Structure Model And Glass Forming Ability Of Al-based Bulk Metallic Glasses

Al-based metallic glasses are potentially industrial materials due to their excellent properties. Ever since the discovery of melt-spun Al-based metallic glass ribbons in 1988, there has been a continuous and relentless pursuit of Al-rich bulk metallic glasses (BMGs). However, Al-based alloys have relatively low glass-forming ability (GFA) compared to other BMG-forming systems because of the particularity of their internal structure, which limits their applications greatly. Therefore, it is a well-known bottleneck to design and develop the high GFA of Al based metallic glasses.This work is focused on elucidating the characterisation of GFA in Al-based metallic glasses. Based on the understanding of the existent structure models and medium-range order of metallic glasses, along with the chemical factor (electrochemical potential equalization principle), new models have been successfully proposeed to design the Al-based metallic glasses with high GFA. The main results are summaried as follows:(1) An efficient atomic packing-chemistry coupled model has been developed for ternary Al-based metallic glasses based upon efficient atomic packing and electrochemical potential equalization principle. For Al-TM (transition metal)-RE (rare earth) system, RE-centered clusters are arranged in a mode of spherical periodic order, and TM atoms locate at the interstitial sites between RE-centered clusters to form an efficient atomic packing. The equalization of electrochemical potential between the RE-centered clusters and the TM-centered clusters accounts for the stability of amorphous structure. Further, complementary inverse structure is utilized to select efficient clusters and solute elements for the best GFA. The validity of this model has been testified in the Al-Ni-RE (Y, Ce, La, Gd) systems. A new Al-Ni-Ho glass with a wedge thickness of about 718 mm was discovered at the predicted composition.(2) To clarify the correlation of medium-range order (MRO) structure with GFA of Al-based metallic glasses, Al86Nii4-aYa (a=2-9 at.%) metallic glasses were analyzed by x-ray diffraction in detail and further verified by synchrotron high-energy X-ray diffraction. The prepeak that reflects the MRO structural evolution was found to be much sensitive to alloy composition. We have proposed an icosahedral supercluster MRO structure model in Al-TM-RE system, which consists of 12 RE(TM)-centered clusters on the vertex of icosahedral supercluster, one RE(TM)-centered clusters in the center, and TM(RE) atoms located at RE(TM)-centered cluster tetrahedral interstices in the icosahedral supercluster. It was indicated that the MRO structural stability mainly depends on the interaction of efficient dense packing and electrochemical potential equalization principle. The Al86Ni9Y(La)5 alloys present good GFA due to the combination of the two structural factors.(3) The indepth understanding of the effect of microalloying La and Co elements on the GFA in the Al-based metallic glasses has been made from electronic structure angle. According to Fermi sphere-Brillouin zone interaction mechanism (A Fermi sphere is in touch with the pseudo-Brillouin zone boundary and the electronic density of states at the Fermi level is minimized so that the stability of the metallic glass is enhanced.) for metallic glasses, the microalloying La and Co elements effect on the GFA in the Al-TM-RE system has been treated from two aspects. One is that the electron hybridization between Al atoms and TM atoms changes the the diameter of the Fermi sphere, this is associated with the microalloying Co element effect on the GFA. The other is that the static structure changes of Al atoms and RE atoms influence the diameter of the pseudo-Brillouin zone, this is associated with the microalloying La element effect on the GFA. The best GFA can be obtained when the Fermi surfaces nearly touch the quasi-Brillouin boundaries. A new composition Al86Ni6.75Co2.25Y3.25La1.75 was found, which can form fully glassy rods of 1.5 mm in diameter. Further, on the basis of the electronic specific heat measured in these alloy, it has been proved that the density of electronic energy states at the Fermi level is indeed closely correlated with the GFA of metallic glasses.