High-Temperature Stability of Phases in Boron Containing Co-Re Alloys for Gas Turbine Applications

P. Strunza, Debashis Mukherjib, P. Berana, R. Gillesc, M. Hofmannc, L. Kargec, J. Röslerb, G. Farkasd aNuclear Physics Institute ASCR, 25068 Řež near Prague, Czech Republic bTechnische Universität Braunschweig, Institut für Werkstoffe, Langer Kamp 8, 38106 Braunschweig, Germany cTechnische Universität München, Heinz Maier-Leibnitz Zentrum (MLZ), Lichtenbergstr. 1, 85747 Garching, Germany dDepartment of Physics of Materials, Faculty of Mathematics and Physics, Charles University, Ovocný trh 5, Prague

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Co-Re-based alloys are candidates for high-temperature applications in gas turbines. The addition of boron largely increases their ductility. The stability of the Co-solid solution matrix and of the strengthening TaC minority phase at the foreseen alloy operation temperatures (≥1200°), potentially influenced by the presence of boron, has to be considered. The hcp to fcc transformation of the Co-solid solution matrix and the TaC minority phase evolution was thus investigated in Co-Re-Ta-C alloy with varying boron addition by in situ neutron diffraction at temperatures up to 1500° in high vacuum furnace. The neutron diffraction showed that the boron addition has no significant influence on TaC volume fraction during heating to high temperatures. In 0, 0.1, and 0.4 at.% B alloys, a strong decrease of the TaC volume fraction was observed above 1300°. Nevertheless, TaC has significant volume fraction up to that temperature and, therefore, can be effective for strengthening. The measurements further showed that while a small amount of boron addition (0.1 at.% B) has no influence on the matrix transformation and its hysteresis, a large boron addition (0.4 at.% B) influences the matrix transformation temperature significantly (lowers it by about 100 K) and, moreover, boron promotes massive Co sublimation at very high temperatures (≥1470°). Further, re-appearance of the hcp phase around 1400° in 0.4 at.% B alloy was observed and explained by a slow sublimation of Co in high vacuum. It is not a problem from the technological point of view as vacuum is not a turbine operation condition, and as the high-temperature components are regularly protected by a coating.

DOI:10.12693/APhysPolA.134.829 PACS numbers: 61.05.fg, 61.66.Dk