Design of functionally graded Fe-based composites from iron to iron borides (Fe2B/FeB) by in-situ powder metallurgy and evaluation of the post-thermal shock behavior

This study investigates the microstructural and mechanical behavior of functionally graded iron-based composites (FGCs) when subjected to thermal shock cycles. The FGCs were fabricated by in-situ powder metallurgy (IPM) with three composition gradients and tested with a newly designed shock device. Despite progressively increasing hardness, 10-layer FGCs were successfully fabricated without any separation between the layers. The stable iron boride phases (Fe2B/FeB) were effectively obtained in each layer dependent on the changing functional gradient. Iron borides also increased the interfacial bonding and hence the bonding ability between the layers. Moreover, in-situ borides assisted in decreasing the coefficient mismatch by balancing the thermal expansion coefficients between the iron matrix and B4C. Thermal cracks of various sizes, particularly in the hard ceramic layer, progressed toward the sublayers as the thermal cycle increased. The ductile and tougher sublayers prevented the transverse propagation of the thermal crack. Increasing the functional gradient accelerated the thermal cracking and weakened the mechanical properties. The n5 gradient exhibited fracture damage after impact with the lowest force and energy. From an overall thermo-mechanical perspective, control of crack propagation by the ductile layers was a major advantage of the layered Fe/Fe2B structures.