New insight into the stable grain size of nanotwinned Ni in steady-state creep: Effect of the ratio of effective-to-internal stress

• Twinning/detwinning mediated grain growth at low stress is observed.
• Grain refinement at high stress is mainly attributed to the detwinning process.
• The effective-to-internal stress ratio can characterize the grain size evolution.
• A dislocation-based model is proposed to quantify the stable grain size and the steady-state creep rate.

Understanding the underlying physical mechanisms of grain growth/refinement in materials, in particular for nanotwinned (NT) metals with high stacking fault energy (SFE), to manipulate their microstructural stability for performance optimization is a grand challenge in the material community. The characteristic stable grain sizes of metals have been modeled in terms of various physical parameters, whereas there remains a lack of quantitative information regarding the correlation of stable grain size with the underlying mechanism(s) in the light of competition between effective and internal stresses. In this work, we systematically investigated the microstructural evolution of high SFE NT Ni at different loading rates during room temperature creep. It is found that both grain growth at low stresses and grain refinement at high stresses achieved via (de)twinning-mediated processes emerge in NT Ni. Unlike the general belief that the steady-state grain sizes are characteristics of single-phase metals, it is appealing that the stable grain size is strongly dependent on the effective stress. The effective-to-internal stress ratio ηStress plays a critical role in the grain size evolution: grains grow at ηStress < 1, while they refine at ηStress > 1. A stable grain size is reached at ηStress = 1. We further developed a dislocation-based unified model to quantitatively predict the stable grain size of NT Ni achieved in steady-state creep and the steady-state creep strain rate from the perspective of effective stress, which gains insight into plastic deformation processes associated with growth or refinement of grains.