Ethan Clarke
Grain boundaries have a significant impact on the plastic deformation of materials, particularly at the nano-scale. The interaction between dislocations and grain boundaries plays a crucial role in the deformation process. Grain boundary dislocations can pile up and annihilate each other, influencing the rate of sliding. Additionally, the structure and misorientation of grain boundaries affect various physical properties, including plasticity. Grain size is also a critical factor, with larger grain sizes promoting twinning during plastic straining and enhancing the plasticity of deformed samples. Recent studies have focused on understanding the mechanisms of plastic deformation in small-grained metals and the influence of grain boundary characteristics on deformation twin nucleation. Overall, the complex interplay between grain boundaries and dislocations, as well as their impact on sliding and strain distribution, is essential to comprehending plastic deformation in various materials.
| Characteristics | Values |
|---|---|
| Grain boundaries | Can influence the deformation of Earth's mantle through the activation of grain-boundary sliding |
| Grain-boundary sliding | Appears to be intimately associated with the generation and motion of lattice dislocations |
| Dislocation-accommodated grain-boundary sliding (disGBS) | Stress concentrations produced by grains sliding past their neighbors are relaxed by dislocation motion |
| Grain-boundary dislocations | Pile up in the boundary, and their mutual annihilation by climb within the boundary limits the rate of sliding |
| Grain-boundary structure and misorientation | Affect various physical properties of materials, including plasticity |
| Grain size | Has a significant impact on the macroscopic deformation behavior and mechanical properties of twinning-induced plasticity (TWIP) steels |
| Grain refinement | Driven by grain subdivision and the geometric effects of strain, which together reduce the overall high-angle boundary spacing with increasing deformation |
| Grain-boundary ledges | Sources for dislocations |
| Grain-boundary dislocations | Can glide along the boundary and reach a suitable glide plane, where cross-slip can occur, forming a ledge in the boundary |
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What You'll Learn
Grain boundary dislocation sources
Grain boundaries are the interfaces between two grains or crystallites in a polycrystalline material. They are two-dimensional defects in the crystal structure and tend to decrease the electrical and thermal conductivity of the material. Grain boundaries are important to many of the mechanisms of creep and are preferred sites for the onset of corrosion. They also disrupt the motion of dislocations through a material, so reducing crystallite size is a common way to improve mechanical strength.
Dislocation–grain boundary interactions have been studied using nanoindentation, which has emerged as a powerful tool to study the local mechanical response in the vicinity of the grain boundary. The interaction of the plastic zone with the grain boundary provides important insight into the dislocation transmission effects of distinct grain boundaries. Grain boundaries play a dual role as dislocation sources and obstacles. On one hand, grain boundaries are obstacles for dislocation glide. On the other hand, the intersection points of grain boundaries with the crack front are assumed to be preferred dislocation nucleation sites.
Grain-boundary sliding appears to be intimately associated with the generation and motion of lattice dislocations. In dislocation-accommodated grain-boundary sliding (disGBS), stress concentrations produced by grains sliding past their neighbours are relaxed by dislocation motion. This process is predicted to be a major creep mechanism in the upper mantle. Dislocation incorporation into grain boundaries is a key process.
The density of dislocations in a low-angle grain boundary is lower than in a high-angle grain boundary. Low-angle grain boundaries, where the distortion is entirely accommodated by dislocations, are Σ1. As the grain is bent further, more and more dislocations must be introduced to accommodate the deformation, resulting in a growing wall of dislocations. If deformation continues, the density of dislocations will increase, reducing the spacing between neighbouring dislocations. Eventually, the cores of the dislocations will begin to overlap, and the ordered nature of the boundary will break down, leading to a high-angle grain boundary.
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Grain size and plastic deformation
Grain boundaries have a significant influence on the plastic deformation of materials, and this effect is particularly evident in the deformation of rocks and metals. The grain boundary is the interface between two grains, and its structure and properties can impact the overall deformation behaviour of the material.
One of the critical factors influenced by grain boundaries is the mobility and motion of dislocations. Dislocations are defects in the crystal lattice structure of a material, and they play a crucial role in plastic deformation. During deformation, dislocations move and interact with grain boundaries, leading to processes such as dislocation annihilation and grain coarsening. The interaction between dislocations and grain boundaries can also result in the activation of grain-boundary sliding, which has been observed to influence the deformation of the Earth's mantle.
The grain size, or the size of the individual crystals within a material, also has a significant effect on plastic deformation. In metals, larger grain sizes have been found to promote twinning during plastic straining and increase the work-hardening index and ability. This is because larger grains can effectively distribute strain at the grain boundaries, relieving stress concentration and improving the plasticity of the material. Additionally, grain size can influence twinning and dislocation proliferation rates, resulting in different work-hardening behaviours.
The work hardening rate of metals generally increases with a decrease in grain size. This relationship is attributed to the plastic strain exerted by rolling and subsequent tensile experiments. At a certain grain size, referred to as the "saturation grain size," a balance is reached between the generation and annihilation of dislocations, leading to a refinement limit. Below this saturation size, further refinement techniques are required to reduce grain sizes.
Overall, the grain size and grain boundaries are intimately linked and play a crucial role in the plastic deformation of materials, particularly metals and rocks. The interaction between dislocations and grain boundaries, along with the grain size, influences the mechanical properties and deformation behaviour of the material. Understanding these relationships is essential for optimizing material performance and designing advanced alloys.
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Grain boundary sliding
GBS can be categorised into two primary types: Rachinger sliding and Lifshitz sliding. Rachinger sliding is characterised by grains retaining their original shape, with internal stress building up until it equilibrates with the external applied stress. On the other hand, Lifshitz sliding is associated with diffusion processes in Nabarro-Herring and Coble creep, where grain shape changes during the deformation process. The diffusion of vacancies accommodates the sliding motion, resulting in grain elongation in the direction of applied stress.
The occurrence of GBS is influenced by the grain boundary structure and the interaction between grain boundaries and dislocations. Dislocations play a crucial role in facilitating GBS by entering the grain boundaries and introducing sliding. Additionally, the rate and extent of GBS are influenced by the boundary shape. At high temperatures, wavy grain boundaries are commonly observed, and the steady-state creep rate increases with higher λ/h ratios.
GBS is a significant contributor to the overall creep strain, especially in fine-grained materials and high temperatures. It is the predominant mechanism for superplasticity, where most grain boundaries participate in the deformation process, allowing for large elongation values. GBS can be observed through metallography by introducing scribe lines or microgrids before testing.
GBS has implications for material failure, particularly in tungsten filaments. The diffusion that occurs at high temperatures in these filaments leads to grain rotation and sliding, causing non-uniform sagging and eventual rupture of the filament. Understanding GBS is essential for developing methods to reinforce metal alloys against creep and material failure.
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Grain subdivision
The grain refinement process, driven by grain subdivision, plays a crucial role in the production of ultrafine-grained materials. Severe plastic deformation (SPD) techniques, such as equal channel angular extrusion, can be employed to achieve grain refinement. SPD processes can produce bulk ultrafine-grained metals with grain sizes of less than 1 μm. However, the mechanism behind grain refinement during SPD is not yet fully understood.
At low temperatures, SPD can lead to the formation of new ultrafine grains with high-angle boundary spacing. The rate of grain refinement is influenced by the balance between the compression of high-angle boundary spacing and dynamic grain coarsening. Grain subdivision results in the formation of deformation-induced boundaries, which contribute to the overall microstructure and texture within the material.
The dynamic recrystallization (DRX) phenomena occurring during thermo-mechanical processing (TMP) are closely associated with grain subdivision. Different types of DRX, including discontinuous, continuous, and geometric dynamic recrystallization, play a role in the evolution of new microstructures. The specific DRX processes depend on factors such as stacking fault energy, initial grain size, TMP conditions, and the presence of second-phase particles.
Additionally, grain subdivision influences the interaction between grain boundaries and dislocations. The incorporation of lattice dislocations into grain boundaries is a key process, impacting the overall strain rate. Models have been proposed to explain how lattice dislocations enter the boundary and how their motion influences grain-boundary sliding. These models contribute to our understanding of the role of grain boundaries in the plastic deformation of materials, such as olivine, and their behaviour in the Earth's mantle.
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Intergranular plasticity
Grain boundaries play a significant role in influencing plastic deformation. The interaction between dislocations and grain boundaries is a key process in understanding how materials deform. This is particularly evident in the deformation of rocks, where grain-boundary sliding is facilitated by the activation of lattice dislocations.
One important aspect of intergranular plasticity is intergranular deformation incompatibility (IDI). IDI occurs when adjacent grains have incompatible deformations due to differences in crystal orientations and slip systems. This incompatibility leads to stress triaxiality fluctuations and ductile damage at the grain boundaries, which can initiate fractures. The Rice-Tracey model is often used to predict ductile damage in these scenarios.
The dislocation-density pileups near grain boundaries are another critical factor in intergranular plasticity. These pileups can lead to stress accumulation and fracture nucleation. High-angle grain boundaries (HAGBs) are more susceptible to intergranular fractures than low-angle grain boundaries (LAGBs) due to higher shear slip activity and stress accumulation.
Additionally, the grain refinement process, which involves reducing the high-angle boundary spacing through strain and thermally activated grain boundary migration, influences the overall plasticity of the material. This process is observed in welding and hot torsion experiments, where the grain structure evolves towards a more equiaxed and slightly coarsened state.
The plasticity of tilt grain boundaries has also been studied, particularly in aluminum alloys. These boundaries act as effective dislocation sources during plastic deformation, and the dislocation emission from these boundaries influences the fracture toughness of the material.
In summary, intergranular plasticity is a complex phenomenon influenced by various microstructural factors, including grain boundary characteristics, dislocation interactions, and deformation incompatibilities. Understanding these mechanisms is crucial for predicting and controlling the plastic deformation and fracture behavior of materials.
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Frequently asked questions
Grain boundaries can influence plastic deformation in a variety of ways. Grain boundary structure and misorientation can affect the physical properties of materials, including plasticity. Grain size is also a significant factor, with larger grain sizes promoting twinning during plastic straining and smaller grain sizes activating intergranular plasticity.
Larger grain sizes have been shown to promote twinning during plastic straining, producing an obvious TWIP effect and suppressing the rate of dislocation proliferation. Smaller grain sizes, on the other hand, activate intergranular plasticity, which involves the motion of dislocations in the grain boundary plane.
Dislocations play a crucial role in grain boundary-induced plastic deformation. Dislocations can be emitted from grain boundary ledges, and their interaction with grain boundaries can lead to sliding and cavitation. In some cases, dislocations pile up in the boundary, and their mutual annihilation limits the rate of sliding.
Grain boundary-induced plastic deformation in metals and alloys can occur through a process called superplastic flow, which involves an "interfacial fluid" that accommodates large plastic strains and high strain rates. This was proposed by Professor J.C.M. Li.
Grain boundary-induced plastic deformation has been observed in various materials, including olivine, austenitic steel, and high-manganese austenitic steel. In olivine, the interaction between dislocations and grain boundaries during deformation is key, with grain-boundary sliding playing a significant role. In austenitic steel, the influence of grain size on twinning behaviour, dislocation proliferation, and mechanical properties is of interest.
