Abstract
The discontinuous reaction in commercial Cu-1.87%Be,-0.257%Co alloys has been observed in a TEM/STEM. Microdiffraction has been used to determine precipitate/ matrix orientation relationships and high resolution X-ray microanalysis has enabled the effect of the Co on the discontinuous and continuous precipitation reactions to be studied.Discs of Cu-Be, solution treated at 800°C for 1 hour were water quenched and aged for varying times between 175°C and 340°C. Thin foils were prepared by jetelectropolishing with a solution of 30% HNO3 in methanol at -50°C and 25 volts. All foils exhibited the discontinuous reaction which occurs at grain boundaries only and consists of alternate lamellae of the α Cu-Be solid solution and the metastable intermediate γ' (CuBe) phase which grow at the expense of homogeneous matrix γ'. A typical STEM micrograph of the reaction is shown in fig. 1. The γ' precipitates, initially ascribed erroneously to the presence of a G.P. zone solves, were later identified as a B2 superlattice structure with a=0.27nm, although evidence for superlattice spots has been disputed.
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