crystals Article Characterization of the Strain-Rate-Dependent Plasticity of Alloys Using Instrumented Indentation Tests Ta-Te Chen 1,2 , Ikumu Watanabe 1,2, * and Tatsuya Funazuka 3   Citation: Chen, T.-T.; Watanabe, I.; Funazuka, T. Characterization of the Strain-Rate-Dependent Plasticity of Alloys Using Instrumented Indentation Tests. Crystals 2021, 11, 1316. https://doi.org/10.3390/ cryst11111316 Academic Editor: Umberto Prisco Received: 30 July 2021 Accepted: 26 October 2021 Published: 28 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Graduate School of Science and Technology, University of Tsukuba, Tsukuba 305-8577, Japan; s1930121@u.tsukuba.ac.jp 2 Research Center for Structural Materials, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan 3 Faculty of Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan; funazuka@eng.u-toyama.ac.jp * Correspondence: WATANABE.Ikumu@nims.go.jp Abstract: Instrumented indentation tests are an efficient approach for the characterization of stress– strain curves instead of tensile or compression tests and have recently been applied for the evaluation of mechanical properties at elevated temperatures. In high-temperature tests, the rate dependence of the applied load appears to be dominant. In this study, the strain-rate-dependent plasticity in instrumented indentation tests at high temperatures was characterized through the assimilation of experiments with a simulation model. Accordingly, a simple constitutive model of strain-rate- dependent plasticity was defined, and the material constants were determined to minimize the difference between the experimental results and the corresponding simulations at a constant high temperature. Finite element simulations using a few estimated mechanical properties were compared with the corresponding experiments in compression tests at the same temperature for the validation of the estimated material responses. The constitutive model and determined material constants can reproduce the strain-rate-dependent material behavior under various loading speeds in instrumented indentation tests; however, the load level of computational simulations is lower than those of the experiments in the compression tests. These results indicate that the local mechanical responses evaluated in the instrumented indentation tests were not consistent with the bulk responses in the compression tests at high temperature. Consequently, the bulk properties were not able to be characterized using instrumented indentation tests at high temperature because of the scale effect. Keywords: strain-rate-dependent plasticity; instrumented indentation test; finite elements; mechani- cal testing 1. Introduction A database of fundamental material properties is essential for effective utilization of existing materials and exploration of new materials. For structural materials, tensile and compression tests among various mechanical tests are the standard testing methods for the characterization of mechanical properties based on the stress–strain curve because of the simple stress state. However, material tests require considerable effort and time for specimen preparation and for conducting tests under various conditions. Accordingly, instrumented indentation tests are an efficient approach for the evalua- tion of mechanical properties, such as effective elastic stiffness and hardness. These tests require less effort for specimen preparation and provide multiple results from a single specimen. In addition, the test method is applicable for the characterization of nano- and microscopic mechanical behaviors through the control of the magnitude of the applied load. Therefore, instrumented indentation tests have been widely employed in material science and engineering, e.g., the studies of scale-dependent plasticity [1–3], microscopic heterogeneity [4–6], and complex deformation mechanisms [7–9]. Crystals 2021, 11, 1316. https://doi.org/10.3390/cryst11111316 https://www.mdpi.com/journal/crystals