| The excellent properties of bulk metallic glass?BMGs?,such as high hardness and strength,large elastic limits,stemming from their unique microstructure,have rendered them as promising candidates of their traditional crystalline counterparts.However,lack of macroscopic plasticity has been one of the vital problem for their application as structural materials.In recent years,the advent of BMG composites with in-situ precipitated crystalline dendrites has provided an effective and promising way to circumvent the brittleness.Among them,Ti-based BMG composites have attracted numerous concern for their high strength,large plastic,as well as low density,high specific strength and moderate young's modulus.In the present study,we focused on the Ti-Zr-Nb-Cu-Be BMG composites.From the pespective of intrisic microstructural optimization,the relationship between macroscopic properties and the intrisic properties of the second phase are investigated.In addition,the effects of pre-deformation on the thermal stability,shear evolution and properties of Ti48Zr20Nb12Cu5Be15?Ti48?BMG composite are also discussed.Based on above studies,the deformation behavior of Ti48 BMG composite within wide range of temperatures and strain rates are investigated systematically.The mechanism of ductile to brittle transition?DBT?at crygonic temperature and high strain rate are uncovered.The main contents and conclusions are listed below:?1?By adjusting the Nb content in?Ti0.48Zr0.20Cu0.05Be0.15?100-xNb12-x,the properties of dendrites and glass matrix can be optimized with holding the same volume fraction and morphology of second phase.The thermal stability of glass matrix increases with the increase of Nb content.The lower shear modulus of dendrites than that of glass matrix is in favor of the impedement and absorption of shear bands.The relationship between yield strength and size of second phase fits the linear relationship:?y?28?1070?10?839D-1.The plasticity of Ti48 BMG composites is proportional to the size of second phase first and then decreases.?2?Proper cold-rolling processing can improve the micro and macro properties of Ti48 BMG composite.After 20%cold rolling,the plasticity of Ti48 BMG composite turns from brittlenss to 8.6%plasticity.The evolution of shear bands morphology during cold rolling are observed.The excessed cold rolling causes the deterioration of whole plasticity due to the pre-existed micro cracks formed along the shear bands.?3?The effects of external condition on the deformation behavior of Ti48 BMG composites are studied.Remarkable compressive and tensile mechanical properties have been firstly obtained in these in-situ BMG composite.At 77K,the tensile yield strength of Ti48 BMG composite can be as high as 2120 MPa together with a fracture strain of about 8%.The deformation inside dendrites,characteristic of fragment of dendrites and multiplication of shear bands and slip bands,play an important role on the effective coordination between dendrites and glass matrix.?4?Ti48 BMG exhibits excellent dynamic mechanial properties within the temperature range from 273K to 573K.The plasticity is about 11%,which can be attributed to the effective accommodation of dendrites to glass matrix by hindering the propagation of shear bands and stimulate their multiplication.At 77K,Ti48 BMG composite suffers from DBT under dynamic loading.Based on the Johnson-Cook model,the constitutive equation between yield strength and temperature under quasi-static and dynamic deformation are established respectively.?5?The influence of strain rate on the ambient,cryogenic and high-temperature deformation behavior of Ti48 BMG composites are discussed.At cryogenic temperature,the thermal softening within the critical shear bands causes the negative strain rate sensitivity.Base on the physical mechanism below the positive to negative strain rate transtion at ambient temperature,the constitutive equation of each stage are established.Ti48 BMG compsite exhibits homogenous deformation within the supercooled liquid region.However,the nanocrystallization at 713 K during lower strain-rate deformation(2×10-4/s and 5×10-4/s)leads to the strain hardening.Ti48 BMG composites show inhomogenous plastic deformation under dynamic loading within supercooled liquid region and the plasticity is about 9%.The relationship between flow stress and temperature follows the linear relationship. |
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