In Situ Assessment of the Complex Microstructural Evolution of Refractory Metal Superalloys

Global energy consumption has tripled in the last fifty years and our usage is predicted to increase. The high power to weight ratio and flexibility of gas turbine engines means that they will continue to see use across all areas of energy generation. As such, it is desirable that their efficiency is improved by enabling operation at higher temperatures, thereby reducing emissions. However, Ni-base superalloys, are reaching the limit of their capability, meaning that new materials are required to achieve enhanced performance.<br/><br/>One promising class of materials are refractory metal superalloys (RSA), which aim to combine the high melting temperatures and low intrinsic diffusivities of the refractory metal elements, with intermetallic reinforcement analogous to those of Ni-based superalloys. However, the majority of RSA compositions typically produce ordered matrices and solid solution precipitates, raising concerns about ductility and toughness, particularly at lower temperatures. Consequently, it has become critical to understand the microstructural formation pathway as this is key to establishing strategies by which the phase configuration can be controlled.<br/><br/>Gaining a deep understanding of the microstructural evolution in these systems requires the use of <i>in situ</i>techniques. In this presentation, data acquired using high energy synchrotron radiation during heating and cooling of a number of RSA will be discussed alongside key observations made via high resolution electron microscopy. These data not only provide direct evidence of the microstructural formation pathway and the critical temperatures at which key transitions occur but also highlight how the evolution of interphase misfit can lead to the formation of novel nanometric assemblies that reduce the interfacial stresses.