Design, Preparation And Microstructure Control Of In-situ Magnesium Matrix Composites

Mg alloys have the advantages of low density, high specific strength and stiffness, good electromagnetic shielding and damping capacities, good machining property and easy recycling capacity. And, there is abundant Mg resource on the earth. Thus, Mg alloys have been considered as the"Green Engineering Materials in 21st century". However, due to low elastic modulus, limited strength, poor abrasion and creep resistance, the field and range of Mg alloys application is restricted. In the meantime, Mg and Mg alloys can't be improved effectively even by using aging strengthening because no phase transformation occurs between solidification temperature and room temperature and the solid solubility of alloying elements is low. It is one of the effective ways to improve the performance of Mg alloys by introducing other reinforcements in Mg alloys. The in-situ techniques have received more and more attentions since in-situ Mg matrix composites exhibit thermodynamic stable reinforcements and cleaner and stronger bonding matrix-reinforcement interfaces. However, in-situ techniques have some problems, such as limited available reaction systems. Moreover, the nucleation and growth of in-situ formed reinforcements are affected by reinforcement properties, alloy compositions and the synthesizing process, the reinforcements may have some disadvantages such as coarse size, shape and uneven distribution. Thus the strengthening effect could be weakened by these factors. In order to develop in-situ Mg matrix composites and to control the size and shape of in-situ formed reinforcements, based on the basic principles of thermodynamics and kinetics, the work of adhesion and wetting between a ceramic solid and a binary alloy melt were investigated; the available reaction systems were investigated; and the effect of alloying elements on in-situ formation of reinforcements was investigated; thereby, in-situ Mg matrix composites have been designed. Furthermore, the evolution of phases during the composites synthesizing process was analyzed; the size, shape and distribution of in-situ formation of reinforcements were controlled. Moreover, a method of controlling the size of in-situ formation of reinforcement was proposed. At the same time, microstructure optimization by extrusion was investigated too. The following works have been developed:A model for quantitative calculation of the work of solid -liquid adhesion has been proposed. According to the present model, the work of adhesion-concentration dependency over the whole concentration range for binary alloy can be theoretically determined. This model, which overcomes the disadvantages of conventional investigation of work of adhesion excessively depending on time consuming and expensive experimental measurements, provides a method of predicting the work of solid-liquid adhesion by using the physical parameters of pure components. The model has been used to calculate the work of adhesion for nine solid-liquid binary alloy systems. The calculated results are in good agreement with experimental results, which proves the validity of the model to some extent. Combining the present model with theoretical calculations of surface tension, the wetting between ceramic reinforcements and Mg-X(X is alloying element) melts has been predicted. The investigation results show that the wetting between S3N4 ceramics and Mg alloy melts is good; the wetting between AlN and pure Mg melt is poor, but it can be increased by adding alloying elements (such as Al) into Mg alloy melts.After fully considering the wetting between the reaction materials and Mg alloy melts, the wetting between the reinforcements and Mg alloy melts, the effective reaction systems and Mg alloys characterization, AlN and Mg₂Si particle reinforced Mg matrix composites has been designed. Si₃N₄ particles and AZ91 alloys were chosen as the reaction materials and the matrix alloy, respectively.Through experimental investigation and optimization, synthesis conditions were determined. And the effects of adding mode and surface modification of the reaction materials, the shape of stirring paddle, the stirring mode and the melting condition on the synthesis of composites and the microstructure were investigated. Finally, synthesizing process, which can control the size of Mg₂Si reinforcements, was determined. The investigation results show that Mg matrix composites with uniform distribution of reinforcements have been successfully prepared by adopting it.Through analyzing the evolution of phases during synthesizing process and the formation mechanism of reinforcements, controls of the size and shape of reinforcements were investigated. The investigation results show that the size of AlN reinforcements can be reduced through decreasing that of Si₃N₄ particles; the size of Mg₂Si reinforcements can be controlled through controlling its nucleation and growth by adding alloying elements; the corners of Mg₂Si particles are passivated through changing the solidification cooling rate. Based on the formation mechanism of reinforcements and the effect of adding alloying elements on the nucleation and growth of reinforcements, combining with the synthesizing process, a method of controlling the size of reinforcement formed through nucleation and growth has been proposed. By using the method, the effects of alloying elements on the formation of Mg₂Si were investigated. The results show that the nucleation of Mg₂Si phase can be delayed and its growth can be promoted with the addition of some alloying elements, among which Ti has the greatest effect, followed by V, Mo, Cr, Fe and Mn. On the other hand, Ge has the greatest effect on promoting the nucleation and hindering the growth of Mg₂Si, followed by Sn, Cu, Zn. The effects of Ti addition on the size of Mg₂Si phase in (AlN+Mg₂Si)/Mg composites were investigated experimentally. The results show that before solidification, the nucleation of Mg₂Si can be hindered with the Ti addition and thus the amount and size of coarse Mg₂Si particle can be reduced. At the same time, the size of Mg₂Si particles formed during solidification could be slightly increased with the Ti addition.Based on compression tests at high temperature, the processing map and activation energy map of (AlN+Mg₂Si)/Mg composites have been developed. The deformation behavior of composites has been investigated. The domains of dynamic recrystallization and flow instability have been determined. Based on these, the secondary processing parameters of composites were chosen. By extrusion, the casting defects such as pore have been greatly decreased and the distribution of reinforcements is thus more homogeneous, the microstructure of (AlN+Mg₂Si)/Mg composites have been successfully optimized.The effect of reinforcement control and microstructure optimization on the compression properties at room temperature and high temperature, creep properties at high temperature and damping capacity of (AlN+Mg₂Si)/Mg matrix composites were investigated. The investigation results show that to optimize the microstructure by extrusion can improve their compression properties; the creep resistance of composites has been increased by introducing AlN and Mg₂Si particles; at room temperature, (AlN+Mg₂Si)/Mg composites reinforced with coarser Mg₂Si particles exhibit higher damping capacity compared with that reinforced with finer ones at low strain amplitude, while composites reinforced with finer Mg₂Si particles exhibit higher damping capacity at high strain amplitude and high frequency.