Effect of Cr Addition on the Phase Equilibria and Oxidation Behavior of NbSi₂

Abstract

Effect of Cr addition on the phase equilibria and oxidation behavior of NbSi₂ was investigated. Although the crystal structures of NbSi₂ and CrSi₂ are both C40, they form separated ranges of homogeneities in the Nb–Cr–Si ternary system. The vertical section passing through the NbSi₂ and CrSi₂ binary edges were experimentally determined. A binary NbSi₂ alloy exhibited poor oxidation resistance, showing pest-like behavior upon cyclic oxidation at temperatures from 800 to 1200°C. On the other hand, Cr addition significantly improved the oxidation resistance of NbSi₂. The oxide layers that developed on the Nb–Cr–Si ternary alloys consisted of NbCrO₄, Cr₂O₃, and SiO₂, and the formation of Cr₂O₃ and NbCrO₄ suppresses the Nb₂O₅ formation, and thereby suppresses the pest-like behavior of the binary NbSi₂ alloy. However large Cr additions that exceed the solubility limit of NbSi₂ should not be suitable, because NbSi₂/CrSi₂ alloys were susceptible to spalling and disintegration of the oxide scales upon cyclic oxidation.

Fig. 8 Change of the appearances of oxidation test specimens tested at 1200°C for various cumulative oxidation durations. Cubic specimens were placed in alumina crucibles.