Phase identification and incipient melting in a cast Co--Cr surgical implant alloy.

Phase relationships in cast Co--Cr surgical implant alloys, heat treated at temperatures from 1180 to 1300 degrees C, are reported. Interdendritic material was identified by selected area diffraction as a quaternary near eutectic mixture between sigma phase, M23C6, M7C3, and fcc Co. Incipient melting and subsequent resolidification of this near-eutectic mixture accounts for observations of behavior at temperatures above 1235 degrees C. At temperatures just below its melting point the interdendritic material initially breaks down to M23C6, which subsequently dissolves in the Co matrix.