Reseaches On Constitutive Relationship And Deformation Of The Bulk Heterogeneous Metallic Glasses Via Finite Element Method

Metallic glass which is obtained by rapidly quenching,is the new amorphous alloy and is totally different with the counterpart of the traditional crystal metal.As their long-range disorder and short-range order in the atom arrangements,the unique atomic-level structure will let them obtain the impressive physical,chemic,magnetic and special mechanical properties.However at the ambient temperature the deformation of bulk metallic glass under the tension or compression will be catastrophic failure without the significant macroscopic plastic deformation,applications as structural materials are currently limited by this inhomogeneous deformation.One considers the methods which can improve the plasticity of the counterpart of crystal metal such as surface treatments(cold rolling,shot-peening,laser-burning etc),thermal treatment and composites and so on.Due to the absence of long-range order or dislocation-like defects as in crystalline alloys,the mechanism of metallic glass is totally different with the crystalline alloys.For the crystalline alloys,the plastic deformation mechanism is dislocation-slip and the plasticity lies on the defect dislocation,grain boundaries and grain sizes;however the microstructure and deformation mechanism of metallic glass are still not clear.For the difference of mechanism,Lots of tries can improve the plasticity or toughness of crystal metal,but fail to change the structures of BMG and improve the plasticity of BMGs.So it is very important to study and research the deformation mechanism of metallic glass before we try to improve the plasticity of metallic glass.The theory should go further than the experiment and give the guideline.However many of the theoretical researches about the deformation mechanism have been proposed,the deformation mechanism is still uncertain.One of most popular theory is the decrease of viscosity by shear stress based on the free volume theory which is proposed by Spaepen.Here we will set the macroscopic constitute relationship based on the micro-deformation mechanism of free volume theory.As we know mechanic property depends on the structure of materials included the BMG and metal.The free volume will be as the intrinsic structure parameter of metallic glass,which can reflect the microstructure of metallic glass.We choose random packing of hard spheres model for the microstruture model.So the totally different initial free volume distribution or initial stress or strain state,which means different structure,can change the mechanical property of metallic glass.In other standpoint,we can do the material computation and material design to improve the plasticity or toughness of metallic glass in our constitute relationship through choosing the different initial free volume distribution.So it is very clear that how important to study and reseach the deformation of metallic glass or metallic glass composite based on the free volume theory.Right here our researches include three parts:First,we should set the macroscopic elasctic-plastic deformation constitute relationship which we combine the free volume theory and Drucker-Prager yield criterion.Second,we will show how the initial statistic distributions of free volume affect the mechanical property of monolithic bulk metallic glass.We are able to see clearly the variation of free volume and its effects on plasticity.As we increase the variance of free volume,the metallic glasses start to exhibit a fundamental shift in their mechanical behaviors.Third,the spatial distributions of free volume(such as gradient distribution,phase decomposition,etc)can change the property of the heterogeneous bulk metallic glasses.To set the constitute relationship in finite element model,initial statistic distributions but randomly spatial distribution can change the deformation from the brittle to plastic,we even see change from strain-soften to the work-harden when the variance of statistic distributions of the free volume is larger enough.In the gradient spatial distribution,we get the free volume gradient effect which means the shear band need more energy to cross the high absolute gradient zone and the work-harden derives from the obstacle and stop of propagation of the shear bands.In the models of Spinodal decomposition metallic glasses,we find that orientation effect of spinodal pattern is good for plasticity,the intrinsic size effect of spinodal pattern is fellow the minimum size compatibility principle.The statistical effect of free volume plays a very key role on the mechanical properties of spinodal decomposition metallic glass composites.In the Binodal of phase ceomposition,we find the matrix effect which means the metallic glass matrix play a very important role in the change the mechanical property,though we always focus on the effect of particles.Also we know the shear band angle depends on the stress state and free colume distribution(microstructures).We see how the shear bands initially form,propagate,branch and stop in the bulk heterogenous metallic glassess.