Neutron and synchrotron probes in the development of Co Re based alloys for next generation gas turbines with an emphasis on the influence of boron additives

Ni based superalloys are the material of choice in the hot section of the current gas turbines but they are reaching temperature limits constrained by their melting temperature range. Co Re alloy development was prompted by a search for new materials for the future gas turbines where the temperature of application will be considerably higher. Addition of the very high melting refractory Re metal to Co can increase the melting range of Co alloys to much higher temperatures than the commercial Co alloys in use today. Moreover, the melting temperature can be tailored by adjusting the amount of Re addition. Rhenium addition stabilizes the low temperature hexagonal close packed amp; 61541; Co phase and thereby makes the Co Re alloys significantly different from the conventional Co based alloys, which has a stable face centered cubic amp; 61543; Co matrix. The Co Re Cr system has complex microstructure with many different phases present in different size scales. Phase transformation and stability of fine strengthening precipitates at high temperatures remain mostly unexplored in the Co Re Cr alloys. In the alloy development program model ternary and quaternary compositions were studied in order to develop basic understanding of the alloy system. In situ neutron and synchrotron measurements at high temperatures were extensively used for this purpose and some selected results from the in situ measurements are presented. In particular, the effect of boron doping in Co Re Cr alloys and the stability of the Cr23C6 and TaC carbide precipitates at high temperatures were investigated. A fine dispersion of TaC precipitates strengthens some Co Re alloys and their stability at the application temperature is critical to long term creep properties. In the beginning the alloy development strategy is very briefly discusse