Static mechanical properties of cast and sinter-annealed cobalt-chromium surgical implants

Porous-surface-layered surgical implants may be produced by sintering at elevated temperatures. An investigation was undertaken to determine the effect of these sintering heat treatments on the tensile properties of the cobalt-chromium casting alloy specified by ASTM F75-76. Specimens which were given a sintering treatment and then rapidly cooled from elevated temperature were found to lack ductility. This was due to the incipient melting of an interdendritic material which was subsequently retained in the grain boundaries as a brittle solid after quenching. Two methods were found which would reduce the amount of this brittle solid: (i) modify the heat treatment to include a slow cooling step to temperatures below that at which incipient melting first occurred; and (ii) reduce the carbon content of the alloy. Reduced-carbon alloys gave the greatest post-sintering ductility, but showed a lower 0.2% yield stress. The techniques of thermal activation analysis were used to investigate the effect of second phases upon the initial low-strain work-hardening rates and the 0.2% yield stress. It was found that the work-hardening rate from the elastic limit to a total strain of about 0.01 to 0.02 depends, in part, directly on the volume fraction of second phase.