Improvement in oxidation resistance of silicide coating on an Nb–Ti–Si based ultrahigh temperature alloy by second aluminizing treatment

•A second aluminizing treatment was performed to modify the siliconized coating.•The structure change of the coatings during the two-step process was studied.•The two-step coating possessed much better oxidation resistance at 1250 °C.•Effect of second aluminizing on the oxidation behavior of the coatings was revealed.

Modified silicide coating was prepared on an Nb–Ti–Si based alloy by a two-step pack cementation process including first siliconizing and then aluminizing. The two-step coating is composed of an Al-rich outermost layer, a (Nb,X)Si2 outer layer and a (Ti,Nb)5Si4 inner layer. The coating possesses an enhanced oxidation resistance at 1250 °C due to the presence of the Al-rich outermost layer. The mass gain of the coating after oxidation at 1250 °C for 100 h is 3.7 mg/cm2, and a duplex scale consisting of an Al2O3 outer layer and a SiO2-matrix inner layer forms.