Chromium and iridium effects on the short-term interdiffusion behaviour between Pt rich γ-γ′ bond-coatings and a Ni-Al-Cr alloy

Abstract

The interdiffusion behaviour of a 5μm thick layer of Pt deposited by electroplating on a γ-Ni-12Al-10Cr model alloy was studied in order to assess the effect of Cr. Heat treatments were performed for 1min up to 1h at 1100°C under argon. Cr addition increased the uphill diffusion of Al to the surface when compared with Pt/γ-(Ni,Al) systems. Al and Cr had a positive chemical interaction in presence of Pt, as shown by the positive values of the DAlCrNi and DCrAlNi diffusion coefficients determined by modelling. Pt had a negative chemical interaction with Al and with Cr in such a way that Pt decreased their activities. According to the diffusion coefficient values, Pt had a greater influence on the Al activity than on the Cr one. Similarly, 2μm of Pt and 3μm of Pt-25Ir were deposited by electroplating on the same model alloy to investigate the effect of Ir. Heat treatments were performed in the same conditions as for Cr. Iridium slowed down the interdiffusion when compared with systems with Pt only. Iridium diffused slower toward the substrate than Pt and a lower Pt+Ir flux toward the substrate was found. As voids formed at the interdiffusion zone/substrate interface due to Kirkendall effect, this lower inward Pt+Ir flux resulted in a lower outward vacancy flux and then Ir reduced Kirkendall voids formation. Moreover, Ir decreased the Pt effect on Al activity by dilution or even gave an opposite contribution to the Pt one. This reduced the uphill diffusion of Al, delaying the α-NiPtAl phase formation. Diffusion paths of each model system were also identified after 15min at 1100°C and all highlighted the α-NiPtAl phase formation and its aptitude to be used in TBC systems.