In situ surface imaging: High temperature environmental SEM study of the surface changes during heat treatment of an Al-Si coated boron steel

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 degrees 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 tau(5) (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 degrees 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 alpha-Al2O3 structure.