Computing the effect of solute hydrogen atoms on aluminum acoustic nonlinearity parameter

Hydrogen embrittlement, a critical concern for the mechanical response of engineering materials, can arise due to an influx of hydrogen atoms at interstitial sites and at grain boundaries. The acoustic nonlinearity parameter (ANP) is used in nondestructive evaluation as a sensitive parameter for the early detection of material degradation. From a measurement perspective, the ANP can be determined from the distortion of elastic waves. From a modeling perspective, the ANP is computed from second and third-order elastic constants. This study investigates the influence of solute hydrogen atoms on the ANP in aluminum using results of density functional theory calculations as input to continuum-scale computations of elastic constants. Based on the sensitivity of the ANP to hydrogen solute atoms, the findings suggest that an additive decomposition of the ANP is not applicable. Additionally, approaches based upon the stress or strain caused by local heterogeneity (such as solute atoms), without including the heterogeneity itself may be misleading with regard to the ANP. Moreover, the general expectation that atomistic and microscale defects increase ANP may not be universally valid because we observed a decrease in ANP due to interstitial hydrogen atoms and grain boundaries. This work provides novel insights into the application of nonlinear acoustics for detecting atomistic-scale defects and lays the groundwork for a more accurate connection between acoustic measurements and hydrogen-related degradation in materials.