3D atomic details of next-generation alloys revealed for first time

Amazing stuff!

"Key takeaways

• A team led by ... researchers used an advanced imaging technique to develop 3D maps of the individual atoms of medium- and high-entropy alloys — a scientific first.
• Medium- and high-entropy alloys, developed about 20 years ago, show the capacity to be both tough and flexible in ways that everyday alloys such as steel aren’t.
• These findings, about how structure affects function in these alloys, may enable engineers to tune the alloys’ properties to produce far more durable objects and technologies.

... about 20 years ago, when researchers first developed medium- and high-entropy alloys, stable materials that combine hardness and flexibility in a way in which conventional alloys do not. (The “entropy” in the name indicates how disorderly the mixture of the elements in the alloys is.) ...
Medium-entropy alloys combine three or four metals in roughly equal amounts; high-entropy alloys combine five or more in the same way. In contrast, conventional alloys are mostly one metal with others intermixed in lower proportions. (Stainless steel, for example, can be three-quarters or more of iron.) ...

[Researchers] focused on a type of structural defect called a twin boundary, which is understood to be a key factor in medium- and high-entropy alloys’ unique combination of toughness and flexibility. Twinning happens when strain causes one section of a crystal matrix to bend diagonally while the atoms around it remain in their original configuration, forming mirror images on either side of the boundary. 

The researchers used an array of metals to make nanoparticles ... Six medium-entropy alloy nanoparticles combined nickel, palladium and platinum. Four nanoparticles of a high-entropy alloy combined cobalt, nickel, ruthenium, rhodium, palladium, silver, iridium and platinum. ...
The scientists liquified the metal at over 2,000 degrees Fahrenheit for five-hundredths of a second, then cooled it down in less than one-tenth that time. The idea is to fix the solid alloy in the same varied mixture of elements as a liquid. Along the way, the shock of the process induced twin boundaries in six of the 10 nanoparticles; four of those each had a pair of twins. ..."

From the abstract:

"Medium- and high-entropy alloys (M/HEAs) mix several principal elements with near-equiatomic composition and represent a model-shift strategy for designing previously unknown materials in metallurgy, catalysis and other fields. One of the core hypotheses of M/HEAs is lattice distortion, which has been investigated by different numerical and experimental techniques. However, determining the three-dimensional (3D) lattice distortion in M/HEAs remains a challenge. Moreover, the presumed random elemental mixing in M/HEAs has been questioned by X-ray and neutron studies, atomistic simulation, energy dispersive spectroscopy and electron diffraction, which suggest the existence of local chemical order in M/HEAs. However, direct experimental observation of the 3D local chemical order has been difficult because energy dispersive spectroscopy integrates the composition of atomic columns along the zone axes and diffuse electron reflections may originate from planar defects instead of local chemical order. Here we determine the 3D atomic positions of M/HEA nanoparticles using atomic electron tomography and quantitatively characterize the local lattice distortion, strain tensor, twin boundaries, dislocation cores and chemical short-range order (CSRO). We find that the high-entropy alloys have larger local lattice distortion and more heterogeneous strain than the medium-entropy alloys and that strain is correlated to CSRO. We also observe CSRO-mediated twinning in the medium-entropy alloys, that is, twinning occurs in energetically unfavoured CSRO regions but not in energetically favoured CSRO ones, which represents, to our knowledge, the first experimental observation of correlating local chemical order with structural defects in any material. We expect that this work will not only expand our fundamental understanding of this important class of materials but also provide the foundation for tailoring M/HEA properties through engineering lattice distortion and local chemical order."

3D atomic details of next-generation alloys revealed for first time | UCLA UCLA-led team maps medium- and high-entropy alloys and identifies potential to tune their properties, making them both stronger and more flexible

Three-dimensional atomic structure and local chemical order of medium- and high-entropy nanoalloys (no public access)

Atomic map of a high-entropy alloy nanoparticle shows different categories of elements in red, blue and green, and twinning boundaries in yellow.

Atomic electron tomography (AET) and its transformative impact on the physical sciences. (Top) Schematic diagram of AET, in which 2D images are measured with advanced electron microscopy by tilting a sample to many different orientations. The 3D structure of the sample is iteratively reconstructed from the images and the coordinates of individual atoms are localized. (Bottom) AET enables 3D imaging of crystal defects such as grain boundaries, stacking faults, dislocations and point defects at atomic resolution. The ability to precisely determine the coordinates of individual atoms allows direct measurements of atomic displacements and the full strain tensor in materials. (Source)