There is a drive to improve the environmental credentials of certain industries, such as aerospace, automotive and power generation in order to meet the stringent environmental target set by regulatory bodies. Amongst the proposed changes is the possibility to operate key components used in these industries at higher temperatures, as this leads to greater fuel efficiency and a reduction in harmful emissions.
Typically, high temperature components have been made from nickel-based superalloys, which emerged in the 1950s. These materials are capable of withstanding temperatures in excess of 1000°C. However, it is estimated that, in order to meet the regulatory requirements, operating temperatures will need to increase to around 1850°C - well in excess of the point that nickel-based superalloys will start to melt.
At the University of Sheffield, researchers in the Department of Materials Science and Engineering are developing ultra-high temperature materials (UHTMs) which have the potential to replace nickel alloys and operate at these higher temperatures. Amongst the materials being investigated are Refractory Metal Intermetallic Compounds, or RMICs.
In particular, researchers are investigating a specific family of RMICs, namely niobium-silicide based alloys, and in a recent research paper, present an in-depth comparison of these alloys with Refractory Metal High Entropy Alloys (RHEAs) and Refractory Metal Complex Concentrated Alloys (RCCAs) for the very first time.
Leading the research is Professor Panos Tsakiropoulos, who presents the research as a tryptic - three connected ‘leaves’ of findings.
In the first part, or leaf, of the tryptic, Professor Tsakiropoulos presents his thoughts on the properties of niobium-based silicides and how alloying can influence these properties. The second leaf examines how processing of these alloys can affect the development of the alloys’ microstructures. Finally, the third leaf looks at the design and development of specific alloys.
In this 'leaf' he briefly describes the constraints and challenges confronted by alloy developers and the opportunities that arise from relationships between parameters of alloys and their phases and between parameters and solute concentrations that are common for Nb-silicide based alloys, HEAs and RCCAs.
Professor Tsakiropoulos relates this back to a new methodology for alloy development design devised at the University of Sheffield. NICE, or Niobium Intermetallic Composite Elaboration uses key quantitative chemical analysis data about alloys and their phases, in particular for refractory metal based systems and for systems with alloying additions that are essential for achieving a balance of properties and oxidation resistance, to help design alloy systems and select those that are worthy of further investigation.
In a paper published in Progress in Materials Science, Professor Tsakiropoulos discusses the capabilities of NICE with emphasis on the design Nb-silicide based alloys and RCCAs to meet specific property goals. For the first time he shows that RCCAs with Nb and other RM additions studied to date can be represented together with Nb-silicide based alloys in specific maps of parameters based on atomic size, electronegativity and number of valence electrons per atom filled into the valence band.
The full paper can be read here: https://www.sciencedirect.com/science/article/pii/S0079642520300785