Electrodeposited nanocrystalline Co–P alloys: Microstructural characterization and thermal stability

Nanocrystalline Co–P has a significantly increased thermal stability compared to other nanocrystalline Ni- and Co-based electrodeposits. By combining observations from transmission electron microscopy in situ annealing with differential scanning calorimetry and tomographic atom probe measurements, the microstructural development of nanocrystalline Co–1.1 at.% P and Co–3.2 at.% P is described. In the as-prepared state, both alloys consist of 10 nm sized hcp-Co grains, and the P-distribution is already inhomogeneous. Upon annealing, P-atoms segregate to the grain boundaries, and grain growth takes place. Initial grain growth is abnormal in both electrodeposits, but occurs slower and more homogeneously in Co–3.2 at.% P. When P-saturation of the grain boundaries is reached, P-rich precipitates form accompanied by rapid (normal) grain growth. Due to the higher initial P-concentration in Co–3.2 at.% P, saturation of P and precipitation occurs earlier leading to a slightly lower thermal stability. Apart from the different P-contents, the effect of impurities, allotropic phase transformation of Co and texture is discussed.