Thermal stability of electrodeposited nanocrystalline Co-1.1at.%P

Nanocrystalline Co-1.1at.%P prepared by pulsed electrodeposition was investigated with respect to thermal stability. Several characterization methods such as differential scanning calorimetry, X-ray diffraction, transmission electron microscopy, field ion microscopy and tomographic atom probe were applied. Between 673 and 733 K, the allotropic phase transformation (hcp-Co to fcc-Co) sets in and abnormal grain growth is observed. It is suggested that there is a synergistic effect between abnormal grain growth and the allotropic phase transformation. Between 733 and 753 K, the P excess of the grain boundaries decreases, Co2P and CoP precipitates form, and normal grain growth occurs. Thus, the P-segregation to the grain boundaries is held responsible for the increased thermal stability of Co-1.1at.%P at temperatures below 753 K. The thermal stability of this alloy can be mainly attributed to the reduction of grain-boundary energy.