Impact of plastic softening of over-aged CuCrZr alloy heat sink tube on the structural reliability of a plasma-facing component

Precipitation-hardened CuCrZr alloy is used in fusion experiments as heat sink material for water-cooled plasma-facing components. When exposed to long-term high-heat-flux (HHF) plasma operation, CuCrZr will undergo over-ageing and thus plastic softening. In this situation, the softened CuCrZr heat sink tube will suffer from substantial plastic straining and thus fatigue damage in the course of the cyclic HHF loads. In this paper, a computational case study is presented regarding the cyclic plasticity behaviour of the over-aged CuCrZr cooling tube in a water-cooled tungsten mono-block divertor component. Finite element analysis was performed assuming ten typical HHF load cycles and using the Frederick-Armstrong constitutive equation together with corresponding material parameters. It was shown that plastic shakedown and low cycle fatigue (LCF) would be caused in the heat sink tube when softening of CuCrZr should occur. On the other hand, neither elastic shakedown nor cumulative plastic strain (ratchetting) was found. LCF design life of the CuCrZr tube was estimated based on the ITER materials handbook considering both hardened and softened states of CuCrZr. Substantial impact of softening of the CuCrZr alloy on the LCF lifetime of the heat sink tube was demonstrated.