Chemical Order Research Articles

Phase dissolution strengthens and ductilizes a high-entropy alloy

Annealing is widely used in the thermomechanical processing of metallic alloys. Usually, ductility enhances while strength decreases because defects annihilate with the increase in annealing temperature. Here, we report a simultaneous improvement in strength and ductility of a TiZrNbTa high-entropy alloy by promoting phase dissolution with increasing annealing temperature. When annealing is performed from 800 to 1250°C, the tensile yield strength increases by 40% to 1003 ± 16 MPa, and the elongation is nearly doubled to 16.79% ± 1.03%. The annealing-induced phase dissolution promotes lattice distortion, contributing to enhanced lattice friction stress and yield strength. The correspondingly decreased interface mismatch alleviates stress concentration, and the stimulated formation of local chemical orders encourages coplanar slip and dislocation multiplication. Both occurrences boost ductility. These results not only expand our understanding of the phase dissolution and annealing effect in metallic materials, but also provide an insight into designing strong and ductile alloys.

Disentangling structural and kinetic components of the α-relaxation in supercooled metallic liquids

The particle motion associated to the α-relaxation in supercooled liquids is still challenging scientists due to its difficulty to be probed experimentally. By combining synchrotron techniques, we report the existence of microscopic structure-dynamics relationships in Pt42.5Cu27Ni9.5P21 and Pd42.5Cu27Ni9.5P21 liquids which allows us to disentangle structural and kinetic contributions to the α-process. While the two alloys show similar kinetic fragilities, their structural fragilities differ and correlate with the temperature dependence of the stretching parameter describing the decay of the density fluctuations. This implies that the evolution of dynamical heterogeneities in supercooled alloys is determined by the rigidity of the melt structure. We find also that the atomic motion not only reflects the topological order but also the chemical short-range order, which can lead to a surprising slowdown of the α-process at the mesoscopic length scale. These results will contribute to the comprehension of the glass transition, which is still missing.

Atomic number and isotopy before nuclear structure: multiple standards and evolving collaboration of chemistry and physics

We provide a detailed history of the concepts of atomic number and isotopy before the discovery of protons and neutrons that draws attention to the role of evolving interplays of multiple aims and criteria in chemical and physical research. Focusing on research by Frederick Soddy and Ernest Rutherford, we show that, in the context of differentiating disciplinary projects, the adoption of a complex and shifting concept of elemental identity and the ordering role of the periodic table led to a relatively coherent notion of atomic number. Subsequent attention to valency, still neglected in the secondary literature, and to nuclear charge led to a decoupling of the concepts of elemental identity and weight and allowed for a coherent concept of isotopy. This concept received motivation from empirical investigations on the decomposition series of radioelements and their unstable chemical identity. A new model of chemical order was the result of an ongoing collaboration between chemical and physical research projects with evolving aims and standards. After key concepts were considered resolved and their territories were clarified, chemistry and physics resumed autonomous projects, yet remained bound by newly accepted explanatory relations. It is an episode of scientific collaboration and partial integration without simple, wholesale gestalt switches or chemical revolutions.

Effect of Gd addition on the structural and magnetic properties of L1-FePt alloy thin films

The impact of Gd addition on the structural and magnetic properties of L10-FePt alloy thin films, which were sputter-deposited on MgO(001) substrates at 800°C, was investigated. A rapid deterioration of L10 chemical ordering along with a strong amorphization effect is observed with addition of Gd. At more than 20 at. % Gd, additional crystalline phases occur. Due to the diminishing L10 chemical order, the perpendicular magnetic anisotropy (PMA) gets strongly reduced from about 3 down to 1 MJ/m3 with addition of 14 at. % Gd. For higher Gd concentrations, the easy axis of magnetization turns fully in-plane due to dominating magnetic shape anisotropy. Furthermore, the saturation magnetization gets reduced with Gd addition due to the antiferromagnetic coupling between the Fe and Gd magnetic moments. Also, the Curie temperature can be lowered. Interestingly, with addition of Gd, a change in film morphology takes place, changing from an isolated island structure to a more continuous film morphology, which is of particular interest for samples with low Gd concentration as these films exhibit still strong PMA but rather low coercive fields.

Structure and solidification of the (Fe0.75B0.15Si0.1)100-xTax (x=0–2) melts: Experiment and machine learning

Fe–B–Si system is a matrix for synthesis of new functional materials with exceptional magnetic and mechanical properties. Progress in this area is associated with the search for optimal doping conditions. This theoretical and experimental study is aimed to address the influence of Ta alloying on the structure of undercooled (Fe0.75B0.15Si0.1)100-xTax (x = 0–2) melts, their undercoolability and the processes of structure formation during solidification. Small concentration of Ta complicates standard ab initio and machine learning investigations. We developed a technique for fast and stable training of machine learning interatomic potential (MLIP) in this case and uncovered the structure of undercooled (Fe0.75B0.15Si0.1)100-xTax (х = 0–2) melts. Molecular dynamic simulations with MLIP showed that at Ta concentration of 1 at.% there is a sharp change in the chemical short-range ordering in the melt associated with a change in the interaction of Ta atoms. This effect leads to a restructuring of the cluster formation in the system. At the same time, our experimental investigation shows that melts with a Ta content of 1 at.% have the greatest tendency to undercoolability. Alloying with Ta promotes the formation of primary crystals of Fe2B, and at a concentration of more than 1.5 at.% Ta, also of FeTaB. Herewith, near 1 at.% Ta, the crystallization of the melt proceeds nontrivially: with the formation of two intermediate metastable phases Fe3B and Fe2Ta Laves phase. Also, the highest tendency to amorphization under conditions of quick quenching is exhibited by a melt with a Ta concentration of 1 at.%. The results not only provide understanding of optimal alloying of Fe–B–Si materials but also promote a machine learning method for numerical design of metallic alloys with a small dopant concentration.

Microstructure and texture evolution in (VCoNi)99.9C0.1 medium entropy alloy during deformed and annealing treatments

The microstructure and texture evolution of (VCoNi)99.9C0.1 medium entropy alloy under homogenized, rolled and annealed conditions are systematically investigated. The equiaxed coarse grains of homogenized samples are elongated along rolling deformation direction, and then transformed to equiaxed fine grains after annealing. The distribution of coarse particles develops along grain boundary for as-homogenized and as-rolled samples; while disappears after annealing. Random texture of homogenized sample is changed to Bs-type texture after rolling, which is ascribed to local chemical ordering induced by planar slip and shear band. Bs-type texture is retained in as-annealed sample due to the absence of preferential orientation nucleation and sluggish diffusion.

Local cation order and ferrimagnetism in compositionally complex spinel ferrites

We present an exploration of a family of compositionally complex cubic spinel ferrites featuring combinations of Mg, Fe, Co, Ni, Cu, Mn, and Zn cations, systematically investigating the average and local atomic structures, chemical short-range order, magnetic spin configurations, and magnetic properties. All compositions result in ferrimagnetic average structures with extremely similar local bonding environments; however, the samples display varying degrees of cation inversion and, therefore, differing apparent bulk magnetization. Additionally, first-order reversal curve analysis of the magnetic reversal behavior indicates varying degrees of magnetic ordering and interactions, including potentially local frustration. Finally, reverse Monte Carlo modeling of the spin orientation demonstrates a relationship between the degree of cation inversion and the spin collinearity. Collectively, these observations correlate with differences in synthesis procedures. This work provides a framework for understanding magnetic behavior reported for “high-entropy spinels,” revealing many are likely compositionally complex oxides with differing degrees of chemical short-range order—not meeting the community established criteria for high or medium entropy compounds. Moreover, this work highlights the importance of reporting complete sample processing histories and investigating local to long-range atomic arrangements when evaluating potential entropic mixing effects and assumed property correlations in high entropy materials.

Laser powder bed fusion of (14 M) Ni-Mn-Ga magnetic shape memory alloy lattices

Recent developments in the additive manufacturing of magnetic shape memory (MSM) alloys have demonstrated the high potential of laser powder bed fusion (L-PBF) process for the manufacture of functional polycrystalline Ni-Mn-Ga-based actuating devices with complex geometries. This research utilises a systematic experimental approach to develop and optimise an L-PBF process for manufacturing Ni-Mn-Ga lattices. Experiments were conducted in two distinctive stages: firstly, to characterise the selective Mn evaporation in bulk samples built; secondly, to investigate the influence of the applied process parameters on the relative density and geometrical integrity of the lattice struts. The lattices manufactured using optimised parameters had a high internal density of ~99% and were heat-treated for chemical homogenisation, grain growth, and atomic ordering. The heat-treated lattices exhibited a seven-layered modulated (14 M) martensite structure at ambient temperature with the phase transformation temperatures and magnetic properties corresponding to the chemical composition. Primarily, the results demonstrate that the beneficial ‘bamboo-grained’ structure can be obtained in individual lattice struts via post-process heat treatment. Plus, they also confirm that the use of thinned-down structures, such as lattices, can effectively prevent the cracking previously observed in bulk samples. Although there remains plenty of room for further research on this topic, these results highlight the high potential of L-PBF for the manufacture of a new generation of MSM-based actuating devices.

Novel BCC Ti-Al-Nb-Zr medium-entropy alloys with ultrahigh specific strength and ductility

Body-centered cubic (BCC) medium-entropy alloys (MEAs) are characterized by high strength, low ductility and often high density due to containing much refractory elements. In this paper, a series of BCC Ti-Al-Nb-Zr medium-entropy alloys were constructed by adding different contents of Al to Ti67Nb16Zr17 base alloy composed of α and β phases. It is shown that an enough addition of Al can depress the formation of α phase, resulting in single BCC structure. (Ti67Nb16Zr17)87Al13 alloy has the best comprehensive mechanical property as the strength-ductility trade-off is well overcome. Its specific strength is up to 230 MPa g−1 cm3 and its elongation is also as high as 18.5% in the as-cast state, better than most existing medium- and high-entropy alloy. It is revealed that abundant nanoscale chemical short-range order (CSRO) in the matrix phase plays an important role in the mechanical response of this alloy.

Short-range order and phase stability of CrCoNi explored with machine learning potentials

We present an analysis of temperature-dependent atomic short-range ordering and phase stability of the face-centered cubic CrCoNi medium-entropy alloy employing a combination of ab initio calculations and on-lattice machine learning interatomic potentials. Temperature-dependent properties are studied with canonical Monte Carlo simulations. At around 975 K a phase transition into an ordered $\mathrm{Cr}{(\mathrm{Ni},\mathrm{Co})}_{2}$ phase (${\mathrm{MoPt}}_{2}$-type) is found. This hitherto not reported state has an ordering energy twice as large than the ordered structures previously suggested. We show that magnetism is not responsible for the observed chemical ordering.

Heavy ion irradiation effects on CrFeMnNi and AlCrFeMnNi high entropy alloys

Co-free but Al-included medium/high entropy alloys (M/HEAs) have gained increasing interests due to their lower cost and the potential to tune the multi-phase microstructure. The irradiation response of two Co-free HEAs, face-centered cubic (FCC) CrFeMnNi with limited Cr enriched α′ phase and body-centered cubic (BCC) AlCrFeMnNi with B2 s phase and nanoprecipitates were explored. Ion irradiations using 5 MeV Fe2+ ions were performed at 500 °C to a peak fluence of 50 and/or 100 displacements per atom (dpa). In dual-phase AlCrFeMnNi, there was no significant radiation induced segregation or chemical intermixing at the coherent matrix (FeCrMn-rich)/second phase (AlNi-rich) boundaries. In CrFeMnNi, limited voids were only detected at the peak damage location of ∼ 50 dpa. On the other hand, voids were widely distributed in AlCrFeMnNi: under 50 and 100 dpa irradiation conditions, voids were found with larger dimension and denser distribution in the FeCrMn-rich matrix, smaller and slightly lower density in an AlNi-rich second phase. In addition, the diameter of the FeCMn-rich nanoprecipitates didn't reveal any tendency of dissolution or growth. This is correlated with their superior structural stability against irradiation. Significant radiation-induced hardening (increases from 3.8 ± 0.2 GPa to 4.7 ± 0.6 GPa) was measured in CrFeMnNi, but only ∼ 4% hardness increase (from 7.4 ± 0.8 GPa to 7.7 ± 0.4 GPa) was noted in AlCrFeMnNi. In addition to the radiation-induced defects, such as voids, dislocation loops and point defects, other factors, such as chemical short-range ordering may play an important role.

Theoretical antiferromagnetism of ordered face-centered cubic Cr-Ni alloys

Contrary to prior calculations, the Ni-rich ordered structures of the Cr-Ni alloy system are found to be antiferromagnetic under semilocal density-functional theory. The optimization of local magnetic moments significantly increases the driving force for the formation of ${\mathrm{CrNi}}_{2}$, the only experimentally observed intermetallic phase. This structure's ab initio magnetism appears well described by a Heisenberg Hamiltonian with longitudinal spin fluctuations; itinerant Cr moments are induced only by the strength of exchange interactions. The role of magnetism at temperature is less clear and several scenarios are considered based on a review of experimental literature, specifically a failure of the theory, the existence of an overlooked magnetic phase transition, and the coupling of antiferromagnetism to chemical ordering. Implications for related commercial and high-entropy alloys are discussed for each case.

Atomistic simulations of dislocation plasticity in concentrated VCoNi medium entropy alloys: Effects of lattice distortion and short range order

Face-centered cubic (fcc) high and medium entropy alloys (H/MEAs) have been shown to display superior mechanical properties at low temperatures, but significant improvement of their strength at high temperatures is required for industrial applications at extreme conditions. Recently, it has been shown that the breakthrough of the MEAs from equiatomic/near-equiatomic to non-equiatomic ratios leads to strong MEAs with good ductility. To design new H/MEAs, we consider two important factors that may influence strength: the chemical composition and chemical short range order (CSRO). In this study, we investigate the depinning stress (σc) as a criterion of strength of several compositions of VCoNi concentrated solid solution alloys (CSSAs) including V0.33Co0.33Ni0.33, V0.35Co0.2Ni0.45, V0.33Co0.17Ni0.5, and V0.17Co0.33Ni0.5 at 5 K and 300 K, using atomistic simulations. The chosen interatomic potential is shown to be reliable by comparing experimental/ab initio values and calculated parameters such as lattice constant, shear modulus, depinning stress, and temperature variation of stacking fault width for equimolar VCoNi. We find a good agreement between experimental friction stress and the depinning stress extracted from our results for equimolar VCoNi. Also, we find that Vclusters are the main pinning points of dislocations, and With a random distribution of atoms, we find that the alloy composition V0.33Co0.17Ni0.5 displays the largest depinning stress at both 5 and 300 K. Furthermore, to investigate how CSRO affects the strength of these alloys, we design CSRO into the microstructure using two different methods: In the first method, hybrid Molecular-dynamics/Monte-Carlo simulations were employed to simulate annealing at various temperatures. We observe that such simulations create CSRO so that it increases with decreasing annealing temperature. Recently, the CSRO motif and its concentration in an equimolar VCoNi have been determined by experiment. By modeling this experiment, we also implemented the CSRO into microstructure as the second method. By using both methods, the effect of CSRO on the magnitude of the depinning stress is discussed. It was shown that in both methods, CSRO significantly influences the strength of non-equimolar VCoNi alloys.

Simultaneous strengthening effect of local chemical ordering and twin boundary on the medium entropy alloy CoCrNi

The interactive strengthening effect of twin boundary and local chemical ordering (LCO) structure in CoCrNi medium-entropy alloy (MEA) was investigated using molecular dynamics (MD) simulations of nanoindentation. The correlation between the two strengthening effects and the hardness was characterized. Meanwhile, underlying strengthening mechanisms were revealed. Results demonstrate that both LCO and twin boundary enhance the hardness of CoCrNi MEA. A concurrent strengthening effect appears when LCO and twin boundary are coupled for CoCrNi MEA with a stronger improvement on the hardness (12.40% at 70 K, 23.23% at 300 K, and 31.79% at 700 K). By comparing the plastic zones underneath the indentation, it is discovered that both LCO and twin boundary inhibit dislocation extension. The twin boundary provides a horizontal slipping path for dislocations. Moreover, since dislocations are pinned by Ni LCO structure on the twin plane, LCO can significantly limit the horizontal slip of dislocations on the twin plane.

Compositional undulation induced strain hardening and delocalization in multi-principal element alloys

Multi-principal element alloys (MPEAs) exhibit outstanding mechanical properties due to their unconventional compositions and chemical structures. Rational design of element distribution in such alloys promises for achieving an unprecedented combination of superior properties. Here, using hybrid molecular dynamics and Monte-Carlo simulation method, atomistic models with compositional periodicity and chemical short-range order (SRO) are designed to investigate the effect of compositional undulation on the tensile behavior of CoCrNi medium-entropy alloy (MEA). Specifically, the differences between MEAs with and without compositional undulation in the evolution of defects, local stress-strain state, dislocation nucleation, slip, and its interaction with planar defects are explored. The results reveal that compositional undulation in MEAs can improve the elastic limit and achieve a strong lattice distortion effect which can enhance the dislocation glide resistance. In addition, obvious strain hardening and stress/strain delocalization are observed in the MEA with compositional periodicity, which promotes strength-plasticity synergy. More interestingly, there are formation of many hcp microbands consists of stacking faults, hcp-like structures and nanotwins owing to the compositional undulation. Such structures induced delocalized activation of slip systems which improves the strength/plasticity and strain hardening ability. The proposed compositional design can further optimize the comprehensive mechanical properties in addition to SRO, which provides a feasible approach to design novel MPEAs with exceptional performance.

Maximum strength and dislocation patterning in multi-principal element alloys.

Multi-principal element alloys (MPEAs) containing three or more components in high concentrations render a tunable chemical short-range order (SRO). Leveraging large-scale atomistic simulations, we probe the limit of Hall-Petch strengthening and deformation mechanisms in a model CrCoNi alloy and unravel chemical ordering effects. The presence of SRO appreciably increases the maximum strength and lowers the propensity for faulting and structure transformation, accompanied by intensification of planar slip and strain localization. Deformation grains exhibit notably different microstructures and dislocation patterns that prominently depend on their crystallographic orientation and the number of active slip planes. Grain of single-planar slip attains the highest volume fraction of deformation-induced structure transformation, and grain with double-slip planes develops the densest dislocation network. These results advancing the fundamental understanding of deformation mechanisms and dislocation patterning in MPEAs suggest a mechanistic strategy for tuning mechanical behavior through simultaneously tailoring grain texture and local chemical order.

Correlating local chemical and structural order using Geographic Information Systems-based spatial statistics

Analysis of nanoscale short-range chemical and/or structural order via (scanning) transmission electron microscopy (S/TEM) imaging is fundamentally limited by projection of the three dimensional sample, which averages informational along the beam direction. Extracting statistically significant spatial correlations between the structure and chemistry determined from these two-dimensional datasets thus remains challenging. Here, we apply methods commonly used in Geographic Information Systems (GIS) to determine the spatial correlation between measures of local chemistry and structure from atomic-resolution STEM imaging of a compositionally complex relaxor, Pb(Mg1/3Nb2/3)O3 (PMN). The approach is used to determine the type of ordering present and to quantify the spatial variation of chemical order, oxygen octahedral distortions, and oxygen octahedral tilts. The extent of autocorrelation and inter-feature correlation among these short-range ordered regions are then evaluated through a spatial covariance analysis, showing correlation as a function of distance. The results demonstrate that integrating GIS tools for analyzing microscopy datasets can serve to unravel subtle relationships among chemical and structural features in complex materials that can be hidden when ignoring their spatial distributions.

Mechanically derived short-range order and its impact on the multi-principal-element alloys

Chemical short-range order in disordered solid solutions often emerges with specific heat treatments. Unlike thermally activated ordering, mechanically derived short-range order (MSRO) in a multi-principal-element Fe40Mn40Cr10Co10 (at%) alloy originates from tensile deformation at 77 K, and its degree/extent can be tailored by adjusting the loading rates under quasistatic conditions. The mechanical response and multi-length-scale characterisation pointed to the minor contribution of MSRO formation to yield strength, mechanical twinning, and deformation-induced displacive transformation. Scanning and high-resolution transmission electron microscopy and the anlaysis of electron diffraction patterns revealed the microstructural features responsible for MSRO and the dependence of the ordering degree/extent on the applied strain rates. Here, we show that underpinned by molecular dynamics, MSRO in the alloys with low stacking-fault energies forms when loaded at 77 K, and these systems that offer different perspectives on the process of strain-induced ordering transition are driven by crystalline lattice defects (dislocations and stacking faults).

Crystal chemistry and thermodynamic modelling of the Al13(Fe,TM)4 solid solutions (TM = Co, Cr, Ni, Pt)

The crystal chemistry of the Al13(Fe,TM)4 (TM = Co, Cr, Ni, Pt) solid solutions has been investigated by combining formation enthalpy measurements by differential scanning calorimetry (DSC), density functional theory (DFT) calculations and thermodynamic modelling. The formation enthalpies of seven alloys of the Al13(Fe,Co)4 solid solution were measured by DSC at 920 K, allowing the determination of the mixing enthalpy of the solution. These measurements are presented here for the first time and highlight the ideal nature of this solid solution. In addition, the mixing enthalpy of the Al13(Fe,TM)4 solid solutions (TM = Co, Cr, Ni, Pt) was determined by DFT at 0 K. These calculated and measured data (in the case of the Al13(Fe,Co)4 solid solution) were used to perform thermodynamic modelling of the solid solutions and better understand their thermodynamic stability. In addition, our modelling was used to calculate the TM occupancy on the Fe sites of the Al13(Fe,TM)4 solid solution structure at different temperatures. These data were used to quantify the chemical ordering of the solid solutions as a function of temperature. While these solid solutions show significant chemical ordering at low temperatures, only the Al13(Fe,Pt)4 solution remains highly ordered at high temperatures. These data are presented for the first time in this paper and have allowed us to design an optimal sublattice (SL) model for the Al13Fe4 solid solutions.

Composition formula of Al-TMs high-entropy alloys derived by cluster-plus-glue-atoms model and its experimental verification

High-entropy alloys (HEAs) are based on solid solutions which characterized by chemical short-range ordering (CSRO), but there is no accurate structural tool to address CSRO characteristic, which obstacles precise composition design for HEAs. In this study, based on the cluster-plus-glue-atoms model, the composition law of Al-TMs (TMs, transition metals) HEAs with BCC and FCC structures was revealed. In BCC structure, with five elements in equi-atomic ratio, the composition formula of Al-TMs HEAs can be expressed by a cluster formula [Al-M14]Al3, and in non equi-atomic ratio can be expressed by [Al-M14]Al, where M is the average atom of the TMs. But in FCC structure, both Al-TMs HEAs with five elements in equi-atomic and non equi-atomic ratios can be expressed by a cluster formula [Al-M12]Al. To confirm the effectiveness of cluster formula for Al-TMs, two alloys were designed, [Al-Ti4V3Nb4Mo3]Mo and [Al-Ti3V3Nb4Mo4]Al. Results show that the [Al-Ti3V3Nb4Mo4]Al alloy has both higher strength and higher plastic deformation at room temperature. Besides, [Al-Ti3V3Nb4Mo4]Al alloy shows slower soften effect at 800 °C, contributed to its higher strength. By substituting Ta for some of Mo, the strength of [Al-Ti3V3Nb4Mo3Ta]Al at room temperature and high temperature drastically decreases, suggesting that Ta element deteriorates the properties of Al-TMs alloys.