Structure of Al-modified silicide coatings on a Nb-based ultrahightemperature alloy by pack cementation process

Al-modified silicide coatings were prepared by pack cementation process on a Nb-based ultrahigh temperature alloy. The structure, phase constituents and formation process of the coatings were studied. The coatings were prepared in a two-stage process: Firstly pack siliconized at 1150°C for 4 h and then pack aluminized at 800-1000°C for 4 h, respectively. The main phase of the pack siliconized coating is (Nb, X) Si₂ (X represents Ti, Cr and Hf). The average content of Al in the (Nb, X) Si₂ layer increases with increase of aluminizing temperature, and the maximam is about 10.84at%. When the aluminizing temperature is higher than 860°C, small Nb₃Si₅Al₂ particles are found to exist in the (Nb, X) Si₂ layer, furthermore an niobium-aluminide layer forms between the pack siliconized layers and the matrix.