Cold rolled tungsten with very high degree of deformation and a very fine-grained structure are a very promising structural material for future fusion reactor applications. Main advantages are a high melting point and excellent mechanical properties (e.g. regarding fracture toughness and a sub-zero °C ductile-to-brittle-transition temperature) compared to bulk tungsten materials. However, this very fine-grained structure is not stable at temperatures above 800 °C, which leads to embrittlement through recrystallisation. This study introduces potassium-doping of tungsten sheets to inhibit recrystallisation at high temperatures, a mechanism well known from incandescent lamp wires.
In a first step we produced potassium-doped tungsten sheets by cold rolling with five different degrees of deformation, up to a maximum of φCR = 4.6, as well as pure tungsten sheets with similar degrees of deformation. These sheets are compared structurally by EBSD imaging and microindentation hardness testing.
As a result, both materials reveal similar grain size distributions in normal direction with a substantial grain refinement and increasing hardness values with raising degree of deformation. The K-doped W-sheet reaches the highest hardness values with HV0.1 = 772, compared to the pure W-sheet with HV0.1 = 711. Having similar properties structurally and mechanically, this allows further studies of the new K-doped material to compare the recystallisation behaviour.
In K-doped sheets small pores are distributed between high angular grain boundaries. These “K-bubbles” are known to form strong high temperature barriers. To evaluate the positive effect of the K-bubbles on recrystallisation, first isochronal annealing experiments were carried out. While both materials suffer a sharp drop in hardness values, K-doped W still maintains mediocre values at temperatures above 1400 °C. SEM imaging reveals that large grain growth is suppressed compared to pure W. Therefore, our work will further concentrate on the understanding of the recrystallisation processes in pure and K-doped Tungsten.