Observations on surface relief effects associated with the formation of θ(CuAl2) rods in Al-4.5 wt pct Cu

Abstract

The surface relief associated with the formation of θ (CuAl2) rods at 450° and 375°C on a prepolished surface of an Al-4.5 wt pct Cu alloy has been examined. Most rods show well-defined V-shaped depressions in the surface of a magnitude consistent with accommodation of the transformation volume change taking place normal to the specimen surface. A few rods show V-shaped relief extending upwards from the surface, which is tentatively considered in terms of the change in lattice dimensions between the matrix and precipitate, and hence a function of the rod habit direction and of the lattice orientation relationships. Prolonged annealing increases the magnitude of the relief. Re-solution of the precipitate results in an increase in downward relief at the rod site consistent with solution taking place by a net diffusion of (copper) atoms away from the precipitate sites.