Temperature-dependent mechanical behavior of three-dimensionally ordered macroporous tungsten
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POROUS METALS: FROM NANO TO MACRO
Temperature-dependent mechanical behavior of three-dimensionally ordered macroporous tungsten Kevin M. Schmalbach1,c) , Zhao Wang2,c), R. Lee Penn2, David Poerschke1 , Antonia Antoniou3, Andreas Stein2,a), Nathan A. Mara1,b) 1
Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, USA Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, USA 3 George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA a) Address all correspondence to these authors. e-mail: [email protected] b) e-mail: [email protected] c) These authors contributed equally to this work. 2
Received: 8 March 2020; accepted: 11 May 2020
Porous metals represent a class of materials where the interplay of ligament length, width, node structure, and local geometry/curvature offers a rich parameter space for the study of critical length scales on mechanical behavior. Colloidal crystal templating of three-dimensionally ordered macroporous (3DOM, i.e., inverse opal) tungsten provides a unique structure to investigate the mechanical behavior at small length scales across the brittle–ductile transition. Micropillar compression tests show failure at 50 MPa contact pressure at 30 °C, implying a ligament yield strength of approximately 6.1 GPa for a structure with 5% relative density. In situ SEM frustum indentation tests with in-plane strain maps perpendicular to loading indicate local compressive strains of approximately 2% at failure at 30 °C. Increased sustained contact pressure is observed at 225 °C, although large (20%) nonlocal strains appear at 125 °C. The elevated-temperature mechanical performance is limited by cracks that initiate on planes of greatest shear under the indenter.
Introduction Nanoporous materials are of interest because their high surface areas can provide enhanced properties benefiting a wide range of technologies. Of particular interest in this study is the fact that the mechanical properties of such materials exhibit length scale effects [1, 2, 3]. In nanoporous gold, stresses approaching the theoretical shear strength of the material (an upper bound of τ = μ/10, where μ is the shear modulus) have been reported as the ligament diameter decreases below 100 nm [2]. In addition, the behavior of porous materials scales according to relative density [4], grain size [1], and representative volume [5], allowing for many degrees of freedom in material design. Nanoporous metals made by dealloying processes have a random arrangement of ligaments and typically possess polycrystalline microstructures and combine the effects of small grain size with the open-pore structure. Open-cell structures made with a number of pure metals, such as Au [1, 2, 5, 6, 7], Cu [8], and W [3] can exhibit very high ligament strengths. The ligament strengths are estimated by inverting property
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scaling laws [2]. Nanoporous tungsten made by high-pressure torsion foll
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