Interfacial Segregation, Pore Formation, and Scale Adhesion on NiAl Alloys

Abstract

Ni-40 and 50at%AI alloys were oxidized at 1000°C for various times in oxygen. Auger electron microscopy was used to study the interface chemistry after scale spallation in ultra high vacuum. The interfacial failure stresses were determined using a tensile pull tester and related to the interface chemistry, pore area and density. Results showed that sulfur started to segregate to areas ofthe Ah03INi40AI interface where the scale was in contact with the alloy after a complete layer of a-Ah03 developed there; the concentration then gradually increased to a steady level of~2 at%. However, sulfur did not segregate to similar areas of the Ah03INi50AI interface even after extended oxidation when it was amply present on interfacial void faces. This behavior demonstrated a strong dependence of interface segregation on NiAI alloy composition. Interfacial failure stress was found to decrease with increasing sulfur content between voids and with higher interface porosity. The level of porosity was strongly related to the sulfur content in the alloy. When Ni40AI was doped with excess sulfur, the segregation behavior did not change, but the interfacial pore density increased significantly. The detrimental effect of sulfur on scale adhesion is two-fold: to weaken the interface and to enhance interfacial pore formation.

Examination of the platinum effect on the oxidation behavior of nickel-aluminide coatings

“…Approximately 1 at.% S at the oxide-metal interface on the NiAl coating appears to be sufficient to weaken the interface and cause oxide spallation. This concentration is 8 similar to that found at the oxide-metal interface on a Ni-40Al alloy, where interface strength dropped rapidly with the first few percent of sulfur [5]. The amount of hafnium present in the CMSX-4 alloy (0.03 at.%) is apparently insufficient to mitigate the effect of sulfur in the NiAl coating.…”

“…Oxidized samples were examined using SEM, and chemistry of the Al 2 O 3 /coating interface was studied after 10h, 100h and 250 h isothermal oxidation using a scanning Auger probe. The interface was exposed by scratching the oxidized samples in ultra-high vacuum [5][6][7]. The scratch plows through the scale into the metal, causing oxide spallation adjacent to the scratch mark.…”

“…In binary NiAl alloys, it has been found that ~2 at.% sulfur could segregate to the oxide-alloy interface during oxidation, and its presence greatly reduced the interfacial strength [5]. Although scale adhesion on nickel-aluminide coatings has been reported to improve with decreasing the amount of sulfur in the underlying alloy [2], no chemical information was obtained from the scale/coating interface.…”

Moisture-Induced Delayed Alumina Scale Spallation on a Ni(Pt)Al Coating

“…Additionally, the crystallographic orientation of β-NiAl has been shown to influence both sulfur segregation and the degree of interfacial void formation (Ref. 8), remarkably allowing voids to form on low indice planes of even desulfurized or Y-doped Ni40Al5Cr (Ref. 15).…”

Void formation and filling under alumina scales formed on Fe–20Cr–5Al based alloys and coatings, oxidised at temperatures up to 1200 °C

“…The formation of voids at the metal/oxide interface at high temperatures >1000 8C has been rarely discussed [16][17][18], but is considered detrimental to the lifetime of a component. The experiments described in this paper are designed to investigate the formation of voids beneath alumina scales formed on FeCrAl alloys and give an indication of the influence of void formation on the behaviour of FeCrAl coatings on selected substrates.…”