Xu Zhen, 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). 2018 Sep 25;11(10):1826. doi: 10.3390/ma11101826.
Alloying with Al, Cr, Sn, and Ti significantly improves the oxidation of Nb silicide-based alloys at intermediate and high temperatures. There is no agreement about what the concentration of Sn in the alloys should be. It has been suggested that with Sn ≤ 3 at.% the oxidation is improved and formation of the brittle A15-Nb₃Sn compound is suppressed. Definite improvements in oxidation behaviour have been observed with 5 at.% Sn or even higher concentrations, up to 8 at.% Sn. The research reported in this paper is about three model alloys with low Sn concentration and nominal compositions Nb-24Ti-18Si-5Cr-2Sn (ZX3), Nb-24Ti-18Si-5Al-2Sn (ZX5), and Nb-24Ti-18Si-5Al-5Cr-2Sn (ZX7) that were studied to understand the effect of the 2 at.% Sn addition on as-cast and heat-treated microstructures and isothermal oxidation in air at 800 and 1200 °C for 100 h. There was macrosegregation of Si and Ti in the alloys ZX3 and ZX5 and only of Si in the alloy ZX7. The Nb was stable in all alloys. Tin and Ti exhibited opposite partitioning behaviour in the Nb. The βNb₅Si₃ was the primary phase in all three cast alloys and had partially transformed to αNb₅Si₃ in the alloy ZX3. Aluminium in synergy with Sn increased the sluggishness of the βNb₅Si₃ to αNb₅Si₃ transformation during solidification. After the heat treatment the transformation of βNb₅Si₃ to αNb₅Si₃ had been completed in all three alloys. Fine precipitates were observed inside some αNb₅Si₃ grains in the alloys ZX5 and ZX7. In the latter alloys the A15-Nb₃X (X = Al, Si, and Sn) formed after the heat treatment, i.e., the synergy of Al and Sn promoted the stability of A15-Nb₃X intermetallic in these Nb-silicide-based alloys even at this low Sn concentration. A Nb + Nb₅Si₃ eutectic formed in all three alloys and there was evidence of anomalous eutectic in the parts of the alloys ZX3 and ZX7 that had solidified under high cooling rate and/or high melt undercooling. A very fine ternary Nb + Nb₅Si₃ + NbCr₂ eutectic was also observed in parts of the alloy ZX3 that had solidified under high cooling rate. At 800 °C none of the alloys suffered from catastrophic pest oxidation; ZX7 had a smaller oxidation rate constant. A thin Sn-rich layer formed continuously between the scale and Nb in the alloys ZX3 and ZX5. At 1200 °C the scales formed on all three alloys spalled off, the alloys exhibited parabolic oxidation in the early stages followed by linear oxidation; the alloy ZX5 gave the smallest rate constant values. A thicker continuous Sn-rich zone formed between the scale and substrate in all three alloys. This Sn-rich zone was noticeably thicker near the corners of the specimen of the alloy ZX7 and continuous around the whole specimen. The Nb₃Sn, Nb₅Sn₂Si, and NbSn₂ compounds were observed in the Sn-rich zone. At both temperatures the scales formed on all three alloys consisted of Nb-rich and Nb and Si-rich oxides, and Ti-rich oxide also was formed in the scales of the alloys ZX3 and ZX7 at 1200 °C. The formation of a Sn-rich layer/zone did not prevent the contamination of the bulk of the specimens by oxygen, as both Nb and Nb₅Si₃ were contaminated by oxygen, the former more severely than the latter.
与铝、铬、锡和钛合金化可显著提高铌硅化物基合金在中高温下的抗氧化性能。关于合金中锡的浓度应该是多少，目前尚无定论。有人认为，当锡含量≤3原子百分比时，氧化性能得到改善，并且脆性A15-Nb₃Sn化合物的形成受到抑制。当锡含量为5原子百分比甚至更高（高达8原子百分比）时，已观察到氧化行为有明显改善。本文报道的研究是关于三种低锡浓度的模型合金，其名义成分为Nb-24Ti-18Si-5Cr-2Sn（ZX3）、Nb-24Ti-18Si-5Al-2Sn（ZX5）和Nb-24Ti-18Si-5Al-5Cr-2Sn（ZX7），研究了添加2原子百分比的锡对铸态和热处理微观结构以及在800℃和1200℃空气中等温氧化100小时的影响。在ZX3和ZX5合金中存在硅和钛的宏观偏析，而在ZX7合金中仅存在硅的宏观偏析。铌在所有合金中都很稳定。锡和钛在铌中表现出相反的偏析行为。βNb₅Si₃是所有三种铸造合金中的主要相，并且在ZX3合金中已部分转变为αNb₅Si₃。铝与锡协同作用增加了凝固过程中βNb₅Si₃向αNb₅Si₃转变的迟缓性。热处理后，所有三种合金中βNb₅Si₃向αNb₅Si₃的转变均已完成。在ZX5和ZX7合金的一些αNb₅Si₃晶粒内部观察到细小析出物。在后者合金中，热处理后形成了A15-Nb₃X（X = 铝、硅和锡），即铝和锡的协同作用促进了这些铌硅化物基合金中A15-Nb₃X金属间化合物即使在这种低锡浓度下的稳定性。所有三种合金中都形成了Nb + Nb₅Si₃共晶，并且在ZX3和ZX7合金中以高冷却速率凝固和/或高熔体过冷的部分存在异常共晶的证据。在以高冷却速率凝固的ZX3合金部分中还观察到非常细小的三元Nb + Nb₅Si₃ + NbCr₂共晶。在800℃时，没有一种合金遭受灾难性的虫蛀氧化；ZX7具有较小的氧化速率常数。在ZX3和ZX5合金中，在氧化皮和铌之间连续形成了一层富锡层。在1200℃时，所有三种合金上形成的氧化皮都剥落了，合金在早期表现出抛物线型氧化，随后是线性氧化；ZX5合金给出的速率常数最小。在所有三种合金的氧化皮和基体之间形成了一个更厚的连续富锡区。在ZX7合金试样的拐角附近，这个富锡区明显更厚，并且围绕整个试样连续分布。在富锡区中观察到了Nb₃Sn、Nb₅Sn₂Si和NbSn₂化合物。在这两个温度下，所有三种合金上形成的氧化皮都由富铌氧化物、铌和富硅氧化物组成，并且在1200℃时，在ZX3和ZX7合金的氧化皮中还形成了富钛氧化物。富锡层/区的形成并不能防止氧对试样整体的污染，因为铌和Nb₅Si₃都被氧污染了，前者比后者更严重。
