Electronic Processes at Dislocation Cores and Crack Tips

Abstract

It does not seem to be possible yet to present an electronic theory of dislocation cores and crack tips in bcc transition metals which are in the center of the fracture problem. Instead the electronic aspect has become more transparent in semiconductors with diamond cubic structure which undergo a macroscopic brittle to ductile transition at about 60% of their absolute melting temperatures [1]. Interestingly enough such crystals like Si and Ge deform more easily when they are n-doped (say with lO¹⁹ cm⁻³ As) than in the intrinsic state, i.e., in a range of substitutional solute concentration which would not affect the brittle ductile transition of metal single crystals. This is clearly an electronic effect, not one of the size misfit of the solute, as is shown by the non-effectiveness of tetravalent solute [2]. The important parameter is the position of the Fermi level as determined by temperature and doping relative to the electronic levels (or better: one-dimensional bands of the dislocations [3]. This parameter determines the line charge of the dislocations which in turn has a large and positive) influence on dislocation mobility. One might say that the line charge destabilizes a straight dislocation which is its minimum energy configuration in the Peierls potential.