Migration Of Low-angle Boundaries Research Articles

Low angle boundary migration of shot-peened pure nickel investigated by electron channeling contrast imaging and electron backscatter diffraction.

Study on recrystallization of deformed metal is important for practical industrial applications. Most of studies about recrystallization behavior focused on the migration of the high-angle grain boundaries, resulting in lack of information of the kinetics of the low angle grain boundary migration. In this study, we focused on the migration of the low angle grain boundaries during recrystallization process. Pure nickel deformed by shot peening which induced plastic deformation at the surface was investigated. The surface of the specimen was prepared by mechanical polishing using diamond slurry and colloidal silica down to 0.02 μm. Sequential heat treatment under a moderate annealing temperature facilitates to observe the migration of low angle grain boundaries. The threshold energy for low angle boundary migration during recrystallization as a function of misorientation angle was evaluated using scanning electron microscopy techniques. A combination of electron channeling contrast imaging and electron backscatter diffraction was used to measure the average dislocation density and a quantitative estimation of the stored energy near the boundary. It was observed that the migration of the low angle grain boundaries during recrystallization was strongly affected by both the stored energy of the deformed matrix and the misorientation angle of the boundary. Through the combination of electron channeling contrast imaging and electron backscatter diffraction, the threshold stored energy for the migration of the low angle grain boundaries was estimated as a function of the boundary misorientation.

Microstructural aspects of superior creep resistance of a 10%Cr martensitic steel

The microstructural evolution and the dispersion of secondary phases were studied in a low-nitrogen 10%Cr martensitic steel with 3% Co and 0.008% B additives at 650°C under an applied stress of 140MPa. It was demonstrated that the superior creep strength of this steel can be attributed to the high resistance of M23C6-type carbides and Nb-rich MX carbonitrides against coarsening, resulting in a stable of the tempered martensite lath structure (TMLS) under short-term creep conditions. The TMLS remains slightly changed under creep: lath coarsening occurs with a two-fold decrease in the lattice dislocation density. M23C6-type carbides were found to give the main contribution in hindering the transformation of interlath boundaries to subgrain boundaries, impeding the migration of low-angle boundaries by exerting a large pinning pressure. A high Zener drag pressure is maintained up to rupture. The precipitation of a Laves phase under creep conditions results in a minor contribution to the overall pinning pressure. V-rich MX carbonitrides tend to dissolve with increasing time. No formation of a Z-phase was detected. M23C6 carbides retain their orientation relationship with ferritic matrix up to rupture. No significant strain-induced coarsening of M23C6 carbides, Laves phase, or MX carbonitrides was observed.

A mechanism of grain growth-assisted intergranular fatigue fracture in electrodeposited nanocrystalline nickel–phosphorus alloy

Quantitative investigation of the grain growth and resultant change in grain boundary microstructure during high-cycle fatigue was performed to understand intergranular fatigue fracture in electrodeposited nanocrystalline Ni – 2.0mass% P alloys by using FE-SEM/EBSD technique. Pre-fatigued specimens had an average grain size of 45nm, a sharp {001} texture and a high fraction of low-angle boundaries and of twin, or Σ3 coincidence site lattice (CSL) boundaries. The considerable grain growth occurred due to the migration of low-angle boundaries induced by shear stress during cyclic deformation. The misorientation angle of those low-angle boundaries increased covering the whole surface of fatigue-fractured specimen. A certain fraction of low-angle boundaries was transformed into high-angle random boundaries resultant from grain growth during high-cycle fatigue. Those random boundaries which surrounded the grown {001}-grains were aligned along shear bands at almost 45° to the stress axis, and formed the diamond-shaped grain configuration, as reported in the literature on high temperature fatigue. The reported increase of the fatigue limit by nanocrystallization is likely reduced due to the cyclic stress-induced grain growth associated with the migration of low-angle boundaries composing nanograin cluster. Moreover, the random boundaries transformed from low-angle boundaries can be preferential sites for crack nucleation and propagation at the positions of initially formed shear bands during fatigue.

Microstructural evolution of ultrasonic-bonded aluminum wires

The evolution of microstructural gradients, especially crystallographic texture gradients, after ultrasonic wire bonding process and after active power cycling (APC) of high purity, heavy aluminum (Al) wires is studied by electron backscatter diffraction (EBSD) and nanoindentation. The results improve the knowledge about microstructural changes and arrangements after wire bonding and during APC. After ultrasonic deformation by wire bonding, the evolution of a distinct rotated cube (RC) textured area within the wedge was proved by EBSD analysis. The RC texture is discussed as a result of shear deformation and oriented grain growth. Decreased hardness within the RC textured area provides evidence for local softening effects during wire bonding. During APC, besides crack propagation, grain coarsening as well as local low angle boundary migration occurs and the wedge texture changes to an overall random orientation. Effects of microstructure on the crack growth behavior were discussed and suggestions for the improvement of wire bond reliability were derived.

Recrystallisation nucleation of nickel base single crystal superalloy under compression

Recrystallisation nucleation of a nickel base single crystal superalloy cold deformed by compression and annealed at various temperatures has been investigated. The specimens were compressed using universal testing machine under an initial strain rate of 5×10−4 s−1 at room temperature. Annealing temperatures were chosen from 1100 to 1300°C. The microstructure changes of the deformed and annealed samples were observed by optical microscopy, SEM with electron backscatter diffraction and TEM. It has been found that the surface and dendritic core are the preferential nucleation sites of recrystallised nucleus. The surface may lead to dislocation loss and relaxation of long range stresses, which will induce the recrystallised grains. At the dendritic core, rapid dissolution of γ′ particles during annealing reduces the resistance of grain boundary migration. Nucleation by low angle boundary migration and bow was confirmed during the recrystallisation of nickel base single crystal superalloy.

Early Stages of Recrystallization in ECAP-Deformed AA1050 Alloy Investigated by SEM Orientation Mapping

Early stages of recrystallization were observed for the technical purity aluminium alloy (AA1050). The samples were severely deformed by equal channel angular pressing and then annealed to obtain the state of partial recrystallization. The nucleation of new grains and the changes in ‘density’ of low- and high- angle boundaries were analysed using scanning electron microscopy equipped with high resolution EBSD system. After deformation in six passes along route A the alloy contained a structure of flat grains. On annealing, the structure coarsened and got transformed into nearly equiaxed grains by both discontinuous and continuous recrystallization. A particular role in the rise of nuclei and the structure spheroidization is attributed to migration of low-angle boundaries.

Early Stages of Recrystallization in Equal-Channel Angular Pressing (ECAP)-Deformed AA3104 Alloy Investigated Using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) Orientation Mappings

Early stages of recrystallization in alloys containing complex structure of second phase particles are of considerable practical interest. They were observed for the AA3104 alloy in which large particles occur against the background of randomly distributed dispersoids. The samples were deformed by equal channel angular pressing and then slightly annealed to obtain the state of partial recrystallization. The highly deformed alloy contained a structure of flat grains with the spacing between high-angle grain boundaries ranging from 100 nm to 1 µm. On annealing, the structure coarsened and got transformed into nearly equiaxed grains by both discontinuous and continuous recrystallization. The nucleation of new grains was observed in statically recrystallized bulk samples using scanning electron microscopy, and during in-situ recrystallization in a transmission electron microscope. Special attention was paid to the nucleation of new grains in areas close to large second phase particles, where a relatively high stored energy was expected to stimulate nucleation. A particular role in the rise of nuclei is attributed to migration of low angle boundaries. During recrystallization at 623 K (350 °C), in most of the observed cases, the growth of grains occurred by coalescence of neighbouring cells and by migration of high-angle grain boundaries. These processes led to nearly equiaxed grains of similar size. Orientation mappings showed that although orientations of new grains were widely scattered, they were not completely random.

Microstructural Evolution in 9%Cr Heat Resistant Steels under Creep Conditions

Microstructural design of a new generation of 9%Cr steels for fossil power plants is considered. It was shown that microstructural stability of 9%Cr steels impairs their creep resistance. Two types of restoration processes can occur in the heat resistance steels under creep conditions: (i) normal grain growth and (ii) dynamic recovery. The first process associates with the migration of high-angle boundaries (HAGB) of blocks of tempered martensite lath structure (TMLS). However, their migration is negligible even during creep deformation. Boundaries of packets and prior austenite boundaries (PAB) are effectively pinned by precipitations of M23C6and Laves phase Fe2(W,N). The second process consists of transformation of lath boundaries to subboundaries and their subsequent migration (subgrain coarsening) under creep. Under aging the migration of low-angle boundaries (LAGB) is retarded by uniformly distributed nanoscale M(C,N) dispersoids and particles of M23C6precipitated on these boundaries under tempering. Under creep the dissolution of M23C6carbides located along LAGBs and coagulation of uniformly distributed M(C,N) carbonitrides facilitates LAGB migration. It was shown that the normal grain growth is not important for deterioration of creep strength. Conversion of the lath boundaries into subgrain boundaries strongly decreases creep rate. In contrast, continuous subgrain coarsening is the main process restricting the ability of the 9%Cr steel for long-range service under creep conditions. Tertiary creep is attained due to the occurrence of subgrain coarsening.

New martensitic steels for fossil power plant: Creep resistance

In this paper, we consider the origin of high-temperature strength of heat-resistant steels belonging to martensitic class developed on the basis of the Fe—9%Cr alloy for the boiler pipes and steam pipelines of power plants at steam temperatures of up to 620°C and pressures to 300 atm. In addition, we give a brief information on the physical processes that determine the creep strength and consider the alloying philosophy of traditional heat-resistant steels. The effect of the chemical and phase composition of heat-resistant steels and their structure on creep strength is analyzed in detail. It is shown that the combination of the solid-solution alloying by elements such as W and Mo, as well as the introduction of carbides of the MX type into the matrix with the formation of a dislocation structure of tempered martensite, ensures a significant increase in creep resistance. The steels of the martensitic class withstand creep until an extensive polygonization starts in the dislocation structure of the tempered martensite(“troostomartensite”), which is suppressed by V(C,N) and Nb(C,N) dispersoids. Correspondingly, the service life of these steels is determined by the time during which the dispersed nanocarbonitrides withstand coalescence, while tungsten and molybdenum remain in the solid solution. The precipitation of the Laves phases Fe2(W,Mo) and the coalescence of carbides lead to the development of migration of low-angle boundaries, and the steel loses its ability to resist creep.

A model for stress-induced migration of tilt grain boundaries in crystals of NaCl structure

We present a qualitative model for stress-induced migration of tilt grain boundaries which we have observed during creep of sodium chloride. The elementary mechanism of the model consist in the propagation of the ledge associated to an extrinsic grain boundary dislocation. The model appears as an extension of the mechanism for growth of deformation twins and is compatible with but more general than Read's and Shockley's model for migration of low angle boundaries.

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.