Investigating the local structure of Ni-33atCr via neutron total scattering

The atomic ordering of alloys is known to influence a host of material properties, including: strengthening, electrical conductance, magnetic properties and radiation resistance. Despite this, our understanding of how changes in the arrangement of atoms affect these properties has not been thoroughly developed, largely due to limitations on our ability to model the atomic scale structures of alloys. Over recent years, developments in diffraction techniques have allowed us to reconstruct models of these atomic structures, which can then be used to understand the mechanisms for changes in physical properties that we see on the macroscale.

Generally we assume our alloy systems to be random solid solutions, where atoms have no preference in their lattice neighbouring. However, in many cases, we know this to be untrue. It is known that many alloys may exhibit short-scale preferences for their atomic neighbouring, often referred to as Short-Range Order (SRO), or even form distinct ordered superlattices coherent to the random matrix.

Nichrome (Ni-Cr) is a common base for many current Ni-superalloys, as well as the emerging Multi-Principle Element Alloy (MPEA) field, largely due to its strengthening capabilities and corrosion resistance. It’s also known to exhibit SRO under some conditions and, after extended ageing, can form a separate Ni2Cr superlattice, both of which are thought to drastically alter the properties of alloys. Despite this, our understanding of how these changes in atomic neighbouring influences alloy properties is limited and has reduced our ability to make both informed alloy design choices and improve current commercial systems.

During my third year of the stream, I’ve been conducting a placement at ISIS Neutron Muon Source, Oxfordshire. Here we’re working to develop some of the existing instruments to expand our capabilities and offer a wider array of experiment bases, including in-situ tensile testing. This on a site where users already come to characterise  everything from metals, battery materials, to biological molecules and archaeological artifacts.

Tom Cole

Total Scattering has emerged as a novel technique for reconstructing models of alloy structures. High precision diffraction data is obtained using national synchrotron facilities. This data can be transformed into Pair Distribution Functions (PDFs), which represent the tendency for atoms to exist at a given distance from one another (typically over ~3 – 4nm range). Reverse Monte Carlo (Large Box) modelling can then be used to reconstruct a visualisation of local atomic configurations which will enable us to develop future models and mechanisms for changing alloy properties through chemical ordering.

However, our ability to accurately reconstruct these large box models is currently limited by the presence of artificial features in the box due to processing and our understanding how metallurgical factors (such as crystallographic texture) may affect the quality of our PDFs.

Current technological advancements – be it high temperature alloys for nuclear fusion technology, or energy storage for green domestic products – can be restricted by the properties of the materials involved. The design of new material systems can help accelerate the process of these advances. Understanding how the atomic scale structures of these materials can influence the material properties is, therefore, critical to our further development. By improving the methods by which we can image and characterise these structures we can begin to understand atomistic mechanisms on a much more fundamental level, opening up the possibility of designing new materials specifically for novel applications from the atomic scale up.

So far, we have managed to demonstrate the use of Total Scattering as a viable technique for reconstructing the atomic structure of alloy systems, and used this to analyse how Nichrome systems evolve under different operating conditions. Potential applications for this new understanding of atomic structures reach to the development of new alloys, and impacts for the aerospace and nuclear industries, which rely heavily on Nichrome as a base for many commonly-used alloys.

Further work is being done to understand how these atomic structures change and develop through prolonged aging and high temperature environments, as well as the strengthening and deformation characteristics for failure mechanisms. By expanding our understanding of this base system to more complex cases, we will be able to better quantify the changing properties of alloys due to local order and qualify material for novel applications.

Equipment used

• ISIS Neutron Muon Source

• Beam experiments are permitted through ISIS beam application processes.

Biography

Tom completed his MEng in Material Science and Engineering at the University of Sheffield where his dissertation focused on the corrosion of CoCrMo alloy in biomedical implants. He remained in Sheffield to conduct a PhD in conjunction with the Advanced Metallic Systems CDT, supported by ISIS Neutron Muon source (Rutherford Appleton Laboratory) where he is also placed to aid in the development of In-situ testing rigs on the Polaris diffractometer.