Abstract:Five types of Cr-Nb alloys with different phase constitutions were fabricated by mechanical alloying and hot pressing. The influence of phase constitution on the oxidation behavior of the hot pressed Cr-Nb alloys were investigated at 950−1 200 ℃ in air. The results show that Cr phase improves the oxidation resistance of Laves phase NbCr₂ based compounds at 950 ℃. However, Nb phase deteriorated the oxidation resistance, moreover, Nb phase results in a catastrophic failure during oxidation. SEM and XRD analyses indicate that the Cr_SS phase (Cr-2.5Nb) exhibits selective Cr oxidation and forms a continuous chromia scale. However, single phase alloys of Cr-33Nb and two phase alloys of Cr-18.5Nb exhibit that internal and external oxidation happen simultaneously for the active element chromium, and their oxide scales are made with a multi-layered scale consisting of an outer layer being a porous, loosely adherent Cr₂O₃ layer and the inner layer being of CrNbO₄. With the exposure temperature increasing up to 1 200 ℃, the oxidation resistance of Cr-rich Cr-Nb alloys decreases due to the continuous vaporization of CrO₃ in the oxide scale. Therefore, it is necessary to apply surface protective coatings on Cr(or Nb)-NbCr₂ alloys to promote their application in the high temperature environment.

Key words: Cr-Nb alloys; Laves phase; ductile phase; phase composition; oxidation behavior