Dongdong Qu

New Approaches to the Computer Simulation of Amorphous Alloys: A Review

In this work we review our new methods to computer generate amorphous atomic topologies of several binary alloys: SiH, SiN, CN; binary systems based on group IV elements like SiC; the GeSe2 chalcogenide; aluminum-based systems: AlN and AlSi, and the CuZr amorphous alloy. We use an ab initio approach based on density functionals and computationally thermally-randomized periodically-continued cells with at least 108 atoms. The computational thermal process to generate the amorphous alloys is the undermelt-quench approach, or one of its variants, that consists in linearly heating the samples to just below their melting (or liquidus) temperatures, and then linearly cooling them afterwards. These processes are carried out from initial crystalline conditions using short and long time steps. We find that a step four-times the default time step is adequate for most of the simulations. Radial distribution functions (partial and total) are calculated and compared whenever possible with experimental results, and the agreement is very good. For some materials we report studies of the effect of the topological disorder on their electronic and vibrational densities of states and on their optical properties.

MECHANICAL PROPERTY OF A NEW Zr-BASED BULK METALLIC GLASS WITH CERTAIN PLASTICITY AT LOW TEMPERATURE

The mechanical property of a new bulk metallic glass Zr50.7Cu28Ni9Al12.3, which owns 5% room-temperature plasticity, is investigated at low temperatures 193K and 123K. It is indicated that the yield strength and the plasticity significantly increase by 12.8% and 50% respectively as testing temperature is lowered from 298K (at room temperature) to 123K (in liquid nitrogen). The dense and multiple shear bands which contribute to the improved ductility are observed on the side surface of these samples fractured at low-temperature. Detailed study of the stress-strain curves for the samples deformed at low temperature show that more serrations and smaller stress drop form due to generation of more much shear bands.

The Interaction of Sn-Ga Alloys and Au Coated Cu Substrates

Ga and Ga-based alloys appear to be promising materials for low temperature soldering in microelectronics. This research involved an analysis of the joint interfaces that resulted from reactions between a eutectic Ga-Sn alloy and Au coated Cu substrates at both room temperature and 100°C. At both temperatures the intermetallic CuGa2 accounted for the majority of the interfacial microstructure. This study has shown the possibility of using eutectic Ga-Sn alloys in low temperature soldering applications, as well as the advantages of Synchrotron XFM techniques in characterising trace element distributions in solder joints.

Effect of Trace Elements on the Liquid Structure of Sn-Cu Alloys Investigated by High Energy X-Ray Diffraction

Liquid metal structure of Sn-Cu alloys studied by synchrotron high energy X-ray diffraction have been performed to investigate the effect of the trace elements Ni and Al. It has been demonstrated that trace Ni and Al additions are able to alter the liquid Sn-4wt%Cu structure. The refinement of the primary Cu6Sn5 phase after trace Al additions is proposed to be caused by the consumption of Cu atoms involved in a AlCu chemical ordering and an associated change in the undercooling for Cu6Sn5 nucleation.