A high-entropy alloy containing a gradient nano-twin structure, characterized by a high population of nano-twins in the surface region and coarser twins with lower density in the inner sample region.Strong, load-carrying alloys have one major shortcoming: these multi-component metallic materials can undergo sudden, catastrophic failure when exposed to hydrogen. Now, however, researchers have found a way to beat hydrogen at its own game and turn this weakness into strength [Luo et al., Materials Today (2018), https://doi.org/10.1016/j.mattod.2018.07.015].
The team from Max-Planck-Institut für Eisenforschung in Germany has developed a new approach to ward off the hydrogen embrittlement problem in a high-entropy alloy (HEA) CoCrFeMnNi. In metallic materials, hydrogen is known to have an effect on how dislocations multiply, move, and coalesce, which can have a significant knock-on effect on mechanical properties. But instead of avoiding hydrogen, the researchers used hydrogen to promote the formation of twins in the alloy.
“We produced a CoCrFeMnNi high-entropy alloy that contains a gradient nano-twin structure, characterized by a high population of nano-twins in the surface region and coarser twins with lower density in the inner sample regions,” explain Zhiming Li and Dierk Raabe, who led the research.
Twin-gradients are well known to improve the mechanical properties of metals by providing a barrier to the propagation or spread of cracks. However, these nanostructures are usually introduced by means of physical processes such as surface mechanical grinding. In this work, however, a chemical rather the physical process is at work. Because hydrogen cannot diffuse very deeply into CoCrFeMnNi, far more nano-twins are created in the surface region than in the core of the material, creating a gradient in the number of twins. The nano-twin gradient counteracts weakening in the material with local strengthening.
“The gradient in the nano-twin population provides additional local strain hardening reserves, suppressing the material’s surface embrittlement,” say Li and Raabe. “With this, we have invented a self-accommodation mechanism: the higher the local hydrogen content (which would otherwise be detrimental), the higher the twin formation rate.”
The result is the complete absence of hydrogen-embrittlement surface cracks when the treated alloy is deformed at low temperatures. CoCrFeMnNi is already one of the most appealing HEAs because of its high thermodynamic stability and excellent mechanical properties at various temperatures, but this approach offers new opportunities for designing novel alloys with even more exceptional mechanical, physical, and chemical characteristics.
“Our findings represent a new strategy in designing hydrogen-tolerant materials for cryogenic applications,” point out Li and Raabe. “Embrittlement-resistant alloys are crucial for modern manufacturing and infrastructure, as well as transport and energy solutions.”
Hydrogen-resistant HEAs able to withstand very cold conditions would be highly desirable for artic, offshore, energy and liquid gas storage applications. The researchers are now using their approach to develop novel ultrahigh strength, corrosion and hydrogen-embrittlement resistant alloys.