High strain rate superplasticity in three contrasting fine grained aluminium alloys

The superplastic properties and microstructures of three contrasting fine grained aluminium alloys were investigated. These included (i) a powder metallurgy MMC, (ii) a severely deformed spray cast alloy, and (iii) the Zn-22%Al eutectoid alloy. The results showed some differences in the details of behaviour between the alloys. One of these was the presence of true work hardening, associated with dislocation activity, in the MMC, and its absence in the microduplex Zn-Al eutectoid alloy. In addition, the powder route MMC had the high threshold stress (up to 10MPa) commonly encountered in such materials, whereas this was not the case with the cast and severely deformed alloy, indicating that the threshold stress was associated with the presence of ceramic particles in the MMC (oxide + reinforcement). In all the alloys, however, the m value tended to increase with temperature, and this led to a corresponding increase in elongation with temperature until the microstructure became destabilised. In the two predominantly single phase alloys studied this destabilisation corresponded to the solvus temperature. The constantly changing rn value indicates that there is no unique value of this parameter. The same was found to be true of the grain size exponent, p. The direct interpretation of apparent activation energies in terms of simple physical processes should be made with care in the light of the present results, as (i) the microstructures of two of the alloys were found to change continuously and significantly with small changes in temperature, and (ii) the fact that m is a function of temperature necessarily implies that the apparent activation energy is a function of stress