In Situ Synthesis And Properties Of TiCp Reinforced Magnesium Matrix Composites

Due to energy shortage and environmental pollution, magnesium alloys are attracting more and more attention in automobile industry for losing the weight of a vehicle,reducing the exhaust emissions and raising fuel efficiency. However, the poor thermal resistance properties of magnesium alloys restrict development and application of magnesium alloys in automobile industry.This research is trying to solve the above problem. The research work aims to synthesize magnesium matrix composites by using in situ technique and obtain higher strength, better thermal resistance properties and better damping capacities than those of magnesium alloys. Compared with ex situ technique, in situ technique exhibits the finer reinforcement,the better distribution and the cleaner reinforcement-matrix interface than ex situ one. So in situ magnesium matrix composites have great potential to obtain excellent mechanical properties and functional properties and extend the application of magnesium alloys in automobile industry.In situ synthesis method has been extensively studied for aluminum, titanium, iron and copper matrix composites. However, this technique is still relatively new for in situ magnesium matrix composites. Therefore it is of great significance to study in situ reaction, fabrication, microstructure, mechanical properties, damping capacities and thermal resistance properties of in situ magnesium matrix composites.In this study, the feasibility of Mg-Al-Ti-C in situ reaction was firstly confirmed thermodynamically. According to the calculation using an extended Miedema model and the Wilson equation, the influences of alloying elements on the free energy of TiAl3 and TiC formation were calculated and the effect of the alloying additions on in situ reaction to synthesize TiC/Mg composites was evaluated. The calculation results show elements Zn and Mn in AZ91D alloys were beneficial to promote TiC formation, as well as hinder the brittle TiAl3 phase formation.Experimental research showed molten magnesium alloy was infiltrated into Al-Ti-C preforms, simultaneously the in situ reaction happened and TiC particles were formed in the liquid of magnesium alloy. Ti reacted with Al to form TiAl3 in the initial stage, and then C reacted with TiAl3 to form TiC. Al in the preforms serves not only as a reactant and participates in the in situ reaction to decrease reaction temperature and TiC particle size, but also as a diluent to to facilitate the diffusion and distribution of TiC particles. The as-cast microstructure of the in situ composites revealed the uniform distribution of TiC particulates with spherical sizes. Microstructural analysis showed high dislocation density around the reinforcements and good and clean interface between TiC particles and matrix.Introducing TiC particles into magnesium matrix improved the mechanical properties of matrix material. The tensile strength of magnesium matrix composites were 20-26% higher than those of magnesium alloy and elastic module were 3-11% higher than magnesium alloy. The strengthen effect of in situ magnesium matrix composites was explained by refine grain strengthen and dislocation strengthen mechanism. The fracture analysis of TiC/AZ91D composites revealed the failure of composites was caused by interfacial debonding and the fracture of matrix.The compressive deformation behaviors of AZ91D alloys and TiC/AZ91D composites were studied in temperature and strain rate range of 25-200°C and 10-3-1s-1. The border of early fracture is close to Z=8×1014s-1 in AZ91D alloys and Z=5×1013s-1 in TiC/AZ91D composites. So this value may represent the limit of hot deformation conditions because a premature fracture occurs for Z>8×1014s-1 in AZ91D alloys and Z>5×1013s-1 in TiC/AZ91D composites. Compared with AZ91D alloy, TiC/AZ91D composites need higher temperature and lower strain rate (lower Z value) to avoid premature fracture.The compressive deformation behavior of TiC/AZ91D composites has also been investigated at strain rates 10-3-8s-1and hot working temperatures 250-400°C. The processing map has been adopted to correlate processing parameters, predict safe regions and instable regions, obtain safe processing conditions and optimize hot forming process of TiC/AZ91D composites.Damping capacities of in situ TiC reinforced magnesium matrix composites with different reinforcement percentage were investigated. Experimental results show TiC reinforced magnesium matrix composites possessed better damping capacities than those of non-reinforced magnesium alloy. Compared to magnesium matrix alloy, an increase in damping capacities of TiC/AZ91D composites was attributed to the dislocation damping mechanism at room temperature. At elevated temperatures, the matrix became relatively soft with respect to ceramic reinforcement and interface sliding between the reinforcement and matrix occurred. Interface damping became a new contributor to the increase of damping capacity.A new bolt load retention evaluation system was set up to simulate automobiles parts served at high temperature and preloads and evaluate the thermal resistance properties of Mg alloys and composites. The effect of the variables such as temperature, preload and different samples on the feasibility of BLR evaluation systems was systematic investigated. BLR behaviors of AZ91D alloys and TiC/AZ91D composites at different temperature and preloads were tested. Results showed that the increases in temperature and preload led to a decrease in BLR as a fraction of preload. Higher preloads led to a higher remaining clamp loads, although a fraction of preloads was lower for higher preloads. The fraction of remaining load to preload of composites was found to be higher than that of AZ91D alloy, which indicated that TiC/AZ91D composites can improve BLR behaviors of magnesium matrix. The combined influence of temperature and preload level can be described by a series of"contour"maps that relate the fraction of remaining load to preload and temperature. This diagram is useful for engineering design.