MICROSTRUCTURAL DEVELOPMENT IN PT-ALUMINIDE COATING ON CMSX-4 SUPERALLOY DURING TMF

The microstructure and the microstructural development of a Pt-alumini de coating on CMSX-4 superalloy during thermal mechanical fatigue (TMF ) over a temperature range from 300 to 1050 degrees C have been charac terised. 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 mu phase in the inner layer of the coating. During TMF testing, the coating extended into the origina l substrate with the initial gamma/gamma' substrate, transforming to a mixture of gamma', beta-NiAl and a certain amount of mu-phase precipi tates. beta-NiAl grains mostly exhibit a lath shape in this region. An other important microstructural change is the precipitation of many si gma-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 di stribution of alloying elements in precipitates, beta-NiAl phase and s ubstrate has been analysed, and their influence on the precipitation o f some phases is discussed. (C) 1998 Elsevier Science S.A. All rights reserved.