Presentation Title

Aluminization of Ni-base Superalloys by Slurry and Pack Cementation Processes

Faculty Mentor

Dr. Vilupanur Ravi, Chemical and Materials Engineering, Cal Poly Pomona

Start Date

17-11-2018 2:00 PM

End Date

17-11-2018 2:15 PM

Location

C301

Session

Oral 3

Type of Presentation

Oral Talk

Subject Area

engineering_computer_science

Abstract

Ni-base superalloys are used in gas turbine applications because of their desirable mechanical strength and oxidation resistance at temperatures up to 1000°C. To increase the efficiency of gas turbines, higher operating temperatures are needed. Protective coatings can increase the longevity of the substrates in high temperature and corrosive environments. Both halide activated pack cementation (HAPC) and and halide activated slurry cementation (HASC) are in situ chemical vapor deposition processes activated by halide salts. The chemical reactions deposit the desired element (aluminum in this case) on the surface of metallic alloys. The coating element subsequently diffuses into the substrate. Under oxidizing conditions, the surface modified alloy forms a passive oxide layer that protects it from high temperature corrosion. In this study, aluminide coatings were produced by HAPC and HASC on Ni-base superalloys (UNS N07208, UNS N06230, and UNS N07718). Alloy coupons were coated in an inert environment at 950°C for 4 hours. The characteristics of the coatings produced, i.e., thickness, microhardness, microstructure and elemental distribution, were studied using a broad range of techniques - X-ray diffraction (XRD), macrophotography, optical and scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS).

Summary of research results to be presented

The choice of coating method affected each alloy differently. For UNS N07718, the coating obtained using the HAPC process had an outer layer made up of Ni2Al3 with small amounts of Ni3Al, possibly existing as a sublayer, while the HASC coating had an outer layer consisting of Ni2Al3 with Al6MoNb. Both processes resulted in similar thicknesses of 160μm, and similar trends in VHS. For UNS N07208, the coatings produced by HAPC and HASC were similar in all characteristics. For UNS N06230, the HAPC coating resulted in a Ni2Al3 outer layer, while the HASC had an NiAl outer layer. The HAPC coating was about 150μm, while the HASC coating was about 70μm, and had a lower average VHS. The morphology of aluminized UNS N06230 was different for the two processes as revealed by SEM and EDS analysis. The outer layer of the HAPC aluminized UNS N06230 had a Ni2Al3 matrix containing chromium and tungsten-rich precipitates. UNS N06230 aluminized by HASC had a NiAl matrix with fewer precipitates. Both of the HAPC and HASC aluminized coupons had tungsten and chromium rich precipitates along the inner diffusion zone. Microhardness measurements showed that, in all cases, the coatings are harder than the substrate. For most of the coatings, the samples aluminized by HAPC had a more consistent microhardness profile.

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Nov 17th, 2:00 PM Nov 17th, 2:15 PM

Aluminization of Ni-base Superalloys by Slurry and Pack Cementation Processes

C301

Ni-base superalloys are used in gas turbine applications because of their desirable mechanical strength and oxidation resistance at temperatures up to 1000°C. To increase the efficiency of gas turbines, higher operating temperatures are needed. Protective coatings can increase the longevity of the substrates in high temperature and corrosive environments. Both halide activated pack cementation (HAPC) and and halide activated slurry cementation (HASC) are in situ chemical vapor deposition processes activated by halide salts. The chemical reactions deposit the desired element (aluminum in this case) on the surface of metallic alloys. The coating element subsequently diffuses into the substrate. Under oxidizing conditions, the surface modified alloy forms a passive oxide layer that protects it from high temperature corrosion. In this study, aluminide coatings were produced by HAPC and HASC on Ni-base superalloys (UNS N07208, UNS N06230, and UNS N07718). Alloy coupons were coated in an inert environment at 950°C for 4 hours. The characteristics of the coatings produced, i.e., thickness, microhardness, microstructure and elemental distribution, were studied using a broad range of techniques - X-ray diffraction (XRD), macrophotography, optical and scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS).