Study On The Correlations Of Plasticity/Toughness With Physical Properties And Structure Heterogeneity In Zr-based Metallic Glasses

Metallic glasses(MGs)have shown great potential applications due to their high strength and elastic limit,excellent corrosion resistance and soft magnetic properties etc.Since the atomic packing in MGs is disordering in long-range but ordering in short-range,many MGs have been found to exhibit poor,or even zero macroscopic plasticity before fracture at room temperature,which restricts their application for structural engineering materials.Therefore,it is of great significance to develop MGs with macroscopic plasticity and to study the intrinsic mechanism of plasticity and toughness.In the present thesis,Zr-based bulk metallic glasses(BMGs)were chosen for the object of study.The correlation of plasticity to the fragility in molten and supercooled liquid region has been studied systematically.By adding the elements which have the positive mixing enthalpy with those in BMGs,the effects of phase separation and ?-relaxation on the compressive plasticity at room temperature has been investigated.The correlation of phase separation and deformation induced nanocrystallization(DINC)to compressive plasticity/toughness of Zr-based BMGs have been further studied.The research work in the present thesis contains the following four parts:(1)In order to explore the origin of the plasticity of BMGs,the physcial parameter which can characterize the properties of glass formation liquid has been employed to study the relationship among the molten liquid properties,fragile-to-strong transition,relaxation,structure and plasticity.It is shown that there is a negative relationship between plasticity(?)and the slope of molten liquid T/TL= 1(T:the temperature,TL:the liquidus temperature).Based on the present experiments measured and previously reported values of fragility,m,for 51 glasses,it has been found that there is a positive relationship between m and ?with the equation of ?= 92exp[-73/(m-15)].The properties of the molten and supercooled liquid can be related to the fragile-to-strong(F-S)transition.The results indicate that there is a negative relationship between the plasticity and the F-S transition temperature.The intrinsic mechanism of the plasticity can be well interpreted in the frame of structural relaxation theory.That is to say,there is a negative relationship between the degree for the F-S transition and the difference in the structure unit for ? relaxation and ? relaxation.The higher degree of F-S transition,the lower difference between the structure unit for ? relaxation and ?relaxation and vice versa.There is negative relationship between the plasticity and the degree of F-S transition,however,it is positive between plasticity and structure units for a relaxation and ? relaxation,which means the structure between a relaxation and ? relaxation is more different(or the F-S transition is more lower),the plasticity is more larger,and vice versa.(2)Zr50Cu41.5 5Al5.5Mo3 BMG with compressive plasticity and strength above 20%and 2500 MPa,respectively,has been prepared by the addition of Mo,which has positive mixing enthalpy with Cu,into Zr-Cu-Al BMG system.The analysis of transmission electron microscope(TEM)and energy spectrum shows that there are Cu-rich and ZrMo-rich zones in this BMG,meaning that phase separation takes place.Dynamic mechanical analysis(DMA)shows that obvious(3-relaxation peak occurs on the spectrum due to the addition of Mo element.It is the synergistic effects of phase separation and ?-relaxation that result in the pronounced plasticity of this BMG.(3)By the minor addition of Nb and adjusting the proportion of Zr and Cu in Zr-Cu-Al-Nb BMGs,new BMGs with different plasticities and obvious work-hardening have been prepared.In these BMGs,the plasticity and fracture toughness of Zr50Cu44Al5.5 Nb0.5 BMG reaches above 40%and 151MPam,respectively.The TEM study indicates that phase separation occurs in as-cast alloy,which is featured by the formation of ZrAl-rich,Cu-rich regions.At the same time,DINC takes place,and the nanocrystals with the size smaller than 5 nm have been formed in the whole region,especially in shear bands(SBs).The tendency of the instability of SBs can be greatly decreased by DINC,and multiple SBs are formed,resulting in the improvement of plasticity or toughness.(4)Two step work-hardening phenomenon has been discovered in Zr54 75Cu21.15Fe4 7Al9.4 4Nb6Co3Mo BMG by the combined addition of Mo,Nb,Fe and Co elements and the adjustment of the proportion of Zr to Cu in the BMG.The first step work-hardening of the alloy occurs at the plastic deformation?5%.Phase separation occurs around the SBs at first step work-hardening,and the size of the second amorphous phase is smaller than 3 nm.The second step work-hardening occurs after plastic deformation-5%,which is caused by DINC.Under the effect of two step work-hardening,the compressive strain of Zr54 75Cu21 15Fe4 7Al9 4Nb6Co3M01 BMG reaches above 20%at room temperature.