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: ϵ_T = f/3(ϵ₁₁ + ϵ₂₂ + ϵ₃₃) + (1 − f )ϵ_a, 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.

Keywords:

• copper–beryllium alloy
• misfit
• dimensional change
• precipitated phase

Acknowledgements

We acknowledge K. Higashimine of the Center for Nano Materials and Technology, Japan Advanced Institute of Science and Technology, for the TEM observation by HITACHI H9000-NAR. A part of this work was conducted in the Kyoto-Advanced Nanotechnology Network, supported by the ‘Nanotechnology Network’ of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. We also thank Professor K. Tazaki, Kanazawa University, for use of the JEOL2010FEF.