Fabrication Of In Situ Particulate Reinforced Magnesium Matrix Composites

With the entrance to 21st century the resource and environment problemshave become the principle problems in the sustainable development of humanbeings. Therefore in the long term, the trends of the industrial development allover the world can be reduced to the saving of energy and resource, and what ismore, the pursuit of natural production. Magnesium alloys are kind of the lighteststructural metal alloys, which have many excellent and unique properties, such ashigh specific strength and stiffness, high heat and electrical conductivity, gooddamping and shock absorption, excellent electromagnetic shielding capability andeasy processing, as well as a good recovery capability, and therefore, increasingattention has been paid to magnesium and its alloys in automotive, communicationapparatus and aerospace applications. Consequently, it is considered thatmagnesium alloys are the most worthy and exploitative "natural engineeringmaterials" with greatly potential application in this century. However, the further development and commercial application of magnesiumalloys have been limited because of its low elastic modulus, poor tensile strengthand creep resisting especially at elevated temperatures, as well as low wearresistance. Moreover, these disadvantages can not be overcome only by using costintensive magnesium alloys. As a result, it has become a bottleneck problem thathow to solve these problems in the field of magnesium alloys. It is widely recognized that excellent properties such as a lower coefficient ofthermal expansion, a higher Young's modulus and a higher wear resistance etc. canbe realized only by magnesium metal matrix composites (MMCs). As aconsequence, the best routes are the selection of magnesium MMCs to solve theproblems of magnesium and its alloys on the basis of the internal attribute ofmaterial system. According to the morphological feature of reinforcement, - VI -吉林大学博士学位论文magnesium MMCs can be grouped under three heads, i.e. fiber, whisker andparticulate reinforced magnesium matrix composites. The strengthening effect offibre reinforced composites is evident along the fiber direction, attracting moreextensive interest; however, the fabrication processing is very complex and theproduction can not be reutilized. Although, the strength of whisker reinforcedmagnesium MMCs is very high and the production with excellent fabricating andreprocessing properties can be reutilized, the range of application of the compositehas been limited due to a high cost starting material. Recently, the particulatereinforced magnesium matrix composites have been becoming one of the hotspotin the research field of magnesium MMCs due to its significant inherent simplicityand potential cost-effectiveness in scale-up manufacturing, as well as its isotropyproperties. Presently, the particulate reinforced magnesium MMCs are mainly fabricatedby ex situ manner, namely, the reinforcements are prepared separately prior to thecomposite fabrication, and then the reinforcements are mixed into the magnesiummatrix by powder blends or directly added to the molten magnesium. In this case,the scale of the reinforcement is limited by the starting powder size, which istypically of the order of microns to tens of microns and rarely below 1.0 μm.Other main drawbacks that have to be overcome are the interfacial reactionsbetween the reinforcements and the matrix, and poor wettability between thereinforcements and the matrix due to surface contamination of the reinforcements.It is a puzzle, therefore, for the materials science researchers to find a solution tothe inherent problems that are associated with conventionally processed MMCs. It is well known that the properties of MMCs are controlled by the type, sizeand volume fraction of the reinforcement phase as well as the nature of thematrix-reinforcement interface. A unique or an optimum set of mechanicalproperties...