Effect of pinning by an orientation gradient on the thermal stability of ultrafine grained Ni produced by equal channel angular pressing

摘要

The effect of thermal annealing on the microstructure evolution in nickel of 99.6 wt. % purity, severely deformed by Equal Channel Angular Pressing using the route BC4, was investigated. The ultrafine grained microstructure was shown to be characterized by numerous deformation bands with an average grain size of about 250 nm. High-temperature annealing induces heterogeneous recrystallization and grain growth. Though, some small grains are found to persist recrystallization in spite of pronounced grain growth in surrounding regions. This stability is suggested to be affected by specific microstructure elements containing rotational defects with a misorientation gradient of about 10°/μm along the band direction and thus perpendicular to the bounding high-angle interfaces. Pinning by orientation gradient prohibits motion of bounding interfaces and thus consumption of the grain by recrystallizing matrix.