Studying The Plastic Flow Mechanical Behaviors Of Glassy Materials Under Different Conditions

Glassy materials,including metallic glasses(amorphous alloys),polymer glasses,belong to the amorphous solids.Their structure are characterized by short-range order and long-range disorder,and they are non-equilibrium systems and energetically metastable.Glassy materials are formed by the rapid cooling of supercool liquids to the glass transition temperature.The dynamical properties of the glassy materials have changed dramatically during this process.Viscosity and relaxation times have increased by several orders of magnitude.Therefore,glassy materials are also referred to as"frozen liquids." Compared with the crystal,the glassy material does not have lattice,grain boundary,dislocation and other structural features,so the special microstructure of the glassy material determines its excellent mechanical,physical and chemical properties,such as high strength,high hardness,wear resistance,corrosion resistance,large elastic limit and good plastic deformation ability.Glassy materials have made a wide range of applications in many areas.Below the glass transition temperature,some of the particles inside the glassy material still have mobility,and these particles spontaneously and slowly move to cause structural relaxation,which is known as aging phenomenon,a ubiquitous physical feature of non-crystallites.Aging can have a significant effect on the flow-defect structure and mechanical properties of glassy materials.The dynamical heterogeneity and spatio-temporal correlation during the temperature and strain rate dependent flow-defect evolution are the two main concerns in the deformation of glassy materials.Therefore understanding the evolution of flow defects and aging effect is important to understand the mechanical behaviors of glassy materials and regulating their macroscopic mechanical properties.In this master's thesis,a dynamic strain control equation for the flow-defect evolution is proposed first.Based on the extended three-parameter Maxwell model,the mechanical behaviors of glassy materials at different strain rates,temperatures and aging times are studied and the effects of these loading conditions are analyzed.Then,we proposed a simplified Maxwell model combining with the concept of free volume defect concentration and the solid phase transformation kinetics theory.The model is used to study the mechanical response of glassy materials under different loading conditions,and the relations between the peak stress,overshoot stress and critical strain and loading conditions in stress-strain curves are analyzed and discussed.The plastic flow model based on the mean-field approximation developed here can capture many mechanical phenomena of deformation of glassy materials,like transition from Newtonian flow to non-Newtonian flow transition,stress overshooting,strain softening.The obtained results in this master's thesis facilitate understanding the plastic flow of glassy materials.