Effect of Thermal Treatment on the Structure and Toughness of the Co,Cr-(Cr,Co)7C3 In-Situ Composite.
DREXEL UNIV PHILADELPHIA PA DEPT OF MATERIALS ENGINEERING
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Work of fracture in the Co,Cr-Cr,Co7C3 in-situ composite has been determined at room temperature in the as-grown condition and following post-solidification isothermal exposure or thermal cycling. Toughness is low in the as-grown condition and is attributed to restricted matrix slip and a low stacking fault energy in the cobalt-rich matrix coupled with the absence of crack deflection and matrix-interface delamination. In general, the heat-treatments did not lead to any major deterioration in toughness, rather in some cases toughness was enhanced by a factor of about two over that in the as-grown composite. Changes in toughness after isothermal exposure are attributed to micro-structural changes involving degeneration of the Cr,Co7C3 into a precipitate of Cr,Co23C6 and to fiber coarsening with an attendant increase in interfiber spacing and fiber diameter. In thermal cycling, thermal fatigue and fiber degradation are superimposed on degeneration and coarsening. A model based on thermal residual stresses and strains resulting from thermal expansion mismatch of matrix and fiber has been developed experimental results for the two cycling regimes examined are in good agreement with the model. The increase in toughness is proportional to the temperature range of the cycle.
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