Interaction between dislocations in bcc iron at high temperature

When a crystal deforms plastically, sources within, such as the Frank–Read source, emit dislocations, which then glide in response to the applied stress. As the dislocations move away from the source, they may encounter an obstacle, for example a grain boundary, impurity atom or locked dislocation, which they cannot overcome. As more dislocations are emitted, they ’pile up’ near the obstacle, until their own stress fields acting back on the source prevent more dislocations from being produced, unless the external applied stress is increased. The properties of these pile-ups strongly influence the deformation of the crystal as a whole giving rise to the Hall–Petch effect [E. Hall, Proc. Phys. Soc. B 64 (1951) 747–753, N. Petch, J. Iron and Steel Institute 173 (1953) 25–28.] relating yield strength to grain size. In this paper we investigate how the observed strong variation of elastic constants as functions of temperature affects the strength of interactions between dislocations, and mechanisms of plastic deformation of iron at elevated temperatures. We find that the observed severe softening of the tetragonal shear modulus C′ at high temperature gives rise to a drastic reduction in the repulsion between parallel like edge dislocations, and hence to a greatly increased number of dislocations in pile-ups, especially those in the 〈1 0 0〉(0 0 1) configuration. The associated increase in plastic strain will lead in turn to a conspicuous reduction in tensile strength as C′ falls. The phenomenon is inherently anisotropic, and this work underlines the importance of including anisotropic elastic effects when modelling Fe at high temperatures.