Search

The effect of pH value on the corrosion behavior and microstructural evolution of carbon fiber reinforced magnesium matrix (Cf/Mg) composites in NaCl solution were investigated. The results of the electrochemical test revealed that the corrosion current density of Cf/Mg composites in pH = 13 solution achieved 3.4 × 10−5 Am/cm2, which was one order and two orders of magnitude lower than that of pH = 7 solution (2.6 × 10−4 Am/cm2) and pH = 1 solution (1.1 × 10−3 Am/cm2), respectively. Electrochemical impedance spectroscopy (EIS) curves implied that the corrosion resistance of Cf/Mg composites was enhanced when the pH value increased. The microstructure of the Cf/Mg composites after corrosion in the solution with pH = 13 first appeared as some point-like bumps, and then gradually grew into a lamellar structure. The corrosion mechanisms and microstructural evolution of the Cf/Mg composites were systematically discussed.

In order to overcome the interfacial incompatibility of carbon fiber reinforced magnesium matrix (Cf/Mg) composites, a double-layer interface (ZrO2–MgO) is designed in this work. Carbon fiber was modified with ZrO2 coating by sol-gel process. Microstructural examination reveals that MgO layer forms on the surface of ZrO2 coating by ZrO2 reacting with Mg during the composite fabrication. Such double-layer interface could inhibit Al4C3 and hence prevent fiber damage. Meanwhile, the wettability was improved for the reaction between ZrO2 and Mg. Thus the tensile strength of ZrO2-Cf/AZ91D composite was 68.0% higher than that of the unmodified one. Due to the fiber bundle pull-out, debonding and crack deflection, the toughness of Cf/Mg composite with double-layer interface is increased simultaneously.

Microstructure evolution and dynamic restoration mechanism of solution-treated Mg-4Y-2Nd-1Sm-0.5Zr alloy have been studied under three TMP parameters consisting of deformation temperatures (350–500 °C), strain rates (0.01–5 s−1), and strains (0.2, 0.4, and 0.8). A strong dynamic softening is observed in all stress-strain curves, even at higher strain rates (1 and 5 s−1) due to an adiabatic heating effect. Various stress-strain curves are applied to construct a processing map and develop an Arrhenius-type constitutive equation. With the prediction of the processing map, an optimal processing domain has been determined to be the temperature range 450–500 °C and strain rate range 0.01–0.1 s−1 at a strain of 0.8. The volume fraction of DRX grains is the largest in the corresponding domain of high temperature and low strain rate. For the effect of TMP parameters on the dynamic restoration, the discontinuous dynamic recrystallization (DDRX) and continuous DRX (CDRX) synergistic effect occur throughout the whole process at high temperature and high strain rate. In terms of high temperature and low strain rate, DDRX characteristics at a low strain and then the DDRX + CDRX synergistic effect is observed at a higher strain. Although the DRX process is weak at low temperature and low strain rate, deformation twins have occurred and provided nucleation sites for DRX grains.

A HfC-TaC composite coating was prepared on SiC-coated C/C composites by supersonic atmosphere plasma spraying (SAPS) to improve their ablation resistance. The ablation resistance was evaluated under oxyacetylene torch with heat flux of 2.38 MW/m2 and 4.18 MW/m2 respectively, and the effect of TaC on microstructure and phase composition of the coating was investigated. Results showed that TaC-modified HfC coating exhibited good ablation resistance under the torch with heat flux of 2.38 MW/m2 and its mass and linear ablation rates were -0.35 mg/s and -1.05 μm/s separately, which is ascribed to the formation of continuous oxide film including phases like Hf6Ta2O17 and Ta2O5. Crack propagation in the coating was inhibited due to the phase transition restriction of HfO2 by Ta2O5, and cracks and holes in the oxide layer could be repaired by molten Ta2O5 and Hf6Ta2O17. However, the occurrence of ablation crater on the surface of HfC-TaC coating is mainly attributed to the unsatisfactory scouring resistance of the oxide film under heat flux of 4.18 MW/m2.

A modified random fiber removal algorithm is proposed to numerically study transverse elastic properties of unidirectional carbon fiber reinforced pyrolytic carbon (C/C) composites. This method can efficiently generate the Representative Volume Element (RVE) of composites with random distribution fibers. On the basis of the numerical homogenization strategy, the effect of interphase between carbon fiber and pyrolytic carbon on the mechanical behaviors of unidirectional C/C composites are investigated. Meanwhile, periodic boundary conditions and linear elastic constitutive materials are taken into account. For validation, numerical calculations are compared with these obtained by atomic force microscopy (AFM) tests, which shows that more accurate results will be pursued when the interphases are considered. In addition, the influences of interphase parameters including the thickness, modulus, and Poisson’s ratio, on the effective transverse behavior of unidirectional C/C composites are studied numerically.The results demonstrate that the effective transverse properties of unidirectional C/C composites are mainly determined by the interphase modulus and independent of the interphase Poisson’s ratio. If the interphase thickness (less than 140 nm) of unidirectional C/C composites are very small, the modulus of interphase beyond 8 GPa makes little effect on the effective transverse properties.