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One of the corner stones of materials research is the design and development of alloys. Particular emphasis has been laid on the creation of alloys that can help pierce the envelope of known limitations of machine performance. Broad-range of examples include development of materials that have long service life in aviation applications performing at supersonic velocities, light yet strong materials that can make commercial, non-stop global flights possible (Read: Hyderabad - Dallas; Hyderabad - Santiago; Melborne - Chicago; to name a few). Life time service of in-body implats, life time service of batteries, high efficiency solar panels, ultra-low activation alloys and so on.
Picking on the last example, ultra low-activation alloys; this has been a subject of continued research interest for five-decades, as visionaries see that future of energy lies in realization of safe nuclear technologies. US Departement of energy has prioritized developing nuclear technology as a sustained and safe source of energy, and has been identified it as a top grand-challenge by the US National Academy of Engineering.
This is an important area of research since the very realization of Gen IV nuclear power-plants and clean energy source depends on availability of materials. Although there is no radioactivity in the fusion reactor core, the structural materials in the vicinity gain radioactivity from prolonged exposure to nuclear transmutation reactions. It may take several hundred years before the activity of these alloys reaches “hands-on level” to be safely recycled.
In cases such as these, the traditional "linear" approach namely casting small sample ingots, testing and commercialization takes too long a time, often spanning decades, and is not suitable for the rapidly changing world and its concurrant needs.
Therefore, new a technique has been proposed to revolutionize and accelerate materials discovery. An integrated materials development approach combining molecular dynamics, parallelly with experiments was proposed to depart from the linear development path to a concurrent development path. For the specific example being discussed, i.e., low-activation alloys high entropy alloy compositions were chosen from a palette of reduced activation elements. These alloys may be grouped as 4-5-6 alloys since the elements are from the namesake groups and periods in the periodic table. This is called first-tier selection.
Several hundreds of MD calculations were carried out to explore the alloy space created by the first tier group, and a group of few tens of alloys were narrowed down to in a span of few weeks. Of these set of alloys a dozen were casted and charecterized for their application worthiness. Two such alloys are shown below:
Overview: Elements selected from their first tier properties, summary of MD simulations predicting a two phase alloy formation in Ti-V-Zr-Ta-W as seen by the two peaks, and a single phase alloy in Ti-V-Zr-Ta-Hf alloy. Transmission electron images taken from the alloys confirm the theoritical predictions.
Electron microscopy images showing two phase and single phase compositions, accompanied by respective EDS maps.
The alloys created based on the MD simulations produced excellent microstructural stability, high formability and very high uniformity in the elemental distribution.
The fascinating observation was the accurate theoretical prediction exactly coinciding with the experimental observation. One alloy showed two b.c.c phases and the other a single b.c.c phase, consntant with the predictions, validating the approach!
As much as the alloy development and its success as a novel approach was gratifying, the ultimate test of the validity of the effort was measuring the response to irradiation. The samples were exposed to high irradiation flux and the properties were measured.
This was accomplised by creating small pillars using FIB milling, and subsequent compression testing.
The tests showed that the alloys showed much lesser irradiation induced damage as compared to SS304 stainless steel.
The results of compression tests are shown in the images. This was the first time serendipity of such a broad range of techniques - namely, MD, alloy casting and heat treatment, irradiation experiments, hardness measuremetns bulk and nanomechanical tests formed an integrated approach for accelarated materials discovery.
Further Reading:
Low activation high entropy alloys for next generation nuclear applications, Materialia Volume 4, December 2018, Pages 99-103.
Ion irradiation response and mechanical behavior of reduced activity high entropy alloy Journal of Nuclear Materials Volume 529, February 2020, 151955
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