Composition Design And Mechanical Properties Of Zr-Ni-Al-based Bulk Metallic Glasses In Terms Of The Cluster-plus-glue-atom Model

Composition design and mechanical properties are two basic issues in the study of bulk metallic glasses (BMGs). Developing BMGs with high glass forming abilities (GFAs) and excellent mechanical properties (e.g. high strength and large plasticity) is one of the targets in this field. Cluster-plus-glue-atoms model, a composition design method developed in our group, has been successfully applied in several metallic glass systems. This model dissociates a glassy structure into a cluster part and a glue atom part. The cluster is the nearest neighbor coordination polyhedron constituted by specific atoms and glue atoms are located between the clusters. The composition and structure information of BMGs can be expressed by a universal cluster formulas [cluster](glue atoms)1,3resulted from this model. In this thesis, the GFAs and mechanical properties of the Zr-Ni-Al and Zr-Al-Ni-Cu-(Nb) BMGs were studied, and the relationship between mechanical properties and glassy structure were discussed from the perspective of the cluster-plus-glue-atom model.The first step for the composition design is to determine the principal clusters, as exemplified by clusters CN11Ni3Zr9, CN12Ni4Zr9, and CN10Ni3Zr8respectively derived from three common devitrification phases Al2NiZr6, NiZr2(NiTi2-type) and NiZr2(Al2Cu-type) in Zr-Ni-Al BMGs. Three alloy series were thus designed, namely Ni3Zr9Alx, Ni3Zr8Al and Ni4Zr9Alx,(x=1,1.5,2,3). Analogically, Ni3Zr6Alx, Ni3Zr7Alx and Ni3Zr10Alx alloy series were designed for comparisons. Alloy rods with different diameters of these compositions were prepared by copper mould suction casting, and their structure and thermal glass parameters were investigated. The experimental results showed that all the rods, except Ni3Zr6Al3, were in amorphous state within a rod diameter of3mm. Composition Ni4Zr9Al2(Zr60Ni26.7Al13.3, at%) could be cast into rods with a critical glass forming diameter of10mm, whose GFA was even higher than that of the reported best glass former Zr6oNi25Al15prepared in the same preparation conditions. Its supercooled liquid region△Tx, reduced glass transition temperature Trg and γm were respective68K,0.579and0.689, which were also higher than those of Zr6oAl15Ni25(△TX=68K,Trg=0.568, γm=0.676). After analysis for electron concentration (e/a) and cluster formula, Ni4Zr9Al2was shown to satisfy a cluster formula based on the Ni3Zr9cluster plus three glue atoms [Ni3Zr9] Al2Ni.Room temperature compression and Vickers hardness tests of the3mm-diameter BMGs were carried out. Their yield strength (σy), Young’s modulus (E) and Vickers hardness (Hv) increased with increasing contents of Al and Ni and were closely related with glass transition temperature (Tg), crystallization temperature (Tx), molar volume (V) and atomic density (ρa). The e/a and cluster formula analysis of these Zr-Ni-Al BMGs showed that they belonged to two glass series respectively expressed by cluster formulas [Ni3Zr9](glue atom)3and [Ni3Zr8](glue atom)1. An estimation of the bonding strength, I, between the three kinds of bonds prescribed by the cluster formulas, i.e. the bonds between the cluster center and the cluster shell (Icenter-shell), between the cluster shells (Ishell-shell), and between the glue atom and the shell (Iglue-shell), were calculated in terms of the enthalpy of mixing. It was found that the mechanical properties of the Zr-Ni-Al BMGs were linearly correlated with1/Icenter-shell+1/Ishell-shell+1/Iglue-shell.The mechanical properties of the Zr-Ni-Al BMGs were mainly dominated by the weakest shell-shell interaction. A spring model was therefore proposed to explain the relationship between structure and strength of metallic glasses.Finally, the mechanical properties of Zr-based Zr-Al-Ni-Cu-(Nb) BMGs which satisfy cluster formula of Ni4Zr9cluster plus one glue atom, and the effect of alloying element Nb on the mechanical properties of Zr65Al10Ni10Cu15BMG, and the thermal properties and mechanical properties of (Zr65Al10Ni10Cu15)97Nb3BMG under different structural conditions were studied. It was shown that the yield strength increased effectively and affect deformation ability with Nb addition. The compression plasticity reached12.7%in the sample with2at.%Nb addition. With annealing treatment, the structure of (Zr65Al10Ni10Cu15)97Nb3BMG became unstable, and the initial crystallization temperature and supercooled liquid region decreased. Meanwhile, the yield strength and Young’s modulus increased, but the compression plasticity decreased from8%to nearly0%.