Crystal metamorphosis at stress extremes: how soft phonons turn into lattice defects

TitleCrystal metamorphosis at stress extremes: how soft phonons turn into lattice defects
Publication TypeJournal Article
Year of Publication2016
AuthorsLiu, X, Gu, J, Shen, Y, Li, J
JournalNpg Asia Materials
Date Published2016/10//
ISBN Number1884-4049
Keywordselectron-gas, energy, homogeneous dislocation nucleation, indentation, instabilities, molecular-dynamics, nanoindentation, solids, strength

At 0 K, phonon instability controls the ideal strength and the ultrafast dynamics of defect nucleation in perfect crystals under high stress. However, how a soft phonon evolves into a lattice defect is still unclear. Here, we develop a full-Brillouin zone soft-phonon-searching algorithm that shows outstanding accuracy and efficiency for pinpointing general phonon instability within the joint material-reciprocal (x-k) spaces. By combining finite-element modeling with embedded phonon algorithm and atomistic simulation, we show how a zone-boundary soft phonon is first triggered in a simple metal (aluminum) under nanoindentation, subsequently leading to a transient new crystal phase and ensuing nucleation of a deformation twin with only one-half of the transformation strain of the conventional twin. We propose a two-stage mechanism governing the transformation of unstable shor-twave phonons into lattice defects, which is fundamentally different from that initially triggered by soft long-wavelength phonons. The uncovered material dynamics at stress extremes reveal deep connections between delocalized phonons and localized defects trapped by the full nonlinear potential energy landscape and add to the rich repertoire of nonlinear dynamics found in nature.

Short TitleNPG Asia Mater.