Shape memory effect of Cable-In-Conduit Conductors

In fusion magnets, Cable-In-Conduit Conductors (CICCs) are foreseen, consis ting of an outer conduit and an inner niobium-tin cable produced by a simul taneous heat treatment. The heat treatment and the different thermal contra ctions of the various conductor materials lead to prestress onto the niobiu m-tin filaments during cool down, degrading the superconducting parameters considerably. These parameters show, during external mechanical loading, a maximum dependence on strain (epsilon). For example, the critical current ( I-c) of steel jacketed CICCs increases about by a factor of 2 at epsilon ap proximate to 0.7 % and decreases again at further straining (B = 13 T, T = 4 K). This characteristic is reversible up to epsilon approximate to 1.2 % in stressed and unstressed condition of the conductor. If the conductor is plastically deformed at 4 K up to a remaining strain (epsilon(r)) and then warmed up to 295 K, epsilon(r) decreases by about a factor of 5, which is c oupled to a phase transition resulting to a change of volume in the microst ructure of the steel conduit. This reduced epsilon(r) remains constant afte r cooling to 4 K again (shape memory) and the critical current corresponds to this decreased epsilon(r). In case of an accidentally over-straining of a CICC, a further temperature cycle of 4 K --> 295 K --> 4 K would recover I-c. Recovery strain results with different jacket materials (aged) of Type 316L, 316LN, Incoloy 908, and titanium are investigated.