Multiscale Research On High Temperature Creep And Interfacial Mechanical Behavior Of Magnesium Alloy Based Hybrid Nanocomposites

The nano-composites based light metals (i.e. magnesium alloy) are regarded as good structural materials due to the low desity, high specific strength and stiffness, and thus they have potential application trends in the aerospace, automotive and other industry. The mechanical properties and failure mechanism of the nano-composites are related not only to macroscopic properties but also to component phase properties and micro-structural features such as the shape and size of reinforcements, the distribution of reinforcements in the matrix and interface characteristics between the reinforcement and matrix. In order to facilitate the optimal design and application of light metal matrix nano-composites, it is necessary to study the relationship between micro-nano structure and macro-micromechanical properties by multi-scale methods. Microstructure, high temperature creep resistance and constitutive equation as well as the interface mechanical properties of hybrid carbon nanotubes and SiC nanoparticles reinforced AZ91magnesium alloy matrix composites have been studied by multi-scale experiments and numerical simulation methods in the present paper.Micro-nano structure and high temperature creep mechanical properties of magnesium alloy matrix nano-composites were experimentally investigated at the macro-level. AZ91magnesium alloy based composites reinforced with carbon nanotubes (CNTs) and SiC nanoparticles have been fabricated by semisolid stirring assisted ultrasonic cavitation and subsequent hot extrusion method. In the nano-composites, the mass fraction was1mass%for CNTs and2.23mass%for SiC nanoparticle respectively, and CNTs and SiC nanoparticles were modified by the nickel-zinc coating and nickel coating separately. The surface modified effects of hybrid nanosized reinforcements and the micro-nano structure of the magnesium alloy nano-composites were characterized by means of X-ray diffraction (XRD), optical microscope (OP), high-resolution scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The room-temperature mechanical properties and high-temperature creep properties of the Mg alloy matrix nano-composites were also studied by using tensile testing machine and high temperature creep testing apparatus. At last, the creep constitutive equation and its parameters for the nano-composites have been determined and the reasons for the excellent mechanical properties of the nano-composites have been discussed. At the microscopic scale,3-D unit cell model of wherein reinforcement orientation is in line with the law of Weibull distribution has been built. Based on the model, the high-temperature creep behavior of the magnesium-based hybrid composite was simulated by finite element method. The creep constitutive equation related to the relationship of the stress, strain rate, temperature, hybrid ratio of reinforcements were obtained. In additon, the coupling effect of thermal residual stress and creep temperature has been taken into account during the establishment of the constitutive equation. It can be seen that the creep strain rate of the matrix in the nano-composites is significantly reduced, and thus the high temperature creep-resistant properties of the nano-composites are improved. The creep constitutive equation obtained can predict the creep behavior of the nano-composites and the influencing law of the constitutive parameters.Atom models of the Mg alloy matrix nanocomposites with interface layer were built at nano-scale. The effects of the diameter, chiral parameters and surface coated nickel atomic layer of CNTs on the interfacial bonding strength and elastic modulus of the nano-composites have been investigated by molecular dynamics method. The calculated results show that the smaller the diameter of CNTs is, the higher the interfacial bonding strength and modulus are. Similary, the more the Ni atomic layers are, the higher the interfacial bonding strength and modulus are.Multi-scale experiments and numerical simulation have shown that enhanced mechanical properties of magnesium alloy matrix nanocomposites should be mainly attributable to the hybrid strengthening of CNTs and SiC nanoparticles, the coefficient of thermal expansion (CTE) mismatch between matrix and hybrid nanosized reinforcements, the dispersive strengthening effects (Orowan strengthening) and the grain refining (Hall-Petch effect).