Effect of prior deformation on dimensional change and precipitation process in a Cu-1.8wt%Be-0.2wt%Co alloy aged at 320°C

Abstract

The influence of prior cold work (90% reduction) on the length change and precipitation behaviour of a Cu-1.8wt%Be-0.2wt%Co alloy aged at 320°C up to 350 h has been investigated. The alloy gradually expands to a maximum and then contracts during ageing. The maximum expansion is attributable to both the expansion due to the loss of Be solute atoms and the contraction due to precipitation of the , γ _I, γ _m and γ′ phases. The subsequent contraction results from the decrease in amounts of the and γ _m phases and the increase in amount of the γ′ phase. The new-found γ _m phase, consisting of alternate Be and Cu matrix layers parallel to the matrix {001}_α, is body-centred monoclinic with a = b = 0.263 nm and c = 0.279 nm and α = 83°, and aligns with the matrix according to the Bain orientation relationship. The Guinier–Preston (GP) zone transforms continuously to the γ _m or γ _I phase via γ″ and . The transformation from the GP zone to γ′ via γ″, and γ _I is retarded in comparison with that for the un-deformed alloy.

Keywords:

• Cu-Be-Co alloy
• dimensional change
• precipitation behaviour
• γ _m phase
• plastic deformation
• misfit

Acknowledgements

We thank Mr K. Higashimine of the Centre 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. This work was partially supported by a Grant-in-Aid for Science Research from The Ministry of Education, Culture, Sports and Technology under Grant No.19560742.