An Investigation On Fabricating B₄C/Mg Composites By Metal-induced Pressureless Infiltration Technique

In all fabrication processes of metal-matrix composites, metal melt infiltration into the ceramic preform is a kind of low-cost, fast, efficient and near net-shape preparation method. However, the prerequisite for achieving this process is good wettability between the ceramic and metal. Generally, the wettability between ceramic and matrix metal is not so well, e.g. in conventional ceramic/magnesium systems Mg-Ti-C, Mg-Ti-B, Mg-B4C In order to improve the wettability of the metal melt with preform, one simple way to be done is to add third phase metal with high melting point such that the surface tension of the melt and the liquid-solid interfacial tension can be reduced. Thus, the liquid-solid interfacial reaction can also be relieved or inhibited. In this thesis, a high melting point and immiscible metal Ti with magnesium was introduced into the B4C/Mg system as an inducer, the wettability between B4C and Mg is thus effectively improved, and light-weight ceramic reinforced metal-matrix composites B4C/Mg was successfully fabricated by this method. Moreover, infiltration dynamics, mechanism and the mechanical behaviors involved in this process were studied.According to the quantitative relation of preform relative density and the volume fraction of the ceramic reinforcement, light-weight particle hybrid composites were firstly fabricated with tailorable ceramic content under experimental conditions, i.e. volume percentage of Ti, reheating temperature, holding time, relative density of the preform and the particle size of the starting particles. This makes the unification of composite design with fabrication. The infiltration dynamics of the as processed composites were analyzed for the relationship between the infiltration depth and influencing factor. Some phenomena were found as follows. A farther distance of magnesium melt infiltration into preform can be obtained if Ti content is higher and reheating temperature is increased and the holding time is prolonged. A higher relative density of the preform and nearly the same size of the starting particles are also beneficial to the spontaneous infiltration. Most importantly, the spontaneous infiltration of magnesium melt into the preform is unlikely to occur without the presence of Ti, even if other preparation conditions are satisfied.As for the B4C/Mg composite system, the influences of Ti content, reheating temperature, holding time, preform relative density and starting particle sizes on the phase composition and microstructures were then studied for the B4C/Mg composites. There exist only four phases under these experimental conditions, Ti?Mg?B4C?MgB2. The existence of these four phases showed that the wettability in Ti-B4C-Mg system is good and there is not any reaction among them. Due to the lower Ti content, it is hard to observe the presence of Ti in the composite microstructure. B4C particles are uniformly distributed in the magnesium matrix, which play the reinforcing role and make the mechanical properties of the magnesium-matrix composites greatly improve.Finally, the wear behaviors, hardness and density of the as fabricated composites were measured and characterized.