Interstitial High Entropy Alloys

What are Interstitial High Entropy Alloys (iHEA) ?

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).

 

Which deformation mechanisms occur in Interstitial High Entropy Alloys ?

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.

Interstitial atoms enable joint twinning and transformation induced plasticity in strong and ductile high-entropy alloys
Scientific Reports | 7:40704 | DOI: 10.1038/srep40704
Li_et_al-2017-Scientific_Reports interst[...]
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Microstructure and elemental distribution in the as-homogenized coarse-grained interstitial high entropy alloy iHEA: Scientific Reports volume 7, Article number: 40704 (2017) doi:10.1038/srep40704 Microstructure and elemental distribution in the as-homogenized coarse-grained interstitial high entropy alloy iHEA: Scientific Reports volume 7, Article number: 40704 (2017) doi:10.1038/srep40704
Interstitial high entropy alloy: (a) XRD and EBSD patterns reveal the f.c.c. matrix and a small fraction of h.c.p. phase prior to deformation. (b) ECC image and EDS maps corresponding to the identical region marked in (a) show that the nano-sized particle Interstitial high entropy alloy: (a) XRD and EBSD patterns reveal the f.c.c. matrix and a small fraction of h.c.p. phase prior to deformation. (b) ECC image and EDS maps corresponding to the identical region marked in (a) show that the nano-sized particle
Interstitial high entropy alloy: GS refers to the grain size. (a) Engineering stress-strain curves; data of Fe50Mn30Co10Cr10 (at%) TRIP-DP-HEAs (ref. 6), single-phase Fe20Mn20Ni20Co20Cr20 (at%) and Fe19.9Mn19.9Ni19.9Co19.9Cr19.9C0.5 (at%) HEAs (refs 7 and Interstitial high entropy alloy: GS refers to the grain size. (a) Engineering stress-strain curves; data of Fe50Mn30Co10Cr10 (at%) TRIP-DP-HEAs (ref. 6), single-phase Fe20Mn20Ni20Co20Cr20 (at%) and Fe19.9Mn19.9Ni19.9Co19.9Cr19.9C0.5 (at%) HEAs (refs 7 and

 

 

What is a bidirectional transformation in metastable high entropy alloys?

We made in two high entropy alloy variants the surprising observation that in these materials not only the multiple well-known deformation mechanisms resulting from phase metastability such as martensitic phase transformation and twinning may occur but also the reverse transformations when exposed to loads. Particularly, the observation of a deformation-driven martensitic transformation from the original FCC host phase into the hexagonal close-packed (HCP) product phase and back is an intersting finding in that context. This means that the deformation-driven athermal transformation can be bidirectional, yet, in the
form of different crystallographic variants. We found this effect in two different types of metastable alloys, viz. a carbon doped and non-carbon doped metastable HEA variant. In the C-free HE alloy the deformation-induced bidirectional transformation observed under tensile loading showed formation of reverted FCC phase from a preceding martensitic HCP region. This effect was related to the reverse motion of the Shockley partials that had been generated to form the HCP martensite. In the metastable carbon-doped iHEA has we find similar effects of this bidirectional transformation induced plasticity (Bi-TRIP) and used it to create bulk nanostructures.

Deformation-driven bidirectional transformation promotes bulk nanostructure formation in a metastable interstitial high entropy alloy
Acta Materialia 167 (2019) 23
Acta 2019 bidirectional transformation n[...]
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Bidirectional Transformation Enables Hierarchical Nanolaminate Dual-Phase High-Entropy Alloys
Adv. Mater. 2018, 1804727
Bidirectional TRIP effect High Entropy A[...]
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Deformation-driven bidirectional transformation promotes bulk nanostructure formation in a metastable interstitial high entropy alloy Deformation-driven bidirectional transformation promotes bulk nanostructure formation in a metastable interstitial high entropy alloy
Bidirectional Transformation Enables Hierarchical Nanolaminate Dual-Phase High-Entropy Alloys Bidirectional Transformation Enables Hierarchical Nanolaminate Dual-Phase High-Entropy Alloys

 

 

 

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
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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Interstitial high entropy alloy: In-situ ECCI observations of SF formation during tensile testing in (a) large (grain size ~7.8 mm); and (b) small (grain size ~3 mm) FCC g grains. The engineering stress strain curves given above indicate the corresponding Interstitial high entropy alloy: In-situ ECCI observations of SF formation during tensile testing in (a) large (grain size ~7.8 mm); and (b) small (grain size ~3 mm) FCC g grains. The engineering stress strain curves given above indicate the corresponding
Overview: Strong and Ductile Non-equiatomic High-Entropy Alloys: Design, Processing, Microstructure, and Mechanical Properties
Li 2017 JOM 2017 overview Non-Equiatomic[...]
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What is the strain hardening mechanism in an interstitial high-entropy alloy under cryogenic conditions?

In this project we investigate the deformation mechanisms of a carbon-doped
interstitial high-entropy alloy (iHEA) with a composition of Fe49.5 Mn30 Co10 Cr10 C0.5 (at. %) at cryogenic conditions. Very good strain hardening of this material at 77 K leads to a substantial increase in ultimate tensile strength (1300MPa) and excellent ductility (50%) compared to the room temperature values. Prior to loading, iHEAs with coarse (100 mm) and fine (6 mm) grain sizes show nearly single face-centered cubic (FCC) structure, while the fraction of hexagonal close-packed (HCP) phase reaches up to 70% in the cryogenically tensile-fractured iHEAs. Such an unusually high fraction of deformation-induced phase transformation and the associated plasticity (TRIP effect) is caused by the strong driving force supported by the reduced stacking fault energy and increased flow stress at 77 K. The transformation mechanism from the FCC matrix to the HCP phase is revealed by transmission electron microscopy (TEM) observations. In addition to the deformation-induced phase transformation, stacking faults and dislocation slip contribute to the deformation of the FCC matrix phase at low strains and of the HCP phase at medium and large strains, suggesting dynamic strain partitioning among these two phases. The combination of TRIP and dynamic strain partitioning explain the striking strain hardening capability and resulting excellent combination of strength and ductility of iHEAs under cryogenic conditions. 

Engineering stress-strain curves and strain hardening rate curve as a function of true strain for CG and FG iHEAs tested at 293 K and 77 K. The strain hardening rate curves for the fine-grained CoCrFeMnNi HEA (17 um) and CoCrNi MEA (16 um) are shown for c Engineering stress-strain curves and strain hardening rate curve as a function of true strain for CG and FG iHEAs tested at 293 K and 77 K. The strain hardening rate curves for the fine-grained CoCrFeMnNi HEA (17 um) and CoCrNi MEA (16 um) are shown for c
On the mechanism of extraordinary strain hardening in an interstitial high-entropy alloy under cryogenic conditions
Journal of Alloys and Compounds 781 (2019) 734
J Alloys Comp 2019 strain hardening inte[...]
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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.

 

Hydrogen enhances strength and ductility of an equiatomic high-entropy alloy
Scientific Reportsvolume 7, Article number: 9892 (2017)
doi:10.1038/s41598-017-10774-4
Luo et al 2017 Sci Rep Hydrogen strength[...]
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Tensile deformation behavior of different metals including the CoCrFeMnNi HEA under various in-situ hydrogen charging conditions. Tensile deformation behavior of different metals including the CoCrFeMnNi HEA under various in-situ hydrogen charging conditions.

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.

Boron doped ultrastrong and ductile high-entropy alloys
Acta Materialia 151 (2018) 366-376
Boron doped ultrastrong and ductile high[...]
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Interstitial high-entropy alloys with TRIP and TWIP effect

We made an approach to utilizing a hierarchical microstructure design to improve the mechanical properties of an interstitial carbon doped high-entropy alloy (HEA) by cold rolling and subsequent tempering and annealing. Bimodal microstructures were produced in the tempered specimens consisting of nano-grains (~50 nm) in the vicinity of shear bands and recovered parent grains (10-35 um) with pre-existing nano-twins. Upon annealing, partial recrystallization led to trimodal microstructures characterized by small recrystallized grains (<1 um) associated with shear bands, medium-sized grains (1-6 um) recrystallized through subgrain rotation or coalescence of parent grains and retained large unrecrystallized grains. To reveal the influence of these hierarchical microstructures on the strength-ductility synergy, the underlying deformation mechanisms and the resultant strain hardening were investigated. A superior yield strength of 1.3 GPa was achieved in the bimodal microstructure, more than
two times higher than that of the fully recrystallized microstructure, owing to the presence of nano-sized grains and nano-twins. The ductility was dramatically improved from 14% to 60% in the trimodal structure compared to the bimodal structure due to the appearance of a multi-stage work hardening
behavior. This important strain hardening sequence was attributed to the sequential activation of transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects as a result of the wide variation in phase stability promoted by the grain size hierarchy. These findings open a broader
window for achieving a wide spectrum of mechanical properties for HEAs, making better use of not only compositional variations but also microstructure and phase stability tuning.

Metastable interstitial high-entropy alloy with TRIP and TWIP effect (Acta Materialia 163 (2019) 40); Hierarchical microstructure design to tune the mechanical behavior of an interstitial TRIP-TWIP high-entropy alloy Metastable interstitial high-entropy alloy with TRIP and TWIP effect (Acta Materialia 163 (2019) 40)
Hierarchical microstructure design to tune the mechanical behavior of an interstitial TRIP-TWIP high-entropy alloy
Acta Materialia 163 (2019) 40-54
Jing SU et al in press January 2019 Acta[...]
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