm in average was uniformly evolved at a cumulative strain ofΣε=2.4, while at 773 K the microstructure was still inhomogeneous. The above experimental results suggest important role of coarse particles to stimulate grain refinement.