Ultra-rapid freezing of Al and Al–Si12: Formation of nanocrystals and metastable solid solutions

We report effects of ultra-rapid melt–freeze cycles on pure aluminum and on aluminum alloy 4047 (which contains ≈12 at. % Si) caused by a series of ≈15 ns-long pulses of 308-nm laser light at a fluence of around 3 J cm−2. This process achieves a calculated cooling rate of about 109 K s−1, which is much faster than splat cooling or melt spinning. The focus here is on the reduction in the lattice parameters of the resulting crystals. In pure aluminum, this reduction is caused by the formation of nanocrystals, whose sizes are determined by the magnitude of the change in the lattice parameter. However, for the alloy, previous investigators have also found a comparable reduction in the lattice parameter but attributed it, instead, to the formation of metastable solutions of Si in Al. We find that the effect in the Al–Si alloy is caused by an unknown combination of size and composition dependence, but it is possible that there is no size dependence in the alloy.