Effect Of Solute Segregation Research Articles

Tailoring the stability of [formula omitted] twins in magnesium with solute segregation at the twin boundary and strain path control

{101¯2}〈101¯1〉 twins in magnesium is commonly activated at the room temperature under mechanical loading to accommodate arbitrary deformation. The effect of solute segregation at twin boundary on the stability of {101¯2} twins was investigated by employing first-principles calculations. A definition of twinning energy under external stress is proposed to predict the stability of twin with solid solutes at twin boundary under strains. The calculations reveal that the stability of {101¯2} twins, which is dependent on the strain path could be tailored by applying external stress with or without solid solutes at boundary. The modeling well matches the previous experimental results qualitatively. Effective solute could be selected based on the electron work function (EWF) to substitute Mg atoms in certain positions along the {101¯2} twin boundary in order to stabilize the twin.

Effect of solute segregation on the intrinsic stacking fault energy of Co-based binary alloys: A first-principles study

Segregation of solute atoms to stacking faults (Suzuki segregation) and their interactions are of longstanding attention since it has a deep impact on mechanical properties. In this study, we systematically investigate the segregation behavior of different solute atoms in fcc Co-based binary alloys and its effect on the intrinsic stacking fault energy (SFE) using first-principles density-functional-theory (DFT) calculations. Interestingly, while Mo, W, Cr, Mn and Al atoms have a strong tendency to Suzuki segregation with some significant energy barriers, Ni atom has no tendency of segregation and only Fe atom has extremely low energy barriers. A strong segregation effect on the intrinsic SFE was observed and it only extends at a few atomic layers in the vicinity of the intrinsic stacking fault plane. The results are consistent with observed solute segregation in similar alloys and commercial superalloys then it would be useful for SFE modeling and Co-based alloys design. Furthermore, the effect of solute concentrations and changes in the electronic features (charge density difference and density of states) are also investigated. A mutual interaction between solutes and stacking faults enhanced local bonding between the solute atom and the adjacent Co atoms, resulted in the lower intrinsic SFE, thus favoring Suzuki segregation.

Atomistic study of the hardening of ferritic iron by Ni-Cr decorated dislocation loops

The exact nature of the radiation defects causing hardening in reactor structural steels consists of several components that are not yet clearly determined. While generally, the hardening is attributed to dislocation loops, voids and secondary phases (radiation-induced precipitates), recent advanced experimental and computational studies point to the importance of solute-rich clusters (SRCs). Depending on the exact composition of the steel, SRCs may contain Mn, Ni and Cu (e.g. in reactor pressure vessel steels) or Ni, Cr, Si, Mn (e.g. in high-chromium steels for generation IV and fusion applications). One of the hypotheses currently implied to explain their formation is the process of radiation-induced diffusion and segregation of these elements to small dislocation loops (heterogeneous nucleation), so that the distinction between SRCs and loops becomes somewhat blurred. In this work, we perform an atomistic study to investigate the enrichment of loops by Ni and Cr solutes and their interaction with an edge dislocation. The dislocation loops decorated with Ni and Cr solutes are obtained by Monte Carlo simulations, while the effect of solute segregation on the loop's strength and interaction mechanism is then addressed by large scale molecular dynamics simulations. The synergy of the Cr-Ni interaction and their competition to occupy positions in the dislocation loop core are specifically clarified.

Designing Mg alloys with high ductility: Reducing the strength discrepancies between soft deformation modes and hard deformation modes

Two Mg alloys containing 0.03 at% and 0.18 at% Nd were designed to investigate the effects of solute concentration, and thermomechanical processing on the strengths of related deformation modes. Annealing treatments were conducted to stimulate grain coarsening for as-extruded alloys. In view of solute concentration effect, we conducted compressive tests and microstructure characterization to reveal and analyze the activities of the related deformation modes and their corresponding strengths. We find that the solute concentration of Nd does not affect the nucleation CRSS for {10-12} twinning. However, with the increase of Nd concentration, the nucleation CRSS for pyramidal <c+a>-slip was greatly reduced. We also use a thermomechanical processing route, in terms of pre-compression, unloading, intermediate annealing, and re-compression, to explore the effect of solute segregation on strengths of related deformation modes. It is found that the CRSS for the migration of {10-12} twinning dislocation increased significantly after the intermediate annealing treatment for Mg-0.18Nd alloy. We conclude that when designing high ductility Mg alloys, from the perspective of reducing the strength discrepancies between soft deformation modes and hard deformation modes, important factors including the type of alloying element, solute concentration, and pre-deformation strain need to be taken into account.

Effect of twin boundary segregation on damping properties in magnesium alloy

The effect of solute segregation at deformation twin boundaries on the strength (hardness) and damping capacity was investigated using an extruded Mg-Y binary alloy. The existence of deformation twin brought about an increase in both the hardness and the damping capacity. However, a subsequent annealing had a different effect, in which segregation at twin boundaries led to a decrease in the damping capacity. This is because the segregation of yttrium stabilized the twin boundaries; as a result, these boundaries inhibited alternate shrinkage and growth of deformation twin, by which the vibration energy is absorbed.

The different effects of solute segregation at twin boundaries on mechanical behaviors of twinning and detwinning

In the present study, a comparative study about the hardening effect of solute segregation at {101¯2} twin boundaries (TBs) on a {101¯2} twinning predominant deformation and a detwinning predominant one was carried out. The influence of the pre-straining and subsequent annealing conditions on mechanical behavior was systematically addressed. Our results show that solute segregation at TBs can occur even at 100°C. The annealing at 100°C for 20min induces a partial segregation at TBs, while that at 150°C or higher temperature for 20min can induce a complete solute segregation. The annealing conditions and pre-strain levels generate quite different effects on deformation by twinning and that by detwinning. Both annealing hardening and annealing softening might happen during the twinning predominant recompression. Annealing hardening occurs only with pre-strains of 3.0% and 5.5% after annealing at 100°C for 6h. A higher pre-strain or a higher annealing temperature or a longer annealing time generate a higher annealing softening effect. However, during the detwinning predominant recompression, all used annealing treatments generate hardening effect in all the pre-strained samples. With a complete solute segregation at TBs, a hardening of about 11–25MPa is generally achieved. It is also found that solute segregation at TBs reduces the strain hardening rate of deformation by TBs migration.

Modeling grain growth kinetics of binary substitutional alloys by the thermodynamic extremal principle

The thermodynamics and kinetics fundaments of grain growth in binary substitutional alloys were analyzed using the thermodynamic extremal principle. Applying the regular solution approximation, a new equation for solute segregation at steady-state diffusion is proposed, which suggests reduced solute segregation as the grain boundary (GB) solute concentration increases, differently from previous models [Acta Mater 2009;57(5):1466, Acta Mater 2012;60:4833, Scripta Mater 2010;63:989] that adopt constant segregation enthalpy. Furthermore, a self-consistent consideration has been carried out to account for the coupled changes in GB energy and GB mobility as a result of solute segregation. On this basis, the quantitative relation is evaluated between the thermodynamic and kinetic effects of solute segregation to determine the dominant role in retarding and even suppressing grain growth, by comparison of the dimensionless GB energy (i.e., the GB energy of alloy over that of pure solvent) and the dimensionless effective GB mobility (i.e., the effective GB mobility over that of pure solvent): the kinetic effect prevails if the dimensionless effective GB mobility is smaller than the dimensionless GB energy, and vice versa. The present model is adopted to describe well the experimental results for Fe–P alloys, and nanocrystalline Ni–P and Pd–Zr alloys.

Microstructural Features Affecting Tempering Behavior of 16Cr-5Ni Supermartensitic Steel

In industrial production processes, the respect of hardness and UTS maximum values of 16Cr5Ni steel is of utmost importance and a careful control of chemical composition and thermo-mechanical treatments is a common practice. Nevertheless, some scatter of properties is often observed with consequent rejection of final components. To better understand the role played by different factors, two heats of 16Cr-5Ni supermartensitic stainless steels with very close chemical compositions but different thermal behavior during tempering have been studied by means of TEM observations, X-ray diffraction measurements, dilatometry, and thermo-mechanical simulations. It has been found that Ms–Mf temperature range can extend below the room temperature and the relative amount of retained austenite in as-quenched conditions plays a significant role in determining the thermal behavior. When present, the γ-phase increases the amount of reversed austenite formed during tempering and accelerates the process kinetics of martensite recovery. Moreover, increasing amounts of retained austenite after quenching lower the critical temperature for austenite destabilization and influence the optimum temperature–time combination to be adopted for controlling final mechanical properties. In the studied cases, the very close chemical composition of the heats was not a sufficient condition to guarantee the same as-quenched structure in terms of retained austenite amount. This was proven to be related to solute segregation effects during solidification of original heats.

Hardening due to dislocation loop damage in RPV model alloys: Role of Mn segregation

The exact nature of the radiation defects causing hardening in reactor vessel pressure steels at high doses is not yet clearly determined. While generally it is attributed to solute-rich clusters (precipitates) and point defects clusters (matrix damage), recent fine-scale experiments and atomistic simulations suggest that solute rich clusters, mainly containing Mn, Ni and Cu, might be the result of the segregation of these elements to small dislocation loops (heterogeneous nucleation), so that the distinction between precipitates and matrix damage becomes blurred. Here, we perform an atomistic study to investigate the interaction of a0/2〈111〉 dislocation loops with moving dislocations and specifically address the effect of solute segregation on the loop’s strength and interaction mechanism, focusing in particular on Mn, alone or with other crucial solute elements such as Cu and Ni. It is found that the enrichment of Mn in the core of dislocation loops causes significant increase of the unpinning stress, especially for small, invisible ones. At the same time, the solute segregation at the dislocation loops enhances their resistance against absorption by moving dislocations.

The effect of solute segregation on strain localization in nanocrystalline thin films: Dislocation glide vs. grain-boundary mediated plasticity

Deformation mechanisms of nanocrystalline Au and AuCu thin films on compliant substrates were investigated by synchrotron-based in situ tensile testing and Automated Crystal Orientation Mapping using transmission electron microscopy. The results demonstrate that intragranular dislocation plasticity, inferred from evolution of deformation texture, is responsible for the formation of periodic and ordered shear bands in AuCu films. In contrast, pure Au films deform homogeneously without shear band formation and without evolution of deformation texture. Cu solutes are deemed to pin grain boundaries thereby enforce dislocation glide, while in pure Au, plasticity is carried by grain boundary shear and grain boundary migration.

A thermokinetic description of nanoscale grain growth: Analysis of the activation energy effect

The inhibition of grain growth by solute segregation in nanoscale materials has often been described using kinetic models (e.g. Acta Mater 1999;47:2143) or thermokinetic models (e.g. Acta Mater 2009;57:1466), in which constant activation energy and a negligible effect of solute segregation on activation energy were assumed. In this paper, an intact thermokinetic model for nanoscale grain growth was developed by incorporating mixed effects of activation energy and grain boundary (GB) energy. By application of the model to nanoscale grain growth in Ni–P, Pb–Zr, Fe–Zr and Ru–Al alloys, the validity of the present model was confirmed, in combination with verification of the initial condition of GB segregation. On this basis, the increase of activation energy and the decrease of GB energy are interrelated and thus the kinetics and the thermodynamics of normal grain growth are linked. Based on a comparison of three characteristic velocities VTK, VGE and VAE of GB derived from the present thermokinetic model, grain boundary energy model and activation energy model, a mechanism of controlled nanoscale grain growth was proposed, which indicated a transition from a kinetic-controlled to a thermodynamic-controlled process.

Effect of solute segregation on thermal creep in dilute nanocyrstalline Cu alloys

The effect of solute segregation on thermal creep in dilute nanocrystalline alloys (Cu–Nb, Cu–Fe, Cu–Zr) was studied at elevated temperatures using molecular dynamics simulations. A combined Monte-Carlo and molecular dynamics simulation technique was first used to equilibrate the distribution of segregating solutes. Then the creep rates of the diluted Cu samples were measured as functions of temperature, composition, load and accumulated strain. In Cu–Nb samples, the creep rates were observed to increase initially with strain, but then saturate at a value close to that obtained for alloys prepared by randomly locating the solute in the grain boundaries. This behavior is attributed to an increase in grain boundary volume and energy with added chemical disorder. At high temperatures, the apparent activation energy for creep was anomalously high, 3 eV, but only 0.3 eV at lower temperatures. This temperature dependence is found to correlate with atomic mobilities in bulk Cu–Nb glasses. Calculations of creep in nanocrystalline Cu alloys containing other solutes, Fe and Zr, show that the suppression of creep rate scales with their atomic volumes when dissolved in Cu.

Thermodynamic stability of binary nanocrystalline alloys: analysis of solute and excess vacancy

Regarding that the excess volume in grain boundaries (GBs) is released as the vacancies which are accommodated by the crystal bulk during grain growth, a free-energy function for binary nanocrystalline solid solution is proposed, based on the pairwise nearest-neighbor interactions. The model, for the given composition and temperature, predicts an equilibrium grain size, subjected to a mixed effect due to solute segregation and due to excess vacancies. Furthermore, excess-vacancy-inhibited grain coarsening can be attained, which plays a minor role in holding the thermal stability of nanocrystalline alloys, as compared to the effect of solute segregation.

Effects of Solute Segregation on Precipitation Phenomena and Age Hardening Response of High-Purity and Commercial AZ91 Magnesium Alloys

In the present study, the continuous and discontinuous precipitation behavior of the commercial AZ91 alloy containing manganese and the AZ91 alloy produced from high-purity elemental components with no manganese addition has been investigated. It was found that for the commercial alloy, the solute manganese segregated within the primary α-Mg dendrites during solidification and played a major role in the initiation of non-uniform distribution of β-precipitates within the matrix grains. In addition, the solute manganese significantly suppressed the growth of discontinuous (lamellar) precipitates. The high-purity alloy also exhibited non-uniform precipitation. However, the precipitation patterns differed from those observed in the commercial alloy due to the inhomogeneous distribution of aluminum, the origin of which was rooted in the solute segregation within the interdendritic regions of the solidification structure. These differences in the precipitation mode caused by the presence or absence of manganese influenced the age hardening of the alloys. The high-purity specimens aged at lower temperatures attained higher peak-hardness owing to greater area fractions of discontinuous precipitation cells.

The Effects of Solute Segregation on the Evolution and Strength of Dislocation Junctions

In this study, the role of solute segregation on the strength and the evolution behavior of dislocation junctions is studied by utilizing kinetic Monte Carlo and 3D dislocation dynamics simulations. The different solute concentrations and the character of the junctions are all included in the simulations in an effort to make a parametric investigation. The results indicate that solute segregation can lead to both strengthening and weakening behavior depending upon the evolution of the dislocation junctions.

Effects of solute segregation on the grain-growth kinetics of orthopyroxene with implications for the deformation of the upper mantle

The kinetics of static grain growth in a pure synthetic orthopyroxene and orthopyroxene of natural composition were investigated to determine the effects of solute segregation on grain growth. In all experiments the grain size of the synthetic orthopyroxene increases considerably; in contrast, the natural orthopyroxene shows no perceptible grain growth at the same conditions. High-resolution X-ray compositional mapping confirms that the natural orthopyroxene has large concentrations of aluminum and calcium cations segregated on and near its grain boundaries. This segregation is believed to be responsible for the slow grain-growth kinetics in the natural orthopyroxene. Slow grain growth may permit long-term deformation in the diffusion creep regime, as has been observed in some deep mantle xenoliths. As such, it may provide a means of weakening the lithosphere.

Effect of solute segregation on the strength of nanocrystalline alloys: Inverse Hall–Petch relation

We have used a high-energy ball mill to prepare single-phased nanocrystalline Fe, Fe 90Ni 10, Fe 85Al 4Si 11, Ni 99Fe 1 and Ni 90Fe 10 powders. We then increased their grain sizes by annealing. We found that a low-temperature anneal ( T < 0.4 T m) softens the elemental nanocrystalline Fe but hardens both the body-centered cubic iron- and face-centered cubic nickel-based solid solutions, leading in these alloys to an inverse Hall–Petch relationship. We explain this abnormal Hall–Petch effect in terms of solute segregation to the grain boundaries of the nanocrystalline alloys. Our analysis can also explain the inverse Hall–Petch relationship found in previous studies during the thermal anneal of ball-milled nanocrystalline Fe (containing ∼1.5 at.% impurities) and electrodeposited nanocrystalline Ni (containing ∼1.0 at.% impurities).

Role of coupling microscopic and macroscopic phenomena during the crystallization of semicrystalline polymers

AbstractIn this work a microscopic cell model of crystallization incorporating solute rejection is coupled with the macroscopic heat transfer using a finite element procedure to study the evolution of crystallinity. The role of coupling is analyzed using two contrasting polymers, polyethylene and polystyrene. Simulations of the solidification were carried out using both the coupled and the uncoupled formulations to elucidate the effect of coupling. Both polymeric systems considered were affected by this coupling on the evolution of the crystallinity; however, the initial solute content has more effect on the slower crystallizing system. An incremental stress model is also applied to predict the residual stresses generated during the crystallization of the two polymeric systems. It was found that the predicted residual stresses are significantly affected by coupling. Good agreement with experimental results was observed for crystallization half‐time results from the coupled solution by incorporating solute segregation effects for high undercooling, but the deviation was more significant at lower undercooling. For the slow‐crystallizing polymer, fair agreement was observed irrespective of the degree of undercooling. The results show that the solute segregation has only a secondary influence on these systems, especially compared to the effect of undercooling.

Top seeded solution growth and characterisation of ferroelectric potassium lithium niobate crystals

We have grown KLN crystals from different melts by the TSSG method at our laboratory. The melting and crystallisation behaviours of the raw materials have been studied by the DTA method. The crystal compositions were measured by chemical analysis and the solute segregation effects between the melt and crystal were discussed. The crystal structure and optical properties were characterised by means of XRD and optical spectrum measurement. The blue SHG effect of the as-grown crystals was also examined by a dye laser.

Combined creep mechanism in an intermetallic alloy Ni-30Al-20Fe

In the present study, a comparative study about the hardening effect of solute segregation at {101¯2} twin boundaries (TBs) on a {101¯2} twinning predominant deformation and a detwinning predominant one was carried out. The influence of the pre-straining and subsequent annealing conditions on mechanical behavior was systematically addressed. Our results show that solute segregation at TBs can occur even at 100 °C. The annealing at 100 °C for 20 min induces a partial segregation at TBs, while that at 150 °C or higher temperature for 20 min can induce a complete solute segregation. The annealing conditions and pre-strain levels generate quite different effects on deformation by twinning and that by detwinning. Both annealing hardening and annealing softening might happen during the twinning predominant recompression. Annealing hardening occurs only with pre-strains of 3.0% and 5.5% after annealing at 100 °C for 6 h. A higher pre-strain or a higher annealing temperature or a longer annealing time generate a higher annealing softening effect. However, during the detwinning predominant recompression, all used annealing treatments generate hardening effect in all the pre-strained samples. With a complete solute segregation at TBs, a hardening of about 11–25 MPa is generally achieved. It is also found that solute segregation at TBs reduces the strain hardening rate of deformation by TBs migration.