Abstract
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).
Highlights
The speed of solidification of a liquid metal affects the nanoand micro-structures of the resulting solid and its mechanical and physical properties
We freeze aluminum with a cooling rate of ≈1.2 × 109 K s−1, which is faster than those in previous experiments, such as splat cooling (
Indirect comparison of our data with the molecular dynamics simulation is possible because, first, Mahata’s smallest value of CR was chosen to be a constant 5.83 × 1010 K s−1, which is ≈50× faster than ours at temperatures near TUC and, second, our quenching is of a melt that is formed in a range of temperatures, which decreases with melt depth z and correspondingly has a large percentage spread of liquid amorphous atoms
Summary
The speed of solidification of a liquid metal affects the nanoand micro-structures of the resulting solid and its mechanical and physical properties. A pulsed laser creates a 1–2 μm thick melt on an aluminum surface, which cools and solidifies. We use multiple laser pulses that create surface coatings, which are analyzed with a standard θ/2θ x-ray diffractometer. Averaged nanoparticle sizes are deduced from shifts in the values of 2θ for those diffraction peaks Such estimates of size and shape have previously been difficult to measure in opaque materials. Our experiments were not designed to explore the effects of fast cooling, but rather to produce hard coatings on an Al–Si alloy, and pure aluminum was laser-treated only in order to create a baseline for that alloy study. We applied multiple laser pulses in order to generate a sufficiently thick surface coating for potential technological use with the hardened alloy. Some other, potentially useful, investigations were not performed
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
