Microstructural modelling solution for complex Co based alloys and coatings

Microstructural changes following oxidation at high temperatures of a complex coating or alloy can significantly influence its properties. This paper investigates possible solutions that can accurately model alloy microstructures that result from such a process in a cobalt-rich CoNiCrAlReY bond coat alloy system. Four different alloys with varying aluminum content were studied after heat treatments at 1000 °C or 1150 °C. SEM, EPMA and XRD were used to characterize the alloy microstructure, and hardness testing was performed to determine the role of microstructure on mechanical properties. Experimental microstructures were modeled using two databases of Thermo-Calc, namely TTNI8 and SSOL5. Effects of Al content on phase changes and materials properties were quantitatively investigated. Data showed that a 1 wt. % increase of Al in the alloy caused an 8% increase in the beta phase, and a Cr and Re rich sigma phase precipitated preferentially at high Al concentrations. In addition, alloy hardness was found to increase by 4 and 81%, respectively, with a 10% increase in the beta and sigma phases. Modeled results using the TTNI8 database showed excellent agreement with experimental observations, but the SSOL5 database produced erroneous information.