Research On Exploring The Glass Forming Ability Of BMGs Based On Electron Transport Property, Internal Friction And Melt Property

Owing to the unique property and application prospect of bulk metallic glasses （BMGs）, inrecent half a century, reseachers are trying to explore new alloy systems and new preparationtechnology continuously, and exploring the essence of glass forming ability （GFA） from physicalproperties of BMG solids-supercooled liquid-equilibrium and overheat liquid. At present, there aregreat progresses on GFA criteria and the empirical rules on selecting compositions of BMGs.However, the acquaintance of the essence of GFA and designing rules of new BMGs still needcontinuously exploring.Multicomponent alloy systems （CuZr-based, LaCe-based, and La-based） with different GFAare selected in this paper, by means of heating and cooling, the temperature behaviors of internalfriction property, electron transport property （resistivity） of BMGs with different GFA are explored,and the relationship and essence between GFA and above mentioned property are discussed based onthe phenomenon and laws. For further, new GFA criterion and designing principle are tried toproposed, and the effectivenesses are tested. Besides, the possibility and mechanism of temperatureinduced structure transition of CuZr binary alloy melt was explored, and the possibility of the effectsof melt temperature on GFA （critical diameter） of BMGs was also studied, meanwhile, the differenceof crystallization behavior and crystallization products of BMGs with different melt temperature isanalyzed.Based on the above results, the physical mechanism of liquid-solid transition andsolid-solid transition are discussed based on the understanding of phase competition, and new CuZrbinary BMGs with largest critical diameter are successfully prepared. The main conclusions andinnovation points are summarized as follows:1. The internal friction results of BMGs on heating condition reveal that the internal frictionvalue （Q-1） and relative modulus （M） have no significant changes before glass transition. Wheneverglass transition appears, the relative modulus decrease obviously which means the material changesfrom anelastic to visoelastic. Accordingly, Q-1begining to increase, and internal friction peak willappear after crystallization, afterwards, the internal friction tend to decrease until the end ofcrystallization. The internal friction peak which starts from glass transition and ends with the final ofcrystallization implies that internal friction peak includes both glass transition process andcrystallization process. The obvious characteristic of “Q-1-T” can be summarized as follows: with thesame vibration frequency, the higher the heating rate the larger the internal friction peak temperature;the internal friction peak value decrease with the increase of frequency.2. By studying the Q-1-T spectrum and frequency spectrum of different BMGs, a new methodfor determining the crystallization temperature of BMGs is proposed: on the Q-1-T curve, Txcan be determined from the inflexion which locates on the left side of internal friction peak （the peaktemperature of d Q-1-dT curve）. By studying internal friction behaviors of a series BMGs withdifferent GFA, the results reveal that there exist certain relationships between GFA and the internalfriction value of inflexion: For BMGs with same base alloy, the higher the GFA, the larger theinternal friction value of inflexion.3. For the BMGs that have been studied in this paper, on heating from room temperature to acertain temperature before glass transition temperature, the temperature coffecient of BMGs isnegative with the temperature elvating. By fitting the electrical conductivity according to lnσ—1/T,the result is well in accordance with the physical model:σ=σ0exp（ΔE/（k BT））, e. g.,corresponding to the electron transport mechanism of semiconductor which proposed by Mott: theelecton which is bounded in the “localized state” can jump to the “extended state” as a result ofthermal activation which will cause the decrease of the resistivity.4. Owing to the special electron transport property of BMGs, new GFA criterions which relatewith electron transport property are proposed: reduced resistivity （ρr=ρamor/ρcrystal） and storageresistivity （Δρ=ρamor-ρcrystal）. The results of resistivity measurements of a series BMGs reveal that thehigher the GFA, the larger the value of ρrand Δρ. Taking the electron transport criterion of BMGs asthe guiding principle of GFA, glass forming ability of Zr₅₇Cu₂₀Al₁₀Ni₈Ti_5-xAg_x（x=0,1,2,3,4,5）BMGs with respect to Ag content are studied by element substitution. The results of resistivitymeasurements reveal that Zr₅₇Cu₂₀Al₁₀Ni₈Ag₅BMG has the largest ρrand Δρ, it implies thatZr₅₇Cu₂₀Al₁₀Ni₈Ag₅BMG has the best GFA. By preparing Zr₅₇Cu₂₀Al₁₀Ni₈Ag₅BMG using coppermould casting method,20mm rod with fully amorphous structure and high strength and plasticitycan be cast.5. Based on the understanding of Cu₁₀Zr₇phase which always competing on the glass formationof Cu₆₄Zr₃₆BMGs, by substituting7.5%Zr with Ti, the precipitation of Cu₁₀Zr₇phase can be fullyrestrained for the new alloy on copper mould casting process, and the competing crystal phase onglass formation the the new alloy change into Cu₅₁Zr₁₄. The competing crystal phase on glassformation of Cu₅₀Zr₅₀BMG is CuZr. By examing the resistivity behavior with continuous heatingand cooling, we found that CuZr phase only exist in the temperature region of higher than988K, andit will decompose into Cu₁₀Zr₇and CuZr2on cooling, however, there exists large temperaturehysteresis of117K on decomposition process. It was believed that the hysteresis temperature isimportant for stabilizing CuZr phase on preparing BMG based composites.6. There are many reports about the effect of overheating on glass formation of BMGs, however,in the present paper, resistivity measurements of CuZr binary alloy melts on overheating conditionare performed on a special high accuracy, high vacuum melt resistivity measuring system. Abnormal temperature behaviors of Cu₅₀Zr₅₀and Cu₆₄Zr₃₆alloy melts are observed using resistivitymeasurement （proved by DTA results）. Based on the results of melt state, the relationships betweenmelt state and GFA/crystallization behavior are explored. And the results are shown as follows:（1）Abnormal temperature behaviors of the melt in the temperature region of300⁴00K higher than theliquid temperature which implies that temperature induced liquid-liquid structure transition appeared,and this phenomenon was proved by DTA results;（2） Several overheating temperatures are selectedaccording to the different region of the melt state and metallic glass ribbons with different melt statewere prepared using single melt spinning method. Experimental results of resistivity and DSCmeasurements based on continuous heating mode reveal that elevating the overheating temperatureof alloy melts can improve the GFA and thermal-stability of CuZr binary BMGs, and crystallizationbehavior of CuZr binary BMGs are various for the CuZr binary BMGs with different meltstate.relevant melt state;（3） As liquid-liquid structure transition occurs, the critical diameter of theCu₅₀Zr₅₀cone shape sample is improved from2mm to3mm, and the critical diameter of the Cu₆₄Zr₃₆cone shape sample is improved from1mm to2.5mm.