This contribution presents a comprehensive analysis of the low temperature deformation behavior of CoCrFeMnNi on the basis of quasistatic tensile tests at temperatures ranging from room temperature down to 4.2 K. Different deformation phenomena occur in the high-entropy alloy in this temperature range. These include (i) serrated plastic flow at certain cryogenic temperatures (4.2 K/8 K), (ii) deformation twinning (4.2 K/8 and 77 K), and (iii) dislocation slip (active from 4.2 K up to room temperature). The importance of deformation twinning for a stable work-hardening rate over an extended stress range as well as strain range has been addressed through the use of comprehensive orientation imaging microscopy studies. The proposed appearance of ε-martensite as well as a previously uninvestigated route of analysis, essentially a quantitative time-dependent, strain-dependent, and stress-dependent evaluation of the serrated plastic flow in CoCrFeMnNi is provided.

The quest for high particle energies of modern super conducting lepton and hadron storage rings assumes a strong minimisation of the non-magnetic interconnection zones when compared to the total magnetic length of the main quadrupoles and the dipoles. The ratio of non-magnetic to magnetic zones in the LHC is close to 3%. Such a low percentage leads to a very compact design of systems and sub-systems situated in the interconnections, like the mechanical compensation system, composed of the bellows expansion joints, that have to compensate for the thermal contraction of the magnets. Given the complexity of the LHC interconnections, the requirements concerning their reliability were considerably increased since the availability of the collider for physics cannot be compromised. The failure modes taken into account in the present analysis are grouped into 3 categories: material failures, structural stability failures and fatigue failures. Optimisation of stainless steel for cryogenic applications has been carried out. Also, a concept of reliability oriented parametric optimisation of the LHC interconnections is presented.