Structural Mechanism For Glass Forming Ability In Zr-based Amorphous Alloys

Amorphous alloy possesses excellent properties due to its unique atomic structure,owning short range order but lacking of long range order.However,because of lack of detailed structural mechanism for glass-forming-ability(GFA),it is still a mystery that small variance of composition or minor addition of alloying elements can drastically change the GFA.In order to guidance preparation of more amorphous alloys with large GFA,it is of great importance to understand the correlation of local structures with GFA.In this work,Zr-based amorphous alloy systems are selected for revealing the microstructural mechanism for GFA.The microstructure of Zr Cu alloys were investigated by calculations based on the data from synchrotron radiation-based X-ray diffraction(XRD)and extended X-ray absorption of fine structure(EXAFS)experiments.Although the cluster-level topological and chemical character is similar in five selected glassy alloys,a relatively high atomic-packing efficiency in the Zr(solute)-centered clusters occurs in the Cu64Zr36 bulk amorphous alloy,and these compact clusters possess the highest regularity,contributing to the structural stability of this glass alloy.Combinations of these structural features in Cu64Zr36 amorphous alloy lead to the best GFA of the compositions studied.The microstructure features of are presentative Zr48Cu45Al7 bulk amorphous alloy were investigated via a series of simulations and calculations coupled with the synchrotron radiation-based experiments.It was revealed that bond shortening occurs in Al-contanting atomic pairs,due to the strong hybridization interaction between the Al dopant atoms and their neighbors.The bond shortening leads to the atomic and cluster level dense packing in the local structures,which should be the structural origin of the relatively high GFA in Al-containing bulk amorphous alloys.Structural mechanisms of the microalloying-induced high GFA in Zr Cu Al-based bulk amorphous alloys was investigated via synchrotron radiation techniques combined with simulations.In the Zr Cu Al Gd quaternary alloy system,it is found that 2 at.% Gd addition increases and stabilizes the solute-centered clusters.This leads to the relatively high atomic-and cluster-level packing efficiency,contributing to the enhanced GFA in Cu46Zr45Al7Gd2 alloy.However,the presence of Al–Gd solute-solute bonding in Cu46Zr42Al7Gd5 decreases the packing efficiency,deteriorating the GFA of this alloy.In the Zr Cu Al Ag quaternary alloy system,it is revealed that microalloying Ag in the Zr Cu Al alloys not only gives rise to a number of Ag-centered icosahedral-like local structures,but also enhances the atomic packing efficiency and structural regularity of Zr-and Cu-centered clusters(the major building blocks).This stabilizes the glassy-state structure,contributing significantly to the great enhancement of GFA.Nevertheless,when excessive Ag atoms are added,the above-mentioned structural contribution gets smaller,leading to the deterioration of GFA.In the Zr Cu Al Fe quaternary alloy system,it is found in Zr60Cu25Fe5Al10 that there is relatively high fraction of clusters having fruitful fivefold symmetrical features,high regularity of clusters,high atomic packing efficiency,and strong interaction between heterogeneous atoms.All these structural contributions results in the enhanced GFA in this composition.When excessive Fe atoms are added in Zr Cu Al,these structural parameters have relatively low values,and the GFA is accordingly deteriorated.The Zr48Cu45Al7 amorphous alloy samples quenched under cooling rates of about 2×106 K/s and 1×102 K/s were prepared by melt spinning and copper-mold suction casting,respectively.Synchrotron radiation-based experiments,combined with a series of calculations,were performed to study the microstructures in both samples.It was found that although the short-range orderings are similar in Zr-centered clusters for both samples,the atom arrangements and distributions in Cu-and Al-centered clusters are very different in terms of atomic-packing efficiencies and regularity of clusters in these two samples.A quantitative analysis revealed that the lower cooling rate leads to the higher packing efficiency and the higher regularity of clusters.This revealed how the cooling rate during quenching fine-tunes the atomic-and cluster-level microstructures in amorphous alloys with the same composition,which may be the structural basis to address the issue why macroscopic properties change with the cooling rate.