High temperature stability of fine grained 7075Al alloy containing nanosized SiC particles

Both an AA7075 alloy and AA7075 Al base metal matrix composite, the latter reinforced with 5 wt-% of 50 nm diameter SiC particles, were deformed by equal channel angular pressing (ECAP) at 350°C to a true effective strain of 4·6. The high temperature microstructural stability of the as deformed materials was investigated mainly by TEM to elucidate the effect of precipitate particles and reinforcement phase on continuous and discontinuous grain coarsening. It was found that ECAP generated a fine equiaxed grain size of ∼2·3 and ∼1·8 μm in the alloy and composite respectively. During annealing at times up to 300 min at 500°C, the fine grained structure of both materials coarsened gradually and substantial grain boundary interactions with MgZn₂ precipitates and oxide particles were found in the alloy, with precipitate, oxide and SiC particles found in the composite. The strong pinning effect exerted by these particles minimised grain growth in both materials with the composite exhibiting a finer (2·3 μm) grain size than the alloy (3·4 μm) after extended annealing. This slightly superior grain stability in the composite was attributed to the SiC particles, which resulted in significant boundary pinning.