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

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)(alpha) plane have enabled estimation of the misfit strains of gamma '',gamma(I)',gamma' and gamma precipitated phases in directions parallel and perpendicular to the GP zone, epsilon(11) = epsilon(22) and epsilon(33). The absolute values of epsilon(33) = -0.09 and -0.08 for the respective gamma' and gamma phases, experimentally estimated, are far smaller than epsilon(33) = -0.25 calculated using lattice parameters of the gamma' and gamma 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 epsilon(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 degrees C. The alloy aged at 320 degrees 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 gamma' and gamma phase. The elongation epsilon(T), of the alloy aged at 500 degrees C shows a gradual decrease with the proceeding discontinuous precipitation reaction, which results in a lamellar aggregate consisting of and phases, and then a constant value. The elongation behaviour during ageing at 320 and 500 degrees C is well represented by the equation: epsilon(T)=f/3(epsilon(11)+epsilon(22)+epsilon(33))+(1-f)epsilon(a), where f is the volume fraction of precipitates and epsilon(a) is the dimensional change due to the loss of Be solute atoms in the matrix.