Grain Boundary Diffusion Mechanism Research Articles

Interdiffusion of Cu substrate/electrodeposits for Cu/Co, Cu/Co-W, Cu/Co/Ni and Cu/Co-W/Ni systems

The interdiffusion of electrodeposited Ni, Co and Co-W with Cu substrate were studied, at temperature ranging from 400 to 800 °C, using energy dispersive X-ray spectroscopy (EDS). Chemical interdiffusion coefficients were calculated using the Boltzmann-Matano analysis. Intermetallic phase formation was studied by X-ray diffraction (XRD) and microstructures were observed with a scanning electron microscopy (SEM). No remarkable interdiffusion was observed in the Cu/Co and Cu/Co-W systems up to 800 °C. However, after annealing at 400–800 °C for the Cu/Co/Ni system, accumulation of Cu atoms was found at the /Ni interface in which Cu had further diffused into the Ni electrodeposit. It is suggested that Cu diffuses through the Co layer by the grain-boundary diffusion mechanism. Interdiffusion of Cu through electroplated Co coating to Ni was compared with electroplated alloy coating of 65 wt.% Co-35 wt.% W. The structure of the Co-W coating has changed from a mixture of Co-like hcp and fcc structures to a solely fcc structure upon annealing at 1100 °C for 90 min. The diffusion barrier properties of Co and Co-W were compared with those of Ni. The results show that Co is a more effective barrier than Ni for Cu diffusion up to 219 h at 400 °C. A Co-W alloy coating, of 36 wt.% W, is a more effective barrier for Cu diffusion than Ni heated up to 72 h at 500 °C. However, interpenetration of Cu through Co is more pronounced than diffusion of Cu through Ni for temperatures ranging from 500 to 800 °C. Interdiffusion of Cu through the Co-36 wt.% W coating is more pronounced than diffusion of Cu through Ni for temperatures ranging from 600 to 800 °C whereas Co-W is a more effective barrier than Co for Cu diffusion.

Novel Electromigration Failure Mechanism for Aluminium-Based Metallization on Titanium Substrate

AbstractIn this paper a phenomenological characterisation of an anomalous grain growth, observed after classical electromigration lifetests, on Al-l%Si-0.5%Cu multigrain stripes, deposited at high temperature (460°C) on a Ti substrate, is reported. Failure analysis, carried out by Scanning Electron Microscopy (SEM) and Focus Ion Beam (FIB), has detected an abnormal single-grain growth in the vertical direction, starting from the Ti/Al interface. Medium Time to Failure (MTF) data are compared with those of stripes on Ti/TiN substrate, on which no grain growth was observed. The growth of the anomalous grains has been related to the Ti/Al interface properties. Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD) and Atomic Force Microscopy (AFM) analyses concurrently show a higher texture of the film on the Ti, compared with the film on the Ti/TiN substrate. The tight columnar orientation of grain boundaries strongly limits the mechanism of grain boundary diffusion during electromigration stress. The atomic flux is then forced to take place at the Ti/Al interface, where the epitaxial growth of the Al single grains is favoured.

On the creep rupture life prediction

Combined effects of stress, σA, and fracture cavitation on the creep rupture life, tR, have been studied in conventionally cast MAR-M 002 alloy tested at 1173 K (900 °C) over a limited range of stress (σA = 200–400 MPa). It is predicted that the creep fracture cavity growth is controlled by the coupled power-law creep with the grain-boundary diffusion mechanism. On the basis of this prediction the Edward–Ashby model overestimates the creep rupture life although this model correctly describes the trend in the data. The observation of a linearity between the cavity density, NA, and the product eRtRσ4A indicates that this relationship can be used to predict the creep time, tR, where eR is the rupture strain. Furthermore, another empirical method is the creep-fracture parameter, Kf = σf(πac)1/2, approach, developed using the modified Griffith–Irwin type of relationship, which can also be used to predict the creep rupture life in the present alloy, where σf is the creep fracture stress (or the applied stress, σA) and ac the crack (or cavity) size.

EFFECT OF SOLID SOLUTION ADDITIVES ON THE DENSIFICATION AND CREEP OF GRANULAR CERAMICS

The effect of solid solution additives on the densification and creep of granular ceramics was investigated for a model system consisting of CeO2 solid solutions with Y2O3 as the additive. In the sintering of powder compacts at 1150°C, the densification rate of CeO2 at a given density decreased significantly with increasing Y3+ concentration. The reduction in the densification rate reached a factor of ≈ 100 for a Y3+ concentration of 6 at.%. Creep of dense specimens was investigated at constant strain rates of 10−5 and 10−4 s−1 in air at 1200°C. After compensation for differences in grain size, the creep rate was also found to decrease significantly with increasing Y3+ concentration. If the creep rate is assumed to be controlled by a mechanism of grain boundary diffusion, then the magnitude of the decrease is in good agreement with that observed in the sintering experiments. The results strongly indicate that it may be possible to predict changes in the steady-state creep behavior from observed changes in the sintering behavior provided that matter transport occurs by the same mechanism. They also indicate that the solid solution approach may have considerable merit for controlling the creep resistance of rare earth oxides that commonly have a high solubility for many cations. © 1997 Acta Metallurgica Inc.

Combined effects of thin-section size, grain size and cavities on the high temperature creep fracture properties of a nickel-base superalloy

The creep fracture characteristics of a conventionally cast (CC) MAR-M 002 superalloy, controlled by the grain-boundary diffusion mechanism, have been investigated at various specimen section-sizes D, and grain sizes, d. It is observed that the creep rupture strain (or ductility, eR, is controlled by the D2/(nGl) ratio, where nG is the number of grains per cross-section of specimen and l is the half-cavity spacing, at the creep conditions (900° C/ 300MPa). A rapid improvement in creep rupture life can be made by reducing the (dC/d)/D ratio [ or, equivalently, the (dCnG)/D2 ratio] below a critical value (∼ 100×10-8 10μm-1), where dC is the cavity size. The thin-section size dependent creep rupture life, tR/D, and creep rupture strain, eR/D, are explained on the basis of grain boundary sliding (GBS) and creep crack growth (CCG) behaviour of the alloy. eR/D and tR/D can be improved by reducing the GBS rate. A large improvement in tR/D can be achieved by reducing the GBS and CCG rates below the critical values of these rates by reducing the crack size through increasing the grain size above a critical value. (Above a critical grain size value the crack size becomes so small that, as a result, a large increment of tR is achieved.)

Preparation of nanocrystalline yttria-stabilized zirconia

Nanocrystalline powder with an average crystallite size of 8–12 nm, which was produced by a combustion synthesis process, was used to prepare dense, nanocrystalline articles. Green compacts of high green density were prepared by dry pressing and densified by a fast-firing process. During fast-firing, the dwell temperature significantly affected the final grain size and final density. On the other hand, the ranges of heating rates and dwell times that were used had a much less significant effect on the final density and final grain size. It was determined, however, that a high final density (>99% ρth) and a very fine final average grain size (<200 nm) can be simultaneously achieved under three different firing conditions. The high densification rates are, in part, a result of the minimal coarsening that the particles undergo when the sample is taken rapidly through the temperature regime in which surface diffusion predominates to the temperature regime in which the densification mechanisms of grain boundary and lattice diffusion predominate.

Alumina scale growth at zirconia-MCrAlY interface: a microstructural study

High-temperature oxide scale growth at the ceramic-metal interface is a major contributor to the thermomechanical resistance of thermal barrier coatings for hot stages of gas turbines. In order to better understand this phenomenon, microstructural observations of the alumina scales formed at 1100 and 1200 °C under air, between low-pressure plasma-sprayed NiCrAlY and air plasma-sprayed ZrO2-8.5 wt % Y2O3, have been performed by classical and analytical transmission electron microscopy on transverse thin foil specimens. The evolution of the oxide grain morphology from the metal-oxide to the oxide-oxide interface suggests that the scale growth principally takes place at the metal-oxide interface. Segregation of yttrium at oxide grain boundaries has been detected as well as significant quantities of zirconium inside the alumina grains. The oxide growth seems to be dominated by a classical grain-boundary oxygen diffusion mechanism. The presence of zirconium inside the alumina grains also suggests that Al2O3 partially forms by chemical reduction of ZrO2 by AI. The comparison between the microstructures observed and that of alumina scales grown under similar conditions on bare MCrAlY alloys gives some insight into how the ceramic top-coat modifies NiCrAlY high-temperature oxidation mechanisms.

The Mechanism of Grain Boundary Diffusion in Condition of Nonequilibrium State of Structure

The Mechanism of Grain Boundary Diffusion in Condition of Nonequilibrium State of Structure

Transient creep during transformation in Cd-Zn alloys

Transient creep of Cd-2 wt. % Zn and Cd-17·4 wt. % Zn alloys has been studied under different constant stresses ranging from 6·4 MPa to 12·7 MPa near the transformation temperature. The results of both compositions showed two transient deformation regions, the low temperature region (below 483 K) and the high temperature region (above 483 K). From the transient creep described by the equationɛ tr=Bt n, whereɛ tr andt are the transient creep strain and time. The parametersB andn were calculated. The parameterB was found to change with the applied stress from 0·3×10−4 to 3×10−4 and from 0·6×10−4 to 18×10−4 for Cd-2 wt. % Zn and Cd-17·4 wt. % Zn, respectively. The exponentn was found to change from 0·8 to 0·95 for both alloys. The parameterB was related to the steady state creep rate $$\dot \varepsilon _{st} $$ through the equation $$B = B_0 \dot \varepsilon _{st}^\gamma $$ , the exponentγ was found to be 0·5 for Cd-2 wt. % Zn and 0·6 for the eutectic composition. The activation energies of transient creep in the vicinity of the transformation regions (above 483 K) were found to be 50·2 kJ/mole for Cd-2 wt. % Zn and 104·7 kJ/mole for the eutectic composition characterizing the mechanisms of grain boundary diffusion and volume diffusion in Cd, respectively.

Redistribution of alloy elements in aging of austenitic chrome-manganese steel containing nitrogen

In interrupted decomposition of the austenite of chrome-manganese steel containing nitrogen in the 600–900°C range the alloy element content in the nitride phase formed changes. The quantity of chromium increases while that of iron decreases significantly. The manganese content in the nitride is low and remains approximately constant. The residual austenite in the grains which have undergone interrupted decomposition is gradually depleted of chromium and nitrogen. The growth of pearlitelike colonies consisting of transformed austenite and type M2N nitride is controlled simultaneously by the mechanisms of grain-boundary and three-dimensional diffusion.

The pressure dependence of grain-boundary diffusion of65Zn in polycrystalline aluminium

Abstract The pressure dependence of the grain-boundary diffusion of 65Zn in polycrystalline aluminium has been measured in the B-kinetic regime at 593 K in the pressure range 0–1·2 GPa. Separating the subgrain and large-angle grain-boundary segments of the penetration plots, it has been found that the activation volumes are (0·86 ± 0·06)Ω and (0·80 ± 0·10)Ω respectively (Ω is the atomic volume). These results are consistent with the vacancy mechanism of grain boundary diffusion

Electron-optical investigation of the sintering kinetics of tungsten powders of spherical particle shape. II. Correlation between contact growth kinetics and shrinkage kinetics

It has been established that the variation of macroscopic and local shrinkages as functions of particle contact size are similar in character. This is evidence that in the sintering of tungsten powders contact growth and densification are due to a single mechanism. A calculation of contact growth involving geometric models on the basis of elementary mass transport mechanisms gives good agreement in the case of mechanisms of viscous flow and surface diffusion. The latter is not a controlling mechanism of contact growth, as is shown by the unique contact size dependence of shrinkage obtained in this work. Surface diffusion, however, does not lead to shrinkage. In [12] a description is given of cases where no mass transport is induced by surface diffusion. In the initial stages of sintering (ΔV/V ≈ 6–8%) there is a correlation between macroscopic shrinkage and shrinkage calculated on the basis of viscous flow and grain-boundary diffusion mechanisms. In the whole temperature and time ranges investigated there is a close match between shrinkage calculated on the basis of a viscous flow mechanism and local shrinkage values.

Annealing kinetics of thin permalloy films

Low-temperature (250-397°C) annealing was found to cause a large decrease in the resistivity of radio-frequency sputter-deposited thin (≤ 500 Å) Permalloy films. A transmission electron microscope was used to investigate a probable microstructural change occurring during the annealing. It was found that the low-temperature annealing induced considerable grain growth in these films. Furthermore, an electron diffraction analysis has shown that this grain growth was accompanied by the formation of the ordered phase (Ni <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Fe). The use of dark-field imaging revealed that the small ordered region appears to be formed around the disordered regions via a grain-boundary diffusion mechanism. The observed resistivity decrease was proposed to occur by the structural ordering as a result of the annealing. The measured activation energy for this process was 0.72 eV. This activation energy appears to be associated with the grain boundary migration of excess vacancies trapped during film formation.

Diffusion, Migration and the Atomic Structure of some Grain Boundaries

AbstractThe atomic structure of representative tilt boundaries in gold has been determined by high resolution transmission electron microscopy. Characteristic and varying regions of decreased density and coordination have been identified and related to mechanisms of grain boundary diffusion and migration

Grain-boundary diffusion in metals

AbstractThe available models for grain-boundary diffusion are reviewed together with the relevant mathematical techniques for extracting the grain-boundary diffusivity from the various types of experimental data. The more useful experimental techniques are described, fo11owed by a presentation of results that shed light on the models and mechanisms of grain-boundary diffusion. A ‘pipe’ mechanism of diffusion based on the well established dislocation model seems most appropriate for small-angle boundaries. Open channels which have atomic configurations somewhat like dislocation cores, probably play a major role in high-angle grain-boundary diffusion. Dissociated dislocations and stacking faults are not efficient paths for grain-boundary diffusion. The diffusion and computer-modelling experiments are consistent with a vacancy mechanism of diffusion by a rather well localized vacancy. The atomic migration is a three-dimensional process with jumps away from, and back towards, the dislocations as well as jumps...

Grain-boundary de-segregation and intergranular cohesion in Si-Al-O-N ceramics

The influence of heat-treatment on high-temperature creep and sub-critical crack growth in hot-pressed β′ Si-Al-O-N ceramics has been analyzed from microstructural evidence and determination of stress exponents and activation energies. The most significant change is the suppression of cavitation during creep and of the cavity-interlinkage mechanism for slow crack propagation. A creep mechanism of grain-boundary diffusion is characterized by stress exponent n=1 and unusually high activation energy >820 kJ mol−1. The microstructural origin of the transformation in grain-boundary dominated properties is mainly the removal of triple-junction glassy residues within which cavities are nucleated. This is caused by grain-boundary diffusion of metallic impurities (Mg, Mn, Ca) into a surface silica oxidation layer, and consequent crystallization of the remaining glass components as β′. There is a continued improvement in grain-boundary cohesion and increased difficulty of grain-boundary diffusion following the stage at which triple-junction glass is removed. The resultant ceramics, in addition to superior mechanical behaviour, have an increased temperature for application due to a marked reduction in susceptibility to dissociation above 1400° C.

ChemInform Abstract: THEORY OF THE DEPENDENCE OF DENSIFICATION ON GRAIN GROWTH DURING INTERMEDIATE‐STAGE SINTERING

AbstractEs wird eine Beziehung zwischen der Sinterzeit und der Porosität einer Probe im intermediären Sinterzustand abgeleitet.

ChemInform Abstract: CHEMICAL DIFFUSION IN POLYCRYSTALLINE AL2O3 AS DETERMINED FROM ELECTRICAL CONDUCTIVITY MEASUREMENTS

AbstractBei Untersuchungen der zeitlichen Änderung der elektrischen Leitfähigkeit von polykristallinem Al2O3 bei verschiedenen Og‐Drücken wurde ein Defektdiffusionskoeffizient ermittelt, der der jeweils schneller diffundierenden Spezies, dem Al‐ oder O‐Ion,zuzuordnen ist.

Chemical Diffusion in Polycrystalline Al2O3 as Determined from Electrical Conductivity Measurements

By analyzing the changes with time of the electrical conductivity of polycrystalline Al2O3 after the O2 pressure was changed, a defect diffusion coefficient was obtained which was assigned to the Al or O ion, whichever is the faster-diffusing species. Both decreased grain size and MgO addition increase the defect diffusion coefficient. The grain-boundary defect diffusion coefficient for the undoped material was estimated to be: and that for the MgO-doped material was over the range 1100° to 1350°C (δ is the effective thickness of the boundary and s the coefficient of segregation of defects to the boundary region). The mechanism of grain-boundary diffusion is discussed in terms of defect mobility.

Initial Sintering with Constant Rates of Heating

Initial sintering of several materials was studied by measuring powder compact densification at constant rates of heating (CRH). The CRH technique was extremely sensitive to the particle size distribution and other characteristics of the compacts. Although the CRH method circumvents several problems encountered in isothermal studies, it cannot be used to identify the mechanism of diffusion. Using the method on carefully prepared alumina powder compacts and assuming a grain-boundary diffusion mechanism, an activation energy of 115±10 kcal/mol was obtained. Zirconia (yttria-stabilized) and titania also exhibited a single densification mechanism with diffusion coefficients which correlate well with values obtained by isothermal measurements.