Unocic Kinga A, Chen Yimeng, Shin Dongwon, Pint Bruce A, Marquis Emmanuelle A
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.
CAMECA Instruments, Inc. 5500 Nobel Drive, Madison, WI 53711, USA; Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136, USA.
Micron. 2018 Jun;109:41-52. doi: 10.1016/j.micron.2018.01.011. Epub 2018 Apr 7.
A thermally grown scale formed on a cast NiCrAl model alloy doped with lanthanum, hafnium, and titanium was examined after isothermal exposure at 1100 °C for 100 h in dry flowing O to understand the dopant segregation along scale grain boundaries. The complex scale formed on the alloy surface was composed of two types of substrates: phase-dependent, thin (<250 nm) outer layers and a columnar-grained ∼3.5 μm inner alumina layer. Two types of oxides formed between the inner and outer scale layers: small (3-15 nm) LaO and larger (≤50 nm) HfO oxide precipitates. Nonuniform distributions of the hafnium, lanthanum, and titanium dopants were observed along the inner scale grain boundaries, with hafnium dominating in most of the grain boundaries of α-AlO The concentration of reactive elements (RE) seemed to strongly depend on the grain boundary structure. The level of titanium grain boundary segregation in the inner scale decreased toward the model alloy (substrate), confirming the fast outward diffusion of titanium. Hafnium was also observed at the metal-scale interface and in the γ' (NiAl) phase of the alloy. High-resolution scanning transmission electron microscopy (STEM) confirmed the substitution of REs for aluminum atoms at the scale grain boundaries, consistent with both the semiconducting band structure and the site-blocking models. Both STEM and atom probe tomography allowed quantification of REs along the scale grain boundaries across the scale thickness. Analysis of the scale morphology after isothermal exposure in flowing oxygen revealed a myriad of new precipitate phases, RE segregation dependence on grain boundary type, and atomic arrangement along scale grain boundaries, which is expected to influence the scale growth rate, stability, and mechanical properties.
对一种掺杂了镧、铪和钛的铸造NiCrAl模型合金上热生长的氧化皮进行了研究。该合金在1100 °C的干燥流动氧气中进行100 h等温暴露后，以了解掺杂剂沿氧化皮晶界的偏析情况。合金表面形成的复杂氧化皮由两种类型的基底组成：与相有关的薄（<250 nm）外层和柱状晶的约3.5 μm厚的内层氧化铝层。在内层和外层氧化皮层之间形成了两种类型的氧化物：小的（3 - 15 nm）LaO和较大的（≤50 nm）HfO氧化物沉淀。观察到铪、镧和钛掺杂剂沿内层氧化皮晶界分布不均匀，在α - AlO的大多数晶界中铪占主导。活性元素（RE）的浓度似乎强烈依赖于晶界结构。内层氧化皮中钛晶界偏析水平朝着模型合金（基底）方向降低，证实了钛的快速向外扩散。在金属 - 氧化皮界面以及合金的γ'（NiAl）相中也观察到了铪。高分辨率扫描透射电子显微镜（STEM）证实了活性元素在氧化皮晶界处替代铝原子，这与半导体能带结构和位点阻塞模型均相符。STEM和原子探针断层扫描都能够对沿氧化皮厚度方向的晶界处的活性元素进行定量分析。在流动氧气中进行等温暴露后对氧化皮形态的分析揭示了大量新的沉淀相、活性元素偏析对晶界类型的依赖性以及沿氧化皮晶界的原子排列情况，这些预计会影响氧化皮的生长速率、稳定性和机械性能。
