Microstructural characteristics and mechanical properties of the Al–Si coating on press hardened 22MnB5 steel

The microstructural characteristics and mechanical properties of the phases in the substrate-coating interdiffusion layer of an Al–10%Si coated 22MnB5 press hardened steel (PHS) were studied by means of electron diffraction, chemical analysis, and nano-indentation. The interdiffusion layer of the Al–Si coated PHS, austenitized at 900 °C for 6.5 min, consisted of α-Fe(Al), Fe3Al, FeAl, Fe2Al5, and an Fe–Al–Si ternary intermetallic phase. The chemical analysis results indicated that the Fe3Al and Fe2Al5 phases were non-stoichiometric, and their composition ranges were not fully consistent with existing Fe–Al and Fe–Al–Si phase diagrams. These results partly explain the significant discrepancies between research groups in phase analysis of the interdiffusion layer. The observed Fe–Al or Fe–Al–Si phases were harder and more brittle compared to the martensitic matrix. With increasing austenitizing temperature and hold time, the thickness of the interdiffusion layer increased and the major constituent in the coating changed from Fe2Al5 to a multi-layered structure of FeAl, Fe3Al, and Fe(Al), leading to significant softening of the interdiffusion layer. Free bend testing was employed to investigate cracking and fracture behavior of the interdiffusion layer. The cracks formed in the thicker coating associated with the highest austenitizing temperature propagated deeper into the substrate material than in the case of lower austenitizing temperatures. Fracture of the interdiffusion layer involved transgranular cleavage and intergranular fracture features. The fracture surface appearance depended on both the phases present and the Al distribution in the coating. FeAl phase, known to be relatively “ductile”, was more susceptible to intergranular fracture than other phases (Fe2Al5, Fe3Al, and α-Fe(Al)). •Microstructure and phase-specific mechanical properties in press-hardened steel Al–10%Si coating are studied.•Composition ranges of the phases in the coating are not fully consistent with existing Fe–Al and Fe–Al–Si phase diagrams.•Cracks formed in thicker coating propagate deeper into the substrate material during free bending test.•Fracture and cracking of the coating depend on the thickness, phases, and compositions of the coating.•FeAl is more susceptible to intergranular fracture than other phases in the coating.