SELF-PROPAGATING HIGH-TEMPERATURE SYNTHESIS OF TIC AND NBC BY MECHANICAL ALLOYING

Titanium and niobium carbides have been synthesized through self-propa gating high-temperature reaction by mechanically alloying the elementa l powder mixtures. It is found that this reaction is very similar to t he conventional self-propagating high-temperature synthesis (SHS) proc ess, but the ignition of the reaction is identified to be the mechanic al collisions instead of heating the materials. Analysis of the produc ts reveals that the final products of the Ti-C system are in good agre ement with the equilibrium phase diagram, showing less relation with t he alloying process. The decrease of the C content shortens the millin g time prior to the SHS reaction of TiC by promoting the intimate mixi ng of the components, but lowers the heat release of the reaction and makes the propagation of the reaction more slowly. The SHS reaction du ring the mechanical alloying of the Nb-C system shows little differenc e, but the decrease of the C content can hardly influence the milling time prior to the reaction. By lowering the heat release of the SHS re action of NbC, it lowers the reaction propagating rate. Mechanical all oying Nb50C50 and Nb55C45 results in the formation of NbC, while mecha nical alloying Nb60C40 results in NbC + Nb instead of NbC + Nb2C accor ding to the phase diagram. It was attributed to the fact that the rapi d SHS reaction favors the formation of NbC, but hinders the occurrence of Nb2C phase through slow diffusion between NbC and the residual Nb. The measurement of the lattice parameters of TiC and NbC for differen t composition affirms the observed results. The particle sizes of obta ined TiC and NbC are very fine at around or even less than I mu m.