Interactions between interstitial and substitutional solutes in an H.C.P. lattice

Abstract

The reported results of interstitial stress-relaxation studies in hexagonal metal systems have been somewhat controversial. Out of the two possible interstitial holes in the α-Ti lattice, viz. octahedral and tetrahedral, only the octahedral ones are large enough to accommodate an oxygen atom. However the octahedral sites remain isotropic under an externally applied stress and therefore would not exhibit a stress-induced relaxation phenomenon. In contradiction to this Pratt, Bratina and Chalmers have reported that the introduction of oxygen into α-Ti caused the appearance of a peak in the relaxation spectrum. In this investigation the authors studied binary (Ti-O) and several dilute ternary (Ti-O-Me) alloys. They attribute these peaks to a localized distortion of the octahedral sites caused by the substitutional solute atoms. The distortion makes one interstitial site energetically favourable to another and hence an interstitial atom will jump from one position to another under the influence of an externally applied stress.