**1. Introduction**

Small scale experiments have provided numerous insights regarding the mechanical properties of confined volumes. This is largely owed to the fact that by testing small volumes, individual constituents of the microstructure can be examined. Since the first report on the intriguing mechanical properties of micron sized metallic objects [1], understanding related size effects was a prime driver of research in the micromechanics community [2,3]. Sparked by this initial discovery, the activities and interests continuously grew and spread into many different routes of research, which cannot be covered within the scope of this article. One of the first subsequent questions that received considerable interest regarded the effect of specific or multiple interfaces within a small probed volume [4–7], while another facet of experimental efforts concerned the deformation mechanisms in more complex crystal structures such as hexagonal-close packed metals (Mg [8,9] or Ti [10,11]).

The deformation of Mg is accommodated by deformation twinning (tensile twin, contraction twin) or dislocation slip (basal <a>, prismatic <a>, and pyramidal <c+a> slip). Each deformation mode has been investigated in order to understand the fundamentals of the anisotropic deformation behaviour in Mg. In the case of <a> basal slip, the deformation mechanism appears quite similar to that of face-centred cubic metals, because <a> dislocations are easily nucleated and glide on basal planes [12]. In the case of <c+a> pyramidal slip, however, the deformation behaviour is quite different compared to <a> basal slip,

**Citation:** -Kiener, D.; Jeong, J.; Alfreider, M.; Konetschnik, R.; Oh, S.H. Prospects of Using Small Scale Testing to Examine Different Deformation Mechanisms in Nanoscale Single Crystals—A Case Study in Mg. *Crystals* **2021**, *11*, 61. https://doi.org/10.3390/cryst11010061

Received: 12 November 2020 Accepted: 11 January 2021 Published: 14 January 2021

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because <c+a> dislocations cannot easily glide on their slip plane due to the large Burgers vector and the high Peierls stress of pyramidal planes [8,9,13–15]. In the case of the - 1012 tensile twin, the twinning mechanism is still not well understood and under debate because of the extremely small twinning shear compared to the other twinning modes [16,17].

Thinking beyond the significant open questions that still exist for pure magnesium, one might be interested, for example, in aspects concerning the critical resolved shear stress of certain deformation mechanisms [12,18,19] or the influence of alloying on solid solution strengthening and precipitation hardening affecting the material twinnability [20,21].

With the increasing complexity of the research in question, it becomes more and more demanding to assess the single crystal properties for a fundamental understanding rather than just examining the bulk response of a new experimental alloy. This is where the strength of small scale testing should be embraced, as it allows comprehensive testing utilising only minimal amounts of material. Thus, the intention of this manuscript is to demonstrate the feasibility to test all fundamental deformation mechanisms of magnesium utilising only a small material volume. Due to the large difference in the activation energy of each deformation mode in Mg, strongly anisotropic deformation behaviours can be investigated depending on loading axes and conditions. While this allows not only the examination of the influence of crystal orientation, but also the impact of crystal size, strain rate and deformation temperature on the resulting mechanical response, we will limit this consideration to the fundamental modes of deformation, the related extensions towards the mentioned other variables are trivial but beyond the scope of this work.

As such, in the remainder we will detail a possible testing scheme using in-situ testing in the scanning electron microscope (SEM), apply it to the defined fundamental hexagonal crystal orientations, and detail the resultant deformation features and their relation to previous works.
