Superalloys interdiffusion

Inertia (friction) welding, a solid state joining method, has been used to join refractory metal alloys to Ni-base superalloys at the Edison Welding Institute (EWI). Successful material combinations of Mo-47.5Re or T-111 to Mar M-247 or Hastelloy X were joined in air with no adverse effects. However, detrimental metallurgical issues found in these joints included the production of small Interdiffusion layers and intergranular cracking. A complete record of the results can be found in Reference 9- 82. 

As the TBCs are applied onto advanced single crystal superalloy substrates, interdiffusion between the bond coat and the substrate has become a serious problem. The local concentration change caused by the interdiffusion would promote the precipitation of detrimental phases in the substrate. Outward diffusion of elements from the substrate, such as W, Mo and Cr, is detrimental to the interfacial bonding between TOO and bond coat, accompanied by volatilization of oxides of those elements at elevated temperatures. In addition, inward diffusion of A1 into the substrate could result in A1 depletion in the bond coat and finally degrade the oxidation resistance of the bond coat in long-term thermal exposure [7,65]. 

Recently, a new bond coat system, EQ coating (equilibrium coating), designed to be in thermodynamic equilibrium with the Ni-based superalloy substrate, has been proposed [72]. The composition of the EQ coating is close to that in the Ni-based superalloy substrate. The interdiffusion between the coating and the substrate was effectively restricted. 

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