Using both ex situ metallographic imaging and in situ X-ray tomographic microscopy, we investigate the kinetics of Al- and Ni-interdiffusion during homogenization at 825–1100 °C for surface-aluminized, 50 μm diameter Ni wires with 0, 10 or 20 wt%Cr. Kirkendall pores, which are created due to imbalanced diffusion of atomic species, are not observed at any of the homogenization temperatures in the Cr-free Ni–Al wires, which equilibrate to Ni-rich β-NiAl. By contrast, during homogenization of the aluminized Ni–10Cr and Ni–20Cr wires to β-NiAl(Cr) at 1000 and 1100 °C, numerous Kirkendall pores are created within the wire volume, indicating that the addition of Cr significantly increases the imbalance between the Ni and Al diffusivities. These pores eventually coalesce into a single cavity, with crescent-shape cross-sections and high aspect ratio aligned with the axis of the wires, so that a tubular β-NiAl(Cr) structure is formed. Tomography shows that the Ni-rich β-NiAl(Cr) reaction layer grows radially within the Ni–10Cr and Ni–20Cr wires annealed in situ at 825, 900, and 1000 °C according to a parabolic law. The growth kinetics of this layer increase slightly with increasing Cr content and obey an Arrhenius relationship, from which an activation energy of ~200 kJ/mol is calculated, in good agreement with literature values for interdiffusion in binary NiAl. The two methods, ex situ metallography and in situ X-ray tomography, are complementary. While tomography very rapidly acquires numerous cross-sectional images showing phase contrast on a single wire, thus replacing interrupted annealing and destructive imaging of multiple samples, metallography has higher spatial resolution and can identify additional phases. In the present case, metallography revealed that α-Cr precipitates form during homogenization of the aluminized Ni–Cr wires due to the limited solubility of Cr in β-NiAl; upon full homogenization, these precipitates re-dissolved in the aluminized Ni–10Cr wires, but remained stable in the Ni–20Cr wires.

同时使用非原位金相成像和原位X射线断层扫描显微镜，我们研究了在825–1100°C下均质化过程中Al，Ni相互扩散的动力学，这些表面渗铝的直径为50μm，含0、10或20 wt％Cr的镍丝。在无铬的Ni-Al丝的任何均质温度下均未观察到由于原子种类扩散失衡而产生的Kirkendall孔，该温度与富Ni的β-NiAl平衡。相比之下，在1000和1100°C的条件下，将镀铝的Ni-10Cr和Ni-20Cr焊丝均匀化为β-NiAl（Cr）时，在焊丝体积内会形成许多Kirkendall孔，这表明Cr的添加会显着增加不平衡性在镍和铝的扩散率之间。这些孔最终聚结成一个单一的腔体，该腔体具有新月形的横截面和与导线轴对齐的高长宽比，从而形成管状的β-NiAl（Cr）结构。断层扫描显示，富Ni的β-NiAl（Cr）反应层在退火的Ni-10Cr和Ni-20Cr焊丝内呈放射状生长根据抛物线定律在825、900和1000°C的温度下进行原位处理。该层的生长动力学随Cr含量的增加而略有增加，并遵守Arrhenius关系，由此可计算出约200 kJ / mol的活化能，与二元NiAl中相互扩散的文献值非常吻合。两种方法，异位金相和原位X线断层扫描，是相辅相成的。层析成像非常迅速地获取了单条线上显示相衬的大量横截面图像，从而取代了多个样品的间断退火和破坏性成像，而金相学则具有更高的空间分辨率，并且可以识别其他相。在本例中，金相分析表明，由于铬在β-NiAl中的溶解度有限，在镀铝的镍-铬丝均质化过程中会形成α-Cr析出物。经过完全均质化后，这些沉淀物重新溶解在镀铝的Ni-10Cr焊丝中，但在Ni-20Cr焊丝中保持稳定。