Multiaxial constitutive model of discontinuous plastic flow at cryogenic temperatures

FCC metals and alloys are massively used in cryogenic applications down to the temperature of absolute zero, because of suitable physical and mechanical properties including high level ductility. Many of these materials undergo at low temperatures a process similar to dynamic strain ageing, reflected by the so-called discontinuous plastic flow (DPF, serrated yielding). The physically based multiaxial constitutive model presented in the paper constitutes a generalization of the previous uniaxial model that proved efficient in describing the plastic flow instabilities occurring at extremely low temperatures. The model takes into account thermodynamic background, including the phonon mechanism of heat transport and thermodynamic instability caused by specific heat vanishing with the temperature approaching absolute zero. The DPF is described by the mechanism of local catastrophic failure of lattice barriers (for instance Lomer-Cottrell locks) under the stress fields related to the accumulating edge dislocations. The failure of LC locks leads to massive motion of released dislocations accompanied by step-wise increase of the strain rate (macroscopic slip). In the present paper the plastic flow discontinuity associated with the proportional loading paths is studied. Identification of parameters of the constitutive model is based on the experimental data collected during several campaigns of tensile tests carried out on copper and stainless steel samples immersed in liquid helium (4.2 K).