Increase in ductility of Pt-modified nickel aluminide coating with high temperature ageing
Résumé
The ductility of β-(Ni,Pt)Al coating on the single-crystal nickel-base superalloy AM1 has been studied after isothermal and cyclic ageing at 1100 °C. Using accurate in-situ surface observations during tensile tests at room temperature and subsequent digital image correlation, the critical local strain required for crack initiation in the coating was obtained. Such an analysis is essential in order to directly identify the cracking of the β-(Ni,Pt)Al coating during tensile tests and to understand the mechanisms responsible for the evolution of coating ductility induced by thermal ageing. It is shown that both isothermal and cyclic ageing lead to increases in critical local strain, as compared to unaged sample. Besides, the sample after cyclic ageing possessed a greater number of cracks for similar ageing durations as those used for isothermal conditions. However, after the longest ageing duration the coating was either not cracked at all (cyclic ageing) or cracks were not observed in-situ by optical microscopy and only identified afterwards by scanning electron microscopy due to their small dimensions. These observations have been discussed using experimental surface profile analysis and microstructural observations combined with finite element analysis. The results suggest that the surface roughness was responsible for the coating cracking for the short ageing durations (when γ′ just starts to appear in the coating) and for lengthy ageing (with increased γ′ phase fraction) the γ′ phase ensured coating ductility.