Phase evolution and microtwins in the Ni–Al multilayers upon annealing and laser mixing

Multilayers of Ni and Al of 1:1 ratio, 50 nm periodicity, were deposited using sputtering. Progress in phase evolution in these films upon annealing and laser mixing has been studied. Characterization of the as deposited and the treated states were carried out with X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) techniques. As-deposited state reveals the presence of elemental Ni and Al with grain size comparable to the film thickness. Phase evolution upon annealing occurs in several stages determined by the kinetics of nucleation and growth. Al3Ni and Al3Ni2 initially form at 673 K and transforms to a mixture of AlNi and Ni3Al at about 873 K. Films were also treated with nanosecond laser pulse of 20–200 mJ energy with spot size of 1 and 1.5 mm. The major phase obtained after laser irradiation is B2 with grain size increasing with increasing imparted pulse energy. In addition to B2, microtwins and tweed microstructures are often observed. The origin and the nature of twins have been analysed in details. Formation of this type of microstuctures in polycrystalline thin films and with Ni content <62 at.% has not been reported previously.