Metallic glasses have long been of interest to materials scientists and product designers alike. Sometimes referred to as ‘vitrified metals’, the properties of these alloys sit somewhere between those of crystalline metal and amorphous glass. Their disordered atomic structure give them exceptionally high yield stress and tensile strength, which has led to their use in everything from surgical instruments to golf clubs. However, the same lack of order can lead to highly-localised deformations, which limits their ability to ‘flow’.
Improving the plasticity of metallic glasses usually involves assembling them into multi-layer composites via Joule heating or friction welding. But the high temperatures used in these techniques can alter the structure of the glass, resulting in unpredictable mechanical behaviour. An increasingly-popular alternative is diffusion bonding, carried out at relatively low temperatures. In addition to reducing the risk of thermal damage, this process also creates pores within the material, which seem to improve the material’s plasticity.
To better understand how diffusion occurs in multilayer, metallic, glassy films, an international team of researchers designed two zirconium alloys of very different metals – one containing copper (Zr-Cu) and the other, palladium (Zr-Pd) – and created a stack of multiple layers of each. Their results were reported in the latest issue of Materials Today Advances [DOI: 10.1016/j.mtadv.2019.01.003].
The multilayer films were deposited by radio frequency magnetron sputtering and were annealed at 400 °C; a temperature chosen to be high enough to activate diffusion, but too low to cause crystallisation. After 1 hour, the layers were imaged using high-resolution transmission electron microscopy (HRTEM). While no crystallization was found in the Zr-Pd layers, copper nanocrystals were found in the Zr-Cu layers, and the observations were confirmed by energy dispersive X-ray (EDX) mapping. These nanocrystals seemed to be thermodynamically unstable, though, as they could no longer be seen after 5 hours of annealing. EDX analysis also showed that, during annealing, Pd and Cu diffused across the interface of the film layers, while the distribution of the Zr atoms hardly changed.
The researchers then demonstrated that reduced radial distribution function (RRDF) analysis of the HRTEM images provided a way to investigate the changes in atomic structure caused by annealing. In addition, anoindentation measurements explored the influence of annealing on film hardness. The most surprising result, however, was that despite the significant differences in atomic radius and mass between the two metals, Pd and Cu were found to have similar diffusion coefficients.
While the diffusion process in metallic glasses was the main focus of this study, the researchers have perhaps proved something even more powerful. They’ve showed that high-resolution transmission electron microscopy could be “…a universal and powerful tool” for such analysis.
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S.V. Ketov ,Yu P. Ivanov, D. Sopu, D.V. Louzguine-Luzgin, C. Suryanarayana, A.O. Rodin, T.Schoberl, A.L. Greer, J. Eckert. “High-resolution transmission electron microscopy investigation of diffusion in metallic glass multilayer films” Materials Today Advances 1 (2019) 100004. DOI: 10.1016/j.mtadv.2019.01.003