Plastic Deformation, Glass Transition, And Prediction Of Glass Transition Temperature For Metallic Glasses

Metallic glasses have received considerable attention due to their unique propertiesand potential applications in various fields. However, these amorphous metals are stillmuch less understood compared to crystalline alloys. Especially, the plasticdeformation mechanism and the process of glass transition are two tough issues inmetallic glass research. Plastic deformation of metallic glasses has inhomogeneousand homogeneous modes. In both modes, the free volume dominantly affects thedeformation of metallic glasses. However, it is unclear how the free volumeinfluences the plastic deformation. Generally, a metallic glass is formed by cooling aliquid fast enough to avoid crystallization. At the process of continuous supercooling,the liquid viscosity increases dramatically, and at some point, the liquid freezescontinuously into a noncrystalline solid. This is termed as the glass transition. Theglass transition has been observed in large number of experiments, and many workshave been tried to explain the phenomena of glass transition. However, glasstransition is still an unknown basic problem. We do not understand how and when aliquid becomes a metallic glass. Beside, the glass transition approaching from thesolid and inducing a transition from the glass state to the supercooled liquid state isindistinct.In this work, plastic deformation, glass transition, and prediction of glasstransition temperature for metallic glasses have been investigated. The maininvestigation results are as follows:(1) Considering the energy conservation between the applied mechanical workand the increased energy of a metallic glass at steady state flow, the steady-state flowstress of a metallic glass only depends on the difference of reduced free volumebetween undeformed state and steady-state of flow. The relationship between thesteady-state flow stress and reduced free volume has been established. Therelationship and the analysis for steady-state reduced free volume suggest that theeffect of temperature or the strain rate on steady-state stress can be described by twosimple linear relationships. The effects of strain-rate and temperature on thesteady-state flow stress of metallic glasses should be attributed to the effects ofstrain-rate and temperature on steady state free volume.(2) The influence of reduced free volume or temperature on the inhomogeneous strength of metallic glasses was discussed. A possible explanation for deformation hasbeen proposed. When the free volume increases to a critical value, the system will beunstable; at same time, the fracture takes place.(3) The shear modulus values of 47 metallic glasses at absolute zero temperature[G(0)] and at glass transition temperatures [G(T_g)] were calculated. The calculatedresults show that the ratios of the shear modulus at absolute zero temperature to theshear modulus at glass transition temperatures [G(T_g)/G(0)] for various metallicglasses are almost same and have a value of about 0.85. This suggests that glasstransition takes place when the shear modulus decreases to a critical value of0.85G(0).(4) Based on the similarities between the melting of metals and glass transitionof metallic glasses, a linear relationship between the glass transition temperature andaverage melting temperature was observed. Since the average melting temperaturecan be calculated easily from the melting temperatures of the constituent elements, therelationship between the glass transition temperature and average melting temperatureoffers a convenient way to predict and design metallic glasses with desired glasstransition temperatures.