It is shown that marked microstructural coarsening can occur when eutectic composites are exposed for relatively short periods to temperatures approaching 0.9 TM (where TM is the eutectic melting temperature in °K). Less well known and, perhaps, of more practical importance, are the microstructural changes and consequent changes in properties which result from prolonged exposure at 'safe' operating temperatures, e.g. 0.5 Tm.
Cd-Zn and Pb-Cd eutectic specimens of various thermal histories were examined immediately on preparation and subsequently re-examined after storage at ambient laboratory temperatures for up to 1000 days. In general, microstructural changes from the as-grown and heat-treated conditions were observed. In some cases for the Pb-Cd alloy this consisted of only slight thickening of the already coarse larnellae terminations. However, in the more rapidly frozen, and therefore microstructurally finer, specimens marked discontinuous coarsening was observed, but confined to eutectic-cell-wall regions. There the coarse precipitates grew at many angles, including being perpendicular to the eutectic habit plane but were not separated by any well-defined large angle grain boundaries.
It was concluded that high temperature degradation occurs mainly by continuous coarsening, whereas at lowtemperatures, microstructural changes are effected mainly by discontinuous coarsening.