Microstructure And Mechanical Properties Of Cf/Mg Composites And Two-dimensional Isotropic Design For Thermal Expansion

Continuous carbon fibre reinforced magnesium matrix composites (Cf/Mg composites) were designed and fabricated for aerospace structural materials research in this paper. Microstructure and properties of Cf/Mg composites were investigated by optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), electronic tensile testing machine and thermal dilatometer. The effects of Cf/Mg interface types and interfacial bonding on mechanical and thermal expansion properties were discussed. Furthermore, an isotropic design method for continuous carbon fibre reinforced metal matrix composites in two-dimensional plane was proposed, based on anisotropic properties of unidirectional composites.The amount of interfacial reactant (Al4C3) and precipitate (Mg17Al12) decreased with the increasing of fiber graphitization degree in Cf/AZ91D composites. Matrix alloy element Nb segregated towards Cf/Mg interface and formed discontinuous blocky Mg12Nd phase in Cf/ZM6 composites, which improved Cf/Mg interfacial bonding strength. Meanwhile precipitate free zone (PFZ) was formed in magnesium alloy near the interface, due to consumption of Nd in the matrix alloy by interface precipitation. And subgrain in the matrix alloy of Cf/Mg composites was generated by annealing treatment; T6 treatment led to the increment of interface reactant quantity, the increment of interface precipitate phase size, and also the improvement of dislocation density in the matrix alloy.After carbon fiber was hybrid reinforced with SiC particles, Blocky Mg17Al12 at SiC-AZ91D interface and feathery NdSi2 phases at SiC-ZM6 interface were formed. Mg17Al12 and NdSi2 phases improved the wettability between reinforcements and magnesium alloys, which increased mechanical properties of composites and decreased their horizontal coefficient of thermal expansion (CTE).From T300, T700 to M40 fibers, graphitization degree increased correspondingly, which decreased the bonding strength of Cf/Mg interface from 10.5 to 6.65 MPa. However, with the decreasing of interface bonding strength, tensile strength of composites increased from 381 to 1267 MPa, tensile modulus was between 219 and 241 GPa, bending strength increased from 896 to 1629 MPa, bending modulus was between 206 to 241 MPa. Cf/Mg composites have better high temperature strength. Bending strength was between 1200~1536 MPa below 300℃, affected little by temperature, however, decreased dramaticly above 300℃.Thermal strain along fibres in unidirectional Cf/Mg composites showed non-linear increment with temperature. And significant platforms were observed in thermal expansion curves, owing to micro-plastic deformation of matrix alloy and thermal stress relaxation by interface slip at medium temperature. CTE decreased with the increasing of fiber graphitization degree. Lower CTE could be obtained by choosing rare earth magnesium alloy as matrix or T6 heat-treating composites.In the process of thermal cycling for Cf/Mg composites, strain hysteresis and matrix alloy residual plastic deformation, lead to the existence of residual plastic deformation in the composites after the end of each thermal cycling. Fabricating composites by choosing rare earth magnesium alloy or T6 heat-treating composites both decreased residual plastic deformation.In this paper, Schapery's equation was modified, which could calculate longitudinal and horizontal direction CTE for continuous carbon fiber reinforced metal matrix composites accurately, and a calculation method for CTE of hybrid reinforced composites was proposed. Based on analyzing the effect of fiber arrangement on anisotropic properties of composites, according to the modified Schapery's equation, a two-dimensional isotropic design method for Cf/Mg composites was established. On the basis of this method, quasi-isotropic 2D-Cf/Mg composites were fabricated by pressure infiltration technology. Microstructure, thermal expansion and mechanical properties were investigated for 2D-Cf/Mg composites; the results showed composites possessed significant isotropic properties.