Boundary Segregation Research Articles

Electrical Conductivity of Ultrafine-Grained Cu and Al Alloys: Attaining the Best Compromise with Mechanical Properties

In recent years, the severe plastic deformation community has developed an interest for metallic materials with high strength and high electrical conductivity, with special focus in Cu- and Al-based alloys, including composite materials. Several processing and metallurgical strategies have been applied to control the influence of microstructure features such as grain refinement, grain boundary condition, defect structures and segregation of secondary phases, over the electrical and mechanical properties. This work summarizes an important body of literature where several strengthening mechanisms and methods to restore the electrical conductivity have been applied to produce ultrafine-grained or nanostructured Cu and Al alloys, mainly by intense imposed strain. A wide variety of alloy systems were studied for their industrial applications in the electrical and electronic market. It can be concluded that the balanced combination of alloying element selection and processing route (mainly attainable under high hydrostatic conditions) could provide high strength with high conductivity and thermal stability materials.

Grain boundary segregation strengthening behavior caused by carbon chain network formation in nanocrystalline NiCoAl alloy

Grain boundary movement is closely related to plastic deformation of materials. In our contribution, C atoms are introduced into the grain boundary and a highly stable C chain network is formed through structural optimization. The results show that the formation of C chains greatly enhances the stability of the grain boundaries and exhibits strong dislocation pinning capability. With the increase of C content, the generation of phase transformation and the nucleation of dislocations are inhibited. The yield strength and tensile strength first increase and then decrease, showing three different deformation mechanisms during tensile deformation. High-density dislocations and unstable grain boundaries jointly control the mechanical properties in the first stage. When the C content comes to 1%, the formation of C chain network greatly enhances the stability of the grain boundaries and effectively hinder and pin the dislocations, inducing a strengthening effect. In the third stage, the further increase of C content severely inhibits the nucleation of dislocations, and leads to a decline in the mechanical properties of alloy. This new strategy of modifying grain boundaries by introducing non-metallic atoms provides an interesting and positive idea for grain boundary engineering.

Modeling the Effect of Grain Boundary Segregations on the Fracture Toughness of Nanocrystalline and Ultrafine-Grained Alloys

A theoretical two-dimensional (2D) model is proposed that describes the effect of grain boundary (GB) segregations on the fracture toughness of nanocrystalline or ultrafine-grained alloys. It is shown that GB segregations can lead to crack curvature, providing both crack surface roughness and crack deflection near the crack tip. Within the model, the growth of cracks along GBs under the action of a tensile load is considered. The effects of brittle GB segregations on the crack surface roughness and crack deflection near the crack tip are analyzed, and the associated increase in the fracture toughness of the material is calculated. It is shown that toughening can be achieved if segregations are very brittle and occupy a moderate proportion of GBs. In particular, a sufficiently large (up to 50%) fraction of GBs containing very brittle segregations can increase the fracture toughness by 30–35%. The results of the model can be applied to thin nanocrystalline or ultrafine-grained films.

Realizing impressive superplasticity in a low-alloyed Mg-Zn-Ca-Al-Mn alloy: The roles of grain boundary segregation and dense β-Mn particles

Achieving impressive superplasticity is an important strategy to manufacture Mg alloy products with complex shapes. In the present study, we report that an excellent superplastic deformation with elongation larger than 500% can be achieved at 623 K and 1.0 × 10−3s−1 in a Mg-1.51Zn-0.59Ca-0.59Al-0.70Mn (wt.%, ZXAM2111) alloy fabricated by equal-channel angular pressing. The superplastic deformation is mainly carried by grain boundary sliding (GBS), accompanied by a grain size growth from ∼3.0 µm to ∼6.0 µm after deformation. Before deformation, the ZXAM2111 alloy is mainly characterized by a strong co-segregation of Zn and Ca atoms at grain boundaries and uniformly distributed β-Mn particles. With deformation proceeding, the β-Mn particles further dynamically precipitate along grain boundaries that parallel the tensile axis, leading to improved resistance to grain coarsening. Although the enhanced stabilizing effects decrease the strain rate sensitivity value, the resulting impressive microstructure stability provides a cornerstone of the active operation of GBS, facilitating the achievement of superplastic deformation. The present work could provide insight into developing low-alloyed Mg alloys with high microstructure thermal stability to achieve superplasticity.

Grain boundary segregation of nickel vacancies and space charge zone formation in NiO through interactions among Ni2+, O2−, and Ni3+

Defects like nickel vacancies (VNi″), holes (Ni3+) with different segregation tendencies to grain boundary (GB) can govern the properties of nickel oxide (NiO) – a promising material for energy applications. An atomistic model of non-stoichiometric NiO bicrystal with θ°001100 GB is developed to understand the segregation of VNi″, Ni3+ to GB from various ionic interactions. According to the model lattice positions available for VNi″ in GB possess lower attractive (from Ni2+-O2− interactions) and higher repulsive (from Ni2+-Ni2+ interactions) energies when compared with those in grain interior. This is due to the differences in these inter-ionic distances in GB and grain interior. Hence VNi″ segregate to GB. Contrarily, grain interior is more favorable for formation of Ni3+ in GB. These Ni3+ form a space charge zone adjacent to the GB. The model helps in understanding VNi″ segregation to GB and space charge zone formation.

Grain boundary segregation in a high entropy alloy

ABSTRACT Grain boundary (GB) segregation was experimentally studied in a bulk high entropy Cantor alloy, which indicated for the first time that Cr strongly segregates to the GBs along with the weak segregation of Mn, implying co-segregation of Cr and Mn to the GBs. The strong segregation of Cr and weak segregation of Mn are explained in terms of the driving forces for GB segregation, where alloy interaction is favourable and stronger in the case of Cr, while elastic strain energy governs Mn segregation.

Role of Ag segregation on microscale strengthening and slip transmission in an asymmetric Σ5 copper grain boundary

Micropillar compression was used to investigate whether Ag segregation to an asymmetric Σ5[001] grain boundary will lead to measurable strength differences compared to the pure copper bicrystal. Ag segregation was accomplished by deposition and subsequent annealing of an Ag thin-film applied on the surface of the Cu bicrystal. Atom probe tomography analysis indicated Ag segregation at the grain boundary with a peak concentration of 2.3 at.%. While the pristine Σ5 grain boundary shows a yield strength of 288 ± 18 MPa when compressing 1 µm diameter pillars along 〈001〉, micropillars containing an Ag-segregated Σ5 grain boundary demonstrated an increased yield strength of 318 ± 17 MPa. In addition, post-deformation electron microscopy was carried out to examine the active slip systems and slip transmission across Ag-free and Ag-containing bicrystals. The results are compared to reference measurements of the adjacent single crystal grains. The 1 µm pillar diameter promoted deformation governed by dislocation-grain boundary interactions for the bicrystalline pillars. This is the first time that changes in flow stress associated with grain boundary segregation have been quantified locally without interference from other mechanisms such as solid solution strengthening, formation of precipitates or changes in stacking fault energy. The results clearly indicate that purely geometrical models for slip transmission are not sufficient as the local atomic structure and composition influence dislocation transmission through grain boundaries.

Development of ultrahigh-strength low-alloy Mg-0.7Al-0.3Ca-0.4Mn (wt.%) alloy with excellent ductility via controlling extrusion temperature

Mg-0.7Al-0.3Ca-0.4Mn (AXM070304, wt.%) alloy was subjected to extrusion with a ram speed of 1 mm/s and an extrusion ratio of 25:1. The extrusion temperature varies from 170 °C to 400 °C. The influence of extrusion temperature on microstructure and mechanical properties of the dilute AXM070304 alloy was systematically explored. With raising the extrusion temperature, the average size of the dynamic recrystallized (DRXed) grains was increased, while the intensity of the basal fiber texture was decreased. When the extrusion temperature was escalated from 170 °C to 400 °C, the tensile yield strength (YS) of the low-alloy AXM070304 extrusion alloy was decreased from 437 to 246 MPa, while the elongation to failure (EL) was increased from 2.9 to 16.9%. The alloy extruded at 200 °C obtains an YS of 412 MPa, an ultimate tensile strength (UTS) of 418 MPa and an EL of 14.8%, exhibiting excellent ductility and ultrahigh strength. The ultrahigh YS was mainly due to the strengthening from ultra-fine recrystallized grains (0.61 μm) with grain boundary segregation of solute Al and Ca atoms. The growth of DRXed grains was inhibited mainly by the co-segregation of Al atoms and Ca atoms, nanosized Al2Ca and β-Mn particle phases at the grain boundaries. Plastic instability occurred under tensile loading in the ultrafine-grained AXM070304 alloy, which may be due to the fact that the grain boundary segregation of Al atoms and Ca atoms requires a high energy barrier for dislocation emission. Once the tensile stress reaches the peak value, the mobile dislocation density increases suddenly.

Effect of bainitic or martensitic microstructure of a pressure vessel steel on grain boundary segregation induced by step cooling simulating thermal aging

During operating lifetime of a nuclear power plant, the pressure boundary components, usually in 16MND5 (SA508 Cl. 3) steel, are subjected to thermal aging that may induce phosphorus intergranular segregation. This is known to decrease brittle fracture stress and induce intergranular fracture, which causes reversible temper embrittlement. Large components such as the reactor pressure vessel presents microstructure heterogeneity due to forging and welding. Tempered martensite can locally be found in the nominally bainitic steel. The question of microstructure susceptibility to reversible temper embrittlement has been raised in literature suggesting that tempered martensite has higher sensibility to phosphorus intergranular segregation than tempered bainite. In this paper, a comparative study of intergranular segregation in tempered bainitic and martensitic 16MND5 steel after thermal aging is conducted using energy dispersive X-ray spectroscopy on transmission electron microscope. Focus ion beam is used to precisely prepare lamellae at prior austenite grain boundaries on step cooled bainitic and martensitic samples. Different types of grain boundaries are identified using precession assisted crystal orientation mapping and then the local chemical composition is analyzed. Phosphorus, nickel, manganese and molybdenum were observed at all types of grain boundaries in both microstructures. The segregation levels are higher in prior austenitic grain boundaries than in other types of boundaries for both microstructures. Taking into account the segregation amount in different types of boundaries, the phosphorus bulk depletion is shown negligible. The results show similar segregation levels in the two studied microstructures, indicating that the susceptibility to thermally induced grain boundary segregation between the tempered bainite and tempered martensite is the same.

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.

Hydrostatic pressure-induced transition in grain boundary segregation tendency in nanocrystalline metals

Impurity segregation at grain boundaries (GBs) can have a profound impact on the mechanical behavior of nanocrystalline metals. In this study, we employ atomistic simulations to investigate the effects of hydrostatic pressure on the GB segregation energy spectra in a series of binary alloy systems. Our results demonstrate that, depending on the alloy system, hydrostatic pressure can either enhance or hinder GB segregation tendencies. However, in specific alloys, such as AlMg and NiNb, hydrostatic pressure can also result in transitions in the preferred segregation sites, characterized by an initial increase followed by a decrease at certain pressures. The underlying mechanisms for these transitions can be attributed to the changes in the elastic component brought about by hydrostatic pressure, which dominate the other contributions to the total segregation energy.

Exceptional thermal stability of nanostructured FeCoNiCrCu high entropy alloy facilitated by unusual grain boundary segregation

Nanocrystalline metals often suffer from grain coarsening at elevated temperatures, which deteriorates their mechanical properties. One important strategy to retard grain growth is solute segregation at grain boundaries, which can reduce excess Gibbs free energy of grain boundaries and thus impede their motion. The strategy has been widely utilized in binary and ternary alloy systems to stabilize their nanostructures. In present work, we report superior thermal stability of nanostructured FeCoNiCrCu high entropy alloy film prepared by ion beam deposition. In contrast to Cu segregation at grain boundaries when the alloy is fabricated by solidification, the nanostructured FeCoNiCrCu film fabricated via ion beam deposition exhibits segregation of Cr at the boundaries. The segregation of Cr persists at grain boundaries even at high annealing temperature and effectively prevents grain growth. This study indicates that multi-elemental alloys can provide plenty of opportunities to obtain nanostructured alloys with high thermal stability.

Atomic behavior of Ti in A15 Nb3Sn and its effects on diffusional growth of Nb3Sn layer

Atomic details of the dramatic increase in the diffusional growth rate of A15 Nb3Sn superconducting compound with Ti addition have been investigated using atomistic simulations and experimental observations. Ti atoms preferentially locate at Nb sites in the Nb3Sn bulk while they do not affect the volume diffusion of Sn significantly. In addition, Ti is predicted to segregate on Nb3Sn grain boundaries, inducing the segregation of Sn, but cannot enhance the grain boundary diffusion of Sn enough to explain the dramatic increase in the growth rate of the Nb3Sn layer. On the other hand, grain refinement is observed when Ti is added, which is expected to substantially promote the effective diffusion of Sn. Consequently, grain refinement due to the grain boundary segregation of Ti is revealed as the most important mechanism underlying the alloying effect of Ti. A basic understanding of the atomic behavior of alloying elements in Nb3Sn and clarified alloying effects on the diffusional growth of Nb3Sn compounds will contribute to further investigation of optimal alloying strategies.

Thermal stability of Ti-Nb-Fe ternary nanocrystalline alloys related to grain boundary segregation

Reducing the grain size of metal materials to nanometer scale is an important method to improve their strength. However, nanocrystalline materials in most cases corresponds to a non-equilibrium state and have a relatively strong tendency to reduce the total grain boundary (GB) area to diminish the energy of the system, which results in significant grain coarsening even at low temperatures. This work is based on the thermodynamic model of the ternary nanocrystalline system, selected TiNbFe alloys as an interesting ternary system and explored its thermodynamic stability. The conspicuous GB segregation tendency of Fe atoms in annealed films is the decisive factor that achieves sufficient thermodynamic stability of nanocrystalline. Ti83.20Nb15.03Fe1.76, Ti79.71Nb15.43Fe4.86, Ti77.28Nb15.88Fe6.85 and Ti75.09Nb15.25Fe9.66 nanocrystalline samples showed excellent thermodynamic stability, which against grain growth up to 400 °C. The calculation of thermodynamic model indicates that Nb solute isn't a GBs segregator in the TiNb binary system, while the Nb element greatly promotes the GBs segregation of Fe atoms in the TiNbFe ternary system. These results enrich the experimental support for solute GB segregation in ternary systems and provide a new strategy for the design of Ti-based nanocrystalline metals with excellent thermal stability.

Atomic-scale understanding of solute interaction effects on grain boundary segregation, precipitation, and fracture of ultrahigh-strength maraging steels

Understanding the fundamental mechanisms of embrittlement and de-embrittlement is crucial for the development of strategies toward advanced steels with improved performance. In this study, the solute interaction effects on grain boundary (GB) segregation, precipitation, and fracture of Fe–Ni–Ti–(Mo) maraging steels were systematically investigated through a combination of experimental and theoretical techniques, including scanning transmission electron microscopy, atom probe tomography, and first-principles calculations. Our results reveal that the Fe–Ni–Ti maraging steel suffers from serious intergranular embrittlement and the mechanism is related to the formation of coarse Ni3Ti precipitates and associated precipitate-free zones (PFZs) at GBs, which facilitate the crack initiation and growth along the GBs. Interestingly, adding Mo to the maraging steel effectively suppresses the intergranular embrittlement, thereby substantially improving the ductility. Atomistic analyses reveal that Mo de-embrittles the GBs by reducing the segregation of Ni and Ti, which substantially inhibits the formation of coarse Ni3Ti precipitates and PFZs at the GBs, thereby alleviating their harmful impact on the GB cracking. In addition, the Mo segregation enhances the GB cohesion, which may play a minor role in suppressing the GB fracture.

Refractory high-entropy nanoalloys with exceptional high-temperature stability and enhanced sinterability

Nanocrystalline alloys (nanoalloys) are prone to grain growth. It is known that grain boundary segregation and precipitation can stabilize nanoalloys, but the stabilization becomes less effective at high temperatures and adding grain growth inhibitors often reduces sinterability. Herein, we have simultaneously achieved exceptional high-temperature stability and improved sinterability for a class of TiNbMoTaW-based refractory high-entropy nanoalloys (RHENs). Bulk pellets of RHENs were fabricated through ball milling and spark plasma sintering, achieving 93–96% relative densities with 50–100 nm grain sizes for three compositions. For example, Ti17.8Nb17.8Mo17.8Ta17.8W17.8Ni6Zr5 sintered at 1300 °C attained ~ 96% relative density with ~ 55 nm mean grain size. Moreover, these RHENs exhibited exceptional stability at 1300 °C. Both Ti17.8Nb17.8Mo17.8Ta17.8W17.8Ni6Zr5 and Ti18.8Nb18.8Mo18.8Ta18.8W18.8Ni6 retained < 150 nm grain sizes after five hours annealing at 1300 °C. Notably, the addition of Ni, a well-known sintering aid for activated sintering of refractory metals such as W and Mo, in high-entropy TiNbMoTaW can promote sintering while maintaining high-temperature stability against rapid grain growth. This may be explained by hypothesized high-entropy grain boundary (HEGB) effects, while we recognize the possible (additional) effects of compositional inhomogeneity and secondary phase (Zener) pinning. These RHENs possess some of the highest temperature stability achieved for nanoalloys and ultrafine-grained metals.

Simultaneously improving yield strength and formability potential of Mg alloy via introducing nano precipitates and texture-weakened heterogeneous grains

Simultaneously improving the yield strength (YS) and formability at room temperature of magnesium (Mg) alloys is one of the key issues to broaden the application further. In this work, we report a new hot-extruded + annealed alloy (i.e., Mg–2Sm–0.8Mn–0.6Ca–0.5Zn) having good combination of yield strength (YS: 167 MPa) and formability potential (UTS−YS: 115 MPa and elongation: 25%), mainly due to formation of nano precipitates and texture-weakened heterogeneous grains. The results indicate that introduction of a certain amount of nano precipitates can enhance the strength of the alloy with little detrimental effect on ductility. The formation of heterogeneous-grained structure with a weak basal texture is mainly related to Sm/Zn/Ca elements grain boundary (GB) segregation. Geometrically necessary dislocations would nucleate at grain boundaries due to uneven deformation behavior caused by heterogeneous-grained structure, obtaining additional hetero-deformation induced hardening and large work-hardening capacity. Formation of heterogeneous-grained structure and solid solution of Sm/Zn/Ca promote the activation of considerable non-basal slips and the suppression of twinning during the deformation process. Meanwhile, GB segregation is conducive to dislocation nucleation from GBs and suppression of the GB cracking. All of these contribute to the high elongation. Thus, we put forwards a new strategy for improving the YS and formability potential via introducing nano precipitates and texture-weakened heterogeneous grains.

Effect of interfacial structure on peel strength between Cu and AlN bonded with a Mg–Ti co-deposited film

Eliminating electrochemical migration of Ag is effective for improving the long-term reliability of power modules for high-voltage applications. In this work, a Cu plate was bonded onto an AlN substrate via Mg–Ti co-deposited films with three different compositions as Ag-free bonding materials between 800 and 950 °C for 0.5 h. The interfacial structures and the peel strength between the Cu and AlN were investigated by using the Surface And Interfacial Cutting Analysis System (SAICAS). Cu was bonded onto AlN through a TiN layer consisting of TiN particles and a Cu-containing grain boundary phase. All fractures at the Cu/AlN interface were observed to occur between the TiN layer and AlN after the SAICAS test. The peel strength at the Cu/AlN interface, which was independent of the thickness of the TiN layer, increased exponentially with increasing bonding temperature. Cu-containing phases, such as grain boundary phases and segregation phases between TiN particles and AlN, were present at the TiN layer/AlN interface. Mg and O were segregated at the interface, which had a high peel strength between the Cu-containing phase and AlN, while at the signal peak position of Mg, a plateau was observed in the Al concentration distribution between the Cu-containing phase and TiN particle. The peel strength of the Cu-containing phase/AlN interface, which increased as the O concentration in the Mg-containing segregation layer increased, dominated that of the TiN layer/AlN interface.

Role of segregation behavior of Cu and Sb in the region of inclusions on initial corrosion

The distribution characteristics of corrosion-resistant elements in low-alloy steel affected the initial corrosion behavior of the matrix. In this article, the segregation behaviors of elements at grain boundaries and pearlite defects in low-alloy steel Q500 and the precipitation behavior of Cu in inclusions were studied. Cu, Ni and Sb had segregation behavior at the boundary of the inclusions, which was beneficial for reducing grain boundary segregation. Cu had the ability to capture Sb and Ni. Cu and Sb involvement in local corrosion process of inclusions were observised by multi-cycle 3.5% NaCl immersion test, during which Cu and Sb were oxidized to corresponding Cu2O and Sb2O3. Under the influences of the segregation of elements at the grain boundaries, the internal part of the crystal was preferentially corroded. Cr was enriched in the pearlite cementite, formed (Cr,Fe)7C3 and contributed to the corrosion process, forming Cr2O3 and CrO3.

Austenite grain boundary segregation and precipitation of boron in low-C steels and their role on the heterogeneous nucleation of ferrite

The addition of boron (B) to steels suppresses the austenite to ferrite phase transformation dramatically, thus increasing their hardenability. It achieves this through grain boundary (GB) segregation at the austenitic GBs that delays the ferrite nucleation. Though the effects of B segregation on hardenability have long been known, the mechanisms of B segregation and how exactly B suppresses the ferrite nucleation remain elusive. We designed a B-containing low-C steel to study the B segregation and precipitation behavior. We conducted heat treatments with different austenitization temperatures and cooling rates. Site-specific atom probe tomography, complemented by high-resolution secondary ion mass spectrometry reveals B segregation at prior austenite grain boundaries (PAGBs) along with carbo-boride precipitation. The B segregation mechanisms are discussed in detail based on these observations considering their dependence on austenitization temperature and cooling rate. Furthermore, we analyzed the impact of GB energy reduction through nucleation kinetics calculations. From our analysis, we conclude that the reduction in GB energy affects the grain corner and grain edge nucleation substantially. However, the retarding effects of carbo-boride precipitation on ferrite nucleation cannot be completely excluded. We provide a detailed account of the possibilities of how B-containing precipitates may be suppressing ferrite nucleation.