Oxidation behaviours of Nb–22Ti–15Si–2Al–2Hf–2V–(2, 14)Cr alloys with Al and Y modified silicide coatings prepared by pack cementation

Al and Y modified silicide coatings on the Nb–15Si–22Ti–(2,14)Cr–2Al–2Hf–2V alloys (where the alloy with 2 at% Cr or 14 at% Cr is hereafter referred to as 2Cr and 14Cr alloy, respectively) was prepared by pack cementation. The microstructural evolution and the oxidation behaviours of the coating 2Cr and 14Cr samples at 1250 °C were studied. The 2Cr alloy consists of Nb solid solution (NbSS) and α-Nb5Si3 silicide, while the Laves C15–Cr2Nb phase arised in the 14Cr alloy. The coating structure of the coating 2Cr sample contained the outer (Nb, X)Si2+(Nb, X)5Si3 layer, the middle (Nb, X)5Si3 layer and the inner undeveloped intermetallic (Nb,Ti)3(Al,X) layer; the structure of the coating 14Cr sample consisted of the outer single (Nb, X)Si2 layer, the middle (Nb, X)5Si3 layer, the transition (Nb,Ti) (Cr,Al) layer and the inner (Cr, Al)2(Nb,Ti) layer. The coating 14Cr sample exhibited better oxidation resistance than the coating 2Cr sample. With an outer single (Nb, X)Si2 layer, a compact oxide scale consisting of SiO2 and TiO2 formed on the coating 14Cr sample, which can efficiently prevent the substrate from oxidising. For the coating 2Cr sample with an outer (Nb, X)Si2+(Nb, X)5Si3 layer, the oxide scale of the SiO2, TiO2, Nb2O5 and CrNbO4 mixture generated, and the scale spalled out from the surface of the sample, resulting in disastrous failure.