Non-eutectic versus eutectic borides in a boride-ferrite composite: Differences in microchemistry, microtexture, and residual stress

This study involved as-cast, albeit heat-treated, boron-containing 9Cr-1Mo steel, which resulted in various composites of bcc-ferrite (α-Fe) and boride ((Fe,Cr)₂B) phases. The heat treatments, normalizing and tempering, were necessary to reduce composite embrittlement. However, they did not significantly alter phase-specific microchemistry, microtexture, and residual stress. An increase in boron content (0.5 and 2.5 to 5 wt%) enhanced, as expected, boride presence. In the lower boron-containing specimens, fine (∼1 μm) post-solidification eutectic borides were observed. Material with 5 wt% boron, on the other hand, contained larger (∼23 μm) non-eutectic and smaller (∼1 μm) eutectic borides in almost equal volume fractions. An increase in boron content thus transformed a metal-matrix composite with ceramic reinforcement (0.5 and 2.5 wt% boron) into a ceramic-matrix composite with a metallic second phase (5 wt% boron). This study extensively used analytical microscopy, particularly multiscale diffraction-based microtexture and residual stress measurements, to differentiate between eutectic and non-eutectic borides. The non-eutectic borides had a morphological alignment typical of directional solidification. They also exhibited faceted and non-faceted interfaces, the former representing a sharper composition gradient. Further, the non-eutectic borides showed stronger crystallographic texture, lower in-grain misorientations, orientation gradients, and significantly lower residual stresses. The non-eutectic borides appeared to originate as primary dendrites during the initial solidification stages. In contrast, eutectic borides and ferrites formed subsequently in the interdendritic arms. Growth selection of the non-eutectic borides and back-stress relaxation at the dendrite boundaries appeared to define the differences in phase-specific microstructures of the ceramic-matrix composite.