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AB 2 Sb 2 -type Zintl phases, especially Mg 3 Sb 2 , have garnered widespread attention in virtue of their nontoxic constituent elements and outstanding thermoelectric performance in n-type materials. However, the p-type counterpart of AB 2 Sb 2 still struggles with a low figure of merit zT , primarily because of the limited valence band degeneracy, large band effective mass, and relatively high thermal conductivity. Here we simultaneously optimize the electrical and thermal transports of p-type AB 2 Sb 2 -based materials through shaping the band edge and distorting the crystal lattice. By alloying Zn at Mg2 site and Ca at Mg1 site, we successfully decorate the crystal lattice of Mg 3 Sb 2 in real space and thereby modify the band structure in reciprocal space. Zn alloying promotes the band degeneracy while Ca alloying reduce the single band effective mass, which largely promotes the transport of charge carriers. Moreover, the distorted crystal lattice effectively blocks the heat-carry phonons to reduce the lattice thermal conductivity κ L. A high zT of 0.81 is finally realized at 750 K in our Zn/Ca co-alloyed samples, which is three times larger than that of Mg 3 Sb 2 matrix. Our work opens new possibilities for the synthetic optimization of both electrical and thermal transports in Zintl phase and other thermoelectric systems. We achieved a high power factor of 8.3 μW cm-1 K-2 and an ultralow κ L of 0.52 W m-1 K-1 in Mg 3–3 x Zn 2 x Ca x Sb 2 solid solutions through band shaping and lattice distortion engneering. [Display omitted] [ABSTRACT FROM AUTHOR]

Local chemical order (LCO) is energetically favorable in multi-principal element alloys (MPEAs) due to the chemical affinity difference among constituent species. Here we show that the increase of LCO is often accompanied by a decrease of lattice distortion in body-centered-cubic (BCC) MPEAs. This leads to a trade-off between LCO and lattice distortion in their effects on retarding the glide of edge and screw dislocations. With a random solid solution without LCO as a reference, the emergence of LCO at the expense of lattice distortion could weaken the local trapping force for dislocation glide and promote dislocation mobility. However, when the LCO becomes sufficiently large and robust, it acts to hinder dislocation glide. We found that the mobility of edge dislocation is more affected by local lattice distortion, while the kink-pair based screw dislocation motion is more governed by LCO. We further quantitatively evaluate the contribution of LCO and lattice distortion to the overall critical shear stress for dislocation glide using analytical models. For the present equiatomic NbTiZr MPEAs, the critical shear stress is dominated by lattice distortion when the LCO is still limited to chemical short-range order (CSRO). However, LCO could even surpass lattice distortion to become a major contributor to the critical shear stress once it grows to a sufficiently large size. Our work indicates that increasing the degree and extent of LCO may provide an effective means to regulate dislocation motion, shedding light on how to use LCOs to improve the strength of BCC MPEAs. [Display omitted] [ABSTRACT FROM AUTHOR]

As the numbers of medium- to high-entropy alloys being studied and impressive structural properties they exhibit increase rapidly, questions regarding the role played by their complex chemical fluctuations rise concomitantly. Here, using a combination of large-scale molecular dynamics (MD), a hybrid MD and Monte-Carlo simulation method, and crystal defect analysis, we investigate the role lattice distortion (LD) and chemical short-range order (CSRO) play in the nucleation and evolution of dislocations and nanotwins with straining in single crystal and nanocrystalline CoCrNi, a medium entropy alloy (MEA). LD and CSRO effects are elucidated by comparisons with responses from a hypothetical pure A -atom alloy, which bears the same bulk properties of the nominal MEA but no LD and no CSRO. The analysis reveals that yield strengths are determined by the strain to nucleate Shockley partial dislocations, and LD lowers this strain, while higher degrees of CSRO increase it. We show that while these partials prefer to nucleate in the CoCr clusters, regardless of their size, they find it increasingly difficult to propagate away from these sites as the level of CSRO increases. After yield, nanotwin nucleation occurs via reactions of mobile Shockley partials and is promoted in MEAs, due to the enhanced glide resistance resulting from LD and CSRO. [ABSTRACT FROM AUTHOR]

Long periodic stacking ordered phases (LPSOs), consisting of various configurations of stacking faults, play an important role in developing ultrastrong Mg alloys with moderate ductility. However, their formation mechanisms are far from clear as no apparent defects are introduced during their formation as it is commonly believed that stacking faults are induced by defects. Here, we present the atomic and electronic basis for lattice-distortion-mediated formation of stacking faults, i.e., localized face-centred-cubic (FCC) structures, within a Mg-Zn-Y alloy with a hexagonal close-packed (HCP) structure. The atomic motion trajectories from ab-initio molecular dynamic simulations show that the Mg atoms occupying the nearest neighbour positions of Zn and Y solute atoms undergo a local HCP-to-FCC transition. It is revealed that a local lattice distortion caused by the solute atoms enables the Mg atoms to move and rearrange into a local FCC configuration, which is validated by high resolution scanning transmission microscopy and in-situ synchrotron X-ray diffraction. Our simulations provide profound insight into the formation mechanism of stacking faults in HCP Mg and their physical nature of phase transformations. This is not only critically important because conventional defects, such as dislocations and vacancies, are important to deformation for Mg and its alloys, but also because they serve as a potential new approach to the design of advanced Mg alloys when defects could be used to facilitate. Image 1 [ABSTRACT FROM AUTHOR]

Abstract The maximization of the mixing entropy with the optimal range of enthalpy in high-entropy alloys (HEAs) can promote the formation of a stable single solid-solution phase with the absence of competing intermetallic compounds. The resultant effects, such as lattice distortion, can contribute to excellent mechanical properties, which has motivated numerous efforts to develop and design single-phase HEAs. However, challenges still remain, particularly on quantifying the lattice distortion and relating it to materials properties. In this study, we have developed a NbTaTiV refractory HEA with a single body-centered-cubic (BCC) structure using an integrated experimental and theoretical approach. The theoretical efforts include thermodynamic modeling, i.e., CALculation of PHAse Diagram (CALPHAD). The microstructural evolutions have been investigated by systematic heat-treatment processes. The typical dendrite microstructure was observed, which is caused by the elemental segregation during the solidification in the as-cast condition. The structural inhomogeneity and chemical segregation were completely eliminated by the proper homogenization treatment at 1200 °C for 3 days. The homogeneous elemental distribution was quantitatively verified by the Atom Probe Tomography (APT) technique. Importantly, results indicate that this HEA exhibits the high yield strength and ductility at both room and high temperatures (up to 900 °C). Furthermore, the effects of the high mixing entropy on the mechanical properties are discussed and quantified in terms of lattice distortions and interatomic interactions of the NbTaTiV HEA via first-principles calculations. It is found that the local severe lattice distortions are induced, due to the atomic interactions and atomic-size mismatch in the homogenization-treated NbTaTiV refractory HEA. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Local off-centering of atoms generally leads to high Grüneisen parameters and thus low lattice thermal conductivity, which is considered to be favorable for thermoelectric materials. Nevertheless, such severely distorted local structures may cause strong scattering for electrons and poor electrical performance. Here we demonstrate the relationship between thermoelectric properties and local structure in AgBiSe 2 dual alloying with PbTe. It is disclosed that the introduction of Te anions in cubic (AgBiSe 2) 1 − x (PbTe) x can release the distortion index of Bi/Ag/Pb cations in a octahedral shell, which substantially improves the local symmetry and meanwhile the weighted mobility in relevant alloys. More interestingly, it is elucidated that dually-alloyed AgBiSe 2 displays a metavalent bonding character, first time reported in n-type cubic TE materials, as collectively verified by atomic coordination number, anomalously increased optical dielectric constant, moderate electrical conductivity, and large Grüneisen parameter. Consequently, a record-high average power factor of 0.52 mW m−1 K−2 between the measurement temperature interval, and a peak zT value of 0.92 at 800 K is obtained for metavalently bonded (AgBiSe 2) 0.75 (PbTe) 0.25. By manipulating the local distortion extent, a record-high average power factor of 0.52 mW m−1 K−2 is attained in metavalently bonded (AgBiSe 2) 0.75 (PbTe) 0.25 between 300 and 800 K. [Display omitted] [ABSTRACT FROM AUTHOR]

Lattice distortions, i.e. small atomic displacements away from the average lattice, are linked to a number of functional and mechanical properties of concentrated metallic alloys, particularly their yield strength. Here we develop an elastic model of lattice distortions where every atom is modeled as an Eshelby inclusion in a homogeneous elastic matrix. Local environment effects are included by considering fluctuating anisotropic eigenstrain tensors associated to the inclusions. The model is tested on several concentrated alloys, face-centered cubic (FCC) AlMg and FeNiCr alloys, as well as a fictitious body-centered cubic (BCC) binary alloy to study systematically and independently the effects of a size and an elastic modulus mismatch between the constituents. The elastic model predicts lattice distortions well when the size and elastic modulus mismatches are typically less than 5% and 25%, respectively. Interestingly, we find that when the size mismatch is small, as in FeNiCr alloys, the lattice distortion is dominated by the fluctuations of the dilatational eigenstrains rather than their average value as often assumed in elastic models of concentrated alloys. Moreover, models usually assume homogeneous elastic constants, while the limit obtained here of 25% is often exceeded in concentrated alloys, particularly with a BCC structure. [Display omitted] [ABSTRACT FROM AUTHOR]

High entropy alloys (HEAs) have triggered significant scientific interest due to their unusual structural stability combined with excellent mechanical and other functional properties. Recently, the high-entropy concept has also been applied to the field of protective coatings. We use ab initio calculations to provide an unbiased insight into their behavior at the atomic level. Namely, we focus on the interplay between high entropy (chemical complexity), sluggish diffusion, and (local) lattice distortions in the example of high configurational entropy sublattice nitride (HESN) ceramics. We show that the high entropy itself is not a guarantee for the sluggishness of diffusion (as quantified by migration barriers). Next, we propose a relation between the local distortions (measured by the width of lattice bond length distributions) and activation energies. The presented results provide examples that the core effects, characterizing the exceptional properties of HEAs, are system-specific and hence highlight the need for analyzing each system of interest individually. [Display omitted] [ABSTRACT FROM AUTHOR]

The crystallographic and electronic structures of (010) APB of L1 2 Co 3 Al 0.75 TM 0.25 are studied by high-resolution transmission electron microscopy and first-principles calculations. Effects of solute atoms (TM = Cr, Hf, Mo, Ni, Re, Ru, Ta, Ti, W and Y) on the formation energy, lattice parameters/distortion, magnetism, and bonding strength of the (010) APB in Co 3 Al 0.75 TM 0.25 are obtained from first-principles calculations. Comparing to the equilibrium volume of Co 3 Al, it is found that the volume change of the Co 3 Al 0.75 TM 0.25 with and without the presence of APB increases linearly with the volume of the corresponding FCC elements, indicating the contribution of the solute atoms on lattice distortion of bulk and (010) APB. Particularly, the strong dependence of the APB energy on the composition is comprehensively discussed together with the available experimental and theoretical data in the literature. The negative (010) APB energy indicates that the formation of (010) APB could stabilize the ordered L1 2 (or the FCC-lattice) Co 3 Al, and the local L1 2 → D0 22 phase transformation can occur. The physical natures of lattice distortions caused by the fault layers of APB and the solute atoms are characterized by bonding charge density. It is found that the solute atoms, occupying Al site of L1 2 phase and its (010) APB, increase the local bonding strength along (010) through the electron redistribution during forming the chemical bonds with Co, revealing an intrinsic solid-solution strengthening mechanism. This work provides an insight into the atomic and electronic basis for solid-solution strengthening mechanism of L1 2 Co 3 Al 0.75 TM 0.25 . [ABSTRACT FROM AUTHOR]

The effect of aluminum addition to the Cantor alloy in the composition range of 0.25–5 atomic percent towards solid solution strengthening of supposedly single-phase HEA was investigated using experiments and first principle simulation-guided constitutive modeling. The continuous increase in yield and tensile strength without significant change in ductility is observed for the alloys with increasing aluminum content. The constitutive modeling of the strengthening has been performed using traditional as well as recently developed models for solid solution strengthening. It indicated a significant contribution (50% increases from Cantor alloy to Cantor alloy containing 5 atom % Al) of solid solution strengthening due to the addition of Al having a relatively larger size (̴12 %) than the size of elements in the Cantor alloy, causing severe local lattice distortion. The experimental yield strength could be best explained based on the large apparent distortion volume of the Al atom acting as a stronger barrier to dislocation motion based on the Varvenne model by incorporating the lattice distortion. First-principles simulations indicate that local and global lattice distortion contributes to an increase in the strength by strong pinning of dislocations by aluminum atom leading to high strength. [Display omitted] [ABSTRACT FROM AUTHOR]

Tracer diffusion of the substitutional components in interstitial (CoCrFeNiMn) 1- x C x high-entropy alloys with nominally x = 0.002, 0.005 and 0.008 (in at. fractions) is measured at elevated temperatures from 1173 to 1373 K. Two different characteristic effects of interstitial carbon addition on substitutional diffusion in these FCC alloys are distinguished. At the highest temperature of 1373 K, alloying by C with relatively low concentrations (x = 0.002) retards diffusion of the substitutional elements with respect to those in the C-free alloy. At lower temperatures and/or higher C concentrations (x ≥ 0.005), an enhancement of the diffusion rates of all substitutional elements is seen. A model is suggested that relates the self-diffusivities in the CoCrFeMnNi–C alloys with the lattice distortion imposed by interstitially dissolved carbon. The experimental results are interpreted in terms of a decrease of the migration barriers for vacancy-mediated diffusion due to the presence of interstitial C atoms. [Display omitted] [ABSTRACT FROM AUTHOR]

The continuous matrices of atomic displacements and lattice distortion from face-centered-cubic (fcc) to body-centered-cubic (bcc) phases compatible with the hard-sphere geometry of iron atoms are calculated for different possible final orientation relationships (ORs), such as Bain, Pitsch and Kurdjumov–Sachs (KS). The angular distortion introduced in the calculations appears as a natural order parameter of the fcc–bcc transitions. The average atomic displacement is lower with Pitsch and KS ORs than for Bain; one of these two distortions probably constitutes the natural mechanism for stress-free isolated small crystals. For matrix-embedded transformations that occur in bulk steels, the distortion associated with KS OR seems very appropriate: it has special mathematical properties and can explain the {2 2 5} habit planes with a simple O-lattice criterion. From these calculations, the fcc–bcc martensitic transformation appears to be “angular distortive”, and not of “shear” type as usually assumed. [ABSTRACT FROM AUTHOR]

Nano-scale phase separation is reported in a nominal single-phase, high-entropy alloy (HEA), which was characterized using scanning transmission electron microscopy (STEM) combined with atom probe tomography (APT). Despite the fact that X-ray diffraction exhibits a single face-centered-cubic (fcc) phase feature of the as-cast alloy prepared by melt spinning, selected area electron diffraction reveals weak L 1 2 ordering in the as-spun alloy. High-resolution STEM shows the presence of two coherent nanophases with distinct L 1 2 and fcc structures, coupling with compositional segregations. The ordering of the L 1 2 domains is enhanced after annealing at 500 °C. Electron energy loss spectroscopy and APT analyses reveal that the L 1 2 nano-phase is enriched with Fe, Co, Cr and Ni, while the fcc domains are a Cu-rich phase. The nano-scale phase separation can effectively minimize the lattice distortions caused by the atomic size difference in the constituent elements, which may offer structural insights into the unusual mechanical behavior and phase stability of fcc HEA. [ABSTRACT FROM AUTHOR]

In this work, the corrosion behavior of Mg-SiO 2 nanocomposites, fabricated by powder metallurgy coupled with hybrid microwave sintering and subsequent hot extrusion, was investigated using electrochemical, hydrogen evolution and weight loss measurements. Localized intergranular corrosion behavior was analyzed based on microstructural characteristics, which were characterized by scanning electron microscopy, energy-dispersive x-ray spectroscopy, and high-resolution transmission electron microscopy (HRTEM). In particular, copious numbers of Mg/MgO misfit dislocations were identified at the interface between the matrix Mg and SiO 2 nanoparticles; indeed, first principles calculations revealed that the MgO/Mg interfaces were relatively weakly bonded as compared to the close-packed interfacial Mg(0001)//MgO(111) structure. Using geometrical phase analysis of the HRTEM images to determine the lattice strains in the Mg/MgO interfacial area, we conclude that the highly strained and weakly bonded Mg/MgO interfaces act to promote significant localized dissolution and thus play a dominant role in intensifying intergranular corrosion in these nanocomposites. Graphical Abstract [Display omitted]. [ABSTRACT FROM AUTHOR]

The lattice distortion of a solute primarily occurs because its atomic size and chemical bonding are different from those of neighboring atoms. The lattice distortion effects in conventional and high-entropy alloys are different; however, a detailed investigation on these effects has yet to be conducted. To fill this research gap, this study produced face-centered cubic-structured dilute solutions (Ni, Ni–2 at.% W, and Ni–4 at.% W) and concentrated solutions (equiatomic CrFeNi and CoCrFeMnNi) and compared their tensile properties. For the two W-containing alloys, lattice distortion occurred only around the large and strong W atoms. However, for the two concentrated solutions, which had a similar interelement atomic size and shear modulus to the aforementioned alloys, lattice distortion occurred at all lattice sites. These two types of lattice distortion had significantly different effects on tensile properties. The strength and ductility of the alloys with a high concentration of distorted lattice points were higher than those of the alloys with a low concentration of distorted lattice points, although the alloys with a low concentration of distorted lattice points had a larger nominal atomic size difference and shear modulus difference. The mechanisms underlying the evolution of different mechanical properties under different types of lattice distortion were examined for the dilute and concentrated alloys. Moreover, the universal solid solution strengthening mechanism was observed. [Display omitted]. [ABSTRACT FROM AUTHOR]

[Display omitted] In this work, molecular dynamics simulations are conducted to investigate the shock responses and corresponding deformation mechanisms in single crystalline (SC) and nanocrystalline (NC) microstructure of the medium entropy alloy (MEA) CoCrNi. The effects of lattice distortion (LD) and chemical short-range order (CSRO) on the shock wave propagation, defect evolution, and the cavitation process are explored to distinguish the unique shock properties of MEA. The results reveal an anomalous anisotropy in the Hugoniot elastic limit different from that seen in pure FCC metals since LD reduces the barrier for Shockley partial (SP) formation but increases the resistance for SP propagation. With sufficient dislocations nucleated in the first shock compression stage, LD aids in the formation of nanotwins by slowing down dislocation propagation in the following release and tension stages. However, because a higher degree of CSRO increases the average intrinsic stacking fault energy above that of the random material, more stacking faults annihilate in the release stage, reducing the chances for nanotwinning. We show that voids prefer to nucleate at Ni segregation sites (with high CSRO) due to the large hydrostatic tensile strain created by the lattice mismatch between the neighboring Ni and CoCr regions, and moreover, the nucleation event favors the grain boundary during spallation in NCs. [ABSTRACT FROM AUTHOR]

Chemical short-range ordering (CSRO) or demixing in solid solution high entropy alloys (HEAs) is a fundamental issue yet to be fully understood. In this work, we first developed a generalized quasi-chemical solid solution model that enables quantitative computation of the local chemical ordering or demixing in solid solution HEAs. After that, we performed synchrotron diffraction experiments, extensive Reverse Monte Carlo (RMC) simulations, and first principles calculations on the CoCrFeNi model alloy to study the development of local chemical environments after long time thermal annealing. The outcome of the combined research demonstrates that the development of local chemical ordering or demixing in CoCrFeNi is not only affected by the heat of mixing between dislike atoms but also coupled with local lattice distortion. [Display omitted] [ABSTRACT FROM AUTHOR]