Mn+ 1AXn（MAX）相属于三元碳化物和氮化物结构，其中M为前过渡金属元素，A为13-16主族元素，X为C或N元素。和传统陶瓷不同的是, 可加工的MAX 相具有独特的物理化学性质，例如低密度，高强度，出色的抗热冲击和耐损伤性能。MAX相由于其高温稳定性，使其适用于极端环境温度中的结构应用，譬如核电和航空航天推进系统。通过调整MAX晶体结构的成分，可以进一步控制这些化合物的化学、机械、磁性和热学性质。然而，在高温和氧化条件下，大多数MAX相都会经历不利的自我维持氧化反应，从而导致机械完整性的破坏。因此，评估MAX相的氧化行为对于将其进一步发展为高温结构或涂层材料至关重要。由于氧化过程非常复杂且计算模型代价大，目前尚没有一种能够快速评估任意MAX相化合物的氧化物相稳定性的计算方法。
来自美国德州农工大学材料科学与工程系的P. Singh和R. Arroyave教授团队发展了一种机器学习驱动的高通量方法，用于快速评估M2AX相的稳定性和氧反应性。这个提出的高通量方案能够快速评估大合金空间的氧化稳定性，并减少设计时选择合金的时间和成本，这种方法比使用DFT常规方法快了几个数量级。
作者们用机器学习驱动高通量范式，快速评估了211种MAX相M2AX的稳定性和氧反应性。所提出的方案通过结合基于机器学习模型的独立筛选的稀疏算法和巨正则线性程序设计，用以评估MAX相在氧化过程中与温度相关的吉布斯自由能、反应产物以及元素的化学活性。通过充分评估Ti2AlC的组成元素对于氧气的热力学稳定性和化学活性，以了解其高温氧化行为。该预测与在Ti2AlC上进行的氧化实验非常吻合。不仅如此，还揭示了在实验上无法合成Ti2SiC的亚稳态是由于竞争相具有更高的稳定性。对于所提方法的一般性，作者们对Cr2AlC的氧化机理作了分析讨论。对氧化行为的了解将有助于更有效地设计和加速发现具有在高温氧化环境中保持性能的MAX相。
该文近期发表于npj Computational Materials 7: 6 (2021)，英文标题与摘要如下，点击https://www.nature.com/articles/s41524-020-00464-7可以自由获取论文PDF。
High-throughput reaction engineering to assess the oxidation stability of MAX phases
D. Sauceda, P. Singh, A. R. Falkowski, Y. Chen, T. Doung, G. Vazquez, M. Radovic & R. Arroyave
The resistance to oxidizing environments exhibited by some Mn+1AXn (MAX) phases stems from the formation of stable and protective oxide layers at high operating temperatures. The MAX phases are hexagonally arranged layered nitrides or carbides with general formula Mn+1AXn, n = 1, 2, 3, where M is early transition elements, A is A block elements, and X is C/N. Previous attempts to model and assess oxide phase stability in these systems has been limited in scope due to higher computational costs. To address the issue, we developed a machine-learning driven high-throughput framework for the fast assessment of phase stability and oxygen reactivity of 211 chemistry MAX phase M2AX. The proposed scheme combines a sure independence screening sparsifying operator-based machine-learning model in combination with grand-canonical linear programming to assess temperature-dependent Gibbs free energies, reaction products, and elemental chemical activity during the oxidation of MAX phases. The thermodynamic stability, and chemical activity of constituent elements of Ti2AlC with respect to oxygen were fully assessed to understand the high-temperature oxidation behavior. The predictions are in good agreement with oxidation experiments performed on Ti2AlC. We were also able to explain the metastability of Ti2SiC, which could not be synthesized experimentally due to higher stability of competing phases. For generality of the proposed approach, we discuss the oxidation mechanism of Cr2AlC. The insights of oxidation behavior will enable more efficient design and accelerated discovery of MAX phases with maintained performance in oxidizing environments at high temperatures.
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