Microstructures and Isothermal Oxidation of the Alumina Scale Forming Nb1.7Si2.4Ti2.4Al3Hf0.5 and Nb1.3Si2.4Ti2.4Al3.5Hf0.4 Alloys

Abstract

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 Nb1.7Si2.4Ti2.4Al3Hf0.5 and Nb1.3Si2.4Ti2.4Al3.5Hf0.4 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-Al2O3 scale that was able to repair itself during oxidation at 1200 °C. In both alloys there was severe macrosegregation of Si, which in Nb1.3Si2.4Ti2.4Al3.5Hf0.4 was almost double that in Nb1.7Si2.4Ti2.4Al3Hf0.5. 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 Nb1.7Si2.4Ti2.4Al3Hf0.5 was within the range and the parameters VEC and δ of both alloys respectively were outside and within the corresponding ranges. The alloy Nb1.3Si2.4Ti2.4Al3.5Hf0.4 exhibited strong correlations between the parameters Δχ, δ and VEC, and the range of values of each parameter was wider compared with the alloy Nb1.7Si2.4Ti2.4Al3Hf0.5. 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.

A Study of the Effect of 5 at.% Sn on the Micro-Structure and Isothermal Oxidation at 800 and 1200 °C of Nb-24Ti-18Si Based Alloys with Al and/or Cr Additions

“…It is suggested that the formation of these two areas was associated with the strong macrosegregation of Si and Ti in the alloy ZX6 (Table 2). It is worth exploring how areas A and B were formed in the alloy ZX6, because similar transitions have been reported in an alumina scale forming complex concentrated alloy (or high entropy alloy) of the Nb-Ti-Si-Al-Hf system that was prepared using arc melting [25].…”

“…These relationships are used in the alloy design methodology NICE [14] to design new Nb-silicide based alloys. NICE is also used to design complex concentrated alloys (CCAs) or high entropy alloys [25]. NICE shows that, for oxidation resistance, the trends of VEC and δ are opposite and that alloy design should aim to decrease the former and increase the latter.…”

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 presence of Ti 2 Nb 10 O 29 and TiNb 2 O 7 in the scale that formed on Nb-39Ti (at.%) (Nb-25 wt.% Ti) after extended (≥100 h) oxidation between 750 and 780 • C was reported by Felten [70]. The CTE values of oxides in scales of Nb-silicide-based alloys were given in Reference [71]. The Ti 2 Nb 10 O 29 has a negative CTE (−3.1 × 10 −7 / • C) compared with the CTE values of TiNb 2 O 7 (4.5 × 10 −7 / • C) and AlNbO 4 (39.7 × 10 −7 / • C) [72].…”

On the Microstructure and Properties of Nb-18Si-6Mo-5Al-5Cr-2.5W-1Hf Nb-Silicide Based Alloys with Ge, Sn and Ti Additions (at.%)

“…In the introduction we discussed that RMICs, RCCAs and RHEAs intended for application in aero engines would require environmental coatings (ECs) of the BC/TGO/TC type [ 36 , 79 ]. BC Nb-Ti-Si-Al-Hf HEAs forming αAl 2 O 3 TGO [ 80 , 81 ] could be suitable and compatible with Nb-Mo-W-Ti-Cr-Hf-Al-Ge-Si-Sn UHTMs. A good starting point for the design of HEA and/or “conventional” alloys for BC application in ECs could be the (Al/Si) alloy versus [Nb/(Ti + Hf)] alloy maps in Figure 13 in [ 81 ], and the Δχ versus VEC, δ versus VEC and Δχ versus δ maps in Figure 13 in [ 82 ].…”