Deformation Behavior Of TiZr-Based Metallic Glass Composites In The Supercooled Liquid Region

The in-situ dendritic phase TiZr-based bulk metallic glass composites(BMGCs)have the characteristics of high amorphous forming ability and excellent comprehensive mechanical properties,showing great potential for engineering application as a structural material.The deformation behavior of BMGCs in the supercooled liquid region(SLR)is important for their processing and formability.However,there are few related studies,and the law and mechanism of how the second phase affects the deformation behavior in the SLR is still unclear,which hinders the development of processing technology and theory of in-situ dendritic phase BMGCs.In view of this,a series of TiZr-based BMGCs with in-situ dendritic phase volume fraction of 10-30%are selected to investigate how the experimental temperature,holding time,strain rate and dendritic phase volume fraction affect the deformation behavior of BMGCs in the SLR.Based on the free volume theory,the constitutive relation of rheology is constructed,and the relationship between deformation behavior and microstructure of the material is e aborated.Temperature plays a key role in the deformation behavior of TiZr-based amorphous composites.With the increase of experimental temperature,the yield strength,flow stress and overshoot stress of Ti35.7Zr35.6Cu8.3Be2.4 BMGC(TN10 for short,volume fraction of the dendritic phase is about 10%,the glass transition temperature Tg=618 K,initial crystallization temperature Tx=698 K)are gradually reduced;the work hardening rate of the material increases gradually and the deformation behavior of the material changes from work softening to work hardening;the fracture mode of the material changes from ductile fracture to brittle fracture;the structure of the material changes significantly during the deformation process:the amorphous matrix gradually crystallizes during the deformation process,and the degree of crystallization increases with the increasing of temperature and stress,and the dendritic phase undergoes martensite transformation during deformation to form ?"-Ti.Since the deformation behavior of the material in the supercooled liquid region is mainly dominated by the amorphous matrix,the dendritic phase moves along with the amorphous matrix during the deformation process,so the crystallization of the amorphous matrix is the main cause of the work hardening effect,and the phase transition of the dendritic phase contributes little to the work hardening rate of the material.Since the dendrite phase is very small and evenly distributed,the distribution of dendritic phases in the material before and after deformation does not show significant changes at the microscopic level.Holding time and strain rate are also important parameters affecting the deformation behavior of TN10 BMGC.The increase of the holding time leads to the BMGC exhibiting higher yield strength,flow stress and overshoot stress.However,the prolonged holding time leads to a significant decrease in the plastic deformation ability and work hardening effect of TN10.The increase in strain rate leads to a gradual increase in the yield strength and flow stress of TN10,but does not have a large effect on the plastic deformation ability.At 673 K,the lower the strain rate,the longer the time required for the same strain during the tensile deformation process,resulting in a more pronounced degree of crystallization of the amorphous phase in the material,thus exhibiting a higher work hardening rate.The dendritic phase volume fraction has an important influence on the deformation behavior of the material.Different volume fractions of T138 6Zr35 6Cu7 6Be18.2 and Ti14.4Zr35.8Cu6.9Be15.9 BMGCs(TN20 and TN30 for short,the dendritic phase volume fractions are about 20%and 30%)are prepared by adj usting the alloy composition,the Tgand Tx of the material don't change significantly as the volume fraction increased.Compared with TN10,the increase of the dendritic phase volume fraction leads to the decrease of the yield strength and flow stress of the material,and the plastic deformation ability of the material decreases;the microscopic inhomogeneity during the deformation of the material increases,resulting in a number of microcracks in the material during the deformation process.The deformation behavior of TN20 and TN30 showed great similarity,and the difference between the two was not obvious.The free volume model can well describe the deformation behavior of TN10 BMGC in the SLR.The strain rate sensitivity coefficient of TN10 increases with the increase of temperature and the decrease of strain rate,and the variation range is 0.37-1.which changes the rheological behavior of TN10 from non-Newtonian to Newtonian rheology.The activation volume Vact of the TN10 amorphous composite is calculated to be in the range of 0.138 nm3~0.151 nm3,and the activation volume gradually increases with the increase of temperature and finally remains stable;the frequency factor ?0 varies from 1.39×10-4 s-1~4.24×10-4 s-1,and the frequency factor increases with increasing temperature.By comparison with the reported amorphOous alloys and BMGCs.it is f-ound that the activation volume and frequency factor of TN10 are between the amorphous alloys and the BMGCs with a higher volume fraction of the second phase,and its value is closer to that of amorphous alloys,indicating that the deformation behavior of the material in the SLR is mainly dominated by the amorphous matrix,and the influence of the second phase dendritic on the rheological behavior of the amorphous matrix is not obvious;the viscosity of TN10 increases with temperature and strain rate.The normalized viscosity and theoretical master curve of TN10 can be well matched in the high strain rate range,but as the strain rate decreases and the temperature increases,the normalized viscosity value of the material gradually deviates from the theoretical main curve.However,the value remains at a stable value,indicating that the material has entered the Newtonian rheological state.