A calorimetric study of mechanically induced phase transformations in NiZr alloys

Abstract

Previously published structural and microstructural observations of mechanically alloyed NiZr mixtures are complemented by differential scanning calorimetric studies. Depending on milling times and initial conditions (mixtures of elemental powders or intermetallic compounds), various enthalpy evolutions are observed which are interpreted with regard to the known microstructure and the local equilibria which may develop in the heterogeneous medium. The interpretations are as follows. 1. (1) At early stages, intimate contact is established among the elemental powders; annealing may induce a “classical” amorphization by solid-state diffusion. 2. (2) At later stages, when amorphous phases and intermetallic compounds coexist, heating results in a release of strain energy followed by chemical (metastable) equilibration between amorphous and crystalline phases and subsequently by crystallization of metastable amorphous phases. 3. (3) When a single amorphous phase results from the overall milling process, structural relaxation followed by crystallization of the amorphous phase is observed. The glass obtained has a much higher stability than that of the melt-spun alloy.