Texture dependent surface energy measurements on magnetostrictive alloys for abnormal grain growth modeling

Galfenol is a magnetostrictive alloy used in sensing, actuating, and energy harvesting devices. Developing protocols for making thin sheet Galfenol with abnormally grown Goss or Cube grains is challenging because the mechanisms that regulate grain boundary mobility and texture development in these alloys are not well understood. Grain boundary energy models do not account for extraneous driving forces caused by the control of surface energy from atmospheric annealing conditions. By characterizing the surface energy of specific Galfenol grains at room temperature, we can develop a more accurate thermodynamic-based framework for modeling abnormal grain growth and texture development. This will be used to understand why a high temperature atmospheric anneal under 0.5%H2S in Ar transforms myriad grains into highly textured, single-crystal-like polycrystalline material while pure Ar does not. To experimentally measure surface energy, we have developed a non-destructive process to directly probe highly-textured and single-crystal Galfenol. The process involves high quality polishing to sub-nanometer roughness, Ar plasma cleaning to remove native oxides, and preserving the bare metal surface by immersion in hydrocarbon liquid. In this bulk hydrocarbon liquid, we use the two-liquid-phase contact angle method to measure surface energy. Experimental surface energy values on single crystals agree with DFT calculations, confirming the validity of this process. We use this method to observe a decrease in surface energy for sulfur contaminated Galfenol.