#intermetallics

Controlling magnetism to unlock better hydrogen storage alloys

Hydrogen is expected to play a central role in future clean energy systems, but storing it efficiently and safely remains one of the biggest challenges to its widespread adoption. Solid-state hydrogen storage, in which hydrogen is absorbed into metals, is considered a promising alternative to high-pressure tanks. However, many hydrogen-storage alloys face a fundamental trade-off between storage capacity and material stability. In a new study published in Chemistry of Materials, a research team led by Distinguished Professor Hao Li of Tohoku University's WPI-AIMR has uncovered a previously underappreciated factor that governs this trade-off: magnetism. The researchers show that by controlling the magnetic properties of hydrogen-storage alloys, it is possible to design materials that are both thermodynamically stable and capable of storing large amounts of hydrogen.

Elastic strain engineering boosts green hydrogen production with affordable catalysts

Researchers from IMDEA Materials Institute have demonstrated improved and more affordable catalytic materials used to produce green hydrogen. In a study published in ACS Catalysis, intermetallic thin films made from three low-cost alloys: silver and indium (Ag₃In), nickel and iron (Ni₃Fe) and nickel and tin (Ni₃Sn), showed significant gains in catalytic efficiency for the hydrogen evolution reaction (HER) when subjected to controlled elastic strains. The findings point to elastic strain engineering as a promising avenue to develop affordable catalysts that could replace platinum-group metals in industrial hydrogen production.

Semimetal-induced covalency achieves high-efficiency electrocatalysis for platinum intermetallic compounds

Compared with other types of batteries, proton exchange membrane fuel cells have the advantages of high discharge power and no pollution, which is also an important carrier for hydrogen energy conversion and utilization. Platinum intermetallic compounds play an important role as electrocatalysts in a series of energy and environmental technologies such as proton exchange membrane fuel cells. However, the process for synthesis of platinum intermetallic compounds needs to be reorganized into ordered Pt–M metal bonds driven by high temperature (~600°C), which usually has great side effects on the structure of the catalyst, such as the uneven distribution of size, morphology, composition and structure, which further affects the performance of the catalyst and batteries. In response to this challenge, Professor Changzheng Wu's group at the University of Science and Technology of China introduced semimetal atoms, such as Ge, Sb, Te into the synthesis process of platinum-based intermetallic compounds. The research is published in the journal National Science Review.

First 3D visualization of an aluminum nanocomposite for the auto industry

Manufacturing cars with strong, lightweight aluminum alloys rather than steel could improve fuel efficiency and extend electric vehicle range, but the material's instability at high temperatures has held the alloys back from widespread adoption. Creating tiny, reinforcing particles of titanium carbide (TiC) directly inside of molten aluminum results in a stronger, more temperature resistant aluminum-based material called a metal matrix nanocomposite. Up to this point, researchers have not understood how these nanoparticles form, or how they interact with other features in the microstructure, hindering production of the material on an industrial scale.

Purdue University material engineers have created a patent-pending process to develop ultrahigh-strength aluminum alloys that are suitable for additive manufacturing because of their plastic deformability. Haiyan Wang and Xinghang Zhang lead a team that has introduced transition metals cobalt, iron, nickel and titanium into aluminum via nanoscale, laminated, deformable intermetallics. Wang is the Basil S. Turner Professor of Engineering and Zhang is a professor in Purdue's School of Materials Engineering. Anyu Shang, a materials engineering graduate student, completes the team. "Our work shows that the proper introduction of heterogenous microstructures and nanoscale medium-entropy intermetallics offers an alternative solution to design ultrastrong, deformable aluminum alloys via additive manufacturing," Zhang said. "These alloys improve upon traditional ones that are either ultrastrong or highly deformable, but not both."

Long post warning!

TL;DR: It depends on the temperature and pressure the diffusion occurs at, as well as the exact composition of the components of the diffusion couple, due to the effect these values have on the diffusion coefficients (and interdiffusion coefficient), which determine the rate at which diffusion occurs. The why is generally (mostly) understood - the what depends heavily on processing conditions and is often unknown.

Okay, this one's more complicated than most of what I post here, so there's some definitions under the cut if you need more info. (Defined terms: diffusion; intermetallics/IMCs; solid state diffusion couples; interdiffusion; activation energy; diffusion coefficient/diffusivity.)

Short answer, yes, it's kinetics. The long answer is that it's dependent on the rate of diffusion of each component (diffusivity), which is dependent on activation energy. Knowing that isn't enough to just predict which IMC forms first though - there's actually still a lot unknown here, if you're talking about a diffusion couple between intermetallic AB and CD (wherein each letter represents an element). The growth kinetics of every element under every possible condition is hard enough, let alone every potential compound. So let's start with a simpler example, diffusion between two pure elements. Here's an example article discussing formation of intermetallics between Al and Cu (2022 article) - even here, this is new research that doesn't fully answer the question for every processing condition, but it does demonstrate how one might go about predicting/determining which IMC forms first.

While diffusion isn't a phase transformation per se, it still involves the formation of new phases, so a phase diagram can tell you which phases are possible. The actual phases that form are heavily dependent on processing. In the above example article, there are five possible intermetallic phases in the temperature range the authors are interested in, and they site a few examples of prior research with different processing, wherein different IMCs formed (anywhere from 2 to all 5). The best way to know what's going to form is simply to run the experiment, although modeling is becoming more and more popular as computational materials science improves.

What it all really comes down to is activation energy (it almost always comes down to energy: whatever costs the least energy, the universe will let happen). The thing is, as stated above, a lot of times we don't know the activation energy. The above example paper experimentally determines the rate constant, which they use to get the activation energy for each intermetallic. Activation energy then influences diffusion coefficients.

But that can't always be used to determine which forms first; and, indeed, in the above example it isn't enough. Changing directions slightly, we can turn to something called the effective heat of formation model, wherein the intermetallic with the lowest value will form first. We could also look directly at Gibbs free energy, which can get... complicated (involving a lot of math, including factors based on electron distribution differences, size differences, etc.). It gets even more complicated too for the second intermetallic, because now you've got two interfaces between the original components and the first intermetallic formed.

Another example article here (2011 article) is the diffusion between a steel alloy pure aluminum, including the effect of silicon (essentially the Fe-Si-Al intermetallic system). They list three reasons: chemical potentials; nucleation conditions; and mobilities of constituent elements. Different terminology, but essentially the same discussion. Chemical potential isn't equal to activity or activity coefficients, but they're very closely linked (resource), and mobility is essentially the diffusion coefficient (again relying on activity). The only new reason is the nucleation conditions, as new phases can't form if they don't have nucleation sites; this can be neglected in some theoretical discussions, but obviously is important in real world situations.

So generally speaking, yes, we know why some form first (lower energy required), but we can't always predict what those are going to be just by knowing the components of a diffusion couple. This is a complicated question, with a lot more factors I haven't touched on, and if my answer is confusing (or I haven't properly answered the question at all!), or you want me to expand on any of the points, please let me known and I'll try to give more info!

Additional reading:

Effective heat of formation model: 1996 article, 1991 article

Complicating things by considering grain size/grain boundaries: 2015 article

Example in the Zr-Mo/W system: 2023 article

Additional aluminum-steel alloy systems information: 2022 article

General articles about diffusion in metals and IMCs: 1996 article, 2009 article

[As always, if you know more about the topic and I've gotten something wrong, please correct me (with sources)!]

Unlocking the secrets of quasicrystal magnetism: Revealing a novel magnetic phase diagram

Quasicrystals are intermetallic materials that have garnered significant attention from researchers aiming to advance condensed matter physics understanding. Unlike normal crystals, in which atoms are arranged in an ordered repeating pattern, quasicrystals have non-repeating ordered patterns of atoms. Their unique structure leads to many exotic and interesting properties, which are particularly useful for practical applications in spintronics and magnetic refrigeration. A unique quasicrystal variant, known as the Tsai-type icosahedral quasicrystal (iQC) and their cubic approximant crystals (ACs), display intriguing characteristics. These include long-range ferromagnetic (FM) and anti-ferromagnetic (AFM) orders, as well as unconventional quantum critical phenomenon, to name a few.

New criteria to determine whether shear bands are beneficial or harmful to crystalline materials

Shear band formation is not typically a good sign in a material—the bands often appear before a material fractures or fails. But materials science and engineering researchers at the University of Wisconsin–Madison have found that shear bands aren't always a negative; under the right conditions, they can improve the ductility, or the plasticity, of a material.

Led by Izabela Szlufarska, a professor of materials science and engineering at UW–Madison, the researchers published details of their work in the journal Nature Materials.

Using a combination of experimental characterization and simulations, the team identified potential strategies for encouraging shear bands. This could lead to new ways of increasing the toughness of a wide array of materials.

"In a previous paper, we demonstrated that shear bands in a material called samarium cobalt could actually be beneficial," says Szlufarska. "That led to the questions, "When do shear bands form?" and "When do they support plasticity versus fracture? When do you want to avoid them and when do you want to promote them?'"