Open-system elasticity: experimental verification of the Larché-Cahn theory and application in functional materials with switcheable stiffness

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Many materials phenomena are governed by the coupling between chemistry and mechanics. The general theory of the action of that coupling in materials was exposed in the 1970s through the seminal work by Francis Larché and John W Cahn on the elasticity of open systems. As a central conclusion of that theory, the coupled chemo-mechanical equilibrium in the limit of small strain can be projected onto a classic problem of continuum mechanics, provided that the classic elastic constants for constant composition of Hooke’s law are replaced by new, open system elastic parameters. These parameters contain the information on the interaction between composition and stress at thermodynamic equilibrium and subject to constant chemical potential. Explicitly or implicitly, the Larché-Cahn theory is contained in many modern approaches at materials modeling. Yet, no direct experimental verification has been published. Specifically, the theory predictions for the elastic parameters of open systems have not been verified by experiment. These parameters can – according to the theory – be highly interesting because they can deviate strongly from the classic elastic constants at constant composition, because they are highly nonlinear in strain and concentration, and because there numerical values can diverge at critical points of the miscibility gap in alloy phase diagrams.With an eye on the above statements, the proposed research pursues the following twofold aim: Firstly, using a model alloy, we aim at verifying the theory of Larché and Cahn through experimental determination of the open system elastic parameters as the function of the composition. Secondly, we aim to demonstrate a materials design which exploits the results of the theory for achieving – for first time – operando tuneability of elastic stiffness in a wide interval. Ideally, it should be possible to reversibly switch the stiffness almost all the way to zero and back to the pure-metal value.As an original approach, we propose to use nanoporous palladium as the model material. External load induces bending moments on the nanoscale struts that define the microstructure. Hydrogen in the Pd crystal lattice redistributes within milliseconds between the tensile and compressive fiber of the struts. Dynamic mechanical analysis will be used for measuring the effective, macroscopic Young’s modulus, and electrochemical potential control affords fast, precise and reversible control over the hydrogen content.