High-Temperature Oxidation and Thermal Expansion Behavior of NbTi–X (X = 5Co, 10Cr, 10Ni, 10CoCrNi) Refractory Medium Entropy Alloys

Abstract The formation of Nb 2 O 5 and other oxides in NbTi– X ( X = 5Co, 10Cr, 10Ni, 10CoCrNi) after exposure to 800 °C accelerates oxidation kinetics, shifting from a parabolic rate law to a power-law exponent of two at 1200 °C. Subsequent oxidation at 800 °C for material initially exposed to oxidation at 1200 °C shows that the retained TiNb 2 O 7 and CrNbO 4 oxide scale substantially improves oxidation kinetics, transitioning from linear to parabolic behavior at this temperature. The oxide scales were heterogeneous, exhibiting regions with thin oxide layers alongside areas of thick, porous scales due to the formation of discontinuous metal and complex oxide structures. Microstructural analysis using scanning electron microscopy (SEM) revealed that the formation of metal oxides and complex oxides plays a crucial role in oxidation resistance. In alloys containing Co and Ni, TiNb 2 O 7 and Nb 2 O 5 layers are observed. Additionally, NbTi–10(CoCrNi) exhibited significant internal corrosion zones composed of various oxides. Quantitative X-ray diffraction (XRD) analysis of the oxide scales formed after 16 hours of oxidation confirmed that Nb 2 O 5 and TiO 2 were the predominant phases when oxidation resistance is relatively poor and TiNb 2 O 7 and CrNbO 4 are present when oxidation weight gain rates are comparatively lower. Furthermore, it is found that these alloys exhibit low, stable thermal expansion, with hysteresis primarily in the as-cast state due to phase transitions and internal stresses, which largely disappear after homogenization due to single-phase stability and reduced partitioning effects. These findings may provide valuable insights into the oxidation behavior, structural evolution, and thermal stability of these alloys, particularly in environments involving thermal cycling between high and low temperatures.