Since the discovery that two or more metals can be combined into an alloy with new and useful properties, humans have endeavored to understand materials and create new ones. Today, metal alloys are still an exciting area of research and despite centuries of study, there remains a lot to be discovered.
Aluminium alloys are in widespread use today in everything from road bikes to spacecraft. There are lots of ways to influence the properties of aluminium alloys, but an especially common technique is precipitation hardening. This is the process of taking a relatively malleable alloy and subjecting it to high temperature and strain to harden it. As the temperature and strain increases, particles within the metal alloy precipitate. When the metal cools, these particles sit inside the metal and prevent the defects in the crystal lattice from moving as freely. This helps to increase the strength of the alloy while also making it less malleable and more rigid.
Synchrotron techniques such as small angle X-ray scattering (SAXS) can be used to study precipitation in metal alloys. By carefully monitoring how the precipitates scatter the X-ray light, it is possible to understand how the precipitation process is advancing. However, there is a problem. In industrial production processes, aluminium alloys are subjected to a combination of high temperature and strain to achieve the desired properties. Most SAXS instruments cannot penetrate through thick metal samples that are a representative of actual manufactured materials. The challenge for researchers is to reproduce the conditions present in alloy processing, while also enabling study of the precipitation process using SAXS.
A research team from the University of Manchester and Diamond recently published an article where they studied how precipitation changes in an aluminium alloy under high strain, similar to the conditions used in alloy manufacture. One reason that it is very difficult to put alloys under high strain in a synchrotron is that very thin metal samples are often used, to allow adequate X-ray transmission through the sample. To increase the strain, the sample needs to be thicker so that it can withstand the force without breaking. In order to gather data from a thick sample, the researchers needed very high energy X-ray beams of over 50 keV. They used Diamond’s Joint Engineering, Environmental and Processing (JEEP) beamline (I12). JEEP’s high energy allowed the team to collect SAXS data using a larger sample that is more representative of real-world conditions. They closely monitored how precipitation formed during a high temperature deformation process at 190 oC and they discovered some surprising results. The data showed that under a high rate of strain, there was a strong acceleration in the evolution of precipitation size and volume. Their results suggest that a new model is needed to accurately explain precipitation in aluminium alloys which could ultimately help improve manufacturing process.
Researchers often use models to perform experiments. Instead of doing experiments that could be time consuming or destructive on real metal alloys as they are being formed, it is far more useful to produce models in the lab where things can be changed and measured on a much smaller scale. This helps science move fast and makes it possible to generate a lot of data in a much smaller time. However, by using models, it is necessary to make trade-offs that take a model a little bit further away from the reality that they are trying to replicate. While essential, such trade-offs mean that models always need to be validated. This study has taken a model that had existed for many years and improved it so that it more closely resembles the real word. The process of improving models and publishing the results means that all future experiments that are done can use a better model that can tell us more about the reality of metal alloy formation and help us produce new materials.
To find out more about the Joint Engineering, Environmental and Processing (JEEP) beamline (I12) at Diamond, or to discuss potential applications, please contact I12’s Principal Beamline Scientist, Thomas Connolley: firstname.lastname@example.org
W. U. Mirihanage et al. Direct observation of the dynamic evolution of precipitates in aluminium alloy 7021 at high strain rates via high energy synchrotron X-rays. Acta Materialia 205, 116532. (December 2020) . DOI:10.1016/j.actamat.2020.116532
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