Peculiarities in the deformation of CoCrFeMnNi at very low temperaturesWednesday (26.09.2018) 11:15 - 11:30 S1/01 - A5 Part of:
We present recent results on the deformation behaviour of the equiatomic CoCrFeMnNi high entropy alloy studied at very low temperatures. CoCrFeMnNi has been lauded for its excellent ductility and increased strength at cryogenic temperatures. Existing work focuses on these properties down to a temperature of 77 K. Consequently, there is a lack of information at lower temperatures. In the present work, the mechanical behaviour and the deformation mechanisms in operation were analysed by means of quasi-static tensile tests conducted at temperatures ranging from the room temperature down to 4.2 K. Laplanche et al.  have clarified the reasons for work hardening stabilization formerly reported at 77 K and have also estimated the onset stress of mechanical twinning. The work hardening stabilization and an equivalent extension of the onset stress, down to a temperature of 4.2 K, is verified by the present study. The stabilization has been attributed to dynamic refinement effected by extensive deformation twinning while the initiation for this extended plateau has been identified as being influenced by orientation evolution. Furthermore, no indication of ε-martensite formation during the deformation is detected as is proposed by stacking fault energy estimations from literature . At testing temperatures of 8 K and 4.2 K, serrated plastic flow was observed as well. The cause and nature of the serrated behaviour was interpreted by a series of tensile tests conducted at 8 K and 4.2 K under varying constraints, namely (i) different cooling media, (ii) strain rates of three different orders of magnitude and (iii) different gauge dimensions. Based on the results, the contributions of two effects are emphasised: (i) The extrinsic effect by virtue of the coupling of the test specimen to the cooling medium, which is driven by low heat capacity and thermal conductivity of metals and alloys at these temperature. (ii) The intrinsic effect caused by avalanche-like deformation events, such as unloading dislocation pile-ups and deformation twinning, both of which have influence on the serrated plastic flow.