Yang Yong in his laboratory at CityU. Photo: City University of Hong Kong.
Yang Yong in his laboratory at CityU. Photo: City University of Hong Kong.

A research team co-led by materials scientists from the City University of Hong Kong (CityU) has recently discovered a new mechanism for increasing both the strength and ductility of a high-entropy alloy – strength and ductility are two properties that normally vary inversely with each other. These findings provide important insights for the future design of strong yet ductile high-entropy alloys and high-entropy ceramics.

The strength-ductility trade-off is a longstanding issue for conventional alloys based on one or two principal elements, with increased strength usually resulting in reduced ductility and vice-versa. Over the past decade, however, a new alloy design strategy has come to the fore: mixing multiple elements to form ‘multi-principal element alloys’ (MPEAs) or ‘high-entropy alloys’ (HEAs). MPEAs exhibit excellent mechanical properties, such as both great ductility and superb strength.

These excellent mechanical properties are believed to originate from severe atomic lattice distortion caused by the random mixing of multiple principal elements with distinct atomic sizes, bonding and crystal structures, which in turn lead to a ‘heterogeneous lattice strain effect’. However, the heterogeneous lattice strain field (a strain field refers to the distribution of strain through part of a body) has been difficult to quantify and characterize, so its impact on strengthening alloys via three-dimensional dynamic dislocation has been ignored until recently.

But these latest experiments and simulations by the research team co-led by Yang Yong at CityU’s Department of Mechanical Engineering and Fang Qihong at Hunan University in China confirm that the heterogeneous strain field can contribute to the enhanced mechanical properties of MPEAs. It does this through new heterogeneous strain-induced strengthening mechanisms, leading to a strength-ductility synergy in the alloys. The team reports its findings in a paper in the Proceedings of the National Academy of Sciences.

“Materials science and engineering textbooks traditionally list four ductility-strengthening mechanisms: dislocation strengthening, solute strengthening, grain boundary strengthening and precipitation strengthening,” explained Yang. “This textbook knowledge has been taught for hundreds of years in universities to students majoring in materials science, mechanical engineering and applied physics. Now we have discovered a new ductility-strengthening mechanism through experiments and numerical simulations, which we call ‘heterogeneous lattice strain strengthening’.”

Unlike traditional strengthening mechanisms, which usually lead to a strength-ductility trade-off, this newly discovered strengthening mechanism promotes strength-ductility synergy, which means researchers can increase the strength and ductility of a HEA at the same time.

“The new findings help explain many recent findings whose mechanisms are under debate and guide the development of new strong, yet ductile metals and ceramics,” Yang added.

In the experiments, the research team first characterized the lattice strains in the HEA FeCoCrNiMn using techniques like geometric phase analysis (GPA) based on high-resolution transmission electron microscopy (TEM). The team then performed micropillar compression tests to study how dislocations glide and cross slip in the alloy. Finally, it performed extensive discrete dislocation dynamics (DDD) simulations by incorporating the lattice strains measured experimentally.

These experiments showed that the lattice strain not only restricted the dislocation motion, thus improving the yield strength, but also promoted dislocation cross slips to enhance ductility. The findings demonstrated the significant effect of the heterogeneous strain field on the mechanical properties of the alloy. They provide a new perspective on the origin of the high strength of HEAs and open up new avenues for the development of advanced crystalline materials.

The combined efforts of the experiments and computer simulations revealed the physical mechanisms that underpin the strength-ductility synergy observed in the experiments. “The findings of this study provide a fundamental mechanism to overcome the strength-ductility trade-off facing traditional alloys,” said Yang.

This story is adapted from material from the City University of Hong Kong, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.