A transmission electron microscopy study of the mechanisms of strengthening in heat-treated Co-Cr-Mo-C alloys |
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Authors: | J B Vander Sande J R Coke and J Wulff |
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Affiliation: | (1) Department of Materials Science and Engineering, M.I.T., 02139 Cambridge, MA |
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Abstract: | Microstructures produced in the Co-Cr-Mo-C alloy H.S.21 were observed by transmission electron microscopy in cast specimens
following solutionizing at 1230°C and aging at 650°C and in low-carbon wrought specimens following solutionizing and aging
at 650°C and 750°C. In all cases, aging was found to promote the formation of fcc stacking faults and to cause an initial
martensitic transformation from the fcc phase to a heavily faulted hep structure. Precipitate formation was observed in hcp
areas of the cast material after 20 h at 650°C and in hcp areas of wrought material after 20 h at 750°C. Prolonged aging at
750°C produced a transformation in the hcp structure of wrought specimens, with a relatively fault-free structure replacing
the heavily faulted martensitic form. Interruption of fcc slip by both fcc stacking faults and bands of hcp phase was found
to be the principal strengthening mechanism activated by aging. Precipitate formation in the hcp plays an increasingly significant
role as aging time is increased. This microstructural information is used to explain the observed tensile properties of these
alloys after the heat treatments mentioned. |
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