Hardening strain and recovery strain in nanocrystalline Ni investigated in tests with multiple stress changes

Purpose: Dynamic recovery is interesting as it limits the maximal deformation strength of crystalline materials. Due to its small grain size, nanocrystalline Ni reaches its maximal strength after small strains < 0.1. It is shown that dynamic recovery contributes to strain and that its kinetics differs from that of hardening strain. Design/methodology/approach: The kinetics of recovery was studied by performing a large stress reduction suppressing thermally activated glide of the hardening type. The transition to a new quasi-stationary state at reduced strain rate and stress was accelerated by incremental increases of stress. Findings: During the transition the kinetics of deformation changes from that of recovery strain to the quasi-stationary one where hardening and recovery are coupled. The results are interpreted in terms of thermally activated hardening strain (in the grains) and thermally activated recovery strain (boundary mediated) linked by internal stresses. The activation volume of the hardening strain rate determined from the small stress increments is not inconsistent with the classical theory of thermally activated dislocation glide. Research limitations/implications: It is proposed to better characterize dynamic recovery by performing small stress changes in the period of dominating recovery strain to quantify the kinetics parameters of recovery strain. Practical implications: Disturbing deformation by sudden changes of stress is recommended as a suitable means to describe the kinetics of dynamic recovery. Recovery strains should enter the modeling of plastic deformation. This holds in particular for cases where dynamic recovery is prominent, e.g. at high stresses, high temperatures, and variable stresses (cyclic deformation, stress relaxation). Originality/value: The stress change method described in this work is generally applicable in deformation testing independent of the type of testing machine, where inelastic strains are measured at the usual accuracy.

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Opracowanie ze środków MNiSW w ramach umowy 812/P-DUN/2016 na działalność upowszechniającą naukę (zadania 2017)