In situ TEM nanoindentation and dislocation-grain boundary interactions: a tribute to David Brandon

Enhancing dislocation emission in nanocrystalline materials through shear-coupled migration of grain boundaries

“…3a and c). The above results indicate that the dislocation emission process is energetically more favorable, as shear-coupled migration is the dominating process compared with pure normal migration and that the coupled shear (i.e., β 4 0) is able to considerably enhance the dislocation emission in NC materials, which is in good agreement with the existing observations made in experiments [27,[38][39][40], MD and quasicontinuum studies [25,26].…”

“…This result indicates that the coupled shear (i.e., the coupling factor β) plays an important role in enhancing the dislocation activity in nanocrystalline materials, which consist of mostly high-angle boundaries (i.e., ω4 0:26) [1]. This finding provides a possible explanation for the abundant dislocation activity observed in the experiments conducted by De Hosson et al [38,39] and Mompiou et al [27,40] for nano-grained and ultrafine-grained Al during nano-indentation or tensioning where grain boundary motions played a significant role. Since the value of β depends on the specific structure of GBs, the results obtained from Fig.…”

“…[25]). Some experiments carried out by De Hosson et al [38,39] and Mompiou et al [27,40] have also shown that dislocations activity was promoted during the motion of grain boundary in bicrystal or ultrafine-grained metals and that the moving GBs acted as a source of dislocations. Most recently, Ovid'ko and Skiba [41] proposed theoretically that dislocations could emit approximately vertically from a disclinated GB fragment generated by GB sliding or migration.…”

Advances in Transmission Electron Microscopy: Self Healing or is Prevention Better than Cure?

“…The subsequent yield behavior is classified as staircase yielding due the aforementioned dislocationbased mechanisms. Staircase yielding has been reported for indentation of both single crystal and ultrafine-grained polycrystalline Al thin films [9,21] and therefore its occurrence is not expected to depend strongly on the presence of grain boundaries in our case. The displacement bursts encountered in Al-Mg have a magnitude of up to 7 nm, which is substantially smaller than those observed in pure Al, being up to 15 nm in size.…”

“…Since the properties of high purity metals such as pure Al are less relevant for the design of advanced materials, here we focus on the indentation behavior of Al-Mg films and the effect of Mg on the deformation mechanisms described above. To this end, in situ nanoindentation experiments have been conducted on ultrafine-grained Al and Al-Mg films with varying Mg contents [19][20][21]. The classification "ultrafine-grained" in this respect is used for materials having a grain size of the order of several hundreds of nanometers.…”