Interstitial high entropy alloys
The recently introduced new materials class of high-entropy alloys (HEAs) consist of multiple principle elements. These materials provide a novel and promising avenue for realizing exceptional mechanical, physical and chemical properties.
We introduced a novel strategy for designing a new class of HEAs incorporating additional interstitial elements.
We refer to these materials as to interstitial high-entropy alloys (iHEAs).
Specifically the use of interstitial carbon doping results in an alloy class which is chartacterized by the joint activation of twinning- and transformation-induced plasticity (TWIP and TRIP) by tuning the matrix phase’s instability in a metastable TRIP-assisted dual-phase HEA. Besides TWIP and TRIP, such alloys benefit from massive substitutional and interstitial solid solution strengthening as well as from the composite effect associated with its dual-phase structure. Nanosize particle formation and grain size reduction are also utilized. The new interstitial TWIP-TRIP-HEA thus unifies all metallic strengthening mechanisms in one material, leading to twice the tensile strength compared to a single-phase HEA with similar composition, yet, at identical ductility.
Scientific Reports | 7:40704 | DOI: 10.1038/srep40704
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Deformation, phase transformation and mechanical twinning mechanisms in a Fe-30Mn-10Co-10Cr-0.5C (at. %) interstitial high entropy alloy
In-situ SEM observation of phase transformation and twinning mechanisms in an interstitial high-entropy alloy: Acta Materialia 147 (2018) 236
Acta Mater 2018 TWIP and TRIP effect in [...]
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Li 2017 JOM 2017 overview Non-Equiatomic[...]
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Hydrogen enhances strength and ductility of an interstitial equiatomic high entropy alloy
Here we demonstrate how even such a dangerous atom as hydrogen can be used as interstitial alloying element in interstitial high entropy alloys.
Since 1874 it is known that the lightest element of all, hydrogen, can be very hamful as it causes catastrophic and unpredictable failure in metallic alloys, a phenomenon referred to as hydrogen embrittlement. All metallic alloys can suffer from it, be it in engineering parts used in vehicles, planes or power plants or in the context of future fusion and hydrogen fuel and energy storage driven industries.
Although these threats and opportunities have motivated nearly one and a half centuries of research, hydrogen remains not only a ubiquitous but also a threatening element in engineering
metallic alloys. Once hydrogen has entered into metals it accumulates in voids and gets trapped at vacancies, dislocations
and internal interfaces, i.e. at lattice defects which determine the physical, chemical and mechanical properties of metals. Once occupying these sites, hydrogen damages the material through enhanced localized plasticity,
decohesion, vacancy stabilization, hydride formation or void coalescence.
Although practically all metals suffer from such phenomena, the high-entropy alloy (HEA) investigated here seems to be not only less prone to hydrogen embrittlement but it even profits from its presence.
As hydrogen obviously occupies the interstitial sites in such materials the current alloy pertains also to the class of interstitial high entropy alloys (iHEAs).
Interstitial high entropy alloys are a new class of materials originally defined as solid metallic solutions composed of five or more principal elements in
equimolar or near-equimolar ratios for yielding high configurational entropy, yet, here alloyed also with interstitial elements. This concept introduces a new path for developing advanced materials with some unique mechanical properties. The base material, the five-component equiatomic CoCrFeMnNi alloy, is one of the most appealing HEAs due to the good thermodynamic stability of its single face-centred cubic (f.c.c.) structure and the excellent mechanical properties
under various temperatures.
Owing to these features we picked this equiatomic CoCrFeMnNi model HEA for studying its changing mechanical tensile behaviour when exposed to hydrogen. We show that this material is not only resistant to hydrogen embrittlement but we even observe its beneficial role as alloying element as it improves rather than
deteriorates both, the material’s strength and ductility. The key idea behind this turnaround lies in decreasing the stability of the f.c.c. lattice structure of the matrix via hydrogen alloying to trigger more intense nanotwinning upon loading, thereby improving strain-hardening of the alloy.
Scientific Reportsvolume 7, Article number: 9892 (2017)
Luo et al 2017 Sci Rep Hydrogen strength[...]
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The Effect of Boron in High Entropy Alloys
Boron is an essential alloying element in High-Entropy Alloys. In this approach we introduced a new HEA design that is based on compositionally conditioning the grain boundaries instead of the bulk.We found that as little as 30 ppm of boron doping in single-phase HEAs, more specific in an equiatomic FeMnCrCoNi and in a non-equiatomic Fe40Mn40Cr10Co10 (at%), improves dramatically their mechanical properties, increasing their yield strength by more than 100% and ultimate tensile strength by ~40% at comparable or even better ductility. Boron decorates the grain boundaries and acts twofold, through interface strengthening and grain size reduction. These effects enhance grain boundary cohesion and retard capillary driven grain coarsening, thereby qualifying boron-induced grain boundary engineering as an ideal strategy for the development of advanced HEAs.