Variation Of Electrical Resistivity And Viscosity Of Metallic Glasses With Their Metastable States And Crystallization Process

Due to the unique structural characteristics without long-range periodic but with short-range order atomic arrangement,the physical and chemical properties of metallic glasses deviate from those found in crystalline materials,which attract extensive attention of researchers.However,the theory for the nature of the metallic glass and glass transition is far from clear.Electrical resistivity is always related to the electronic structure of alloys and sensitive to structural changes,while viscosity is the most important factor in determining the kinetics of glass transition.Therefore,the thesis researched on several metallic glass systems with good glass forming ability and thermal stability,such as Cu-Zr,Cu-Zr-Al,Zr-Cu-Al-(Ni,Ag)-(Ti,Nb),Ti-Zr-Cu-Ni-Be,to investigate the variations of electrical resistivity and the viscosity of metallic glasses with their metastable states and crystallization process,and to analyze the relationship between these two physical properties and structure evolution.The electrical resistivity variations of metallic glasses during crystallization are related to crystallization kinetics,paths,as well as crystal structure.The disorder to order transition of atomic arrangement changes the band structure of the metallic glasses,leading to sharp decline of electrical resistivity during crystallization process.The ribbons of Cu46Zr46Al8 exhibit analogous tendency,but samples with diameter larger than 1mm crystallize through a two-step process:abnormal increase followed by normal decline of electrical resistivity.More icosahedral clusters are frozen in the sample with lower cooling rate,which act as sites for high-density site-saturated nucleation.High-density nanocrystalline phases with slower growth in the matrix bring out lots of nanometer grain boundaries,which enhance the scattering of electron and finally result in abnormal increase before the normal decline of electrical resistivity.In the Ti4OZr25Cu12Ni3Be20 metallic glass with glass to quasicrystalline transformation,the composition of fine quasicrystalline phase is close to the exact composition of glass matrix,which suggests that this transformation proceeds without large scale atomic rearrangement and can easily overcome crystallization resistance.Meanwhile,the property of electron transport between these two phases is also quite similar leading to a long and continuous decrement of electrical resistivity until the completion of first-step crystallization.The slower phase transformation in Zr52.5Cui7 9Nii4.6Al10Ti5 metallic glass brings out large amount of grain boundaries,which also enhances the scattering of electron and leads to abnormal increase of electrical resistivity at the end of crystallization process.The viscous flow of metallic glasses above Tg under tensile mode is affected by the applied stress.The deformation of crystal phases under high stress results in rapid decline of viscosity after initial increase due to crystallization at Tx.The VFT equation can describe the variation of viscosity with temperature in the supercooled liquid region but deviate below Tg.The viscosity variation during the glass to quasicrystalline transformation is determined by the fraction and size of quasicrystalline.In the metallic glass with small fraction of quasicrystalline,deformation is dominated by the glass matrix,and the viscosity decreases as the temperature increases.However,when the fraction of quasicrystalline becomes large enough and the particles begin to adjoin each other,the viscosity increases sharply.The quasicrystalline with smaller size leads to decrement of viscosity even in temperature higher than Tx.The variations of electrical resistivity can accurately reflect the content of free volume in metallic glasses.When annealing below Tg,the electrical resistivity increases due to the annihilation of quenched-in free volume.The decrement of electrical resistivity in the supercooled liquid region results from free volume reproduction.According to the changes of electrical resistivity during heating-cooling process,the electrical resistivity relaxation can be divided into four stages.In addition,the temperature Tr in which the electrical resistivity begins to increase under continuous heating,is found to be related with?-relaxtion of metallic glasses.Taking advantage of unique feature of either negative or positive temperature coefficient of electrical resistivity(TCR),the metallic glass can be tailored to have near constant electrical resistivity(TCR?0)through controlling the volume of crystalline phases or selective oxidation process.Such structural changes can be utilized to produce precision wire resistors or thin film chip resistors,which further expand functional applications of metallic glasses.