Structural phase transition in Al-Cu-Mg-Si alloys by transmission electronmicroscopy study on an Al-4 wt% Cu-1 wt% Mg-Ag alloy reinforced by SiC particles

The precipitation state of an Al-4 wt% Cu-1 wt% Mg-Ag alloy reinforced with SIC particles fabricated by squeeze casting is studied by transmission ele ctron microscopy. As a consequence of an interfacial reaction the precipita tion of the characteristic phases of the unreinforced alloy, Omega and S', is suppressed and substituted by a new precipitation consisting of isolated nanoscaled rod-shaped precipitates and theta' plates decorated with a furt her type of:rod-shaped precipitates. These precipitates are the Q-Al5Cu2Mg8 Si6 phase (hexagonal with a = 1.035 nm and c = 0.405 nm), and two other new phases also containing Al, Cu, Mg and Si, called QC (hexagonal with a = 0. 670 nm) and QP (hexagonal with a = 0.393 nm). These three phases are based on the same latent lattice, the QP lattice. A phase transition model is pro posed for these phases. It implies an ordering of the structures given by d isplacements of atoms from a lattice based on the common QP lattice. This a pproach is very promising for a better understanding of the precipitation i n Al-Mg-Si-Cu alloys.