On the Microstructure and Isothermal Oxidation at 800 and 1200 °C of the Nb-24Ti-18Si-5Al-5Cr-5Ge-5Sn (at.%) Silicide-Based Alloy

The research presented in this paper aspired to understand how the simultaneous addition of Ge and Sn in an Hf-free Nb-silicide-based alloy affected its oxidation resistance. Results are presented for the Nb-24Ti-18Si-5Al-5Cr-5Ge-5Sn alloy (at.%) which was studied in the as-cast and heat-treated (1400 °C/100 h) conditions and after isothermal oxidation in air at 800 and 1200 °C. There was macrosegregation in the cast alloy, in which the Nb(ss) formed at a low volume fraction and was not stable after heat treatment at 1400 °C. The βNb(5)Si(3), A15-Nb(3)Sn, and C14-NbCr(2) were stable phases. The alloy did not undergo pest oxidation at 800 °C, and there was no spallation of its scale at 1200 °C. There was enrichment in Ge and Sn in the substrate below the scale/substrate interface, where the compounds Nb(3)Sn, Nb(5)Sn(2)Si, (Ti,Nb)(6)Sn(5), and Nb(5)Ge(3) were formed. After the oxidation at 1200 °C, the solid solution in the bulk of the alloy was very Ti-rich (Ti,Nb)(ss). Improvement of oxidation resistance at both temperatures was accompanied by a decrease and increase, respectively, of the alloy parameters VEC (valence electron concentration) and δ, in agreement with the alloy design methodology NICE (Niobium Intermetallic Composite Elaboration). The elimination of scale spallation at 1200 °C was attributed (a) to the formation of Ti-rich (Ti,Nb)(ss) solid solution and (Ti,Nb)(6)Sn(5), respectively, in the bulk and below the scale, (b) to the low concentration of Cr in the scale, (c) to the absence of GeO(2) in the scale, (d) to the formation of αAl(2)O(3) in the scale, and (e) to the presence (i) of Nb(5)Ge(3) below the scale/substrate interface and (ii) of oxides in the scale, namely, SiO(2), Al(2)O(3), TiO(2), and SnO(2), and Ti(2)Nb(10)O(29),TiNb(2)O(7), and AlNbO(4), respectively, with a range of intrinsic thermal shock resistances and coefficient of thermal expansion (CTE) values that reduced stresses in the scale and the substrate below it.