Small-Scale Mechanical Testing of Single Crystal Fe at Low Temperatures
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Nowadays, the increasing exploration and production in the Arctic region poses an urgent challenge to the mechanical properties of materials. The lack of knowledge of the “cold-brittleness” raises the need to better understand the link between microstructure and fundamental plasticity mechanisms at the micrometer and nanometer dimensions. At these length scales, the plastic behavior of materials is not well described by the classical theory of plasticity. Hence, experimental testing at small scales revealing underlying plasticity mechanisms is critical to the overall knowledge in the design of reliably structural applications. At the same time, the development of computational techniques is necessary to interpret the experimentally observed phenomena. Developments in micro- and nanomechanical testing have enabled the possibility of measuring constitutive materials properties and studying intrinsic size effects on mechanical behavior. Additionally, improvements within computational methods have been extensively used to study the local evolution of dislocation behavior. In this work, nanomechanical compression testing, complemented with Molecular Dynamics (MD) simulations have been performed to study the micromechanical aspects of low-temperature deformation mechanisms and the related mechanical properties of body-centered cubic (bcc) iron (α-Fe). Together with a literature review that addresses the relevant plasticity theory and previous findings from small-scale testing, the results from three different research papers on this topic are presented. Fe pillars with [1 ̅49], [2 ̅35], ,  and  orientations and diameters ranging from 400 nm to 4 μm, were produced with focused-ion beam (FIB) milling. Uniaxial compression tests were performed at room temperature, 200 K, 135 K and 40 K, using a custom-built in-situ cryogenic nanomechanical tester. Pillars with 1 μm-diameters compressed along [1 ̅49], [2 ̅35],  and  at room temperature and 200 K, revealed enhanced yield strengths and strain hardening at 200 K. The increased strength with decreasing temperature was attributed to the temperature dependent mobility of screw dislocations and a suppressed dynamic recovery rate, qualitatively in accordance with TEM and MD observations. Yield- and flow stresses revealed an orientation dependent temperature sensitivity. In particular, [1 ̅49] oriented pillars exhibited near-room temperature deformation behavior at 200 K. These deviations in temperature sensitivity between orientations are discussed in terms of non-Schmid effects influencing the initiation of yield stress and suppress dislocation formation to initiate a slip. Contradicting plasticity mechanisms in experiments (dislocation slip) and MD simulations (twinning) were observed for the [1 ̅49] oriented pillars. These findings are discussed in correlation with the necessary high strain rates employed in MD simulations. Suppression of size effect was observed with decreasing temperature from 298 K to 40 K for  oriented Fe pillars with diameters ranging from 400 nm to 4 μm. The changed behavior at cryogenic temperatures was attributed to an increased accumulation of dislocations for the largest pillars. The low-temperature testing enhanced the dislocation density due to the incompatibility of dislocation mobility and reduced dynamic recovery rate, consistent with the post-mortem TEM analysis. The smallest pillars have reduced capability to store dislocations due to the small volume, image force effect and the necessity of high stresses. Consequently, the temperature effect is not that prominent for the smallest pillars and plasticity is controlled by surface nucleation. The current work demonstrates how new insights of temperature-dependent plastic behavior in pure Fe at micro- and submicron scale at can be revealed using uniaxial compression testing, in combination with MD simulations. The related approaches can be applied for a broad range of materials and presents opportunities for direct comparison to atomistic or other micromechanical models. The current work opens up for a broad range of new topics and discussions. Additionally, it also presents challenges and open questions; hence, the outcome of such methods should always be assessed with care.