Prospects of Using Small Scale Testing to Examine Different Deformation Mechanisms in Nanoscale Single Crystals—A Case Study in Mg

Abstract

The advent of miniaturised testing techniques led to excessive studies on size effects in materials. Concomitantly, these techniques also offer the capability to thoroughly examine deformation mechanisms operative in small volumes, in particular when performed in-situ in electron microscopes. This opens the feasibility of a comprehensive assessment of plasticity by spatially arranging samples specifically with respect to the crystal unit cell of interest. In the present manuscript, we will showcase this less commonly utilised aspect of small-scale testing on the case of the hexagonal metal Mg, where, besides dislocation slip on different slip planes, twinning also exists as a possible deformation mechanism. While it is close to impossible to examine individual deformation mechanisms in macroscale tests, where local multiaxial stress states in polycrystalline structures will always favour multiple mechanisms of plasticity, we demonstrate that miniaturised uniaxial experiments conducted in-situ in the scanning electron microscope are ideally suited for a detailed assessment of specific processes.

Deformation mechanisms of Mg-Ca-Zn alloys studied by means of micropillar compression tests

“…The curves show gradual yielding of the micropillar with the applied strain, that is followed by a region where deformation progresses at constant stress. Large strain bursts are observed in this plateau region and this behavior is similar to that observed in Mg, Mg-Al, Mg-Zn and Mg-Y micropillars deformed in equivalent orientations [24,33,36,42]. The yield stress (to determine the initial CRSS for plastic deformation) was taken from the critical points (denoted by black stars in Fig.…”

“…The morphology of the deformed micropillars is very similar to that reported in Mg, Mg-Al, Mg-Zn and Mg-Y micropillars suitably oriented for basal slip [24,33,36,42]. The orientation of the basal slip plane (indicated by the blue plane) and of the relevant slip directions (marked by the red arrows) were identified using VESTA [53] for both lateral surfaces of the micropillar and are plotted in Figs.…”

“…These experimental results were carried out in polycrystalline samples and it was difficult to determine the actual values of the CRSS for each slip system and twinning because of the influence of different contributions (texture, grain size, cross-slip) to the overall mechanical response. However, novel micromechanical testing techniques have been recently used to assess the fundamental deformation mechanisms of single crystals of Mg [24,[33][34][35][36][37][38][39][40][41][42], Ti [43,44], Al [45,46], and Ta [47] alloys as well as of intermetallic compounds [48,49]. In particular, micropillar compression tests have been used to measure the CRSS for different slip systems as well as CRSS for twin nucleation and growth in pure Mg [33][34][35][36][37] and different Mg alloys [24,[38][39][40][41][42] at ambient and elevated temperature as a function of the solute or precipitate content.…”

“…However, novel micromechanical testing techniques have been recently used to assess the fundamental deformation mechanisms of single crystals of Mg [24,[33][34][35][36][37][38][39][40][41][42], Ti [43,44], Al [45,46], and Ta [47] alloys as well as of intermetallic compounds [48,49]. In particular, micropillar compression tests have been used to measure the CRSS for different slip systems as well as CRSS for twin nucleation and growth in pure Mg [33][34][35][36][37] and different Mg alloys [24,[38][39][40][41][42] at ambient and elevated temperature as a function of the solute or precipitate content. This methodology is applied in this investigation to elucidate the intrinsic effect of solid solution atoms on the deformation mechanisms of Mg-Ca-Zn alloys.…”

“…13. (a) Ratio between the CRSS for twin nucleation and basal slip in pure Mg[33][34][35]60] and Mg alloys as a function of alloy content[24,41,42,60]. (b) Idem for twin growth and basal slip.…”