Abstract Al Si coated boron steels have been of interest for many years because of their superior mechanical properties and their excellent oxidation resistance. These high strength steels are particularly used in hot stamping processes during which the Al Si coating undergoes multiple microstructural transformations. In this study, morphology and structure transformations are investigated using an original approach. Most of the time, the coating evolution is studied in cross section, by ex situ characterization of its different layers after heat treatment. In this work, the evolution of the surface is explored for the first time using in situ high-temperature Environmental Scanning Electron Microscopy (HT-ESEM). The austenitization step reproduced in the ESEM chamber allows a precise description of several surface changes occurring, depending on the temperature range. Among them, the main change in morphology is occurring between 650 and 800 °C and is linked to different reactions between aluminium and bi- and ternary Fe-Al-Si phases. The heating rate is also pointed out as a key parameter that affects the surface morphology. In fact, with low heating rates the surface is mostly composed of hexagonal and rectangular elements coming from τ5 (Fe2Al8Si) structuration, while needle-shaped FeAl3 structures are found for higher heating rates. In addition, it is observed that the heating rate also affects the surface roughness depending on the surface morphology. Finally, the origin of the presence of micrometrics pores appearing between 800 and 900 °C at the coating surface is discussed. Our hypothesis supported by ex situ characterization is that they result from formation of a crystallized oxide layer, probably close to α-Al2O3 structure.

中文翻译：

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原位表面成像：Al Si 涂层硼钢热处理过程中表面变化的高温环境 SEM 研究

摘要 Al Si 涂层硼钢因其优异的机械性能和优异的抗氧化性而备受关注多年。这些高强度钢特别用于热冲压工艺，在该工艺中，铝硅涂层会经历多次微观结构转变。在这项研究中，使用原始方法研究形态和结构转换。大多数情况下，通过热处理后不同层的非原位表征，在横截面中研究涂层演变。在这项工作中，首次使用原位高温环境扫描电子显微镜 (HT-ESEM) 探索了表面的演变。在 ESEM 室中再现的奥氏体化步骤可以精确描述发生的几种表面变化，取决于温度范围。其中，形态的主要变化发生在 650 至 800 °C 之间，并且与铝与二元和三元 Fe-Al-Si 相之间的不同反应有关。加热速率也被指出是影响表面形貌的关键参数。事实上，在低加热速率的情况下，表面主要由来自 τ5 (Fe2Al8Si) 结构的六边形和矩形元素组成，而针状 FeAl3 结构则用于更高的加热速率。此外，据观察，加热速率也会影响表面粗糙度，具体取决于表面形态。最后，讨论了涂层表面出现在 800 到 900 °C 之间的测微孔的起源。