Microstructures and Isothermal Oxidation of the Alumina Scale Forming Nb(1.7)Si(2.4)Ti(2.4)Al(3)Hf(0.5) and Nb(1.3)Si(2.4)Ti(2.4)Al(3.5)Hf(0.4) Alloys

Nb–silicide based alloy will require some kind of coating system. Alumina forming alloys that are chemically compatible with the Nb–silicide based alloy substrate could be components of such systems. The intermetallic alloys Nb(1.7)Si(2.4)Ti(2.4)Al(3)Hf(0.5) and Nb(1.3)Si(2.4)Ti(2.4)Al(3.5)Hf(0.4) were studied in the cast, heat treated and isothermally oxidised conditions at 800 and 1200 °C to find out if they are alumina scale formers. The alloys were designed using the alloy design methodology NICE and were required (i) not to have stable solid solution phase in their microstructures; (ii) not to pest and (iii) to form alumina scale. Their microstructures consisted of silicides and aluminides. Both alloys satisfied (i) and (ii) and formed thin scales at 800 °C. At 1200 °C the former alloy suffered from internal oxidation and formed alumina intermixed with Ti rich oxide beneath a thick “layered” scale of mixed oxides that contained Ti and/or Al and/or Si. There was no internal oxidation in the latter alloy that formed a thin continuous well adhering α-Al(2)O(3) scale that was able to repair itself during oxidation at 1200 °C. In both alloys there was severe macrosegregation of Si, which in Nb(1.3)Si(2.4)Ti(2.4)Al(3.5)Hf(0.4) was almost double that in Nb(1.7)Si(2.4)Ti(2.4)Al(3)Hf(0.5). The severe macrosegregation of Si contributed to the formation of a “layered” structure in the former alloy that was retained at 800 and 1200 °C. Both alloys met the “standard definition” of High Entropy Alloys (HEAs). Compared with the range of values of the parameters valence band (VEC), δ and Δχ of bcc solid solution plus intermetallic(s) HEAs, only the Δχ of the alloy Nb(1.7)Si(2.4)Ti(2.4)Al(3)Hf(0.5) was within the range and the parameters VEC and δ of both alloys respectively were outside and within the corresponding ranges. The alloy Nb(1.3)Si(2.4)Ti(2.4)Al(3.5)Hf(0.4) exhibited strong correlations between the parameters Δχ, δ and VEC, and the range of values of each parameter was wider compared with the alloy Nb(1.7)Si(2.4)Ti(2.4)Al(3)Hf(0.5). There was a strong correlation only between the parameters Δχ and δ of the latter alloy that was similar to that of the former alloy.