Formation of Nanocrystalline Matrix Composite during Spray Forming of Al₈₃La₅Y₅Ni₅Co₂

In the present investigation, a multicomponent glass-forming Al₈₃Y₅La₅Ni₅Co₂ (at. pct) alloy was spray deposited on a copper substrate to produce an 8-mm-thick plate. The substrate was 30-mm thick and heated to a temperature of 160 °C prior to spray deposition. The temperature of the substrate and the deposit was measured during and after deposition. The deposits as well as oversprayed powders were characterized in terms of the microstructural features by optical microscopy and scanning electron microscopy (SEM). The phase constitution and transformation were studied by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The oversprayed powder revealed different microstructural characteristics showing crystalline, partially crystalline, and fully featureless particles. The spray deposit showed large fraction of featureless regions with embedded dendrites of 1- to 10-μm size intermetallic phases. These regions were observed to have a nanocrystalline structure with an average grain size of approximately 100 nm. The XRD analysis also revealed the nanocrystallinity in terms of a halo and peak broadening. These microstructural features have been attributed to the deposition of undercooled liquid on a highly conductive copper substrate and rapid heat extraction from the droplets due to proper metallic contact between the deposit and the substrate. These results have been discussed in light of processing conditions and the microstructural evolution of droplets in flight and during deposition.