Boundary Self Research Articles

Grain boundary diffusion and grain boundary phase transition in tungsten in the temperature range of activated sintering

Abstract Grain boundary self- and solute (cobalt) diffusion in tungsten was found [Lee et al., Scr. Metall, 1988; Lee et al., Col. de Physique, 1990] to exhibit discontinuities in the Arrhenius behavior at the homologous temperatures of 0.36 < T/T m < 0.4 that surprisingly match the activation sintering temperature of W (T m is the melting point). In the present work, this unusual grain boundary diffusion phenomenon is discussed in terms of a fundamental grain boundary phase transition in W. The experimental data are analysed with respect to predicted segregation-induced grain boundary phase transformation. Competing co-segregation of impurity elements (carbon and phosphor) might induce a discontinuous grain boundary segregation and invoke a grain boundary phase transition which modifies the grain boundary mobilities of substitutional atoms. The improved understanding of grain boundary phase transitions is expected to provide a breakthrough in interpreting the exact mechanism of W-activated sintering.

Grain boundary self- and Mn impurity diffusion in equiatomic CoCrFeNi multi-principal element alloy

Grain boundary self-diffusion of Co, Cr and Fe and impurity diffusion of Mn are measured in a coarse-grained equiatomic CoCrFeNi multi-principal alloy. The tracer diffusivities are determined in a wide temperature range of 643 K to 1273 K, which encompasses both the C- and B-type kinetic regimes of grain boundary diffusion in polycrystalline materials after Harrison’s classification. At higher temperatures (T>800 K), only one short-circuit (grain boundary) contribution is observed, while the existence of two distinct contributions is elucidated by thorough analysis of the penetration profiles corresponding to the C-type kinetic regime (643–703 K). The latter observations are explained in terms of a grain boundary phase decomposition after prolonged annealing below 700 K. The product of the segregation factor and the grain boundary width is found to be about 0.5 nm for all constituting elements. The grain boundary diffusion data indicate that Mn does not reveal a strong (if any) segregation in the equiatomic CoCrFeNi alloy.

A Clone Selection Based Real-Valued Negative Selection Algorithm

Excessive detectors, high time complexity, and loopholes are main problems which current negative selection algorithms have face and greatly limit the practical applications of negative selection algorithms. This paper proposes a real-valued negative selection algorithm based on clonal selection. Firstly, the algorithm analyzes the space distribution of the self set and gets the set of outlier selves and several classification clusters. Then, the algorithm considers centers of clusters as antigens, randomly generates initial immune cell population in the qualified range, and executes the clonal selection algorithm. Afterwards, the algorithm changes the limited range to continue the iteration until the non-self space coverage rate meets expectations. After the algorithm terminates, mature detector set and boundary self set are obtained. The main contributions lie in (1) introducing the clonal selection algorithm and randomly generating candidate detectors within the stratified limited ranges based on clustering centers of self set; generating big-radius candidate detectors first and making them cover space far from selves, which reduces the number of detectors; then generating small-radius candidate detectors and making them gradually cover boundary space between selves and non-selves, which reduces the number of holes; (2) distinguishing selves and dividing them into outlier selves, boundary selves, and internal selves, which can adapt to the interference of noise data from selves; (3) for anomaly detection, using mature detector set and boundary self set to test at the same time, which can effectively improve the detection rate and reduce the false alarm rate. Theoretical analysis and experimental results show that the algorithm has better time efficiency and detector generation quality according to classic negative selection algorithms.

Conceptualization of the terms of self and the sense of identity and their disorders in the course of schizophrenia in the works of Polish authors

Abstract A defined and formed identity is the foundation of human mental functioning. The sense of one’s own existence as an individual, together with the conscious image of oneself, plays a fundamental role in regulating behaviors. The ability to see the limits of one’s own body is a significant milestone in human development, because it allows the development of a global sense of self, and that is the basis for the process of identifying one’s own identity. The paper presents the contribution of Polish researchers (psychiatrists and psychologists) to understanding the phenomenon of the sense of self, identity, and their disorders with particular emphasis on schizophrenia. Method: A review of the available literature by Polish authors on the subject was undertaken by searching the following data-bases: Polish Bibliography of Medicine, CEJS, NUKAT and National Library Database, using the keywords: identity, self, personality, schizophrenia, ego, within time frame: 1970–June 2017. The article includes only those works which made the original contribution of Polish authors to research on the issue of self and the sense of identity, with particular emphasis on their disorders in schizophrenia. Results: The works of three Polish authors: J. Reykowski, J. Kozielecki and Z. Zaborowski deserve particular attention in the context of the definition of self and the sense of identity. The works of A. Kępiński, in turn, have contributed to understanding the problem of disorders of self in schizophrenia. Kępiński’s psychopathological concept was based on the analysis and description of the pathology of self, the pathology of the boundary self–the world, disorders of time-space order and the hierarchy of values. In these terms, he considered the structure of the psychotic world, especially in schizophrenia and melancholy. In his opinion, schizophrenic psychosis manifests itself primarily in the form of the blurring of the boundary self–the world and the pathology of self. The paper also presents the contribution of J. Zadęcki, M. Rzewuska, J. Wciórka, S. Steuden, M. Huflejt-Łukasik to understanding the disorders of the sense of self in schizophrenia. Conclusions: The views of Polish authors presented in the paper are an interesting contribution to world literature on research into disorders of self in schizophrenia.

Kinetics of Hot Deformation in Mg/Nano-Al2O3 Composite

The hot deformation behavior of extruded Mg/nano-Al2O3 composite has been studied in the temperature and strain rate ranges of 300-500°C and 0.0003-10 s-1. In the lower strain rate regime (<0.1 s-1), the apparent activation energy evaluated is much higher than that for lattice self-diffusion. At higher strain rates, the behavior of the composite is similar to that of the matrix material and is controlled by grain boundary self diffusion. The prior particle boundaries in the composite, which are decorated by the nano-alumina particles, are stable and only kink under compression parallel to the extrusion direction.

Atomistic study on the activation enthalpies for interface mobility and boundary diffusion in an interface-controlled phase transformation

A multi-lattice kinetic Monte Carlo atomistic simulation method has been used to simulate the austenite to ferrite interface-controlled transformation in pure iron. By performing simulations with different amounts of “free volume” at the interface, quantitative relations between the activation energies for interface mobility, boundary self diffusion and bulk self diffusion were investigated. The effect of different interface orientations on the distribution of free volume over the interface, as determined by interface crystallography, and its consequences for interface mobility activation energy was also evaluated. The essential role of thermal fluctuations in free volume distribution at the interface has been shown. The activation enthalpy for boundary diffusion is shown to be larger than the interface mobility activation enthalpy.

Effect of Oxygen Content on the Processing Maps for Hot Deformation of OFHC Copper

Processing map for the hot deformation of high purity oxygen free high conductivity (OFHC) copper (2ppm oxygen) has been developed in the temperature range 600-950°C and strain rate range 0.001-100s−1. The map is compared with those published earlier on OFHC copper with higher oxygen contents (11ppm and 30ppm) with a view to evaluating the effect of oxygen content on the dynamic behavior of OFHC copper and the mechanism of hot deformation. The maps reveal that dynamic recrystallization (DRX) occurs over a wide temperature and strain rate range and is controlled by different diffusion mechanisms. In OFHC copper with 2ppm oxygen, the apparent activation energy for the DRX domain in the strain rate range 0.01-10s−1 and temperature range 600-900°C is estimated to be about 137kJ∕mole which suggests dislocation core diffusion to be the rate controlling mechanism. However, this domain is absent in the maps for OFHC copper with higher oxygen content due to the “clogging” of dislocation pipes by the oxygen atoms thereby preventing this short circuit diffusion process. At strain rates in the range 1-100s−1 and temperatures >700°C, the apparent activation energy is 73kJ∕mole suggesting that DRX is controlled by grain boundary self diffusion, and this domain expands with higher oxygen content in OFHC copper. At strain rates <0.01s−1 and temperatures >750°C, lattice self-diffusion is the rate controlling mechanism and this lower strain rate domain moves to lower temperatures with increasing oxygen content.

Studies of deformation mechanisms in ultra-fine-grained and nanostructured Zn

The temperature, strain rate, grain size and grain size distribution effects on plastic deformation in ultra-fine-grained (UFG) and nanocrystalline Zn are systematically studied. The decrease of ductility with the decrease of average grain size could be an inherent effect in nanocrystalline materials, that is, not determined by processing artifacts. The superior ductility observed in UFG Zn may originate from both dislocation creep within large grains and grain boundary sliding of small nanograins. The stress exponent for dislocation creep is about 6.6. The activation energy for plastic deformation in UFG Zn is close to the activation energy for grain boundary self diffusion in pure Zn.

Grain Boundary Self- and Solute Diffusion and Segregation in General Large Angle Grain Boundaries in Copper

Grain Boundary Self- and Solute Diffusion and Segregation in General Large Angle Grain Boundaries in Copper

Mechanism and kinetics of type II discontinuous coarsening in a Zn-4 at% Ag alloy

Discontinuous coarsening (DC) may succeed discontinuous precipitation (DP) either at the same (DCI) or another temperature (DCII). The present study concerns mechanism and kinetics of DCII in a Zn-4 at% Ag alloy in the range 353–513 K following DP at 393 K for 60 h. DCII colonies prefer to initiate either from one or both sides of the interfaces between the former DP colonies. A suitable comparison of the kinetic data reveals that interlamellar spacing (λ,) and steady-state growth velocity (v) values in DCII are significantly different than those in DP. On the other hand, the kinetics of DCIvis-a-vis DCII in terms of λ andv are comparable to each other, though the calculated values of the driving forces between them differ marginally. A detailed kinetic analysis of DCII through the Livingston-Cahn model leads to an underestimation of the activation energy (Qb) of grain boundary chemical diffusion of Ag in Zn-Ag (=30.7 kJ mol−1), whereas the same obtained from the modified Petermann-Hornbogen model (=61.0 kJ mol−1) compares well with that for DP/DCI (reported elsewhere by us), and grain boundary self diffusion of Zn. Considering thatQb in DCII is nearly 50% of the activation energy for volume/matrix diffusion of Ag in Zn, it appears that DCII in the present alloy is a boundary diffusion controlled process.

Comparative Cu Diffusion Studies in Advanced Metallizations of Cu and Al-Cu Based Thin Films

ABSTRACTAvailability of diffusion data is important in the evaluation of the prospect of substituting Cu for Al-Cu metallization for improving electrical and electromigration performance. Measurements have been made of 67Cu radioactive tracer diffusion in Cu, Cu-0.4Zr and several Al-Cu thin films of commonly used compositions. Grain boundary self diffusion in Cu is described by δDb= 1.5xl0-9 exp( — 0.92eV/kT) cm3/ sec. The activation energy for Cu diffusion in Al, Al-l%Cu and Al-0.5Cu-0.15Ti thin films depends on the amount of Cu present and varies in the 0.4 - 1.0 eV range. The measured diffusion parameters in the two alloy systems are compared and contrasted with those available from electromigration studies.

Grain Growth and Reaction in Nickel‐Silicon Thin‐Films

AbstractThin‐film reactions in nickel‐silicon bilayers were studied by differential scanning calorimetry and transmission electron microscopy. Interesting particularities of thin‐film reactions were observed: a small exothermic effect before the main peak of reaction is due to grain growth of polycrystalline nickel. The activation energy of the process ((220 ± 50) kJ/mol) is in the same order of magnitude as the activation energy of grain boundary self diffusion of nickel. A mean grain boundary enthalpy of nickel ((0.7 ± 0.3) J/m2) is calculated. The main exothermic peak is due to the formation of di‐nickel silicide; its molar enthalpy of formation (‐(155 ± 20) kJ/mol)) and the energy of activation of the reaction ((150 ± 30) kJ/mol) are in good agreement with literature values. The activation energy is comparable to the activation energy of grain boundary diffusion of nickel in δ‐Ni2Si.

GRAIN BOUNDARY SELF AND IMPURITY DIFFUSION IN TUNGSTEN IN THE TEMPERATURE RANGE OF ACTIVATED SINTERING

Grain boundary self- and 57Co impurity diffusion were measured in sintered tungsten on a large temperature interval. Additionally the diffusion of 185W was investigated in Ni-doped tungsten grain boundaries in the context of the activated sintering process. In al1 three cases an unusual discontinuity was obseved in the Arrhenius plot occurring around 1370 K with quite different magnitudes for self and impurity diffusion. The results were discussed in terms of the grain boundary phase transition model of Militzer and Wieting.

Grain Boundary Self- and Impurity (In, Sn, Sb, Te) Diffusion in Nickel over Large Temperature Ranges

Grain Boundary Self- and Impurity (In, Sn, Sb, Te) Diffusion in Nickel over Large Temperature Ranges

Cavity sintering in pure nickel

Experimental measurements of the kinetics of cavity elimination by annealing are reported for pure nickel. A sample of nickel was crept to implant creep cavities in the grain boundaries. The cavity radius distribution in the sample was obtained after measuring approximately 3000 cavity diameters over an area of 4.7 × 10 −5 m 2. The sample was then vacuum annealed for 7 days at 800°C to cause the cavities to shrink, and the radii distribution of the remaining cavities was determined. From the cavity radii distributions, converted from area to volume, it was possible to determine the rate of cavity shrinkage as a function of the cavity radius. It is shown that the rate of cavity shrinkage is driven by the surface free energy, possibly affected by the internal gas pressure, and controlled by grain boundary self diffusion.

Bulk and grain boundary self diffusion of silver in Ag-O solid solutions

We have studied the role of oxygen in grain boundary (g.b.) and bulk diffusion of silver between 450° and 800°C. The role of this solute, in the bulk as in the grain boundary is not affected by the initial purity of samples characterised by residual resistivity ratio of 300 and 1800. Oxygen enhances silver bulk diffusion over the range of temperature we have studied: this effect is explained by a strong oxygen-vacancy interaction which decreases both the formation and migration energies of vacancies. The diffusion parameters of the pairs “oxygen-vacancy” have been calculated. The g.b. diffusion is unaffected by oxygen and metallic residual impurities. This result may suggest that an enhancement of the nominal purity does not necessarily have an effect on g.b. chemical composition, or that silver diffusion occurs on sites forbidden to oxygen with a small binding energy between oxygen and vacancy linked to the relaxed structure of the g.b.

Compositional effects on the high temperature ductility of 1 Cr-1.25 Mo-0.25 V Steel

This paper describes the results of slow strain rate (e = 4.4 × 10-5 s-1) tensile tests performed at temperatures between 25 and 700 °C on a high purity CrMoV steel containing various dopants. The materials all had a bainitic microstructure, a hardness of RC28, and a grain size of ASTM 0. Some samples were step cooled prior to tensile testing. Four different compositions were tested: undoped (HP), Mn + P doped (MnP), P doped (P), and Sn doped (Sn) materials. All four materials failed in a low ductility cleavage mode at low temperatures and by a low ductility grain boundary cavitation mode at high temperatures. At intermediate temperatures, around 500 °C, the MnP material showed the highest ductility, the HP and Sn materials showed the lowest, and the P material was intermediate. The beneficial effects of both Mn and P on the creep ductility are rationalized in terms of their control of the sulfur concentration on prior austenite boundaries. In addition, it is suggested that P on the grain boundaries can reduce the cavitation rate by reducing the grain boundary self diffusion rate.

Autodiffusion en volume et intergranulaire du fer dans des alliages fer-soufre

Bulk and grain boundary self diffusion of 59Fe in FeS alloys (10 to 120·10 −4 wt% S) are measured in the paramagnetic field: with the increase sulphur content we notice a decrease in self diffusion data; however the effect is stronger in grain boundaries. The grain boundaries diffusion data were analyzed in relation to an experimental equilibrium grain boundary segregation of sulphur: so the sulphur effect on segregation rates and on self or heterodiffusion coefficient could be separated.

A model for creep based on the climb of dislocations at grain boundaries

A model for creep based on the climb of dislocations at grain boundaries is presented. It is shown that when a sliding interface or slip band intersects a grain boundary, a traction distribution is established on the boundary. The diffusional flow induced by these tractions results in a steady state creep process. This slip band model predicts an activation energy corresponding to grain boundary self diffusion, with a stress exponent and a grain size dependence which increase and decrease, respectively, as the applied stress increases. The theoretically determined creep rates are in good agreement with the data for metals and alloys which deform by grain boundary sliding and exhibit superplastic flow properties. Other models for creep controlled by grain boundary diffusion are briefly reviewed and compared with the present model. It is concluded that superplastic deformation involves both purely diffusional flow and dislocation motion.

Precipitation in a nickel-beryllium alloy

Precipitation was studied in a nickel 1.89 wt% beryllium alloy over the temperature range 400 to 750° C. Below 650° C, the precipitation sequence is analogous to that in the widely studied Cu-Be system: $$\alpha (supersaturated solid solution) \to \alpha + GP zones \to \alpha + \gamma '_{NiBe} \to \alpha + \gamma _{NiBe} $$ where the equilibrium γ NiBe phase is formed by a discontinuous reaction consuming the intermediate γ∼' NiBe. The morphology of the discontinuous γ NiBe was too irregular to assign a habit plane. Numerous orientation relations were observed between the α Ni and the discontinuous γ NiBe. The kinetics of the reaction were found to follow a linear growth with time. The activation energy of 58±3 kcal mol−1 is less than bulk diffusion of nickel but greater than grain boundary self diffusion. Above 650° C, the discontinuous reaction front is arrested before completion because the equilibrium γ NiBe forms by continuous coarsening of the γ′ NiBe phase. The continuous γ NiBe has a habit plane of {113}α. Three closely related orientations were observed at constant ageing parameters for the equilibrium continuous phase. Each has been separately proposed in the literature as a unique relation for the copper-beryllium system. For nickel-beryllium either all three are uniquely present or the accuracy of selected area diffraction technique is such that the small differences in orientation cannot be distinguished.