Simulation and modelling of forest hardening in body centre cubic crystalsat low temperature

In body centred cubic (bcc) crystals at low temperatures, the thermally act ivated motion of screw dislocations by the kink-pair mechanism governs the yield properties and also affects the strain hardening properties. In this work, the average strength of dislocation junctions is derived and numerica lly estimated in the case of Mb and Ta crystals. This allows us to extend a n existing simulation of dislocation dynamics in bcc crystals to the case o f the motion of a screw dislocation line through a random distribution of f orest obstacles. Numerical results are presented in the case of Ta crystals and at two temperatures, 160 K and 215 K. They are complemented by a simpl e model that applies quite generally to bce metals at low temperatures. It is shown that forest hardening is made up of two contributions, a free-leng th effect that depends on the length of the mobile screw segments and whose dependence on forest obstacle density is logarithmic and a Line tension ef fect linearly proportional to the obstacle density. As a result of the ther mally activated character of screw dislocation mobility, the relative weigh t of the two contributions to forest hardening depends on the temperature a nd strain rate.