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Thu Jul 23, 2020, 04:21 PM

Strength through disorder

Obviously the title of this post does not refer to the senile racist orange bastard in the White House who has made America weak through disorder, but instead refers to the subtitle of this paper: Ultrahigh-strength and ductile superlattice alloys with nanoscale disordered interfaces (Yang, Zhao et al., Science 24 Jul 2020: Vol. 369, Issue 6502, pp. 427-432)

I generally avoid thinking about cobalt, since its mining is such a tragedy, although it is possible to imagine synthetic cobalt in some distant future, not one any of us will live to see, but I'm inclined to make an exception here.

The alloy discussed here is a nickel based alloy - as pretty much all superalloys are - but with admixtures of the aforementioned cobalt, iron, aluminum, titanium and boron.

High strength corrosion resistant alloys are essential for the operation of high temperature turbines, which will be a key tool if we are to have even a slight chance of addressing climate change.

I am not sure of all the properties of this alloy, but its main feature is the use of disorder to generate strength, as implied in the subtitle, and perhaps less strictly tied to composition.

From the paper:

Alloys with atomically close-packed ordered structures constitute a class of structural materials that bridge the gap between ordinary metals and hard ceramics, with potentially novel mechanical properties (13). These long-range ordered superlattice alloys have strong chemical binding and associated low atomic mobility, which make them very attractive to high-temperature structural applications for achieving increased energy efficiencies in a wide range of engineering fields, such as aerospace, automotive, gas turbine engine, and many other industries (46). Since the 1950s, considerable efforts have been devoted to the research and development of bulk ordered alloys. Despite many advances that have been made, widespread application of these alloys remains elusive, largely limited by the irreconcilable conflict between strength and ductility at ambient temperatures. As the crystalline structure of alloys becomes highly ordered, catastrophic brittle failure takes place easily at ambient temperatures and demolishes the most desirable properties of the alloys. Consequently, most conventional ordered alloys with ultrahigh strengths (gigapascal levels) are found to be extremely brittle during tensile deformation, which severely limits their potential use in structural applications. For example, alloys with the topologically close-packed (TCP) structure (e.g., the Laves-phase ordered alloys) offer appreciable strengths (or hardnesses) at elevated temperatures, but their insufficient number of operating slip systems allows for negligible tensile elongations (5, 7). A respectable ductility exists in a few cubic ordered alloys (813), such as binary Ni3Al, FeAl, and NiAl aluminides. However, their yield strengths remain quite limited at ambient temperatures, and they are insufficiently strong for use in many engineering fields. Furthermore, a lack of sufficient thermal stability at elevated temperatures is another concern for their practical usage...


Another class of materials that bridge ceramics and metals and which are highly ordered, I feel behooved to mention as they fascinate me and I'm always mentioning them to my son, are the MAX phases, which are generally composed of early period transition metals, carbon or nitrogen, and silicon. But that's a different discussion...

Further on the authors continue...

...We departed from the traditional high-temperature alloy design strategy by focusing on nanoscale interfacial disordering. This presents a different direction for tuning the microstructures through synergistic modulations of structural and chemical features of bulk ordered alloys to target superb mechanical properties and exceptional thermal stability. By controllably incorporating multiple elements, we synthesized a Ni43.9Co22.4Fe8.8Al10.7Ti11.7B2.5 [in atomic percentage (at %)] alloy by using arc melting and thermomechanical processing (14). The superlattice materials (SMs) that we developed have nanoscale disordered interfaces (NDI-SMs) with a polycrystalline morphology (11.0 7.5 μm average grain size) with an unusual structural feature composed of the micrometer-scale ordered superlattice grain (OSG) capsulated with a disordered interfacial nanolayer (DINL). The L12-type ordered structure (a close-packed A3B-type ordered structure) of the grain interior was identified using bright-field transmission electron microscopy (TEM) (Fig. 1A)...


Figure 1:



The caption:

Fig. 1 Unusual nanoscale interfacially disordered structure of the superlattice materials.
(A) Bright-field TEM image showing the polycrystalline morphology. (Inset) A corresponding selected-area electron diffraction pattern collected from the grain interior, which shows the L12-type ordered structure. (B) Atomic-resolution HAADF-STEM image and corresponding EDX maps taken from the inner L12-type OSG, revealing the sublattice occupations. (C) High-resolution HAADF-STEM image revealing the ultrathin disordered layer at the grain boundaries with a nanoscale thickness. The images on the right show the corresponding fast Fourier transform (FFT) patterns. (D) EDX maps showing the compositional distribution of the DINL. (E) Schematic illustration highlighting the nanoscale interfacially disordered structure. FCC, face-centered cubic.


We used three-dimensional atom probe tomography (3D-APT) to provide a quantitative compositional analysis at the atomic scale. This technique is especially important to quantify the light element boron (Fig. 2A). The L12-type ordered superlattice of the grain interior is compositionally homogeneous without elemental clustering. We identified it as the (Ni,Co,Fe)3(Al,Ti,Fe)-type compositionally complex ordered superlattice with a small amount of boron occupying the interstitial positions (16). By contrast, we found that the Fe and Co are strongly enriched inside the DINL accompanied with codepletion of Ni, Al, and Ti elements, which is consistent with our TEM analyses. We were also able to clearly identify local boron enrichment within the DINL with 3D-APT, as EDX is not capable of accurately determining the boron locations...


DINL = disordered interfacial nanolayer.

...We used three-dimensional atom probe tomography (3D-APT) to provide a quantitative compositional analysis at the atomic scale. This technique is especially important to quantify the light element boron (Fig. 2A). The L12-type ordered superlattice of the grain interior is compositionally homogeneous without elemental clustering. We identified it as the (Ni,Co,Fe)3(Al,Ti,Fe)-type compositionally complex ordered superlattice with a small amount of boron occupying the interstitial positions (16). By contrast, we found that the Fe and Co are strongly enriched inside the DINL accompanied with codepletion of Ni, Al, and Ti elements, which is consistent with our TEM analyses. We were also able to clearly identify local boron enrichment within the DINL with 3D-APT, as EDX is not capable of accurately determining the boron locations. Two-dimensional compositional contour maps across the interface revealed a distinctive multielement cosegregation of Fe, Co, and B inside the DINL (Fig. 2B)...


Figure 2:



The caption:

Fig. 2 Three-dimensional compositional distributions and nanoscale interfacial cosegregation of the NDI-SMs.
(A) Atom maps reconstructed using 3D-APT that show the distribution of each element. Fe, Co, and B are enriched at the DINL, whereas Ni, Al, and Ti are depleted correspondingly. (B) Two-dimensional compositional contour maps revealing the multielement cosegregation behaviors of Fe, Co, and B elements within the DINL. (C) One-dimensional compositional profile that quantitatively reveals the elemental distributions across the OSG and DINL.


OSG = ordered superlattice grain

Figure 3:



The caption:

Fig. 3 Mechanical properties and thermal stability of the NDI-SMs.
(A) Tensile stress-strain curve of the NDI-SM tested at 20C in air. The stress-strain curve of high-strength Ni3Al-type (Ni3Al-2.5 at % B) alloy (9) is also plotted for a direct comparison. (Inset) Tensile fractography showing the ductile dimpled structures. (B) Yield strength (σy) versus uniform elongation (εu) of the present NDI-SM compared with various conventional bulk ordered alloys (810, 12, 13, 17, 18, 2226). (C) Variations of Vickers hardness (HV) of the NDI-SM at elevated temperatures compared with those of conventional ordered alloys (27, 28). (D) Grain size variations as a function of heating durations at a high temperature of 1050C. (Inset) A typical EBSD inverse pole figure (IPF) map showing the grain size of the sample annealed at 1050C for 120 hours. The NDI-SMs exhibit an exceptionally high resistance against the thermally driven softening and grain coarsening.


EBSD = electron backscatter diffraction.

This temperature should be sufficient for many applications. Most modern turbines are coated with thermal barrier coatings that allow them to operate at temperatures higher than their melting points.

I like to imagine and dream about systems that operate at temperatures about 1450C because of my fondness for thermochemical carbon dioxide spitting catalysts, but that's just me.

The conclusion:

The composite architecture of our superlattice alloy, especially the multielement cosegregation-induced interfacial disordering, can be utilized to design high-strength ultrafine-grained or nano-grained materials with enhanced grain-boundary stability and associated coarsening resistance. We anticipate that this approach should be applicable to many other metallic systems, particularly the compositionally complex ordered alloys. This may lead to families of high-temperature structural materials that might avoid some of the drawbacks of high-temperature alloys currently in use. These superlattice materials will be of great interest for a broad range of aerospace, automotive, nuclear power, chemical engineering, and other applications.


A very cool paper out of repressed and dying Hong Kong and increasingly imperial China.

I trust you are doing everything to keep safe and finding space to enjoy life as well.

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Reply Strength through disorder (Original post)
NNadir Jul 2020 OP
CatLady78 Jul 2020 #1

Response to NNadir (Original post)

Fri Jul 24, 2020, 03:57 AM

1. Graphene defect

Hi NNadir,
It's not precisely the same sort of thing but there was an article out about how a defect in graphene apparently increases its charge transfer rate:

https://www.engineering.com/ElectronicsDesign/ElectronicsDesignArticles/ArticleID/20429/Defective-Graphene-Could-Boost-Electrochemical-Sensor-Efficiency.aspx

Similar to the extent that a structural defect improves
its utility.

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