Destructive Effect of Water Vapour on an In Situ Diffusion Barrier Layer within an Aluminide Coating on IN738 Alloy

High-temperature interdiffusion within a hot-dipped aluminide (Al-10 wt.% Si) coating on an IN738 superalloy was investigated at 1050 °C in air and in air plus water vapour. The resulting morphology of in situ diffusion barrier layer (DBL) within the aluminide coating is affected by oxidizing atmospheres; DBL can effectively retard the interdiffusion of aluminium within the coating. The location of the in situ DBL is governed by the partial pressure of oxygen at different depths from the oxide scales in both atmospheres. Meanwhile, the diffusion fluxes of different elements led to DBLs with different morphologies in the aluminide coating on the Ni-based alloy.

High-Temperature Thermal Cyclic Oxidation Behavior of Wide-Gap Brazed IN738 Superalloy

The microstructure and thermal cyclic oxidation resistance of the wide-gap region brazed with different filler metal powder (BNi-3 and DF 4B) comparing with that of Ni-based IN738 alloy were investigated. The microstructure characterization showed that Cr borides with a blocky morphology were existed in the brazed region in both filler metal powder. The normalized weight gain with cyclic oxidation showed that weight loss of the specimen brazed with BNi-3 filler metal occurred after 600 cycles. However, the specimen brazed with DF 4B filler metal had no obvious weight loss until 700 cycles. It was observed that the oxidation kinetics of the all oxidized specimens followed the quasi-parabolic law, and the oxide layer was mainly composed of NiO, Al2O3 and NiCr2O4.

Novel Approaches to the Repair of Vane Segments

Thermal fatigue and craze cracks in a set of industrial vane segments were successfully repaired using an advanced LPM™ powder metallurgy process. Unlike conventional braze repair processes that rely solely on chemical cleaning to reduce the oxides, the LPM™ approach will physically remove the cracked or damaged areas in a manner similar to weld repairs. Consequently, large areas in the fillet radius, airfoils, and shrouds were mechanically prepared using conventional hand tools and then replaced with matching or alternate alloys in a powder metallurgy form. In this application, it was decided to use a stronger nickel-based IN738 alloy to repair the cobalt-based X-45 vanes. The patented LPM™ process uses conventional vacuum furnace heat treatments to produce dense, wide gap repairs that are well bonded to the parent metal. The results of the metallurgical and mechanical tests are presented, along with the observations during final inspection of the parts.