Boundary Segregation Research Articles

The Cause of Intergranular (IG) Fracture by Thermal Embrittlement in SA508 of Reactor Pressure Vessel (RPV) Steel

Intergranular(IG) fracture due to thermal treatment has been reported in a reactor pressure vessel(RPV) steel of Russian light water reactor in last decade. This is attributed to grain boundary segregation of phosphorus (P) or precipitation of carbide, etc.. This is a finding a difference in microstructure before and after IG cracking; this cannot explain the cause of the IG embrittlement. This old paradigm follows only correlation. Recently, a mechanism in which IG embrittlement occurs due to a decrease in entropy of a material has been reported at a temperature where atomic diffusion is possible. It is anticipated that new paradigm can explain the IG embrittlement of RPV based on a causal relationship. Thus, the thermal treatment at 350-420 oC was applied to RPV steel of SA508 and IG cracking was confirmed. DSC analysis was applied to confirm whether a decrease in entropy due to a short range ordering reaction occurs in SA508. It was possible to quantify the entropy change(⊿S= Q/T) through DSC measurement. A lattice changes due to thermal treatment were confirmed using XRD analysis in aged specimens. The results showed that lattice contraction by aging causes a reduction of fracture toughness. The internal stress formed inside the material due to entropy reduction can be calculated by multiplying the exothermic energy per unit mass by the density. This relationship is expressed by a equation of stress(σ) = exothermic heat(⊿Q) x density(ρ).

Unprecedented age-hardening and its structural requirement in a severely deformed Al-Cu-Mg alloy

A gradient nanostructured surface layer was produced in an Al-Cu-Mg alloy (Al 2024) by means of a surface sliding friction treatment carried out at liquid nitrogen temperature. After aging treatment, unprecedented age-hardening was achieved at the surface layer, where microhardness values of >320 HV, more than twice that achievable by conventional precipitation hardening (150 HV), and well above the previously reported maximum value (280 HV) for this alloy, were achieved. Transmission electron microscopy and atom probe tomography analysis revealed that the formation of a nanograin structure at the deformed surface layer resulted in enhanced segregation of Cu and Mg during aging to narrowly spaced grain boundaries, with a corresponding suppression of precipitation. The enhanced grain boundary segregation instead of precipitation is considered to be responsible for the substantial age-hardening, although the underlying hardening mechanisms resulting from grain boundary segregation require further investigation.

Grain boundary segregation induced precipitation in a non equiatomic nanocrystalline CoCuFeMnNi compositionally complex alloy

Compositionally complex alloys (CCAs) in a nanocrystalline state often involve complex and poorly understood phase transformations which can consequently result in grain growth even at low temperatures. A detailed study of the microstructure and phase stability in CCAs is challenging due to the presence of multiple principal components. In view of these challenges the objective of the present study is to establish a systematic understanding of the phase evolution in a face centered cubic non equiatomic nanocrystalline CCA (CoCuFeMnNi). To accomplish this objective, we employed in-situ transmission electron microscope heating in combination with automated crystal orientation mapping (ACOM) and energy filtered transmission electron microscopy (EFTEM) to elucidate the sequence of phase decomposition of the high-pressure torsion (HPT) processed CoCuFeMnNi. Our analysis reveals a complex succession of grain boundary segregation and depletion steps leading to the formation of a FeCo-rich secondary phase. Our results show that prior to the formation of the secondary phase, Cu, Ni and Co segregate and Fe and Mn deplete at the grain boundaries. After the FeCo precipitation is triggered, Mn segregates to the grain boundaries along with Ni and Cu, whereas Fe and Co are depleted. The FeCo precipitates have a B2 crystal structure and typically exhibit a Kurdjumov-Sachs (K-S) and/or Nishyama-Wasserman (N-W) orientation relationships with adjacent fcc grains. Ex-situ heat treated CoCuFeMnNi analyzed by atom probe tomography (APT) revealed a highly heterogeneous segregation of the different elements to different grain boundaries. The FeCo-rich precipitates contain trace amounts of Ni, whereas Cu is rejected leading to the formation of a separate Cu rich phase. This complex segregation phenomenon is assisted by the high fraction of grain boundaries and triple junctions in the nanocrystalline material, which are critical for the phase evolution in this alloy, which is not frequently observed in the corresponding coarse-grained material.

Surface segregation of 3 mol % yttria-doped tetragonal zirconia particle studied by atomic-resolution scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy

Atomic structure of the surfaces in commercial 3 mol % yttria-doped tetragonal zirconia (TZP) nano-particulate powders were studied by atomic resolution scanning transmission electron microscopy (STEM), and the surface segregation of Y were investigated by STEM energy-dispersive X-ray spectroscopy (EDS) at atomic scale. STEM-EDS observations revealed that the Y cations tend to segregate on the top surface layer, forming a monolayer segregation structure. Static lattice calculations revealed that the surface segregation of Y is energetically favorable. These results indicate that Y surface segregation occurs in the TZP powders, which might partially account for the grain boundary segregation in the sintered body of TZP ceramics.

Unprecedented Age-Hardening and its Structural Requirement in a Severely Deformed Al-Cu-Mg Alloy

A gradient nanostructured surface layer was produced in an Al-Cu-Mg alloy (Al 2024) by means of surface sliding friction treatment carried out at liquid nitrogen temperature. After aging treatment, unprecedented age-hardening was achieved at the surface layer, where microhardness values of >320 HV, more than twice that achievable by conventional precipitation hardening (150 HV), and well above the previously reported maximum value (280 HV) for this alloy, were achieved. Transmission electron microscopy and atom probe tomography analysis revealed that the formation of a nanograin structure at the deformed surface layer resulted in enhanced segregation of Cu and Mg during aging to narrowly spaced grain boundaries, with a corresponding suppression of precipitation. The enhanced grain boundary segregation instead of precipitation is considered to be responsible for the substantial age-hardening, although the underlying hardening mechanisms resulting from grain boundary segregation require further investigation.

Improved Al-Mg alloy surface segregation predictions with a machine learning atomistic potential

Various industrial/commercial applications use Al-Mg alloys, yet the Mg added to Al materials, to improve strength, is susceptible to surface segregation and oxidation, leaving behind a softer and Al-enriched bulk alloy. To better understand this process and provide a systematic methodology for investigating dopants that can mitigate corrosion, we have developed a robust atomistic deep neural net potential (DNP) using a dataset generated with first-principles density-functional theory (DFT). The potential, validated systematically against DFT values, has been shown to have a high fidelity in calculating different elemental and intermetallic Al-Mg systems' properties. Our calculations predict a linear trend in the formation energy of the Al-Mg alloy and its density as a function of temperature, consistent with experimental literature. Employing the DNP within a hybrid Monte Carlo and molecular dynamics (MC/MD) approach, we predict anisotropic surface segregation for Al-Mg alloys such that (111)(100)(110), with (111) surfaces displaying the lowest segregation enthalpies and Mg enrichment. Furthermore, we model the segregation tendencies by adapting a recently introduced isotherm model for grain boundary segregation. Our results show that this model describes the MC/MD segregation profiles with higher fidelity than the McLean and Fowler-Guggenheim isotherm models.

Entropy-Driven Grain Boundary Segregation: Prediction of the Phenomenon

The question is formulated as to whether entropy-driven grain boundary segregation can exist. Such a phenomenon would be based on the assumption that a solute can segregate at the grain boundary sites that exhibit positive segregation energy (enthalpy) if the product of segregation entropy and temperature is larger than this energy (enthalpy). The possibility of entropy-driven grain boundary segregation is discussed for several model examples in iron-based systems, which can serve as indirect evidence of the phenomenon. It is shown that entropy-driven grain boundary segregation would be a further step beyond the recently proposed entropy-dominated grain boundary segregation as it represents solute segregation at “anti-segregation” sites.

Segregation competition and complexion coexistence within a polycrystalline grain boundary network

Interfacial segregation can stabilize grain structures and even lead to grain boundary complexion transitions. However, understanding of the complexity of such phenomena in polycrystalline materials is limited, as most studies focus on bicrystal geometries. In this work, we investigate interfacial segregation and subsequent complexion transitions in polycrystalline Cu-Zr alloys using hybrid Monte Carlo/molecular dynamics simulations. No significant change in the grain size or structure is observed upon Zr dopant addition to a pure Cu polycrystal at moderate temperature, where grain boundary segregation is the dominant behavior. Segregation within the boundary network is inhomogeneous, with some boundaries having local concentrations that are an order of magnitude larger than the global value and others having almost no segregation, and changes to physical parameters such as boundary free volume and energy are found to correlate with dopant concentration. Further, another alloy sample is investigated at a higher temperature to probe the occurrence of widespread transitions in interfacial structure, where a significant fraction of the originally ordered boundaries transition to amorphous complexions, demonstrating the coexistence of multiple complexion types, each with their own distribution of boundary chemical composition. Overall, this work highlights that interfacial segregation and complexion structure can be diverse in a polycrystalline network. The findings shown here complement existing computational and experimental studies of individual interfaces and help pave the way for unraveling the complexity of interfacial structure in realistic microstructures.

Thermodynamics and design of nanocrystalline alloys using grain boundary segregation spectra

Solute segregation at grain boundaries (GBs) is a key mechanism to stabilize nanocrystalline alloys. To date, the standard approach to design and screen for nanocrystalline stability uses a simplified representation that treats the GB network as a single entity, and thus, uses a single “average” segregation energy to characterize solute GB segregation in an alloy. This simplification, however, fails to capture the highly anisotropic nature of GBs, which results in a spectrum of segregation energies that can be very broad. Here, we remove this simplification, and outline more formally correct thermodynamic criteria to screen for thermodynamic stability of polycrystalline structures, accounting for the spectral nature of GBs. We proceed to apply the developed criteria to screen over 200 alloy combinations based on embedded atom method potentials. Among its benefits, this spectral approach enables strict enforcement of the third law of thermodynamics, where an average segregation energy does not. The results of the screening are in general agreement with experimental observations.

Correlation between grain size and carbon content in white etching areas in bearings

Premature failure of bearings during rolling contact fatigue is often associated with the formation of white etching cracks (WECs). Crack surface rubbing of WECs transforms the original bainitic/martensitic microstructure into white etching areas (WEAs), comprised of nanocrystalline ferrite. The grain size and carbon content vary within the WEA. Here, we show by atom probe tomography and scanning electron microscopy, that there is an inversely proportional relationship between grain size and carbon content in WEAs formed in 100Cr6 bearings that failed by WECs in service. We explain this phenomenon by the reduction of grain boundary energy through carbon segregation. Depending on the carbon content, this reduces the driving force for recrystallization and grain coarsening, thereby stabilizing the nanocrystalline microstructure. No such effect is observed for the substitutional element chromium. The smallest grain size (< 10 nm) is found directly next to decomposing cementite precipitates, which act as carbon sources, leading to carbon contents as high as ~9.5 at% in ferrite. Correspondingly, the WEA segments with the lowest carbon contents exhibit the largest grain sizes. Increasing carbon contents in sub regions of WEAs do not only lead to smaller grain sizes but also to higher average carbon contents at the grain boundaries as well as in the grain interior. Our results show that the mechanisms of ferrite microstructure stabilization through carbon grain boundary segregation shown in model experiments are also valid for the microstructure alterations associated with WEC failure occurring in practical bearing applications of the technical alloy 100Cr6.

Spinodal Decomposition in Nanocrystalline Alloys

For more than half a century, spinodal decomposition has been a key phenomenon in considering the formation of secondary phases in alloys. The most prominent aspect of the spinodal phenomenon is the lack of an energy barrier on its transformation pathway, offering an alternative to the nucleation and growth mechanism. The classical description of spinodal decomposition often neglects the influence of defects, such as grain boundaries, on the transformation because the innate ability for like-atoms to cluster is assumed to lead the process. Nevertheless, in nanocrystalline alloys, with a high population of grain boundaries with diverse characters, the structurally heterogeneous landscape can greatly influence the chemical decomposition behavior. Combining atom-probe tomography, precession electron diffraction and density-based phase-field simulations, we address how grain boundaries contribute to the temporal evolution of chemical decomposition within the miscibility gap of a Pt-Au nanocrystalline system. We found that grain boundaries can actually have their own miscibility gaps profoundly altering the spinodal decomposition in nanocrystalline alloys. A complex realm of multiple interfacial states, ranging from competitive grain boundary segregation to barrier-free low-dimensional interfacial decomposition, occurs with a dependency upon the grain boundary character.

Atomic-scale evidence for the intergranular corrosion mechanism induced by co-segregation of low-chromium ferritic stainless steel

The composition distribution around grain boundaries of low-chromium ferritic stainless steels was investigated by the techniques of three-dimensional atom probe and focused ion beam. It was found that Cr atoms segregated largely (up to as much as 22.4 at.%) at grain boundaries with co-segregation of other solute atoms in the absence of Cr-carbides precipitation. The co-segregation induced the formation of a Cr-depleted zone (down to only 9.3 at.%) leading to intergranular corrosion. Based on the new mechanism, a novel concept of reducing or even preventing the grain boundary segregation to prevent intergranular corrosion at the level of dynamics/thermodynamics was proposed.

Influences of TiO2 or Y2O3 doping on the homogeneity of polycrystalline Al2O3 produced by pulsed electric current sintering

ABSTRACT Influences of TiO2 or Y2O3 dopants on pressure-assisted sintering of polycrystalline Al2O3 were investigated to control the microstructure of sintered Al2O3 as well as to improve the homogeneity of transparent Al2O3 samples. The transparent polycrystalline Al2O3 samples with and without the dopants were fabricated by two-step pulsed electric current sintering (TS-PECS). By the grain boundary segregation by the dopants, both TiO2 and Y2O3 depressed the densification and grain growth processes during the sintering of polycrystalline Al2O3. The principle originalities of this work include the relationship between the homogeneities of microstructure and of optical transmittance of large-size polycrystalline Al2O3 and the consequent doping strategy for controlling of homogeneity of transparent polycrystalline Al2O3 prepared by two-step PECS. Doping of Y2O3 dramatically reduced the grain size of polycrystalline Al2O3 and improved its optical transparency. The homogeneity of the microstructure and the optical transparency of the large-sized polycrystalline Al2O3 with 0.1 mol% Y2O3 was improved. The addition of TiO2 also slightly retarded densification and reduced the grain size of the sintered Al2O3 samples. However, TiO2 dopant is not good for applications of transparent Al2O3 because it caused a dark color in transparent polycrystalline Al2O3 samples by uncertain reasons.

Stabilized Nanocrystalline Alloys: The Intersection of Grain Boundary Segregation with Processing Science

Processing science for nanocrystalline metals has largely focused on far-from-equilibrium methods that can generate many grain boundaries with excess defect energy. Conversely, the science of stabilizing nanocrystalline alloys has largely focused on the lowering of that excess defect energy through grain boundary segregation, bringing nanocrystalline structures closer to equilibrium. With increasing technological adoption of stabilized nanocrystalline alloys, there is a substantial need for research at the intersection of these two fields. This review lays out the basic thermodynamic issues of the two subfields and surveys the literature on the most common processing methods, including severe plastic deformation, ball milling, physical vapor deposition, and electrodeposition. We provide an overview of studies that have examined grain boundary segregation through each of these methods and identify general themes. We conclude that there is substantial scope for more systematic work at the intersection of these fields to understand how nonequilibrium processing affects grain boundary segregation.

Realization of synergistic enhancement for fracture strength and ductility by adding TiC particles in wire and arc additive manufacturing 2219 aluminium alloy

TiC reinforced Al-matrix composites were fabricated by wire and arc additive manufacturing (WAAM) method. The effects of TiC content on the evolution of grain boundary structure, phase transformation, grains distribution, interface mismatch, and mechanical properties of the WAAM-deposited 2219 Al–Cu alloys were investigated. The columnar grains in the inner-layer zone were transformed to equiaxed grains by the addition of TiC particles, which eventually induced the decrease of free energy for the Al-matrix, promoting an opportunity for nuclei to grow on the TiC substrate. TiC particles played an important role in generating the nucleation supercooling (ΔTns, 3.8 °C), multi-site heterogeneous nucleation with a lower value of ΔTns inhibited the constitutional supercooling (ΔTcs) caused by segregation of Cu. The degree of grain boundary segregation was greatly weakened, and the α-Al+θ-CuAl2 phase was semi-coherent with the matrix at the grain boundary, i.e. [1–31]Al2Cu//[0–11]Al, (211)Al2Cu//(100)Al. The fine spot-like phase possessed a strong interfacial bonding with the Al-matrix. The Al2Cu and TiC possessed a coherent interface with crystal orientation relations, i.e. [00–1]TiC//[01-1]Al2Cu, (110)TiC//(100)Al2Cu. Besides that, the fine spot-like phases and the well-dispersed TiC particles within the grains promoted the formation of dislocations. Therefore, the strength and toughness of the deposited 2219 alloy were improved synergistically. Eventually, the deposited sample exhibited a tensile strength around 384 MPa, and the elongation was up to 18.3%.

Ludwig–Soret effect formulated from the grain-boundary-phase model

The Ludwig–Soret effect is a phenomenon wherein thermal diffusion is induced by a temperature gradient. The governing differential equation to explain this phenomenon has been derived phenomenologically based on the Onsager theorem in non-equilibrium thermodynamics. In this study, we applied the grain-boundary-phase (GBP) model to the Ludwig–Soret effect. This model has been originally proposed for calculating the amount of grain boundary segregation in alloys. The flux equation for the thermal diffusion of vacancies was reasonably derived through parallel-tangent construction of the Gibbs energy curves utilized in the GBP model. Moreover, the thermal vacancy diffusion in a pure metal was simulated. The results illustrated that the excess vacancies in the pure metal preferentially moved to the high-temperature region. The direct application of the thermodynamic Gibbs energy parameters in the CALPHAD method is essential to analyze the thermal diffusion phenomenon. Furthermore, the oxygen vacancy diffusion in Zr(O,Va)2 under a considerably large temperature gradient was calculated, and a similar result was obtained, wherein the excess oxygen vacancies moved to the high-temperature region. This finding may explain the rapid atom diffusion observed during the flash sintering process.

Synergic grain boundary segregation and precipitation in W- and W-Mo-containing high-entropy borides

The structures of W- and W-Mo-containing high-entropy borides (HEBs) are systematically studied by combining atomic-resolution transmission electron microscopy imaging, electron diffraction, and chemical analysis. We reveal that W or W-Mo addition in HEBs leads to segregation of these elements to the grain boundaries (GBs). In the meantime, W- or W-Mo-rich precipitates also form along the GBs. Crystallographic analysis and atomic-scale imaging show that the GB precipitates in both W- and W-Mo-containing HEBs have a cube-on-cube orientation relationship with the matrix. With further strain analysis, the coherency of the precipitate/matrix interface is validated. Nanoindentation tests show that the simultaneous GB segregation and coherent precipitation, as a supplement to the grain hardening, provide additional hardening of the HEBs. Our work provides an in-depth understanding of the GB segregation and precipitation behaviors of HEBs. It suggests that GB engineering could be potentially used for optimizing the performance of high-entropy ceramics.

Factors controlling segregation tendency of solute Ti, Ag and Ta into different symmetrical tilt grain boundaries of tungsten: First-principles and experimental study

In previous reports, experimental studies have shown that both thermal stability and strength can be controlled by grain boundary (GB) segregation. In this study, we investigate the segregation behavior of solute (Ti, Ag and Ta) atoms to low/high-angle symmetric tilt grain boundaries (STGBs) of W using density functional theory (DFT) calculations and supported by TEM experiments. We found no segregation preference for Ti or Ta at low-angle STGBs; however, they exhibit a slight segregation tendency to the core of high-angle STGBs. In contrast, Ag is more prone to segregate in and all around the GB plane. We estimated the mechanical and electronic contributions to solution energy and found that the electronic contribution is dominant. Furthermore, the role of d−valence electrons of solute and W atoms, was analyzed using the local density of states (PDOS). We found that substantial d−valence electrons hybridization in the case of Ta plays an important role in stabilizing W-Ta bonds, while the anisotropic nature of W-Ti bond contributes to stabilize surrounding W atoms. Charge transfer analysis revealed that Ti and Ta lose electrons to W atoms. Contrary to the electronegativity rule, Ag atoms gain charge from neighboring W atoms and excellent s−s hybridization may explain the increased GB segregation of Ag atoms.

Effect of Heat Treatment on Micro Hardness and Microstructural Properties of Al 6063 Alloy Reinforced with Silver Nanoparticles (AgNps)

The demand for light weight metals in engineering design has led to the increasing use of aluminum and its alloys. However, conventional high-strength casting aluminum alloys usually exhibit low ductility and toughness, as a result of boundary segregation and coarse dendritic grains which result in its limited application in large-sized and complex shaped work pieces. Also, the possession of a significant low resistance to corrosion of Al 6063 alloys has been a major challenge in applications involving acidic and alkaline media. In this study, silver nano-particle was extracted from a fixer solution. Al 6063 was impregnated with silver Nano-particle at different percentage ratio to form a metal matrix nano composite and the Hardening heat treatment of the reinforced metal matrix nano composite was performed. The elemental composition of Al 6063 prepared from scrap was determined using Light Emission Polyvac Spectrometer. The hardness and microstructural analyses of the composites were evaluated. The micro hardness of the composites was positively influenced by the addition of AgNps and the hardening heat treatment process it was subjected to. The percentage increase in micro hardness of the sample not heat treated but impregnated with 3% and 9% weight nano particles are 20% and 22% respectively. Subjecting these samples to heat treatment further increased the micro hardness to 33% and 37% respectively. The morphology of the composites revealed reasonably homogenous distribution of the reinforcement in the matrix of Al 6063 alloy. It can be concluded that both percentage increase in weight fraction of AgNps and hardening has significantly increased the micro hardness and refined the grain morphology of the Al-AgNps composites.

Role of vanadium additions on tensile and cryogenic-temperature charpy impact properties in hot-rolled high-Mn austenitic steels

It is well known that the vanadium addition can effectively enhance yield strength of high manganese austenitic steel, and numerous fine VC particles can precipitate in matrix in a relatively low annealing temperature range of 600–800 °C. Unfortunately, a relatively low annealing temperature strongly deteriorates cryogenic impact toughness at -196 °C due to grain boundary segregation and precipitation. Hence, it is of significance to understand the role of vanadium additions in high manganese austenitic steel, which is a hopeful cryogenic material. It has been determined that an effective strength increment can be attained as the vanadium addition reaches 0.6 wt.%, and the double strengthened modes of precipitation strengthening and hardened structure strengthening are found due to vanadium additions in the studied Fe-24Mn-0.6C-1.9Al high manganese austenitic steel. The results show that there is a minor increase in yield strength by the vanadium addition of 0.31 wt.%, but the high vanadium addition of 0.60 wt.% can greatly enhance the yield strength by ~113 MPa, and the cryogenic toughness can still achieve a high level of 121 J. The grain refinement and solid solution strengthening effects of vanadium are rather weak, whereas the high vanadium addition strongly suppresses recrystallization and recovery. Moreover, high-density dislocations in hardened structures suppress the twinning activities, leading to sparse twins of the V-0.6 steel. Due to the sparse twins and small carbide size, the retardation effect of carbides on twinning is considered to be small. The negative effects of carbides and grain size on cryogenic toughness are relatively small compared to that of high-density dislocations in the V-0.6 steel. To summarize, the V-0.6 steel exhibits a better combination of yield strength and cryogenic toughness compared to the reported high manganese austenitic steels.