Mechanisms of microstructure evolution during cryomilling in the presence of hard particles

The present study was undertaken to provide insight into the mechanisms that govern the evolution of microstructure in Ni powder during cryomilling with nitride particles. The AlN particles are distributed in Ni powder particles after cryomilling, and the particles with initial size of 2 μm are fractured into smaller size, 50∼300 nm, during cryomilling. The distribution of particles is uniform, and some extremely small particles, size range of ∼20 nm, are also observed by TEM after cryomilling. With addition of AlN particles, the Ni powder particle size after cryomilling is reduced, and contamination of iron and gaseous atoms, N and O, is increased. For the grain size of Ni, the present results show that, in the presence of 2 wt% (5 vol%) AlN particles, the Ni grain size is reduced to 37 nm after 8 h of cryomilling. In contrast, the grain size of Ni cryomilled under identical conditions but without particles exceeded 100 nm. In terms of volume fraction, the results show an increase in the rate of grain size refinement with increasing volume fraction of AlN particles for the range studied, i.e. 1.2–5.0 vol%. The grain size is also reduced to 25 nm with increasing impeller speed up to 340 rpm, which provides higher kinetic energy, and longer cryomilling time of 20 h. This observation is rationalized on the basis of a mechanism involving the interactions of dislocations with hard, non-deformable nitride particles, and thermally induced dislocation generation due to the thermal expansion coefficient difference between the Ni matrix and the nitride particles.