Boundary Self-diffusion Research Articles

Grain boundary diffusion and segregation of Cr in Ni [formula omitted] bicrystals: Decoding the role of grain boundary defects

Grain boundary diffusion of Cr in a near Σ11(1̄13)[110] Ni bicrystal is measured over a temperature interval between 503 K and 1203 K using the radiotracer technique. The grain boundary diffusion coefficients, Dgb, and the triple products, P=s⋅δ⋅Dgb, are determined in the C- and B-type kinetics regimes, respectively, with s being the segregation factor and δ the grain boundary width. Opposite to expectations, two distinct contributions to short-circuit diffusion along the nominally single interface are distinguished and related to the existence of two macroscopic facets with distinct grain boundary inclinations and, as a result, distinct structures. The experimental results indicate that the segregation factor of Cr in Ni is about unity, which is fully supported by ab initio calculations. Using classical atomistic simulations, Ni grain boundary self-diffusion coefficients are calculated for the symmetric and asymmetric facets. The computational simulations reveal accelerated self-diffusion kinetics along the asymmetric facet, attributing this phenomenon to the presence of disconnection-like defects. This elucidates the experimentally observed diffusion dynamics of chromium atoms, thereby corroborating the heterogeneous mechanisms governing atomic migration across distinct facets.

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.

ATOMISTIC SIMULATION OF SELF- DIFFUSION AND DIFFUSION Co ALONG SYMMETRIC TILT GRAIN BOUNDARIES [2¯1 ¯1 0] IN α-Ti

The structure, point defects, self-diffusion, and diffusion of Co for four energetically preferred grain boundaries (GB) with the tilt axis [21 1 0] in α-Ti are being investigated by computer modeling methods. The structure and energies of the boundaries and the energies of the formation of point defects in GB, were calculated by molecular static modeling. The dependencies of point defect formation energies on the distance from the grain boundary plane are demonstrated. The coefficients of grain boundary self-diffusion are calculated by the method of molecular dynamics. The results of self-diffusion modeling are compared with the available experimental data. The simulation of grain boundary diffusion of the impurity Co in titanium is also performed. It is shown that the structure of GB affects the parameters of grain-boundary diffusion both in the case of self-diffusion and in the case of impurity diffusion, and the coefficients of grain-boundary diffusion may differ by several orders of magnitude depending on the structure.

Solid-state dewetting of Ag/Ni bi-layers: Accelerated void formation by the stress gradient in the bottom Ni layer

Solid-state dewetting (SSD) of the immiscible Ag/Ni bi-layers was studied. After annealing at 400 °C for 1 min, the Ag film was dewetted on the Ni film, and this is the first observation about the SSD of one metal film on another metal film. The easier dewetting of Ag than Ni was attributed to its lower melting point, faster grain boundary self-diffusion and poor wettability between them. At 500 °C, the void formation in the bottom Ni layer was highlighted and compared to Ni single layer: many voids in the former while no visible voids in the latter, indicating that the presence of Ag accelerated the SSD of Ni. It was attributed to the vertical stress gradient in the bottom Ni film of Ag/Ni bi-layers, which accelerated the Ni diffusion and formation of the voids in the underlying Ni film around and below the Ag particles. Besides, voids were more easily formed below the Ag particles than between them due to the large lattice mismatch at the Ag/Ni interface and the possible formation of Ag-Ni alloys. The destabilization of the Ag on the Ni film contributes to the understanding of dewetting kinetics, which is beneficial to realize the controllable nanofabrication.

Simulation of structure of special tilt boundary and grainboundary self-diffusion in Ti

Simulation of structure of special tilt boundary and grainboundary self-diffusion in Ti

Grain boundary self-diffusion in α-iron of different purity: effect of dislocation enhanced diffusion

Article Grain boundary self-diffusion in α-iron of different purity: effect of dislocation enhanced diffusion was published on October 1, 2004 in the journal International Journal of Materials Research (volume 95, issue 10).

Migration of an incommensurate intercrystalline boundary and boundary self-diffusion

Migration of an incommensurate intercrystalline boundary and boundary self-diffusion

Effect of ultrasound on microstructure evolution of friction stir welded aluminum alloys

The dynamic recrystallization (DRX) in ultrasound affected friction stir welding (UFSW) was investigated using Al–Cu alloy and compared with another joint produced by the same setup but without applying the ultrasound. The effect of the ultrasound on the evolution of recrystallized grains was significantly dependent on the local thermal cycle. For the stir zone (SZ) exposed to high temperatures, both the nucleation rate and growth rate of equiaxed grains during DRX were significantly promoted by the ultrasound. Compared with the SZ without the effect of the ultrasound, the bulging frequency of the local high angle boundaries in the SZ of UFSW joint was increased to 2–4 times, producing a considerable number of fine nuclei. Excess vacancies introduced by the ultrasound were the intrinsic reason for the mentioned phenomena. The sub-grain mobility was radically accelerated due to the enhanced dislocation climb by the excess vacancies and thus the normalized nucleation criterion in the UFSW was reduced, contributing to a larger nucleation rate during DRX. Grain boundary sliding and migration were both facilitated by the ultrasound due to the enhanced movement of grain boundary dislocations and grain boundary self-diffusion, leading to a larger growth rate of equiaxed grains in the SZ of UFSW joint.

Ultrafast Atomic Diffusion Paths in Fine-Grained Nickel Obtained by Spark Plasma Sintering

Short-circuit diffusion in fine-grained Ni samples processed by Spark Plasma sintering has been investigated by the radiotracer technique. Ni grain boundary self-diffusion is measured in samples sintered from commercial as-received powder and from a powder processed by mechanical milling (MM). Both samples displayed high penetration of the radiotracer and ultrafast diffusion rates, which exceed the diffusivity along general high-angle grain boundaries as they are present in pure polycrystalline Ni. A distinct profile was observed for each sample, dependent on the precursor powder. A stable penetration profile after pre-annealing at 773 K was observed when using commercial powder whereas a decrease in the grain boundary diffusion coefficient was depicted for the sample prepared from MM powders. The latter observation was interpreted in terms of partial relaxation of non-equilibrium grain boundaries generated by MM. Sample preparation by focused ion beam enabled the observation of interconnected porous paths in the sample prepared from commercial powders, which represent the main ultrafast diffusion path. A measurement of the surface diffusion coefficient through the pores was attempted considering a B–C-type kinetic regime. The isolated pores observed in the sample prepared from MM powder suggest a complex hierarchy of diffusion paths.

Study of grain boundary self-diffusion in iron with different atomistic models

We studied grain boundary (GB) self-diffusion in body-centered cubic iron using ab initio calculations and molecular dynamics simulations with various interatomic potentials. A combination of different models allowed us to determine the principal characteristics of self-diffusion along different types of GBs. In particular, we found that atomic self-diffusion in symmetric tilt GBs is mostly driven by self-interstitial atoms. In contrast, in general GBs atoms diffuse predominantly via an exchange mechanism that does not involve a particular defect but is similar to diffusion in a liquid. Most observed mechanisms lead to a significant enhancement of self-diffusion along GBs as compared to diffusion in the bulk. The results of simulations are verified by comparison with available experimental data.

Study of Grain Boundary Self-Diffusion in Iron with Different Atomistic Models

We studied grain boundary (GB) self-diffusion in body-centered cubic iron using ab initio calculations and molecular dynamics simulations with various interatomic potentials. A combination of different models allowed us to determine the principal characteristics of self-diffusion along different types of GBs. In particular, we found that atomic self-diffusion in symmetric tilt GBs is mostly driven by self-interstitial atoms. In contrast, in general GBs atoms diffuse predominantly via an exchange mechanism that does not involve a particular defect but is similar to diffusion in a liquid. Most observed mechanisms lead to a significant enhancement of self-diffusion along GBs as compared to diffusion in the bulk. The results of simulations are verified by comparison with available experimental data.

Textural Changes in Hot Compression of Disintegrated Melt Deposition (DMD)–Processed AZ31-1Ca-1.5 vol. % Nano-Alumina Composite

Abstract The development of texture in AZ31-1Ca-1.5 volume percent (vol. %) nano-alumina composite subjected to uniaxial compression is studied over large ranges of temperature and strain rate, and correlated with operative slip systems in the various domains of its processing map. The initial rod, synthesized via disintegrated melt deposition and subsequently extruded, has a fine grain size (2–3 μm) and basal texture with (0001) planes parallel to the extrusion direction. The processing map exhibits four domains: Domain 1: 250–350°C and 0.0003–0.01 s−1, Domain 1A: 350–410°C and 0.0003–0.01 s−1, Domain 2: 410–490°C and 0.002–0.2 s−1, and Domain 3: 325–410°C and 0.6–10 s−1. Microstructures in these four domains revealed dynamic recrystallization, although the mechanisms of slip and recovery are different. In Domain 1, basal slip is the dominating mechanism that produced strong basal textures. Recovery occurs via dislocation climb controlled by lattice self-diffusion, which is promoted by the fine grain size in the starting material. In Domain 1A, prismatic slip is the major deformation mechanism and the basal texture is reduced, and the prismatic planes are tilted towards the compression axis. At higher temperatures of Domain 2, in addition to basal and prismatic slip, pyramidal slip occurs, and cross-slip among the multiple intersecting slip planes is the recovery mechanism that destroys the initial basal texture. At higher strain rates, at which Domain 3 occurs, non-basal slip (prismatic and pyramidal) activity is higher than that for basal slip, and the basal texture is reduced, giving way to favorable prismatic slip orientations. The recovery in this domain occurs via dislocation climb, which is controlled by grain boundary self-diffusion. The activation parameters, tensile ductility, and fracture features further support the conclusions on the rate-controlling mechanisms occurring in each domain.

A superplastic bimodal grain-structured Mg–9Al–1Zn alloy processed by short-process hard-plate rolling

Superplasticity is usually obtained in uniform fine-grained materials processed by severe plastic deformation or multi-pass controlled rolling. However, we highlight a superplastic bimodal grain-structured Mg–9Al–1Zn (AZ91) alloy processed by hard-plate rolling (HPR). The present bimodal grain-structured alloy exhibits a notable elongation of ∼580% upon tension at 573 K and 1 × 10−3 s−1. The superplasticity is accommodated by the cooperative process of continuous dynamic recrystallization (CDRX) within coarse grains and grain boundary sliding (GBS) in fine-grained regions at early tension stage, followed by GBS controlled by grain boundary (GB) self-diffusion being the dominant deformation mode at late stage.

Influence of Grain Size and Thermo-Mechanical Conditions on the Activation Energy for Super Plastic Flow in Ti-6Al-4V Alloy

The essential objective of this work is to establish the influence of grain size and thermo-mechanical conditions on the activation energy for super plastic flow (QSPF) in Ti-6Al-4V alloy by applying the quantum mechanics and relativistic model (QM-RM) proposed by Muñoz-Andrade, in the framework of the unified physics. The QM-RM allows the direct determination of the QSPF in advanced materials at instantaneous thermo-mechanical material working conditions. By applying, the QM-RM on the experimental results reported previously by some authors, it is shown for grain size of 6.1μm, that the calculated QSPF for grain boundary sliding is about 193 and 178 kJ/mol, at 850°C with an efficiency of power dissipation, η=0.65. These results are in closed agreement with the values of 204 and 174 kJ/mol reported previously for grain boundary self-diffusion energy of α-Ti. Nevertheless, for grain size of 0.6μm the calculated QSPF is 142 kJ/mol at 650°C, with an efficiency of power dissipation, η=0.61. As well, in order to understand the phenomenology and mechanics of SPF in Ti-6Al-4V alloy, the variation of the activation energy with the temperature; stress and strain rate is analyzed in association with coupled mechanisms during SPF, such as grain boundary sliding, cooperative grain boundary sliding and self-accommodation process related to the microstructure. In summary, the results of QSPF obtained in this work, by the QM-RM are in closed agreement with results reported previously by using the theoretical and conventional methodology set up by Mohamed and Langdon.

Workability Characteristics and Deformation Mechanisms of Die-Cast AM60 and AZ91 Magnesium Alloys: Correlation with Processing Maps

Thermo-mechanical deformation behavior of the die-cast AM60 and AZ91 alloys was assessed by performing uniaxial compression tests on Gleeble 3800 simulator under different temperatures and strain rate conditions. Low-temperature deformation behavior of both alloys was accessed at 25, 100, and 150 °C, at the strain rates of 0.1 and 1 s−1. Processing maps at a true strain of 0.5 were generated to study the intrinsic hot workability of AM60 and AZ91 alloys. Optimum processing windows were identified, and the evolution of microstructures and textures was correlated with the imposed deformation conditions. The processing map for the AM60 alloy exhibited three dynamic recrystallization (DRX) domains in the ranges: (1) 320-400 °C and 0.2-1.5 s−1, (1A) 300-325 °C and 0.1-0.2 s−1, and (2) 360-400 °C and 3-5 s−1. In Domain 1, DRX occurred predominantly by pyramidal slip and recovery mechanisms by cross-slip as confirmed by the randomized textures. Deformation in Domain 1A is by the combination of basal and prismatic slip. In Domain 2, dynamic recovery was a softening mechanism controlled by the climb. The processing map for AZ91 alloy exhibited two DRX domains in the ranges: (1) 260-310 °C and 0.1-0.5 s−1 and (2) 325-350 °C and 0.8-3 s−1. The rate-controlling mechanism was climb by lattice self-diffusion in Domain 1 and by grain boundary self-diffusion in Domain 2. Both alloys exhibited flow instability at 200 and 250 °C, and therefore, they were unsuitable for processing at these temperatures.

Effect of Calcium on the Hot Working Behavior of AZ31-1.5 vol.% Nano-Alumina Composite Prepared by Disintegrated Melt Deposition (DMD) Processing

AZ31-based nanocomposites are produced by disintegrated melt deposition (DMD) processing. In this investigation, the influence of the addition of Ca to AZ31-1.5 vol.% nano-alumina composite (base) on its hot working behavior is studied to develop a processing route for manufacturing components with these composites. A processing map for the base composite in the temperature range 250–500 °C and strain rate 0.0003–10 s−1 is compared with those for composites with 1% Ca and 2% Ca. The grain size of the base composite is refined by Ca addition and the <10 1 ¯ 0> texture is strengthened. Besides nano-alumina particles, the Ca-containing composites have intermetallic particles (Mg,Al)2Ca present at grain boundaries as well as in the matrix. All the three nanocomposites exhibit three DRX domains, with one of them at high strain rate that facilitates high productivity. Addition of Ca mitigates the occurrence of wedge cracking that occurs in AZ31-1.5NAl composite. Increasing of Ca addition to 2% prevents dynamic recrystallization (DRX) at lower temperatures and strain rates and causes only dynamic recovery. At lower temperatures and higher strain rates, DRX occurs by basal + prismatic slip along with recovery via climb controlled by grain boundary self-diffusion promoted by very fine grain size in the composites.

DGM News

Abstract The hot-deformation behavior of oxygen-free high-conductivity (OFHC) copper has been studied in the temperature range 300–950 °C and strain rate range 0.001 – 100 s−1 using hot-compression tests. The dynamics of the behavior is studied with the help of processing maps, and the-rate controlling processes are evaluated using the standard kinetic rate equation. The processing maps exhibited two domains: (1) in the temperature range 600 – 900 °C and strain rate range 0.01 – 10 s−1, (2) in the range 750 – 950 °C and 30 – 100 s−1. From kinetic analysis of the flow stress data, apparent activation energy values of 159 kJ/mole and 75 kJ/mole have been estimated in the above two domains, respectively. These values suggest that dislocation core self-diffusion and grain boundary self-diffusion are the rate-controlling mechanisms. In both these domains, the average grain diameter varies linearly with the Zener–Hollomon parameter.

Low-Temperature Anelastic Property of Nanocrystalline Ag Fabricated by Gas Deposition Method

It was reported that the internal friction (Q-1) of nanocrystalline (n-) Au and n-Cu showed a rapid increase linearly with temperature above ~200 K. Since the rapid increase in Q-1 decreased with the progress in the grain growth, it was suggested that the anelasticity of grain boundaries in n-Au and n-Cu was thermally activated above ~200 K. In order to pursue the intrinsic behavior of the grain boundaries in n-metals, the internal friction of n-Ag was measured and the result was compared with those of n-Au and n-Cu. Similar to n-Au and n-Cu, Q-1 of n-Ag was also showed the increase linearly with temperature above ~200 K. However, the onset temperature for the linear increase in Q-1 of n-Ag was slight lower than those of n-Au and n-Cu and it can be attributed to the lower activation energy of the grain boundary self-diffusion in Ag.

A theoretical model of grain boundary self-diffusion in metals with phase transitions (case study into titanium and zirconium)

The paper offers a model describing the process of grain boundary self-diffusion in metals with phase transitions in the solid state. The model is based on ideas and approaches found in the theory of non-equilibrium grain boundaries. The range of application of basic relations contained in this theory is shown to expand, as they can be used to calculate the parameters of grain boundary self-diffusion in high-temperature and low-temperature phases of metals with a phase transition. The model constructed is used to calculate grain boundary self-diffusion activation energy in titanium and zirconium and an explanation is provided as to their abnormally low values in the low-temperature phase. The values of grain boundary self-diffusion activation energy are in good agreement with the experiment.

44Ti self-diffusion in nanocrystalline thin TiO2 films produced by a low temperature wet chemical process

Grain boundary self-diffusion of titanium in nanocrystalline TiO2 films is investigated in an extended temperature interval by the radiotracer technique applying the 44Ti isotope. The diffusion annealing treatments are performed at low and high oxygen partial pressures. At low oxygen partial pressure Ti grain boundary self-diffusion follows an Arrhenius type temperature dependence with the pre-exponential factor of 7.25×10−12m2/s and the activation enthalpy of 74.7kJmol−1. At high oxygen partial pressure Ti diffusion follows an Arrhenius type temperature dependence below 773K and changes drastically the behavior above 773K indicating a possible grain boundary structure transition.