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In order to clarify spalling mechanism of oxide scale on NiAl, microstructures in the vicinity of NiAl-X(Hf, Zr)/oxide scale interfaces were observed by using TEM. The oxidation tests were done at 1373 K in air for up to 360 ks. In NiAl, many voids were formed at the substrate/Al2O3 interface, leading to spalling of the alumina scale. On the other hand, addition of Hf or Zr to NiAl leads to suppression of the void formation, resulting in prevention of spalling of alumina scale. In addition, a Ni-Al-O layer tends to be formed at the interface in NiAl. This demonstrates appreciable increase in oxygen partial pressure and nickel activity at the interface. In fact, GDS measurements indicated depression of the Al concentration in the substrate close to an alumina scale. Consequently, it was speculated that the addition of Hf or Zr leads to increase in diffusivity of Al in the substrate and suppression of void formation at NiAl/alumina scale, resulting in prevention of spalling of an alumina scale.

Two new different methods for coating aluminide intermetallic compounds on metals have been proposed : one is based on exothermic reaction between droplets of aluminum liquid and metal powder, while the other is based on arc surface alloying with aluminide forming metals. The feasibility of the former method was experimentally investigated using NiAl and SUS 316L stainless steel as demonstration coating material and base material, respectively. As soon as an aluminum droplet was dropped onto the nickel powder that had been placed on steel surface, the droplet and powder exothermically reacted and produced a superheated NiAl liquid bead on the steel surface, and the heat of the liquid bead melted the contact surface of the steel. The superheated NiAl liquid bead solidified on the steel surface, leading to NiAl coating on the steel. On the other hand, the feasibility of the latter method was experimentally confirmed by NbAl3 coating on niobium. When tungsten arc was used to melt an aluminum sheet placed on a niobium plate, the niobium surface was also melted and a melt pool of Al-Nb alloy was formed on the niobium plate. The melt pool solidified into layers of niobium aluminides on the surface of the niobium plate, forming a thick NbAl3 layer on the top surface of the niobium aluminides layers.