MATCHING LOCAL COMPOSITION AND MECHANICAL PROPERTY ANISOTROPY OF PREOXIZED (Ni, Pt)Al BOND COATS AFTER THERMAL CYCLING TESTS

Abstract

Pt-modified nickel aluminide, B2 (Ni,Pt)Al, coatings are used to protect the surface of turbine airfoils against oxidation [1]. However, the lack of ductility of these systems presents a drawback in their performance. At high temperatures several elements diffuse from the superalloy substrate and become incorporated into the B2 phase, which can modify its mechanical properties. This diffusion process also results in a distribution of elements throughout the coating that can change from grain to grain. Few reports on the local, micromechanical properties of B2 (Ni,Pt)Al coatings measured by nanoindentation, taking into account crystal orientation, have been published. No anisotropy effect of elastic modulus in the B2 (Ni,Pt)Al was observed by Durst et al. [2], but their finding remains to be understood, and the effect of local chemical composition is yet to be thoroughly explored. In this work, coupons of Rene-N5 superalloy coated with a B2 (Ni,Pt)Al bond coat were pre-oxidized at different temperatures and oxygen partial pressures (pO2) in order to generate a protective α-Al2O3 scale. An yttria-stabilized zirconia (7YSZ) top-coat layer was subsequently deposited by EB-PVD to complete the thermal barrier coating (TBCs). Finally, the TBCs were subjected to thermal cycling tests, during which the (Ni,Pt)Al was observed to transform from pure B2 to a combination of L10 (martensitic), L12 (ordered fcc), and B2 (ordered bcc) phases. Electron backscattering diffraction (EBSD) was used to identify the crystal orientation of the different phases. The mechanical properties of grains oriented in the [001], [101], and [111] directions were measured by nanoindentation with two different indenter geometries, Berkovich and cono-spherical. No anisotropy of mechanical properties was found using the Berkovich indenter, whereas slight differences were observed with the cono-spherical indenter. Moreover, strong differences in the elastic limit were observed when measuring with the cono-spherical within the same grain, which is attributed to differences in the local chemical composition. Finally, the mechanical properties of the L10 and L12 phases are presented and discussed. References: [1] J. Esslinger and E.E. Affeldt, Transactions of the ASME, Journal of Engineering for Gas Turbines and Power, 121 (1998): 687-690. [2] K. Durst, O. Franke and M. Göken, Proceedings Superalloys (2004): 467-476.