Metastable microstructures in laser ablation deposited aluminium-50 at.% nickel thin films

Thin films of Al-50 at.% Ni composition were deposited by laser ablation technique on single crystal NaCl substrate at room temperature. The films consist of vapour quenched matrix with embedded solidified round particulates of ablated liquid. Transmission electron microscopy (TEM) investigation proved that amorphisation of this composition was only closely approached by vapour quenching as well as in quenching of a few tens of nanometer sized liquid droplets. In both cases, a metastable fcc phase co-exists with the amorphous phase. Co-existence of such a fine scale nanocrystalline fcc phase is a first observation. Ordered B2 phase forms in larger particulates. The grain size of the ordered phase scales with particulate size. The effect of varying cooling rate is reflected through the increasing extent of crystallinity with increasing particulate size. It was interesting to note the constant tendency for ordering in this system, even during the very initial stage of rapid solidification.