Phonon stability of fcc crystals under hydrostatic loading

Phonon stability of fcc crystals under hydrostatic loading

abstract

en

The theoretical strength corresponds to the stress associated with failure of crystal structure, i.e. the first occurrence of mechanical instability. The theoretical strength under hydrostatic tension represents property of material that can be obtained solely by convenient computational approaches. On the other hand, their values are useful, e.g., for the crack stability assessments in solid crystals since the stress state at the crack tip corresponds to triaxial tension. In this work we study stability of four fcc crystals (Al, Ir, Pt, and Au) under isotropic (hydrostatic) tensile loading from first principles. The aim is to show that cubic crystals, when subjected to isotropic tensile loading, can become dynamically or elastically unstable before the maximum isotropic stress is reached. A relevant analysis of elastic stability conditions is also performed and the results obtained by means of both approaches are compared. Method. The first principles code Abinit is employed for this purpose. Study of dynamical stability is based on analysis of phonon spectra which are calculated using the linear response method and the harmonic approximation. The results reveal that, contrary to former expectations, strengths of all the studied crystals are limited by instabilities related to soft phonons with finite or vanishing wave vectors. The critical strains associated with such instabilities are remarkably lower than those related to the volumetric instability. On the other hand, the corresponding reduction of the tensile strength is by 20 % at the most.

The theoretical strength corresponds to the stress associated with failure of crystal structure, i.e. the first occurrence of mechanical instability. The theoretical strength under hydrostatic tension represents property of material that can be obtained solely by convenient computational approaches. On the other hand, their values are useful, e.g., for the crack stability assessments in solid crystals since the stress state at the crack tip corresponds to triaxial tension. In this work we study stability of four fcc crystals (Al, Ir, Pt, and Au) under isotropic (hydrostatic) tensile loading from first principles. The aim is to show that cubic crystals, when subjected to isotropic tensile loading, can become dynamically or elastically unstable before the maximum isotropic stress is reached. A relevant analysis of elastic stability conditions is also performed and the results obtained by means of both approaches are compared. Method. The first principles code Abinit is employed for this purpose. Study of dynamical stability is based on analysis of phonon spectra which are calculated using the linear response method and the harmonic approximation. The results reveal that, contrary to former expectations, strengths of all the studied crystals are limited by instabilities related to soft phonons with finite or vanishing wave vectors. The critical strains associated with such instabilities are remarkably lower than those related to the volumetric instability. On the other hand, the corresponding reduction of the tensile strength is by 20 % at the most.

The theoretical strength corresponds to the stress associated with failure of crystal structure, i.e. the first occurrence of mechanical instability. The theoretical strength under hydrostatic tension represents property of material that can be obtained solely by convenient computational approaches. On the other hand, their values are useful, e.g., for the crack stability assessments in solid crystals since the stress state at the crack tip corresponds to triaxial tension. In this work we study stability of four fcc crystals (Al, Ir, Pt, and Au) under isotropic (hydrostatic) tensile loading from first principles. The aim is to show that cubic crystals, when subjected to isotropic tensile loading, can become dynamically or elastically unstable before the maximum isotropic stress is reached. A relevant analysis of elastic stability conditions is also performed and the results obtained by means of both approaches are compared. Method. The first principles code Abinit is employed for this purpose. Study of dynamical stability is based on analysis of phonon spectra which are calculated using the linear response method and the harmonic approximation. The results reveal that, contrary to former expectations, strengths of all the studied crystals are limited by instabilities related to soft phonons with finite or vanishing wave vectors. The critical strains associated with such instabilities are remarkably lower than those related to the volumetric instability. On the other hand, the corresponding reduction of the tensile strength is by 20 % at the most.

phonon instability, theoretical strength, isotropic loading

phonon instability, theoretical strength, isotropic loading

02.04.2012

Oxford (UK)

ESIS TC2 (Micromechanisms) Oxford 2012

37–37

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