Precipitate hardening in an aluminium-copper alloy

Abstract

In Al-1·7 at. % Cu the critical resolved shear stresses, tension stress-strain curves, and the dependence of the flow stress on strain rate were determined from 4·2 to 273 or 373°K for the following structures: supersaturated α solid solution, α + GP I, α + GP II, α+ θ′, and α+θ. The difference between the CRSS at 4·2 and 273°k is large for the first two structures, but small with GP II, θ′ or θ precipitates. With α, α + GP I, and α + GP II, a single slip system is operative over a significant portion of the stress-strain curve, and the rate of work hardening is about the same as that for pure Al. With θ or θ′ multiple slip is immediately detected beyond the yield stress, the work-hardening rates are initially extremely rapid, and fall to small values after some per cent strain so that a plateau occurs in the stress-strain curves. For supersaturated α, the strength is attributed to clustering in the solid solution. In the cases of α + GP I and α + GP II, the zones must be sheared during plastic deformation and the strengthening arises from the atomic scrambling which occurs and the local strains near a zone which must be overcome when a dislocation moves through a zone. The variation of critical resolved shear stress with temperature and the strain-rate data are discussed in terms of the theory of thermally activated slip. Activation energies computed from the data are larger than those predicted on the basis of atomic scrambling alone. With θ′ or θ precipitate dislocations bend between the particles.