Aluminum Alloy Superheated Melt Fragility And Glass Forming Ability

The Al-based amorphous alloys have wide potential applications as structural and functional materials due to their special excellent properties. The present thesis systematically investigated the relationships between the fragility of superheated melts and the glass-forming ability(GFA) as well as the medium-range order(MRO) in Al-based amorphous alloys. It also presented the works on probing the heterogeneity of microstructure in Al-based glasses.The viscosities of Al-Co-Ce and Al-RE(Ce,Nd,Pr) glass-forming melts were measured using an oscillating viscometer. Combining the differential scanning calorimetry (DSC)method and the viscosity data, the values of M, the fragility index of superheated melts, were calculated for two alloy systems. It was found that the GFA decereases with the increase of M in Al-Co-Ce or Al-RE alloys system, however, this trend between M and GFA can not extend to multi-alloy systems. So we suggest that M can be used as a parameter to evaluate the GFA in single alloy system.The values of M in Al-Ni-Nd alloys were also calculated using above methods. The parameters T_rx,γwhich denote the GFA of alloys were calculated as well. It was found that T_rx orγdecreases as M increases in the alloys with similar crystallization temperatures. So M can be used as an indicator for GFA in the alloys with similar crystallization temperatures. The total structure factors of the Al-Ni-Nd alloys were calculated using XRD results. The prepeak denoting the existence of MRO was founded generally. The negative correlation between the prepeak strength and M was found in Al-Ni-Nd alloys, which suggested that M reflects the stability of MRO clusters in melts when the glass is formed. In addition, the mechanism that how M influences the GFA of alloys was illustrated through TTT curves.A distinct hump peak was observed in the X-Ray diffraction(XRD) pattern of rapidly quenched amorphous Al₈₇Co₁₀Ce₃ alloy, which suggested that the heterogeneity of microstructure exists in this alloy. The composition dependence of hump peak and its correlation with the GFA of alloy were studied for the first time. It was found that the intensity of the hump peak increases as the Co/Ce atomic ratio increases, while the prepeak becomes weak. The existence of the hump peak is not favorable to the glass formation in the Al-Co-Ce system. The Al-Co interaction plays an important role in the formation of the hump peak. From the XRD and DSC experiments, amorphous Al₈₇Co₁₀Ce₃ alloy is devitrified by the simultaneous precipitation of fcc-Al and the intermetallic compound Co₂Al₉. The grain growth process in the crystallization of Al₈₇Co₁₀Ce₃ alloy is identified by the isothermal DSC experiment, which indicates the presence of pre-existing nuclei of fcc-Al and Co₂Al₉. the presence of pre-existent nuclei of Co₂Al₉ is presumed to be associated with the formation of hump peak.