Complex Intermetallics Research Articles

On the Temperature and Composition Dependence of Non‐basal Stacking Faults in C14 Laves Phases

Activation of non‐basal slip is essential in improving the deformability of hexagonal crystals. However, for complex intermetallics such as Laves phase, the mechanism of non‐basal slip remains largely unknown. In this work, the prismatic slip systems of C14 Laves phase crystals and possible metastable states along the slip paths are assessed using atomistic simulations. Multiple prismatic stacking fault states with the same projected positions of atoms but different site occupancies and chemical distributions are identified at different inter‐atomic layers, temperatures and chemical compositions. The formation of energetically favorable prismatic stacking faults involves short‐range diffusion which implies the thermally activated nature of prismatic slip. The outcomes of this work advance the understanding of temperature and composition‐dependent non‐basal slip in Laves phases.

Architecture of new {Ca, Eu, Yb}4CuMg complex intermetallics based on polyicosahedral clusters

The crystal structures of R4CuMg (RCa, Eu, Yb) complex intermetallics, solved by single crystal X-ray diffraction, were found to represent a new trigonal prototype (hR144-h7c2ba, space group R3‾m), being the first examples of 4:1:1 rare earth-rich phases not crystallizing as cF96-Gd4RhIn, adopted by more than 200 compounds known so far. The divalent character of the R component, together with Cu atomic size could contribute to explain the observed structural peculiarity.Indeed, the occurrence of fragments typical of compositionally similar compounds, namely Cu-centered trigonal prisms and Mg-centered core-shell polyicosahedral clusters with R at vertices, prompted a search for significant structural relationships. In this work, a description of the rhombohedral R4CuMg crystal structure is proposed as a linear intergrowth along the c-direction of the two types of slabs R9.5CuMg3.5 (parent type: hP28-kh2ca, SG 194) and R13Cu6Mg (parent type: hR60-b6a2, SG 160). The ratio of these slabs in the studied structure is 2:1, corresponding to the simple equation 6×R9.5CuMg3.5+3×R13Cu6Mg = 24×R4CuMg, normalized to the unit cell content.According to the proposed description, symmetry properties of the slabs (depicted by the p3m1 layer group) were used for screening of possible space groups for other potential members of the same/similar intergrowth families.

First-principles based computational framework for the thermal conductivity of complex intermetallics: The case study of MgZn2 and Mg4Zn7

Complex intermetallics usually exist as second phases in metal alloys. How these second phases can affect the thermal conductivity of alloys is generally unknown because the intrinsic thermal transport properties of these complex intermetallic compounds are quite less explored. In this work, we propose a computational framework based on first-principles calculations to study the electron and phonon thermal transport in complex intermetallics. Two typical intermetallics, i.e., MgZn2 and Mg4Zn7, are studied as prototypes. The rigorous mode-level first-principles calculations are first carried out to study the thermal transport of MgZn2. The calculations not only provide accurate thermal conductivity results, but also allow to prove that the constant relaxation time approximation and the Slack model work quite well in complex intermetallics. Then these two models are combined with first-principles calculations to predict the thermal transport properties for Mg4Zn7. Our results show that the directional average thermal conductivities for MgZn2 and Mg4Zn7 are 53.9 and 21.9 W/mK, significantly smaller than those of their elemental counterparts. Electrons are found to be the main heat carriers in these compounds, leading to a nearly temperature-independent thermal conductivity. Phonon thermal conductivity is negligible due to large unit cells and weak metallic bondings. Our work provides reliable thermal conductivity values for MgZn2 and Mg4Zn7. The computational framework developed in this work can also be further extended to study the electrical and thermal transport of other complex intermetallics.

Uranium-mercury complex antiferromagnet: UHg6.4

Crystallographically complex compounds containing $4f$ and $5f$ electrons often have peculiar chemical and physical properties. In this work we present discovery and characterization of a new material ${\mathrm{UHg}}_{6.4}$, located at the border to complex intermetallics. This material crystallizes in the ${\mathrm{LaHg}}_{6.4}$ structure type, which can be represented by La/U-centered polyhedra with coordination numbers of 13 and 14. Much like the ${\mathrm{LaHg}}_{6.4}$ analog, ${\mathrm{UHg}}_{6.4}$ shows strong crystallographic disorder, in particular for Hg atoms in the channels along [001]. The ${\mathrm{UHg}}_{6.4}$ compound orders antiferromagnetically below ${T}_{\text{N1}}=35.5\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and displays another transition at ${T}_{\text{N2}}=47.3\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is likely also antiferromagnetic. Both transitions are only slightly affected by magnetic field. Given high air sensitivity of ${\mathrm{UHg}}_{6.4}$, an exceptional experimental environment for sample synthesis and characterization was necessary in order to comprehensively describe the chemical and physical properties of this system.

Architecture of rare earth-rich complex intermetallics based on polyicosahedral clusters

Architecture of rare earth-rich complex intermetallics based on polyicosahedral clusters

Atomistic mechanism of phase transformation between topologically close-packed complex intermetallics

Understanding how topologically close-packed phases (TCPs) transform between one another is one of the challenging puzzles in solid-state transformations. Here we use atomic-resolved tools to dissect the transition among TCPs, specifically the μ and P (or σ) phases in nickel-based superalloys. We discover that the P phase originates from intrinsic (110) faulted twin boundaries (FTB), which according to first-principles calculations is of extraordinarily low energy. The FTB sets up a pathway for the diffusional in-flux of the smaller 3d transition metal species, creating a Frank interstitial dislocation loop. The climb of this dislocation, with an unusual Burgers vector that displaces neighboring atoms into the lattice positions of the product phase, accomplishes the structural transformation. Our findings reveal an intrinsic link among these seemingly unrelated TCP configurations, explain the role of internal lattice defects in facilitating the phase transition, and offer useful insight for alloy design that involves different complex phases.

Refractory Metal Intermetallic Composites, High-Entropy Alloys, and Complex Concentrated Alloys: A Route to Selecting Substrate Alloys and Bond Coat Alloys for Environmental Coatings.

This paper considers metallic ultrahigh-temperature materials (UHTMs) and the alloying behaviour and properties of alloys and their phases by using maps of the parameters δ (based on atomic size), Δχ (based on electronegativity), and valence electron concentration (VEC), and discusses what connects and what differentiates material groups in the maps. The formation of high-entropy or complex concentrated intermetallics, namely 5-3 silicides, C14 Laves and A15 compounds, and bcc solid solutions and eutectics in metallic UHTMs and their co-existence with “conventional” phases is discussed. The practicality of maps for the design/selection of substrate alloys is deliberated upon. The need for environmental coatings for metallic UHTMs was considered and the design of bond coat alloys is discussed by using relevant maps.

From high temperature phase formation to transition metal substitution in the Fe/Al9Co2(001) system

We report the formation of several complex intermetallics as surface alloys upon the adsorption of Fe on Al9Co2(001) for different dosing conditions. Up to 4 monolayers equivalent (MLE) of Fe deposited on the substrate held at 593 K, the low energy electron diffraction pattern consists of a (2×2)R45° phase with two additional domain types rotated by ±8∘ from it. The scanning tunneling microscopy (STM) measurements show that these three types of domains have the same crystallographic structure. The lattice parameters and structural motifs point towards the formation of the high-temperature Al8Fe5 phase (γ-brass of Zn8Cu5-type structure). For Fe deposition between 593 K and 873 K, we have identified the formation of two phases, tentatively assigned to a ternary disordered Al9(Co,Fe)2 overlayer and to the monoclinic Al13Fe4(100). The stoichiometric evolution of the grown structures have been characterized by angle-resolved x-ray photoelectron spectroscopy measurements. Density functional theory based calculations have been performed to model the Al8Fe5(100)/Al9Co2(001) interface, its structural stability and to simulate the corresponding STM images.

On the complex intermetallics in an Al-Mn-Sc based alloy produced by laser powder bed fusion

The recently-developed Al-Mn-Sc based alloys specific for laser powder bed fusion (LPBF) have shown excellent mechanical performance. However, the complicated intermetallic particles in the microstructure remain to be identified, hindering the deep understanding of their effects on mechanical properties and further property improvement. In this work, a range of phases in a LPBF-built Al-Mn-Sc based alloy have been systematically studied by atom probe tomography. The results clarify characteristic intermetallic phases in two different grain-size regions in the microstructure. In the fine grain (FG) region, three distinct phases have been identified. A Sc-rich phase having an average composition of Al3.3(Sc0.8Zr0.2) is observed at both the grain boundary (GB) and grain interior, but they have different morphologies. The Mn-rich phase with an average composition of Al4.3(Mn0.9Fe0.1) is only observed along GBs. In addition, Mg-rich oxide has been observed either with a separate distribution or attached to Sc-rich particles. In the coarse grain (CG) region, the GB particles exhibit different size and composition from FG region. The Sc-rich GB particles contain Mg enrichment rather than Zr and the composition is determined to be Al3.4(Sc0.75Mg0.25). The Mn-rich GB particles in the CG region, with higher Fe contents, are Al4.3(Mn0.8Fe0.2) along the GBs and Al4.5(Mn0.85Fe0.15) inside the grains. Many smaller Mg-rich oxides and Sc-rich particles are also observed in the internal grains of the CG region.

Quasicrystals: A New Class of Structurally Complex Intermetallics

Quasicrystals: A New Class of Structurally Complex Intermetallics

Experimental studies and DFT calculations to predict atomic arrangements at twin boundaries and distribution behaviors of different solutes in complex intermetallics

HAADF-STEM observations illustrate that the probabilities of appearance of (111) microtwins with different twinning structures in μ phase were different. Based on this, 8 possible (111) twinning models with different twin boundary structures are established and discussed via DFT. Experimental characterization and DFT calculations reveal a close relation between the probabilities of appearance of these (111) microtwins and the interface energy at the twin boundaries: the smaller the energy is, the easier the twinning structure is to form and exist stably. TB5, with the smallest interface energy, is exactly the abundantly-existing twinning structure observed in HAADF. Moreover, via DFT simulation, distribution behaviors of the solute elements Cr, Mo, Re, Ni at the twin boundary of TB3 and the atomic arrangement at (111) twin boundary of C15–Cr2Nb crystal have been predicted and analyzed. The methods of DFT simulation and analysis on the twin boundaries provide a new strategy to study the twinning structures of complex-structured crystals and preferred distribution of different solutes at the twin boundary, etc.

Site preferences and ordering in Nb(Al1-xMx)2 (M = Ni or Cu) ternary Laves phases

The properties of binary Laves phases can often be improved by incorporation of additional elements into the crystal structure. In some cases, only a ternary Laves phase exists in an alloy system but with a substantial homogeneity range. Knowledge of the site preference in these compositionally complex intermetallics is necessary to tune the properties for improved performance. The site preferences and substitutional ordering behavior of two ternary C14 Laves phases Nb(Al1-xNix)2 and Nb(Al1-xCux)2 are studied with first-principles cluster expansion and Monte Carlo simulations. The ternary Laves phases show extended solubility ranges with constant Nb concentration from experiments, but do not have binary Laves analogues AB2 in their respective systems. At low alloying concentrations the alloying elements Ni and Cu have preferences for the 6h B-sites in the respective Nb(Al1-xNix)2 and Nb(Al1-xCux)2 Laves phases. At higher alloying concentrations (x>0.25), Cu atoms are distributed with a slight preference for the 6h B-sites, but the Ni atoms show higher preferences for the 2a B-sites. Pair correlation analysis from the Monte Carlo simulations show that both Laves phases prefer heteroatomic 1st and 2nd nearest neighbor pairs. Drop-synthesis calorimetry experiments and first-principles calculations of formation enthalpies as a function of composition are found to be in good agreement. The Monte Carlo simulation also show that the site preferences of the alloying element depend on the preference of homoatomic bond pair-types on the Kagome layers formed by 6h atoms.

Application of Ce for scavenging Cu impurities in A356 Al alloys

Secondary (recycled) Al is increasingly used in castings due to its lower cost and carbon footprint relative to primary Al. However, the presence of certain impurity elements in secondary Al degrades its properties, often requiring dilution or downgrading. Scavenging impurities with reactive alloying additions can limit impurity impact and broaden the use of secondary Al. This report will detail the initial results of using Ce to microstructurally tie up Cu impurities present in a model secondary Al alloy (A356) with high Cu impurity. The results show that targeted Ce additions to A356 can be used to scavenge Cu impurities by forming complex intermetallics and inhibit intergranular corrosion susceptibility associated with Cu impurities.Supplemental data for this article is available online

Machine learning formation enthalpies of intermetallics

Developing fast and accurate methods to discover intermetallic compounds is relevant for alloy design. While density-functional-theory (DFT)-based methods have accelerated design of binary and ternary alloys by providing rapid access to the energy and properties of the stable intermetallics, they are not amenable for rapidly screening the vast combinatorial space of multi-principal element alloys (MPEAs). Here, a machine-learning model is presented for predicting the formation enthalpy of binary intermetallics and is used to identify new ones. The model uses easily accessible elemental properties as descriptors and has a mean absolute error of 0.025 eV/atom in predicting the formation enthalpy of stable binary intermetallics reported in the Materials Project database. The model further predicts stable intermetallics to form in 112 binary alloy systems that do not have any stable intermetallics reported in the Materials Project database. DFT calculations confirm one such stable intermetallic identified by the model, NbV2, to be on the convex hull. Furthermore, an adaptive transfer learning method is used to generalize the model to predict ternary intermetallics with a similar accuracy as DFT, which suggests that it could be extended to identify compositionally complex intermetallics that may form in MPEAs.

Growth interactions between icosahedral quasicrystals

We investigate the solidification pathways of a population of icosahedral quasicrystals in a liquid through in situ synchrotron x-ray tomography. The wealth of three-dimensional space- and time-resolved data enables us to test the predictions of various models and theories of crystallization on a quasi-crystal-forming alloy. Remarkably, we find the general evolution equation---that accounts for the competing effects of growth, coalescence, and coarsening---fits well our experimental data on surface area concentration. Furthermore, we quantify the orientation selection, screening length, and coarsening rate of the dodecahedra, and compare these results to that of periodic crystals. The latter is a full order of magnitude smaller than that of elemental metal dendrites in the limit of zero volume fraction, a reflection of the low solid-liquid interfacial energy of the icosahedral phase. Our paper provides the critical input data for microstructural models used for integrated computational materials engineering of complex intermetallics, including quasicrystals.

Catalytic properties of Al13TM4 complex intermetallics: influence of the transition metal and the surface orientation on butadiene hydrogenation

ABSTRACT Complex intermetallic compounds such as transition metal (TM) aluminides are promising alternatives to expensive Pd-based catalysts, in particular for the semi-hydrogenation of alkynes or alkadienes. Here, we compare the gas-phase butadiene hydrogenation performances of o-Al13Co4(100), m-Al13Fe4(010) and m-Al13Ru4(010) surfaces, whose bulk terminated structural models exhibit similar cluster-like arrangements. Moreover, the effect of the surface orientation is assessed through a comparison between o-Al13Co4(100) and o-Al13Co4(010). As a result, the following room-temperature activity order is determined: Al13Co4(100) < Al13Co4(010) < Al13Ru4(010) < Al13Fe4(010). Moreover, Al13Co4(010) is found to be the most active surface at 110°C, and even more selective to butene (100%) than previously investigated Al13Fe4(010). DFT calculations show that the activity and selectivity results can be rationalized through the determination of butadiene and butene adsorption energies; in contrast, hydrogen adsorption energies do not scale with the catalytic activities. Moreover, the calculation of projected densities of states provides an insight into the Al13TM4 surface electronic structure. Isolating the TM active centers within the Al matrix induces a narrowing of the TM d-band, which leads to the high catalytic performances of Al13TM4 compounds.

Nanocluster model and its application for crystal structure prediction of complex intermetallics

Nanocluster model and its application for crystal structure prediction of complex intermetallics

Microstructural Studies of Cobalt Based Microwave Clad Developed on Martensitic Stainless Steel (AISI-420)

AISI-420 stainless steel is widely used to manufacture hydraulic and gas turbine components. The present work is concerned with microstructural studies of cobalt based clad developed through microwave energy in a microwave applicator. A domestic microwave oven was successfully used to develop clads at 2.45 GHz frequency. The developed clads were evaluated using optical metallography, scanning electron microscope, energy dispersive X-ray spectroscopy and X-ray diffraction (XRD) to determine the microstructure, phases and elemental composition of developed surface. The microstructure study reveals that the developed clad shows good metallurgical bond between substrate without any interfacial cracks. XRD study confirms the presence of various complex metal carbides and intermetallics like Cr7C3, Cr3C2, M23C6, Cr2Ni3 which are formed during microwave heating. The average microhardness of the developed clad surface is 807 ± 96 HV, which is 132% higher than that of unclad surface (348 ± 7 HV).

Special Issue “Complex intermetallics – structures and properties”

Special Issue “Complex intermetallics – structures and properties”

Microcompression of brittle and anisotropic crystals: recent advances and current challenges in studying plasticity in hard materials

Recent years have seen an increased application of small-scale uniaxial testing—microcompression—to the study of plasticity in macroscopically brittle materials. By suppressing fast fracture, new insights into deformation mechanisms of more complex crystals have become available, which had previously been out of reach of experiments. Structurally complex intermetallics, metallic compounds, or oxides are commonly brittle, but in some cases extraordinary, though currently mostly unpredictable, mechanical properties are found. This paper aims to give a survey of current advances, outstanding challenges, and practical considerations in testing such hard, brittle, and anisotropic crystals.