AbstractPhase relationships in cast CoCr surgical implant alloys, heat treated at temperatures from 1180 to 1300°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°C. At temperatures just below its melting point the interdendritic material initially breaks down to M23C6, which subsequently dissolves in the Co m