Study On Thermal Expansion And Structural Relaxation Behavior Of Bulk Metallic Glasses

Since the first preparation of bulk metallic glasses in 1960,their excellent properties have attracted lots of interest and focus of researchers.Because of having an amount of free volume and the uniquely disordered structure in long range order,the thermal expansion and relaxation behavior of bulk metallic glasses have been the concerned hot spots in scientific field.So far,although plenty of previous work has been researched in this aspect,the conclusions summarized have not been consistent yet.Neither,few work has connected these two natures with the glass transition temperature and free volume of bulk metallic glasses.Thus,it's worth to continue to probe the thermal expansion and relaxation behavior more deeply.In this work,we utilize the dilatometer and circle-heating method to determine the different thermal expansion coefficient before and after structural relaxation.Consequently,in the linear-expansion stage,the thermal expansion coefficient before relaxation is uniformly less than the one after relaxation in each bulk metallic glass.Furthermore,the decrement has negative correlation with the atom packing density and the density variation,which shows that the thermal expansion behavior is intimate with free volume content in metallic glasses.After crystallization,we find although the density increases,the linear-expansion thermal expansion coefficient variation has no obvious rule comparing to amorphous state.The probable reason is that free volume vanishing,generation of grain boundary and the expansion behavior superposition of poly phases inside exert conjunct impacts on the expansion of materials.Using the dilatometer to isothermally anneal the samples,we can attain the length variation curves of various metallic glasses in isothermal structural relaxation.By fitting the length variation curves,we find that,in our observation time,the samples' length variation roughly accord with the logarithmic form decline during the isothermal annealing.Then,we linearly fit the ending stage of the logarithmically fitted curves to obtain the relaxation rates in the quasi-steady stage and utilize the rates to calculate the structural relaxation activation energy of different composition and states.Consequently,the structural relaxation activation energy rises slightly with the increase of glass transition temperature.After the isothermal temperature is reduced by glass transition temperature,we discover that the relaxation rates of different BMGs in quasi-steady stage have the similar tendency and even almost overlap in some area,which shows the structural relaxation behavior correlates with glass transition temperature in a certain extent.Besides,comparing the as-cast samples to the as-annealed ones,it's shown that structural relaxation activation energy increases sharply after annealing.By measuring the as-cast and as-annealed samples'differential scanning calorimeter curves and mechanical parameters,it can be found that the free volume content is less in the bulk metallic glasses and the material is in a more steady state.These results demonstrate comparing to glass transition temperature,the free volume annihilation and redistribution can be the more important factor account for the change of structural relaxation activation energy.In summary,this thesis further probes how the thermal expansion coefficient and relaxation are affected by glass transition temperature and free volume by means of determining the thermal expansion coefficient and structural relaxation activation energy of bulk metallic glasses.