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Ferrous medium-entropy alloys (FeMEAs), leveraging deformation-induced martensitic transformation (DIMT), demonstrate excellent strain-hardening ability attributed to the transformation-induced plasticity (TRIP) effect. To improve the low yield strength of FeMEAs, the initial microstructure was controlled by utilizing partial recrystallization. The intricate initial microstructure, a blend of recrystallized (ReX) and non-recrystallized (non-ReX) regions, results in complex deformation behavior where DIMT in both the ReX and non-ReX regions are simultaneously activated, posing significant analytical challenges. In this paper, we perform in-situ synchrotron X-ray diffraction during the tensile loading on a partially recrystallized metastable Fe 57.5 Co 18 Cr 13 Ni 7.5 Mo 3 C 1 (at%) FeMEA to quantitatively analyze each deformation mechanism. The innovative idea of peak deconvolution enables separate tracing of the deformation behavior of the ReX and non-ReX FCC domains, revealing the stress partitioning between them. DIMT kinetics in each domain are investigated by the evolution of domain fractions, and we provide a detailed discussion on how both of them exhibit rapid DIMT kinetics. Furthermore, we measure the contributions of DIMT occurring in each domain on the global strain-hardening rate. The results suggest that the predominant contribution shifts from DIMT in the ReX domain to DIMT in the non-ReX domain as deformation progresses, highlighting the distinctive strain-hardening mechanisms between the ReX and non-ReX domains. This work demonstrates how a partially recrystallized metastable FeMEA exhibits superior mechanical properties and provides insights into analyzing the complex deformation behavior. [Display omitted] [ABSTRACT FROM AUTHOR]

The grain boundary (GB) segregation of alloying elements in an ultralow carbon Fe–P alloy was investigated at different annealing temperatures (500 °C, 600 °C, and 700 °C) using a three-dimensional atom probe. The impact of the segregated phosphorus and carbon concentrations on intergranular corrosion was also assessed in a sodium sulfate (Na 2 SO 4) aqueous solution. Comparative analysis of specimens annealed at 600 °C and 700 °C revealed that intergranular corrosion occurred only in the specimen annealed at 600 °C, which exhibited a higher concentration of phosphorous segregation at the GBs. This suggests that intergranular corrosion resistance deteriorates with increased phosphorous segregation at GBs. Additionally, the progression of intergranular corrosion was found to depend on GB misorientations, increasing up to 30° and stabilizing at higher GB misorientation angles. Carbon segregation was observed only in the specimen annealed at 500 °C, which showed no intergranular corrosion, despite having the highest concentration of phosphorus at the GBs. This indicates that carbon, identified as an impurity in the alloy, can inhibit intergranular corrosion induced by high phosphorous concentration at the GBs. This study provides insights into preventing the macroscopic intergranular corrosion in iron alloys by controlling the elemental segregation of impurities at GBs. [Display omitted] [ABSTRACT FROM AUTHOR]

The yield strength of random metal alloys, i.e. alloys with random occupation of the crystalline lattice sites by the elemental constituent atoms all considered as solutes, is primarily understood as controlled by solute/dislocation interactions. Solute-solute interactions exist and provide the energetic driving force for both short-range and long-range order but can then also affect yield strength even in the random alloy. Here, a recent theory for random alloys is extended to include solute-solute interactions described by pair-wise interactions. The new theory involves the standard deviation in total solute-solute interaction energies as a dislocation segment glides through the material, which changes specific solute-solute pairs across the glide plane at every pair distance. An analytic expression is derived for the above standard deviation and validated against numerical simulations on a wide range of model random alloys consisting of 2–5 elements interacting via Lennard–Jones pair potentials. The theory is applied to the bcc MoNbTaW high entropy alloy, using solute-solute interactions computed via first-principles, and a model NbTaV alloy, described by EAM potentials, where the strength increases negligibly by 2% and 0.45%, respectively. Application to random dilute fcc Ni-Al, where the first-neighbor Al-Al interaction is very strongly repulsive, shows significant strengthening of 60–100% at 10% Al, depending on the origin of the inputs. Some connections to literature atomistic simulations on Ni–Al are also presented. Overall, the present theory provides a quantitative framework for assessing the relative roles of solute-dislocation and solute-solute interactions on strengthening in random alloys. [ABSTRACT FROM AUTHOR]

This present work simulated the coherent BCC/B2 microstructures existing in body-centered-cubic (BCC)-based Al-Ni-Co-Fe-Cr high entropy alloys (HEAs) in light of the phase-field method. These coherent BCC/B2 microstructures contain spherical or cuboidal nanoprecipitates, or exhibit a weave-like spinodal decomposition in experiments. Based on the Chan-Hilliard equation, a two-dimensional phase field model was established using the COMSOL Multiphysics software to reveal the coherent microstructural evolutions of the present HEAs. It was found that the simulations of spherical/cuboidal nanoprecipitations and weave-like spinodal decomposition are well consistent with the experimental results. Both the lattice misfit and the anisotropy difference of Young's moduli between the precipitated phase and matrix phase affect the precipitate morphology, in which the cuboidal nanoprecipitation is especially susceptible to the modulus anisotropy. The coarsening behavior of precipitates is also discussed with the aid of the simulation results. The phase-field simulation will provide an important technique to predict the microstructural evolution and to assist the composition design of new HEAs. Image, graphical abstract [ABSTRACT FROM AUTHOR]

This work presents an X-ray absorption spectroscopy study on a single-phase state of the Al 8 Cr 17 Co 17 Cu 8 Fe 17 Ni 33 compositionally complex alloy, focused on the local crystal structure around each alloying element. The comparison of 1st shell bond lengths, obtained by the analysis of extended X-ray absorption fine structure (EXAFS) measured at the K-edges of each alloying element, indicates that Al 8 Cr 17 Co 17 Cu 8 Fe 17 Ni 33 crystallizes in a distorted arrangement of an fcc lattice. A modest bond length dependence of the alloying elements with increasing atomic number is observed, with minima and maxima at Cr/Co, and Al/Cu, respectively. 1st shell bond lengths spread over ~0.03 Å; consequently, such variations cannot result in lattice distortions greater than ~0.04 Å. EXAFS results clearly indicate short-range order in the alloy: pairing of Al with Ni and Cu is favored, correlating well with a γ' precipitate composition (Al-Ni-Cu rich) reported in previous work, while Al-Cr bonding is unfavored and no Al-Al pairs are observed. Electronic structure information was obtained through comparison between near-edge regions of alloying elements and corresponding pure metals. Intensity comparison of K-edge features agree with a charge variation of p states in Al 8 Cr 17 Co 17 Cu 8 Fe 17 Ni 33 , where Ni and Cu act as p states electron acceptors, suggesting an orbital hybridization with Al, responsible for a shrinkage in Al metallic radius in the alloy by ∼0.17 Å. Image, graphical abstract [ABSTRACT FROM AUTHOR]

This work investigates solute transport due to self-interstitial defects and radiation induced segregation tendencies in dilute ferritic alloys, by computing the transport coefficients of each system based on ab initio calculations of binding energies, migration rates, as well as formation and migration vibrational entropies. The implementation of the self-consistent mean field method in the KineCluE code allows for the calculation of transport coefficients extended to arbitrary interaction ranges, crystal structures, and diffusion mechanisms. In addition, the code gives access to the diffusion and dissociation rates of small solute-defect clusters – in this case, vacancy- and dumbbell-solute pairs. The results show that the diffusivity of P, Mn, and Cr solute atoms is dominated by the dumbbell mechanism, that of Cu by vacancies, while the two mechanisms might be in competition for Ni and Si, despite the fact that the corresponding mixed dumbbells are not stable. Systematic positive radiation-induced segregation (RIS) at defect sinks is expected for P and Mn solutes due to dumbbell diffusion, and for Si due mainly to vacancy drag. Vacancy drag is also responsible for Cu and Ni enrichment at sinks below 1085 K. The RIS behavior of Cr is the outcome of a fine balance between enrichment due to the dumbbell diffusion mechanism and depletion due to the vacancy one. Therefore, for dilute Cr concentrations global enrichment occurs below 540 K, and depletion above. This threshold temperature grows with solute concentration. The findings are in qualitative agreement with experimental observations of RIS and clustering phenomena, and confirm that solute-defect kinetic coupling plays an important role in the formation of solute clusters in reactor pressure vessel steels and other alloys. [ABSTRACT FROM AUTHOR]

Enthalpy of mixing is among the key materials parameters to determine phase stability and phase transformations in solid solutions. The possibility to predict it from first principles in the framework of the density functional theory is one of the corner stones of the modern materials modeling and the future data-driven materials design. Here we have considered body-centered cubic (bcc) Ti-V alloys, a system with high potential for aerospace, automotive biomedical and energy applications, which is known to exhibit the dynamical instability of the crystal lattice for Ti-rich alloys at low temperature. We have calculated the mixing enthalpies ΔH of bcc Ti-V alloys in the whole interval of concentration at high temperature using ab initio molecular dynamics (AIMD) simulations. A comparison with state-of-the-art static calculations at temperature 0 K shows drastic difference between the two methods: while AIMD predicts positive values of ΔH in the whole range of concentrations, the static zero-temperature simulations result in negative values of ΔH for Ti-rich alloys. We have measured the mixing enthalpy of bcc Ti-V alloys experimentally at 1073 K using an isoperibol high temperature Tian-Calvet calorimeter and found that the enthalpies are positive, in agreement with our finite temperature AIMD calculations. We attribute the failure of the standard static calculations of ΔH to lattice distortions associated with the dynamical instability of bcc Ti-V alloys at zero temperature and argue that the effect should be generally important in theoretical predictions of thermodynamic properties, especially for systems with dynamical instability. Image, graphical abstract [ABSTRACT FROM AUTHOR]

We studied grain boundary (GB) self-diffusion in body-centered cubic iron using ab initio calculations and molecular dynamics simulations with various interatomic potentials. A combination of different models allowed us to determine the principal characteristics of self-diffusion along different types of GBs. In particular, we found that atomic self-diffusion in symmetric tilt GBs is mostly driven by self-interstitial atoms. In contrast, in general GBs atoms diffuse predominantly via an exchange mechanism that does not involve a particular defect but is similar to diffusion in a liquid. Most observed mechanisms lead to a significant enhancement of self-diffusion along GBs as compared to diffusion in the bulk. The results of simulations are verified by comparison with available experimental data. [ABSTRACT FROM AUTHOR]

In the present atomistic study, layer-specific segregation to the surface is investigated for an exemplary (100) surface in iron–chromium alloys using an embedded-atom potential. Through a continuous variation of the chemical potential difference in the semi-grandcanonical ensemble, the full composition range is explored at temperatures between 600 K and 1400 K. The obtained layer-specific segregation curves demonstrate the well-known limitations of the widely used McLean model for interface segregation, i.e. a monolayer model imposing ideal behavior. However, keeping the original idea of McLean, the segregation model is extended in two ways in order to provide a complete analytical description of segregation: Firstly, the entire surface is hypothetically replaced by an equivalent single layer with identical segregation properties, which accounts for all subsurface layer effects, but also enables the application of an effective monolayer model with only one single energy of segregation. Secondly, directly following from the general treatment assuming non-ideal solution behavior, the energy of segregation becomes composition-dependent. The validity and accuracy of the proposed analytical model is confirmed by fitting the composition-dependent energy of segregation to the thermodynamically unambiguous solute excess. The change in surface formation energy according to the interfacial adsorption equation can be described excellently over the entire composition range for all investigated temperatures. The model can be applied to experimental data and directly transferred to grain boundaries. [ABSTRACT FROM AUTHOR]

A phenomenon of "phase inversion", presumably the first ever experimental evidence in metallic alloys, is shown in a refractory high entropy alloy (RHEA), Al 0.5 NbTa 0.8 Ti 1.5 V 0.2 Zr. Phase inversion in crystalline solid systems is driven by the differences in elastic modulus of the two phases. Quenching from a high temperature single phase field, the RHEA exhibits a co-continuous mixture of a disordered BCC and an ordered B2 phase, that upon isothermal annealing at 600 °C develops via spinodal decomposition into a continuous B2 matrix with discrete cuboidal BCC precipitates aligned along the 〈001〉 directions. Longer term annealing at 600 °C results in the development of necking constrictions along the B2 channels, eventually pinching-off these channels and making the BCC phase continuous with discrete B2 precipitates. This inversion process can be related to the simultaneous operation of two processes: (i) spheroidization of the initially discrete cuboidal BCC precipitates driven by a reduction in the total interface energy and (ii) an increase in the stiffness of the B2 phase, relative to the BCC phase, due to chemical composition changes during annealing, forcing the B2 regions to become discrete driven by the reduction in the total elastic strain energy. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) calculations were used to model G-phase precipitates of formula X 6 M 16 Si 7 where X is Cr, Hf, Mn, Mo, Nb, Ta, Ti, V, W and Zr and M is either Fe or Ni. It was found that the occupancy of the d-orbital is correlated to the formation enthalpies of each structure. Past thermal expansion coefficient data was used to predict the lattice misfit between each G-phase and body centred cubic (BCC) Fe. All except Hf and Zr containing G-phases were predicted to have zero misfit between 581−843 K. Of the Ni containing G-phases, Mn 6 Ni 16 Si 7 was predicted to have the most similar elastic properties to BCC Fe. DFT calculations of the substitution energies of Al, Cr Cu, Fe, Ge, Hf, Mo, Nb, P, Ta, Ti, V, Zr, and vacancies onto the Mn 6 Ni 16 Si 7 G-phase from BCC Fe were performed. It was predicted that Cu, P and vacancies favour G-phase substitution. Suppression of the G-phase is predicted when Si content is reduced by half, at which point the BCC phase is favoured. It is hypothesised that including Zr to form a (Mn,Zr) 6 Ni 16 Si 7 precipitate will allow for higher ageing temperature and expediate nucleation in an Fe alloy. Thermocalc was used to predict that a mixture of Fe bal Cr 9 Ni 4 Si 2 (Mn 0.6 Zr 0.4) 1.2 (at.%) will produce a G-phase strengthened Fe alloy with potential for a good balance of strength, ductility and oxidation/corrosion resistance at room temperature. This alloy composition was experimentally determined to precipitate the G-phase in ≤24 h with cube-on-cube orientation to the BCC Fe matrix. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Random body-centered-cubic (BCC) "High Entropy" alloys are a new class of alloys, some having high strength and good ductility at room temperature and some having exceptional high-temperature strength. There are no theories of strengthening of screw dislocations in BCC metals that span naturally from the dilute limit to the multi-component, non-dilute concentrations typical of the high-entropy domain. Here, such a theory is developed and validated. Unlike low-temperature elemental BCC metals and very dilute BCC alloys, strength is not controlled by the kink-pair nucleation mechanism. Rather, screw dislocations naturally adopt a kinked structure as the minimum total energy configuration in the field of random alloying atoms. The characteristic length and energy scales for the low-energy kinked screw dislocation are derived for random alloys, leading to a characteristic spacing of both kinks and cross-kinks that depends on the kink formation energy and a characteristic collective solute/screw dislocation interaction energy parameter. Glide motion of this initially-kinked screw dislocation occurs via Peierls-type motion, lateral kink glide, and failure of cross-kinks. All these features are observed in molecular dynamics simulations. The resulting strength versus temperature, strain rate, and composition is analytic. The theory is validated by comparison to experiments on non-dilute Fe-Si, Nb-Mo, Nb-W, and Ti-Nb-Zr-based high entropy alloys versus composition and temperature. The theory provides a framework for tailored design of new high-performance BCC alloys. [ABSTRACT FROM AUTHOR]

The modification of the matrix of ductile cast irons by heat treatments has been of interest of researchers for many years. Among these treatments, in the last years the Quenching & Partitioning (Q&P) process has emerged as a viable way to produce microstructures containing controlled amounts of martensite and retained austenite, providing a good combination of strength and ductility. In this work, the different mechanisms of phase transformations occurring during the Q&P heat treatment applied to a ductile cast iron alloy is investigated. Microsegregation, inherent to cast irons, was analyzed by means of Electron Probe Microanalysis (EPMA). Microstructural characterization was performed with Scanning Electron Microscopy (SEM) and Electron Backscattered Diffraction (EBSD), while kinetics of carbon redistribution and competitive reactions were studied using dilatometry and in situ synchrotron X-ray diffraction. It was found that either transition carbides or cementite precipitate in martensite depending on the partitioning temperature. Despite of carbides precipitation, evidence of carbon partitioning from martensite to austenite was obtained. Formation of bainitic ferrite occurs during the partitioning step, further contributing to carbon enrichment of austenite. The experimental results are compared with a local field model that computes the local kinetics of carbon redistribution by simultaneously considering carbides precipitation and growth of bainitic ferrite. Results showed that kinetics of carbon partitioning from martensite to austenite depends on the carbides free energy. More stable carbides do not dissolve and prevent the escape of carbon from martensite. Fast carbon partitioning occurs by dissolution of less stable carbides, but it is slowed down as growth of bainitic ferrite proceeds. This result is explained by the overlapping of the diffusion fields (soft impingement) of the carbon partitioned from martensite and the carbon rejected from growth of bainitic ferrite. Image 1 [ABSTRACT FROM AUTHOR]

The formation of a metastable and nitrogen-supersaturated f.c.c. interstitial solid solution layer on Fe–Cr–Ni austenitic stainless steel at a moderate temperature around 650–720 K is not entirely understood. In the present work, three groups of austenitic Fe–Cr–Ni alloys, containing systematic variations of chromium, nickel and iron contents were nitrided by plasma-based low-energy ion implantation at 653 K for 4 h and investigated with light-optical microscopy (LOM), electron probe microanalysis (EPMA), (grazing incidence) X-ray diffraction (XRD) and transmission electron microscopy (TEM). Commercial AISI 304L was included for comparison. For the austenitic alloys with a Cr content below 12 wt.%, a duplex layer is observed in the nitrided case where the interface between the top layer, consisting of γ'-Fe 4 N like ordered γ' N , and the disordered γ N (nitrogen enriched austenite) zone underneath is associated with a decrease in N content. For the alloys with a Cr-content over 12 wt.%, a featureless continuous zone is observed with LOM and a gradual decrease in nitrogen content is measured with EPMA. Nevertheless, a similar duplex structure of outer γ'-Fe 4 N like ordered γ' N and inner γ N is confirmed by XRD and TEM for all nitrided alloys, irrespective of the Cr content. The results are discussed in terms of the short-range order (SRO) promoted by the Cr–N interaction and long-range order (LRO) caused by the Fe–N interaction. Image 1 [ABSTRACT FROM AUTHOR]

FeAl-based MIL composites of various iron alloys were fabricated with an innovative "multiple-thin-foil" configuration and "two-stage reaction" strategy. Alternating stacked metal foils were reactive sintered via SPS at 600 °C and 1000 °C to grow intermetallics. The "multiple-thin-foil" configuration reduces reaction time, enables local chemical composition control and allows metal/intermetallic combinations, which cannot be produced via the conventional methods. Fe–FeAl, 430SS-FeAl, and 304SS-FeAl MIL composites can be synthesized with desired metallic/intermetallic ratios, where FeAl is the single intermetallic phase present in the composites. Microstructure analysis via SEM, EDS, and EBSD confirms phase identification and reveals the formation of transition layers. The transition layer, which incorporates the composition gradient between the metal (Fe, 430SS or 304SS) and the FeAl intermetallic phase, provides a gradual change in mechanical properties from the metal to intermetallic layers, and further functions as a chemical barrier into which other undesired intermetallics dissolve. Driven by diffusion-controlled growth, grains in the transition layers and FeAl regions exhibit ordered arrangement and sintering textures. Hardness profiles from the metal layer to FeAl region reveal the correlation between local mechanical properties and local chemical compositions. In compression testing, the compressive strength can reach 2.3 GPa with considerable plasticity, establishing the best mechanical properties of any MIL composites synthesized to date. Image 1 [ABSTRACT FROM AUTHOR]

In this work, we report a crystalline-amorphous-crystalline (CAC) two-step solid-state phase transformation (SSPT) in a 15Cr-2Ni iron-based alloy that results in nanostructures. Microstructurally, the CAC process leads to a localized supra-nano structure having almost equal content of an amorphous matrix and nanocrystalline inclusions less than 6 nm. Dynamically, it is manifested as a glass-transition-like variation of Young's modulus with a sharp surge in internal friction. Through in situ heating and Neutron Scattering experiments, a reduction in the intensities of the austenite diffraction peaks is observed, which is not associated with any intensity increase in other peaks, suggesting the collapse of austenite lattice during heating. The CAC two-step SSPT occurs at temperatures far below the melting point, and the introduction of the dislocations is the indispensable condition giving rise to such a transition. Based on experimental results, we suggest the physical picture of CAC two-step SSPT and a phenomenological model to describe it. Through the analysis, we argue that the CAC two-step SSPT provides a new route to fabricate nanostructured alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

The influence of substitutionally dissolved V in ferritic Fe-4 at.% V alloy, on the structure and morphology of iron-carbonitrides ε-Fe 3 (N, C) 1+ x and γ'- Fe 4 (N , C) 1-y was investigated upon salt bath nitrocarburizing. An unusual plate-type morphology of γʹ and ε carbonitrides development was evidenced during nitrocarburizing of Fe-V alloy as against to the usual layer-type growth of γʹ and ε nitrides reported for nitrided Fe-V alloys. The ε phase developed first along ferrite matrix grain boundaries, followed by its formation as plates on the plates of γʹ within the grains of ferrite matrix and finally as a layer at the surface. In contrast to ε, γʹ has developed only as plates within the ferrite matrix grains either with Kurdjumov-Sachs or Pitsch orientation relationship. Initially the inwardly diffusing N gets consumed for the development of VN precipitates in the ferrite matrix while C gets enriched at the ferrite-matrix grain boundaries leading to the development of ε carbonitride. Once all the V precipitates as VN, the development of γʹ occurs as plates within the grains and upon further increase in the N content lead to the development of ε within grains. This demonstrates that as a consequence of drastically different solubility and diffusivity of C and N in ferrite, ε and γʹ phases can lead complex microstructural evolution upon nitrocarburizing of Iron-based alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract When metals are mechanically loaded at elevated temperatures for extended periods of time, creep damage will occur in the form of cavities at grain boundaries. In the present experiments it is demonstrated that in binary iron-tungsten alloys creep damage can be self healed by selective precipitation of a W-rich phase inside these cavities. Using synchrotron X-ray nano-tomography the simultaneous evolution of creep cavitation and precipitation is visualized in 3D images with a resolution down to 30 nm. The degree of filling by precipitation is analysed for a large collection of individual creep cavities. Two clearly different types of behaviour are observed for isolated and linked cavities, where the isolated cavities can be filled completely, while the linked cavities continue to grow. The demonstrated self-healing potential of tungsten in iron-based metal alloys provides a new perspective on the role of W in high-temperature creep-resistant steels. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract A FeCoNiCrTi 0.2 high-entropy alloy strengthened by two types of coherent nano-precipitates but with the same composition was fabricated, and its tensile properties at room (293 K) and cryogenic temperatures (77 K) and the corresponding defect-structure evolution were investigated. Compared with the single-phase FeCoNiCr parent alloy, the precipitation-strengthened FeCoNiCrTi 0.2 high-entropy alloy exhibits a significant increase in yield strength and ultimate tensile strength but with little sacrifice in ductility. Similar to the single-phase FeCoNiCr high-entropy alloy, the deformation behavior of this precipitation-strengthened FeCoNiCrTi 0.2 high-entropy alloy shows strong temperature dependence. When the temperature decreases from 293 K to 77 K, its yield strength and ultimate tensile strength are increased from 700 MPa to 860 MPa and from 1.24 GPa to 1.58 GPa, respectively, associated with a ductility improvement from 36% to 46%. However, different from the single-phase FeCoNiCr high-entropy alloy with a twinning-dominant deformation mode at 77 K, multiple-layered stacking faults with a hierarchical substructure prevail in the precipitation-strengthened FeCoNiCrTi 0.2 high-entropy alloy when deformed at 77 K. The mechanism of twinning inhibition in this precipitation-strengthened high-entropy alloy is the high energy barrier for twin nucleation in the ordered γ′ nano-particles. Our results may provide a guide for the design of tough high-entropy alloys for applications at cryogenic temperatures through combining precipitation strengthening and twinning/stacking faults. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract In this work the global and local mechanical properties of the magnetocaloric intermetallic LaFe 11.2 Si 1.8 alloy are investigated by a combination of different testing and characterization techniques in order to shed light on the partly contradictory data in recent literature. Macroscale compression tests were performed to illuminate the global fracture behavior and evaluate it statistically. LaFe 11.2 Si 1.8 demonstrates a brittle behavior with fracture strains below 0.6% and widely distributed fracture stresses of 180–620 MPa leading to a Weibull modulus of m = 2 to 6. The local mechanical properties, such as hardness and Young's modulus, of the main and secondary phases are examined by nanoindentation and Vickers microhardness tests. An intrinsic strength of the main magnetocaloric phase of at least 2 GPa is estimated. The significantly lower values obtained by compression tests are attributed to the detrimental effect of pores, microcracks, and secondary phases. Microscopic examination of indentation-induced cracks reveals that ductile α-Fe precipitates act as crack arrestors whereas pre-existing cracks at La-rich precipitates provide numerous 'weak links' for the initiation of catastrophic fracture. The presented systematic study extends the understanding of the mechanical reliability of La(Fe, Si) 13 alloys by revealing the correlations between the mechanical behavior of macroscopic multi-phase samples and the local mechanical properties of the single phases. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract High-entropy alloys (HEAs) are a newly emerging class of materials that show attractive mechanical properties for structural applications. Particularly, face-centered cubic (fcc) structured HEAs and medium-entropy alloys (MEAs) such as FeMnCoNiCr and CoNiCr alloys, respectively, which exhibit superior fracture toughness and tensile properties at liquid nitrogen temperature, are the potential HEA materials available for cryogenic applications. Here, we report a ferrous Fe 60 Co 15 Ni 15 Cr 10 (at%) MEA exhibiting combination of cryogenic tensile strength of ∼1.5 GPa and ductility of ∼87% due to the multiple-stage strain hardening. Astonishingly, detailed microstructural observations at each stage reveal the sequential operation of deformation-induced phase transformation from parent fcc to newly formed bcc (body-centered cubic) phases. No compositional heterogeneity is observed at phase boundaries, indicating diffusionless phase transformation, as confirmed by atom probe tomography. The transformation to bcc phase occurs predominantly along grain boundaries (GBs) at the early stage of plastic deformation. Simultaneously, numerous deformation-induced shear bands (SBs) having stacking faults associated to the Shockley partial dislocations and thin hcp plates, form within fcc grains. Further deformation leads to the intense nucleation and growth of the bcc phase at the intersections of SBs within fcc grains. These micro-processes consecutively enhance the strain hardening rate, which play a key role in the multiple strain hardening behavior. The in-situ neutron diffraction studies make it clear that the martensite formation and the concurrent load partitioning between the fcc and bcc phases play an important role in the increase in strength. Furthermore, replacing high-cost alloying elements cobalt and nickel with iron, as well as introduction of metastability-engineering at liquid nitrogen temperature, distinguishes the new ferrous MEAs from previously reported equiatomic HEAs. This result underlines insights to provide expanded opportunities for the future development of HEAs for cryogenic applications. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Grain size stabilization is crucial for the production and application of nanocrystalline (NC) materials. The mechanically alloyed (MA) NC Fe-Zr system is known as a very successful NC system as it exhibits excellent thermal stability at elevated temperatures. The grain size stabilization of this system has been previously ascribed to its reduced grain boundary (GB) energy by Zr segregation and Zener pinning of Zr-rich intermetallic precipitates. In this work, we report a different mechanism that significantly contributes to grain size stabilization of this NC alloy system using two MA-produced NC Fe-Zr alloys (Fe-1 at.% Zr and Fe-5 at.% Zr) as examples. We show by using atom probe tomography and Cs-corrected transmission electron microscopy that highly dispersed coherent Fe-Zr-O nanoclusters, with a number density up to 10 24  m −3 , form in ferrite matrix after annealing at certain temperatures. Our first-principles calculations indicate that the formation of these nanoclusters is caused by the ordering of Zr and O-impurity in ferrite matrix. We analyzed the underlying mechanism of grain size stabilization in terms of the experimental results and the Zener pinning theory, and suggest that the pinning effect exerted by these nanoclusters significantly contributes to grain size stabilization of the NC Fe-Zr alloys. [ABSTRACT FROM AUTHOR]

Using ab initio calculations, we investigate the elastic properties of paramagnetic Al x CrMnFeCoNi (0 ≤  x  ≤ 5) high-entropy alloys (HEAs) in both body-centered cubic (bcc) and face-centered cubic (fcc) structures. Comparison with available experimental data demonstrates that the employed approach describes accurately the elastic moduli. The predicted lattice constants increase monotonously with Al addition, whereas the elastic parameters exhibit complex composition dependences. The elastic anisotropy is unusually high for both phases. The brittle/ductile transitions formulated in terms of Cauchy pressure and Pugh ratio become consistent only when the strong elastic anisotropy is accounted for. The negative Cauchy pressure of CrMnFeCoNi is due to the relatively low bulk modulus and C 12 elastic constant, which in turn are consistent with the relatively low cohesive energy. The present findings in combination with the experimental data suggest anomalous metallic character for the HEAs system. [ABSTRACT FROM AUTHOR]

Diffuse scattering under grazing incidence of a Fe-27.8 at.% Pt (110) surface was measured at RT and 424 K to explore the impact of the Invar behavior in the near-surface region. The quenched-in short-range order does not change between these two temperatures. The presence of an oscillatory segregation profile is indicated through a directional dependence of the nearest-neighbor site occupation. Size-effect scattering is found to be lower at RT than at 424 K. The species-independent displacement parameters of the nearest and next-nearest neighboring shells are larger than in the bulk, their change with temperature shows the same behavior as in the bulk. This analogous behavior of the near-surface microstructure to the bulk is a strong support that the Invar behavior is retained in the near-surface region. [ABSTRACT FROM AUTHOR]

Microstructures of magnetocaloric Ni-Mn-In-based Heusler alloys, Ni 50.2 Mn 35.0 In 14.8 and Ni 46.1 Mn 37.9 Fe 3.0 In 13.0, were studied to understand the origin of a large difference in thermal hysteresis in these two alloys. In-situ transmission electron microscopy (TEM) observation showed that the Fe containing sample with a large hysteresis shows a discontinuous phase transition due to the existence of nano-scale Fe-rich bcc phase, along with Fe-lean B2 and L2 1 phases in the austenite state. The Fe-free sample with a low hysteresis shows a uniform phase transition from martensite to austenite initiated by the nucleation of austenite at the twin boundaries. Ni segregation was found at the twin boundaries of the low hysteresis sample that is considered to facilitate the nucleation of the austenite. The phase transition progresses by the growth of the nucleated austenite to the neighboring twins. 5M and 7M modulated martensites in the low hysteresis sample give rise to a slight difference in the phase transition temperatures in the twin bands contributing to the small hysteresis of 4.4 K in the Fe-free sample. Based on these results, we conclude that to minimize the thermal hysteresis of the Ni-Mn-In based magnetocaloric compounds, one of the key factors is to achieve a uniform composition and crystal structure in the alloy. [ABSTRACT FROM AUTHOR]

Nitric acid oxidation has been used to fabricate insulation coatings of the FeSiAl soft magnetic composites (SMCs). Growth mechanism of the coatings obtained with varied HNO 3 concentration has been systematically studied based on careful analysis of the coating thickness, microstructure and composition. The oxidation process for the powders reacted with 10 wt% HNO 3 corresponds to the steady-state passivation, while pitting corrosion occurs with raised HNO 3 concentration of 30 wt%. Origin of the pitting corrosion has been analyzed based on the point defect model. Evolution of the coatings has been revealed under different growth conditions and correlated to the magnetic performance of the SMCs. The samples oxidized with 10 wt% HNO 3 mainly form a thin and compact oxidation layer with dominating Al 2 O 3 , AlO(OH) and a small amount of Fe 3 O 4 , while increased HNO 3 concentration (30 wt%) gives rise to mixed Al 2 O 3 , AlO(OH), Fe 2 O 3 and Fe 3 O 4 in the thicker coating layers. Enhanced magnetic performance of the FeSiAl SMCs can be achieved by optimized thickness and composition of the coatings ( μ e  = 180.4, P cv  = 327.2 mW/cm 3 for the SMCs oxidized by 10 wt% HNO 3 and μ e  = 157.3, P cv  = 241.2 mW/cm 3 via oxidization by 30 wt% HNO 3 for 5 min). [ABSTRACT FROM AUTHOR]

This paper presents a numerical analysis of the influence of spinodal decomposition on the strength of Fe-Cr alloys using the dislocation dynamics (DD) method. In the DD simulations, the structure of a spinodally decomposed chromium distribution is approximated using a cosine function. Using the equation for internal stress distribution, the interaction between a dislocation and the internal stress distribution is precisely accounted for in the DD simulations. The structure of the spinodally decomposed chromium distribution is parameterized using four variables, including the magnitude of chromium concentration, wave length of chromium distribution, position of the slip plane of dislocation, and the dislocation character (angle between the dislocation line and Burgers vector). Using these variables, the influence of the structure of chromium distribution on the critical resolved shear stress (CRSS) is studied. Furthermore, we focused on two major slip systems of the BCC structure, { 110 } 〈 111 〉 and { 112 } 〈 111 〉 , and discuss the difference in the influence between the different slip systems. In the { 110 } 〈 111 〉 slip system, the Δ CRSS appears only for a mixed dislocation with θ  = 54.7°, because of the stripe pattern of the resolved shear stress distribution. On the other hand, in the { 112 } 〈 111 〉 slip system, the dislocations with θ  = 39.2° and 90° have a large Δ CRSS. There is a plateau of Δ CRSS in a range of 39.2 ∘ < θ < 90 ∘ . The slip plane position does not change the Δ CRSS. There is a dependence of Δ CRSS on the wave-length of the chromium distribution. The dependence of Δ CRSS on the wave-length can be found not with a straight dislocation but with a curved dislocation using the DD simulations. The information is a new finding, and is meaningful in understanding the relationship between the material strength and the structure of spinodal decomposition. [ABSTRACT FROM AUTHOR]

The ferritic Fe-Ni-Al-Cr-Mo steel (FBB8) has good creep properties up to 700 °C due to B2-NiAl nanoscale precipitates and its creep resistance can be further improved by additions of 2 or 4 wt.% Ti, as a result of sub-precipitates within the main precipitates. Here, the hierarchical structure of the precipitates is studied in the light of phase separation via transmission electron microscopy (TEM) and atom probe tomography (APT). For FBB8-2Ti (with 2% Ti added) exhibiting B2-NiAl precipitates with L2 1 -Ni 2 AlTi sub-precipitates, APT analysis shows strong partitioning of Ni, Al and Ti from the ferritic matrix into the B2/L2 1 precipitates and, within the precipitates, partitioning of Ti and Fe within the L2 1 sub-precipitates. Based on the published pseudo-binary phase-diagram between (Ni,Fe)Al and (Ni,Fe)Ti, this hierarchical precipitate microstructure is discussed based on the known miscibility gap between the B2 and L2 1 phases, due to partitioning of Ti into the L2 1 phase and ordering of Al and Ti on the Al sub-lattice of the B2 structure. For FBB8-4Ti (with 4% Ti added), by contrast, the L2 1 precipitates exhibit bcc sub-precipitates rich in Fe and Cr, with a composition close to that of the matrix; the absence of the B2 structure is consistent with an increase of Ti and Fe concentrations, to 18.2 and 19.3 at.% respectively, as measured via APT, in the L2 1 precipitates. [ABSTRACT FROM AUTHOR]

Mechanisms governing thermal stability of nanocrystalline metal alloys have typically been classified as either “kinetic” or “thermodynamic”, although both should generally be expected to manifest at the same time and either act constructively, or compete. The aim of this work is to present a quantitative study of thermal stability in ball-milled Fe-Mg alloys with two concurrent stabilization mechanisms at play—grain boundary segregation and Zener pinning by oxide nanoparticles—to assess their relative potency and to explore the interplay between them. To that end, we studied the alloys' configuration and attendant thermal stability by varying their composition, annealing time, temperature and atmosphere. At the mesoscale, both grain growth and oxide evolution were tracked by in-situ annealing during x-ray diffraction, as well as by electron microscopy and atom probe tomography. At the nanoscale, the local grain boundary chemistry was also probed. We discuss our results, which lie between the expectations from each separate mechanism, in terms of two primary interactions them. These results also suggest a method to control both grain and oxide precipitate size within a unified framework, and thus helps elaborate the design space for nanocrystalline alloys. [ABSTRACT FROM AUTHOR]

Grain boundary engineering of Fe-based austenitic stainless steels and other materials has been successful in producing a large increase in twin and twin related grain boundaries from a wide range of thermomechanical treatments. However, the exact mechanisms and effective grain boundary network descriptors to create the heavily twinned microstructures are yet to be fully understood. In this study, we provide insight into the grain boundary engineering process by examining sequential progression of the same spatial location of a twin related microstructure through thermomechanical processing. The results show that clusters of twin related grain boundaries called twin related domains form during primary recrystallization. The size of the twin related domains increases as the level of strain falls toward the critical strain for recrystallization. Growth of twin related domains during recrystallization results in the formation of twin boundaries behind the migrating grain boundary front. Formation of higher order twin boundaries occurs when two separate grain boundary fronts of the same twin related domain impinge upon each other. We also present relevant microstructural descriptors with emphasis on twin related domain statistics to recrystallization phenomena in grain boundary engineering materials. [ABSTRACT FROM AUTHOR]

The orientation of austenite (γ) reverted from lath martensite has been studied using electron backscatter diffraction (EBSD) in Fe-2%Mn-1.5%Si-0.3%C (mass%) alloy. Two morphologies of γ, acicular and globular, are formed during reversion. Nucleation sites for acicular γ are lath, block and sub-block boundaries, while those for globular γ are prior γ grain boundaries, as well as inside packets. Both acicular and globular γ hold a near Kurdjumov-Sachs (K-S) orientation relationship with at least one of its surrounding tempered martensite blocks. Most acicular γ have almost identical orientations with the prior γ, while only a part of acicular γ have a specific twin-related variant with the prior γ. Furthermore, grain boundary globular γ has almost identical orientations with the prior γ into which it does not grow. The formation of acicular and grain-boundary globular γ was attributed to strong variant selection during γ nucleation at lath, block and sub-block boundaries. On the other hand, orientation relationship analyses among intragranular globular γ, tempered martensite and cementite suggest that the nucleation of γ at cementite and tempered martensite interfaces causes weakening of the variant selection for the formation of intragranular globular γ. [ABSTRACT FROM AUTHOR]

The effects of Si concentration and austenite grain size (AGS) on the tensile properties, stacking fault energy ( γ ), and deformation mode of Fe-18Mn-0.6C (wt.%) steel were investigated to improve the yield strength (YS) of twinning-induced plasticity (TWIP) steel. The 3% Si-added steel revealed the higher YS than previous TWIP steels at the same level of AGS. In particular when the AGS was ∼6.8 μm, its YS reached ∼593 MPa, which is comparable to the YS (613 MPa) of transformation-induced plasticity steel with a tensile strength of 980 MPa. The measured γ of 3% Si-added steel was exponentially decreased with grain coarsening primarily due to the reduction of micro-strain, finally reaching its intrinsic γ ( γ int ) at the AGSs above ∼70 μm. This indicates that the γ int measurement by means of X-ray diffractometry must be performed using coarse-grained specimens with the AGSs above ∼70 μm. Critical resolved shear stresses for twinning ( τ twin ) and ε-martensitic transformation ( τ ε-mart ) were evaluated as a function of AGS in (0-3%) Si-added steels. Whereas the τ twin value was slightly decreased with increasing AGS or Si concentration, the τ ε-mart value was more significantly reduced. This indicates that ε-martensitic transformation precedes mechanical twinning with increasing AGS or Si concentration. As a result, a transition of deformation mode from mechanical twinning to ε-martensitic transformation occurred with grain coarsening in Si-added steels. A critical AGS for the transition of deformation mode was reduced from ∼69 μm to ∼15 μm with increasing Si concentration from 0.5 wt.% to 3.0 wt.%. [ABSTRACT FROM AUTHOR]

The effect of cooling rate after solution treatment on the initial structure of concentrated binary Fe-Cr alloys and the effect of the initial structure on phase separation during subsequent aging has been investigated. The nano-scale compositional fluctuations in the bulk of the alloys are studied using small-angle neutron scattering and the results are compared with simulations using the Cahn-Hilliard-Cook (CHC) model. The alloys investigated represent different mechanisms of phase separation and at higher Cr content, when spinodal decomposition (SD) is favored, the initial Cr compositional fluctuations due to slow cooling after solution treatment reduce the kinetics of phase decomposition, whereas, at lower Cr composition when nucleation and growth is favored, the kinetics of phase decomposition is more rapid. Regardless of the nominal Cr composition of the alloy, the phase decomposition after extended aging up to 300 h at 748 K is always larger for the more non-random initial structure. The CHC modeling of the cooling process and subsequent initial aging (below 10 h) is in reasonable qualitative agreement with the experimental results for the Fe-40 wt.% Cr alloy decomposing via SD. However, the modeling approach must be refined for accurate quantitative modeling of the full SD process, including coarsening. [ABSTRACT FROM AUTHOR]

Kinetics of the austenite (γ) to ferrite (α) transformation and the reverse ferrite (α) to austenite (γ) transformation in a series of Fe-X (X = Ni, Mn and Co) binary alloys has been experimentally and theoretically investigated. A transition from partitioning to partitionless transformation has been predicted to occur during both the γ→α and α→γ transformations by a so called Gibbs Energy Balance (GEB) model, in which the chemical driving force is assumed to be equal to the energy dissipation due to interface friction and diffusion of X inside the migrating interfaces. The transition temperature is found to depend on the kind of X and its concentration, which is in good agreement with experimental results. The intrinsic mobility of the α/γ interface has been derived from the kinetic curves of both the γ→α and α→γ transformations in the investigated alloys, and its value seems to be marginally affected by the transformation direction and alloying elements. [ABSTRACT FROM AUTHOR]

Iron-Nickel alloys are perspective alloys as nuclear energy structural materials because of their good radiation damage tolerance and mechanical properties. Understanding of experimentally observed features such as the effect of Ni content to radiation defects evolution is essential for developing predictive models of radiation. Recently an atomic-scale modelling study has revealed one particular mechanism of Ni effect related to the reduced mobility of clusters of interstitial atoms in Fe-Ni alloys. In this paper we present results of the microsecond-scale molecular dynamics study of point defects, i.e. vacancies and self-interstitial atoms, diffusion in Fe-Ni alloys. It is found that the addition of Ni atoms affects diffusion processes: diffusion of vacancies is enhanced in the presence of Ni, whereas diffusion of interstitials is reduced and these effects increase at high Ni concentration and low temperature. The role of Ni solutes in radiation damage evolution in Fe-Ni ferritic alloys is discussed. [ABSTRACT FROM AUTHOR]

A quantitative multi-phase-field model, with an anti-trapping current and involving diffusion in the solid, is proposed to simulate the peritectic phase transition in Fe-C alloys. An interface field method with a newly defined step function is adopted to formulate the governing equation of the multi-phase-field variables. The proposed model is applied to simulate the γ -platelet tip growth during the peritectic reaction and the subsequent peritectic transformation ( γ -platelet thickness growth) in large computational domains of the experimental length scale. It is found that the local liquid concentration at the triple junction point L / δ / γ is slightly higher than the liquid concentration in equilibrium with the δ -phase, but lower than the liquid concentration in equilibrium with the γ -phase. This leads to slight melting of the δ -phase in the vicinity of the triple junction, while the γ -platelet growth continues. Higher tip velocities at higher undercoolings produce a steeper carbon concentration gradient along the γ -platelet's thickness direction, and thus yield a higher thickness growth velocity. It is also found that the ratio of tip and thickness growth velocity of the γ -platelets increases at higher undercoolings, leading to decreasing tip radius and platelet thickness. Good agreement between the simulations and the experimental data reported in literature is achieved, demonstrating the quantitative prediction capabilities of the proposed model, and confirming the diffusion control for the peritectic phase transition close to equilibrium conditions. [ABSTRACT FROM AUTHOR]

The objective of the present study was to elucidate the complicated interrelationship between necking, post-uniform elongation (e pu ), strain rate sensitivity (SRS), fracture mechanism and Al concentration in Fe-18Mn-0.6C-xAl twinning-induced plasticity steels. Many tensile tests were conducted for in- and ex-situ observations of necking, fracture surfaces, crack propagation and the density and size of micro-voids with the assistance of a high-speed camera and X-ray tomographic equipment. The addition of Al increased e pu , SRS and reduction ratios in dimension of the neck part of tensile specimens, and also changed fracture mode from quasi-cleavage to ductile fracture at the edge part. The quasi-cleavage surface of Al-free specimen was induced by edge and side cracks occurring along grain boundary junctions and twin boundaries within the edges and side surfaces where local deformation bands meet. The ductile-fracture surface of 1.5 %Al-added specimen was formed by the coalescence of micro-voids. While the side-to-middle crack propagation occurred in Al-free and 1 %Al-added specimens due to side cracks, the middle-to-side crack propagation was observed in 1.5 %Al-added specimen. The Al-free specimen had the larger size of the 20 largest voids compared to the 1.5 %Al-added specimen despite its lower void density and local strain due to the accelerated growth of voids near the tips of side cracks. Evaluating the negligible e pu of Al-free specimen by SRS is not deemed to be reasonable due to its inappreciable necking and side cracks. The improvement of e pu in 1.5 %Al-added specimen is primarily due to disappearance of edge and side cracks. [ABSTRACT FROM AUTHOR]

A detailed study of Fe 48 Rh 52 samples heat treated at different temperatures (1123 K, 1273 K and 1423 K) has been performed using scanning and transmission electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction and magnetic measurements. The microstructure of all samples consisted of α′ (B2) and γ (A1) phases. Although the chemical composition, chemical ordering and the lattice parameter of the magnetic α′ phase in all three samples did not vary with heat treatment temperature, the temperature of the metamagnetic transition between the antiferromagnetic (AFM) and ferromagnetic (FM) states varied by 50 K. No gradients in chemical composition on a length scale larger than approximately 10 nm exist at the α′/γ phase boundaries. Due to the 1% volume expansion of the α′ phase, which occurs during the AFM-FM transition, stress fields around the α′/γ phase boundaries are expected. The composition, lattice parameter, size, shape and distribution of the γ phase was shown to change with heat treatment temperature. Finite element models indicated that these microstructural parameters influence the magnitude and extent of the stress fields at the phase boundaries. The variation in the transition temperature is therefore ascribed to the differences in the stress fields. [ABSTRACT FROM AUTHOR]

We present a systematic microstructure oriented mechanical property investigation for a newly developed class of transformation-induced plasticity-assisted dual-phase high-entropy alloys (TRIP-DP-HEAs) with varying grain sizes and phase fractions. The DP-HEAs in both, as-homogenized and recrystallized states consist of a face-centered cubic (FCC) matrix containing a high-density of stacking faults and a laminate hexagonal close-packed (HCP) phase. No elemental segregation was observed in grain interiors or at interfaces even down to near-atomic resolution, as confirmed by energy-dispersive X-ray spectroscopy and atom probe tomography. The strength-ductility combinations of the recrystallized DP-HEAs (Fe 50 Mn 30 Co 10 Cr 10 ) with varying FCC grain sizes and HCP phase fractions prior to deformation are superior to those of the recrystallized equiatomic single-phase Cantor reference HEA (Fe 20 Mn 20 Ni 20 Co 20 Cr 20 ). The multiple deformation micro-mechanisms (including strain-induced transformation from FCC to HCP phase) and dynamic strain partitioning behavior among the two phases are revealed in detail. Both, strength and ductility of the DP-HEAs increase with decreasing the average FCC matrix grain size and increasing the HCP phase fraction prior to loading (in the range of 10–35%) due to the resulting enhanced stability of the FCC matrix. These insights are used to project some future directions for designing advanced TRIP-HEAs through the adjustment of the matrix phase's stability by alloy tuning and grain size effects. [ABSTRACT FROM AUTHOR]

In this first-principles study, we investigate the effect of many kinds of substitutional solute species on screw dislocation motion in bcc-Fe, dominating the strength of dilute Fe alloys. Most of the solute species show a significant interaction with the dislocation core, while only several solute species among them, such as Si, P, and Cu, significantly lower the Peierls potential of the screw dislocation motion. A first-principles interaction energy with the “Easy-core” structure excellently correlates with the change in the γ -surface caused by solute atoms (i.e., chemical misfit). Based on the interaction energy, we predicted the effect of each species on macroscopic critical resolved shear stress (CRSS) of the dilute Fe alloy. The CRSS at low and high temperature for various alloys basically agree with experiment CRSS. These results provide a novel understanding of the interaction between a screw dislocation and solute species from the first-principles. [ABSTRACT FROM AUTHOR]

Fe-Ga alloys exhibit unique functional properties such as magnetostriction that can be varied from the highest positive values among iron-based alloys to negative values including zero magnetostriction, if proper compositions and heat treatments are chosen. This remarkable behavior is related to rather complex phase transformation sequences in this alloy family that are still unresolved. In earlier studies, the phase transformations in Fe-Ga alloys were studied by X-ray diffraction, which provides structural information limited to the near-surface sample area. In this paper, we use electron back-scatter diffraction and in situ neutron diffraction to characterize the D0 3 and L1 2 phases that originate from the fcc and bcc phases in the Fe-27Ga type bulk alloy, respectively. Different ratios between these phases, characterized by magnetostriction values of different signs, were achieved using an isothermal annealing treatment that produces an intrinsic composite in the alloy. Depending on the relative fraction of the D0 3 and L1 2 phases, the magnetostriction values of the alloy, λ S , vary from +100 to −50 ppm, including the value of λ S = 0 for the alloy with L1 2 :D0 3 = 2:1 achieved after 600 min annealing at 400 °C, thus demonstrating the controlled adjustment of magnetostriction in these advanced alloys. [ABSTRACT FROM AUTHOR]

Fundamental insights into the mechanism of breakaway oxidation in Fe9Cr1Mo steel are deduced, through advanced characterisation and modelling. Degradation at 600 °C/∼42 bar CO 2 for ∼20,000 h is emphasised: conditions relevant to components such as the finned superheater tubes used for advanced gas-cooled nuclear reactors. It is shown that such conditions are sufficient to cause carbon saturation of the metallic substrate, as confirmed by direct observation of extensive carbide precipitation but also numerical analysis of the carbon transport. Thus the observation of graphite precipitation close to the scale/metal interface is rationalised. Nonetheless, the activity of carbon at the scale/metal interface does not reach unity – with respect to graphite – at time zero. A modelling method is proposed which accounts for this kinetic retardation of the attack; this can be used to interpolate across the regimes within which breakaway oxidation is prevalent. It is a plausible model for extrapolation to the lower temperatures relevant to service conditions and is suitable for lifetime estimation – so-called ‘remnant life analysis’ – of such safety-critical components when prone to this form of attack. [ABSTRACT FROM AUTHOR]

In the context of dendritic rapid solidification, the goal of this work was to develop a model that describes growth of the stable phase in the presence of a preexisting metastable phase without requiring in-depth knowledge of the local geometry of the growing dendrite or the growth environment. Results facilitate predictive computational materials modeling of the transformation sequence, solidification path and microstructural evolution during industrial casting operations. The model was evaluated using mushy-zone growth velocity data from Fe–Co and Fe–Cr–Ni alloys. The Fe–Co alloys that were tested are hyper-peritectic compositions, while the Fe–Cr–Ni alloys were evaluated as pseudobinary hypoeutectic compositions. The results show that for a given heat flux there will be a minimum undercooling for which dendritic growth can be supported. The predicted growth velocity which corresponds to that minimum undercooling agrees reasonably well with the measured experimental growth velocity data suggesting that the growth of the stable phase into the mushy zone occurs under the minimum conditions required to support dendritic growth. [ABSTRACT FROM AUTHOR]

The thermal stability and the quantification of the different transformation processes involved in the overall crystallization of the Fe 50 Cr 15 Mo 14 C 15 B 6 amorphous alloy were investigated by several characterization techniques. Formation of various metastable and stable phases during the devitrification process in the sequence α-Fe, χ-Cr 6 Fe 18 Mo 5 , M 23 (C,B) 6 , M 7 C 3 , η-Fe 3 Mo 3 C and FeMo 2 B 2 (with M = Fe, Cr, Mo), was observed by in-situ synchrotron high energy X-ray diffraction and in-situ transmission electron microscopy. By combining these techniques with differential scanning calorimetry data, the crystallization states and their temperature range of stability under continuous heating were related with the evolution of the crystallized fraction and the phase sequence as a function of temperature, revealing structural and chemical details of the different transformation mechanisms. [ABSTRACT FROM AUTHOR]

Chromium alloys are being considered for next-generation concentrated solar power applications operating > 800 °C. Cr offers advantages in melting point, cost, and oxidation resistance. However, improvements in mechanical performance are needed. Here, Cr-based body-centred-cubic (bcc) alloys of the type Cr(Fe)-NiAl are investigated, leading to 'bcc-superalloys' comprising a bcc-Cr(Fe) matrix (β) strengthened by ordered-bcc NiAl intermetallic precipitates (β'), with iron additions to tailor the precipitate volume fraction and mechanical properties at high temperatures. Computational design using CALculation of PHAse Diagram (CALPHAD) predicts that Fe increases the solubility of Ni and Al, increasing precipitate volume fraction, which is validated experimentally. Nano-scale, highly-coherent B2-NiAl precipitates with lattice misfit ∼ 0.1% are formed in the Cr(Fe) matrix. The Cr(Fe)-NiAl A2-B2 alloys show remarkably low coarsening rate (∼102 nm3/h at 1000 °C), outperforming ferritic-superalloys, cobalt- and nickel-based superalloys. Low interfacial energies of ∼ 40/20 mJ/m2 at 1000/1200 °C are determined based on the coarsening kinetics. The low coarsening rates are principally attributed to the low solubility of Ni and Al in the Cr matrix. The alloys show high compressive yield strength of ∼320 MPa at 1000 °C. The Fe-modified alloy exhibits resistance to age softening, related to the low coarsening rate as well as the relatively stable Orowan strengthening as a function of precipitate radius. Microstructure tailoring with Fe additions offers a new design route to improve the balance of properties in "Cr-superalloys", accelerating their development as a new class of high-temperature materials. [Display omitted] [ABSTRACT FROM AUTHOR]

High-performance but low-cost structural materials with nanoscale precipitations are vitally important for engineering applications in advanced industry systems. Thus far, the classical nucleation mechanism assumes a critical nucleus of a homogeneous composition up to a sharp boundary with a large energy barrier of nucleation. A fine dispersion of nanoscale precipitates often requires strong solute supersaturations with expensive alloying additions or otherwise deteriorated strength-ductility, weldability, and fabricability. The simultaneous satisfaction of all these mutually exclusive requirements is the longstanding issue for the design of advanced materials for structural applications. In this alloy design, we have adopted a revolutionary approach of ultra-strong iron-based alloys that have resolved all these key issues through non-classical nanoscale precipitations and multi-elemental partitioning. Such an alloy design strategy offers a unique approach to control nanoscale precipitates with low solute supersaturations. Fine precipitations so designed lead to the strong enhancement of both the precipitate-dislocation interactions and work-hardening capacity, resulting with a sharp increase of the yield strength (YS) to as high as 1500 MPa and tensile ductility up to 15%. Furthermore, our design assures outstanding fabricability together with superior post-weld properties through dense nanoscale reprecipitations (∼1024 m −3) in a well economical manner for production. The non-classical actions unfold brand new pathways for precipitation of various intermediate structures and chemistries with low energy barriers, especially useful for the design of sustainable and economical modern alloy systems. (a) The all-in-one ultra-strong iron-based alloy design through careful non-classical nanoscale multi-elemental partitioning and precipitate nucleation controls. Such a strategy turns the embrittling equilibrium precipitates at grain boundaries to fine dispersion of nanoscale multi-component precipitates, especially important in advanced alloy designs with low solute supersaturations. (b) The outstanding mechanical properties of our all-in-one ultra-strong iron-based alloys. (c) Our all-in-one ultra-strong alloys outperform other high-specific strength alloy systems such as Ti-6Al-4 V and Al 7075. A significant cost reduction of our alloys by more than 90% in comparison with the conventional ordered intermetallic strengthened steel alloys, Marage 200 for instance. (d) The superb post-weld mechanical performances with zero strength loss of our alloys due to nano-reprecipitations. [Display omitted] [ABSTRACT FROM AUTHOR]

Grain boundary (GB) segregation can drastically affect both the thermal stability and fracture strength of nanocrystalline metals. Underlying physics of the correlation between the above two aspects, however, remains unclear, since a comprehensive study addressing a variety of properties and involving a wide range of solute types seems not available. In this work, we theoretically modeled the GB segregation and strengthening energies considering both the chemical and elastic contributions of segregated solutes. Then using first-principles method, the GB segregation behaviors of twenty-six 3d, 4d and 5d transition metal atoms at three typically symmetrical tilt GBs (∑3(112), ∑5(210) and ∑5(310)), with iron as representative, were studied, where the GB segregation and strengthening energies of solutes were calculated, thus revealing their dependence on GB structures, group number and solute parameters (such as atomic radius and electronegativity). Arising from the competition between the chemical (related to the electronegativity) and the elastic (related to the atomic radius) contributions from solutes, a concave-down (concave-up) parabolic-like dependency of the GB segregation (strengthening) energy on the group number has been discovered. Furthermore, the stabilizing and the strengthening mechanisms of the segregants were revealed by analyses of bond length and charge density. The strengthening energy generally decreases with increasing the GB segregation energy, as reflected by a so-called trade-off relationship between the strengthening effect and the stabilizing effect. This work could be beneficial for understanding the GB segregation and the strengthening behaviors of the segregants in iron-based alloys and designing the iron-based alloys with comprehensively desired performances. [Display omitted] [ABSTRACT FROM AUTHOR]

We investigate the mobility of screw dislocations decorated by carbon solutes in body-centred cubic iron based on density functional theory (DFT) calculations and transmission electron microscopy (TEM) observations of in-situ straining experiments. We focus on the high-temperature domain, between 500 and 800 K, where plasticity is controlled by the slow glide of decorated screw dislocations with a mobility similar to the Peierls mechanism existing below 300 K, at variance with the athermal regime observed at intermediate temperatures. We propose that due to the strong pinning of reconstructed dislocation lines by carbon solutes, dislocation glide occurs by the formation and migration of kinks, both controlled by the jumps of carbon atoms. We calculate the associated kink-pair nucleation and formation energies as well as the migration energies of isolated kinks and use these DFT values to predict the dislocation velocity as a function of temperature, applied stress and dislocation length. The validity of the mobility law is assessed by the comparison with dislocation velocities measured in TEM observations at different temperatures and local shear stresses. The quantitative agreement between the experimental measurements and the analytical model supports the proposed glide mechanism and the DFT values obtained for kink energies. [Display omitted] [ABSTRACT FROM AUTHOR]

The effects of cold rolling followed by annealing on the mechanical properties and dislocation substructure evolution of undoped and 1.1 at. % carbon-doped Fe 40.4 Ni 11.3 Mn 34.8 Al 7.5 Cr 6 high entropy alloys (HEAs) have been investigated. X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atom probe tomography (APT) were employed to characterize the microstructures. The as-cast HEAs were coarse-grained and single phase f.c.c., whereas the thermo-mechanical treatment caused recrystallization (to fine grain sizes) and precipitation (a B2 phase for the undoped HEA; and a B2 phase, and M 23 C 6 and M 7 C 3 carbides for the C-doped HEA). Carbon, which was found to have segregated to the grain boundaries using APT, retarded recrystallization. The reduction in grain size resulted in a sharp increase in strength, while the precipitation, which produced only a small increase in strength, probably accounted for the small decrease in ductility for both undoped and C-doped HEAs. For both undoped and C-doped HEAs, the smaller grain-sized material initially exhibited higher strain hardening than the coarse-grained material but showed a much lower strain hardening at large tensile strains. Wavy slip in the undoped HEAs and planar slip in C-doped HEAs were found at the early stages of deformation irrespective of grain size. At higher strains, dislocation cell structures formed in the 19 μm grain-sized undoped HEA, while microbands formed in the 23 μm grain-sized C-doped HEA. In contrast, localized dislocation clusters were found in both HEAs at the finest grain sizes (5 μm). The inhibition of grain subdivision by the grain boundaries and precipitates lead to the transformation from regular dislocation configurations consisting of dislocation-cells and microbands to irregular dislocation configurations consisting of localized dislocation clusters, which further account for the decrease in ductility. Investigation of the formation mechanism and strain hardening of dislocation cells and microbands could benefit future structural material design. [ABSTRACT FROM AUTHOR]

The effect of neutron irradiation on tensile deformation of a Fe-9wt%Cr alloy was investigated using in situ high-energy synchrotron X-ray diffraction during room-temperature uniaxial tension tests. New insights into the deformation mechanisms were obtained through the measurements of lattice strain evolution and the analysis of diffraction peak broadening using the modified Williamson-Hall method. Two neutron-irradiated specimens, one irradiated at 300 °C to 0.01 dpa and the other at 450 °C to 0.01 dpa, were tested along with an unirradiated specimen. The macroscopic stress–strain curves of the irradiated specimens showed increased strength, reduced ductility and reduced work-hardening exponent compared to the unirradiated specimen. The evolutions of the lattice strain, the dislocation density and the coherent scattering domain size in the deformation process revealed different roles of the submicroscopic defects in the 300 °C/0.01 dpa specimen and the nanometer-sized dislocation loops in the 450 °C/0.01 dpa specimen; the dislocation loops were more effective in dislocation pinning. While the work hardening rate of stage II was unaffected by irradiation, significant dynamic recovery in stage III in the irradiated specimens led to the early onset of necking without stage IV as observed in the unirradiated specimen. [ABSTRACT FROM AUTHOR]