Seiichi Nishikawa1, Kazuo Nagata1 and Sigeyoshi Kobayashi1
1Institute of Industrial Science, University of Tokyo, Tokyo
Decomposition processes of Cu-Co binary supersaturated solid solutions containing 2.85, 1.05 and 0.51 wt%Co, solution treated at 1000°C for 1 hr, have been investigated by electrical resistivity measurements, hardness tests, optical and transmission electron micrographs.
The results are summerized as follows: (1) Electrical resistivity of 2.85 wt%Co alloy dropped very rapidly in the early stage of high temperature ageing. For example, when aged at 600°C, about 60% of the whole decomposition process had been completed within 10 sec, but without any indication of hardening and any traces of micrographical changes. (2) Increment in electrical resistivity due to solute clustering or zone formation was not observed in the early stage of ageing. (3) Hardening began to appear at a fairly later stage than the electrical resistivity change, and the hardness curve showed only a simple single-peak. (4) As a result of the microscopical observation, it was clarified that in this system both continuous and discontinuous precipitations proceed simultaneously, but the amount of the latter was very small. In the former, sherical Co-rich precipitates completely coherent with matrix grew, but in the latter, normal lamellar precipitates appeared. (5) On the application of such an overall transformation kinetics equation as f=1-exp (-btn), the decomposition process of the Cu-Co supersaturated solid solution was divided into three stages with different n values. But for some reasons, it seemed to be probable that the first and third stages might be put out of the application of the above equation.
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