Hongbo Cao

Bulkier glass formability enhanced by minor alloying additions

In this study, we present a computational thermodynamic strategy to obtain a minor but optimum amount of additional element into a base alloy to improve its glass-forming ability, through thermodynamically calculating the maximum liquidus depressions caused by various alloying addition (or replacement) schemes. We demonstrate the successful use of Zr56.2Cu31.3Ni4.0Al8.5 as the base alloy with the addition of 4.9% Ti, by observing a significant increase in the glass-forming ability of more than 100% in terms of the diameter of the glass formed from the base alloy to the one with the addition of 4.9% Ti. The approach presented here can be considered as a universal method to synthesize novel and bulkier metallic glasses not only of scientific interest but also potential technological applications.

Change in Activation Volume for Plastic Deformation of Zr-based Bulk Metallic Glass following Annealing

We performed nanoindentation creep experiments on the bulk metallic glass Zr54Cu38Al8 in an effort to measure the scale of the individual deformation events responsible for plastic deformation. From a nanoindentation creep experiment, we can determine V*, the activation volume, which we interpret as the volume of a shear transformation zone (STZ) multiplied by the shear strain undergone by the STZ during thermal activation. For the as-cast alloy hardness, H, is 5.33 ± 0.06 GPa, and V* is 87 ± 5 A. The alloy was then annealed near Tg for 24 hr and retested. No crystallization occurred during annealing as verified by XRD and TEM. Following annealing H increased to 7.36 ± 0.08 GPa and V* increased to 160 ± 10 A. We interpret the change in V* as arising from an increase in the number of atoms involved in the STZ.

Nanometer-scale Structural Relaxation in Zr-based Bulk Metallic Glass

We investigated the influence of annealing on the nanometer-scale medium-range order in Zr 54 Cu 38 Al 8 bulk metallic glass using fluctuation electron microscopy. Fluctuation microscopy experiments probing structure at a length scale of 1 nm show that the as-cast Zr bulk metallic glass contains significant medium range order. That structure is unchanged by annealing at 87% of the glass transition temperature for 24 hours, although that anneal does significantly change the differential scanning calorimetry trace.

Thermodynamic Database of Mg‐Al‐Ca‐Sr: A Resource for Alloy Development and Improvement

A thermodynamic database of Mg-Al-Ca-Sr, a basic quaternary for a number of commercial magnesium alloys such as AXJ530 (Mg-4.5Al-3.0Ca-0.14Sr, weight percent hereafter, unless noted otherwise) was developed. Calculated isopleth from Mg-5Al-3Ca to Mg-5Al-3Sr agrees with experimental data reported in the literature. Moreover, using this database and a simple kinetic model proposed by Scheil, the calculated solidification paths of several alloys are likewise in accord with experimental data. Thus, the availability of this database enables one to exploit new alloy compositions for promising properties which can be rapidly validated with few experiments.