The effect of dispersed particles on the primary recrystallisation of aluminium zirconium alloys.

The isothermal and isochronal recrystallisation characteristics of a series of aluminium-zirconium dispersion alloys have been investigated as a function of the particle size, the interparticle spacing, the matrix grain-size and the amount of prior cold-deformation. The investigation was based on the use of hardness measurements and metallographic examinations at various stages of the recrystallisation process. Particle characteristics were determined by quantitative metallography techniques. Recrystallisation was accelerated as the particle size increased (for approximately similar centre-to-centre nearest neighbour spacings), or as the number of particles increased (for similar particle sizes) when the dispersion consisted of widely spaced coarse particles (for centre-to-centre nearest neighbour distances in the range 7 to 15 microns). This particle-accelerated recrystallisation was found to be associated with an increased nucleation rate, a relatively unchanged growth rate, and a decreased sensitivity to changes in the matrix grain-size as the particle content increased (i. e. as the interparticle spacing decreased). In the case of finer dispersions it was found that recrystallisation was severely retarded as the interparticle spacing was decreased below approximately 3 microns. In particular, an alloy containing fine particles, at a centre-to-centre nearest neighbour distance of 1.75 microns, was not fully recrystallised after having been held at 375°C for a week following a room temperature rolling reduction of 60% in thickness. This retardation of recrystallisation was found to be associated with marked reductions in both the nucleation and growth rates of the recrystallising grains. It was found that the final recrystallised grain-size of the alloys decreased as the particle content increased in the acceleration region, but increased as the particle content increased in the retardation region. The finest grain-size was, therefore, produced in an alloy for which the recrystallisation rate was the most rapid. The results are shown to be consistent with the Mould-Cotterill hypothesis for the recrystallisation of dispersion alloys.