Crystallization Kinetics and Fragility of Al-Based Amorphous Alloy

Crystallization among amorphous alloy is a crucial study since it generally affects it properties, which may detrimental or beneficial, depending in the intended application of the materials. Controlling crystallization is crucial for obtaining the desired properties. The crystallization study was performed using differential scanning calorimeter (DSC). Samples were heated at heating rate between 20 and 40 K·min-1. Structural evolution during crystallization was studied under X-ray diffraction (XRD). Apparent activation energy for each temperature characteristics was determined using Kissinger’s equation. Local Avrami exponent was investigated using modified Johnson-Mehl-Avrami-Kolgomorov equation. Liquid fragility, which indicates the strength of the glass formation, was predicted using temperature characteristics instead of its viscosity. It was found that upon crystallization both as-cast samples crystallize to cubic-Al, Al2CuMg and Al2Cu and Al3Ni. Alloy with composition of (Al75Cu17Mg8)95Ni5 shows superior activation energy at every temperature characteristics than alloy with composition of Al75Cu10Mg8Ni7. Local Avrami exponent and local activation energy for (Al75Cu17Mg8)95Ni5 show high values at the beginning and at the end of crystallization process. From liquid fragility, it was predicted that the samples are stronger glass former than previous studied Al-amorphous alloys.

Revisiting the Dependence of Poisson’s Ratio on Liquid Fragility and Atomic Packing Density in Oxide Glasses

Poisson’s ratio (ν) defines a material’s propensity to laterally expand upon compression, or laterally shrink upon tension for non-auxetic materials. This fundamental metric has traditionally, in some fields, been assumed to be a material-independent constant, but it is clear that it varies with composition across glasses, ceramics, metals, and polymers. The intrinsically elastic metric has also been suggested to control a range of properties, even beyond the linear-elastic regime. Notably, metallic glasses show a striking brittle-to-ductile (BTD) transition for ν-values above ~0.32. The BTD transition has also been suggested to be valid for oxide glasses, but, unfortunately, direct prediction of Poisson’s ratio from chemical composition remains challenging. With the long-term goal to discover such high-ν oxide glasses, we here revisit whether previously proposed relationships between Poisson’s ratio and liquid fragility (m) and atomic packing density (Cg) hold for oxide glasses, since this would enable m and Cg to be used as surrogates for ν. To do so, we have performed an extensive literature review and synthesized new oxide glasses within the zinc borate and aluminoborate families that are found to exhibit high Poisson’s ratio values up to ~0.34. We are not able to unequivocally confirm the universality of the Novikov-Sokolov correlation between ν and m and that between ν and Cg for oxide glass-formers, nor for the organic, ionic, chalcogenide, halogenide, or metallic glasses. Despite significant scatter, we do, however, observe an overall increase in ν with increasing m and Cg, but it is clear that additional structural details besides m or Cg are needed to predict and understand the composition dependence of Poisson’s ratio. Finally, we also infer from literature data that, in addition to high ν, high Young’s modulus is also needed to obtain glasses with high fracture toughness.