Mechanical Property Design Of Metallic Glasses And Their Fracture Behaviors Under Complex Loading Conditions

Metallic glasses(MGs)have been regarded as promising structural materials owing to their desirable mechanical properties such as high strength,high hardness and superior elasticity.However,engineering applications as structural components under complex loading conditions demand not only outstanding mechanical properties of materials but also the developed fracture theory in order to guarantee the safety and reliability.In this thesis,a comprehensive study was conducted to understand the deformation and fracture behaviors of MGs under room temperature and quasi-static loading conditions,as well as complex loading conditions including high temperature,cryogenic temperature and high strain rate.Firstly,the inherent relationships between compositions of MGs and their mechanical properties were established.Secondly,the roles of complex stress state,temperature and strain rate in the deformation mechanisms and ductile-to-brittle transition(DBT)of MGs were clarified,based on which the variations of dynamic strength and dynamic hardness were discussed.The understanding of above questions should provide important insights into designing MGs with attractive mechanical properties and ensuring the safety in engineering applications of structural components.By taking into account both the ellipse criterion and the temperature-based glass-forming ability(GFA)criterion,here we built quantitative correlations among compositions,elastic constants,GFA and mechanical properties of MG,which enable one to predict the GFA,fracture strength and fracture surface simultaneously in advance once the composition of MG is given.Experimental data confirms the validity of this approach in prediction.Finally,a strategy for designing MGs with optimal combinations of strength,toughness and GFA is proposed,which allows for discovering glass formers with excellent mechanical properties and selecting optimal materials in engineering applications.In order to uncover the effect of normal stress on the deformation and fracture behaviors of MG,both the tensile and compressive tests were carried out in a wide range of temperature(173 K-623 K)by using a TiZr-based MG.With decreasing temperature,the resistance against shear-band cracking was enhanced by compressive normal stress and cryogenic temperature,which results in the fracture mode transition from instant shear fracture to gradual shear band cracking,accompanying with improved compressive plasticity.At high temperatures,the tensile normal stress can largely promote the transition from shear localization to homogeneous deformation while the compressive one may slightly suppress it.Hereby,for the first time the deformation maps under both compression and tension for an identical MG were constructed.Moreover,the tensile normal stress would facilitate the diffusional relaxation induced by high temperature,leading to the strain hardening behavior in supercooled liquid region.These results emphasize the critical role of stress state in the deformation and fracture processes of MGs at different temperatures.The DBT behaviors of a TiZr-based MG caused by cryogenic temperature and high-speed loading were comprehensively studied by using uniaxial tension,compression and notched tension tests.On the one hand,with decreasing the testing temperature,the MG shows obvious increments in tension-compression strength asymmetry and deviations of shear-fracture angle from 45°,as well as the tendency for normal fracture in notched tensile samples,implying that the MG is prone to brittle fracture at cryogenic temperatures.Thereafter,the low-temperature failure criterion was established and applied to various MGs,the variations of critical normal fracture stress and critical shear fracture stress with temperature were constructed,demonstrating that the low-temperature induced brittle fracture is the result of increased critical shear fracture stress but decreased critical normal fracture stress.On the other hand,as increasing strain rate,the MG displays profound decrement of fracture strength and normal fracture phenomenon under both tension and notched tension,indicating that the fracture mode is transformed from shear fracture to normal fracture.The DBT at high-speed loading is essentially caused by the significant decrease of critical normal fracture stress.The above results suggest that the occurrence of DBT in MG is primarily determined by the competition between shear deformation and normal fracture.A systemic investigation on compressive strength,Vickers hardness and failure mechanism in a wide range of loading rate was conducted by using Zr-based MGs with different toughness levels and AISI 4340 high-strength steels with different strength levels.It was found that the MGs show strain-rate softening behavior while the steels exhibit strain-rate hardening behavior.However,the strain rate sensitivity,characterized by the normalized dynamic strength(NDS)or the normalized dynamic hardness(NDH),were found to rely on not only materials but also loading mode.For MGs,NDS increases with decreasing toughness while NDH shows the opposite trend.This is because the fracture under uniaxial compression is primarily governed by the critical normal fracture stress,especially for the brittle MG,but the deformation under indentation is mainly controlled by the critical shear fracture stress.For high-strength steels,with the increase of material strength,the NDS significantly increases yet the NDH decreases slightly.The inconsistency between the variations of NDS and NDH is because the hardness is governed by not only the yield strength but the strain-hardening ability.As rising strain rate,although the yield strength increases,the strain-hardening ability reduces obviously,particularly for high-strength materials,in which a transition from homogeneous deformation to shear fracture is also caused.The variations of NDS and NDH could reflect the effects of strength and toughness on resisting dynamic impact,respectively,allowing to propose a criterion for locating the optimal strength-toughness combination against dynamic perforation,which was further verified by the ballistic penetration testing results.Moreover,similar trends of NDS and NDH in various other alloys were also found,which may suggest the extensive applicability of the proposed criterion in engineering field.This study unveils the effects of intrinsic material properties(including chemical composition and microstructure)and extrinsic factors(including temperature,strain rate and complex stress state)on the fracture behaviors of MGs,which not only implies that mechanical property and processability could be improved via adjusting chemical compositions,microstructures and stress states,but also will be helpful for avoiding brittle fracture induced by cryogenic temperature and high-speed loading.