Interfacial Segregation Research Articles

Grain boundary structural variations amplify segregation transition and stabilize co-existing spinodal interfacial phases

Grain boundaries (GBs)’s role in determining the functional and mechanical properties of polycrystalline materials is inscribed in both their structure and chemistry. Upon solute segregation, the structure and composition of a GB can change concurrently. We study the co-evolution of GB’s structure and segregation by enhancing the density-based phase-field model to account for the in-plane structural variations in the GB. Significant mutual coupling is revealed between the GB’s chemical and structural states during Mn segregation in Fe-Mn alloys. We found that the structural degrees of freedom in a GB (the ability of the GB structure to respond to the chemical variation) amplifies Mn segregation transition, even when the GB structure stays unchanged. When the GB structure is not uniform, that is the usual case, the coupling between GB structure and segregation evolution also enables the spinodally formed low- and high-Mn phases (upon segregation transition) to co-exist within the GB region. These findings explain the stabilizing mechanism of pronounced interfacial segregation fluctuations, experimentally evidenced in Fe-Mn GBs, and give new insights on the structural sensitivity of GBs’ segregation phenomena and the mutual chemo-structural interplay.

First-principles calculations on brazed diamond with FeCoCrNi high entropy alloys doped with strong carbide-forming elements

FeCoCrNi alloy is one of the most widely studied high-entropy alloys due to its excellent properties such as extreme hardness, excellent wear resistance, and good chemical stability. However, the feasibility of using FeCoCrNi alloy as the filler for brazing diamond and the related interfacial bonding mechanisms are unclear. In this work, the interfacial bonding behaviors between FeCoCrNi filler and diamond, and the influences of strong carbide-forming elements X (X = Mo, Mn, Ti, V, W, Zr, Nb, Hf, Ta) doping into FeCoCrNi filler on the brazed diamond were systematically investigated through first-principles calculations method. The calculated interfacial segregation work and interfacial energies show FeCoCrNi/diamond interface has a higher interfacial bonding strength and stability than Ni–Cr/diamond interface. The calculated embedding energies show that these strong carbide-forming elements X are more likely to segregate in the interfacial region close to the diamond. The doping of X, especially Hf, Zr, Ta, and Nb, significantly enhances the interfacial bonding strength and stability of the FeCoCrNi/diamond interface. Combining with further analysis of electronic structures, it is suggested that the FeCoCrNiNb is an ideal high entropy alloy filler for brazing diamond due to its dual effects on interface strengthening of FeCoCrNi/diamond and graphitization inhibiting of diamond.

On the interaction between Mn, Sb and O during the annealing and selective oxidation of Fe-Mn-Sb alloys

The effect of Mn on the interfacial segregation of Sb for a series of cold-rolled Fe- (2–10) Mn-0.03Sb (at. pct) alloys was determined. Using Atom Probe Tomography (APT), it was found that increasing the substrate Mn content increased Sb segregation at both internal and external Mn-oxide/substrate interfaces, which in turn decreased the excess oxygen content at these interfaces. This is attributed to the reduction of Sb solubility in α-Fe with increasing Mn concentration and positive interactions between Sb and Mn. APT analysis revealed that the excess interfacial oxygen content decreased with increasing Sb segregated to the interfaces due to site competition between O and Sb.

Understanding Fe-Containing Intermetallic Compounds in Al Alloys: An Overview of Recent Advances from the LiME Research Hub

Control of Fe in Al alloys is a severe challenge for the full metal circulation to produce the recycled alloys with mechanical and physical performance as high as the primary alloys. The high restriction of Fe content is mainly due to the deterioration caused by the large-scale Fe-containing intermetallic compounds (FIMCs) in Al alloys. In this paper, recent knowledge gained regarding nucleation, formation, and technical developments on microstructural control and refinement of FIMCs are overviewed. Specific characteristics of the multiple types of FIMCs in Al alloys are presented in two- and three- dimensional (2D and 3D) form. Phase relationships between the FIMCs in different structures, such as primary phase, binary eutectic, and ternary eutectic, formed at different solidification stages are studied. Phase transformations between the FIMCs with or without intermediate phases during the solidification process are examined in different Al alloys, with the mechanisms being clarified. Various approaches to microstructural control of FIMCs are proposed and validated. Significant refinement of FIMCs has been achieved through inoculation of TiB2 particles that had been previously modified with deliberately interfacial segregation of desirable alloying elements, leading to the development of the novel “compositional templating” concept.

Defeating hydrogen-induced grain-boundary embrittlement via triggering unusual interfacial segregation in FeCrCoNi-type high-entropy alloys

Metallic materials are mostly susceptible to hydrogen embrittlement (HE), which severely deteriorates their mechanical properties and causes catastrophic failures with poor ductility. In this study, we found that such a long-standing HE problem can be effectively eliminated in the Fex(CrCoNi)1-x face-centered-cubic (fcc) high-entropy alloys (HEAs) by triggering the localized segregation of Cr at grain boundaries (GBs). It was revealed that increasing the Fe concentration from 2.5 to 25 at. % leads to substantially improved HE resistance, i.e., the ductility loss decreases from 70% to 6%. Meanwhile, the fracture mode transformed from the intergranular to the transgranular mode. Multiscale microstructural analyses demonstrated that the Fe2.5Cr32.5Co32.5Ni32.5 and Fe25Cr25Co25Ni25 alloys show negligible differences in the phase structure, grain size, and grain-boundary (GB) character. However, interestingly, the near atomic-resolution elemental mapping revealed that an increased Fe concentration promotes the nanoscale Cr segregation at the GBs, which is primarily motivated by the strong repulsive force between Cr and Fe and the low self-binding energy of Cr. Such unusual interfacial segregation of Cr, which has not been reported before in the Fe25Cr25Co25Ni25 alloy, helps enhance the GBs’ cohesive strength and suppresses the local hydrogen segregation at GBs due to the deceased GB energy, leading to the outstanding HE resistance. These findings decipher the origins of the vastly-improved HE resistance in current FeCrCoNi-type HEAs, and meanwhile, provide new insight into the future development of novel high-performance structural alloys with extraordinary immunity to hydrogen-induced damages.

Manipulating Nucleation Potency of Substrates by Interfacial Segregation: An Overview

During solidification of metallic materials, heterogeneous nucleation occurs on substrates, either endogenous or exogenous. The potency of the substrates for nucleation is mainly dependent upon the atomic arrangements on the substrate surface, which are affected by the lattice misfit between the substrate and the nucleated solid, the surface roughness at atomic scale, and the chemical interaction between the substrates and the melt. Extensive examinations on metal/substrate (M/S) interfaces at atomic scale by the state-of-the-art aberration (Cs) corrected STEM and associated EDS and EELS have shown that alloying elements in liquid melts tend to segregate at the interfaces, leading to the formation of various 2-dimensional compounds (2DCs) or 2-dimensional solutions (2DSs), depending upon segregation behavior of the elements. For instance, Al3Ti 2DC and Ti2Zr 2DC at the Al/TiB2 interface, Y2O3 2DC at the Mg/MgO interface, and a Si-rich 2DS layer at Al-Si/TiB2 interface have been identified. Such interfacial segregations significantly affect nucleation potency of the substrates, resulting in either promoting or impeding the heterogeneous nucleation process during solidification. In this paper, we present an overview of the current studies of interfacial segregation behavior, the structure and chemistry of interfaces, and their impacts on the subsequent heterogeneous nucleation and grain initiation processes. Our focus is on the advances made in the understanding of the mechanisms for nucleation and grain refinement. It is demonstrated that it is feasible to manipulate heterogeneous nucleation by modifying nucleation potency of a substrate through deliberate interfacial segregation of desirable elements, achieving effective control of the grain structure of cast metallic materials.

The interplay of precipitation of ordered compounds and interfacial segregation in Al‐Cu‐Hf‐Si alloys for high-temperature strength

Achieving high yield strength (250-270 MPa) at elevated temperatures (250 °C and above) for precipitation strengthened aluminium-copper (Al-Cu) alloys still remains a challenge for the alloy designers. This paper presents a systematic progression of the microstructure evolution in a binary Al-0.3Hf, ternary Al-0.3Hf-0.3Si and quaternary Al-2Cu-0.3Hf-0.3Si (in at%) alloys. A quantitative understanding of the role that Hf and Si plays in promoting ordered precipitates and the metastable Al-Cu intermetallic compounds θ"/θ' has been attempted. The initial microstructure is guided by the initial precipitation of primary phases, triggering the formation of Hf-rich dendrites and finally, the Si/Cu segregation at boundaries. Ageing of binary Al-Hf alloy results in precipitation of L12 ordered phase via discontinuous (cellular) process. Si, however, promotes a continuous precipitation process, resulting in the formation of coherent and spherical L12 ordered phase in the matrix. The role of these ordered precipitates in influencing the precipitation of θ"/θ' compounds in quaternary copper containing alloy was explored in detail, including their coarsening and mechanical properties at elevated temperatures. The compact morphology developed in the alloy results in yield strength of 445 ± 8 MPa at room temperature (25 °C). The yield strength evolves through the interaction of L12and θ"/θ' ordered phases with the α-Al matrix. The coarsening of θ' phase was slowed down due to Hf segregation at a semi-coherent interface and led to a thermally stable microstructure that resulted in yield strength of 256 ± 8 MPa at 250 °C.

The segregation behavior of elements at the Ti/TiFe coherent interface: First-principles calculation

The segregation behavior of elements at the Ti/TiFe coherent interface were studied by first-principles calculation. The effect of elements on nucleation, growth, and coarsening of the TiFe precipitate were analyzed by calculation of segregation energy, interfacial energy and diffusion rate. The results show that Ni, Co, Cu, V and Mo atoms have the obvious tendency to segregate towards the interface. Cr and Mn prefer to accumulate in the TiFe precipitate while Al to be energetically more favorable to enrich in Ti matrix. The mechanism of the above different segregation phenomena of the elements at the coherent Ti/TiFe interface was also analyzed and the corresponding thermodynamic driving force was proposed. The interfacial segregation results in the reduction of interface energy. Titanium alloys containing with V and Co elements are more conducive to the nucleation of TiFe precipitate from the Ti matrix. The coarsening of TiFe precipitate is effectively modified by addition of the Co, Ni and Al.

Solute Effect on Grain Refinement of Al- and Mg-Alloys: An Overview of the Recent Advances Made by the LiME Research Hub

Grain refinement is of importance for metallic materials since it provides multiple benefits, such as improved castability, reduced casting defects and improved mechanical properties. From extensive research carried out in the past decades, it has been widely accepted that solute is one of the crucial factors for achieving grain refinement. However, grain refinement is a complex phenomenon, depending on not only solutes in the melt to provide growth restriction but also the physical and chemical nature of the nucleant particles (either endogenous or exogenous). Although significant progress has been made on the subject, some critical questions still remain open, and a comprehensive understanding of the mechanisms of solute effect on grain refinement is still desirable. In this paper, we present an overview of the solute effect on grain refinement based on our recent advances made in the LiME Research Hub. This covers the effect of solute on nucleation potency of nucleant particles due to interfacial segregation, columnar to equiaxed transition (CET), growth restriction and eventually on the overall grain refinement.

Synergistic reinforcement effect of Fe and in-situ synthesized MgAlB4 whiskers in Al matrix composites

Interfacial bonding is an important factor affecting the mechanical properties of composites. The interface between the in-situ synthesized ceramic phase and the metal matrix is considered ideal, so little attention is paid to the interfacial modification. In this work, we found that interfacial segregation affects the growth of the MgAlB4 whisker and the interface of reinforcement and matrix in the composites. Systematic experiments and theoretical calculations indicate Fe solute prefer to segregate at the MgAlB4-{101‾0}/Al interfaces in in-situ synthesized MgAlB4/Al composites, which is helpful to reduce system free energy. On the one hand, Fe segregation induces the whisker coarsening and the formation of planar defects with a sandwich structure formed by a layer of Fe atoms and two layers of Al atoms in coarse-whiskers, on the other hand, it improves the interfacial strength. Although extra Fe aggravates the whisker coarsening, the synergistic reinforcing effect of the whiskers and the extra Fe is greater than the sum of their individual reinforcing effects. This work demonstrates that interfacial segregation can be used to modulate the size of reinforcements and improve interfacial strength.

Enhanced heterogeneous nucleation of Al6(Fe,Mn) compound in Al alloys by interfacial segregation of Mn on TiB2 particles surface

In this study, the composition templating theory for heterogeneous nucleation was applied to achieve TiB2 particles with Mn segregation on the surface, which supplied the initial composition for the heterogeneous nucleation. The interfacial segregation of added alloy element Mn and the other common impurities, such as Fe and Si, was investigated with scanning transmission electron microscopy (STEM). The modified TiB2 particles was applied in Al-2.0Mn-1.0Fe alloys to test its effects on grain refinement of Al6(Fe,Mn) compound. The interfaces between Al6(Fe,Mn) particles and the engulfed TiB2 particles were examined with TEM observation.

Atomistic Simulation Informs Interface Engineering of Nanoscale LiCoO2.

Lithium-ion batteries continue to be a critical part of the search for enhanced energy storage solutions. Understanding the stability of interfaces (surfaces and grain boundaries) is one of the most crucial aspects of cathode design to improve the capacity and cyclability of batteries. Interfacial engineering through chemical modification offers the opportunity to create metastable states in the cathodes to inhibit common degradation mechanisms. Here, we demonstrate how atomistic simulations can effectively evaluate dopant interfacial segregation trends and be an effective predictive tool for cathode design despite the intrinsic approximations. We computationally studied two surfaces, {001} and {104}, and grain boundaries, Σ3 and Σ5, of LiCoO2 to investigate the segregation potential and stabilization effect of dopants. Isovalent and aliovalent dopants (Mg2+, Ca2+, Sr2+, Sc3+, Y3+, Gd3+, La3+, Al3+, Ti4+, Sn4+, Zr4+, V5+) were studied by replacing the Co3+ sites in all four of the constructed interfaces. The segregation energies of the dopants increased with the ionic radius of the dopant. They exhibited a linear dependence on the ionic size for divalent, trivalent, and quadrivalent dopants for surfaces and grain boundaries. The magnitude of the segregation potential also depended on the surface chemistry and grain boundary structure, showing higher segregation energies for the Σ5 grain boundary compared with the lower energy Σ3 boundary and higher for the {104} surface compared to the {001}. Lanthanum-doped nanoparticles were synthesized and imaged with scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) to validate the computational results, revealing the predicted lanthanum enrichment at grain boundaries and both the {001} and the {104} surfaces.

Segregation behavior, microstructure and properties of TA2/AA7005 composite plates manufactured by horizontal twin-roll casting under the effect of an electromagnetic oscillation field

Three TA2/AA7005 composite plates were manufactured by horizontal twin-roll casting (HTRC) under various external fields, and the formation mechanism of segregation in the cast-rolling clad process as well as the effect of external fields on the segregation behavior, microstructure and properties of the composite plates were studied. In the non-field (NF) plate, due to the high cooling rate and asymmetric heat transfer in the cast-rolling clad process, a central macro-segregation band and an interface segregation band were formed in the aluminum matrix. After applying the electromagnetic oscillation field (EMOF), the electromagnetic oscillation stirring effect was induced in the melt of the cast-rolling zone. The dendrites grown at the solidification front were broken by a shearing force and re-entered the melt. In addition, the temperature gradient at the solidification front decreased, which inhibited the growth of dendrites and avoided blocking the equilibrium distribution of solute elements due to the “bridging” of dendrite arms. After applying the EMOF, the central and interface segregation bands of the composite plate were nearly eliminated and the microstructure became more homogeneous. As a results, under the action of the EMOF, the tensile strength, yield strength, effective elongation, and interfacial bonding strength of the TA2/AA7005 cast-rolling composite plate were 303.8 MPa, 204.7 MPa, 21.6%, and 25.3 N/mm respectively, which were significantly improved in comparation with the NF plate.

Effect of alloying elements on the interface of fcc-Fe/Ni3Al by first principle calculations

The effect of alloying element segregation on the interface properties of the fcc-Fe/Ni3Al was investigated using the first-principles approach based on density functional theory, revealing the element segregation mechanisms from the perspectives of atoms and electrons. The results show that the alloying elements tend to replace position 1 on the interface of fcc-Fe phase side, and the order of stability after replacement is Ti, Nb, Mo, Cr and Mn. The state density diagrams show that the 4d orbital electrons of Mo and Nb contribute to increase the peak of Al atom 2p orbital at −5 eV, while the 3d orbital electrons of Cr and Ti tend to weaken the peak of Fe atom 3d orbital at 1 eV. The degrees of electron cloud enrichment and consumption increase between interface atoms of the difference charge diagrams indicate that the bonding strength increases and the interface stability is improved. The result of bond population is consistent with the differential charge density diagram. The study has a certain guiding significance for interfacial segregation behavior of alloying elements in heat-resistant steels.

Solute interface segregation measurement: Cross comparison between four different analytical methods

Phosphorus intergranular segregation is known to influence the fracture properties of steels by decreasing grain boundary cohesion and induce intergranular fracture. Different techniques such as Angle-Resolved X-ray Photoelectron Spectroscopy (AR-XPS), Wavelength Dispersive Spectroscopy (WDS), Energy Dispersive X-ray Spectroscopy coupled with Scanning Transmission Electron Microscope (STEM-EDX), and Atom Probe Tomography (APT) can be used to quantify intergranular segregation. Although many studies of this phenomenon were conducted over the last decades, there are rarely direct comparisons between different techniques and there is still a need of reliable and comparable quantification methods for grain boundary segregation. This study cross-compares four available techniques (AR-XPS, WDS, STEM-EDX, and APT) to quantify phosphorus interfacial segregation within the same grain of a sample. This was done by fabricating a Fe-P-Fe sandwich specimen with phosphorus segregated at the interface. Attention was paid to the way of expressing the results of the different techniques so that they can be compared with one another. The quantification results from the different techniques show reasonable agreement.

The effect of ultrafast heating rate on the elemental distribution between phases in a low carbon steel

This work focuses on the effect of heating rate, i.e. conventional heating (CH) and ultrafast heating (UFH) rates on the elemental distribution between phases in a low carbon steel. Microstructural characterization was carried out using an Electron Backscatter Diffraction (EBSD) and Atom Probe Tomography (APT) technique. Nanohardness of individual microconstituents was measured. It is shown that the applied heat treatments result in the formation of multiphase microstructures consisting of a ferritic matrix with embedded martensite and retained austenite. The ferritic matrix of the CH material was fully recrystallized, whereas both recrystallized (coarser) and non-recrystallized (finer) ferritic grains were present in the matrix of the UFH material. APT analysis indirectly confirmed that recrystallized grains after both heat treatments have a lower carbon content, when the non-recrystallized grains after UFH have a higher carbon content. It correlates with the nanohardness results, i.e. non-recrystallized grains show higher hardness, while recrystallized grains have a lower hardness. The segregations of C and Mn atoms at the martensite/ferrite interface were observed after both treatments. It is hypothesized that the segregations are formed under negligible partitioning local equilibrium condition during CH, whereas the solute drag effect results in the formation of interface segregation during UFH.

Interfacial Selective Study on the Gelation Behavior of Aqueous Methylcellulose Solution via a Quartz Crystal Microbalance.

It is important to understand the interfacial structure and physical properties of a polymer material to improve its function. In this study, we used a quartz crystal microbalance (QCM) and neutron reflectivity (NR) measurements to evaluate the viscoelasticity and structure of an aqueous methylcellulose solution near the gold interface. The apparent shear modulus, which was calculated from the complex frequency, was used to assess gelation behavior. The apparent shear modulus determined via the QCM suggested high-frequency rheological properties that reflected the relaxation of skeletal stretching and rotational motion of polymer segments, as well as cooperative motion of the various functional groups. The gelation temperature was found to be lowered at the interface in comparison with that of the bulk. It is suggested that the QCM can evaluate the shear modulus accompanying the gelation near the interface. The interfacial segregation on the gold substrate caused by the surface free energy and long-range van der Waals interaction was observed from NR measurements.

Effect of Oligomer Segregation on the Aggregation State and Strength at the Polystyrene/Substrate Interface.

The interfacial strength of polystyrene (PS) with and without PS oligomers in contact with a glass substrate was examined to determine the relationship between the interfacial aggregation state and adhesion. The shear bond strength and adsorbed layer thickness of neat PS exhibited a similar dependence on the thermal annealing time: they increased to constant values within almost the same time. This implies that the adhesion of the polymer is closely related to the formation of an adsorbed layer at the adhesion interface. Nevertheless, in the case of PS with a small amount of oligomer, the shear bond strength decreased, while the adsorbed layer thickness was almost the same as that of neat PS. Based on the results of interfacial analyses, we propose that the interfacial segregation of the oligomer reduced the entanglement between the interfacial free chains in the adsorbed layer and the bulk chains.

Cu-assisted austenite reversion and enhanced TRIP effect in maraging stainless steels

Control of the formation and stability of reverted austenite is critical in achieving a favorable combination of strength, ductility, and toughness in high-strength steels. In this work, the effects of Cu precipitation on the austenite reversion and mechanical properties of maraging stainless steels were investigated by atom probe tomography, transmission electron microscopy, and mechanical tests. Our results indicate that Cu accelerates the austenite reversion kinetics in two manners: first, Cu, as an austenite stabilizer, increases the equilibrium austenite fraction and hence enhances the chemical driving force for the austenite formation, and second, Cu-rich nanoprecipitates promote the austenite reversion by serving as heterogeneous nucleation sites and providing Ni-enriched chemical conditions through interfacial segregation. In addition, the Cu precipitation hardening compensates the strength drop induced by the formation of soft reverted austenite. During tensile deformation, the metastable reverted austenite transforms to martensite, which substantially improves the ductility and toughness through a transformation-induced plasticity (TRIP) effect. The Cu-added maraging stainless steel exhibits a superior combination of a yield strength of ∼1.3 GPa, an elongation of ∼15%, and an impact toughness of ∼58 J.

Interfacial Segregation, Structure, and Diffusion ofn-Alkane Mixture Films Adsorbed on Smooth and Rough Gold Surfaces

The compositional, structural, and diffusional properties of equal-weight mixtures of liquid n-hexane and n-hexadecane films supported on atomically smooth and rough gold surfaces are explored with the use of temperature-controlled, canonical, molecular dynamics simulations at 315 K. Preferential adsorption of n-hexadecane molecules is found in the region next to the solid surface. The selective molecular segregation effect is much stronger at the smooth Au(111) surface, where it is accompanied by pronounced layering exhibited in the density profile of the adsorbed film normal to the surface, which shows the formation of two well-defined interfacial liquid layers. For both the smooth and rough solid surfaces, in the region of the first layer located about 5 Å from the solid surface, the alkane molecules order with their axis oriented parallel to the adsorbing surface, thus maximizing the attractive physisorption energy. The orientational ordering is accompanied by the formation of ordered domains in the first liquid-film layer, with the molecules in each domain lying parallel to each other. The interfacial domain ordering is frustrated in the alkane mixture film adsorbed on the rough surface. Reduced diffusive motion is found for molecules located near the solid surfaces. However, in the region of the first adsorbed layer of the liquid film adsorbed on the smooth surface, the rate of diffusion, particularly of the hexane molecules, is larger than at the rough surface. It is found that the smaller, hexane, molecules diffuse through channels at the boundaries between the ordered domains formed by the hexadecane molecules. For the rough surface case, where the domain ordering is frustrated, such interdomain boundary channels do not form and consequently the domain boundary-assisted diffusion of the smaller molecules is not operative there, resulting in a smaller diffusion rate.