Nucleation and Growth of Twin Interfaces in FCC Metals and Alloys


Presence of coherent interfaces within crystallites, referred to as «annealing twins,» has many effects on the properties of several fcc metals and alloys. The mechanism of the formation of these twins have been subject of numerous studies over fifty years. Although much progress has been made in recent years, a universally accepted view on the formation of these twins has not yet emerged. This brief review will describe recent studies at Naval Research Laboratory to understand several aspects of the annealing twins including nature of these twins, the mechanism of their formation, and the compositional parameters which affect their densities. These studies have established a relation between twin density and grain size, temperature and material properties. A model of the mechanism of their formation based on the emergence of Shockley partial loops on consecutive {111} planes during grain migration has also been developed. It is argued that various experimental and theoretical results obtained over the years can be consistently and satisfactorily explained by this model. This study has analyzed two aspects of annealing twins in details viz, a) role of boron additions in reducing twin density in nickel and b) the effect of annealing twins in HallPetch relation.