Non-destructive characterization of dislocation density in annealed pure iron using nonlinear ultrasonic combined with recurrence quantification analysis

To achieve a non-destructive, high-sensitivity, and in situ characterization of the dislocation density in metals, an integrated strategy is proposed based on nonlinear ultrasonic (NLU) and recurrence quantification analysis (RQA) techniques. Acoustic nonlinearity generated by dislocation-induced finite-amplitude ultrasonic distortion is analyzed in a chaotic behavior. The decreasing dislocation density in annealed pure iron samples is confirmed by transmission electron microscopy and X-ray diffraction analysis, as well as the densities for edge and screw types, respectively. They are positively correlated with a recurrence rate-based NLU parameter, β_RQA, which is more effective in discriminating slight variations than the traditional NLU parameter β, with an increased sensitivity of 43%. The NLU response under different fractions of edge dislocation is numerically calculated, and a noticeable difference is observed compared with the experiments. A stress-modified dislocation model for harmonics generation is then put forward on the basis of the weakened long-range and short-range interactions of dislocations in the annealing system. The proposed integrated strategy can provide a better insight into the quantitative diagnosis of dislocation density in metals.