Hernández-Negrete Ofelia, 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 Aug 8;12(16):2531. doi: 10.3390/ma12162531.
Nb-silicide-based alloys have the potential to replace Ni-based superalloys in future aero engines to enable the latter to meet environmental and performance targets. These new alloys, like the Ni-based superalloys that are currently used, will require environmental protection with a coating system that should be chemically compatible with the substrate. A challenge for alloy development is to discover αAlO scale forming coating alloys and in particular to find out whether such alloys could be "compatible" with other coating alloys for environmental coating systems for the Nb-silicide-based alloys. This paper focuses on these challenges. The alloy Al-25.5Nb-6Cr-0.5Hf (at %) was studied in the cast and heat-treated (1400 °C) conditions and after isothermal oxidation for 100 h in air at 800, 1200 and 1300 °C. The microstructure consisted of the alloyed NbAl and C14-NbCr compounds, both of which were stable at least up to 1400 °C, a eutectic of the two compounds and very small volume fractions of (Cr,Al,Nb) and HfO. The prior eutectic microstructure was stable at T ≤ 1200 °C and the solid solution was not stable at T < 1200 °C. At 800 °C the alloy did not pest, but exhibited external and internal oxidation, with AlNbO, CrNbAlO, and αAlO in the former and deeper oxidation along the NbAl/Laves phase boundaries in the latter At 1200 and 1300 °C there was only external oxidation and the scale consisted of two layers, the outer was (Al,Cr)NbO intermixed with αAlO and the inner was continuous αAlO. At all three oxidation temperatures, no NbAl was observed below the alloy/scale interface and Hf acted as a reactive element forming HfO that enhanced the adhesion of the scale. The alloy exhibited good correlations with αAlO scale forming silicide and silicide + aluminide intermetallic alloys in maps of the parameters (related to atomic size), (related to electronegativity), and VEC (number of valence electrons per atom filled into the valence band) that should assist the design of bond coats that do not pest and form αAlO in their scales.
基于铌硅化物的合金有潜力在未来航空发动机中取代镍基高温合金，以使后者能够满足环境和性能目标。这些新型合金，与目前使用的镍基高温合金一样，需要用与基体化学相容的涂层系统进行环境保护。合金开发面临的一个挑战是发现能形成αAlO 氧化膜的涂层合金，特别是要弄清楚这类合金是否能与用于基于铌硅化物合金的环境涂层系统的其他涂层合金“相容”。本文聚焦于这些挑战。对合金 Al-25.5Nb-6Cr-0.5Hf（原子百分比）在铸造和热处理（1400°C）条件下以及在 800、1200 和 1300°C 的空气中等温氧化 100 小时后的情况进行了研究。微观结构由合金化的 NbAl 和 C14-NbCr 化合物组成，这两种化合物至少在 1400°C 以下都是稳定的，还有这两种化合物的共晶以及极少量体积分数的 (Cr,Al,Nb) 和 HfO。原始共晶微观结构在 T≤1200°C 时是稳定的，而固溶体在 T<1200°C 时不稳定。在 800°C 时，合金没有发生脆化，但出现了外部和内部氧化，前者中有 AlNbO、CrNbAlO 和 αAlO，后者中沿着 NbAl/拉夫斯相界有更深的氧化。在 1200 和 1300°C 时只有外部氧化，氧化膜由两层组成，外层是 (Al,Cr)NbO 与 αAlO 混合，内层是连续的 αAlO。在所有三个氧化温度下，在合金/氧化膜界面以下均未观察到 NbAl，Hf 作为活性元素形成 HfO，增强了氧化膜的附着力。在与原子尺寸相关、与电负性相关以及价电子浓度（填充到价带中的每个原子的价电子数）等参数的图谱中，该合金与能形成 αAlO 氧化膜的硅化物以及硅化物 + 铝化物金属间化合物合金表现出良好的相关性，这有助于设计不发生脆化且在其氧化膜中形成 αAlO 的粘结层。
