Oxidation Behavior and Pack Siliconized Oxidation-resistant Coatings of an Nb-based Ultrahigh Temperature Alloy

摘要

The halide-activated pack cementation method was utilized to deposit silicide coatings on a multicomponent Nb-Ti-Si based alloy. The siliconized temperature was 1150 ℃ and the holding time was 10h. Both the specimens with siliconized coatings and without coatings were oxidized at 1250℃ for 5, 10, 20, 50 and 100h respectively. The coating possessed a double layer structure with the composition of (Nb,X)Si_2 (X represents Ti, Cr and Hf), and the outer layer was denser. The major structure in the outer layer was composed of columnar crystals perpendicular to the interface between the coating and the substrate, and that in the inner layer was mainly composed of equiaxed crystals. A transitional layer about 5 μm thick was found between the coating and the substrate. After oxidation at 1250℃, the major constituents in the scale were SiO_2 and TiO_2 and the mole ratio of these two phases was about 2:1. The thickness of the (Nb,X)Si_2 layer decreased and that of transitional layer increased as the oxidation time prolonged. The siliconized coating exhibited excellent oxidation-resistance at 1250℃ within 50 hours.