The effect of hot zone velocity and temperature gradient on the directional recrystallization of polycrystalline nickel

Abstract

Cold-rolled polycrystalline nickel sheets were directionally annealed at 1000°C with either a large (1000°C/cm) or a small (50°C/cm) temperature gradient ahead of the hot zone. Columnar grains were produced at hot zone velocities from 2 to 100 mm/h for the large temperature gradient with only two ~6 mm wide grains across the width at 5, 10 and 30 mm/h. With increasing hot zone velocity above 30 mm/h, the columnar grains became narrower. For the same range of hot zone velocities with a small temperature gradient, equiaxed grains were produced, with the grain size decreasing with increasing hot zone velocity. This difference arises because for the low temperature gradient recrystallization followed by substantial grain growth occurs ahead of the hot zone. Both interrupted directional annealing runs and runs (with the large temperature gradient) of nickel in which the nickel had been primary recrystallized at low temperature showed that the columnar grain structure developed by secondary recrystallization.