Structural characteristics and inverse Hall–Petch relation in high-purity nickel irradiated with nanosecond infrared laser pulses

Mechanically polished nickel specimens (99.99% purity) were irradiated in air with Nd:YAG laser for laser shots ranging from 1000 to 5000 with an increment of 1000. The structural and mechanical characterization of the specimens was done using an X-ray diffractometer (XRD) and a Vickers hardness tester. The XRD patterns exhibit variation in the intensity of diffraction peaks pointing to significant structural changes on laser irradiation. Stacking fault probability was found to increase on irradiation with laser shots up to 2000, and later on it remained constant. Crystallite size, uniform deformation energy density, lattice deformation stress, and lattice strain were determined by Williamson–Hall analysis using uniform deformation energy density model. All the four structural parameters referred to were found to increase linearly with laser shots. On laser irradiation, surface hardness of nickel first increases up to 3000 laser shots, and later on it decreases up to 5000 laser shots. Crossover from classical Hall–Petch relation to inverse Hall–Petch relation occurs at about 69 nm in the observed crystallite size range of 81–44 nm.