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Effects of heat treatments on near surface elemental profiles of Fe-15Cr polycrystalline alloy

机译：热处理对Fe-15Cr多晶合金近表面元素轮廓的影响

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摘要

For the development of optimised surface modification of Fe-Cr alloys, the surface chemistry of an Fe-15 at.- percentCr alloy specimen during the initial stage of oxidation was investigated as well as the effects of hydrogen during annealing and following oxidation. Samples were exposed to various heat treatment atmospheres at 800 deg C: annealing in N_2-H_2 gas mixtures, oxidation in air and annealing then oxidation. After the heat treatments, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were employed to characterise the morphology, elemental depth profiles and the chemical states of the elements. In the initial stage of oxidation, the chemistry of the oxide layer on the surface transformed in the following sequence: Fe rich oxide -> Duplex oxide layer (outer Fe rich and inner Cr rich) -> Cr rich mono-phase layer (M_2O_3). In the transformation, the reaction rate for the formation of the Cr rich oxide layer was controlled by the diffusion of cations. Annealing in H_2-containing atmospheres increased the Cr content at the surface. An increase in hydrogen content in the atmosphere further increased both the Cr to Fe ratio in the near surface region, and the thickness of the layer affected by the heat treatment. Selective oxidation of chromium occurred as internal Cr_2O_3 formation, as a function of the Cr content rather than the oxygen partial pressure. Hydrogen facilitated the diffusion of chromium, probably by cleaning of fast diffusion paths. The addition of hydrogen to the annealing atmosphere before the oxidation was beneficial to obtain a Cr rich oxide layer on the surface.
机译：为了开发优化的Fe-Cr合金表面改性方法，研究了Fe-15 at。％Cr合金样品在氧化初期的表面化学性质以及退火和氧化后氢的影响。将样品暴露于800摄氏度的各种热处理气氛中：在N_2-H_2气体混合物中退火，在空气中氧化，然后退火。热处理后，采用扫描电子显微镜（SEM）和X射线光电子能谱（XPS）表征元素的形貌，元素深度分布和化学状态。在氧化的初始阶段，表面上的氧化层的化学性质按以下顺序转换：富铁氧化物->双面氧化物层（富铁内部和富铬内部）->富铬单相层（M_2O_3） 。在转变中，通过阳离子的扩散来控制形成富Cr氧化物层的反应速率。在含H_2的气氛中退火会增加表面的Cr含量。大气中氢含量的增加进一步增加了近表面区域中的Cr / Fe比，以及受热处理影响的层的厚度。铬的选择性氧化是内部Cr_2O_3的形成，它是Cr含量而不是氧分压的函数。氢可能通过清洁快速扩散路径来促进铬的扩散。在氧化之前将氢添加到退火气氛中有利于在表面上获得富Cr的氧化物层。

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