Microstructural Transformation in Platinum Aluminide Coated on CMSX-4 Superalloy

Abstract

Microstructural evolution induced by isothermal exposure at 1100°C in a platinum aluminide coating was investigated. Specimens of CMSX-4 alloy have been platinum aluminised to give a single phase (Ni,Pt)Al coating. Coated alloys were exposed in an oxidising environment for 188, 350, 750 and 1500 hours at a temperature of 1100ºC. After isothermal oxidation of the single phase (Ni,Pt)Al coatings the surface is shown to progressively roughen. The compositional development assisted by chemical diffusion during isothermal oxidation at 1100°C has been related to the transformation of the coating from its original β-NiAl (B2) structure to a Ni-rich (L1 0 ) martensite γ′. The transformation took place due to aluminium depletion in the coating from the formation of the Al 2 O 3 scale and interdiffusion between the coating and alloy substrate. The effects of phase changes on coating surface character are considered.

Isothermal Oxidation Behavior of Ru Modified Aluminide Coating on a Fourth Generation Single Crystal Superalloy

“…In the simple aluminide coating, the transformation from its original structure β-NiAl to γ'-Ni 3 Al had occurred during the oxidation. The transformation occurs from Al depletion in the coating layer due to Al consumption by the formation of Al 2 O 3 scale and also due to Al diffusion into the substrate [8], resulting in the degradation of oxidation resistance since γ' is less protective in Al concentrations too low to form a protective α-Al 2 O 3 scale [9]. It is noteworthy that the γ' or other Al depleted phase was not formed in the Ru-modified coating even after 150 h oxidation.…”

Isothermal Oxidation of Pt Modified and Ru Modified Aluminide Coating on a Fourth Generation Single Crystal Superalloy

“…The Al-rich phase remained about half in the surface part of the Ptmodified coating, while the surface part of the simple aluminide coatings was a completely Al-poor phase after the exposure at 1373 K. The phase transformation in the coating from its original structure of β to γ', that is a well-known phenomenon, occurs by the depletion of Al due to the Al 2 O 3 scale formation and due to its inward diffusion [8]. This induces the degradation of oxidation resistance, since γ' is less protective in which the Al concentration is too low to form a protective α-Al 2 O 3 scale [9]. So, the reduction of the γ' area at the surface may largely contribute to restricting the local oxidation attacks.…”