S. Kuroda

Effects of Ga and Sn Additions on the Creep Strength and Oxidation Resistance of Near-α Ti Alloys

The effects of Ga and Sn additions, with an almost constant value regarding Al equivalence, on the creep properties and oxidation resistance of the near-α Ti alloy Ti-5Al-4Zr-1Mo were investigated. The creep strain rate increased due to the replacement of Sn with Ga, which was accompanied by an increase in the volume fraction of primary equiaxed α phase. In addition, the replacement of Sn with Ga decreased the activation energy of the steady-state creep rate for the similar volume fraction of the equiaxed α phase. Ga addition improved the oxidation resistance without Ga segregation at the TiO2/substrate interface, whereas Sn promoted oxide spallation with metallic Sn segregation at the interface. Ga uniformly dissolved into the internal TiO2 and into the external Al2O3 as a result of solid solution formation between Ga2O3 and the aforementioned oxides, which suppressed oxide growth. The formation of (GaAl)2TiO5 in TiO2 and Al2O3 was also discussed on the basis of the phase relationships in the TiO2-Al2O3-Ga2O3 system.

Steam oxidation resistance of thermal sprayed coatings for USC boiler applications

Abstract Plasma sprayed coatings of 50Ni–50Cr as undercoat coupled with an Al topcoat were investigated with the aim of filling the pores in the Ni–Cr coating with the Al topcoat at high temperatures. For comparison purposes, a thick 50Ni–50Cr coating (∼450 μm) with sealant according to BS 5493 was also applied. The results showed that both the two layer coating of Ni−Cr+Al and the thick coating with the sealant had excellent resistance against steam oxidation at 650°C for up to 3000 h of testing. It is interesting to note that the two layer coating showed a similar performance, despite the lower coating thickness compared to the thick coating. Both Ni and Cr diffused into the Al coating structure. Diffusion of Al into the Ni–Cr system (coating) was slower than that into the Fe–Cr system (substrate).