In situ synthesis of TiB_2-TiC particulates locally reinforced medium carbon steel-matrix composites via the SHS reaction of Ni-Ti-B_4C system during casting

摘要

The fabrication of medium carbon steel-matrix composites locally reinforced with in situ TiB_2-TiC particulates using self-propagating high-temperature synthesis (SHS) reaction of Ni-Ti-B_4C system during casting was investigated. X-ray diffraction (XRD) results reveal that the exotherm of 1042 deg C initiated by heat release of the solid state reaction in the differential thermal analysis (DTA) curve is an incomplete reaction in Ni-Ti-B_4C system. As-cast microstructures of the in situ processed composites reveal a relatively uniform distribution of TiB_2-TiC particulates in the locally reinforced regions. Furthermore, the particulate size and micro-porosity in the locally reinforced regions are significantly decreased with the increasing of the Ni content in the preforms. For a Ni content of 30 and 40 wt. percent, near fully dense composites locally reinforced with in situ TiB_2 and TiC particulates can be fabricated. Although most of fine TiB_2 and TiC particulates which form by the reaction-precipitation mechanism during SHS reaction are present in the locally reinforced region, some large particulates which form by the nucleation-growth mechanism during solidification are entrapped inside the Fe-rich region located in the reinforcing region or inside the matrix region nearby the interface between matrix and reinforcing region. The hardness of the reinforcing region in the composite is significantly higher than that of the unreinforced medium carbon steel. Furthermore, the hardness values of the composites synthesized from 30 to 40 wt. percent Ni-Ti-B_4C systems are higher than those of the composites synthesized from 10 to 20 wt. percent Ni-Ti-B_4C systems.