NUCLEATION AND GROWTH OF OMEGA PHASE IN AL-4.0 CU-0.5 MG-0.5 AG ALLOY - AN INSITU HOT-STAGE TEM STUDY

An in situ hot-stage TEM study was conducted in an Al-4.0 Cu-0.5 Mg-0.5 Ag (wt%) alloy to study nucleation and growth of the OMEGA-phase. Nucleation of OMEGA-phase was found to occur uniformly with no apparent preference for defects present in the as-quenched microstructure and preferential growth along the {112}alpha-parallel-to[100]OMEGA direction was observed. There was a strong interaction between matrix dislocations and the OMEGA-precipitates. The thickening kinetics were accelerated by the presence of these matrix dislocations at the interface, apparently due to both diffusion and a dislocation dissociation mechanism of a/2[110] --> a/3[111] + a/6[112BAR], where the Frank partials accommodate the large misfit normal to the habit plane. During growth, impingement of twin-related variants of OMEGA-produces both planar and faceted twin boundaries, depending on the orientation of the boundary plane. Very large (almost-equal-to 2900 nm) and thick (almost-equal-to 300 nm) plates of OMEGA-were found to co-exist with equilibrium theta precipitates at 400-degrees-C.