Kinetics Of Grain Boundary Migration Research Articles

Kinetics of grain boundary migration in nanosized Al polycrystals

Classical theories of grain growth assume that grain boundary curvature and velocity are linearly correlated. However, recent experimental observations seem to imply that the relationship is more complex in polycrystalline materials. Here, we determined the velocity and curvature of approximately 12000 grain boundaries from molecular dynamics simulations of the annealing of randomly textured nanosized polycrystalline Al. The grain boundary kinetics was studied at different scales and compared with theoretical predictions. At the grain level, that is, attributes averaged over all grain boundaries of a grain, the kinetics behavior showed a linear correlation of curvature and velocity. When studying grain-boundary-level behavior (i.e., attributes of individual grain boundaries are considered), we found a clear correlation between the sign of the grain boundary curvature and the direction of migration. Nevertheless, considering all the grain boundaries, the correlation coefficient between the magnitude of velocity and curvature was found to be low (0.34) although a strong linear correlation was observed for a subset of high angle, long-lived, initially large grain boundaries with uniform curvature distribution and similar 5 crystallographic degrees of freedom. Furthermore, some grain boundaries that are close in crystallographic space showed different kinetics. This elucidates the complexity of grain boundary migration in the nanocrystalline ensemble as a result of complex dependence of grain boundary mobility and energy on grain boundary character, and the existence of other driving forces that affect the behavior of grain boundary migration. Among others, defect distributions and variation in stress distribution between neighboring grains are found to influence grain boundary migration.

Impurity diffusion behavior study of electroplated copper films annealed by linear shaping laser mobile scanning system

Impurities due to additives in electroplated copper interconnects diffuse during grain growth. High-resolution electron backscatter diffraction and TOF-SIMS (Time-of-flight secondary ion mass spectrometer) results reveal that the electroplated copper films after laser annealing by linear shaping laser mobile scanning system (LALS) have higher recrystallization fraction and less impurity content than traditional thermal annealing. Impurity diffusion behavior and grain boundary migration are inseparable. The effect of LALS on the impurity diffusion of electroplated Cu films is the result of the introduced local temperature gradient, which works collaboratively on the thermodynamics and kinetics of grain boundary migration. And the amount of impurity diffusion is related to the proportion of different species of impurities. The findings are meaningful for the metallization of electroplated copper interconnects.

Shock-induced migration of asymmetry tilt grain boundary in iron bicrystal: A case study of Σ3 [110]**Project supported by the Fundamental Research for the Central Universities of China, the National Key Laboratory Project of Shock Wave and Detonation Physics of China, the Science and Technology Foundation of National Key Laboratory of Shock Wave and Detonation Physics of China, the National Key

Many of our previous studies have discussed the shock response of symmetrical grain boundaries in iron bicrystals. In this paper, the molecular dynamics simulation of an iron bicrystal containing Σ3 [110] asymmetry tilt grain boundary (ATGB) under shock-loading is performed. We find that the shock response of asymmetric grain boundaries is quite different from that of symmetric grain boundaries. Especially, our simulation proves that shock can induce migration of asymmetric grain boundary in iron. We also find that the shape and local structure of grain boundary (GB) would not be changed during shock-induced migration of Σ3 [110] ATGB, while the phase transformation near the GB could affect migration of GB. The most important discovery is that the shock-induced shear stress difference between two sides of GB is the key factor leading to GB migration. Our simulation involves a variety of piston velocities, and the migration of GB seems to be less sensitive to the piston velocity. Finally, the kinetics of GB migration at lattice level is discussed. Our work firstly reports the simulation of shock-induced grain boundary migration in iron. It is of great significance to the theory of GB migration and material engineering.

Linking stress-driven microstructural evolution in nanocrystalline aluminium with grain boundary doping of oxygen.

The large fraction of material residing at grain boundaries in nanocrystalline metals and alloys is responsible for their ultrahigh strength, but also undesirable microstructural instability under thermal and mechanical loads. However, the underlying mechanism of stress-driven microstructural evolution is still poorly understood and precludes rational alloy design. Here we combine quantitative in situ electron microscopy with three-dimensional atom-probe tomography to directly link the mechanics and kinetics of grain boundary migration in nanocrystalline Al films with the excess of O atoms at the boundaries. Site-specific nanoindentation leads to grain growth that is retarded by impurities, and enables quantification of the critical stress for the onset of grain boundary migration. Our results show that a critical excess of impurities is required to stabilize interfaces in nanocrystalline materials against mechanical driving forces, providing new insights to guide control of deformation mechanisms and tailoring of mechanical properties apart from grain size alone.

Phase field modeling of ideal grain growth in a distorted microstructure

In this work, we perform phase field simulations of ideal grain growth in a distorted polycrystalline aggregate. The distorted microstructure is assumed to be generated by homogenous deformation of an initially equiaxed polycrystalline aggregate. Phase field theory of curvature driven grain boundary migration under distortion is developed and the phase field kinetic relations are implemented in a numerical code based on an explicit time integration procedure. As a benchmark example, phase field simulations of the shrinkage of a distorted circular grain are performed and compared with that of an undistorted elliptical grain to validate the phase field theory and its numerical implementation and also to study the effect of distortion on the kinetics of grain boundary migration. The non-equiaxed distorted grain evolves towards an equiaxed geometry by curvature-driven boundary migration. The evolution rate towards equiaxity is proportional to the distortion magnitude. However, the distortion regardless of its magnitude has no effect on the decrease rate of the distorted grain surface area. We also investigate the evolution of a distorted polycrystalline microstructure by phase field simulations. The elongated distorted grains grow and evolve towards equiaxed grains by ideal grain growth. It has been observed that the distortion influences the directional measure of average grain size to a large extent, but the nondirectional average characteristics of the microstructure are not affected.

Transition State Model for Grain Boundary Motion During Ion Bombardment

ABSTRACTIon bombardment of polycrystalline Ge, Si, and Au films leads to rates of grain boundary motion that greatly exceed rates of thermally-induced motion at the same temperature and which exhibit a weak temperature dependence. The enhanced migration rate is proportional to the rate of energy deposition in nuclear collisions at or very near the grain boundary. Experimental work is reviewed, and a transition state model is presented which accounts for the observed kinetics of grain boundary migration during bombardment. This model suggests that the rate limiting step in grain boundary motion may be thermally-induced migration of a bombardment-generated defect across the boundary. Also, the ratio of atomic jumps at grain boundaries to the local collision-induced Frenkel defect generation rate is shown to be characteristic of each material, but independent of ion mass and ion flux. The model is extended to the motion of an interface between two phases, and applications to crystallization during ion bombardment are discussed.

Kinetics of grain boundary migration in dilute CuSi bicrystals

Kinetics of grain boundary migration in dilute copper-silicon bicrystals have been investigated by the reversed-capillary technique at temperatures and silicon solute additions ranging from 275–400°C and 50–680 ppm, respectively. Grain boundary displacements increase with increasing time and temperature and are characterized by an apparent activation energy, which increases from 27 to 35 kCal/mole with increasing silicon content. Results are in excellent quantitative agreement with theories of grain boundary migration and indicate that the controlling mechanism is thermally activated transport of silicon across the moving boundary.

Formation of annealing twins, faceting, and grain boundary pinning in copper bicrystals

In the course of a recent investigation of the kinetics of grain boundary migration in copper bicrystals, formation of annealing twins, faceting and grain boundary pinning have been observed. Stability and frequency of formation of annealing twins are related to boundary misorientation and temperature of anneal. Tendency for grainboundary faceting decreases with increasing temperature indicating that anisotropy of grain boundary energy is more pronounced at lower temperatures. In general, orientations of faceted boundaries corresponded to higher-order twin planes with respect to shrinking grains. At relatively high temperatures (950°C), specimen-thickness-dependent pinning effects also are observed. These observations are described and analyzed in terms of grain boundary structure, energy, and migration behavior.

Kinetics of grain boundary migration in copper bicrystals with [001] rotation axes

Kinetics of grain boundary migration in high-purity copper bicrystals with common [001] rotation axes have been investigated as a function of driving force, angle of misorientation and temperature. In all cases, grain boundary displacements obey a parabolic rate law and concomitant velocities are linearly related to driving force implying that intrinsic mobilities have been measured. Migration of low-angle boundaries is characterized by an activation energy of 49 kcal/mole. Close agreement between this value and 47 kcal/mole for lattice self diffusion in copper indicates that motion of low-angle boundaries is controlled by dislocation climb via a lattice self-diffusion mechanism. Migration of random high-angle boundaries, on the other hand, is characterized by activation energies ranging from 26 to 30 kcal/mole indicating that motion is controlled by direct atom transfer across the boundary. For a given misorientation, it has been possible to study boundary migration either in a pure tilt configuration or with varying degrees of twist character merely by varying orientation of the [001] rotation axes relative to the specimen surfaces. It is observed that values of the critical misorientation for transition from low to high-angle migration kinetics are about 9° and 13° for mixed and pure tilt boundaries respectively. Differences in mobility occur between the two configurations even at large misorientations, presumably due to the anisotropic nature of boundary diffusion.

Altering the time cycle of heat treatment by preannealing prior to grain growth

Preannealing is found to alter the time cycle of grain growth in lead and lead-tin alloys by modifying the kinetics of grain boundary migration. Aging at a low temperature prior to annealing at a high temperature shows the influence of residual strains in the early stage of grain growth. A model is proposed to explain the deviation of the growth kinetics from the theoretical t1/2 law when the samples are annealed immediately after primary recrystallization in terms of a superposition of a strain-induced and a surface tension-induced grain boundary migration. Preannealing by using multiple anneals at the same temperature enables simulation of many different alloys by changing the amount of impurities segregated at grain boundaries. In zone refined material, a good agreement between the Cahn theory and the experimental results is observed; in lead-tin alloys, grain growth kinetics are understood in terms of a superposition of the influences of the solute and of other lattice defects.