Energetics and formation kinetics of helium bubbles in metals

Abstract

Recent theoretical work on the energetics and formation kinetics of helium bubbles in metals is reviewed. The energetics are discussed in particular for bubbles with radii between 10 and 1000 Å containing helium under very high pressures (nonideal gas bubbles). For this size range, the bubble formation free energy is split into three parts: a helium bulk free energy which is considered for solid and fluid helium (high-density equation of state), a bubble-matrix interface free energy for which curvature corrections are introduced, and a relaxation energy which is treated within the elastic continuum approach. The formation kinetics are considered for two extreme cases. For high helium production rates and low temperatures, helium clustering is associated with athermal processes such as metal interstitial emission and dislocation loop punching. For dislocation loop punching, an interpolation between computer simulation and continuum-theory results is discussed. For low helium production rates and high temperatures, helium clustering is connected with vacancy absorption. For this case, nucleation kinetics are briefly sketched while growth kinetics are analyzed in some detail with special reference to the effects of the high helium densities in the bubbles. In particular, the transition from gas-driven to vacancy supersaturation-driven bubble growth and its role for the evolution of bimodal bubble size distributions are analyzed. Reliable criteria to to discriminate between the two main bubble-coarsening mechanisms, bubble coalescence and Ostwald ripening, are suggested. The procedure to derive the activation energy of gas permeation from bubble coarsening, identified as Ostwald ripening is discussed.