Characterization and modeling of a martensitic transformation in a platinum modified diffusion aluminide bond coat for thermal barrier coatings

Phase transformations in a platinum modified nickel aluminide bond coat were investigated by in situ high temperature X-ray diffraction analysis. Three phases, L10 martensite, B2 (β-(Ni,Pt)Al) and L12 (γ′-Ni3Al), were identified at different temperature ranges. The martensite is stable at temperatures below 620 °C, and the β-phase is stable at elevated temperatures. The reversible transformation, M↔β, is the principal reaction occurring throughout the bond coat layer during thermal cycling. Quantitative measurements indicate that the molar volume of the β-phase is approximately 2% larger than that of the martensite. Finite element simulations incorporating the volume change associated with this transformation indicate that the transformation significantly influences the distribution of stresses and strains in TBC systems. The effect of the martensite on TBC life is sensitive to the transformation temperatures relative to the creep strength of the bond coat.