Brazed graphite/refractory metal composites for first-wall protection elements

The peak surface heat flux deposition on divertor elements of near term fusion devices is expected to exceed 10 MW/m². The needed reliability of brazed plasma interactive components, particularly under abnormal operating conditions with peak surface temperatures well beyond 1000°C, makes refractory metallic substrates and brazes with a high melting point very attractive. TZM, a high temperature alloy of molybdenum, and isotropic graphite, materials very closely matched in their thermal expansion, were brazed with four high-temperature brazes. The brazes used were Zr, 90Ni/10Ti, 90Cu/10Ti and 70Ag/27Cu/3Ti (nominal composition prior to brazing, wt%). The resulting composite tiles of 50 × 50 mm² with a TZM thickness of 5 mm and a graphite thickness of 10 mm have been tested in high heat flux simulation for their thermal fatigue properties. Up to 600 loading cycles were carried out with an average heat flux of 10 MW/m² for 0.5 s pulses. The maximum surface temperature was 1100°C. In support of the experiment, the thermal response and temperature gradients of the samples were investigated using a finite element model.