Ultra-rapid cooling of aluminum: Homogeneous solidification to anisotropic nanocrystals

Aluminum fcc-crystal surfaces (110) are irradiated by series of ≈15 ns-long pulses of laser light. Each pulse is calculated to produce an ≈2 µm thick surface layer of liquid and quasi-liquid whose temperature decays rapidly, becomes supercooled liquid until ≈168 K below the nominal melting temperature, and then freezes homogeneously into fcc nanocrystals and amorphous atoms. The cooling rate is ≈1.2 × 109 K s−1 in the undercooled solidification region, which we call ultra-rapid because it is faster than that in experiments involving splat-cooling or melt-spinning. However, it is slower than those in a molecular-dynamics simulation with a million aluminum atoms, which was described by Mahata et al. [Model. Simul. Mater. Sci. Eng. 26, 025007 (2018)]. Standard θ/2θ x-ray diffraction is applied to the resulting solid. The magnitude and location of the diffraction peaks yield estimates of the anisotropy and the sizes of the nanocrystals. The sizes, between about 4 nm and 50 nm, are on the order of “critical” as defined in classical nucleation theory. The anisotropy is caused by a difference in growth rates among various crystal faces, which is in qualitative agreement with theoretical predictions. For example, the loosely packed (311) face grows much faster than that of the close packed (111).