Grain boundary diffusion of hydrogen in nickel

Grain boundary diffusion of hydrogen in nickel was quantified through permeation measurements performed on fine-grained foils produced by electrodeposition. The permeation data were analyzed with a modified version of the Hart equation. The grain boundary diffusion coefficient at 30°C is at least 3 × 10⁻¹² m²/s, which is a factor of 40 greater than the lattice diffusion coefficient; however, the analysis indicates that a 1000-fold increase may not be unreasonable. The activation energy for grain boundary diffusion in this system is 30 kJ/mole, which is approximately three-fourths of the activation energy for hydrogen diffusion in single-crystal nickel. These results indicate that grain boundary diffusion should be considered in models of hydrogen transport during hydrogen-induced cracking of nickel and nickel-based alloys. Formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

Grain boundary faceting and abnormal grain growth in nickel

A correlation between grain boundary faceting and abnormal grain growth has been observed in recrystallized polycrystalline Ni at varying annealing temperatures, with or without C added. Carburized Ni specimens deformed to 50 pct show faceted grain boundaries and abnormal grain growth when annealed at temperatures below 0.7 T _m, where T _m is the melting point of Ni in absolute scale. When annealed at or above 0.7 T _m, the grain boundaries are smoothly curved and, therefore, have a rough structure, and normal grain growth is observed. In the specimens annealed in vacuum without carburization, all grain boundaries are faceted at 0.55 T _m, and some of them become defaceted at higher temperatures. The specimens annealed in vacuum at temperatures between 0.55 and 0.95 T _m show abnormal grain growth. When the grain boundaries have a rough structure and are, therefore, nearly isotropic, normal grain growth is indeed expected, as shown by the simulation and analytical treatment. When all or a fraction of the grain boundaries are faceted, with the facet planes corresponding to the singular cusp directions in the variation of the boundary energy against the inclination angle, abnormal grain growth can occur either because some grain boundary junctions become immobile due to a torque effect, or the growth occurs by a step mechanism.     

Grain boundary faceting and abnormal grain growth in nickel

A correlation between grain boundary faceting and abnormal grain growth has been observed in recrystallized polycrystalline Ni at varying annealing temperatures, with or without C added. Carburized Ni specimens deformed to 50 pct show faceted grain boundaries and abnormal grain growth when annealed at temperatures below 0.7 T _m , where T _m is the melting point of Ni in absolute scale. When annealed at or above 0.7 T _m , the grain boundaries are smoothly curved and, therefore, have a rough structure, and normal grain growth is observed. In the specimens annealed in vacuum without carburization, all grain boundaries are faceted at 0.55 T _m , and some of them become defaceted at higher temperatures. The specimens annealed in vacuum at temperatures between 0.55 and 0.95 T _m show abnormal grain growth. When the grain boundaries have a rough structure and are, therefore, nearly isotropic, normal grain growth is indeed expected, as shown by the simulation and analytical treatment. When all or a fraction of the grain boundaries are faceted, with the facet planes corresponding to the singular cusp directions in the variation of the boundary energy against the inclination angle, abnormal grain growth can occur either because some grain boundary junctions become immobile due to a torque effect, or the growth occurs by a step mechanism.     

Role of grain-boundary diffusion in the grain growth in nanocrystalline nickel

The grain growth in a nanocrystalline nickel during nonisothermal annealing is studied by differential scanning calorimetry (DSC) and transmission electron microscopy. Nanocrystalline nickel is prepared by electrodeposition at a pulsed voltage. Its nonisothermal annealing is accompanied by anomalous grain growth; at a temperature corresponding to a DSC peak, a bimodal grain structure forms. The processing of DSC signals in terms of the Avrami formalism permits the grain growth activation energy to be determined, which is found to be close to the activation energy of grain-boundary self-diffusion. The anomalous grain growth creates conditions such that grain-boundary diffusion is a controlling stage of the process.     

Analysis of mean square penetration depth in grain boundary diffusion

Exact and approximate expressions for mean square penetration depth (MSPD) in grain boundary diffusion are derived. The MSPDs in boundary diffusion are functionally different from those in lattice diffusion. The quantitative results obtained provide more reliable estimates for the penetration depth of a diffusant in polycrystalline materials, thin films, and bicrystals.     

Dy gradient and coercivity in grain boundary diffusion processed Nd-Fe-B magnet

By controlling Dy vapor deposition process, the amount of Dy that diffused into the magnet was increased gradually from 0.1 wt.% to 0.3 wt.%. Compared with the original status, the coercivity increment was not proportional to the Dy diffusion amount. Subsequent H_(cj) and Dy content gradient data showed that slope of the 0.3 wt.% sample gradient was bigger than that of 0.1 wt.% one, and the gaps between outer flakes and inner flakes enlarged with the increasement of Dy diffusion amount. Although Dy mostly enriched in triple-junction regions in electron-probe microscope analysis(EPMA) images, the following Auger depth graph showed that Dy content was as high as 3.0 at.% in 1.5 mm deep center. It proved that Dy tended to get into the main phase rather than stayed in the grain boundary during the diffusion process, and over-diffusion of Dy in the main phase was unhelpful for the coercivity enhancement.     

Equilibria between cerium or neodymium and oxygen in molten iron

Autoradiographs show that there is an obvious reaction between Ce and Nd in liquid iron and the MgO/CaO crucible wall. For reaching the Ce-O, Nd-O equilibrium, a long melting period and the addition of rare earth elements (RE’s) in several batches were needed to ensure the full reaction between the RE’s in the melt and crucible wall. The dissolved Ce or Nd content in iron was measured by means of radioactive isotopes¹⁴¹Ce or¹⁴⁷Nd and electrolysis in the organic electrolyte. The oxygen activity was measured by solid electrolyte sensors made of ZrO₂(MgO) tube. The relationships between the equilibrium constants and the temperatures for reactions Ce₂O₃ (s) = 2[Ce] + 3[O], CeO₂ (s) = [Ce] + 2[O], and Nd₂O₃ (s) = 2[Nd] + 3[O], as well as the corresponding thermodynamic parameters, have been determined. Formerly Graduate Student at the University of Science and Technology Beijing     

An analysis of transient cavity growth controlled by grain boundary diffusion

An analysis of the elastic transient associated with grain boundary diffusion controlled cavity growth has been made for both two-dimensional and three-dimensional (axisymmetric) cavities using a technique developed by Raj. We find that characteristic times for the elastic transient,t _E, and for the steady state diffusional growth process,t _D, can be used to formulate expressions for the initial growth of a cavity of unit volume as follows:     

Utility and influence mechanism of densification modulation on grain boundary diffusion in NdFeB magnets

Grain boundary diffusion technology is pivotal in the preparation of high-performance NdFeB magnets.This study investigates the factors that affect the efficiency of grain boundary diffusion,starting from the properties of the diffusion matrix.Through the adjustment of the sintering process,we effectively prepared magnets with varied densities that serve as the matrix for grain boundary diffusion with TbH,diffusion.The mobility characteristics of the Nd-rich phase during the densification stage are leveraged to ensure a more extensive distribution of heavy rare earth elements within the magnets.According to the experimental results,the increase in coercivity of low-density magnets after diffusion is significantly greater than that of relatively high-density magnets.The coercivity values measured are 805.32 kA/m for low-density magnets and 470.3 kA/m for high-density magnets.Additionally,grain boundary diffusion notably enhances the density of initial low-density magnets,addressing the issue of low density during the sintering stage.Before the diffusion treatment,the Nd-rich phases primarily concentrate at the triangular grain boundaries,resulting in an increased number of cavity defects in the magnets.These cavity defects contain atoms in a higher energy state,making them more prone to transition.Consequently,the diffusion activation energy at the void defects is lower than the intracrystalline diffusion activation energy,accelerating atom diffusion.The presence of larger cavities also provides more space for atom migration,thereby promoting the diffusion process.After the diffusion treatment,the proportion of bulk Nd-rich phases significantly decreases,and they infiltrate between the grains to fill the cavity defects,forming continuous fine grain boundaries.Based on these observations,the study aims to explore how to utilize this information to develop an efficient technique for grain boundary diffusion.     

Characterization of diffusion induced grain boundary migration in the Ag/Cu system

The phenomenon of diffusion induced grain boundary migration (DIGM) when silver diffuses along copper's grain boundaries was confirmed through the characterization of the microstructure and the morphology of the migrated boundaries, the discontinuity and asymmetric character of the concentration profile, the dislocation configuration and the kinetics of migration by means of SEM, EPMA, TEM and AEM. The experimental results were discussed to prove the existence and the characteristics of DIGM in Ag/Cu system.     

On the chemistry of grain boundary segregation and grain boundary fracture

This paper considers the problem of impurity segregation in metals and the effect of these impurities on grain boundary cohesion. The primary goal of this paper is to provide a physical model that will allow us to think about these two processes. We describe both of them in chemical terms. Segregation is treated as a distribution of a solute between two phases. In this way, it is a typical example of heterogeneous equilibrium. We also consider the various driving forces for solute segregation and find that the correlation between decreased solubility and increased segregation, first proposed by Hondros and Seah,^[9] is still an adequate one. We introduce the discussion of grain boundary fracture by pointing out that as the impurity enters the boundary, it establishes chemical bonds with the structural units of the boundary. The segregated boundary can then be thought of as a string of molecular units with bonds of different types. Some of these bonds will be weaker than others, and they will be the ones that eventually fracture when a stress is applied. We consider the cause of these weak bonds and suggest that the primary reason for them is the transfer of electronic charge from the metal atoms to the impurity, as proposed in previous work.^[3] However, some of the ideas in the earlier models should be amended based on new results obtained from the quantum mechanical analysis of bonding in metals presented by McAdon and Goddard.^[10,11] We also suggest that intergranular brittleness of intermetallic compounds such as Ni₃Al, which occurs in the absence of impurity segregation, can be explained by the charge distribution present at the grain boundary. Finally, we provide a critique of other models that have been used to describe grain boundary fracture and segregation. This paper is based on a presentation made in the symposium “Interface Science and Engineering” presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26–29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee.     

A numerical study of cavity growth controlled by coupled surface and grain boundary diffusion

The diffusive growth of both two dimensional and axisymmetric cavities initially having equilibrium shapes and located on grain boundaries loaded in tension is studied using finite difference techniques. The shape evolution and growth kinetics of individual cavities as well as the time required for adjacent cavities to grow together is studied as a function of applied stress and the ratio of grain boundary to surface diffusivity. A key feature of this treatment is that the diffusional processes in the grain boundary and on the cavity surface are coupled by boundary conditions at the tip of the cavity. When surface diffusion is much slower than grain boundary diffusion, the cavities become crack-like during growth, and the fracture time varies reciprocally with the third power of the applied stress. When grain boundary diffusion is the slower process, the cavities remain rounded during growth, and the fracture time varies reciprocally with the first power of the stress. The transition between these limiting kinds of behavior is described and the results are compared with previous treatments of these problems.     

Effect of Zn concentration on diffusion induced grain boundary migration in Cu -Zn system

Cu-Zn alloy system with three different compositions has been chosen to study the time, temperature and composition dependence of the Diffusion Induced Grain boundary Migration (DIGM) in the temperature range of 277–427°C. The grain boundary migration follows parabolic rate law as a function of time. The diffusivity, D_bα, was calculated from concentration-distance profile using growth rate, v. The activation energy for diffusion is found to be 101kJ/mol which is nearly half of the activation energy required for volume diffusion indicating that preferential grain boundary diffusion is more favorable than volume diffusion leading to grain boundary migration in Cu-Zn system.     

The intergranular embrittlement of nickel by hydrogen: The effect of grain boundary segregation

The mechanical behavior of polycrystalline nickel specimens that were deformed in tension and cathodically charged with hydrogen simultaneously was investigated with particular emphasis on the fracture of such electrodes. This procedure leads to definite, if, however, weak serrated yielding and also markedly reduces the elongation at fracture compared to polycrystals unexposed to hydrogen. Moreover, in contrast to hydrogenated nickel monocrystals which neck down to give a chisel-edge fracture typical of ductile metals, hydrogenated polycrystal fractures are brittle and intergranular. The embrittlement of nickel by hydrogen is shown by means of Auger electron spectroscopy to be associated with the segregation of hydrogen recombination poisons to the grain boundaries. In essence, it is suggested that the entry of hydrogen into the nickel specimens occurs preferentially in the proximity of grain boundary intersections with the free surface, due to the presence therein of Sb and Sn which act as hydrogen recombination poisons and stimulate the absorption of hydrogen by the metal. The presence of such impurities in the grain boundaries suggests that a pressure mechanism is not involved in the intergranular cracking.