Oxidation behavior of Nb–24Ti–18Si–2Al–2Hf–4Cr and Nb–24Ti–18Si–2Al–2Hf–8Cr hypereutectic alloys at 1250°C

Nb-24Ti-18Si-2Al-2Hf-4Cr and Nb-24Ti-18Si-2Al-2Hf-8Cr alloys were prepared by arc melting in a water-cooled crucible under argon atmosphere.Microstructural characteristics and oxidation resistance of the alloys at 1250 ℃ were investigated.The results show that,when the Cr content is 4 at%,the microstructures consist of （Nb,Ti）ss and Nb5Si3;as Cr content increases to 8 at%,C14 Laves phase Cr2Nb is formed.The isothermal oxidation tests show that the oxidation kinetics of the two alloys follow similar features.The weight gains of the two alloys after oxidation at 1250 ℃ for 100 h are 235.61 and 198.50 mg.cm-2,respectively.During oxidation,SiO2,TiO2,Nb2O5 and CrNbO4 are formed at first.Then,Ti2Nb10O29 is formed after oxidation for 20 min and begins to change into TiNb2O7 as the oxidation proceeds.SiO2 is formed as solid state at first but later evolves into glassy state to improve the cohesion of the scale.After oxidation for 100 h,oxidation products consist of SiO2,TiNb2O7,Nb2O5 and CrNbO4.