Synthesis of carbide-boride composites and their structure control using the solid-state displace ment reaction

采用固态置换反应法合成碳化钛-硼化钛复合材料，方法在1000 ~ 1300 ° C温度范围内研究了B_4C与Ti在固态下的反应机理，在反应初期，生成了TiC_X和TiB。生成的TiC_X为非化学计量比，其成分为与Ti平衡的成分。随着反应的进行，TiB_2开始生成，TiB含量减少，TiC_X中碳含量增加。这些结果可以通过以下反应机理很好地解释：第一步反应为B_4C+（4+ 1/x）Ti_4TiB +1/xTiC_x，第二步反应为yB_4C+4 TiB +1/xTiC*（2+ 2 y）TiB_2+（2+ 2 y）TiC+（2- 2 y +1/x）Tic_<（y+1）（2- 2 y +1/x）>为获得致密的B_4C/Ti复合材料烧结体，采用放电等离子烧结法对B_4C/Ti粉末进行致密化处理。通过测定其相对密度，确定了最佳烧结温度和保温时间。然后对烧结体进行金相检查，并测量杨氏模量、维氏硬度和断裂韧性的机械性能。当B_4C和Ti混合粉末在1400 ℃以上烧结时，在烧结初期生成TiB、TiB_2、Ti_3B_4和TiC_X，最后全部转变为TiB_2和TiC_X。在1800 ℃及以上的温度下，获得了相对密度高于99%的复合材料。在1800 ℃及以下烧结的复合材料具有独特的显微结构：TiB_2相在TiC相周围形成网状结构。在较高的温度下，网络结构消失。具有高于99%的相对密度的复合材料具有杨氏模量514 GPa、泊松比0.178、维氏硬度2090和断裂韧性3.8MPam-1的机械性能<0.5>。

To synthesize titanium carbide-boride composites by solid-state displacement reaction method, method the reaction mechanism between B_4C and Ti in the solid state was examined at temperatures of 1000 to 1300ﾟC, In the early stage of the reaction, TiC_X and TiB formed. The TiC_X formed was nonstoichiometric, the composition of which was that in equilibrium with Ti. As the reaction proceeded, TiB_2 began to form with reducing TiB and increasing carbon contents in TiC_X. These results were well explained by the following reaction mechanism : as the first step, the reaction B_4C+(4+l/x)Ti4TiB+1/xTiC_x proceeded, and as the second step, the reaction yB_4C+4TiB+1/xTiC*(2+ 2y)TiB_2+(2+2y)TiC+(2-2y+l/x)Tic_<(y+1)(2-2y+1/x)> followed.To obtain dense sintered bodies of B_4C and Ti composites, a reactive spark plasma sintering method was applied to densification from B_4C and Ti powders. The most suitable sintering temperature and holding time were first surveyed and determined by measuring their relative densities. Sintered bodies were then examined metallographically and mechanical properties were measured for Young's modulus, Vickers hardness and fracture toughness. When mixtures of B_4C and Ti powders were sintered above 1400ﾟC, in the early stage TiB, TiB_2, Ti_3B_4 and TiC_X were produced and finally they were entirely transformed into TiB_2 and TiC_X. Composites with relative densities higher than 99 % was obtained at 1800ﾟC and above. The composites sintered at 1800ﾟC and below had unique microstructure : TiB_2 phases formed networks surrounding TiC phases. The network structure disappeared at higher temperatures. The composites with relative densities higher than 99 % had mechanical properties of Young's modulus 514 GPa, Poisson's ratio 0.178, Vickers hardness 2090 and fracture toughness 3.8 MPam^<0.5>.