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

Abstract

Electrodeposited nanocrystalline Co offers a relatively unique opportunity to study the interaction of two fundamentally different elementary solid state reactions: grain growth and ε (HCP) to α (FCC) allotropic phase transformation. Samples were isothermally annealed at temperatures above and below the equilibrium transformation temperature (T_εα = 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 ε to α transformation. Unexpectedly, however, simultaneous grain growth and ε to α transformation also occurred during isothermal annealing at temperatures as low as 573 K, i.e. 122 K below the expected equilibrium T_εα. It was observed that non-equilibrium α-Co formed within a matrix of nanocrystalline ε-Co via abnormal grain growth, and is therefore fundamentally different from the ε to α transformation typically observed in conventional polycrystalline Co.

Acknowledgements

The authors would like to acknowledge the fruitful discussions with Prof. Uta Klement (Chalmers University of Technology, Sweden) and the contributions of Dr. Jonathan McCrea and Mr. Fred Smith (Integran Technologies Inc., Canada). Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and Integran Technologies Inc. is gratefully acknowledged. One of the authors (GDH) was supported by a NSERC Postgraduate Scholarship.