Microstructural Formation And High Temperature Oxidation Resistance Of Pack Siliconized Coatings On An Nb Silicide Based Alloy

The multielement Nb silicide based alloy with the nominal composition of Nb-24Ti- 6Cr- 3Al- 16Si- 4Hf- 1.5B- 0.05Y (in at.%) was prepared by vacuum consumable arc-melting method. Oxidation-resistant silicide coatings on the surfaces of Nb silicide based alloy were prepared by pack cementation technique. The holding temperatures were 1050, 1150 and 1250℃ and the holding time was 2, 5, 10, 15 and 20h respectively. The Structure, phase constituents and compositional distribution in the coatings have been analyzed using SEM, EDS and XRD techniques. The formation mechanism and growth kinetics of the coatings have been discussed and measured. Both of the bare Nb silicide based alloy and coated sample were oxidized in an oxidizing furnace at 1250℃ for 5, 10, 20, 50 and 100h respectively. The Structure, phase constituents and compositional distribution of the oxidation products were analyzed and the oxidation kinetics of the two kinds of specimens were compared with each other. Furthermore, the oxidation mechanism was discussed.It was found that the phase constituent was (Nb, X)Si₂ (X represents Ti, Cr, Hf and Al elements) in the coatings. There were double layers in the coatings, and the outer layer was denser. The major structure in the outer layer was columnar crystals whose growth direction was vertical to the interface between the coating and the substrate, and the microstructure in the inner layer was mainly composed of equiaxed crystals. A transitional layer and diffusion zone formed between the coating and substrate. This kind of composite structure coatings can improve the oxidation- resistance of Nb silicide based alloy. The transitional layer combined tightly with both the coatings and substrate. The phase constituent of the transitional layer was (Nb, X)₅Si₃ and (Nb, X)Si₂, and (Nb, X)₅Si₃ was the major phase. All the coatings prepared at different temperatures and/or for different holding time possessed similar morphologies and structures. The coating growth kinetics respectively at 1050, 1150 and 1250℃ were all fitted well to parabolic laws.The phase constituent in the oxide scales of bare Nb silicide based alloy oxidized at 1250℃ was TiO₂, SiO₂, TiNb₂O₇ or Ti₂Nb₁₀O₂₉. Pure Nb₂O₅ and other oxides of sole Nb were not detected both in the external scale and the internal oxide zone. For the...