Promotion of Ultrafine Grain Evolution by Coarse Particles during Multidirectional Forging of Ni-Fe Alloy

A Ni-Fe alloy containing coarse particles was multidirectionally forged at temperatures of 773 and 873 K at a true strain rate of 1 x 10(-3) s(-1) in vacuum. For comparison, a Ni-Fe alloy without dispersed particles was also prepared and multidirectionally forged. In both alloys, clear flow softening took place at the high strain region during the multidirectional forging at 873 K, while at 773 K, work hardening followed by steady-state like flow occurred. The average grain size became gradually smaller with increasing cumulative strain. The microstructural evolution, however, depended strongly on temperature and the presence of particles. Uniform evolution of ultrafine grains was dramatically promoted by coarse particles especially at a higher temperature. The Ni-Fe alloy containing particles multidirectionally forged at 873 K exhibited a homogeneous evolution of equiaxed fine grains of about 0.8 mu m only at a cumulative strain of I I pound epsilon = 2.4, while Ni-Fe alloy without such particles showed a much coarser and inhomogeneous microstructure.