Grain Boundary Regions Research Articles

Revealing Local Grain Boundary Chemistry and Correlating it with Local Mass Transport in Mixed-Conducting Perovskite Electrodes.

Grain boundary (GB) mass transport, and chemistry exert a pronounced influence on both the performance and stability of electrodes for solid oxide electrochemical cells. Lanthanum strontium cobalt ferrite (LSCF6428) is applied as a model mixed ionic and electronic conducting (MIEC) perovskite oxide. The cation-vacancy distribution at the GBs is studied at both single and multi-grain scales using high-resolution characterization techniques and computational approaches. The accumulation of oxygen vacancies ( ) in the GB region, rather than necessarily at the GB core, results in an enhancement of the oxygen diffusivity by 3 - 4 orders of magnitude along the GBs (Dgb). At 350°C, the oxygen tracer diffusion coefficient (D*) is measured as 2.5 × 10-14 cm2s-1. The Dgb is determined to be 2.8 × 10-10 cm2s-1 assuming a crystallographic GB width (δcrystal) of 1nm, and 2.5 × 10-11 cm2s-1 using a chemically measured δchem of 11.10nm by atom probe tomography (APT). The origin of the concomitant changes in the cation composition is also investigate. In addition to the host cations, strong Na segregation is detected at all the GBs examined. Despite the low (ppm) level of this impurity, its presence can affect the space charge potential (Φ0). This, in turn, will influence the evolution of GB chemistry.

Ordering-facilitated lower hydrogen embrittlement sensitivity in a prototype high-entropy alloy

The ageing treatment (450 °C/60 h) led to the spinodal decomposition at grain boundaries (GBs) and the development of chemical short-range orders (CSROs) in the alloy matrix, increasing the strength and effectively reducing the HE sensitivity of the equiatomic FeMnNiCoCr alloy. Firstly, the emergence of NiMn- and Cr-rich orders at GB regions reduces hydrogen diffusion rates along GBs and limits hydrogen enrichment at these sites. Secondly, CSROs promote hydrogen accumulation within the grain interior and impede hydrogen penetration, resulting in a shallower hydrogen-affected depth in the aged sample. Moreover, the modulation of hydrogen distribution by microstructure initiates transgranular cracking in the aged sample.

Effects of ZnO additive on the electrical and densification properties of A-site deficient barium cerate perovskite

The dual strategies of A-site deficiency and addition of ZnO sintering aid have been employed to optimise stability and densify the high-temperature proton conductor Ba0.95Ce0.8Y0.2O3-δ at 1200 °C (B95CYZn-1200). Structural analysis performed by Rietveld refinement based on X-ray diffraction data indicated perovskite phase with monoclinic symmetry (space group, I2/m). Raman spectroscopy revealed the presence of ZnO for material sintered at 1200 °C, which most likely partially resides in grain-boundary regions, leading to enhanced electrical transport observed by impedance spectroscopy at low temperature. Anomalous and reproducible frequency-dependent impedance behaviour below 500 °C, observed in various atmospheres, is suggested to be attributable to the varistor-type properties of ZnO. The electrical conductivity of B95CYZn-1200 in the range 500–900 °C is typical of perovskite proton conductors but with greater conductivity than both Ba-stoichiometric BaCe0.8Y0.2O3-δ and ZnO-free Ba0.95Ce0.8Y0.2O3-δ in wet O2 and N2, reaching ∼ 1.1 S m−1 at 550 °C in wet O2. Resistance to carbonate formation and mechanical breakdown was demonstrated via prolonged impedance measurements in CO2- and H2-containing atmospheres in the range 600–700 °C.

Structural evolution and mechanical response of multiple Al Σ5(210) grain boundaries with segregation of solute atoms: First-principles study

Grain boundary (GB) segregation of solute atoms is a key factor affecting the microstructure and macroscopic properties of materials. In this study, first-principles calculations were carried out to investigate the effect of solute atoms (Mg and Cu) segregation on the structural evolution and mechanical response of Al ground-state Σ5(210) GB (GB-Ⅰ) and metastable Σ5(210) GBs (GB-Ⅱ and GB-Ⅲ). The GB energy, segregation energy, and the theoretical tensile strength of the multiple Σ5(210) GBs were calculated. The results show that both Mg and Cu atoms tend to segregate in the boundary plane, thus reducing GB energy and improving GB stability. The segregation of Cu atoms reduced the GB energy more significantly than that of Mg atoms. The segregation of solute atoms can change the symmetric GB structure into an asymmetric one or induce GB phase transformation. The theoretical strength of GB-I is weaker than that of the metastable GB-II with higher GB energy, suggesting that grain boundaries with higher energy are not necessarily unstable. In addition, the effect of solute atom segregation on the GB strength depends not only on the type of element but also on the specific GB structure. The weakening effect of Mg segregation in GB-I and GB-II is due to the weak MgAl bond which enlarges the low charge density region of GB and increases the free volume of GB. The strengthening of GB-I and GB-II by Cu segregation mainly depends on the stronger AlCu bond than AlAl bond along the GB fracture path. The enhanced strength of GB-III with Mg and Cu segregation is attributed to the GB structural phase transformation caused by the segregation of solute atoms. The calculation results further elucidate the relationship between element segregation and grain boundary characteristics.

Understanding the influence of boron in additively manufactured GammaPrint®-700 CoNi-based superalloy

Boron is commonly added to superalloys in small amounts to enhance creep resistance, but can lead to cracking at high concentrations, especially during the additive manufacturing process. Two variants of CoNi-based GammaPrint®-700 superalloy with different B contents (0.08 at% vs 0.16 at%) were printed via laser powder bed fusion (LPBF) with the same printing parameters, with only the high B alloy exhibiting solidification cracking. Atom probe tomography (APT) revealed stronger segregation behaviors in the high B alloy compared to the low B alloy at both the inter-dendritic regions and grain boundaries (GBs). The segregation behavior at inter-dendritic regions was well captured with Scheil simulation and can correlate with the existing cracking susceptibility index (CSI) on cracking tendencies, although high angle GBs are where cracking occurs according to electron backscatter diffraction (EBSD) measurements. Additionally, the extent of GB segregation was compared between the high B and low B alloy. Higher B additions led to significantly more GB B segregation in the high B alloy compared to the low B alloy. For the high B alloy, the cracked region of one GB exhibited higher levels of B compared to the uncracked region of the same GB. However, much higher B contents were also found in two other uncracked GBs in the high B alloy, which demonstrates that higher GB B concentrations are not fully responsible for the cracking. A much larger variance in GB B segregation content was found in the high B alloy compared to the low B alloy. These phenomena were explained with a solidification model with the GB segregation content expressed explicitly by a modified Langmuir-McLean equation. This model linked the GB segregation content with solidification undercooling, which can be used as quantitative cracking criteria for future builds.

Highly reliable and large-scale simulations of promising argyrodite solid-state electrolytes using a machine-learned moment tensor potential

The high ionic conductivity of argyrodite makes it an attractive candidate for solid-state electrolytes (SSEs) in all-solid-state Li-ion batteries (ASSBs). Although great effort has been devoted to using ab initio molecular dynamics (AIMD) to evaluate ionic conductivity and elucidate the Li-ion diffusion mechanism of argyrodite-based SSEs, limitations in system size, simulation temperatures, and time associated with AIMD make accurate predictions and analysis of Li-ion diffusion challenging. Here, we present a reliable, large-scale computational approach to realistic simulation of SSEs in the bulk and at the grain boundary (GB) based on moment tensor potentials (MTPs) trained at the van der Waals optB88 level of theory. MTPs enable sufficiently large-scale and long-time simulations that reflect all possible configurational disorder of experimental crystal structures and provide accurate ionic conductivities that are close to values measured experimentally in halogenated Li-argyrodite (Li6PS5X [X = Cl, Br, I]). Our simulations show that the vibrational motion of a PS4 polyhedron has a positive effect on ionic conductivity. We also developed an accurate MTP using an active-learning approach to exploring Li-ion diffusion at the GB in polycrystalline SSEs. Simulations of the molecular dynamics of large ∑5100021 (>10,000-atom) GB models reveal that Li-ion accumulation around the GB region retards ionic conductivity and extends into an interior region approximately 20 Å from the GB interface. This work provides a practical approach to realistic large-scale and interfacial GB simulations that are otherwise inaccessible through ab initio calculations by developing accurate machine-learned MTPs.

The distribution and segregation behaviors of helium in tungsten Σ5(310)/[001] grain boundary region: A first-principles study

In this work, the distribution and segregation behaviors of helium (He) and the effect of yttrium (Y) on these behaviors in the symmetrical tilt tungsten (W) Σ5(310)/[001] grain boundary (GB) region were studied using first-principles calculations. The results revealed that the GB has a significant impact on the behaviors of He in W. The solution and segregation energies of He in the W Σ5(310)/[001] GB region increase with increasing the distance from He to the GB and are inversely proportional to the effective electrons of He. The density of states analysis showed that the GB can suppress partial hybridization between He and W atoms. In addition, we find that the strengthening element Y facilitates the dissolution of He in the W GB region.

Study on Ce2Fe14B-based sintered magnets with Ce/RE> 80%

Up to now, the development of Ce/RE> 80% sintered permanent magnets still faces a challenge, owing to their seriously deteriorated microstructure. In this article, the Ce25.5∼31.5Nd0∼5Febal.B1.25M1.15 (wt.%) sintered magnets with Ce/RE > 80% were investigated, the roles of minor Nd substitution were explored. The typical island-like phase common existed in the grain-boundary (GB) regions of Ce2Fe14B-based SC alloys (which destroys the continuity of RE-rich GB phase), was confirmed as a tetragonal B-rich phase (fct-RE5Fe18B18), and probably generated by the peritectic reaction of “L + Ce2Fe14B → CeFe2 + B-rich” at 797 °C. It was found that: the deteriorated microstructures of high Ce sintered magnets were hardly improved (Nd element was not enriched in the main phase as it was expected), and the coercivity increments were far below expectations by directly adding 3–5% Nd in alloy designs. However, the GB phase distribution was more uniform and continuous, Nd-rich shells formed, and magnetic properties were remarkably promoted by blending 2% NdHx powders into the Ce27.5Nd3Febal.B1.25M1.15 (Ce27.5Nd3) magnet during JM milling, compared with the Ce25.5Nd5Febal.B1.25M1.15 (Ce25.5Nd5) magnet with similar composition. The good comprehensive magnetic properties of Hcj = 1.714 kOe, Br = 9.395 kG, and (BH)max = 11.16 MGOe have been reached in the Ce27.5Nd3+2% (NdHx) magnet (Ce accounted for 84.5 wt% of total rare earth). The present work deepens our understanding of metallurgical behavior, process/microstructure designing for Ce/RE> 80% sintered magnets, and shed a light on the large-scale utilization of Ce element in a permanent magnet.

Revealing the deformation mechanisms of 〈110〉 symmetric tilt grain boundaries in CoCrNi medium-entropy alloy

The synergy of multiple deformation mechanisms responsible for the excellent mechanical properties attracts an increasing interest in CoCrNi medium entropy alloy (MEA). In this study, we employed molecular dynamics simulations to investigate the chemical properties of grain boundaries (GBs) and their influence on deformation mechanisms in CrCoNi MEA, with particular emphasis on role of lattice distortion and short-range order (SRO) in dislocation plasticity. After aging, we found Ni element segregate at GB through short-range diffusion driven by a more negative segregation enthalpy. The degree of SRO within the GB region is significantly correlated with the Ni concentration, and it markedly influences the structure and local stress distribution of the GB. Compared to the lattice distortion, SRO can effectively suppress the GB dislocations nucleation and slip, thereby increasing yield strength of the material. During the plastic stage, although the deformation microstructure is intimately linked to the active slip systems and grain orientation, the presence of SRO, as well as the resultant rugged dislocation pathways and increased slip resistance lowers the propensity for stacking faults and phase transformation. The current findings can enhance a fundamental comprehension of diverse deformation mechanisms in CoCrNi MEA and suggest that the chemical characteristics of GB could serve as a pivotal approach for modifying the mechanical properties of MEAs.

Multiplicity of grain boundary structures and related energy variations

Ideal minimum-energy grain boundary (GB) structures are usually the only ones considered when characterizing GBs. This limited perspective provides an incomplete view of the possible structural variability of GBs. In the present study, phase field crystal (PFC) simulations are employed in a systematic search of GB states based on γ-surface sampling, demonstrating PFC to be a capable tool for this purpose. It is also shown that a set of microscopic degrees of freedom (DOF) must be considered in addition to the GB’s five macroscopic DOF when identifying variants in GB structure. This set of microscopic DOF comprises three components of relative crystal translation, plus one DOF describing the atomic density in the GB region. GB energy is taken as an example of a key property which is shown to exhibit a considerable spread due to variations in these microscopic DOF, at constant macroscopic DOF. The findings underline the need to consider a spectrum of GB energies for each macroscopic GB configuration, rather than a single value corresponding to an idealized minimum-energy structure. As part of the study, some computationally efficient strategies for setting up the PFC simulation model are also proposed.

Localized engineering of grain boundary morphology by electro-nano-pulsing processing

We report a novel electro-nano-pulsing (ENP) processing method to achieve localized engineering of grain boundary (GB) morphology in polycrystalline metallic materials. ENP is extraordinarily capable of generating intense nanopulse electric current with a current density greater than a few to several hundreds of 1010A/m2 and a pulse duration on the order of a few 100ns. Such a level of current density is ∼3–5 magnitudes higher than that is usually achieved during the Spark Plasma Sintering process. Using the Nichrome-80 superalloy as a model material, we observed a variety of GB roughening phenomena at multiple length scales, resulting in the generation of diverse forms of atomistic facets, nanoscale serrations, and nanoscale step-like GB morphologies after the ENP processing. We think that the excessive GB heat localization and electron wind force or stress are the main factors contributing to the GB morphological changes during the ENP processing. The ENP processing provides a new unique grain boundary engineering strategy to manipulate the GBs with the changes localized at the GB region, without altering its adjacent grains.

Microstructure, thermophysical properties and neutron shielding properties of Gd/316 L composites for spent nuclear fuel transportation and storage

In this paper, (0.5–5.0 wt%)Gd/316 L composites were prepared by vacuum arc melting, and the microstructure, thermophysical properties, and neutron shielding properties were analyzed deeply. The results showed that the Gd/316 L composites started solidification in primary ferrite mode and terminated solidification by a peritectic-type reaction forming (Fe, Ni, Cr)3Gd that is rich in Ni and Gd. In the peritectic-type reaction, local convection in the Ni-rich and Gd-rich peritectic liquid phase leads to the formation of multiple variants of (Fe, Ni, Cr)3Gd. The higher the Gd content, the greater the amount of (Fe, Ni, Cr)3Gd and ferrite, and the harder it is to keep the austenitic matrix of 316 L stainless steel. The solidification temperature range of Gd/316 L composites is 1074–1460 °C, which severely limits the ability to further process it by high-temperature deformation techniques such as hot rolling or hot forging. (Fe, Ni, Cr)3Gd preferentially nucleates and grows in the triangular region of austenite grain boundaries at 1074 °C. At 50–500 °C, with increasing Gd content, thermal expansion and thermal diffusion coefficients become decrease, and thermal conductivity and specific heat capacity increase. The trend of thermophysical property changes is favorable for SNF transportation and storage applications. The thermal neutron shielding properties of 1.0 wt%Gd/316 L composite is comparable to that of 4.45 wt% B content boron steel and 15 wt%B4C/Al composite. The Gd content in Gd/316 L composites material should not exceed 2.5 wt%. If the Gd content continues to increase, it is of little significance to improving shielding properties and material thinning. A shielding thickness of 3 mm is more appropriate for Gd/316 L composites. Too thin would not provide sufficient mechanical properties and increase processing and manufacturing costs.

Mobile dislocation mediated Hall-Petch and inverse Hall-Petch behaviors in nanocrystalline Al-doped boron carbide

The Hall-Petch and inverse Hall-Petch behaviors in nanocrystalline ceramics are not well understood because dislocation activities, which are important in metals, are usually limited in these materials. In this study, we use molecular dynamics simulations with a machine-learning force field to investigate the shear deformation behaviors of nanocrystalline Al-doped boron carbide (n-B12-CAlC) as the grain size varies. We find a transition from the Hall-Petch to inverse Hall-Petch behavior in n-B12-CAlC when the grain size reaches its critical value of 6.09 nm, with a maximum shear strength of 14.93 GPa. Interestingly, mobile dislocation nucleated from grain boundaries (GBs) is activated in n-B12-CAlC due to the breakage of weakened C-Al chain bonds, which plays a significant role in its Hall-Petch behaviors. As the grain size decreases, the increasing GB regions homogenize the shear stress, which suppresses dislocation nucleation from GBs and strengthens the material, as observed in the conventional Hall-Petch relationship. However, with a further decrease in the grain size (< ∼ 6 nm), the increasing GB regions provide more favorable sites and a higher possibility for dislocation nucleation, reducing the shear strength and triggering a transition into the inverse Hall-Petch behavior. These findings shed light on deformation behaviors and mechanisms of nanocrystalline ceramics and provide an explanation for the transition from Hall-Petch to inverse Hall-Petch behaviors.

A hydrogen diffusion model considering grain boundary characters based on crystal plasticity framework

The influence of grain boundaries (GBs) on hydrogen transportation is a crucial factor in understanding the mechanisms of hydrogen embrittlement (HE) in polycrystalline alloys. Crystal plasticity models have the advantages in simulating the effect of grain microstructure on hydrogen diffusion in polycrystals, but they have not yet been coupled with hydrogen diffusion models that can take into account GB characters. In this study, a hydrogen diffusion model that considers the effect of GBs on the transportation of hydrogen was developed to study the local hydrogen diffusion behavior in bi-crystalline and polycrystalline alloys. The simulation results showed the capability of the model in describing the influence of hydrogen on the material deformation. In addition, this study found that the hydrostatic stress determined the hydrogen concentration in the far-GB region, whereas the GB energy played a control role in and near the GB regions. Notably, hydrogen exhibits a preference for localization near high-angle grain boundaries (HAGBs) and triple junctions. In alloys with medium grain size, hydrogen typically had low average concentration with a more uniform distribution. Furthermore, the hydrogen-assisted crack growth mechanism was analyzed as the continuous coalescence of primary and secondary cracks along GBs. This work presents a hydrogen diffusion model that considers GB effects, and the proposed model has advantages in predicting the deformation and fracture of polycrystalline alloys in hydrogen environments.

Screening and manipulation by segregation of dopants in grain boundary of Silicon carbide: First-principles calculations

The effect of segregation behavior of non-metallic dopants (H/He/O) and metallic dopants (Be/Al/Mg/Y) on the performance of grain boundary (GB) in SiC has been systematically investigated by first-principles calculations. Firstly, the GB energy and excess volume of different GBs have been studied to evaluate the stability of GB and the capacity to accommodate dopant atoms. The solution energies of dopant atoms greatly reduce in the GB region compared with those in the bulk, which makes the dopant atoms inside the grain tend to segregate and aggregate near the GB. The driving force of GB on dopant segregation generally decreases with the increase of distance from GB plane, and the preferential site of dopant is closely correlated with the atomic size of dopant. In addition, H and Y atom possesses the lowest segregation energy at the interstitial and substitutional site near the GB, respectively. Next, the segregation of single dopant induced the changes in the strength and stability of GB have been explored. It is found that non-metallic dopants have the significant embrittlement effects on GB strength. However, the segregation of most metallic dopants could strengthen the GB and Mg atom has the most significant strengthening effect on the GB. The stability of GB can be greatly improved by segregation of Al and Y dopants. Besides, the aggregation of H atoms has the obvious embrittlement effect on the GB. Furthermore, the co-segregation behavior of different dopants has also been explored. Be and Mg dopants have the most significant inhibition effect on the segregation of detrimental impurities H/He/O due to the repulsive interaction between dopant atoms. The present results provide a new insight into the effect of dopant segregation on GB properties and are expected to be a useful guidance for screening the chemical composition and manipulating the performance of SiC-based ceramics.

Mixing enthalpy near grain boundaries in Fe-Cr alloy: The results of atomistic simulation

Energy characteristics of symmetric tilt (twin) grain boundaries (GBs) Σ5(210) and Σ5(310) with the tilt axis [001] and boundaries Σ3(111) and Σ3(112) with the tilt axis [11¯0] in a bcc random binary FeCr alloy have been studied using atomistic simulation. The simulation has been carried out based on DFT calculations and two-band semi-empirical potential. We have proposed the formula for calculating the mixing enthalpy near GBs. The relationship between the mixing enthalpy and the energy of the boundary in the alloy has been revealed. Both simulation methods have shown a decrease in the mixing enthalpy near a GB compared to the ideal crystal. Moreover, DFT has predicted a stronger decrease in the mixing enthalpy compared to estimates based on semi-empirical interaction. Based on the obtained dependences of the mixing enthalpy on the alloy composition, phase diagrams for the considered GB regions have been constructed. It has been shown that the GB region is characterized by a higher equilibrium concentration of chromium, and the precipitates of the α'-phase have a lower concentration of chromium compared to the bulk. The smallest difference in equilibrium compositions has been observed for the Σ5(310) and Σ3(112) boundaries, and the largest one for the Σ3(111) boundary, which has the highest energy among considered GBs.

Effect of strain rate on dynamic recrystallization mechanism of Mg-Gd-Y-Sm-Zr alloy during hot compression

Strain rate plays an important role in the hot deformation process of materials. In this work, hot compression experiments were conducted on Mg-Gd-Y-Sm-Zr alloy at the strain rates of 0.001–1 s−1 by Gleeble thermal simulation machine. The deformation behavior and microstructure evolution were studied in detail, and the dynamic recrystallization (DRX) mechanism was discussed. The results show that the flow stress of the alloy increases with the increasing strain rate. Deformation bands (DBs) are formed inside some grains at the strain rates of 0.1–1 s−1, which increase interfaces in the microstructure and hinder the dislocation movement. DRX grains are formed at the regions of DBs and original high angle grain boundaries (HAGBs). The DRX mechanism at high strain rates includes DBs induced DRX and discontinuous dynamic recrystallization (DDRX). At low strain rates and small strain, the dislocations first accumulate at the original HAGBs, and DDRX occurs in the alloy. The subgrains with low angle grain boundaries (LAGBs) are formed inside the initial grains through sufficient dynamic recovery (DRV) as strain increases, and LAGBs will gradually transform into HAGBs with the increasing misorientation angle. The DRX mechanism shows the synergistic effect of DDRX and continuous dynamic recrystallization (CDRX) at low strain rates.

Influence of lattice vibrations and phonon interactions on the ion transport properties of grain boundary tailored Li1.3Al0.3Ti1.7(PO4)3 solid-state electrolyte ceramics

Lithium ions shuttle between electrodes through the ceramic solid electrolyte across the boundary regions in a solid-state Li-ion battery. This work demonstrates how phonon vibrations get altered by sintering conditions, and grain boundaries (GBs) could be useful in enhancing the ionic conductivity of solid electrolytes. GB engineered Li1.3Al0.3Ti1.7(PO4)3 (LATP) ceramics are prepared using a sol-gel process and performed sintering under different conditions, viz., spark plasma sintering (SPS) and conventional isothermal sintering (CIS). The former exhibits GB regions with amorphous characteristics, whereas the latter shows a sharp boundary between crystalline grains. LATP-SPS ceramic shows two orders of higher ionic conductivity (σ = 1.02 × 10−5 S/cm at 300 K and 100 Hz) than LATP-CIS. We investigate the interrelation between lattice vibration and lithium-ion migration by monitoring the changes in vibrational mode characteristics of LATP ceramics through temperature-dependent Raman spectroscopy. Raman modes of LATP-SPS exhibit a higher Raman shift (∼2 cm−1 at 123 K) due to increased defects, preferentially from grain boundary regions, compared to the LATP-CIS pellet. Most of the vibrational modes undergo a red shift (∼10 cm−1) with increasing temperature, except for the O–P–O bending mode [A1g(3)], which exhibits a blue shift (∼3 cm−1). These observations correlate with interstitial ionic migration in the LATP framework. Force constant of the observed Raman modes suggests that lithium-ion migration is assisted significantly by dynamic structural changes of the (PO4)3− sublattice. Anharmonicities observed from temperature-dependent changes in Raman profiles are explained using three-phonon and four-phonon scattering processes, which lower the migration barrier and, hence, contribute to higher ionic conductivity.

Unrevealing grain boundary mobility in the precipitate hardening high entropy alloys

Exploring the interactions between precipitates and grain boundaries (GBs) is of great important to tailor the high-temperature stability and mechanical property of precipitate hardened alloys. However, the influence of precipitate on GB stability in high entropy alloys (HEAs) is still uncovered at nanoscale. In this work, we investigate the effects of precipitate size and temperature on grain boundary migration (GBM) using molecular dynamic simulations. The results show that the GB stability is improved with the increase of the precipitate size, especially at low temperatures. Compared with the traditional dilute alloys, the strong atomic lattice distortion exits in HEAs, resulting in the localized high tensile/compressive stresses. Thus, GB bends towards the indeterminate direction due to the lattice distortion, different with the case bending towards precipitate in the dilute alloys. In addition, the lattice distortion makes dislocation nucleate at random GB region in HEA, which is different from that in dilute alloys. High temperature reduces lattice distortion and relieves the localized high stresses, thereby inhibits the nucleation rate of dislocations from GB. These results provide the fundamental messages for designing novel HEAs with excellent thermal stability.

Grain boundary- and triple junction-induced martensitic transformations: A phase-field study of effects of grain boundary width and energy

An original thermodynamically consistent large strains-based general multiphase phase-field (PF) approach of Ginzburg–Landau type is developed for studying the grain boundary (GB)- and triple junction (TJ)-induced martensitic transformations (MTs) in polycrystalline materials at the nanoscale considering the structural stresses within the interfaces. In this general PF approach, N+1 order parameters are used for describing the austenite (A)↔martensite (M) transformations and N(>1) martensitic variants and other M order parameters are used to describe M(>1) grains in the polycrystalline samples. Neglecting the kinetics of the GBs and TJs and assuming a uniform temperature of the sample, the system of coupled mechanics and Ginzburg–Landau equations for the order parameters related to MTs is derived. Distinct energies for the GBs in austenitic and martensitic phases due to their structural rearrangement are considered by using variable energy for the GB regions as a function of the order parameter related to the A↔M transformation and misorientations. The strong effects of the austenitic GB width (size-effect), the difference of GB energies of two phases, and applied strains on heterogeneous nucleation of the phases and their subsequent growth are explored in bicrystals with a symmetric planar tilt GB during the forward and reverse transformations. A rich plot for the temperatures of transformations between A, premartensite (the intermediate states between A and M), and M in a bicrystal is plotted for varying austenitic GB width exhibiting a strong GB size effect. The TJ energy and the energy and width of the adjacent GBs are also shown to significantly influence the nucleation and microstructures using the tricrystals having three symmetric planar tilt GBs meeting at 120° dihedral angles. The elastic and structural stresses across the interfaces are plotted, which is essential for understanding the role of GBs and TJs in material failure. The plausible reasons for the nonintuitive behaviour of the GBs on martensite nucleation observed in experiments and atomistic studies are explained.