In situ small-scale mechanical testing under extreme environments
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Introduction Small-scale mechanical testing (SSMT) at the micro- and nanoscales has become an integral part of materials science and is widely used as a tool to evaluate either a material’s mechanical properties changes due to microstructural alterations or to obtain new insights about its physics. It has become common to utilize small-scale mechanical testing even for engineering applications, for evaluating coatings, irradiated samples, individual grains, welds, and joints, as examples. For better engineering performance and to better understand materials behavior, there is a currently a push to test materials under in operando conditions, such as temperature, high strain rate, or hydrogen environment to assess property changes of the materials under these environments. In addition, expanding the testing conditions to these areas allows new insights to be gleaned into multidimensional environmental space. This article adumbrates current efforts in using small-scale mechanical testing in extreme environments to push the envelope on what is possible using SSMT.
High and low temperatures Variable temperature nanoindentation is one of the more developed capabilities in the area of small-scale mechanical
testing under extreme conditions. As illustrated in Figure 1,1,2 several commercially available nanoindentation systems have been brought to the market in the last few decades, which are capable of operating at variable temperatures from approximately–1403 up to 1000°C,4–9 including in situ in the scanning electron microscope.10–12 These developments offer new measurement capabilities that enrich our scientific understanding of fundamental material response, including strain rate and temperature effects on deformation mechanisms at the micro- and nanoscale.13 This allows testing relevant technological materials under real operating conditions, widening the applicability of these techniques to important industrial applications, such as the hard-coating industry, high-temperature metallic alloys, and corrosion layers.14,15 The major difficulties faced when testing under variable temperature conditions include instrument stability, accounting for thermal gradients, management of thermal drift, and chemical stability, not only of the specimen itself, but also of the indenter tip material. Several recent reviews discuss these issues in detail.2,14 The key developments that have allowed pushing the temperature limits are the heating/ cooling of the indenter tip and the specimen to ensure thermal equilibrium at the contact16 and the development of more advanced environmental control, either by enclosing the
Afrooz Barnoush, Department of Mechanical and Industrial Engineering, Norwegian University of Science and Technology, Norway; [email protected] Peter Hosemann, Nuclear Materials Group, Department of Nuclear Engineering, University of California, Berkeley, USA; [email protected] Jon Molina-Aldareguia, IMDEA Materials Institute, Spain; [email protected] Jeffrey M. Wheeler, Laboratory for Nanometallurgy, ETH Zü
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