Simulation Studies On Interfacial Mechanical Properties And Dynamic Tensile Behavior Of Magnesium Matrix Nanocomposites

As a kind of lightweight composites,magnesium(Mg)matrix nanocomposites have good application prospects in aerospace,automotive,military and other fields due to their high specific strength,specific stiffness and excellent damping capacity.Nanosized reinforcements play a key role in improving the interfacial stress transfer,the interfacial and overall mechanical properties of Mg matrix composites,but much less attention has been paid to the interfacial mechanical behavior of the Mg matrix nanocomposites.In the thesis,the interfacial mechanical behavior and dynamic tensile response of the Mg matrix nanocomposites reinforced with hybrid silicon carbide(SiC)nanoparticles and carbon nanotubes(CNTs)were studied in depth by using a trans-scale and multi-step simulation approach based on molecular dynamics(MD)and finite element(FE).The interfacial debonding and its effect on the dynamic damage evolution of the composites were systematically analyzed.In addition,the prediction method of interface effects on the dynamic performance of the Mg matrix composites was explored.The specific research topics and main results of this paper are as follows:Two different kinds of coherent Mg/SiC interfaces models were established and analytical expressions of interfacial pair potentials as a function of the interfacial adhesive energy have been derived by the lattice inversion method.Ab initio calculations of the adhesive energies for Mg/SiC interfaces were carried out and the C-Mg and Si-Mg pair potentials were obtained by the inversion formula..The self-consistency and transferability of the inversed pair potentials were validated by comparing adhesive energies predicted by the ab initio method with those calculated by the inversed pair potentials.The results show that the inversion method is self-consistent and the obtained pair potentials are of good transferability for some other interface models.Based on the inversed pair potentials,the crack propagation and fracture mechanics of Mg/SiC interfaces were investigated by MD simulations and cohesive zone models(CZMs)of the interfaces were established under different loading angles.Under the pure tensile conditions,four typical asymmetric crack propagation modes of the Mg/SiC interfaces were found:(1)the crack propagates rapidly along the interface,the crack tip is sharp and the separated interface is smooth;(2)the microcracks are formed and gathered at the front of the main crack during the main crack propagation and the separated interface is relatively rough;(3)the crack tip blunts and the crack almost does not propagate due to dislocation nucleation and emission from the crack tip;(4)the twinning is formed directly at the prescribed crack tip after elastic deformation,and the microcracks at the end of the twin boundaries spread into the matrix and propagate along the twinning boundary.The results show that the interfacial orientation and lattice mismatch have a significant effect on the crack propagation of the Mg/SiC interface.In addition,the influence rules of interfacial bonding strength,temperatures,strain rates and loading angles on the interfacial crack behavior were analyzed respectively.It is thought that the interfacial bonding strength and loading angles are the dominant factors influencing the brittle-to-ductile transitions in the interface fracture.The Rice-Thomson model was also used to explain the brittle to ductile transition mechanism in the Mg/SiC interface fracture.The interfacial debonding behavior of the Mg/CNT interface was studied by MD simulations by taking into account different CNT inner diameters,wall numbers and surface characteristics as well as matching directions with Mg matrix.A novel CZM for describing the traction-separation law of the Mg/CNT interface has been built and the CZM key parameters were calculated.The initial stress of Mg/CNT interface decreases with the increasing wall numbers but increases with the increasing inner diameters.For the interface between the multi-walled CNT and Mg matrix,the peak stress decreases while the work of separation increases with the increasing inner diameter.On the whole,both the peak stress and work of separation are relatively high when the inner diameter is 1.356 nm.In addition,interfacial adhesion ability for the model with CNT axis along the Mg[0001]direction on the Mg(1010)plane is more outstanding comparing with the model with CNT axis along the Mg[2110]direction on the Mg(0001)plane.Compared with the above two kinds of interface models without a Ni layer,the average work of separation values for the interface models with a Ni layer are increased by 82.28%and 85.99%respectively,and the average peak stress values are increased by 100.92%and 127.22%respectively.It indicates that the addition of a Ni modified layer on the surface of CNTs can effectively enhance the adhesion capacity of the Mg/CNT interface.The Mg/SiC and Mg/CNT interface constitutive models were both implemented in a user-defined subroutine VUMAT of ABAQUS code,and the FE numerical simulation of the dynamic tension process for the Mg based hybrid nanocomposites containing SiC nanoparticles and CNTs at elevated temperatures and different strain rates was carried out using the commercial software ABAQUS 6.12/Explicit.The FE simulation focuses on the effects of microstructure factors such as the interface bonding strength,reinforcement volume fraction,size,distribution and hybrid ratio on the dynamic tensile response and damage evolution of the composites.The corresponding influence laws were thus summarized.Moreover,the simulation results were compared with the experimental results to verify the accuracy and applicability of simulation models.The numerical simulation and experimental results are in good agreement within a certain range of error.Based on the above analysis,a quantitative characterization method for the reinforcement distribution in the composites microstructures has been proposed and the effect of reinforcement distribution on the properties of nanocomposites with a high reinforcement volume fraction was successfully predicted.The analytical method will provide an important reference to predict the dynamic mechanical properties of the hybrid composites.