Study of grain boundaries in polycrystalline Cr, Ta and W using nuclear gamma-resonance spectroscopy: Formation enthalpies of vacancy-oxygen complexes and single vacancies in grain boundaries, and dynamic properties of the grain boundary core in polycrystals of BCC metals

Abstract

The formation of complexes of vacancies with oxygen atoms (complexes-VacO) in the core of grain-boundary (GB) leads to a dependency of the isomer shifts δ1 of components-1 in NGR emission spectra of 57Co(57Fe) atomic probes localized in the GB core on the annealing temperature of polycrystalline metals in a technical vacuum. The formation enthalpies of complexes-VacO, Qcmpl, 1, and single vacancies, QVac, 1, in the GB core (states-1) in Cr, Ta and W polycrystals have been measured for the first time. Dynamic contributions to formation enthalpies QVac, 1 of single vacancies localized in the GB core in the state-1 and dynamic contributions to formation enthalpies Qcmpl, i of complexes-VacO, which were localized in the lattice regions adjacent to GB’s (Adjacent Lattice Regions, ALR’s), states-2, and in the GB core, states-1, have been separated. The dynamic contribution to the formation enthalpy QVac,1 of vacancies and Qcmpl, 1 of complexes-VacO in the GB core was several times smaller than the contribution to the formation enthalpy QVac, vol of vacancies in the bulk of the crystallites because of low-frequency resonance modes of local collective vibrations of intrinsic atoms in the GB core.