If the rotation axis is perpendicular to the GB plane normal, this GB is called a tilt boundary, whereas if the rotation axis is parallel to the GB normal, the GB is defined as a twist boundary. Although tilt and twist GBs occupy only a small fraction of the GB phase space, they are frequently observed experimentally, which suggests that they are energetically favored over other types of GBs [ 15 ]. Numerous theoretical efforts have been made to characterize the GB structures.
The widely used models to analyze and predict the atomistic structures of GB include the coincidence site lattice CSL model and the structural unit SU model. While in the SU model, those GBs with specific misorientation angles called favored GB is constituted only from one type of structural units.
Any intermediary GB between two favored GBs can be described by a linear combination of SUs comprising one or several neighboring favored boundaries [ 16 , 17 ]. The structures are viewed along the tilt axis [1 1 0], with the open and filled circles indicating atomic positions in alternate 2 2 0 planes . The advent of transmission electron microscopy TEM , especially in the development of high-resolution transmission electron microscopy HRTEM has provided us with a very powerful tool to explore the atomic structure of internal interfaces.
TEM characterization results indicate that the GB of NC material is essentially the same as that of the coarse-grained materials. It should be noted that GB structures of NC materials are highly dependent on the alloy composition and processing steps in the manufacturing of NC materials.
Although NC materials show superior mechanical properties as compared to the coarse-grained counterpart, their application has been severely limited by their microstructural instability under the monotonic mechanical deformation [ 20 — 33 ]. For pure NC metallic materials, the grain coarsening process often occurs even at ambient temperatures [ 34 — 36 ]. Ames et al. They found a transition from an initially self-similar slow growth to abnormal grain growth.
However, they argued that abnormal grain growth is a transient state since a monomodal grain size distribution was observed in the late stage of coarsening. Discontinuous grain growth in NC materials during the deformation processes seems to be a common phenomenon. Zhang et al. However, the increased rate of growth at cryogenic than at the room temperature indicates that the growth is primarily mechanical, not diffusion-driven.
In order to elucidate the effect of stress and strain on mechanically induced grain growth, Rupert et al. Statistical results of grain size in horizontal-hole specimens showed that grain growth occurred at both high strain region and high stress region. However, grain size was greater in the high stress region, indicating that the grain growth is driven by stress. Statistics results of grain size of angled-hole specimens demonstrated that grain growth was scaled with shear stress.
Gianola et al. The specimens that exhibit high strength maintain their NC microstructure, the limited elongation, and the dramatic strain softening.
Doherty, Drexel University, Philadelphia, B. Cantor, University of Oxford. Subjects: Condensed Matter Physics, Nanoscience and Mesoscopic Physics, Materials Science, Engineering. Chapter 1 - The general problem of the stability of microstructure. ultemeege.tk: Stability of Microstructure in Metallic Systems (Cambridge Solid State Science Series) (): J. W. Martin, R. D. Doherty, B. Cantor.
By contrast, specimens that undergo discontinuous grain growth show intermediate strengths and the unexpected development of a region of extended plasticity. Statistical results of grain size outside of the deformed region of specimens that exhibited the grain growth are similar to the initial state indicating that the grain growth is directly tied to the applied stress or deformation in the sample [ 37 ].
Grain growth is also observed during the cyclic deformation of NC materials [ 38 — 50 ]. Early fatigue experiments by Witney et al.
Fe 50 Ti The prediction of the number of transition phases, their crystal structure and composition has not yet been achieved for any alloy system, even on an empirical basis. It is not usually profitable to divide microstructural transformations into separate classes - those giving desirable microstructures and those in which desirable microstructures degrade. Any misfit in the atomic spacings of the two structures in the interfacial plane is accommodated by elastic 'coherency strains'. Weatherly and Nicholson observed that the flat faces of the 6' phase in Al-Cu alloys lose coherency by the nucleation and growth of small dislocation loops inside the precipitate, which then climb into the precipitate-matrix interface. Thus, by affording the best responses in relation to the raw material in question, this drying condition was used to obtain the powdered mango for the stability study.
They found that the protrusions stick out on the order of a micrometer which is far greater than the initial grain size, and extend several microns parallel to the surface, similar to extrusions formed during the fatigue test of a coarse-grained Cu. Detailed TEM studies of NC Ni-Fe alloy under cyclic deformation demonstrates that the grain coarsening is accompanied by the fatigue crack growth.
Stress concentration at the crack tip causes the lattice reorientation under the cyclic deformation [ 41 ]. GB dislocations play a critical role in grain rotation and in the formation of subgrain in larger grains. They found localized regions of grain growth during fatigue loading.
Coarsened Ni grains did not favor any particular orientation, while Ni Their observations also suggest that grain stability is an important factor affecting the crack initiation and propagation process in these NC alloys. Meirom et al. They found a clear evidence of increased grain size in the crack wake and ahead of the crack tip. Coarsened grains underwent a nearly one order of magnitude increase in size compared with the as-received one.
Recently, Zhang et al. However, the grain growth is greatly suppressed in the annealed NC Cu film, which leads to the enhanced resistance to the fatigue cracking as compared with that of the as-deposited one. The enhanced fatigue strength of Cu film after annealing may be related to the GB structure readjustment during the annealing process. Although the mechanical grain growth has been experimentally observed under various mechanical conditions, the underlying microstructural and atomic scale mechanisms are still open for debate.
As mentioned in the introduction section, in an NC material, the fraction of atom residing at the GBs increases as the grain size decreases. Thus, GBs in NC materials promote the total free energy of the system. The reduction of this excess free energy through the removal of grain boundary area represents a large driving force for the grain growth. Grain growth in NC materials can be due to the rotation and coalescence of adjacent grains, as well as normal grain boundary movements.
There are several theoretical models that have been proposed to describe the mechanically driven grain growth behavior, for example, stress-coupled GB migration [ 51 — 56 ] and grain rotation-induced grain coalescence [ 57 ]. The stress-coupled GB migration model is based on the argument that shear stress causes tangential movement of grains along GBs GB sliding , and this produces a coupling with the normal motion of GBs GB migration [ 51 ]. In the initial state, these GBs form two triple junctions. The motion of the disclination produces rotational plastic deformation. The same is true for migration of two high-angle tile GBs with large angle gaps.
In all cases, GB migration leads to the unstable growth of a grain at the expense of its neighbors. Energy methods calculation indicates that critical shear stresses strongly depend on the elastic modulus of the material, as well as on the strength of disclination-like defects appearing at the GB junctions in the process of GB migration. Schematic of stress-induced grain boundary GB migration a and b low-angle and c and d high-angle GB GB3 by grain rotation through the glide of lattice dislocations b or motion of a dipole of wedge disclinations d , respectively .
For the case where the translational mode is mainly represented by GB sliding, Wang et al. The grain rotation-induced grain coalescence model can be understood as follows [ 57 ]: with the applied force, GB dislocations glide results in the relative translational motion of GBs. However, the triple junctions impede the motion of GB dislocation.
The blocked GB dislocation dissociates from the two climbing GB dislocation at the triple junction. With further plastic deformation, the dislocation split process happens repeatedly and the climbing GB dislocations form two dislocation walls along the GBs, which results in the rotation of the central NC grain.
Multiple rotations bring the orientation of abutting grains closer together and reduce the GB misorientation angles, and even eliminate the GBs, leading to coalescence of smaller grains into larger ones. Combined grain boundary GB sliding and rotational deformation mode. Although theoretical models give some clues about the grain growth processes during mechanical deformation of NC materials, the exact microstructural scale and atomic scale mechanisms of the GB migration is still unclear.
Quantitative information aimed to identify atomic scale mechanisms that reveal the influences of GB structure on the GB migration can be obtained by using in situ HRTEM and molecular dynamics MD simulation methods. Haslam et al.
They found that grain rotation decomposes a GB into multiple but distinct dislocations, which can then move by dislocation slip; these dislocations can annihilate at GBs or remain embedded within grains if the applied stress is relieved. Bulk metallic glasses and their composites : composition optimization, thermal stability, and microstructural tunability. Abstract A design protocol utilizing common elements for bulk metallic glass formation has been employed to develop novel, low cost Fe-, and Ti- based bulk metallic glasses.
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