Complex metal alloys have physical properties that keep them sturdy even under intensely extreme conditions. They can withstand the operating conditions of jet engines, for instance, where extraordinarily hot temperatures would melt most typical metals; or the cores of nuclear reactors, where radiation would degrade their ‘pure’ counterparts.
Refractory high-entropy alloys (RHEAs) are a prime example: the combination of several heavy-duty elements enables them to endure in the most challenging industrial environments. These exceptional qualities of RHEAs are, paradoxically, a product of their intrinsic imperfections.
“In common metals, all atoms are identical, leading to predictable behaviour,” said Zachary Aitken, a Senior Scientist at A*STAR’s Institute of High Performance Computing (IHPC). Conversely, RHEAs are a mosaic of different atoms in a random arrangement. This randomness fosters short-range ordering (SRO), which introduces localised zones with a propensity for dislocations: tiny irregularities in the metal’s crystal structure that affect its strength and flexibility.
Understanding these nuances is crucial for predicting RHEA behaviour under stress. According to Aitken, prior research offered contradictory insights into how two key dislocation types—edge and screw—proliferate in alloys.
“Experiments have indicated that unlike conventional metals, which have less complex compositions, RHEAs have a prevalence of edge dislocations,” said Aitken.
Working with researchers from the University of Hong Kong, the City University of Hong Kong, and the University of Tennessee, USA, Aitken and colleagues took a synergistic approach to investigate an alloy composed of molybdenum, tantalum, titanium, tungsten and zirconium. Combining density-functional theory, Monte Carlo simulations and molecular dynamics, this approach enabled them to conduct an unprecedented, atomic-level examination of dislocation dynamics in the presence of SRO.
Their findings revealed that SRO prompts the emergence of a pseudo-composite microstructure within RHEAs which not only comprised clusters with distinct energy levels, but promoted unexpected types of dislocations.
“The low-energy clusters, being more stable, resist deformation and contribute to the alloy’s strength,” said Aitken. “On the other hand, the high-energy clusters, due to their instability, facilitate plasticity, while the medium-energy clusters form the alloy’s matrix.”
The team’s discovery of how SRO affects the dominance of dislocations in RHEAs may be instrumental for engineering alloys that marry strength with ductility, overcoming the long-standing trade-off between strength and brittleness.
Moving forward, Aitken and colleagues aim to extend these findings to practical applications, focusing on how SRO and dislocation dynamics at high temperatures can be harnessed to refine RHEAs for even broader applications, such as in advanced turbine technologies.
The A*STAR-affiliated researchers contributing to this research are from the Institute of High Performance Computing (IHPC).
