Nanomechanical studies of high-entropy alloys
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Nanomechanical studies of high-entropy alloys Yu Zoua) Department of Materials Science and Engineering, University of Toronto, Toronto, ON M5S 3E4, Canada (Received 1 April 2018; accepted 2 May 2018)
In the past decade, nanomechanical techniques have become ubiquitous for mechanical measurement concurrently with the discovery of high-entropy alloys (HEAs). Different from large-scale testing, small-scale measurements offer quantitative details about mechanical behavior of materials at the micro/nanoscale, presenting new opportunities to probe fundamental nature of HEAs. This article will review the literature on using versatile nanomechanical tools for HEA studies, including nanoindentation, microcompression, high-temperature deformation, fracture measurement, and in situ electron microscopy. With these approaches, many interesting phenomena and properties of HEAs have been unveiled, for example, properties about incipient plasticity, strain-rate sensitivity, creep, diffusion, size-dependent strength, and fracture, which are difficult, or impossible, to be measured in macroscopic experiments. Despite current literature only focusing on a few HEA compositions and several methods, as nanomechanics and HEAs are developing rapidly, a new avenue of research is to be exploited. The article concludes with perspectives about future directions in this field.
I. INTRODUCTION AND OVERVIEW
More than a century has passed since the initial scientific study on the mechanical behavior of materials.1,2 Still today, research on mechanical properties plays an essential role in materials science, especially for metallic materials. As the most widely used structural materials in the world, alloys contain complex structures [e.g., phases, grains, grain boundaries (GBs), dislocations, vacancies, and interstitials] across many length scales, extending from the macroscopic scale to the atomistic size, as illustrated in Fig. 1(a). Over the past three decades, significant advances have been made in fundamentally understanding mechanical behavior of materials at the micrometer and nanometer scales, owing to the invention of instrumented nanoindentation.3,4 Particularly, in the past decade, the emerging techniques [e.g., focused ion beam (FIB), lithography, advanced atomic force microscopy, in situ electron microscopy, and new sensors and actuators of nanoindenters] have led to a large variety of nanomechanical testing methods, including micro/nanocompression, microtension, microbending, high-temperature/ cryogenic nanoindentation, high-strain-rate indentation, and in situ electron microscopy [Fig. 1(b)], as reviewed in Refs. 5–10 These techniques have generated a second wave of more sophisticated experiments and theories on a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.155
small-scale plasticity: So far, over a thousand reports have been published on a large variety of materials, including regular metals,11 ordered intermetallics,12 ionic crystals,13 semiconductors,14 ceramics,15 quasicrystals
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