Microstructure of a high-velocity oxy-fuel thermal-sprayed nanostructured coating obtained from milled powder

The mechanisms responsible for the different types of microstructure present in a nanostructured coating prepared by high-velocity oxy-fuel (HVOF) spraying of the Y2O3-reinforced milled FeAl powder have been established. Nano- and microsized grains were present both in the melted splats and the retained unmelted powder particles. In the unmelted powder particles, equiaxed grains formed by recrystallization from the heavily deformed and disordered structure present in the initial powder. Their size was controlled by the degree of advancement of the recrystallization process and the larger grains contained ribbonlike antiphase boundaries (APBs) separating large, ordered domains. In the melted zones, the grain size (and shape) was controlled by the local efficiency of the heat extraction during solidification. The Al evaporation during thermal spraying led to a modification of the local chemistry that resulted in the formation of the Fe3Al and FeAl phases containing fine networks of APBs. These APBs formed by sequential ordering from the high-temperature disordered FeAl phase that formed from the melt. Despite the high-temperature processing and recrystallization processes, both the melted and unmelted zones contained dislocations that result from residual stresses as well as from peening by the powder particles during spraying.