Research On Room-Temperature Rheology-Mechanical Behavior Of Bulk Metallic Glasses At Micro-and Nano-Scale And Its Mechanism

Bulk metallic glass （BMG） offers novel physical, chemical and mechanicalproperties and has perspective potential applications. As one of the hot topics offrontier science and technology, BMG has received extensive attention both fromscientific community and material engineering. Fundamental problems in amorphousmaterials, e.g. structure, glass forming and deformation mechanism etc. are significantissues of current research. Furthermore, rheological behavior of BMGs provides newresearch topic for rheological mechanics. In this thesis, several BMGs were chosen asobjects of the research. Based on the micro-structure and physical essence of thematerials, micro-nano rheological mechanical behavior and related mechanism ofBMGs were studied both by theoretical and experimental methods. Meanwhile, thecorresponding rheological model was established. The main results are summarized asfollows:1. A fractal model was proposed for describing the distribution features of thestructural heterogeneities of metallic glasses （MGs） ranging from the scale of mediumrange order to¹0nm or even larger scale （may reach^μm）. The validity of themodel was proved by comprehensive analysis and fractal dimension calculation.2. Based on the fractal distribution features of the structural heterogeneities ofMGs, fractional-order differential rheological model which can match fractal treenetwork structure was introduced to study the room-temperature rheologicalbehaviors of BMGs. The properties of this model were discussed according to theRiemann-Liouville definition of the fractional derivatives. Practical simplifiedexpressions for the creep compliance and relaxation modulus were given as well asthe range of parameters of the model under the thermodynamic constraints. Theproblem of spherical indenter into fractional viscoelastic half-space was solved byutilizing the constitutive equations of the fractional rheological model. The responsesof the fractional viscoelastic body by step-loading and constant-loading-ratenanoindentation were derived.3. Room-temperature viscoelastic nanoindentation tests were performed onPd₄₀Cu₃₀Ni₁₀P₂₀，Zr₄₈Cu₃₄Pd₂Al₈Ag₈and(Fe_0.432Co_0.288B_0.192Si_0.048Nb_0.04)₉₆Cr₄BMGs.The results show that the fractional rheological model can characterize the delayedelastic behavior of these BMGs accurately and reflect the overall structureinformation of the materials properly and effectively. The rheological parametershave clear relationship withTg T and own clear physical significance. 4. A new fractional-differential rheological model was presented forcharacterizing the room-temperature nanoindentation creep behavior of BMGs byconsidering the fractal growth of the structural heterogeneities caused by yielding.This model contains the traditional rheological model. By appropriate simplification,the empirical equation used to characterize the nanoindentation creep behavior can bederived from the proposed rheological model. Well-designed nanoindentation creepexperiments were carefully performed in (La_0.5Ce_0.5)₆₅Al₁₀Co₂₅BMG to charactize therecoverable viscoelastic deformation and unrecoverable viscous flow respectivelyfrom the room-temperature creep deformation. Combing the nanoindentation creepexperimental results of several BMGs with different constituents, effects of the creepload （Pm ax） and loading rate （P） on the creep behavior as well as the trends of themodel parameters were investigated. It shows that the proposed rheological modelwith few parameters possesses high fitting accuracy and clear physical significance.AsPm axchanging, the viscoelastic deformation shows some of linear nature, whileapparent creep indentation size effect is clearly observed in viscous flow deformation.The effect of P on creep behavior is pronounced and reflected by the trends of therheological model parameters. Trends of parameters e.g. viscosity index （η）,fractional order （α0） and relation betweenα0and creep stress exponent （n） wereanalyzed with great emphasis. The results show thatα0exhibits similar trend as nchanging with P. η is a loading history dependant index which characterizes theinstant viscosity of the materials when creep is starting.α0reflects the internalstructural information and flowing trend during the creep period. The controversialtrends of n changing with P reported in the literatures is explained reasonably onthe base of the physical significance of the model parameters. Taking Mg₆₅Cu₂₅Y₁₀and Mg₈₅Cu₅Y₁₀BMGs as examples, effects of relaxations on the creep behaviorwere discussed, which further verified the physical meanings of the proposedrheological model.5. An expression correlating Nanoindentation hardness （H） with strain rate（ε （t））, free volume concentration （c f（t）） and loading time （t） was derived based onfree volume theory. The micro-mechanism of indentation size effect （ISE） in creepand hardness was proposed as strain softening caused by applied load. The strainsoftening is affected by the initial free volume concentrationcf, ε （t）and t. Withthese in mind, the controversy in literatures about the mechanism of ISE in BMGswas explained in reason. Finally, the strain softening effect was confirmed by nanoindentation performed with atomic force microscopy.