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Discontinuous carbide formation along the grain boundaries was found to be a desirable condition for obtaining ASTM-recommended strength and ductility levels in a sintered alloy containing 0.3% carbon by weight and meeting the requirements of the ASTM Specification for Cast Cobalt-Chromium-Molybdenum Alloy for Surgical Implant Applications (F 75–82). Excessive carbides in the grain boundaries of this alloy caused a severe reduction in ductility, while relatively carbide-free grain boundaries produced acceptable ductility but a marked reduction in the ultimate and 0.2% yield strengths. This behavior was also true for an ASTM F75 alloy with a lower carbon level (0.24% by weight). However, the latter alloy failed to meet the strength levels specified in ASTM Specification F 75–82. Subsequent aging of similar specimens led to carbide precipitation within the grains and dissolution of grain boundary particles, which caused an immediate loss of ductility. In addition, there was evidence for the precipitation of intermetallic, topologically close-packed (TCP) phases, which have been shown to have deleterious effects on the room-temperature ductilities of cobalt-base alloys. This phenomenon occurs in alloys with both high (0.3%) and low (0.24%) carbon contents. The precipitate population and the severity of ductility loss were time and temperature dependent.
porous implants, ASTM Specification F 75–82, cobalt-base alloys, porous coatings, sintering, tensile strength, ductility, carbide formation, sigma phase, carbon depletion
Technical director, Astro Met Associates, Inc., Cincinnati, OH