Cyclical phase transformations and dynamic equilibrium in mechanical alloying

Abstract

Mechanical alloying (MA) often synthesizes a mixture of equilibrium and non-equilibrium phases. In many instances, the relative phase fractions are apparently invariant after long milling times. The situation is termed dynamic equilibrium. Recent work, however, shows evidence of cyclical phase transformations taking place during MA. Cyclical phase transformations resemble dynamic equilibrium in the sense that two phases are simultaneously present during milling. On the other hand, phase fractions vary during cyclical transformations. In this paper we rely on thermodynamic and kinetic concepts to establish connections between cyclical transformations and dynamic equilibrium. We find that dynamic equilibrium represents a true steady state, and cyclical transformations are a transient precursor to this situation.

Acknowledgements

The senior author, Professor Thomas H. Courtney passed away on 13 January, 2003 after he made a last editorial touch on this manuscript. The authors are grateful for discussions with Prof. Stephen A. Hackney. The Army Research Office (Dr William Mullins, Grant Monitor) supported the efforts of T.H.C. Both Prof. M. Sherif El-Eskandarany and Prof. Carl C. Koch were kind enough to provide us with preprints of references [18] and [21], respectively.