Thermodynamic And Kinetic Characteristics Of Glass Transition Of Vit1 And Ce₆₉Al₁₀Cu₂₀Co₁ Alloys

Glass transition is one of the key problems to be solved in the condensed matter physics,and it is also a difficult question in science.Deeply understanding the glass transition plays an important role in revealing the nature of amorphous materials.In the field of metallic glass(MG),in order to avoid crystallization and solidify into solid glass by glass transition,the high temperature melt needs to be cooled at a high cooling rate which is equal or higher than the critical cooling rate.However,due to the limitation of technical means,it is difficult to obtain the real glass transition temperature by the cooling experiment during the liquid solidification process.Besides,the glass transition of MG during the calorimetric experiment corresponds to the relaxation process of the alloy.At present,however,the methods(e.g.,viscosity,diffusion and internal friction,etc.)that are used to evaluate the relaxation time near the glass transition temperature usually require a large amount of experiments or have a great difficulty in experiments due to limited time scale.Therefore,it is of great scientific significance to establish a mathematical model which can quantitatively solve the glass transition temperature during the solidification process of MG and to develop a new method which can accurately calculate the relaxation time of alloy system near the glass transition temperature by only performing with simple calorimetric experiments.On the other hand,the thermal history conditions of the cooling rate of alloy melt and the subsequent thermal processing,such as annealing,quenching and cryogenic treatment,all have important effects on the glass transition of MG.However,the quantitative analysis or systematic study on the effect of the cooling rate of melt and the thermal processing of annealing combined with rapid quenching and cryogenic treatment with liquid nitrogen on the glass transition were rarely reported.In this work,different alloys were used to explore the above problems of glass transition and relaxation of MGs expressed above.The progresses of this work are mainly as follows:Taking a copper mold injected Vit1 MG sample in diameter of 8 mm with constant cooling history as a research object,the kinetic glass transition of the sample was firstly calorimetriclly analyzed at different heating rates by using DSC.The dependence of the glass transition temperature on the heating rate was fitted with the extended VFT equation.In addition,the relationship between the specific heat capacity and temperature of the MG and its supercooled liquid was established based on the results of calorimetric experiments.Then,based on the theory that the total enthalpy change during the cooling process of the vitrification is equal to the one during the followed heating process,a thermal equilibrium model was established which can be used to quantitatively calculate the glass transition temperature during the liquid metal solidification process by only using calorimetric experiments.Based on the thermal equilibrium model,the extrapolated glass transition temperature of the investigated Vit1 metallic glass during its liquid solidification is ranged from 618 K to 595 K.To verify that the thermal equilibrium model is suitable for accurately calculating the glass transition temperature range during liquid metal solidification,the thermodynamic parameters,including the specific heat capacity and thermal conductivity,were adjusted by using the glass transition temperature range that was determined at the heating rate of 20 K min^-1and the one extrapolated by the thermal equilibrium model.Then the adjusted thermodynamic parameters were used to simulate the cooling temperature curves of the investigated MG sample with the diameter of 8mm during its preparation process.The simulated cooling temperature curves were then compared with the measured curves.The simulation results show that the thermodynamic parameters adjusted by the glass transition temperature range which was extrapolated by using the thermal equilibrium model can reflect the cooling process of the MG sample more accurately as prepared.The result indicates that the thermal equilibrium model proposed in this work is suitable for quantitatively calculating the glass transition temperature range during the liquid to glassy solid phase transformation for MG which has a constant cooling history.To analyze the effect of melt cooling rate on the glass transition of MG,three groups of Vit1 MG samples with different orders of magnitude of melt cooling rate were prepared by different methods.Then the kinetics of glass transition and the enthalpy of structural relaxation of the three groups of MG samples were analyzed by using DSC.The results show that the apparent activation energy of the glass transition,E,the fragility parameter of the alloy,m,and the enthalpy of structural relaxation,?H_rel,are all affected by the melt cooling rate.The E,m,and?H_reltend to decrease with the decrease of melt cooling rate in the order of magnitude,and they are approximately in a linear relationship with the natural logarithm of cooling rate within the range of the experimental cooling rates.Besides,the limiting fictive temperatures,_fT',of the three groups of MG samples and the solidification glass transition temperature ranges of the samples during their amorphous solidification processes were extrapolated.The results show that the_fT'of the three groups of MG samples are all close to the onset temperatures of the solidification glass transition,T^l_sg,extrapolated by the thermal equilibrium model.In addition,the_fT'andT^l_sg all move to the low temperature region with the decrease of melt cooling rate,which is consistent with the theoretical viewpoints.In the work,it is found that the dependence of the onset and end temperatures of the glass transition of Vit1 MG in heating on the heating rate can be well fitted by the extended VFT equation.Based on this phenomenon,a mathematical model was proposed,which can be used to quantitatively calculate the relaxation time of MG near the glass transition temperature by using DSC calorimetric experiments.Then,the relaxation times of the Vit1and Pd₄₀Cu₃₀Ni₁₀P₂₀MGs were calculated by using the mathematical model,and the results were compared with the ones calculated by using the viscosity and diffusion coefficient methods.The comparison results show that the proposed calorimetric model can accurately calculate the relaxation time of MG in a large temperature range near the glass transition temperature.Finally,taking the Ce₆₉Al₁₀Cu₂₀Co₁MG as the research object,the effects of different thermal processing on the glass transition of MG were analyzed.The effects of annealing temperature,cooling rate after annealing(including slow cooling and liquid nitrogen quenching)and cryogenic treatments on the glass transition temperature,T_g,kinetic parameters of glass transition(e.g.,E and m),and apparent enthalpy change of glass transition of the MG were investigated.The results show that the higher the annealing temperature,the higher the stability of the thermodynamics and kinetics of glass transition of the relaxed sample.The liquid nitrogen quenching after annealing can reduce the effect of annealing on the glass transition of the sample,and it will facilitate the MG to transform from the relaxed state to the rejuvenated state.Besides,the cryogenic treatment after liquid nitrogen quenching will cause the different thermodynamic states and the glass transition kinetic parameters of the samples quenched at different annealing temperatures to change towards a unified trend.