Microstructural Analysis of Plasma Sprayed Al-based Amorphous Alloys: A Comparison with Melt-spinning Technique

The Al85Y8Ni5Co2 coatings produced by atmospheric plasma spray (APS) exhibit both an inhomogeneous microstructure and yttrium distribution. The microstructures vary from completely amorphous to mixtures of amorphous and alpha-Al nanocrystalline particles that are globular, spherical or dendritic in shape. The glass transition temperature is conspicuously absent during differential scanning calorimetry (DSC) of the APS coating, but does show a shallow exothermic reaction from 120 to 325 DGC. The effects of alloy composition, quenching rate, and annealing on microstructural development in the APS coating was investigated using transmission electron microscopy (TEM) and DSC of the melt-spun amorphous alloys (Al85Y8Ni5Co2, Al87Y6Ni5Co2 and Al89Y4Ni5Co2). The primary crystallization temperature was found to depend upon yttrium content and quench rate in the melt-spun ribbons. This study shows that the microstructures and calorimetry results from the APS coating can be explained by assuming a combination of amorphous structures with different compositions and quench rates. The presences of multi-compositional amorphous phases with associated morphologies of nanocrystalline alpha-Al particles may be explained by the formation of icosahedral clusters in the metastable liquid, where the density of the clusters is proportional to yttrium concentration