Ghadyani Mohammad, Utton Claire, Tsakiropoulos Panos
Department of Materials Science and Engineering, Sir Robert Hadfield Building, The University of Sheffield, Mappin Street, Sheffield S1 3JD, UK.
Materials (Basel). 2019 Jan 10;12(2):222. doi: 10.3390/ma12020222.
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 NbSiTiAl₃Hf and NbSiTiAlHf 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 NbSiTiAlHf was almost double that in NbSiTiAl₃Hf. 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 NbSiTiAl₃Hf was within the range and the parameters VEC and δ of both alloys respectively were outside and within the corresponding ranges. The alloy NbSiTiAlHf exhibited strong correlations between the parameters Δχ, δ and VEC, and the range of values of each parameter was wider compared with the alloy NbSiTiAl₃Hf. 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.
基于铌硅化物的合金将需要某种涂层体系。与基于铌硅化物的合金基体具有化学相容性的形成氧化铝的合金可能是此类体系的组成部分。对金属间化合物合金NbSiTiAl₃Hf和NbSiTiAlHf在铸造、热处理以及800℃和1200℃等温氧化条件下进行了研究，以确定它们是否能形成氧化铝氧化皮。这些合金是使用合金设计方法NICE设计的，要求（i）其微观结构中不存在稳定的固溶体相；（ii）不发生脆化；（iii）形成氧化铝氧化皮。它们的微观结构由硅化物和铝化物组成。两种合金均满足（i）和（ii），并在800℃形成了薄氧化皮。在1200℃时，前一种合金发生了内氧化，在含有钛和/或铝和/或硅的混合氧化物的厚“层状”氧化皮之下形成了与富钛氧化物混合的氧化铝。后一种合金没有发生内氧化，形成了薄的连续且附着良好的α-Al₂O₃氧化皮，该氧化皮在1200℃氧化过程中能够自我修复。两种合金中硅均存在严重的宏观偏析现象，在NbSiTiAlHf中的硅宏观偏析程度几乎是NbSiTiAl₃Hf中的两倍。硅的严重宏观偏析导致前一种合金形成了在800℃和1200℃时均保留的“层状”结构。两种合金均符合高熵合金（HEAs）的“标准定义”。与体心立方固溶体加金属间化合物高熵合金的价带（VEC）、δ和Δχ参数值范围相比，只有合金NbSiTiAl₃Hf的Δχ在该范围内，而两种合金的VEC和δ参数分别超出和处于相应范围。合金NbSiTiAlHf的Δχ、δ和VEC参数之间表现出强相关性与合金NbSiTiAl₃Hf相比，每个参数的值范围更宽。后一种合金仅在Δχ和δ参数之间存在与前一种合金类似的强相关性。
