Peculiarities of work hardening of high-purity tungsten single crystals below 800 K

The work-hardening behaviour of high-purity tungsten single crystals was studied below and above a transition temperature T-t, which is about 0.1-0.2 of the melting temperature and is known to subdivide temperature regimes with different work-hardening behaviour. Tensile tests were performed below 800 K with specimens oriented for single slip applying the following procedures of incremental straining tests: (i) pre-straining below, re-straining above and below T-t, (ii) pre-straining above and re-straining below T-t, and (iii) pre-straining above T-t and re-straining below, above, and below T-t. The results show that a flow-stress increase due to work hardening induced either below or above T, is not preserved if subsequent straining takes place at higher (> T-t) or lower (< T-t) temperatures, respectively. These phenomena clearly indicate that work hardening below and above T-t must originate from different work-hardening modes and are interpreted in terms of the low mobility of screw dislocations below T-t and nearly equal mobilities of screw and edge dislocations above T-t in body-centred cubic metals. (C) 2004 Elsevier B.V. All rights reserved.