Segregation Of Impurity Atoms Research Articles

Processing and microstructure development of reactive sintered (Ti-Zr-Nb-Hf-Ta)B2 + (Ti-Zr-Nb-Hf-Ta)C high-entropy ceramics

A novel dual-phase (Ti-Zr-Nb-Hf-Ta)B2 + (Ti-Zr-Nb-Hf-Ta)C high-entropy, compositionally complex composite was synthesized by reactive spark plasma sintering using commercial ZrB2, NbB2, HfB2, TaB2 and TiC powders as starting materials. The composite with high density, consists predominantly of boride (∼39.7 vol%) and carbide (∼56.4 vol%) phases and with residual amount of (Hf-Zr)O2. The microstructure is homogenous and relatively fine due to the reaction and solid solution coupling effect with average grain sizes of 3.4 μm and 2.2 μm of the boride and carbide phases, respectively. Large-area SEM analyses confirmed no microcracks or micropore formation and no presence of large grains or clusters. Investigation from micro to nano and atomic level, focused on local chemical composition and spatial distribution of constituent elements, revealed that Ti preferentially dissolves into the boride and all other elements to the carbide phase creating a (Ti0.82Zr0.04Nb0.08Hf0.03Ta0.03)B2 + (Ti0.49Zr0.12Nb0.13Hf0.11Ta0.15)C system. The analysis of grain and phase boundary interfaces revealed a continuous, <1.5 nm wide, segregation of impurity atoms of Fe and Co. Continuous (>20 nm) and atomically flat basal interfacial termination planes (001) decorated with monoatomic Zr enriched layer in the boride phase were observed. Triple points at grain and phase boundaries exhibit metal-rich composition due to impurities from the starting raw powders.

Critical Examination of the Representativeness of Austenite Grain Growth Studies Performed In Situ Using HT-LSCM and Application to Determine Growth-inhibiting Mechanisms

This contribution critically addresses the potential of HT-LSCM experiments for in situ observations of austenite grain growth (AGG). By quantifying AGG for various alloys, the impact of impurity induced solute drag effects (SDE) and second phase precipitation Zener pinning forces (PZ) on AGG can be estimated. Also the grain boundary mobility (GBM) can be determined. The measured arithmetic mean of the time-resolved grain size distributions as a function of temperature and chemical composition is the most important value for quantification. The obtained data is then used to contribute to mathematical models of classical grain growth theory and to allow conclusions on parametrization of SDE and PZ. In this contribution, grain size measurements at the sample surface (in situ and ex situ) are compared with ex situ bulk measurements and experiments on grain growth in the single-phase austenite region (γ-Fe) under isothermal annealing conditions at different temperatures are presented. Grain growth results include high-purity Fe (Fe > 99.98%), binary Fe‑P, Fe‑C, and quaternary Fe-C-Nb‑N systems. For the alloys investigated, it is assumed that grain growth in high-purity Fe occurs without the influence of solute drag or precipitation mechanisms. In Fe‑P, it is shown that grain growth is inhibited by the segregation of impurity atoms at the grain boundaries (GB), which allows conclusions to be drawn about the influence of SDE. In the case of Fe-C-Nb‑N systems, the influence of Nb(C,N) precipitation on grain growth due to Zener pinning forces is presented.

Structure and thermal stability of austenitic steel Fe-17%Cr-12%Ni-2%Mo-0.02%C synthesized by selective laser melting

The structural features of the Fe-17%Cr-12%NI-2%Mo-0.02%C alloy manufactured by selective laser melting (SLM) and the thermal stability of the formed structures are investigated. It is shown that crystallization cells form at SLM, the boundaries of which are volume plexes of dislocations, fixed by segregation of impurity atoms. The microhardness of such a structure is one and a half times higher than that of an alloy of the same composition in a standard hardened state. During annealing up to 800 °C, this cellular structure maintains stability while maintaining hardness indices.

The influence of the heating rate of cemented constructional steels on the growth of austenitic grain in the process of high-temperature holding

The influence of the heating mode of samples of constructional cemented steels 20ХН3А, 20ХГНР and 15ХГН2ТА on the value of austenite grain after high-temperature isothermal aging at 1000 °С is studied. It is shown that the heating of steels at a rate of 1.2–3.0 °C / min in the phase-transformation interval stabilizes the grain structure of the steels and leads to a slowing down of the kinetics of the growth of austenite grains during prolonged high-temperature aging, which makes it possible to increase the temperature of the chemical-thermal treatment of steels. It is concluded that the stabilization of the grain structure of steels is associated with the formation of segregation of impurity atoms and particles at grain boundaries with high-angle disorientation during slow heating, which prevents migration of grain boundaries in the process of prolonged high-temperature aging. A high-temperature chemical-thermal treatment of a batch of billets from steel 20ХН3А under experimental conditions with stepwise heating in the phase-transformation interval provided a qualitative fine-grained structure of the cemented layer.

Effects of prestrain and grain boundary segregation of impurity atoms on bake hardening behaviors of Ti+V-bearing ultra-low carbon bake hardening steel

The distribution pattern of solute atoms, interaction between C atoms and dislocations, and bake hardening behaviors of the P-added and P-free ultra-low carbon bake hardening (ULC-BH) steels were investigated, which were annealed at 810 °C for 2 min and subsequently treated with prestrains and baking. The three-dimensional atomic probe (3DAP) and internal friction results revealed that C atoms were homogeneously distributed in the matrix of the P-added ULC-BH steel, but they were strongly segregated in the P-free ULC-BH steel after annealing at 810 °C for 2 min. The solute C concentration of the P-added ULC-BH steel was higher than that of the P-free ULC-BH steel. The addition of P greatly strengthened the BH behavior of the ULC-BH steel. The variation in the BH behavior with the prestrain was apparently different between the P-added and P-free ULC-BH steels. The BH value of the P-added ULC-BH steel gradually increased with an increase in the prestrain ranging from 1% to 10%. However, the value of the P-free ULC-BH steel increased by increasing the prestrain up to 8% and decreased subsequently. The BH behavior showed higher sensitivity to the solute C concentration for the ULC-BH steel than for the low carbon (LC) steel. The BH value either increased or decreased with an increase in the prestrain, which mainly depended on the solute C concentration, i.e., whether the solute C concentration was sufficient or not to saturate the dislocation generated by the prestrain during the baking process.

Deformation of nanocrystalline binary aluminum alloys with segregation of Mg, Co and Ti at grain boundaries

The influence of the temperature and sort of alloying element on the deformation of the nanocrystalline (NC) binary Al alloys with segregation of 10.2 at % Ti, Co, or Mg over grain boundaries has been studied using the molecular dynamics. The deformation behavior of the materials has been studied in detail by the simulation of the shear deformation of various Al bicrystals with the grain-boundary segregation of impurity atoms, namely, Ti, Co, or Mg. The deformation of bicrystals with different grain orientation has been studied. It has been found that Co introduction into grain boundaries of NC Al has a strengthening effect due to the deceleration of the grain-boundary migration (GBM) and difficulty in the grain-boundary sliding (GBS). The Mg segregation at the boundaries greatly impedes the GBM, but stimulates the development of the GBS. In the NC alloy of Al–Ti, the GBM occurs actively, and the flow-stress values are close to the values characteristic of pure Al.

{100} texture evolution in bcc Fe sheets - Computational design and experiments

A computational and experimental study has been carried out to produce highly value-added {100} textured steel sheets. This study is based on an idea that an anisotropic surface segregation tendency of impurity atoms would cause an inversion of relative surface energy between {110} and {100} surfaces in bcc Fe and may induce the formation of {100} texture in steel sheets during grain coarsening. A phase field model that can consider surface segregation of impurity atoms (and resultant decrease of surface energy) and grain growth simultaneously is newly developed. The phase field model and surface property data obtained from an atomistic approach are used to simulate grain coarsening behavior of phosphorus-containing iron and to find an optimum process condition that can induce the {100} texture in steel sheets. The phase field simulation indicates that the surface segregation and resultant change in the relative difference in surface energy should be accomplished before the start of grain coarsening for an effective formation of the {100} texture. A strong {100} texture is indeed generated experimentally in phosphorus-containing bcc Fe-3.5wt% Si steel sheets through a two-step annealing process, one at a relatively low temperature where only surface segregation can occur but not the grain coarsening and the other at a relatively high temperature where the grain coarsening can also occur.

Effect of grain boundary segregations of Fe, Co, Cu, Ti, Mg and Pb on small plastic deformation of nanocrystalline Al

The paper studies the effect of grain boundary (GB) segregation of impurity atoms (Fe, Co, Cu, Ti, Mg and Pb) on the stress–strain relations of nanocrystalline Al and its alloys subjected to both shear plastic deformation and hydrostatic pressure at room temperature using molecular dynamics (MD) simulations. It is found that the GB segregations of Fe and Co atoms in the nanocrystalline Al–Fe and Al–Co alloys, respectively, strengthen their GBs, while those of Cu and Pb atoms in GBs of the nanocrystalline Al–Cu and Al–Pb alloys have the opposite effect. Furthermore, the GB strengthening effect of the mentioned impurity atoms in both twist and tilt GBs of Al bicrystals are analyzed via molecular statics (MS) simulations. Similar to the MD simulations, the results show that the GB segregations of Co atoms have a noticeable positive effect on the strength of the GBs while those of Pb atoms have a negative effect. Moreover, both MD and MS studies reveal that the Co, Mg and Pb atoms tend to occupy GBs of polycrystalline Al, while Ti, Fe and Cu atoms do not.

Retardation of Grain Growth and Grain Boundary Pinning in Athermal Block Copolymer Blend Systems

The effect of filler addition on the grain coarsening characteristics of block copolymer materials is analyzed for the particular case of a lamellar poly(styrene-b-isoprene)-type block copolymer and polystyrene as well as polystyrene-grafted nanoparticle fillers. Filler addition is shown to reduce the rate of grain growth and to induce grain size distributions that deviate from the log-normal type that is characteristic for pristine block copolymer systems. The retardation of grain growth is shown to be associated with the segregation of filler additives into high energy grain boundary defects—a process that bears similarities to the segregation of impurity atoms within grain boundary structures in ceramics or metals. The analysis of grain boundary energy, grain size distribution, and grain coarsening kinetics suggests two major mechanisms for the interference of filler additives with grain coarsening: First, the segregation of fillers into boundary regions lowers the relative grain boundary energy and hence the driving pressure for grain growth. Second, the formation of particle aggregates along grain boundaries gives rise to a “pinning pressure” that counteracts grain growth and that limits the ultimate grain size during thermal annealing. This is in contrast to pristine block copolymer systems in which continuous grain growth is observed during thermal annealing. The results highlight the fundamental differences between structure evolution in pristine and mixed block copolymer systems and suggest that thermal annealing (in the absence of structure-guiding fields) is an inefficient path to facilitate the controlled growth of large grains in athermal block copolymer blend materials.

Modelling of grain boundary impurity segregation during high temperature plastic deformation

A modified theoretical model is proposed to predict the grain boundary segregation of impurity atoms during high temperature plastic deformation. The model is based on the supersaturated vacancy-impurity complex created by plastic deformation and involves quasi-thermodynamics and kinetics. Model predictions are made for phosphorus grain boundary segregation during plastic deformation in ferrite steel. The results reveal that phosphorus segregates at grain boundaries during plastic deformation. At a given temperature, under a certain strain rate the segregation increases with increasing deformation amount until reaching a steady value, and at the same deformation amount it increases with increasing strain rate. The predicted results are compared with the available experimental values, indicating that there is a reasonable agreement between the theoretical predictions and the experimental observations.

Doping and segregation of impurity atoms in silicon nanowires

Abstract Raman peaks were observed at about 618 and 643 cm −1 for SiNWs synthesized by using a Si target with B. The peak frequencies are in good agreement with those of a local vibrational mode of B in Si crystal. The Fano broadening due to a coupling between the discrete optical phonon and a continuum of interband hole excitations was also observed in the optical phonon peak, which indicates heavily B doping. These results prove that B atoms were doped in substitutional sites of the crystalline Si core of SiNWs. ESR measurements were also performed to investigate defects and P donor/conduction electrons in P-doped SiNWs. The observation of ESR signal due to conduction electrons clearly showed that P atoms were doped in substitutional sites of the crystalline Si core of SiNWs. The segregation behaviors of B and P were investigated by using B local vibrational peaks and Fano broadening for B-doped SiNWs, while an ESR signal of conduction electrons was used for P-doped SiNWs.

Efficiency potential of thin film polycrystalline silicon solar cells by silane-gas-free process using aluminum-induced-crystallization

Abstract Analyzing the performance of thin film polycrystalline silicon solar cells fabricated by silane-gas-free process including the aluminum-induced-crystallization technique by using the device simulation program “PC1D”, we have estimated the efficiency of them. In addition, we have discussed the issues to make the silane-gas-free process practical. In the cell fabrication by silane-gas-free process, segregation of impurity atoms at the grain boundaries of the Si film is one of the serious problems. By suppressing the impurity inclusion and optimizing the cell parameters, the simulated efficiency is to be about 13% in single-junction cells.

Thermal stability of nano-RuAl produced by mechanical alloying

The thermal stability of as-milled nano-RuAl has been studied by isothermal annealing at high temperatures. All three kinds of structural evolutions in as-milled powders upon high temperature exposure, namely reordering, strain relaxation and grain growth, show signs of stagnation. The total quantity of impurities, mainly 15 at.% Fe has been analyzed as being dissolved substitutionally in RuAl and also segregated to grain boundaries. Upon grain growth, lattice diffusion and segregation of impurity atoms in grain boundaries have been verified by the lattice parameter variation. The incremental apparent activation energy for grain growth at different temperatures is related to the accumulation of impurities in grain boundaries. Reordering and strain relaxation processes that accompany grain growth could consume a certain part of the driving force for grain growth.

The effect of the solute atomic size on the swelling of vanadium alloys

Addition of impurities to vanadium can substantially enhance its swelling. However, the swelling is accelerated only by `undersized' and not by `oversized' impurities. In this report we propose a mechanism of swelling acceleration in vanadium, which is based on the impurity effect on the elastic interaction of dislocations with point defects. It is shown that the segregation of impurity atoms at dislocations modifies the dislocation bias, the sign of the bias correction being opposite to that of the impurity misfit. Such bias modification should affect the swelling in qualitative agreement with the existing experimental observations for vanadium alloys.

Voids in fast-neutron-irradiated Cu, Ni and Cu–Ni concentrated alloys studied by TEM and positron annihilation methods

The effect of concentrated Ni and Cu solute atoms in the Cu–Ni system on the formation of voids has been examined using Cu, Cu–8 at.% Ni, Ni–8 at.% Cu and Ni irradiated with fast-neutrons in the FFTF-MOTA. Both solute atoms introduced smaller voids in the grains of the concentrated alloys than voids in the normal grains of pure-Cu and pure-Ni. Slight increase of irradiation temperature and the higher dose of fast-neutrons induced coalescence of voids in the grains of Ni–8 at.% Cu, but it resulted in the abrupt decrease of the concentration of small voids in the grains and the formation of heterogeneously distributed larger voids near grain boundaries in Cu–8 at.% Ni. Heterogeneous distribution of larger voids was also observed in other materials. Annealing at higher temperatures induced segregation of impurity atoms at a void surface in Ni–8 at.% Cu.

Influence of Cr Additive on the Threshold Stress for Superplastic Flow in Al-33%Cu Alloy

The effect of Cr additive on the threshold stress in region I superplastic behavior of Al-Cu eutectic alloy is investigated using high-purity alloys with various Cr contents ranging from 0.002 to 0.26 mass%. The threshold stress increased largely with increasing Cr content up to 0.05 mass% and exhibited a tendency toward a possible saturation. It decreased with increasing temperature and grain size. The temperature dependence was similar to that of grain boundary segregation of impurity atoms. Transmission electron microscopy did not reveal the presence of fine precipitates at the grain and phase boundaries. Therefore, it was proposed that segregating Cr atoms are pinning grain boundaries, inhibiting grain boundary migration that accompanies grain boundary sliding and bringing about the threshold stress.

Influence of Silicon, Carbon and Phosphorus on Intergranular Corrosion of High Purity Austenitic Stainless Steels Under Transpassive Conditions

Precipitate-free Fe-Cr-Ni f.c.c. alloys exhibit strong intergranular corrosion in acid solutions at electrochemical potentials from the transpassivity range. Segregation of impurity atoms to grain boundaries is generally considered to be responsible for this specific kind of localized damage. A study of the influence of silicon, phosphorus and carbon on the intergranular transpassive corrosion of the solution treated Fe-17Cr-13Ni alloy in the 2N sulphuric add at a fixed electrochemical potential is presented. No localized attack occurs in a high purity base alloy nor in one containing phosphorus. The amount of intergranular corrosion is determined by the silicon content : according to previous results a maximum is observed for about 1 wt.% (2 at.%) Si. Carbon additions between 100 and 200 ppm cause some intergranular corrosion in silicon-free alloys and amplify the corrosion intensity in those containing silicon. No localized corrosion occurs for silicon contents higher than 2 wt%. An analysis of the segregation kinetics of silicon is presented and the relation between intergranular corrosion and bulk silicon concentration is discussed.

A particular case of electronic behaviour of the Sigma =3 grain boundary in p-type Si

Electrical conductivity measurements combined with transmission electron microscopy observations were performed on a p-type Si bicrystal containing a Sigma =3 coincidence site lattice grain boundary. An increased conductivity observed in the surroundings of the grain boundary is due to the formation of a potential well for the holes. The potential well is found to be due to the segregation of impurity atoms, which saturate existing free bonds in the core of the observed dislocations at the interface.

Effect of impurity content on superplastic flow in the Zn-22% Al alloy

A detailed investigation was conducted on the superplastic Zn-22% Al alloy to study the effect of impurity content on the creep behavior in region II (intermediate-stress region) and region I (low-stress region) of the sigmoidal plot between stress and strain rate which was previously reported for the alloy. In conducting the investigation, three grades of Zn-22% Al, containing different levels of impurities, were used. The experimental results show that at intermediate stresses, where region II exists, the creep rates of the three grades are similar and that the stress exponent, n, and the activation energy, Q, are insensitive to impurity level; for all three grades, n = 2.5 and Q ∼ Q gb , where Q gb is the activation energy for grain boundary diffusion. By contrast, the experimental results reveal that there are significant differences in the creep behavior of the three grades at low stresses and that the emergence of region I at low stresses, along with its stress exponent and activation energy, is affected by the purity level of Zn-22% Al. Analysis of the experimental data for the three grades of the alloy suggests that the creep behavior in region I is a consequence of the presence of a threshold stress which decreases strongly with increasing temperature and which depends on impurity content. These two characteristics of the threshold stress appear to be compatible with those of a threshold stress for sliding whose origin is related to strong segregation of impurity atoms at boundaries.

On the cleavage strength of grain boundaries in Ni 3Al: Effect of impurity segregation

The cleavage strength of grain boundaries in Ll 2 type of intermetallic compounds (Ni 3Al) is investigated using the tight-binding (TB) electronic theory of s, p and d basis orbitals. It is shown that cleavage strength of the grain boundaries depends strongly on the segregation of impurity atoms at the grain boundaries, in agreement with experimental results. The stoichiometry effect of the Ll 2 compound is also discussed.