Nelson Jack, 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). 2018 Sep 1;11(9):1579. doi: 10.3390/ma11091579.
In Nb-silicide based alloys Al, Cr, Hf, and Ti additions are crucial for achieving balance of properties. It is not known how the simultaneous addition of Hf with Al and Ti, or Hf with Al, Cr, and Ti affects macrosegregation, and how the alloying affects hardness, Young's modulus and bulk alloy oxidation, and contamination of the solid solution Nb and the Nb₅Si₃ compound by oxygen. Two alloys with nominal compositions (at.%) Nb-24Ti-18Si-5Al-5Hf (alloy NbSiTiHf-5Al) and Nb-24Ti-18Si-5Al-5Cr-5Hf (alloy NbSiTiHf-5Al-5Cr) were studied in the as-cast and heat-treated conditions and after isothermal oxidation at 800 and 1200 °C and were compared with similar alloys without Hf. In both alloys there was macrosegregation of Si and Ti, which was more severe in NbSiTiHf-5Al. Both alloys formed Nb+βNb₅Si₃ eutectic. The Nb was stable and its Al and Cr concentrations increased with increasing Ti concentration. In both conditions the βNb₅Si₃ was observed in the alloys NbSiTiHf-5Al and NbSiTiHf-5Al-5Cr, and the γNb₅Si₃ only in the alloy NbSiTiHf-5Al. In both heat-treated alloys, separate Hf-rich Nb₅Si₃ grains were formed. The Si and Al concentrations in Nb₅Si₃ respectively decreased and increased with increasing Ti concentration. Al and Cr had a stronger hardening effect in the Nb than Al, Cr, and Hf. Al, Cr, and Ti had a stronger negative effect on the Young's modulus of the Nb compared with Al, Cr, Hf, and Ti. When Nb was substituted by Ti, Cr, and Hf, and Si by Al in the βNb₅Si₃, the Young's modulus was reduced compared with the unalloyed silicide. At 800 °C both alloys did not exhibit catastrophic pest-oxidation after 100 h. The Nb and Nb₅Si₃ were contaminated by oxygen in both alloys, the former more severely. At 1200 °C the scales spalled-off, more severely in the alloy NbSiTiHf-5Al, where substrate that was heavily contaminated by oxygen below the scale also spalled-off. In both alloys the contamination of Nb₅Si₃ and Nb by oxygen was more severe compared with 800 °C, but the silicides were not contaminated by oxygen in their bulk. The Nb was not contaminated by oxygen only in the bulk of the alloy NbSiTiHf-5Al-5Cr.
在基于铌硅化物的合金中，添加铝、铬、铪和钛对于实现性能平衡至关重要。目前尚不清楚同时添加铪与铝和钛，或者铪与铝、铬和钛如何影响宏观偏析，以及合金化如何影响硬度、杨氏模量和块状合金氧化，以及氧对固溶体铌和Nb₅Si₃化合物的污染。研究了两种名义成分（原子百分比）的合金，即Nb-24Ti-18Si-5Al-5Hf（合金NbSiTiHf-5Al）和Nb-24Ti-18Si-5Al-5Cr-5Hf（合金NbSiTiHf-5Al-5Cr），研究了它们的铸态、热处理状态以及在800和1200℃等温氧化后的情况，并与不含铪的类似合金进行了比较。两种合金中都存在硅和钛的宏观偏析，在NbSiTiHf-5Al中更为严重。两种合金都形成了Nb+βNb₅Si₃共晶。铌是稳定的，其铝和铬浓度随着钛浓度的增加而增加。在两种条件下，在合金NbSiTiHf-5Al和NbSiTiHf-5Al-5Cr中都观察到了βNb₅Si₃，而γNb₅Si₃仅在合金NbSiTiHf-5Al中观察到。在两种热处理合金中，都形成了单独的富铪Nb₅Si₃晶粒。Nb₅Si₃中的硅和铝浓度分别随着钛浓度的增加而降低和增加。铝和铬在铌中的硬化作用比铝、铬和铪更强。与铝、铬、铪和钛相比，铝、铬和钛对铌的杨氏模量有更强的负面影响。当在βNb₅Si₃中用钛、铬和铪替代铌，用铝替代硅时，与未合金化的硅化物相比，杨氏模量降低。在800℃时，两种合金在100小时后都没有出现灾难性的虫蛀氧化。两种合金中的铌和Nb₅Si₃都被氧污染，前者更严重。在1200℃时，氧化皮剥落，在合金NbSiTiHf-5Al中更严重，在氧化皮下方被氧严重污染的基体也剥落。与800℃相比，两种合金中Nb₅Si₃和铌被氧的污染更严重，但硅化物的整体未被氧污染。仅在合金NbSiTiHf-5Al-5Cr的块状中铌未被氧污染。
