Chapter 12 - Phase transformations

Summary

Introduction

Ion irradiation of crystalline metallic alloys causes structural changes. Crystalline phase can become amorphous or can change to a different crystalline structure. The transformation can be to metastable or equilibrium phases. The driving force for an irradiation induced transformation is provided by the energy to the lattice during the penetration and subsequent stopping of an energetic ion (see Chapter 7).

To understand irradiation induced phase transformations on a thermodynamic level, let us consider an A–B binary phase diagram and the associated free energy of three of the components (Fig. 12.1). We choose A and B to be fcc metals, AB₃ to be a line compound with many atoms per unit cell, AB to be a simple CsCl structure with an extended phase field (the CsCl structure has two atoms per unit cell) that exists over a 10 to 20% range in composition, and the region α to be an fcc solid solution. A free energy curve with small curvature (wide phase field) indicates that the irradiation induced deviations from equilibrium do not result in large increases in the compound's free energy. Large curvature (narrow phase field) indicates just the opposite; small increases in the equilibrium defect concentration or changes in composition produce large increases in the free energy. As a result, compounds with large phase fields can tolerate a higher degree of irradiation damage and should be more irradiation stable relative to compounds with narrow phase fields (Brimhall et al., 1983; Hung et al., 1983).