Study Of Mechanical And Thermal Expansion Properties Of Gr_f/ZM6 Composites

Magnesium matrix composites reinforced by 60vol.%fiber and by 55vol.%fiber and 5vol.%SiC particle hybrid were successfully fabricated by pressure infiltration technology, respectively. Based on the study of unidirectional fiber reinforced matrix magnesium, two dimensional graphite fiber fabric reinforced magnesium composites with low coefficient of thermal expansion(CTE), approximate isotropy and dimensional stability were designed and fabricated by means of the good designable ability of fiber reinforced composites.The microstructure, mechanical and thermal expansion properties, as well as dimensional stability were studied by optic microscope, scanning electron microscopy(SEM), transmission electron microscopy(TEM), electron tensile machine and thermal analyzer. The effect of types of reinforcements, heat treatment method and way of fiber layup on composites'properties were discussed.Microstructure observation shows that the composites have a relatively high density, in which fibers are well bonded with the matrix. Massive discontinuous Mg12Nd precipitates are observed at the fiber-matrix interface, which leads to the enrichment of matrix alloy element Nd at the interface. There are many dislocations at or around the fiber-matrix interface in the as-cast composites.The tensile and bending strengths of unidirectional M40/ZM6 are 420.6MPa and 739.2MPa, respectively. When 5%SiC particles are introduced, the tensile and bending strengths of the composites increase 24.5% and 32.6%, respectively. From 20℃to 100℃, the CTEs of M40/ZM6 composites are 19.0×10-6/K in 90°direction and 1.0×10-6/K in 0°direction. The introduction of SiC particles makes the CTEs increase in 0°direction and decrease in 90°direction. The CTEs of composites decrease with annealing treatment.Graphite fiber fabric reinforced magnesium matrix composites have low coefficients of thermal expansion. From 20℃to 100℃, the CTEs in 0°~90°direction are between 3.6×10-6/K and 4.1×10-6/K of orthogonal M40 fabric/ZM6 composites. By the addition of 5%SiC particles, the CTEs of composites decrease, and the values are between 3.1×10-6/K and 3.8×10-6/K. With the temperature increasing, the CTEs of the composites decrease. The method of graphite fiber fabric as reinforcement can improve the CTEs'anisotropy of single direction fiber reinforced magnesium matrix composites effectively, and the CTEs can be isotropy by further optimistic design.For the materials studied in this dissertation, theΔof M40/ZM6 composites in 0°direction is 0.86 during the cold/hot blowing test and its dimensional stability is the best. The dimensional stability of (M40+SiC)/ZM6 composites in 0°direction is better and theΔis 0.97. And the dimensional stability of (M40 fabric+SiC)/ZM6 composites in 0-90°direction is less better and theΔis 1.08.