二元合金选择性晶间氧化模型
Modeling Selective Intergranular Oxidation of Binary Alloys
摘要：
在水热条件下合金的晶间侵蚀是一个复杂的问题，它取决于金属和氧通过固态和通道状途径向前进的氧化前沿的传输动力学。实验揭示了镍基二元合金的氧化前沿取决于微量元素的晶间攻击和微量元素耗尽距离非常不同的速率。例如，对于Ni-5Cr二元合金，在晶界裂纹尖端之前高达9微米处记录了显著的Cr耗尽，与Ni-5Al的相对中等的Al耗尽（约100 nm）形成对比。我们提出了一个数学动力学模型，适应瓦格纳的厚膜生长模型的二元合金的晶间攻击。从文献中估计了元素O、Ni、Cr和Al在块体合金中和沿晶界的输运系数。对于平面表面氧化，可以从其中次要元素的氧化物变得主导于主要元素的模型确定次要元素的临界浓度。这种简单的晶界氧化的通用模型可以预测的氧化渗透速度和微量元素耗尽的距离前进的前锋，是可比的实验数据。与Ni-5Al中的局部Al耗尽相比，Ni-5Cr中的Cr耗尽的显著距离可以由该模型解释，这是由于相对于Al，Cr沿晶界的相对较快的扩散和在体晶粒中的较慢的扩散的组合。
英文摘要：
Intergranular attack of alloys under hydrothermal conditions is a complex problem that depends on metal and oxygen transport kinetics via solid-state and channel-like pathways to an advancing oxidation front. Experiments reveal very different rates of intergranular attack and minor element depletion distances ahead of the oxidation front for nickel-based binary alloys depending on the minor element. For example, a significant Cr depletion up to 9 microns ahead of grain boundary crack tips were documented for Ni-5Cr binary alloy, in contrast to relatively moderate Al depletion for Ni-5Al (~100s of nm). We present a mathematical kinetics model that adapts Wagner's model for thick film growth to intergranular attack of binary alloys. The transport coefficients of elements O, Ni, Cr, and Al in bulk alloys and along grain boundaries were estimated from the literature. For planar surface oxidation, a critical concentration of the minor element can be determined from the model where the oxide of minor element becomes dominant over the major element. This generic model for simple grain boundary oxidation can predict oxidation penetration velocities and minor element depletion distances ahead of the advancing front that are comparable to experimental data. The significant distance of depletion of Cr in Ni-5Cr in contrast to the localized Al depletion in Ni-5Al can be explained by the model due to the combination of the relatively faster diffusion of Cr along the grain boundary and slower diffusion in bulk grains, relative to Al.
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