Theoretical Study On The Deformation And Failure Behaviors Of Bulk Metallic Glasses And Their Composites

Due to their unique metastable structure,the bulk metallic glasses(BMGs)possess many excellent mechanical properties,such as high strength and hardness,large elastic limit and so on.However,owing to lacking the hardening mechanism,a significantly narrow and single shear band is easily formed during the deformation of BMGs at room temperature.Thus,the BMGs exhibit a macroscopic brittleness and often fail catastrophically at room temperature,when the applied maximum shear stress reaches to their yield strength.Because the inherent brittleness of BMGs restricts their applications in engineering structures,many researchers tried to introduce toughening phases into the BMGs to obtain the BMG based composites(BMGCs)with an excellent toughness.Although the toughness of BMGCs is better than that of their BMG matrix,it is still a great challenge to manufacture the BMGCs with an excellent tensile plasticity.Thus,a systematic study on the deformation and failure behaivors of BMGs and BMGCs is extremely necessary.Therefore,in this dissertation,the deformation and failure behaviors of BMGs and BMGCs are numerically and theoretically studied as follows:(1)Based on the existing failure mechanism of BMGs and the free volume theory,a 3-dimensional constitutive model of BMGs is developed within the framework of continuum mechanics;and the concentration of nano-voids in the BMGs is introduced into the constitutive model as an internal variable whose evolution characterizes the nucleation and coalescence of nano-voids during the deformation.Moreover,to describe both the brittle and ductile failure modes presented during the tensile and compressive deformations of BMGs,respectively,a failure criterion is established by introducing a stress triaxiality,which can describe the failure characteristics of BMGs under different stress states.Finally,through the finite element implementation of the developed constitutive model,the reasonability and effectiveness of the model are validated by comparing the simulated results with the experimental ones of BMGs;in addition,the nucleation and coalescence of nano-voids in the BMGs during the tensile and compressive deformations are also predicted.(2)The above-proposed model is simplified,and then implemented into a finite element code as a user material subroutine(UMAT)to describe the uniaxial tensile and compressive deformations of BMGs.Based on such a UMAT,a systematic numerical simulation is performed to investigate the deformation behaviors of BMGCs subjected to uniaxial tensile and compressive loadings.In these simulations,the effects of the volume fractions,shapes,orientations and yielding strengths of toughening particles on the deformation of BMGCs are investigated,and the obtained results are helpful for developing the meso-mechanical constitutive models of BMGCs.Moreover,the simulated uniaxial tensile deformations of the BMGCs containing different volume fractions,shapes,orientations and yielding strengths of toughening particles are compared with the compressive ones,and such a comparison helps us to understand the tension-compression asymmetry of BMGCs.(3)On the basis of existing experiments and obove-mentioned numerical study,a new two-leveled homogenization method is developed to reflect the local failure occurring in the composites by extending the traditional Mori-Tanaka's method.Based on the new two-leveled homogenization method and the failure-mechanism based constitutive model of BMGs proposed above,a meso-mechanical constitutive model is constructed to describe the deformation and failure of BMGCs under the monotonic tensile and compressive loading conditions.The accuracy and effectiveness of the developed model are futher validated by comparing the predicted deformation and failure of BMGCs under monotonic tension and compression with the corresponding experimental results.