Finding and Characterising Active Slip Systems : A Short Review and Tutorial with Automation Tools

James Samuel Kwok-Leon Gibson, Risheng Pei, Martin Heller, Setareh Medghalchi, Wei Luo, Sandra Korte-Kerzel
2021 Materials  
Publisher's Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Abstract: The behaviour of many materials is strongly influenced by the mechanical properties of hard phases, present either from deliberate introduction for reinforcement or as deleterious precipitates. While it is, therefore, self-evident that these phases should be studied, the ability to do so-particularly their plasticity-is hindered by their small sizes and lack of
more » ... sizes and lack of bulk ductility at room temperature. Many researchers have, therefore, turned to small-scale testing in order to suppress brittle fracture and study the deformation mechanisms of complex crystal structures. To characterise the plasticity of a hard and potentially anisotropic crystal, several steps and different nanomechanical testing techniques are involved, in particular nanoindentation and microcompression. The mechanical data can only be interpreted based on imaging and orientation measurements by electron microscopy. Here, we provide a tutorial to guide the collection, analysis, and interpretation of data on plasticity in hard crystals. We provide code collated in our group to help new researchers to analyse their data efficiently from the start. As part of the tutorial, we show how the slip systems and deformation mechanisms in intermetallics such as the Fe 7 Mo 6 µ-phase are discovered, where the large and complex crystal structure precludes determining a priori even the slip planes in these phases. By comparison with other works in the literature, we also aim to identify "best practises" for researchers throughout to aid in the application of the methods to other materials systems. Here, we will focus on single crystalline BCC [8] and intermetallic crystals [15, 16] as examples, but the methods, analyses and code can be applied to any material that forms sufficiently clear traces of plasticity on its surface and in which these traces are governed by the crystal structure. With respect to the intermetallics taken as an example here [15, 16] , there are not only a great number that remain to be studied, but many different applications in which their mechanical properties need to be known beyond a simple stiffness or hardness. They are used as phases for alloy reinforcement [1], they form unintentionally in modern materials with high alloying contents, such as the superalloys [17, 18] , and for the optimistic researcher they may still be hiding a new class of bulk alloys amongst their numbers that combines safety as a structural material with great resistance to harsh environmental conditions. The study of intermetallic phases is a steadily increasing field of research, with over 3350 papers published on this topic in 2019, according to a Web of Science search for the topic. Research on their mechanical properties is, however, far less prominent, comprising on average less than half of this body of work. This contrast comes in part from several experimental challenges: (i) due to the complex crystal structure of these phases, they typically show very limited room-temperature plasticity [5, 19, 20] ; (ii) as a direct result of the poor ductility, these phases are only tolerated either as small precipitates, or introduced as small reinforcing phases, such that the properties of the parent alloy are not significantly reduced [2, 21] . This small size consequently limits the tests that can be performed; (iii) finally, as only small amounts of these phases are formed or used, it is difficult for the researcher to even obtain experimentally useful volumes of known chemical composition and with known thermomechanical history. Similar limitations exist with respect to fundamental studies of plasticity in single or bicrystals of pure metals and alloys [22] . The draw-backs of small-scale testing, such as size effects on quantitative values and the danger of affecting dislocation nucleation, multiplication and interaction in a given setting [22] [23] [24] , are commonly outweighed by the difficulty in preparing single crystals large enough for macroscopic testing, particularly of alloys. In the case of bicrystals, the difficulties are even more severe as the included grain boundary must be both well-defined and straight, and bi-phase samples with a single or few phase boundaries can most of the time not be synthesised at all. These limitations are compensated for by the strengths of nanoindentation and micromechanical testing. These enable the investigation of small volumes of material [22, [25] [26] [27] , allowing intermetallic phases to be tested within the parent alloy or even small fragments of specially prepared alloys through arc melting, while boundaries or specific crystal orientations can be targeted straightforwardly in any metallic alloy or metallic-intermetallic composite. Perhaps the greatest strength of micro-mechanical testing, however, is the suppression of brittle fracture, even in uniaxial microcompression tests [4, [28] [29] [30] . These subsequently allow the study of plastic properties essential for the performance of many modern engineering alloys in which a governing microstructural length is often at the same length scale of microcompression tests; that is, from 100 s of nm to a few µm. The experimental space occupied by metallic alloys and intermetallic compounds is enormous, covering binary, ternary and subsequent systems, the resultant crystal structures and their orientation within the sample, local chemistry, impurities, and precipitations, and how deformation is shared across interfaces. In addition come the influences from the 'traditional' global variables: effects of temperature and strain rate [31, 32] . It can be, therefore, safely assumed that, in the case of intermetallics in particular, no starting point exists for the characterisation of the vast majority of these phases, or at least many unknowns remain concerning their mechanical and particularly plastic behaviour. The following questions, therefore, need answering for a new compound of interest:
doi:10.18154/rwth-2021-01658 fatcat:jlm7li6fdzdnlopo34es3zfa5m