Phase transitions in alumina films during post-sparking anodising of Al alloys
Phase transitions in alumina films during post-sparking anodising of Al alloys
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DOI：
10.1016/j.actamat.2022.118587
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发表时间：
2022-12
期刊：
影响因子：
9.4
通讯作者：
E. Bousser;A. Rogov;P. Shashkov;A. Gholinia;Nicolas Laugel;T. Slater;P. Withers;A. Matthews;A. Yerokhin
中科院分区：
文献类型：
--
作者：
E. Bousser;A. Rogov;P. Shashkov;A. Gholinia;Nicolas Laugel;T. Slater;P. Withers;A. Matthews;A. Yerokhin
高温α-Al 2 O3相的形成在等离子体电解氧化过程中的铝在环境中的本体温度已被先前归因于局部微放电事件提供多个熔化-固化循环的微体积的表面氧化层。在这项工作中，它被证明，即使在特定的工艺条件下的微放电被完全抑制，可以形成α相。采用FIB、TEM、EBSD、EDS和GDOES等技术对火花后阳极氧化过程中产生的氧化层进行了研究，揭示了γ-Al_2O_3向α-Al_2O_3转变过程中氧化层的微观结构和化学成分的变化。研究结果表明，在γ-Al_2O_3基体中，在适当的温度、晶粒尺寸和杂质分布条件下，α相可以在氧化物区域自发形成，从而使α/γ晶界具有足够的流动性。氧化物中的离子迁移和相关微孔网络中的水热溶解/沉淀促进了晶界处的物种迁移，从而降低了γ→α转变活化的临界温度。总体而言，它建议，氧化层的生长可以被认为是在一个相对简单的活塞流反应器模型。这可以帮助预测相变动力学，取决于关键的工艺参数，如电流密度和脉冲极化频率，从而实现对特定应用要求的涂层微观结构的最佳控制。
Formation of the high-temperature α-Al2O3phase during Plasma Electrolytic Oxidation of aluminium at ambient bulk temperatures has been previously attributed to local microdischarge events providing multiple melting-solidification cycles in micro-volumes of the surface oxide layer. In this work, it is demonstrated that the α phase can be formed even if the microdischarge is fully suppressed under specific processing conditions. Oxide layers produced in the post-sparking anodising mode were studied by FIB, TEM, EBSD, EDS and GDOES techniques to reveal microstructural and chemical evolutions that accompany the γ to α alumina transition. Our results provide strong evidence that the α phase can form spontaneously in regions of oxide with the appropriate temperature, grain size and impurity distributions in the γ-Al2O3matrix that allow sufficient mobility of α/γ grain boundaries. Ionic migration within the oxide and hydrothermal dissolution/precipitation in the associated microporous network that facilitate species mobility at the grain boundaries allow the critical temperature for activation of γ→α transition to be reduced. Overall, it is suggested that oxide layer growth can be considered in terms of a relatively simple Plug Flow Reactor model. This can help predict the phase transition kinetics depending on key processing parameters such as current density and frequency of pulse polarisation, thus enabling optimum control of coating microstructure for specific application requirements.