Structural changes of single crystals of low-alloyed tungsten alloys at thermal cycling

Abstract

Parameters of sub-structure of single crystals of low-alloyed tungsten alloys from the system W-Nb and W-Mo were investigated after thermal cycling in argon protective atmosphere. The mode of thermal cycling of tungsten based single crystals consisted of heating to the temperature of 2500 °C and of cooling to the temperature of 500 °C with dwell of 3.5 s at those temperature. It was established with use of X-ray topography, OLM and SEM, that after 100 thermocycles dislocations were re-distributed, sub-boundaries of the 2nd order were “washed away” (disappeared), and the size of sub-grains of the 1st order and their angle disorientations increased – as a result of heat stresses. After 500 thermocycles formation of new boundaries was observed in the structure as a result of polygonisation, i.e. by ordering of dislocations of the same sign. Application of 2000 thermocycles led to an increase of size of new grains and that of their angle disorientations. All the tested samples preserved at the thermal cycling within the interval from 0 to 2000 thermocycles their single crystalline structure. Formation of oxides on the surface of single crystals is connected with residual content of oxygen in argon.