Steel, comprising less than 2% carbon added to iron (with additional elements sometimes added), illustrates a classical strategy for improving the strength, hardness, ductility, corrosion resistance, or other mechanical property of structural materials; namely, the addition of alloying elements in small quantities to a principal element. Lately, researchers have achieved some success with high-entropy alloys (HEA), those with five or more elements in roughly equal proportions. Chain Tsuan Liu of the City University of Hong Kong together with colleagues in Hong Kong and other institutions in China have now brought the spirit of HEAs to another materials frontier, nanoparticles, to make quinary alloys laced with multicomponent intermetallic nanoparticles (MCINPs), thereby simultaneously achieving high strength and ductility. They reported their findings in a recent issue of Science.

A structural material has to be strong enough to support a useful load and ductile enough to be shaped into a useful configuration by a mechanical operation. “Achieving strength and ductility together is one of the core challenges of structural materials because both are often needed, but the mechanisms that increase strength tend to reduce the ductility and vice-versa,” says C. Cem Tasan of the Massachusetts Institute of Technology. “The authors have designed a strategy for enhancing both. While it is not the only recent proposal with this goal, their approach is an immensely important and exciting one that will motivate more research in this direction.”

When a ductile metal is stressed by pulling, compressing, or bending, it first goes through a reversible elastic deformation stage; beyond the yield point, it enters the realm of irreversible plastic deformation where dislocation motion allows the material to deform without breaking. Once in this realm, the metal may continue to deform with no additional stress applied and fail rather quickly by means of plastic instability or necking. If the dislocation motion is hampered in some way, the metal can pass through a work-hardening region (also known as strain-hardening or cold-working), where the strength increases with strain before the material ultimately fails. “Liu’s group has demonstrated that a sequence of work-hardening mechanisms, each taking over when the previous one loses steam as the strain increases is behind their ability to achieve high ductility in an alloy strengthened by second-phase MCINPs,” Tasan says.

Liu says that the research group’s general approach was to start with a ternary iron-cobalt-nickel alloy with moderate strength and ductility. To boost the strength into the gigapascal (GPa) range where ductility has always plummeted in the past, the researchers considered adding hard second-phase intermetallic compounds. Knowing that micron-sized carbide-based particles reported previously tended to crack under load, leading to low ductility because the material fractured, they added aluminum and nickel to the starting mix with the aim of making ductile nanoparticles with a stable intermetallic structure.

The preparation process starts with arc melting all five elements together at once. The mix of five elements solidifies in a two-phase region as a face-centered-cubic (fcc) matrix and MCINPs with an A₃B structure (L1₂ phase) embedded in it. The near-spherical MCINPs (about 30~50 nm in size) were uniformly distributed in the matrix with a high volume fraction up to 50–55%. All five alloying elements are present in both phases, but while they were randomly distributed in the matrix, they were ordered in the L1₂ phase. The researchers controlled the particle size and amount by heat-treating the alloys.

The two model alloys explored have the overall compositions (FeCoNi)₈₆Al₇Ti₇ and (FeCoNi)₈₆Al₈Ti₆. Of the two, the (FeCoNi)₈₆Al₇Ti₇ alloy had better properties, supporting a stress up to 1.4 GPa before failure at a strain near 50%, whereas the (FeCoNi)₈₆Al₈Ti₆ alloy suffered from a plastic instability beyond 1.2 GPa and failed at a strain near 35%. By comparison, the base iron-cobalt-nickel alloy had a maximum strength of about 0.5 GPa and failed at a bit over 30% strain.

“Our alloys have both excellent strength and ductility because the alloy matrix is ductile, and the second-phase particles are also very strong and ductile. The superior mechanical properties come from the fact that dislocations initiated in the matrix are difficult to move because they are blocked by hard and ductile particles. Also, the particles do not initiate microcracks during deformation; as a result, they do not initiate any fracture processes in the alloy,” Liu says, summarizing the remarkable behavior of the alloys.

In contrast to traditional alloys, the MCINP alloys exhibited a distinctive multi-stage work-hardening behavior. The group used both theory and experiments to identify the deformation processes underlying the various stages, breaking them down into three general regions, all marked by characteristic dislocation structures. The first two are common to both alloys, whereas the third region was unique to the (FeCoNi)₈₆Al₇Ti₇ alloy. This region was characterized by deformation-induced microbands in the matrix, where tangled dislocations form substructural walls during plastic deformation and are known to give rise to so-called microband-induced plasticity in manganese-based alloys. In the MCINP alloys, this plasticity appears to allow work-hardening to continue to higher strains and hence retain ductility.

Looking to the future, the MCINP strategy offers a paradigm that could be applied to many other alloy systems to develop next-generation materials for structural applications. In particular, Liu foresees an opportunity to apply the MCINP approach to superalloys that must function at high temperatures. “We have a good base for a new class of superalloys based on high-entropy alloys,” he says.

Read the abstract in Science.