Microstructural development in Pt-aluminide coating on CMSX-4 superalloy during TMF

The microstructure and the microstructural development of a Pt-aluminide coating on CMSX-4 superalloy during thermal mechanical fatigue (TMF) over a temperature range from 300 to 1050 °C have been characterised. Two kinds of precipitates have been observed in the as-coated condition; submicron-sized tungsten structured particles in the outer layer of the coating and micron-sized μ phase in the inner layer of the coating. During TMF testing, the coating extended into the original substrate with the initial γ/γ' substrate, transforming to a mixture of γ', β-NiAl and a certain amount of μ-phase precipitates. β-NiAl grains mostly exhibit a lath shape in this region. Another important microstructural change is the precipitation of many σ-phase needles in the substrate adjacent to the interface with the coating. The microstructural changes became more evident as the number of TMF cycles increased. No detrimental effect on TMF lives in terms of crack initiation and growth from these TCP phases was found. The distribution of alloying elements in precipitates, β-NiAl phase and substrate has been analysed, and their influence on the precipitation of some phases is discussed.