In-Situ Hip Synthesis Of TiB₂-TiC_x Composites

In this paper, TiB2-TiCx composites are fabricated with TiH2 and B4C powders by in situ synthesis under hot isostatic pressing (HIP). Effects of powder size, n(?)/nB4C, sintering temperature on the sintering properties, micro structure and mechanical properties are investigated. The changes of impurities in B4C powders before/after treatment are investigated by scanning electron microscopy (SEM) and energy diffraction (EDX). Phase, grain size, grain appearance, fracture mechanism of TLB2-TiC composites are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM). The results on their properties of relative densities, micro hardness, fracture toughness and three-point bend strength show that TiB2-TiC composites have good performances by in-situ synthesis under hot isostatic pressing (HIP).The results on the treatment of B4C powders show that the changes of powder sizes during treatment are small, The results on EDX show that the impurities of B4C powders have mainly Fe besides Ca, Mg, Al, Si, Ti, Cr, Mn, and so on.The results on XRD indicate that the reactants with 3TiH2-B4C systems have only TiB2 and TiC phases, while the reactants with 11TiH2-3B4C systems have not only TiB2 and TiC phases, but also Ti3B4 phases.Ti3B4 phases form at the expense of TiB2, with the extra Ti coming from TiC0.6 The accurate lattice parameters tests show that the values of the test in the lattice parameters of the TiB2 and TiC phase are almost equal to the values of the documents. The TiCx is nonstoichiometric with a range of stoichiometry, therefore effects of x value on lattice parameters are very large. Lattice parameters of stoichiometric TiC are 0.432516nm and 0.432201nm, respectively; while Lattice parameters of nonstoichiometric TiC are 0.431688nm and 0.431773nm,respectively.The results on SEM analysis indicate that the grains of TiB2-TiCx composites by in-situ synthesis under HIP are very tiny and well distributed, but some bulky grains arc found in some part area. During the crack propagation, when the crack encounters the small TiB2 grains area, The crack propagation path is border-grain because of its high strength and modulus, and the fracture faces are very coarse; when it encounters the bulky grains, the fracture patterns in some bulky TiB2 grains area are cross-grain fracture, and the fracture faces are very smooth, sometimes some steps are found in fracture face. Crack deflection, crack zigzag, crack bridging, particles pullout are the toughening mechanism of TiB2-TiCx composites, these mechanism make their strength and fracture toughness be higher than mono-phase TiB2and TiC ceramic.The results on transmission electronic microscopy (TEM) indicate that theshape of TU?2 crystal grains is regular-polygon and wattle, while TiC grains is irregular. The interface between TiBj and TiC is very smooth and clean. The interface strength is improved without large holes being found.The results on the mechanical properties show that effects of powder size on the relative density and mechanical properties of sintered samples are very large.In a same system, the relative densities , micro-hardness and fracture toughness of the samples sintered at 1600癈 are higher than these of the samples sintered at 1400 癈. Under a same sintered temperature, the micro-hardness with STfflb-B^jC systems is higher than that with HTiH2-3B4C systems, but the relative density three-point bend strength and fracture toughness are just contrary. From the experiment, we may conclude that the relative density and mechanical properties of the sintered samples are the best performance with w28 B4C (powder size 28 y m).