Sigma phase formation in irradiated tungsten, tantalum and molybdenum in a fusion power plant

After five years in a fusion power plant, neutrons will have transmuted a plasma-facing tungsten component into an alloy of about 75 W, 13 Os and 12 Re (atomic percent), a composition close to the homogeneous σ field of the phase diagram. Since the σ phase is extremely brittle, it is important to know whether the transformation will occur. To avoid this problem, the use of tantalum or molybdenum, as alternatives, is proposed.