Effect of alloy preheating on the mechanical properties of as-cast Co-Cr-Mo-C alloys
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I.
INTRODUCTION
INVESTMENT castings made out of Co-Cr-Mo alloys have been widely employed in the manufacture of surgical prostheses due to their high corrosion resistance and adequate mechanical properties.[1] However, in the as-cast condition, preferential distributions of extensive interdendritic and grain boundary carbides significantly effect the alloy ductility. Thermal processing has been commonly used in Co alloys to modify the potential carbide precipitation reactions. Various carbide microstructures[2] can be produced which in turn might lead to improved alloy strength. In particular, the development of stacking faults and twins during alloy heat treating seems to play a key role in the resultant carbide distribution and morphology.[2] The work of Rajan and Van der Sande[3] shows that aging of as-cast or wrought ASTM-F75 Co alloys promotes the formation of fcc stacking faults as well as a martensitic transformation from fcc to a heavily faulted hcp phase. In their work, preferential precipitation was found at intrinsic stacking faults (ISFs), which manifested as intragranular striations. This was further confirmed by Taylor and Waterhouse,[4] who found that aging between 650 7C and 1150 7C a previously solutionized Co alloy resulted in preferential precipitation of M23C6, carbides at intragranular striations. Apparently, the observed striations consisted of densely packed ISFs, which became copious at lower aging temperatures. Aging at elevated temperatures where the stability of the fcc matrix is significantly improved reduces the extent of intragranular striations. At the higher temperatures, the developed striations are relatively coarse and discontinuous, which in turn affects the active precipitation reactions. In particular, Taylor and Waterhouse found that C. MONTERO-OCAMPO and H. LOPEZ, Associate Professors, and M. TALAVERA, Student, are with CINVESTAV-IPN, Unidad Saltillo, Apdo. Postal 663, 25000 Saltillo, Coahuila, Mexico. Manuscript submitted November 7, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
aging above 925 7C favors the precipitation of ‘‘blocky’’ type carbides due in part to the absence of appreciable amounts of ISFs. Room-temperature plastic straining can also induce the fcc-hcp martensitic transformation and the development of intragranular striations.[3] This in turn might influence the plastic flow properties of Co alloys. Rajan and Vander Sande[3] suggested that the high work hardening rates exhibited by Co alloys arise from their low stacking fault energies. In their work, they found that the interactions of dislocations with ISFs or fcc-twins provide the largest contributions to work hardening. Moreover, Van der Sande et al.[5] suggested that in previously aged alloys, the dominant strengthening mechanisms arise from the interruption of dislocation motion in the fcc matrix by both fcc stacking faults and hcp bands. Furthermore, Kilner et al.[6] showed that both the initial work-hardening rates and the yield strength are mainly controlled by the presence of coarse carbides.
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