Interfacial reactions of a MAX phase/superalloy hybrid

Abstract

Oxidation resistant, strain tolerant MAX phase coatings are of general interest for high temperature applications. Accordingly, Cr 2 AlC MAX phase coupons were vacuum diffusion bonded to an advanced turbine disk alloy at 1100°C for compatibility studies. The interface revealed an inner diffusion zone consisting of~10 μm of β-Ni(Co)Al, decorated with various γ′ (Ni,Co) 3 Al, Ta(Ti,Nb)C, and W(Cr,Mo) 3 B 2 precipitates. On the Cr 2 AlC side, an additional~40-μm Al-depletion zone of Cr 7 C 3 formed an interconnected network with the β-Ni(Co)Al. On the superalloy side, enhanced carbide precipitation developed over a depth of~80 μm. Subsequent annealing for 100 h and 1000 h at 800°C coarsened some features, enhanced TCP precipitation in the superalloy, but only enlarged the diffusion layers by~5 μm at most. Because of Al depletion from the MAX phase and corresponding Al enrichment of the alloy, the reaction zone displayed similarities to an oxidized Cr 2 AlC surface and an aluminized superalloy, respectively. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.

Cr2AlC MAX phase as bond coat for thermal barrier coatings: Processing, testing under thermal gradient loading, and future challenges

“…However, diffusion of elements such as Co, Ti, and Mo from the substrate are detected in the carbide and Cr 2 AlC phases. Diffusion of metallic elements from the substrate has been already reported by Smialek's and Barsoum's groups under other thermal conditions and using other superalloys . Among the different tested MAX phases, Cr 2 AlC shows the lowest diffusion rate, but it is still the main problem to use these compounds in real applications at high temperature.…”

“…Finally, the self‐healing capability at high temperature of Al‐based MAX phases may play an important role in case of a crack penetrating into the bond coat layer . Nevertheless, one of the potential drawbacks for using MAX phases as bond coat is the inner diffusion of metallic elements from the superalloy substrate . In that case, Cr 2 AlC is the most promising composition since it exhibits lower diffusion rates and better compatibility with superalloys than Ti 2 AlC …”

Compositional effects on the cyclic oxidation resistance of conventional superalloys

“…According to the prior analysis, this will be sufficient to skew oxidation behaviour significantly towards non-protective Ti, Cr-rich rather than protective Al-rich external scales expected for single crystals. 20,21 For example, Equation 6 predicts a weight loss of ~270 mg/cm 2 for LSHR compositions and implies much more severe oxidation than ~1 mg/cm 2 projected for single crystals. This provides another incentive for a hybrid disk concept proposing a bonded outer rim of single crystal elements.…”

Elastic properties, thermal stability, and thermodynamic parameters of MoAlB

“…The MoAlB single crystals were shown to have a relatively low hardness (10-13 GPa) compared to most TMBs [19,20] and high room-temperature electrical conductivity [16,19]. In addition to its excellent oxidation resistance, our recent work on bulk MoAlB has shown that it has high electrical and thermal conductivities, room-temperature compressive strengths that are close to 2 GPa, and a thermal expansion coefficient of −9.6 × 10 −6 K −1 -that is comparable to many other engineering materials (e.g., alumina, yttria-stabilized zirconia, and Ni superalloys) [13,21]. In combination, these properties bode well for the use of MoAlB in HT applications.…”