Study On Microstructure And Behavior Of Interfaces Between Metallic Glasses And Nano-crystals By Atomistic Simulations

Metallic glasses have attracted great attention from both scientists and engineers because of their superior properties,such as high strength,large elastic limit,wear and corrosion resistances,etc.However,there is an Achilles’heel for this material,room-temperature fragility,which hinders their wide spread application in engineering.Lots of work has been done in order to understand the deformation mechanism of metallic glasses and to improve their room-temperature plasticity.Recently,it was found experimentally that the plasticity of some metallic glasses could be improved effectively by introducing second phases in-situ or ex-situ to the amorphous matrix.Nonetheless,the underlying mechanism is unclear yet due to limited experimental means,especially for the interfacial microstructure,properties and the role of interfaces during the deformation process.In this work,by taking the Cu-Zr binary system that has a high glass-forming ability as an example,we therefore examined the microstructure,thermodynamics and kinetics properties as well as compression performance at low temperature of the B2-CuZr crystal/Cu₅₀Zr₅₀ amorphous interface systems.In this way,we try to throw light on the mysterious physics of crystal/amorphous interfaces and promote the development of metallic glasses.The main results are as follows:The crystalline/amorphous interface is not a simple two-dimensional curved surface,but a three-dimensional transition zone with a certain thickness and unique microstructure.Due to the complexity of the interface,the interfacial thickness is not consistent using different characterizing methods.For example,in the interface against B2-CuZr（110）,atomic number density analysis shows that the transition zone spans 5-6atomic layers,while it is found to be just about 2-3 atomic layers in terms of the in-plane structure of the interfacial atomic layers.Based on analysis on the local order parameter an interfacial thickness of 1.8?is measured while it is 8.0?by Voronoi polyhedron index method.Therefore,the interfacial thickness should be determined according to the physical essence involved in simulations like phase field modelings.Analysis on flat and spherical interfaces shows that the average atomic volume and local order of the interfacial atoms is in between that of the crystalline and the amorphous phases.No composition segregation is observed in the interfacial regions,while several kinds of polyhedral only abundent in the interfaces are identified,such as Cu-centered<0,5,2,6>and Zr-centered<0,4,4,6>,characterizing the special microstructure of the interfaces.From the viewpoint of topology,these two polyhedra are actually distorted crystalline polyhedron<0,6,0,8>which has to adjust its geometry configuration so as to accommodate the amorphous disordered structure.Moreover,such configurational adjustment facilitates the structural transition between the two distinct phases and hence reduces the interfacial energies.It turns out that the interfacial structure,interfacial energy and so on depends on the orientation of the crystalline phase.Among the three crystalline planes（100）,（110）and（111）,the interface against（110）has the smallest thickness,the most prominent layered ordering,the highest fraction of characterizing polyhedrons<0,5,2,6>and<0,4,4,6>,and the lowest interfacial energy（¹65 mJ/m²）.The secret lies in the resonance between the crystalline interlayer spacing and the inherent periodicity of the metallic glass.Temperature is another factor that influences the interfacial structure and properties.Annealing of the interface systems at high temperatures reveals that as the temperature increases,the interfacial atoms are well relaxed,and the fraction of characteristic polyhedra<0,5,2,6>and<0,4,4,6>increases,meanwhile the interfacial energy decreased.When temperature is above 600 K,crystallization is observed in the interfaces.The Arrhenius relationship is found to be obeyed by the crystallization rate and temperature.Moreover,the lower interfacial energy,the higher the crystallization activation energy.It should be noted that the increased temperature does not eliminate/change the anisotropy in the interfacial structure and properties.Notable size effect is observed in the microstructure and properties of interfaces between spherical nano-crystallite and amorphous matrix.As the nano-crystallite size increases,the fraction of characteristic polyhedra in the interfacial region decreases.In turn,the interfacial energy increases while the crystallization activation energy reduces.Finally,the structure and properties for spherical interfacesgradually converge to that of flat interfaces.The presence of interfaces changes dramatically the deformation behavior of the amorphous alloys.Uniaxial compression tests at low temperature demonstrate that crystalline-amorphous nano-laminateshave higher plasticity than the pure amorphous alloy.That is because the interfaces not only inhibit the formation of one main shear band and promote stress uniform distribution in the amorphous region,but also induce crystalline dislocation motion,which finally trigger more shear transformation zones in the amorphous regions and therefore improves the plasticity of the amorphous phase.In general,interface can effectively improve the amorphous plasticity by virtue of suppressing shear band formation and growth before strain localization,and promote the shear band propagation after strain localization.