Evolution of interfacial nanostructures and stress states in Mg matrix composites reinforced with coated continuous carbon fibers

Magnesium (Mg) matrix composites reinforced with 45 vol.% continuous carbon fibers (Cf) were fabricated using a pressureless infiltration process in vacuum. In order to modify the interface between the Cf and the Mg matrix, the Cf were coated with 5.0 mol.% yttria stabilized zirconia (YSZ) in sol–gel route. The Cf/Mg composite exhibited a tensile strength of 1.08 GPa which reached 90% of the theoretical prediction by means of the rule of mixture. Microstructural examinations revealed the occurrence of interfacial reaction between the YSZ coating and the Mg matrix, producing a ∼20 nm thick interfacial reaction layer consisting of nanocrystalline particles, which include MgO particles, remaining ZrO2 particles and a small quantity of ZrC particles. Interfacial reaction could induce a large compressive stress in the interfacial layer. As a result, some nanostructured defects, such as edge dislocations, were formed in interfacial layer due to the compressive stress. In the cooling process of fabricating the composite, the phase transformation of the remaining ZrO2 from tetragonal to monoclinic could relax the thermal residual stress of interfacial layer.