Grain Boundary Solute Segregation Research Articles

Constructing heterogeneous micro/nano multilayer structure in Al alloys via solute grain boundary segregation for superior strength-ductility synergy

A heterogeneous micro/nano multilayered (HM/NMed) Al/Al-0.5 wt%Cu composite, composed of alternating coarse-grained pure Al layers and nanostructured Al-0.5 wt%Cu alloy layers containing a nanolamellar structure (NLS) with lamellar thickness as small as ∼60 nm, was developed by accumulative roll bonding (ARB) followed by low temperature annealing. Deformation induced Cu atoms grain boundary (GB) segregation was observed in Al-0.5 wt%Cu layer, which is responsible for the formation and stabilization of the NLS in the Al-0.5 wt%Cu alloy layers. This resultant HM/NMed Al/Al-0.5 wt%Cu composite with dramatic difference in microstructure and strength/hardness between neighboring pure Al and Al-0.5 wt%Cu alloy layers can achieve an exceptional strength-ductility combination: a yield strength close to the NLSed Al-0.5 wt%Cu alloy, and a tensile ductility comparable to coarse-grained pure Al. Strong hetero-deformation-induced (HDI) strengthening associated with the activation of stacking faults, are responsible for this exceptional strength-ductility synergy. The results presented here demonstrate a general design strategy based on solute GB segregation that enables the fabrication of HM/NMed Al-based materials with multi-scale hierarchical microstructures and improved mechanical properties.

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.

Revealing the structure-property relationships of amorphous carbon tribofilms on platinum-gold surfaces

Nanocrystalline metal alloys have shown great promise as electrical contact materials, given their mechanical and tribological properties. In particular, platinum-gold (Pt–Au) nanocrystalline alloys have demonstrated coefficients of friction as low as 0.01 and specific wear rates on the order of 10−9 mm3 N−1 m−1, largely due to the formation of carbon-based tribofilms at the sliding interfaces. In this study, we advance our understanding of the Pt–Au tribofilm structure-property relations and growth mechanisms via high-throughput and high-resolution measurements as a function of Pt–Au composition. As the solute content increased from 0 at. % to 10 at. % Au, cross-sectional and plan-view transmission electron microscopy demonstrated a decrease in average grain size d and an accompanied increase in grain boundary (GB) segregation. The decrease in d and increase in GB solute segregation translated to a decrease in modulus Er and an increase in hardness H as determined via nanoindentation; the Er trend was mainly described using a rule-of-mixtures approximation, whereas the H trend was ascribed to solid solution strengthening and GB stabilization. The steady state-friction μ and wear rate decreased with the addition of Au; low Au-content films showed substrate wear, while high Au-content films showed stable tribofilm growth in both macroscale and nanoscale friction tests. The carbon bonding configuration of the tribofilms was investigated by near-edge X-ray absorption fine structure spectroscopic analyses and found to be similar to that of hydrogenated amorphous carbon films. Altogether, the study provided insight into the mechanistic origins of the tribofilms, thus opening the door to tunable properties ranging from mitigation for electrical contacts to the creation of self-healing films for solid lubricants.

Ni solute segregation and associated plastic deformation mechanisms into random FCC Ag, BCC Nb and HCP Zr polycrystals

Different types of grain-boundary (GB) solute segregation in nanocrystalline alloys have been described in the literature as being either homogeneous or heterogeneous, which differently impacts their mechanical properties. In heterogeneous segregation, solute atoms tend to cluster along certain GB areas, while other GB regions remain solute-free. This phenomenon is evidenced with the segregation of Ni solute in nanocrystalline FCC Ag. Yet the physical origin for this segregation behavior and its existence in non-FCC alloys are not fully determined. Here, hybrid Monte-Carlo and molecular dynamics simulations were used to study the segregation of Ni solutes (4 at.%) into nanocrystalline FCC Ag, BCC Nb and HCP Zr metals at their solubility limit temperature and the same homologous temperature (0.405·Tm), respectively. A range of segregation configurations was found: Fully heterogeneous segregation in Ag96Ni4 at 500 K, homogeneous segregation with second-phase precipitates in Nb96Ni4 at 1110 K, homogeneous segregation with dispersion of small-scale Ni clusters in Nb96Ni4 at 1564 K and Zr96Ni4 at 464 K, and fully homogeneous segregation with amorphous intergranular films in Zr96Ni4 at 1118 K. We rationalize these changes by quantifying the Ni solute interactions at GBs. Furthermore, significant variations in mechanical behavior and associated plastic deformation mechanisms are shown for each alloy due to their different segregation behaviors. It is found that strain-localized shear banding is the most significant at interfaces with homogeneous solute segregation containing second-phase precipitates and absent with segregation leading to amorphous intergranular films. These findings underscore the importance of solute interactions in profoundly altering segregation and mechanical behavior in stable nanocrystalline alloys.

Local atomic environment analysis of short and long-range solute-solute interactions in a symmetric tilt grain boundary

We develop an efficient molecular-statics algorithm for predicting local solute-solute interaction effects on grain-boundary segregation. This algorithm is applied to the segregation and interaction of Ni and Cu solutes in a Σ9 (221) symmetric tilt grain boundary in Ag. It is found that the strength of interaction between two solutes directly correlates with the change of average segregation energy in the local atomic environment of a fixed grain-boundary solute. It is shown that local interaction effects at grain boundaries can be either attractive or repulsive wherein long-range attractive effects increase the propensity for solute cluster nucleation. This study underscores the atomic nature of grain-boundary solute segregation and provides a tool that could yield meaningful insight into heterogeneous solute segregation phenomena in polycrystalline alloys beyond their dilute limit.

Hall–Petch strengthening limit through partially active segregation in nanocrystalline Ag-Cu alloys

The breakdown from grain-size strengthening to softening mechanisms is generally well understood for high-purity nanocrystalline materials when the mean grain size decreases to the nanometer range. In nanocrystalline alloys, however, the stabilization of nanosized grains by grain-boundary solute segregation complicates the above mechanisms. Moreover, current segregation models have little predictive power for determining the optimal solute content that maximizes Hall-Petch strengthening effects. In this article, using large-scale hybrid Monte-Carlo/molecular dynamic simulations, we present a systematic study of the Hall–Petch breakdown in Cu-segregated Ag alloys with grain sizes ranging from 8 nm to 59 nm, where three concentration-dependent regimes of plasticity are described: (1) Classical segregation strengthening behavior at low solute contents, (2) shear band-induced softening at high solute contents, and (3) a previously unknown, but extended plateau of maximum strengths for intermediate solute contents from 4 to 15 at.%, which we term as nanocrystalline Sterling silver. We find that flow strengths in nanocrystalline Sterling alloys naturally exhibit a zero-slope limit at the smallest grain sizes that is well below the ideal Hall–Petch strengthening trend. This phenomenon results from partially active grain-boundary segregation that acts to influence interfacial plasticity in some, but not all, grain boundary regions. Our findings amplify the atomic nature of solute segregation and interaction at grain boundaries and its complex roles on grain boundary-mediated plasticity mechanisms in nanocrystalline alloys.

CALPHAD-informed phase-field modeling of grain boundary microchemistry and precipitation in Al-Zn-Mg-Cu alloys

The grain boundary (GB) microchemistry and precipitation behaviour in high-strength Al-Zn-Mg-Cu alloys has an important influence on their mechanical and electrochemical properties. Simulation of the GB segregation, precipitation, and solute distribution in these alloys requires an accurate description of the thermodynamics and kinetics of this multi-component system. CALPHAD databases have been successfully developed for equilibrium thermodynamic calculations in complex multi-component systems, and in recent years have been combined with diffusion simulations. In this work, we have directly incorporated a CALPHAD database into a phase-field framework, to simulate, with high fidelity, the complex kinetics of the non-equilibrium GB microstructures that develop in these important commercial alloys during heat treatment. In particular, the influence of GB solute segregation, GB diffusion, precipitate number density, and far-field matrix composition, on the growth of a population of GB η-precipitates, was systematically investigated in a model Al-Zn-Mg-Cu alloy of near AA7050 composition. It is shown that the GB solute distribution in the early stages of ageing was highly heterogeneous and strongly affected by the distribution of GB η-precipitates. Significant Mg and Cu GB segregation was predicted to remain during overageing, while Zn was rapidly depleted. This non-trivial GB segregation behaviour markedly influenced the resulting precipitate morphologies, but the overall precipitate transformation kinetics on a GB were relatively unaffected. Furthermore, solute depletion adjacent to the GB was largely determined by Zn and Mg diffusion, which will affect the development of precipitate free zones during the early stages of ageing. The simulation results were compared with scanning transmission electron microscopy and atom probe tomography characterisation of alloys of the similar composition, with good agreement.

Effects of precipitates versus solute atoms on the deformation-induced grain refinement in an Al–Cu–Mg alloy

With prior knowledge of alloying additions beneficial to grain boundary (GB) stability of nanostructured metals, here we demonstrate that an artificially aged Al–Cu–Mg alloy exhibits superior grain refinement capability compared to its solution-treated counterpart after surface severe plastic deformation processing. The concurrent precipitate dissolution and GB solute segregation, together with the strong precipitate-dislocation interaction enhanced dislocation multiplication but reduced dislocation mobility, is the fundamental reason for improved grain refinement. This result offers a novel insight into the role of precipitate in deformation-induced grain refinement and a new design pathway towards nanostructured materials with smaller grains and higher hardness.

On the thermal stability and grain boundary segregation in nanocrystalline PtAu alloys

Grain boundary (GB) solute segregation has been proposed as a new mechanism to stabilize nanocrystalline (NC) metals. In this study, we investigate the thermal stability and GB solute segregation in a noble metal alloy system (Pt–Au). Thermal stability of the Pt.90Au.10 alloy system was evaluated by annealing a thin film (∼20 nm in thickness) at 500 °C and 700 °C as well as a thick film (∼2 µm in thickness) at a temperature range from 200 °C to 700 °C. The remarkable stability of the Pt.90Au.10 alloy system was demonstrated by comparing its thermal stability to that of pure Pt films processed under identical conditions. Although presence of voids in the GBs may contribute to thermal stability, the enhanced thermal stability of the Pt.90Au.10 alloy is mainly attributed to preferential Au segregation to GBs in the alloy film, which is revealed by aberration-corrected scanning transmission electron microscopy. Our results show that Au segregation to GBs is heterogeneous, with variation in solute content between different GBs as well as non-uniformity along individual GBs. The heterogeneity is dependent on the annealing temperature and is less pronounced at a higher processing temperatures (e.g., 700 °C). By using the noble Pt–Au system, which avoids oxidation and impurities, this study validates the mechanism of GB solute segregation and provides further understanding of the thermodynamics and kinetics underlying NC stabilization.

Grain-boundary segregation of 3d-transition metal solutes in bcc Fe: ab initio local-energy and d-electron behavior analysis

We present ab initio calculations of grain-boundary (GB) segregation of the series of 3d transition-metal (TM) solutes in bcc Fe, taking advantage of the local-energy analysis. For 11(3 3 2) and 3(1 1 1) symmetrical tilt GBs, the segregation behaviors of 3d-TM solutes can be classified into three groups. The early TMs (Sc, Ti and V) are preferentially segregated to the looser sites of GBs antiferromagnetically, the late TMs (Co, Ni and Cu) are preferentially segregated to the tighter sites of GBs ferromagnetically, and the middle TMs (Cr and Mn) are segregated antiferromagnetically without fixed site preference. TMs at both ends of the 3d series show larger segregation-energy gains, while Mn shows a cusp at the center, which is similar to the ab initio interaction energies between the 3d-TM solutes and a screw-dislocation core in bcc Fe. By the local-energy analysis combined with the local densities of states, the segregation of the early TMs is mainly attributed to the stabilization of surrounding Fe atoms by the TM solute at the looser sites of GBs, and that of the late TMs is mainly attributed to the stabilization of the TM solute itself from bulk Fe to GB sites and the destabilization of Fe atoms around the TM solute in bulk Fe. The cusp at Mn is mainly caused by the destabilization of Fe atoms around the Mn solute in bulk Fe, due to nearly-localized high-spin d states of Mn, in contrast to substantial d-d hybridization for Mn in GBs. For each group of 3d-TM solutes, the effects on the magnetic and mechanical properties of Fe GBs are also analyzed by the d-electron behavior in common with the segregation mechanism.

Segregation effects of Y, Ti, Cr and Si on the intergranular fracture of niobium

Abstract The intergranular fracture of materials is sensitive to the grain-boundary segregation of solutes, which is not yet clear for niobium alloys, a vital promising high-temperature material in the aerospace industry. In this study, first-principles calculations are performed to study the segregation effects of four mostly-used solutes (Y, Ti, Cr and Si) on the cohesion of two distinct grain boundaries (GBs) of Nb. Based on Rice-Wang Model, it is found that Y embrittles the GB; while Ti and Si strengthen it; the effect of Cr on the GB cohesion is structure-dependent. The strengthening/embrittling of the GBs induced by the segregation are attributed to the local structural distortion and charge redistributions around GBs. The underlying physical origin of different alloying effects on the intergranular fracture are identified to be the variation of bonding characters between the host Nb Nb atoms across the GB regions.

Grain boundary segregation in immiscible nanocrystalline alloys

Grain boundary (GB) solute segregation has been proposed as a route to mitigate grain growth in nanocrystalline (NC) metals and stabilize their grain structures. An interesting effect emerges in immiscible NC alloys due to the intertwined roles of GB segregation and bulk alloy phase separation. Based on a diffuse interface model, we examine grain growth dynamics in immiscible NC alloys, where both GB solute segregation and bulk phase separation act in conjunction. Analytical treatments identify regimes, where the reduction in GB energy due to segregation is significant. Simulation results reveal that microstructural evolution and enhanced stability are a manifestation of the competing effects of GB heat of segregation and that of bulk heat of mixing. More specifically, in systems with low GB segregation precipitation of solute-rich domains and associated GB (Zener) pinning effects lead to sluggish grain growth rates. In contrast, GB solute segregation plays a more dominant role as the heat of segregation increases in comparison with the bulk heat of mixing. On the whole, this modeling framework provides an avenue to explore the role of bulk alloy and interfacial effects on the microstructural evolution of NC metals.

Modeling solute segregation in grain boundaries of binary substitutional alloys: Effect of excess volume

The effect of excess volume in grain boundary (GB) on GB solute segregation was quantitatively evaluated by means of thermodynamic modeling. Helmholtz free energies of bulk (grain interior) and GBs in binary substitutional systems with fcc (face centered cubic) and bcc (body centered cubic) crystal structures were derived as functions of composition, temperature and atomic volume by applying Embedded-atom-method (EAM) to standard thermodynamic relations under random site occupation assumption. The equilibrium solute distribution between GBs and bulk was then obtained in terms of the parallel tangent construction between bulk and GB phases’ Helmholtz free energy curves. By modeling the CuAg (fcc) and WNb (bcc) systems, an enhanced GB segregation with increased GB excess volume was predicted. The segregation enthalpy and the solute enrichment ratio were further calculated, showing good agreements with the experimental and simulated results.

Stabilization of nanocrystalline alloys via grain boundary segregation: A diffuse interface model

Recent experimental and theoretical findings suggest that nanocrystalline binary alloys can be stabilized against interface-driven homogenization processes via grain boundary (GB) solute segregation mechanism. However, a detailed understanding of this process requires detangling the thermodynamic aspect, GB energy, from the kinetic one, GB mobility. In this work, we present a diffuse-interface model of GB segregation in binary metallic alloys that is capable of accounting for bulk thermodynamics, interfacial energies, and the interaction of alloying elements with GBs. In addition, the model presented herein extends current treatments by independently treating solute–solute interactions within both the bulk grain and GB regions, allowing for deviations from dilute and ideal systems and the ability to account for phase separation processes occurring in conjunction with grain growth. Starting with the analytical treatment of one-dimensional (1D) systems, we investigate the dependence of the GB energy, and subsequently the driving force for grain growth, on the segregation model parameters. More specifically, classic GB segregation isotherms are recovered in the limit of 1D infinite grains. Simulation results of two-dimensional systems reveal regimes of increased thermal stability, and highlight the importance of the thermodynamic model parameters of both bulk grain and GBs on grain growth processes. In broader terms, our modeling approach provides further avenues to explore GB solute segregation and its role in stabilizing polycrystalline aggregates.

Duplex nanocrystalline alloys: Entropic nanostructure stabilization and a case study on W–Cr

Abstract

Solvent grain boundary diffusion in binary solid solutions: a new approach to evaluate solute grain boundary segregation

A new method to estimate solute grain boundary (GB) segregation from solvent GB diffusion measurements in binary solid solutions is proposed for the systems with restricted solubility and intermediate stoichiometric phases. The method is based on the model of complex formation: solvent atoms (A) interact with solute ones (B) at GB with the formation of AB chemical bond. A description of solvent GB diffusion is then given. Using such an approach, a new equation for the dependence of solvent GB diffusion coefficient on solute bulk concentration is derived for the systems with simple atomic attraction with AB complex formation in GB. Theoretical results predict non-linear dependence of solvent GB diffusion coefficient on solute bulk content, which is in agreement with earlier published experimental data regarding the ease of grain boundary diffusion in alloys of different chemical compositions. More importantly, the equation deduced permits to evaluate, on one hand, the characteristics of solute GB segregation from solvent GB diffusion measurements and, one the other hand, GB diffusion coefficient in pure solvent. Such a numerical data analysis for Fe–Sn system is in agreement with experimental results. In special cases, such an analysis can be done analytically.

Thermodynamics of stable nanocrystalline alloys: A Monte Carlo analysis

A Monte Carlo simulation method is used to study the energetics and configuration of binary alloys when grain boundary states are included as potential equilibrium features. For certain sets of alloy properties, a nanostructured grain assembly is found to be the most energetically favorable state, and is stabilized by grain boundary segregation of solute. The conditions for stability against grain coarsening and the ``grain boundary energy'' requirement are clarified, with emphasis on the closed system conditions that prevail in nanostructured alloys. Two thermodynamic parameters, the grain boundary area potential and the grain boundary formation energy, are quantitatively disentangled and shown to differently reflect grain stability and the energy state of interfaces. These discussions provide insights on how alloying can be used to actively manipulate nanocrystalline grain sizes.

Quantitative grain boundary analysis of bulk samples by SIMS

Solute grain boundary (GB) segregation is an important metallurgical phenomenon that has been extensively studied over the last 40 years, especially by Auger spectroscopy of fractured surfaces. More recently, it has been demonstrated that high‐resolution SIMS (NanoSIMS) analysis can detect solute GB segregation on a simple polished cross‐section. The aim of the work presented here was to demonstrate the use of SIMS to achieve quantitative analysis of GB segregation, including taking into account the inclination of the individual boundaries to the bulk sample surface. Copyright © 2012 John Wiley & Sons, Ltd.

Graphical representation for isothermal kinetics of non-equilibrium grain-boundary segregation

Abstract A model of non-equilibrium grain-boundary segregation of solute is expressed with graphs for the isothermal aging at various temperatures after quenching from a solution temperature. It is found from the graphical representations that when the samples are aged for a certain time at various temperatures there is a maximum value in the segregation concentration at a certain temperature. The aging time is equal or close to the critical time of non-equilibrium segregation corresponding to this temperature. This finding is experimentally confirmed with sulfur in an Ni-base superalloy with the aid of Auger electron spectroscopy. As an application of the new finding, a mechanism for intermediate temperature embrittlement of alloys is suggested and identified experimentally with tension tests of the superalloy.

Irradiation-Induced Grain-Boundary Solute Segregation and Its Effect on Ductile-to-Brittle Transition Temperature in Reactor Pressure Vessel Steels

Abstract A study on grain-boundary segregation and embrittlement in terms of the Charpy ductile-to-brittle transition temperature (DBTT) has been performed for the neutron-irradiated A533B steels with typical contents of impurities of Japanese reactor pressure vessel ones. The neutron irradiation was conducted at 563 K to a fluence of 1.3× 1024n/m2 (E>1 MeV) using material testing reactors. The neutron irradiation induced the P and Ni segregation and the reduction in C in some cases at grain-boundaries. The increase in the P segregation at high fluence (>5×10 23n/m2, E>1 MeV) was less than 0.1 in monolayer coverage for the steels with the bulk content of P not exceeding 0.02 wt%. The hardening more strongly affected the DBTT shift than the P segregation for those steels. The reduction in segregated C that enhances the grain-boundary cohesion by neutron fluence is not large enough to cause the DBTT shift.