Boundary Segregation Behavior Research Articles

Grain Boundary Segregation Behavior and Thermodynamic Analysis Based on CALPHAD Method for B Added High Carbon Steel

Grain Boundary Segregation Behavior and Thermodynamic Analysis Based on CALPHAD Method for B Added High Carbon Steel

Exploring solute behavior and texture selection in magnesium alloys at the atomistic level

This study advances our understanding of how chemical binding and solute distribution impact grain boundary segregation behavior and subsequent annealing texture modification in lean Mg-X-Zn alloys (X = RE or Ca). Notably, differences in Ca and Gd solute behavior at grain boundaries were revealed, where Ca exhibited stronger binding to vacancy sites than Gd, resulting in elevated Ca segregation and an RD-TD-type texture. The introduction of Zn showed significant synergistic effects on solute clustering, with Gd-Zn pairs forming more favorably than Ca-Zn pairs, leading to a strong synergy between Zn and Gd. This promoted their co-segregation and high concentration at the grain boundary, generating a unique TD-spread texture. In contrast, weaker binding in Ca-Zn pairs did not affect Ca segregation but influenced Zn segregation, which underscores the importance of solute binding behavior in alloy design concepts. Additionally, the combined atomic-scale experiments and ab initio predictions provide strong evidence that selective texture development in Mg alloys is tied to heterogeneous solute-boundary interactions, where the sensitivity of the binding energy to volumetric strain affects solute segregation at grain boundaries, resulting in varying grain boundary mobilities and specific texture component growth. It also emphasizes that solute behavior in clustering and segregation is influenced not only by atomic size but also by chemical binding strength with vacancies or co-added Zn.

Grain boundary segregation behavior of Mo atoms in nanocrystalline Ni–Mo alloy

In this paper, molecular dynamics method was used to design the grain boundary (GB) segregation structure of solute atom Mo in nanocrystalline nickel, and the effect of the segregation structure on the migration and deformation mechanism of nanocrystalline Ni–Mo alloy boundaries was studied. The results indicate that the addition of solute atom Mo can cause segregation at GBs, and the yield strength and tensile strength of nanocrystalline Ni are significantly increased through solid solution strengthening. Mo atoms segregation result in an increase of the GB thickness and stability of the GB. In addition, by labeling GB atoms and tracking their diffusion trajectories, it was found that after adding Mo atoms, the probability of atomic diffusion at GBs decreased. This indicates that Mo atoms reduce GB energy and improve GB stability. Meanwhile, as the Mo content increases, the degree of atomic disorder increases, and the probability of GB migration decreases. This leads to the inability of grains to merge and inhibit their growth, effectively improving the mechanical properties of the material. As the strain increases, the number and length of dislocations increase, and a large amount of entanglement occurs at GBs. With the increase of Mo content, the number of dislocations decreases sharply, with Shockley dislocations having the highest number. Shockley dislocations interact with other dislocations and hinder their generation and movement, forming a more stable dislocation system structure and increasing the strength of the alloy. Our work focuses on observing the influence of GB segregation structure on the mechanical properties and deformation mechanism of nanocrystalline polycrystalline Ni–Mo alloys, establishing the mechanism of the influence of segregation structure on the stability and coarsening of nanocrystalline metal GBs, examining the influence of segregation structure on dislocation motion and GB migration process during deformation, and proposing positive research and development ideas and theoretical basis for designing nanocrystalline metal materials with excellent performance.

Grain Boundary Segregation Behavior and Thermodynamic Analysis Based on CALPHAD Method for B Added High Carbon Steel

The segregation behaviors on the prior austenite grain boundaries for the B added-high C case hardening steel (Fe-0.82C-0.22Si-0.86Mn-0.02P-1.1Cr-0.21Mo-0.005B (mass％)) were evaluated by three-dimensional atom probe (3DAP). The results revealed the intense segregation of C, Mo, Cr, and B on the grain boundaries. In contrast, it was confirmed that the segregation of P known as a strong segregation element for steel was suppressed. The thermodynamic analysis based on the parallel tangent law by McLean-Hillert was carried out in order to validate the segregation of each element. To evaluate the chemical potentials taking the interaction with multiple elements into account, the CALPHAD (CALculation of PHAse Diagram) method was employed, where liquid phase was adopted to estimate the Gibbs free energy of grain boundaries. The calculation results represented the segregation tendencies obtained from 3DAP. Detailed investigations of the interaction effects of C, B, and Mo on the other elements were also conducted. The results showed the suppression of the P segregation by increasing B content. Therefore, the efficiency of the segregation prediction method that implemented the CALPHAD method for the B added high C steel was demonstrated.

α鉄におけるホウ素，炭素の粒界偏析に及ぼす合金元素の影響

To clarify the effect of alloying elements (M) on the grain boundary segregation behavior of boron (B) and carbon (C) in α-iron, the grain boundary segregation of B, C and alloying elements was evaluated thermodynamically for the Fe–B–1.0 at.%M and the Fe–C–1.0 at.%M ternary systems (M: Al, Ti, V, Cr, Mn, Nb, Mo) using the parallel tangent law proposed by Hillert. In this calculation, the Gibbs energies of the liquid phase in the Fe–B–M and Fe–C–M ternary systems were applied to those of the grain boundaries. According to the calculated results, in the Fe–B–M ternary systems, co-segregation of Ti, V, Mn or Nb with B was predicted, while no co-segregation behavior was confirmed in the case of Al, Cr or Mo addition; in the Fe–C–M ternary systems, co-segregation of Ti, V, Nb or Mo with C was predicted, while no co-segregation behavior was confirmed in the case of Al, Cr or Mn addition. These co-segregation tendencies correspond well with the formation tendencies of metal borides or metal carbides. Although the present calculated results were based on the assumption that substitutional elements can diffuse sufficiently in addition to interstitial elements B and C, we proposed an equation for the parallel tangent law under paraequilibrium condition in which no partitioning of substitutional elements occurs.

Grain boundary segregation behavior in Fe-rich Sm-Co-Fe-Cu-Zr magnets

Grain boundary (GB) deficiencies, including magnetically softer phase and precipitate-free-zones (PFZs), have been recognized as important microstructural origins for the lower-than-ideal energy product of precipitate-hardening Sm-Co-Fe-Cu-Zr magnets with high Fe content. However, the underlying co-aggregations or co-depletions of the interactive compositional variables and their effect on the heterogeneous precipitation near GBs are still unclear, being one of the key issues to optimize the microstructure towards high magnetic performance. Herein, the GB metallurgical segregation behavior during decomposition process of a commercially sintered magnet Sm25Co42.9Fe23.5Cu5.6Zr3.0 (wt.%) was systematically investigated. The concurrently-happened co-aggregations of Sm/Zr/Cu and co-depletions of Co/Fe near GBs lead to further growth and new formation of Zr-stabilized Smn+1Co5n-1-type (n = 2 for 1:3R, n = 3 for 2:7R, n = 4 for 5:19R) sheet-like precipitates, which in turn strongly affect the precipitation behavior surrounding them. The further growth of primary Smn+1Co5n-1 sheet-like precipitates leads to misaligned Sm-rich PFZs (without 1:5H and 1:3R precipitates that are normally formed within grain interiors), while the formation of secondary Smn+1Co5n-1 sheet-like precipitates leads to Sm-lean PFZs, characterized by misaligned regions without 1:5H and 1:3R precipitates or coherent band-like regions without 1:5H precipitates. Further magnetic domain studies and magnetic measurements revealed that the GB segregation-induced Smn+1Co5n-1 soft phases and misaligned PFZs not only reduce the efficient domain wall pinning sites but also weaken the [001] grain texture, being detrimental to both coercivity and remanence. To achieve high magnetic performance, the processing condition should be carefully controlled to balance the competitive contributions between the segregation-induced GB deficiencies and the normal cellular nanostructure within grain interiors.

Grain boundary segregation in Si-doped B-based ceramics and its effect on grain boundary cohesion

Boron carbide and boron suboxide are attractive for their low densities and ultrahigh hardness values, yet they exhibit poor fracture resistance. Strategies to improve mechanical performance aim to control bulk microstructures and properties by incorporating Si-based additives. A related approach to toughen boron carbide is tailoring the phase-like states of grain boundaries by applying the concept of grain boundary complexion engineering. This work directly investigates the potential of grain boundary complexion engineering to improve fracture resistance by systematically evaluating grain boundary segregation behavior of a polycrystalline silicon hexaboride / boron carbide diffusion couple using aberration-corrected scanning transmission electron microscopy and ζ-factor microanalysis. A main result was that all grain boundaries in the diffusion couple exhibited Si segregation that, depending on the bulk Si concentration, approached up to 3 monolayers of coverage, thereby resulting in nanolayer complexions (oftentimes known as intergranular films). Furthermore, upon analyzing 110 distinct grain boundaries throughout the diffusion zone and an impurity boron suboxide region, free energies of Si segregation in boron carbide and boron suboxide were experimentally determined using the Brunauer, Emmett and Teller (BET) multilayer grain boundary segregation theory. Subsequently, changes in grain boundary energy due to Si segregation were quantified and a maximum energy reduction of 51% and 81% were observed in boron carbide and boron suboxide, respectively, which led to decreases in works of adhesion of 18% and 17%, respectively. In summary, this work uncovers grain boundary processing-structure-property relationships that can be used to improve the fracture resistance in icosahedral boron-based ceramics.

First-principles analysis of the grain boundary segregation of transition metal alloying elements in γFe

In this study, first-principles calculations were carried out for the first time to systematically investigate the grain boundary segregation of transition metal alloying elements (Ti, V, Cr, Mn, Co, Ni, Cu, Nb, and Mo) in paramagnetic γFe and its dependence on the grain boundary character. The segregation energies of each element and site were comprehensively calculated for nine [001] symmetric tilt grain boundary models with Σ values from 5 to 41; the paramagnetic state was simulated by the antiferromagnetic double-layer (AFMD). By calculating the effective segregation energy for each grain boundary model from the obtained segregation energies, the grain boundary segregation behavior for each alloying element and its dependence on the grain boundary character were investigated. The segregation energy of transition metal elements is dominated by the Voronoi volume of Fe at the segregation site and arises from the elastic energy derived from the difference in atomic radii between the host and solute metals. Ti, Cu, Nb, and Mo have negative effective segregation energies (indicating a tendency toward segregation) at all investigated grain boundaries, and the absolute values of the effective segregation energy increase in the order of Cu < Mo < Ti < Nb in all of the grain boundary models. In contrast, the sign of the effective segregation energies for V, Ni, and Co changes depending on the grain boundary, and the effective segregation energies for Cr and Mn are positive at all grain boundaries. These results corresponded well with the experimental results. The results obtained in this study provide important basic data for the material design of high-strength steels and are useful for understanding the effects of alloying elements in γFe.

Effect of grain boundary migration on intergranular fracture in the weld metal of steels

ABSTRACT The relationship between grain boundary segregation behaviour of sulphur and intergranular fracture in 10% nickel steel weld metal solidified with austenite single phase was investigated. In the weld metal with sulphur added, sulphur is segregated at the grain boundaries, causing intergranular fracture, and the toughness is significantly deteriorated. The austenite columnar grain boundaries formed during solidification cause grain boundary migration during cooling after solidification, and sulphur segregated at the columnar grain boundaries is diffusely migrated along with the grain boundary migration. Intergranular fracture of the weld metal is not a fracture that occurs at the columnar grain boundaries formed during solidification but is a fracture that occurs at prior austenite grain boundaries, where sulphur is segregated, formed by migration after solidification.

Microstructural origin of the outstanding durability of the high nitrogen bearing steel X30CrMoN15-1

The high nitrogen bearing steel X30CrMoN15-1, marketed under commercial names such as Cronidur30, VC444, NitroMax and N360, is well-known for its outstanding rolling contact fatigue and corrosion properties. More importantly, there are no reports about failure by white etching cracks for this alloy, although this premature failure mode is frequently observed in conventional high carbon bearing steels. To understand the origin of this superior performance, we analyze in the current paper its microstructure prior to rolling contact fatigue by means of scanning electron microscopy, atom probe tomography and nano-indentation, and relate this to its thermal processing history. The origin of the exceptional white etching crack resistance of this alloy is rationalized in terms of the mechanical and thermodynamic stability of the precipitates, the different grain boundary segregation behavior between nitrogen and carbon as indicated by the atom probe results, and the cleanliness of the steel.

First-principles calculations of solute-segreagtion of W-In alloys at grain boundaries

In a tungsten-based alloy system, the appropriate solute elements are selected to produce strong segregation effect to reduce the interfacial formation energy, which can effectively improve the mechanical property and thermal stability of the system. Based on the first principles calculation, the solute segregation model of tungsten-based alloys is constructed. The W-In alloy is taken for example to study the grain boundary segregation behavior and bonding characteristics of solute at different concentrations. The bonding of the W-In system is revealed from the electronic structure, and the variation of the interface stability of the W-In system with the solute concentration is predicted. Based on the electronic structure analysis of bond population, differential charge density and density of states, the bond transition characteristics of solute atoms in the W-In system in the segregation process are found, and the microscopic mechanism of the W-In bond transitioning from the ionic bond inside the grain to the strong covalent bond in the grain boundary region is elucidated: the difference between the grain boundary and the intragranular structure leads to a decrease in the valence state of the W atom in the grain boundary and the oxidizability is weakened, eventually leading to the W-In bond transition. The non-monotonic variation of the intrinsic segregation energy of the solute with the concentration of In in the W-In system is obtained. The mechanism of the influence of solute concentration on the intrinsic segregation energy is revealed by analyzing the bond interaction and energy: the solute concentration remarkably affects the bond strength before and after the W-In bond segregation, resulting in a significant decrease in the segregation ability when the solute concentration is close to 0.0976, and finally the variation of the segregation energy with solute concentration is obtained. Based on the analysis of the phase mechanical stability and the solute segregation in the grain boundary, without considering the vacancy concentration, the optimal solute concentration range and the range that needs to be circumvented in the W-In alloy system with high thermal stability are predicted by the calculations of the model, which are 0.106−0.125 and 0.0632−0.106, respectively. This study provides theoretical basis and quantitative guidance for designing and preparing the tungsten-based alloy materials with high thermal stability.

Effects of neutron irradiation and post-irradiation annealing on the microstructure of HT-UPS stainless steel

Microstructural changes resulted from neutron irradiation and post-irradiation annealing in a high-temperature ultra-fine precipitate strengthened (HT-UPS) stainless steel were characterized using transmission electron microscopy (TEM) and atom probe tomography (APT). Three HT-UPS samples were neutron-irradiated to 3 dpa at 500 °C, and after irradiation, two of them were annealed for 1 h at 600 °C and 700 °C, respectively. Frank dislocation loops were the dominant defect structure in both the as-irradiated and 600 °C post-irradiation-annealed (PIAed) samples, and the loop sizes and densities were similar in these two samples. Unfaulted dislocation loops were observed in the 700 °C PIAed sample, and the loop density was greatly reduced in comparison with that in the as-irradiated sample. Nano-sized MX precipitates were observed under TEM in the 700 °C PIAed sample, but not in the 600 °C PIAed or the as-irradiated samples. The titanium-rich clusters were identified in all three samples using APT. The post-irradiation annealing (PIA) caused the growth of the Ti-rich clusters with a stronger effect at 700 °C than at 600 °C. The irradiation caused elemental segregations at the grain boundary and the grain interior, and the grain boundary segregation behavior is consistent with observations in other irradiated austenitic steels. APT results showed that PIA reduced the magnitude of irradiation induced segregations.

Effects of hot compression on carbide precipitation behavior of Ni—20Cr—18W—1Mo superalloy

The effect of hot compression on the grain boundary segregation and precipitation behavior of M6C carbide in the Ni–20Cr–18W–1Mo superalloy was investigated by thermomechanical simulator, scanning electronic microscope (SEM) and X-ray diffraction (XRD). Results indicate that the amount of M6C carbides obviously increases in the experimental alloy after hot compression. Composition analyses reveal that secondary M6C carbides at grain boundaries are highly enriched in tungsten. Meanwhile, the secondary carbide size of compressive samples is 3–5 μm in 10% deformation degree, while the carbide size of undeformed specimens is less than 1 μm under aging treatment at 900 and 1000 °C. According to the thermodynamic calculation results, the Gibbs free energy of γ-matrix and carbides decreases with increase of the compression temperature, and the W-rich M6C carbide is more stable than Cr-rich M23C6. Compared with the experimental results, it is found that compressive stress accelerates the W segregation rate in grain boundary region, and further rises the rapid growth of W-rich M6C as compared with the undeformed one.

Hydrogen segregation to inclined twin grain boundaries in nickel

Low-mobility twin grain boundaries dominate the microstructure of grain boundary-engineered materials and are critical to understanding their plastic deformation behaviour. The presence of solutes, such as hydrogen, has a profound effect on the thermodynamic stability of the grain boundaries. This work examines the case of a grain boundary at inclinations from . The angle corresponds to the rotation of the (coherent) into the (lateral) twin boundary. To this end, atomistic models of inclined grain boundaries, utilising empirical potentials, are used to elucidate the finite-temperature boundary structure while grand canonical Monte Carlo models are applied to determine the degree of hydrogen segregation. In order to understand the boundary structure and segregation behaviour of hydrogen, the structural unit description of inclined twin grain boundaries is found to provide insight into explaining the observed variation of excess enthalpy and excess hydrogen concentration on inclination angle, but the explanatory power is limited by how the enthalpy of segregation is affected by hydrogen concentration. At higher concentrations, the grain boundaries undergo a defaceting transition. In order to develop a more complete mesoscale model of the interfacial behaviour, an analytical model of boundary energy and hydrogen segregation that relies on modelling the boundary as arrays of discrete disconnections is constructed. Furthermore, the complex interaction of boundary reconstruction and concentration-dependent segregation behaviour exhibited by inclined twin grain boundaries limits the range of applicability of such an analytical model and illustrates the fundamental limitations for a structural unit model description of segregation in lower stacking fault energy materials.

Influence of boron on phosphorus grain boundary segregation behaviour in high-strength interstitial free steels

Boron and phosphorus segregated at grain boundary are investigated by auger electron spectroscopy in high-strength interstitial free steels. The results show that the segregated concentration of phosphorus at grain boundaries reduces greatly with increasing the contents of boron in the high-strength interstitial free steels. The segregation concentration of phosphorus at grain boundary is about 14 at.-% for the boron-free high-strength steel. For the boron-bearing high-strength interstitial free steel, the segregation concentration of phosphorus at grain boundary is about 2–4 at.-%, and the boron concentration at grain boundary is about 5–14 at.-%. The fracture morphology of the steels indicates that almost all grains are intergranular fracture for the boron-free high-strength interstitial free steel. The intergranular fracture grains reduced greatly for the boron-bearing interstitial free steel. The number of low-angle grain boundaries increased with the increasing of boron in the high-strength interstitial free steels.

Segregation mechanism of phosphorus in Ti-stabilized interstitial-free steel

The grain boundary segregation of phosphorus in an interstitial-free (IF) steel aged for different times at 600 and 720°C was investigated using Auger electron spectroscopy. Results revealed that a significant degree of phosphorus segregation occurred during annealing for Ti-stabilized IF steel with 0.045wt% bulk content of phosphorus. In addition, the segregation degree increased with increased holding time. The segregation behavior of phosphorus in IF steel was analyzed based on the theory of grain boundary segregation, along with the present experimental data and previous results. The phosphorus segregation had equilibrium segregation characteristics, and the bulk concentration of phosphorus and holding temperature strongly influenced the final grain boundary segregation concentration of phosphorus in IF steel during annealing. Furthermore, the segregation level of phosphorus substantially decreased with increased annealing temperature or decreased bulk concentration of phosphorus. Results of theoretical calculation were consistent with the experimental ones, indicating that the grain boundary segregation behavior of phosphorus in IF steel during annealing can be qualitatively and quantitatively explained by the theory of equilibrium segregation.

Grain Boundary Segregation Behavior of Boron in Low-Alloy Steel

The boron concentration profiles around prior austenite grain boundaries in Fe-0.05C-0.5Mo-0.001B (mass pct) are examined using aberration-corrected STEM-EELS. In order to obtain the precise distribution of boron around the boundaries, tilt series measurements with thin specimens (<30 nm) are performed and the EEL spectra are analyzed by principal component analysis (PCA) and multivariate curve resolution (MCR). The boron concentration profile changes with the cooling rate from the solid solution temperature. The concentration at grain boundaries is maximized at a medium rate (30 °C/s), where the concentration reaches 8 at. pct, and it decreases at a larger (250 °C/s) or smaller (5 °C/s) rate. On the other hand, the boron distribution becomes wider as the cooling rate becomes smaller. The current results suggest that the boron segregation in the alloy is formed by the “non-equilibrium segregation mechanism.”

Grain Boundary Fracture Behavior and Segregation of P under Low Tensile Stresses in Steel 2.25Cr1Mo and 12Cr1MoV

Experimental investigation about the grain boundary fracture behavior and segregation behavior of phosphorus under low tensile stresses in steel 2.25Cr1Mo and 12Cr1MoV was carried out in this paper. AES (Auger electron spectroscopy) experiments and dynamic analyses on the non-equilibrium grain-boundary segregation (NGS) of phosphorus and the SEM photos of grain boundary fracture in Auger specimens of both steels were obtained. The variation of phosphorus segregation level in grain boundary at different aging time was studied. Results show that the non-equilibrium segregation of phosphorus occurred at the grain boundaries in the two steels while subjected to a tensile stress of 30MPa and held at 500°C. The corresponding critical time was about 1 hour for steel 2.25Cr1Mo and 3.5hour for steel 12Cr1MoV respectively. SEM photos of grain boundary fracture in Auger specimens of the test steels show that the grain boundary fracture rate increased with increasing concentration of phosphorus, and that the fracture toughness of steel 12Cr1MoV is much lower than the one of steel 2.25Cr1Mo.

Direct correlations between fracture toughness and grain boundary segregation behavior in ytterbium-doped magnesium aluminate spinel

The effect of ytterbium (Yb) on magnesium aluminate spinel (MgAl2O4) grain boundary structure and strength has been evaluated using mechanical testing and electron microscopy. Bicrystal specimens were prepared by hot pressing. Certain interfaces were doped with Yb prior to bonding. Interfaces were characterized using atomic-resolution high-angle annular dark-field imaging. Interface fracture toughness was determined using a microscale cantilever beam test. A 30% increase in fracture toughness was found to correlate with grain boundary Yb segregation.

Effect of Carbon and Nitrogen on the Hall-Petch Coefficient of Ferritic Iron (Review on the Hall-Petch Relation in Ferritic Iron)

Grain size dependence of yield strength was reviewed for polycrystalline ferritic iron and low carbon steel. Yielding of polycrystalline low carbon steels was characterized by a clear yield point (upper yield point) and such a yielding behavior is taken over to ultra fine grained steel with the grain size below 1m. Yield strength (y) of polycrystalline low carbon steels obeys the Hall-Petch relation: y[MPa]=+600×d[m]-1/2 . The Hall-Petch coefficient ky is around 600 MPa･m1/2 for the commercial low carbon steels but it is lowered to about 100 MPa･m1/2 for interstitial fee steel. Besides, it is known in industrial pure iron (Fe-30ppmC) that ky increases with aging at 363K. The value of ky is also increases with increasing the amount of solute carbon content. The ky is enlarged from 100 MPa･m1/2 to 550 MPa･m1/2 by adding 60ppm of solute carbon and then levels off at around 600 MPa･m1/2 in the carbon concentration region above 60ppm. On the other hand, nitrogen hardly influences the ky value. Difference between C and N in the contribution to ky is probably due to the difference in grain boundary segregation behavior. Macroscopic yielding of polycrystalline ferritic iron is reasonably explained by the Hall-Petch model considering dislocation pile-up against grain boundary and dislocation emission from the grain boundary where stress concentration has been generated by piled up dislocations. It is seemed that the segregated carbon stabilized the dislocation emission site at grain boundary and this leads to the increase in ky.