Tungsten-based bcc-superalloys: Thermal stability and ageing behaviour

Tungsten is considered as a primary material for the divertor and first wall in many fusion reactor designs. There has been further interest in nano-structured multi-phase tungsten alloys and composites, such as oxide dispersion strengthened alloys, where interfaces may be harnessed as defect sinks to improve irradiation resilience, whilst also improving base mechanical strength, and potentially ductility. Here we further investigate the concept of tungsten-based 'bcc-superalloys' within the W-Ti-Fe ternary system, comprising W-TiFe, A2-B2, beta-beta' nano-structures. Alloys were produced by arc melting and the microstructure controlled via thermal heat treatments, by solutionising at 1250 degrees C, followed by 750 degrees C ageing.The alloys were characterised using electron microscopy, including composition measurements, alongside hardness measurements. Building on our previous work, we have demonstrated that nano-scale B2 TiFe(W) forms within A2(W,Ti,Fe) in the W-Ti-Fe alloys, creating localised regions of the targeted A2-B2 (beta-beta') precipitate reinforced structure. Further, here we evaluated ageing at 750 degrees C, where within the interdendritic domains decomposition consistent with B2TiFe(W) -> B2 + A2 and A2(Ti,Fe,W) -> A2 + A3 is proposed. An experi-mentally validated preliminary W-Ti-Fe ternary phase diagram has been produced, helping to understand the stable phases present and instructing onward optimisation of W-superalloys as a candidate material for fusion energy.