Nanoscale combined reactions: non-equilibrium α-Co formation in nanocrystalline ϶-Co by abnormal grain growth

Electrodeposited nanocrystalline Co offers a relatively unique opportunity to study the interaction of two fundamentally different elementary solid state reactions: grain growth and e (HCP) to α (FCC) allotropic phase transformation. Samples were isothermally annealed at temperatures above and below the equilibrium transformation temperature (Teα = 695 K) and quenched to ambient for subsequent characterization by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Isothermal annealing above 695 K resulted in concurrent grain growth and e to α transformation. Unexpectedly, however, simultaneous grain growth and e to α transformation also occurred during isothermal annealing at temperatures as low as 573 K, i.e. 122 K below the expected equilibrium Teα. It was observed that non-equilibrium α-Co formed within a matrix of nanocrystalline e-Co via abnormal grain growth, and is therefore fundamentally different from the e to α transformation typically observed in conventional polycrystalline Co.