Study On The Glass Formation And Local Atomic Structure Of Al-based Metallic Glasses

Metallic glasses (MGs), as a new member of amorphous materials, exhibit many excellent properties due to their long-range disordered and short-range ordered atomic structures as compared to traditional crystalline metals or alloys. Among them, Al-based MGs have been one of hot issues for their low density and high mechanical strength. However, the poor glass forming ability (GFA) and limited critical size of Al-based MG are seriously restraining their industrial applications. Therefore, this study is mainly focused on the aspects of GFA and atomic structure of Al-based MGs. The achieved results are as follows:(1) By partially substituting La for Y, Y for La and Co for Ni, the effects of similar element substitution on the GFA of Al-based MGs are studied. The role of Ca addition in Al86Ni7Y4.5Co1La1.5 metallic glass is also studied in this work. The results suggest that Al89Y11 exhibits higher GFA than Al91.5La8.5. Moreover, it shows positive effect on the GFA enhancement by partial substitution Y for La in Al91.5La8.5, but limited GFA improvement of Al89Y11 when using La to replace Y. It is found that Co replacing Ni atoms might not be effective to GFA enhancement of (Al90Y6.6La3.4)92Ni8. However, we find that the Ca addition in Al86-xNi7Y4.5Co1La1.5Cax (x=0-5 at.%) can significantly improve the thermal stability and the GFA due to the large size difference and negative heat of mixing between Ca and Al elements.(2) Al89Y11, Al90Y6.5La3.5 and Al82.8Y6.07Ni8La3.13 MGs are systematically studied by using differential scanning calorimetry, high energy X-ray diffraction and X-ray absorption spectroscopyexperiments, combined with ab initio molecular dynamic simulation and reverse Monte Carlo simulations. The local atomic structures of Al89Y11, Al90Y6.5La3.5 and Al82.8Y6.07Ni8La3.13 indicate that X-Al (X=A1, Y, La or Ni) pairs are most dominant in these three MGs and Y-La pairs are found in Al90Y6.5La3.5 MG. As for quarternary Al82.8Y6.07Ni8La3.13 MG, there’s at least one Ni atom locating in the shell of polyhedra centered by Y and La atoms, indicating the existence of Ni-Y and Ni-La pairs in this MG. It is demonstrated that the partial replacement of Y atoms by La has limited improvement of the GFA although La atoms reduce the ordering around Y atoms and also the fractions of icosahedron-like polyhedra centered by Al atoms. In contrast, Ni atoms can significantly improve the GFA, which are inclined to locate in the shell of polyhedra centered by Al, Y and La atoms, mainly forming Ni-centered icosahedron-like polyhedra to enhance the spatial connectivity between clusters and suppress the crystallization.