A THERMODYNAMIC MODEL FOR THE NANOCRYSTAL TO GLASS-TRANSITION OF INTERMETALLIC COMPOUNDS SUBJECTED TO HIGH DEFORMATION BY MECHANICAL ATTRITION-APPLICATION TO L1(2) PHASES

It has been proven that mechanical grinding of intermetallic compounds can lead, in certain cases, to complete or partial amorphisation. The present work is an attempt to explain such behavior on the basis of a thermodynamic model. The approach is based on the hypothesis of wetti ng of atomic disorder at the nanograin boundaries as the nanograins ar e formed during mechanical grinding. Using a Landau-Ginsburg free ener gy for the evaluation of the nanograin boundary energy and introducing a topological order parameter in order to describe the nanocrystal to glass transition, it is found that the formation of a glassy layer is expected below a certain nanograin size. When grinding yields further lowering of the nanograin size it is shown that a capillary instabili ty can lead the sample to complete amorphization. An application to Zr 3Al allows to establish a phase diagram in the coordinate system: nano grain size versus the long distance order parameter of the intermetall ic. Such a phase diagram exhibits three domains: two domains where nan ocrystalline state and amorphous state are metastable, and a region wh ere the two phases coexist. (C) 1997 Acta Metallurgica Inc.