Role of cold work and SiC reinforcements on the β′/β precipitation in Al-10 pct Mg alloy

Elevated temperature (above 100 degrees C) precipitation behaviors were studied in Al-10 wt pet Mg alloy and the same alloy reinforced with SIC particles through electrical resistivity, hardness, differential scanning calorimetry (DSC), and microscopy. Two distinct hardness peaks/resistivity drops, as associated with two precipitation events, were identified: (1) alpha (solid solution) --> beta' (metastable hex precipitate) --> beta (Al3Mg2, stable complex cubic precipitate); and (2) alpha --> beta. Equilibrium beta precipitates, transformed from metastable beta', were observed to possess a wide variety of orientation relationships with the matrix and were often observed to be twinned. A more restricted orientation relationship (only three variants) between beta and matrix was observed in direct decomposition of cu to beta, and beta precipitates, within these orientation relationships, were never observed to be twinned. In a predominantly binary Al-Mg system, direct precipitation of beta was observed to dominate. However, the presence of trace amounts of boron nitride and/or boron (or a large supply of matrix dislocations) either from cold work, or (as in case of composites) from the thermal mismatch between the SiC and Al matrix, produced both precipitation events with event 1 dominant.