Improvement of High Temperature Oxidation Behavior of ‘w’ Free Y strengthened Co-based Superalloys through Alloying Addition
The discovery of coherent g-g’ microstructure in Co-Al-W and Co-Al-Mo-Nb/Ta systems represents new possibilities for turbine engines materials. Their chemical and mechanical properties are similar to commercially successful Ni base superalloys. The present work aims at understanding the oxidation behavior of ‘W’ free γ’ strengthened Co-based superalloys. To understand the oxidation kinetics, mass change of the alloys was recorded with time during isothermal exposure at elevated temperature (650-950°C) in static air. To elucidate the oxidation mechanism in these alloys, in-depth analysis of kinetic data along with the characterization of the oxide scales have been carried out. The high temperature isothermal oxidation behavior of the basic Co-30Ni-10Al-5Mo-2Ta (at%) alloy has been studied in a static atmosphere to provide benchmark information on the oxidation behavior. Further, a 2 atom% Ti was added to this alloy to evaluate the consequences of Ti addition. Additionally, systematic addition of Cr (5at% and 8at%) in Co-30Ni-10Al-5Mo-2Ta-2Ti (at%) is evaluated. In the series ‘Mo’ free Co-30Ni-10Al-2Nb-x (x: 8 and 12) Cr-y (y: 2 and 4)Ti (at%) alloy system possess superior high temperature properties. Thus, the effect of Ti and Cr on this alloy system is studied from the point of view of environmental degradation through oxidation. Three different types of oxide layers were observed. The outer layer comprises of cobalt rich oxides followed by mixed oxides in the middle and inner layers. Oxides of Ta, Al, Mo, Ti, Cr, Nb were detected in the middle layer. Phase transformation of alumina oxide from amorphous to crystalline α-Al2O3 is observed with the addition of Ti and Cr in the base alloy. All the alloys show the γ-γ’ microstructural stability at the oxide-matrix interface during oxidation. No oxidation induced secondary phase is observed at the alloy side of the oxide-matrix interface as compared to the formation of Co3W phase in ‘W’ containing alloys.