Mechanical stability of cubic crystals under hydrostatic and uniaxial loading

abstrakt

angličtina

Theoretical strength corresponds to a stress associated with failure of a crystal lattice, i.e., with the first occurrence of mechanical instability. Its value sets an upper limit of attainable stresses in a solid crystal without defects and imperfections. The goal of this study is to advance our fundamental understanding of microscopic mechanisms that limit region of mechanical stability of fcc Ni and fcc Ir under hydrostatic (isotropic) and uniaxial [001] loading. The stability is assessed by analysing phonon spectra that are calculated for different values of strain from first principles. Two methods (linear response method and supercell method) are employed for computation of phonon dispersion curves and their results are compared. A relevant analysis of elastic stability conditions is also performed. Although most of the previous studies of isotropic loading predicted that first instabilities in crystals correspond to macroscopic (elastic) instabilities we found soft phonons of finite wave vectors at lower strains (and stresses). Such instabilities were confirmed by our models of microspic deformation. Elastic instabilities predicted under hydrostatic and uniaxial loading correspond well to those associated with soft phonons with vanishing wave vectors.

ab initio calculations, elastic stability, phonon instability, theoretical strength, hydrostatic loading, uniaxial loading

ŘEHÁK, P.; ČERNÝ, M.; ŠOB, M.

1. 9. 2014

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