The directional recrystallisation process which leads to the growth of highly anisotropic 'new' grains in an oxide dispersion strengthened nickel base superalloy is investigated to elucidate the mechanism of recrystallisation and the process variables important in controlling microstructural evolution. It is found that after extrusion and hot deformation, the alloy consists of an ultrafine equiaxed grain microstructure characteristic of a material which has undergone dynamic primary recrystallisation. Thus, the directional recrystallisation which follows when the hot rolled sample is annealed in a moving temperature gradient (zone annealed) is really a secondary recrystallisation phenomenon. By comparing grain boundary mobility with the rate of zone annealing, it was theoretically predicted and experimentally observed that directional recrystallisation should give way to equiaxial recrystallisation when the interface velocity is less than the speed of zone annealing. The variables involved in the anisothermal zone annealing process appear to rationalise when the effects of temperature and time are consolidated into a single parameter, the kinetic strength of the heat treatment. Directional recrystallisation is found to occur only when a critical value of the kinetic strength is exceeded, irrespective of the peak temperature or tranverse speed during zone annealing.
Materials Science and Technology, Vol. 6, 1990, pp. 1236-1246.
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