Study Of The Fabrication And Room-temperature Mechanical Properties Of In-situ Ti-based Bulk Metallic Glass Composites

Bulk metallic glasses（BMGs）,which exhibit a series of excellent mechanical and physical properties because of its short-range order and long-range disorder of the structural characteristics,are currently taken to be candidate of structural materials and been widely investigated.However,a serious problem for their potential engineering applications is the rapid development of localized shear bands and subsequent catastrophic failure upon loading,which lead to a negligible plasticity especially in uniaxial tension at room temperature.In order to circumvent this drawback,the micrometer dendrite phase was introduced into the glass matrix to prevent the unstable extension and promote the multiple initiations of the shear bands,thus forming the in-situ BMG composite.Recently,many Ti-based and Zr-based BMG composites have been developed.Nevertheless,most of the present studies about in-situ BMG composites mainly focused on the effects of the volume fraction and size morphology of the dendrite on the mechanical properties of BMG composites.However,the existing experimental results indicate that the macro-mechanical properties are not only denpendent on the microstructure of the BMG composites,but also on the intrinsic mechanical properties.Moreover,because the macroscopic strain-softening phenomenon with an early onset of necking always appears upon the loading process,the potential application of such in-situ BMG composites has been restrained.Therefore,it is both physically meaningful and industrially useful to study the relationship between intrinsic mechanical properties and macro-mechanical properties of the BMG composites.Based on the above reasons,a series of Ti-based BMG composites with excellent room temperature plasticity were synthesized by copper-casting method,and the intrinsic mechanical properties of the dendrite and the glass matrix of BMG composites were systematically studied by the nanoindentation and the resonant ultrasound spectroscopy,then the underlying relationship between the intrinsic mechanical properties and macro-mechanical properties were also been discussed.The main conclusions are as follows:（1）The effect of low modulus elements on the macroscopic mechanical properties of Ti-based BMG composites were studied in detail by adding low modulus element Sn into the Ti₄₇Zr₂₅Nb₆Cu₅Be₁₇ alloy.It is found that the Sn element is distributed in the dendrite and glass matrix,and the intrinsic mechanical properties of the two phases are obviously changed.The addition of Sn elements does not obviously change the average size of the dendrite,however,when the content of Sn is more than 3 at.%,the Zr₅Sn₃ intermetallic compound phase is precipitated,and then the compressive plasticity of the Ti-ased BMG composite is distinctly decreased.With the increase of the content of Sn,the yield strength of the BMG composites decreases gradually.Based on the above results,it is proposed that the macroscopic mechanical properties of BMG composites can be controlled by adding low-modulus elements to tailor the intrinsic mechanical properties of the dendrite and glass matrix.（2）The stress-strain response and shear band propagation behavior of Ti₄₅Zr₂₅Nb₆Sn₂Cu₅Be₁₇ BMG composite at different strain rates were systematically studied.It is found that the Ti₄₅Zr₂₅Nb₆Sn₂Cu₅Be₁₇ BMG composite has negative strain rate sensitivity,i.e.higher loading strain rate leads to lower yield strength and the compression plasticity.At high strain rate,the adiabatic shear causes more obvious thermal effect shear stress and critical shear stress,and finally the yield strength is decreased.At low strain rate,the shear toughness of dendrite is higher than that of the glass matrix,so that the dendrite can hinder and stabilize the propagation of the shear bands,and that the compression plasticity of the Ti₄₅Zr₂₅Nb₆Sn₂Cu₅Be₁₇ BMG composite is mainly determined by the extended mode of the senondary shear bands.Based on the above results,the mechanism of the effect of shear band extension on the macroscopic mechanical properties of BMG composites is determined.（3）The effect of intrinsic mechanical properties on the macroscopic compressive mechanical properties of BMG composites was systematically studied by replacing Cu elements in Ti₄₅Zr₂₅Nb₆Sn₂Cu₅Be₁₇ BMG composite with transition metal Ni,Co and Fe elements.It was found that the transition metal elements were mainly distributed in the glass matrix and the intrinsic mechanical properties were obviously changed,while the average size and volume fraction of the dendrite were not significantly changed.The yield strength of the BMG composites was obviously determined by the yield strength of the glass matrix.The increase of the average stress concentration factor of the dendrite phase can promote the multiple initiation and stable extension of the shear bands,and then improve the compression plasticity of the BMG composites.Based on the above results,the design method of plastification for BMG composites is proposed:the Young’s modulus of glass matrix and dendrite are controlled to ensure elastic matching.This method is consistent with the experimental data of the existing Ti-based,Zr-based BMG composites,thus indicating the universality of this method.（4）The effects of intrinsic mechanical properties of the dendrite and the glass matrix on the mechanical properties of macroscopic tensile properties in Ti BMG composites were systematically studied.The results show that the tensile plasticity and Poisson’s ratio of the BMG composites are not directly related.When Young’s modulus of the dendrite and the glass matrix was matching,the BMG composite has more than 8%tensile plasticity;the lower the shear modulus,the more obvious the hardening ability of BMG composites,which is mainly due to the fact that lower shear modulus of dendrite can dissipate the cumulative stress at the interface during the deformation through the dislocation slip in the dendrite.Then the shear bands in glass matrix will not be premature initiation and rapid extension caused by shear softening.Based on the above results,the design principle of Ti-based BMG composites with work hardening ability is proposed:the shear modulus and Young’s modulus of dendrites should be reduced by alloying method,which makes them have better plastic deformation ability.