Misfit strains of precipitated phases and dimensional changes in Cu–Be alloys

Abstract

Length-change measurements using Cu–0.9 wt% Be alloy single crystals containing only the disc-shaped Guinier–Preston (GP) zones parallel to the Cu matrix (001)α plane have enabled estimation of the misfit strains of γ″, , γ′ and γ precipitated phases in directions parallel and perpendicular to the GP zone, ε 11 = ε 22 and ε 33. The absolute values of ε 33 = −0.09 and −0.08 for the respective γ′ and γ phases, experimentally estimated, are far smaller than ε 33 = −0.25 calculated using lattice parameters of the γ′ and γ phase and the Cu matrix. The presence of the array of misfit dislocations at the interfaces between the phases and matrix results in the smaller absolute values of ε 33. Length-change measurements have also been carried out for Cu–1.8 wt% Be–0.2 wt% Co alloy polycrystals aged at 320 and 500°C. The alloy aged at 320°C initially exhibits a decrease in length change (elongation), then plateau behaviour, a subsequent slight increase and, finally, a plateau. The first contraction and the slight expansion are caused by precipitation of the γ′ and γ phase. The elongation, ε T, of the alloy aged at 500°C shows a gradual decrease with the proceeding discontinuous precipitation reaction, which results in a lamellar aggregate consisting of γ and α phases, and then a constant value. The elongation behaviour during ageing at 320 and 500°C is well represented by the equation: ϵT = f/3(ϵ11 + ϵ22 + ϵ33) + (1 − f )ϵa, where f is the volume fraction of precipitates and ε a is the dimensional change due to the loss of Be solute atoms in the matrix.