Vacancy loops and stacking-fault tetrahedra in copper

Direct transformation of equilateral hexagonal Frank vacancy loops to stacking fault tetrahedra under thermal fluctuation

Stacking fault tetrahedra (SFTs) are highly interesting three-dimensional vacancy defects in quenched, plastically deformed or irradiated face-centered-cubic metals and have a significant impact on the properties and subsequent microstructural evolution of the materials. Their formation mechanism and stability relative to two-dimensional vacancy loops are still debated. Equilateral hexagonal Frank vacancy loops (faulted, sessile) observed in microscopy have been considered unable to directly transform to SFTs due to separation of Shockley partial dislocations as well as embryonic stacking faults. Here using sufficiently long (up to tens of nanoseconds) molecular dynamic simulations, we demonstrate that such a transformation can in fact take place spontaneously at elevated temperatures under thermal fluctuation, reducing potential energy of defected atoms by <0.05 eV/atom. The transformation becomes easier with increasing temperature or decreasing loop size.

Historical Perspective on Diffraction Line-Profile Analyses for Crystals Containing Defect Clusters

Deviations of crystal diffraction line profiles from those predicted by the dynamical theory of diffraction for perfect crystals provide a window into the microscopic distributions of defects within non-perfect crystals. This overview provides a perspective on key theoretical, computational, and experimental developments associated with the analysis of diffraction line profiles for crystals containing statistical distributions of point defect clusters, e.g., dislocation loops, precipitates, and stacking fault tetrahedra. Pivotal theoretical developments beginning in the 1940s are recalled and discussed in terms of their impact on the direction of theoretical and experimental investigations of lattice defects in the 1960s, the 1970s, and beyond, as both experimental and computational capabilities advanced. The evolution of experimental measurements and analysis techniques, as stimulated by theoretical and computational progress in understanding the distortion fields surrounding defect clusters, is discussed. In particular, consideration is given to determining dislocation loop densities and separate size distributions for vacancy and interstitial type loops, and to the internal strain and size distributions for coherent precipitates.