The creation and annihilation of vacancy-solute complexes during irradiation of Ni(In) and Ni(Au) crystals

Abstract

The displacement of solute atoms from normal lattice sites, which results from the trapping of irradiation-induced defects by the solute atoms, can be determined in ion channeling studies by measuring the yield of reaction products resulting from ion-solute collisions. In the present investigation, the backscattering of 1.5 MeV 4He ions from Ni crystals containing ≈ 0.1 at % In or Au was used to monitor the fraction f ds 〈 lmn〉 of solute atoms displaced into various ⩽lmn> channels following 4He ion irradiations and annealing treatments. Also, the rate at which the channeled ions were dechanneled in the Ni host lattice served as a simultaneous measure of the total irradiation-induced defect concentration. Subsequent to 35 K irradiations, annealing in the range ≈ 250–400 K produced appreciable displacements of both In and Au atoms from their normal lattice sites with f ds 〈100〉 > f ds 〈110〉 > f ds 〈111〉 . There were also a peaking of solute atom yield for angular scans through a 〈100〉 direction. The displacements are believed to arise from the trapping of vacancies at the solute atoms, thus forming relatively simple vacancy-interstitial solute atom complexes. These vacancy-solute complexes remain quite stable up to approximately 600 K and then break up in the region 600–750 K. In addition, the vacancy-solute complexes can be annihilated during irradiations at 80 K where only interstitials are mobile. The results provide strong evidence for the formation of tetravacancy-solute atom complexes in which an interstitial solute atom is surrounded by a tetrahedron of four vacancies. In such a complex the solute atom is displaced into the centre of the 〈100〉 channel, i.e. into a tetrahedral lattice site. Trivacancy-interstitial solute atom complexes, in which the solute atom is at the centre of a triangle of three nearest-neighbour vacancies in a {111} plane, may also be present.