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 Tt, 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 Tt, (ii) pre-straining above and re-straining below Tt, and (iii) pre-straining above Tt and re-straining below, above, and below Tt. The results show that a flow-stress increase due to work hardening induced either below or above Tt is not preserved if subsequent straining takes place at higher (>Tt) or lower (<Tt) temperatures, respectively. These phenomena clearly indicate that work hardening below and above Tt must originate from different work-hardening modes and are interpreted in terms of the low mobility of screw dislocations below Tt and nearly equal mobilities of screw and edge dislocations above Tt in body-centred cubic metals.