EBSD study of crystallographic identification of Fe-Al-Si intermetallic phases in Al-Si coating on Cr-Mo steel

5Cr-0.5Mo steel was coated by hot-dipping in a molten Al-10 wt.% Si bath at 700 °C for 10, 60, 120 or 180 s. The identification of the phases in the Fe-Al-Si intermetallic phases formed in the aluminide layers during hot-dipping was carried out by using a combination of scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). The EDS results show a τ5(H)-Al7Fe2Si phase, which exhibited 2 distinct morphologies, small particles widely dispersed and a continuous layer. Also revealed by EDS were τ6-Al4FeSi and τ4-Al3FeSi2 phases, which showed plate-shaped morphology, in an Al-Si topcoat. However, the XRD results show the intermetallic phases in the aluminide layer were composed of outer cubic τ5(C)-Al7(Fe,Cr)2Si and inner hexagonal τ5(H)-Al7Fe2Si. EBSPs and mapping functions in EBSD helped to clarify the confused phase identifications yielded by EDS and XRD. In this way, the small intermetallic particles and the continuous intermetallic layer were identified as cubic τ5(C)-Al7(Fe,Cr)2Si and hexagonal τ5(H)-Al7Fe2Si, respectively, and the plate-shaped intermetallic phase was identified as monoclinic τ6-Al4FeSi and tetragonal τ4-Al3FeSi2 with the same metallographic morphology. EBSD proved to be a very effective technique for local phase identification of aluminide layers with complicated multiphase morphologies.

Research highlights▶ EBSD provides crystallographic information regardless of the content of materials. ▶ Mapping and EBSPs functions in EBSD provide a reliable phase identification. ▶ A phase constitution in the aluminide layer has been described in detail. ▶ Cubic τ5(C)-Al7(Fe,Cr)2Si and hexagonal τ5(H)-Al7Fe2Si are identified.