Challenges of nanoparticle-reinforced NiAl-based coatings processed by in situ synthesis of the aluminide.

NiAl intermetallic-based hardfacing coatings were processed by in situ synthesis from elemental Ni and Al powders mixtures, by Directed Energy Deposition - Plasma Transferred Arc (DED-PTA). The exothermic nature of NiAl synthesis promotes the formation of columnar grains, which do not contribute to mitigate the low toughness of the material at room temperature, limiting its use. Refining the microstructure is a path to overcome this limitation and the approach used took advantage of tungsten carbide nanoparticles (WC-NPs) to limit grain growth during solidification. During preparation of powder mixtures, the adhesion of 1 wt.%WC-NPs to the surface of Ni particles (which act as carriers) helps to reduce the tendency to agglomeration. However, this approach proved to be ineffective for the in situ synthesis of NiAl, since the NPs form a film of molten material on the Ni particle surface as they cross the plasma arc, limiting the interdiffusion of Ni and Al, compromising the synthesis of the aluminide alloy. The present study assessed this challenge by investigating the amount of NP carriers, i.e, the proportion of Ni particles that carry the NPs, that allows both for the synthesis of NiAl, and grain refining due to the WC-NPs. Results show that β-NiAl phase was successfully formed in multi-layer coatings, processed in situ by DED-PTA. Powder mixtures containing 50% and 70% NP carriers promoted a progressive reduction in grain size by the addition of 1 wt.% WC-NPs in the Ni+Al powder mixture, followed by an increase in hardness. [ABSTRACT FROM AUTHOR]