Abstract
Three alloys were investigated. These were the industrial alloy Hiduminium RR58 containing a bimodal particle distribution of both coarse intermetallic and fine S phase particles, together with two specially prepared alloys, each of which contained one of the particle regimes in isolation. The alloy with only the coarse intermetallic dispersion was designated Alloy C, whilst the alloy containing only the fine S phase was designated Alloy F. An option in the specimen preparation schedule made it possible to either induce or prevent a limited amount of further precipitation during the annealing of RR58 and Alloy F. All of the alloys were given an equal cold deformation prior to isothermal annealing at four temperatures: 270°, 300°, 330° and 360°C. The progress of the softening mechanisms was followed using hardness measurements and by optical metallographic techniques developed specifically for this study. It. was found that in Alloy C, the coarse widely-spaced particles acted as recrystallisation sites, however, on decreasing the annealing temperature, it was found that an increasingly large proportion of the softening was due to the recovery processes preceding recrystallisation. In Alloy F, the fine precipitate promoted softening by recovery mechanisms, although at the higher annealing temperatures, recrystallisation occurred without progressing to impingement. The behaviour of the bimodal RR58 was a compromise between the behaviours of the two unimodal alloys. Recrystallisation was initiated at the coarse particles, but was gradually suppressed by the influence of the fine particles as the annealing temperature decreased. The effect of precipitation during annealing was found to be small, principally slowing the recovery and nucleation processes.