Innovative Al–Ni–Ir alloy for bond coats: Microstructure, phase analysis and oxidation behaviour

Iridium modified nickel alluminides are considered promising bond coats due to the capability to promote α-Al2O3 scale growth and to develop the formation of Ir layer acting as oxygen diffusion barrier (ODB). An innovative Al–Ni–Ir alloy (Al 60 %at, Ni 35 %at, Ir 5 %at) was formulated, taking into account the bond coat appropriate requirements and the cost standpoint. A detailed and systematic microstructural investigation of both powder and bulk samples was reported, comparing the phase composition, thermal stability and oxidation behaviour of the proposed system with respect to the Ir free one. The analysed AlNiIr system was composed of Al3Ni2 and AlNi3, in addition to the β-NiAl, detected in the reference one. The Ir presence seemed to promote the alumina scale growth, starting at around 1000 °C. Both as cast and oxidised AlNiIr bulk samples showed Ni-poor and Al-rich islands, in which Ir resulted more concentrated, suggesting an iridium higher affinity towards Al with respect to Ni. After the oxidation at operative temperature (i.e. 1150 °C), the α-Al2O3 scale growth was observed, increasing the thickness with the dwelling time. Neither Ir ODB nor Ir rich islands at the interface between the alloy and the Al2O3 scale were identified, due to the low Ir amount. However the presence of metallic Ir and of the compound Al2.75Ir was detected in the powder after thermal treatment at 1000 °C.