Double HCP Phase in Cobalt Alloys with Dilute Contents of Iron

The crystal structure and the phase transformation of cobalt alloys containing less than 8 at% Fe have been investigated by means of X-ray diffraction, electron microscopy and heat capacity measurements. A double hcp structure of the α-La type is found in the range 1.5 to 4.8 at% Fe at room temperature and the alloys of 0.7 to 0.9 at% Fe transform from hcp through dhcp to fcc with increasing temperature. The stacking fault energy in the dhcp phase at 2 at% Fe is estimated to be about 18 erg/cm 2 from electron microscopic observation of extended dislocation nodes, and this value agrees well with an evaluation from calorimetric data.