HETEROGENEOUS MICROSTRUCTURES IN A356-PERCENT-SICP AND IN A356 ALLOY(15VOL.)

The microstructures developed and the characteristics during hot worki ng of a liquid mixed composite (MMC, A356 + 15 vol.-%SiCp) and its mat rix alloy (Al-7.0Si-0.35Mg-0.2Cu) were determined over the range 300-5 00 degrees C and 0.1-5.0 s(-1). Through isothermal hot torsion tests, flow stresses of the metal matrix composites (MMC) were found (a) to b e generally higher than the alloy but with the difference declining at higher temperatures and (b) to depend on the strain I ate through a s inh function and on temperature in an Arrhenius term with activation e nergies of 263 and 161 kJ mol(-1) for MMC and alloy respectively. The matrix of the MMC undergoes additional strain hardening compared to th e alloy because of the constraints imposed by the rigid 12 mu m SiC pa rticles. From examination by TEM in both bright and darkfield, it was established that dynamic recovery gives rise to some regions of subgra ins in the matrix of the MMC but they al e smaller and less polygonise d than those in the alloy. In other regions, very high dislocation den sities were observed and in some cases gave rise to dynamic recrystall isation (DRX) nuclei; these cellular features with I educed internal d islocation density were confirmed to have a range of high misorientati ons in dark field. However; the DRX did not progress into the growth s tage, thus no rapid work softening was noted in The flow curves. Ducti lity of the composite, about 25% below the alloy, i rose by a factor o f 4 between 400 and 500 degrees C to leach the equivalent of about 95% reduction in area. The limited ductility of A356 resulted i om linkin g up of cracks nucleated at large Si particles, whereas in the MMC wit h much refined Si particles, the decohesion voids initiated at the SiC . MST/4025.