Plastic relaxation in tantalum single crystals strained far from equilibrium

Relaxation is a basic yet fundamental behavior observed in a wide range of natural phenomena. In physical systems, relaxation is observed in response to a perturbation in otherwise equilibrium thermodynamic conditions or external fields, e.g. dielectric, magnetic, mechanical, and in numerous other settings. At variance with the common view of relaxation as a process leading the system back to its equilibrium, here we present computational experiments in which model physical systems are driven far from equilibrium and then observed to relax to other driven stationary states that too are far from equilibrium. We base our study of far-from-equilibrium relaxations on recent observations of stationary plastic flow in tantalum crystals subjected to continuous compressive straining. Once the flowing crystal attains a state of stationary flow, the rate is changed abruptly - increased or decreased – and a new state of flow is attained. When relaxation after one such sudden jump in the straining rate is interrupted by another sudden jump in the opposite direction, we observe distinct “memory” effects: the stress first deviates from its ultimate stationary level and only then gradually returns back to it. Similar in appearance to complex relaxations observed in glasses after temperature jumps, here the relaxation is taking place between stationary states of an open system driven far from equilibrium in which mechanical energy of straining is converted into heat via continuous plastic deformation.