Grain Boundary Migration Process Research Articles

Grain boundary segregation behavior of Mo atoms in nanocrystalline Ni–Mo alloy

In this paper, molecular dynamics method was used to design the grain boundary (GB) segregation structure of solute atom Mo in nanocrystalline nickel, and the effect of the segregation structure on the migration and deformation mechanism of nanocrystalline Ni–Mo alloy boundaries was studied. The results indicate that the addition of solute atom Mo can cause segregation at GBs, and the yield strength and tensile strength of nanocrystalline Ni are significantly increased through solid solution strengthening. Mo atoms segregation result in an increase of the GB thickness and stability of the GB. In addition, by labeling GB atoms and tracking their diffusion trajectories, it was found that after adding Mo atoms, the probability of atomic diffusion at GBs decreased. This indicates that Mo atoms reduce GB energy and improve GB stability. Meanwhile, as the Mo content increases, the degree of atomic disorder increases, and the probability of GB migration decreases. This leads to the inability of grains to merge and inhibit their growth, effectively improving the mechanical properties of the material. As the strain increases, the number and length of dislocations increase, and a large amount of entanglement occurs at GBs. With the increase of Mo content, the number of dislocations decreases sharply, with Shockley dislocations having the highest number. Shockley dislocations interact with other dislocations and hinder their generation and movement, forming a more stable dislocation system structure and increasing the strength of the alloy. Our work focuses on observing the influence of GB segregation structure on the mechanical properties and deformation mechanism of nanocrystalline polycrystalline Ni–Mo alloys, establishing the mechanism of the influence of segregation structure on the stability and coarsening of nanocrystalline metal GBs, examining the influence of segregation structure on dislocation motion and GB migration process during deformation, and proposing positive research and development ideas and theoretical basis for designing nanocrystalline metal materials with excellent performance.

Deformation-enhanced recrystallization of titanite drives decoupling between U-Pb and trace elements

Titanite is a common accessory mineral that is useful in determining both age (U-Pb isotopes) and pressure-temperature (P–T) conditions (trace-element composition: Zr, rare earth elements (REE)). However, titanite has a propensity to recrystallize during metamorphism, fluid flow, and deformation, which can result in modifications to its isotopic and trace-element compositions. This modification has implications for the interpretation of titanite dates and the evaluation of pressure–temperature–time paths. The impact of deformation and recrystallization on trace-element mobility in titanite is investigated through microstructural and compositional mapping of titanite crystals from a sheared orthogneiss within an ultrahigh-pressure domain of the Western Gneiss Region (WGR), Norway. Results show that optically coherent titanite single crystals deformed in the dislocation creep regime and recrystallized by the process of grain-boundary migration, forming aggregates of titanite grains. Some of the aggregate grains record Caledonian-exhumation dates, whereas others have an inherited isotopic composition. Individual grains within the aggregate, regardless of their U-Pb isotopic composition, contain patchy zoning that formed during syn- to post-recrystallization fluid alteration and that is characterized by generally decreasing Ca and Ti and increasing Al and Fe from cores to rims. However, Zr and Sr concentrations are broadly zoned with respect to the long axis of the host crystal, without regard for the aggregate grain boundaries. REE do not show any obvious correlation with microstructure or age. These results indicate that many trace elements in titanite are unaffected by multi-stage, deformation-driven recrystallization; in contrast, Pb is variably mobile in these deformed titanite crystals. The combination of microstructural and high-spatial resolution geochemical and isotopic data reveals the potential extent of decoupling between the U-Pb isotopic system and the behavior of trace elements as pressure–temperature conditions change through time.

Phase-field modeling of grain-boundary grooving and migration under electric current and thermal gradient

Grain-boundary migration, void formation as well as associated hillock formation are important mechanisms which lead to the failure of interconnects in the microelectronic packages. An understanding of the underlying physics of each of the phenomena can allow better design of interconnects. In this paper, we formulate a new phase-field model based on a grand-potential formalism for studying the phenomena of grain-boundary grooving under the combined influence of pure diffusion controlled transport, electric current and thermal gradient. We separately investigate the contributions of each of the stimuli towards the process of grain-boundary migration and hillock formation, by performing phase-field simulations as well as comparing with analytical theories. Additionally, we qualitatively reproduce the phenomena observed in experiments on polycrystalline metals, wherein electromigration and thermomigration may act in unison or against each other towards their contributions in grooving, hillocking and void growth.

In situ observation of stress induced grain boundary migration in nanocrystalline gold

In this study, the plastic behaviors of nanocrystalline Au with an average grain size of 18nm were investigated in situ using a home-made tensile device in a transmission electron microscope. We provide the direct experimental results revealed the process of grain boundary migration. The results show that dislocation behaviors are prevalent for large grains. However, for grain sizes below ~15nm, grain boundary migration occurs frequently. The results of our statistical analyses show that grain boundary migration occurs more frequently than grain boundary sliding and rotation in nanocrystalline Au.

Effect of cooperative grain boundary sliding and migration on crack growth in nanocrystalline solids

A new mechanism of fracture toughness enhancement in nanocrystalline metals and ceramics is suggested. The mechanism represents the cooperative grain boundary (GB) sliding and stress-driven GB migration process near the tips of growing cracks. It is shown that this mechanism can increase the critical stress intensity factor for crack growth in nanocrystalline materials by a factor of three or more and thus considerably enhances the fracture toughness of such materials.

Cooperative Grain Boundary Sliding and Migration Process in Nanocrystalline Solids

A new physical mechanism or mode of plastic deformation in nanocrystalline metals and ceramics is suggested and theoretically described. The mode represents the cooperative grain boundary (GB) sliding and stress-driven GB migration process. It is theoretically revealed that the new deformation mode is more energetically favorable than "pure" GB sliding and enhances the ductility of nanocrystalline solids in wide ranges of their structural parameters.

Recrystallization mechanism for the grain refinement in undercooled DD3 single-crystal superalloy

An investigation of the grain refinement in the bulk undercooled DD3 single-crystal superalloy is performed. The application of a molten salt denucleating technique combined with thermal cycle enables such investigation over a wide range of undercooling up to 210 K. The microstructure formation has been discussed with respect to undercooling, nucleation and recalescence as well as recrystallization during recalescence and post-recalescence, respectively. Furthermore, TEM and SEM techniques are employed to reveal the recrystallization mechanism for grain refinement occurring at the critical undercooling Δ T *, and to explain the influence of ϒ′ precipitation and solute segregation on the recrystallization, i.e. grain-boundary migration process.

Diffusional penetration during diffusion-induced grain-boundary migration process in an Al-Zn couple

The existence of diffusion-induced grain-boundary migration (DIGM) has been re-examined by electron probe micro analysis and analytical electron microscopy in the aluminium substrate of Al-Zn diffusion couple annealed in the temperature range 395–535 K. The investigation revealed two basic kinds of DIGM: laminar and turbulent. The laminar kind occurs over the whole temperature range and is characterized by a small migration distance and large migration depth. The zinc enrichment at a sample surface is 4.0–5.0 wt% and gradually decreases with increasing depth. The turbulent kind is limited to annealing temperatures above 450 K. In this case, the width of the alloyed zone is much greater, close to the surface of sample and then dramatically decreases, showing a behaviour similar to the laminar morphology. The zinc content at the surface of sample is 8.0–9.0 wt%. The diffusivities of DIGM calculated based on Cahn's equation agree well with the values of stationary grain boundary in diluted AlZn alloys. Evidence for the existence of DIGM was the asymmetry of the zinc profile with regard to the final position of the boundary. Microanalytical scan across the alloyed zone showed an abrupt change of the zinc concentration at the moving boundary. This suggests that the role of volume diffusion during DIGM is not so important and a considerable chemical contribution to the total driving force should exist.

Recrystallization in internally oxidized Cu-Ag-Al alloys

The isothermal recrystallization kinetics of rolled Cu-Ag alloys containing up to 0.9 vol % alumina have been followed through hardness measurements; and nucleation rates have been obtained by X-ray back-reflection techniques. Electron microscopic studies have been carried out to characterize the as-deformed structures and the structural changes occurring during the annealing process. The recrystallization of the alloys was either accelerated or retarded as the alumina content increased. This effect has been found to depend both on the dispersion parameters and the degree of deformation. The recrystallization behaviour of these alloys is controlled primarily by the nucleation process. Nucleation occurred after an incubation period and involved the formation of subgrains from the cell structure and their subsequent growth to a viable size. The difficulty of the formation of viable nuclei has been attributed to the particle restraint on sub-boundary migration and to the lack of heavily distorted regions in the defect structure. The rate of nucleation was a function of time and found to depend on the dispersion, initial deformation and the temperature of anneal. The process of grain boundary migration during recrystallization was anisotropic.

Recrystallization studies in solid-solution alloys of Cu-Ag-Al

The isothermal recrystallization kinetics of rolled Cu-Ag alloys containing up to 1.2 at.% Al have been followed metallographically and by hardness measurements and the nucleation rates measured by X-ray back-reflection techniques. Electron-microscopic studies have been carried out to characterize the as-deformed structures and the structural changes occurring during the annealing process. The recrystallization of the alloys was retarded as the aluminium content increased. Nucleation occurred both in the bulk and along the existing grain boundaries and the rates of nucleation varied with time. These effects have been found to be dependent on the deformation structure, alloy composition and the temperature of anneal. The process of grain-boundary migration during recrystallization was very anisotropic and this has been shown to be caused primarily by the nature of the deformation structure.