This work is consisted with four topics on SiC/2124Al metal matrix composites. This thesis is focused on the four main topics, i.e. the load transfer phenomena, relationship between fabrication process conditions and mechanical properties, aging characteristics, and high temperature creep properties on SiC/Al metal matrix composite.
In chapter III, the load transfer efficiency of a cylindrical whisker and spherical particle in metal matrix composite was analyzed. Assuming that both the shear stress on side surface and the normal stresses on end surfaces of reinforcement contribute to the load transfer from matrix to reinforcement. A new parameter defined as effective aspect ratio was derived as a function of aspect ratio and misorientation angle for more accurate calculation of load transfer between reinforcement and matrix in metal matrix composites. The effective aspect ratio were decreased with increasing misorientation angle, and its rate were higher when the aspect ratio of whiskers are higher. The effective aspect ratio, the load transfer efficiency, of reinforcements depends on the misorientation angle and geometrical shapes of reinforcements simultaneously. The load transfer efficiency of distributed whiskers in metal matrix composites were calculated as integration form. The load transfer efficiency of distributed whiskers depends on the degree of whisker alignment and aspect ratio. By using the effective aspect ratio, the strength and anisotropy of metal matrix composite were explained more correctly.
In chapter IV, the effects of processing parameters, i.e., vacuum hot pressing temperature, vacuum hot pressing pressure, extrusion temperature and extrusion ratio, on mechanical properties of $SiC_w$/2124Al composites fabricated by powder metallurgy process were investigated using a statistical technique known as Taguchi method. The vacuum hot pressing temperature was found to be the most sensitive parameter to the tensile strength. As the vacuum hot pressing temperature increased up to 570℃, the tensile strength increased due to the enhanced densification of composite and increased aspect ratio of SiC whiskers, which were resulted from the reduced strength and increased amount of liquid phase in matrix with increasing vacuum hot pressing temperature. The optimum vacuum hot pressing pressure was considered to be 70MPa, below which the remaining voids reduced the tensile strength of $SiC_w$/2124Al composite. The tensile strength increased with increasing volume fraction of SiC whiskers. However, the volume fraction of whiskers more than 20% was not helpful to increase the tensile strength since the whiskers tend to be non-uniformly distributed. The extrusion temperature need to be higher than the solidus temperature of 2124Al matrix to reduce the damage of whiskers and to improve the alignment of whiskers. The extrusion ratio of 15:1 resulted in the highest tensile strength. The alignment of whiskers improved with increasing extrusion ratio, while the aspect ratio of whiskers decreased due to the damage of whiskers. A modified phenomenological equation describing the tensile strength of $SiC_w$/2124Al composites as a function of microstructural parameters was proposed by introducing the effective aspect ratio in substitute for the average aspect ratio of SiC whiskers.
In chapter V, the precipitation mechanism during the aging treatment and its effect on the mechanical properties of 2124Al alloy and $SiC_p$/2124Al metal matrix composites(MMC), which were fabricated by powder metallurgy process, were investigated. The major precipitate phase was identified as S'($Al_2CuMg$) phase and was grown along <100> direction of matrix in both 2124Al alloy and $SiC_p$/2124Al MMC. Due to a large difference in thermal expansion coefficients between SiC reinforcement and 2124Al matrix, the dislocation density in $SiC_p$/2124Al was much higher than that in 2124alloy. The high dislocation density in $SiC_p$/2124Al composite accelerated the aging process and resulted to form finer precipitates by providing faster diffusion path with more nucleation sites of precipitates. The peak yield strength was obtained after 8hr aging at 190℃ in both 2124Al alloy and $SiC_p$/2124Al composites. Comparing the precipitation behavior of $SiC_p$/2124Al and 2124Al alloy, the volume fraction of precipitates increased rapidly in $SiC_p$/2124Al composites, while the coarsening rate of precipitates was slower in $SiC_p$/2124Al composites compared to 2124Alalloy. It is analyzed that the precipitates influence the yield strength on the point of two different strengthening mechanisms, i.e. shearing of precipitates by dislocation and dislocation bowing between precipitates. The peak strength was observed when the two strengthening effects from shearing and dislocation bowing are balanced.
In chapter VI, the creep behavior of $SiC_p$/2124Al alloy and $SiC_w$/2124Al composites was investigated and its relationships with microstructural parameters were quantitatively analyzed. It is suggested that the creep deformation of SiC/2124Al composites proceeds in 2124Al matrix and the steady state creep rates of composites are directly dependent on the effective stress on the matrix. The microstructural parameters of reinforcements, such as aspect ratio and alignment, influence the creep behavior by changing the effective stress through the load transfer mechanism. The steady state creep rates and sub-grain sizes were measured similar under the identical effective stress on 2124Al matrix. It is suggested that the creep deformation of composite proceed by the deformation of matrix and the role of SiC reinforcement is to reduce the effective stress acting on matrix through the load transfer from matrix to reinforcement. A modified power law creep equation bsaed on the effective stress was proposed to describe the creep behavior of SiC/2124Al composites.
