Heterogeneous microstructures in A356 + 15 vol.-%SiCp and in A356 alloy

Abstract

The microstructures developed and the characteristics during hot working of a liquid mixed composite (MMC, A356 + 15 vol.-%SiCp) and its matrix alloy (AI-7.0Si-0.35Mg-0.2Cu) were determined over the range 300–500°C and 0.1-5.0 S-l. Through isothermal hot torsion tests, flow stresses of the metal matrix composites (MMC) were found (a) to be generally higher than the alloy but with the difference declining at higher temperatures and (b) to depend on the strain rate through a sinh function and on temperature in an Arrhenius term with activation energies of 263 and 161 kJ mol-1 for MMC and alloy respectively. The matrix of the MMC undergoes additional strain hardening compared to the alloy because of the constraints imposed by the rigid 12 11mSiC particles. From examination by TEM in both bright and dark field, it was established that dynamic recovery gives rise to some regions of subgrains in the matrix of the MMC but they are smaller and less polygonised than those in the alloy. In other regions, very high dislocation densities were observed and in some cases gave rise to dynamic recrystallisation (DRX) nuclei; these cellular features with reduced internal dislocation density were confirmed to have a range of high misorientations in dark field. However, the DRX did not progress into the growth stage, thus no rapid work softening was noted in the flow curves. Ductility of the composite, about 25% below the alloy, rose by afactor of 4 between 400 and 500°C to reach the equivalent of about 95% reduction in area. The limited ductility of A356 resultedfrom linking up of cracks nucleated at large Si particles, whereas in the MMC with much refined Si particles, the decohesion voids initiated at the SiC.