Using the room temperature creep to strengthen nanotwinned Ni: the scaling behavior between the twin thickness and the grain size

Abstract

How to design ultra-strong face-centered cubic (FCC) metals is a grand challenge in materials community. In general, nanotwinned (NT) FCC metals manifest the maximum strength as the intrinsic size ratio of twin thickness–to–grain size is achieved, above which softening caused by detwinning occurs, whereas below this intrinsic size ratio strengthening caused by dislocation–twin boundary interactions occurs. In this work, we used the room temperature creep as an effective pathway to strengthen NT-Ni with the high stacking fault energy by controlling microstructure in terms of the intrinsic size ratio. We experimentally discovered in NT-Ni at the room temperature steady-state creep stage that there is a critical grain size of ∼230 nm for the intrinsic size ratio transition from 0.04 in the large grain size regime to 0.14 in the small grain size regime. In particular, this intrinsic size ratio can be taken as a signature to estimate the microstructural stability, which linearly scales with the twinnability of FCC metals. Compared with the as-deposited NT-Ni foils, their counterparts undergoing the steady-state creep stage manifest significantly enhanced strength without losing their elongation to failure. This work provides new insights into designing the strongest NT metals via the creep process.