Study On Glass-Forming Ability And Liquid Fragility Of Bulk Metallic Glasses

In the present thesis, the rare earth-based bulk metallic glasses were mainly chosen for the study of glass-forming ability (GFA), behaviors of thermal expansion and fragility of liquids. The cluster behaviors of liquid metals with temperature were investigated by means of X-ray diffraction measurement. A concept of the structural stabilization factor of liquids has been proposed, and its correlation with the glass-forming ability has also been discussed.The GFA of RE₅₅Al₂₅Co₂₀ (RE = Ce, Pr, Nd, Sm, Gd) alloys was studied by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). When the alloys are cast into rods up to 2 mm in diameter, only the Sm₅₅Al₂₅Co₂₀ and Gd₅₅Al₂₅Co₂₀ alloys show entirely amorphous structures; the Pr₅₅Al₂₅Co₂₀ and Nd₅₅Al₂₅Co₂₀ alloys are mixtures of amorphous and crystal structures; and the Ce₅₅Al₂₅Co₂₀ alloy shows mostly crystal structure. As a result, among the five alloys, the Sm₅₅Al₂₅Co₂₀ and Gd₅₅Al₂₅Co₂₀ alloys exhibit the highest GFA and the Ce₅₅Al₂₅Co₂₀ alloy shows the lowest GFA. Based on the results, the GFA of the Sm₅₅Al₂₅Co₂₀ and Gd₅₅Al₂₅Co₂₀ alloys was further investigated. By sucking-cast into a Cu-mold, the Sm₅₅Al₂₅Co₂₀ glassy rod with a diameter of 4mm and the Gd₅₅Al₂₅Co₂₀ glassy rod with a diameter of 2mm have been prepared for the first time. Compared with the Gd₅₅Al₂₅Co₂₀ alloy, the Sm₅₅Al₂₅Co₂₀ alloy with a higher GFA shows a smaller fragility parameter of supercooled liquids and a smaller Gibbs free energy difference at glass transition temperature (ΔG^1-x(T_g)).The fragility parameters of supercooled liquids, m, of Gd-based glass-forming alloys were calculated by thermal scanning method and the values of m rank as follows: Gd₅₅Al₂₅Cu₁₀Co₅Ni₅(m=37) < Gd₅₅Al₂₅Co₁₀Cu₁₀(m=45) < Gd₅₅Al₂₅Co₁₀Ni₁₀ (m=58) < Gd₅₅Al₂₅Co₂₀(w=74). Among the Gd-based alloys, the alloy with a larger m exhibits a higher GFA. Based on the dilatometric measurements, a positive linear correlation is found between the fragility parameter of supercooled liquids and the average thermal expansion coefficient of glassy solids. Negative linear correlations are obtained between m and the nearest-neighbor atom distances (r_l)as well as the depths of the effective pair potentials (V₀) in the Gd-based bulk metallic glasses (BMGs). These relationships help to study GFA and understand the essence of glass formation from the view of supercooled liquid fragility. In addition, an abnormal thermal expansion phenomenon can be observed in the Gd-based BMGs. There is no increase of thermal expansion coefficient derived from glassy state to supercooled liquid state. This behavior has a close correlation with the viscous flow ability of the sample in the supercooled liquid region.The viscosities of superheated liquids for a series of La- and Sm-based BMGs were measured by an oscillating vessel method. After Cu element is substituted partly by Ni element in the La₅₅Al₂₅Ni₂₀ alloy, the viscosities of the liquid alloys decrease, from low to high as follows: La₅₅Al₂₅Ni₅Cu₁₅< La₅₅Al₂₅Ni₁₅Cu₅< La₅₅Al₂₅Ni₂₀. According to the fragility concept of superheated liquids, the fragility parameters of superheated liquids, M, for the La-based alloys were calculated as follows: La₅₅Al₂₅Ni₅Cu₁₅(M=1.4509)55Al₂₅Ni₁₅Cu₅(M=1.6428)55Al₂₅Ni₂₀(M=1.7747). Among the three alloys, the La₅₅Al₂₅Ni₂₀ alloy with the highest value of M shows the lowest GFA. Also, the fragility of superheated liquids for the Sm-based glass-forming alloys was investigated. The fragility parameters M of the Sm-based alloys rank as follows:Sm₅₅Al₂₅Co₁₀Ni₁₀(M=1.8260)< Sm₅₀Al₃₀Co₂₀(M=1.9588)< Sm₅₅Al₂₅Co₁₀Cu₁₀ (M=2.1005). A negative correlation between GFA and M is found in the Sm-based alloys. According to the results of formula deduction and experiments, it is found that the fragility parameter of supercooled liquids, m, is proportional to the product of the reduced glass transition temperature and the reciprocal of fragility parameter of superheated liquids ((T_g/T_l)×(1/M)).The microstructure characteristics of the Au-Si alloys were studied by X-ray diffraction measurement. A sub-peak reflecting the local atomic structural ordering is observed on the right side of the main peak in the pair correlation function curves. In the Au-Si alloys, the height of the sub-peak increases with the Si content increasing and the Au₇₅Si₂₅ alloy with the most obvious sub-peak has the highest GFA. In addition, a concept of structural stabilization factor of liquids, B, has been proposed based on the theories of classical homogeneous nucleation and thermodynamics. It is defined as,B=dr_c/dTwhere r_c is the correlation radius of clusters in liquids and T is the absolute temperature. The structural stabilization factor of liquids reflects the change in cluster size with temperature in different liquids. A liquid with a low absolute value of B should more likely keep its liquid structure, which favors the formation of glass, so it can be used to predict GFA. A liquid with a low absolute value of B tends to have a high GFA. The structural stabilization of liquids and the fragility of superheated liquids in the Al-Cu alloys were also studied. It is found that |B| increases with the increase of Cu content, indicating that the change in cluster size with temperature in liquids accelerates with the addition of Cu element. From a macroscopical point of view, the increase of |B| can be reflected by the fast change in the viscosity with temperature around liquidus temperature (the increase of M). Among the Al-Cu alloys, there is a positive linear correlation between |B| and M, which correlates macrokinetics properties with microkinetics properties of liquids.