Preparation and oxidation resistance of silicide/aluminide composite coatings on an Nb–Ti–Si based alloy

• Silicide/aluminide composite coatings were prepared on Nb–Ti–Si based alloy.
• A two-step process including siliconizing firstly and then aluminizing was applied.
• Al moves across the as-siliconized coating and an Al3(Nb,X) layer forms beneath it.
• A dense and double layer scale forms on the coating after oxidation at 1250 °C/50 h.

Silicide/aluminide composite coatings were prepared on an Nb–Ti–Si based alloy by a two-step pack cementation process including siliconizing firstly and then aluminizing. The siliconized coating is composed of a thick (Nb,X)Si2 outer layer and a thin (Ti,Nb)5Si4 inner layer. During the aluminizing process, the original (Nb,X)Si2 outer layer retains and Al moves through it to form an Al3(Nb,X) layer beneath it. Besides, a thin Al3(Nb,X) outermost layer also forms above the (Nb,X)Si2 layer. This composite coating possesses good oxidation resistance at 1250 °C within 50 h, and a double-layered scale with an Al2O3 outer layer and a SiO2-matrix inner layer forms.