Temperature insensitivity of the flow stress in body-centered cubic micropillar crystals

Abstract

Plasticity of body-centered cubic (bcc) crystals is known to have a strong dependence on temperature, as a direct consequence of the thermally-activated process of kink pair nucleation and migration with a high energy (Peierls) barrier. Here we demonstrate that, in the sub-micron size scale, such strong temperature dependence of the flow stress must disappear. We explore the flow stress and hardening behavior of micro-pillar sizes in the range 200–2000 nm at temperatures of 150–900 K. Discrete Dislocation Dynamics (DDD) simulations reveal that the weak temperature sensitivity can be rationalized in terms of the weak role of screw dislocations in controlling plasticity; unique to small crystals of finite size. It is shown that finite, sub-micron samples have limited ability to store screw dislocations. The necessity of applying high stress in sub-micron crystals is demonstrated to greatly enhance the mobility of screw dislocations, rendering it close to that of edge dislocations. This leads to a transition of the dislocation mobility mechanism from being thermal-activated kink dominated to being phonon-drag dominated. Thus, the flow stress gradually becomes not governed by the mobility of screw dislocations (as determined by the critical resolved shear stress), giving rise to the weak temperature sensitivity of the flow stress. A dislocation mechanism map in the temperature-size space is proposed to further illustrate this phenomenon in tungsten micropillars.