Twinning/detwinning-mediated grain growth and mechanical properties of free-standing nanotwinned Ni foils: Grain size and strain rate effects

Grain Boundary Segregation in Nanocrystructured Metallic Materials

“…Gianola et al [6,7] concluded that stress-driven grain growth appears to have preceded dislocation activity and involved GB migration and grain coalescence and becomes an active RT deformation mode in abnormally ductile NC Al thin films, based on coupled microtensile thin-film testing, in situ synchrotron diffraction experiments, and postmortem TEM observations. However, the twinning-mediated grain grow mechanism unveiled in stretched NT Ni [37,47] and compressed NT Cu [34] is radically different from these grain growth mechanisms aforementioned above. Figure 4 shows the atomic evidence of twinning-mediated grain growth in NT Ni, essentially being the consequence of nanotwin-assisted GB dissociation and local grain coarsening.…”

“…Though this model was originally proposed to explain the H-P d-dependent twinning in CG metals (e.g., Ti), latter in situ TEM observations in stretched Au nanowires clearly demonstrated the stimulated slip of partials is operative at nanoregime [36]. Actually, just opposite to the trend of twinning, its reverse process, i.e., detwinning, also manifests the double-inverse d-dependence, as revealed in NT Ni [37].…”

“…by twinning, leading to some parts of G1 being transformed into G2, thus some localized segments of GB coalesce and disappear, see Figure 4(b) and (c). Consequently, the repetitive formation of nanotwins induces some local segments of a high-angle GB are transformed into low-angle GB segments by storage of residual dislocations generated from dislocation reactions [37]. These recurrent interactions between partials/twins and GBs would facilitate the two adjacent nanograins to gradually coalesce into one larger grain with nanotwins.…”

“…Gianola et al [6] has uncovered that the stressassisted grain growth has a dynamic effect on the macroscopic mechanical properties of free-standing NC Al thin films; extended ductility can be realized along with a concurrent loss in strength in comparison to tests in which no grain growth was observed. Therefore, this twinning-mediated grain growth mechanism unveiled in NT Ni [37,47] seems to synergically combine the merits of (deformation/growth) nanotwins and grain growth mentioned above, being a novel and promising method to enhance the tensile ductility of NS metals for their performance optimization.…”

Effects of loading strain rate and stacking fault energy on nanoindentation creep behaviors of nanocrystalline Cu, Ni-20 wt.%Fe and Ni

“…Some glide partial or full dislocations would be accumulate near TBs and would trigger the activation of secondary twins by partial dislocation emission from the other side of the TBs [47][48][49]. As a result, a high dislocation storage rate or a high density of dislocations would be expected in the same loading condition.…”