What is the Difference Between Low-angle and High-angle Grain Boundary

Low-angle and high-angle grain boundaries are fundamental features within crystalline materials, delineating the interfaces between adjacent grains. Understanding the difference between low-angle and high-angle grain boundary is crucial for comprehending their roles in material properties, such as mechanical strength, electrical conductivity, and diffusion behavior.

What is the difference between low-angle and high-angle grain boundary? A low-angle grain boundary occurs when the misorientation angle between adjacent grains is small, whereas high-angle grain boundaries have a larger misorientation angle, typically greater than a few degrees.

Key Areas Covered

1. What is Low-angle Grain Boundary
      – Definition, Features, Applications
2. What is High-angle Grain Boundary
      – Definition, Features, Applications 
3. Similarities Between Low-angle and High-angle Grain Boundary
      – Outline of Common Features
4. Difference Between Low-angle and High-angle Grain Boundary
      – Comparison of Key Differences
5. FAQ: Low-angle and High-angle Grain Boundary
      – Answers to Frequently Asked Questions

Key Terms

Low-angle Grain Boundary, High-angle Grain Boundary, LAGB, HAGB

What is Low-Angle Grain Boundary

A low-angle grain boundary (LAGB) refers to the interface between two crystalline grains in a material that is misoriented by only a small angle relative to each other. These boundaries play a crucial role in determining the mechanical, electrical, and thermal properties of polycrystalline materials.

In contrast to high-angle grain boundaries where the misorientation between adjacent grains is significant, LAGBs typically have misorientation angles less than 15 degrees. Despite their small angular deviation, LAGBs can still influence material properties significantly.

One of the key effects of LAGBs is their impact on mechanical properties. They can impede the motion of dislocations, which are the primary carriers of plastic deformation in crystalline materials. This hindrance to dislocation movement can lead to increased strength and hardness in materials, as well as improved resistance to fatigue and creep deformation.

Moreover, LAGBs can also affect the electrical and thermal conductivity of materials. The presence of LAGBs introduces scattering centers for charge carriers and phonons, reducing the overall conductivity of the material. This effect is particularly important in electronic and thermal management applications, where maximizing conductivity is crucial.

Furthermore, LAGBs can influence the nucleation and propagation of defects such as voids and cracks, affecting the material’s fracture behavior and durability. Understanding the behavior of LAGBs is, therefore, essential for designing materials with tailored properties for specific applications.

Figure 1: Micrograph of a Polycrystalline Metal, with Grain Boundaries Shown by Acid Etching

What is High-Angle Grain Boundary

A high-angle grain boundary (HAGB) is a type of interface that separates adjacent crystalline grains in a material. It occurs when the orientation difference between the neighboring grains is significant, typically greater than 10-15 degrees. In contrast, low-angle grain boundaries (LAGBs) have smaller orientation differences.

These boundaries play a crucial role in determining the mechanical, electrical, and thermal properties of materials, particularly polycrystalline metals and ceramics. The term “high angle” refers to the large misorientation angle between adjacent grains, indicating a significant deviation from the ideal lattice alignment.

At HAGBs, the atomic arrangement undergoes abrupt changes, leading to disruptions in the crystal lattice continuity. This disruption can affect various material properties such as strength, ductility, corrosion resistance, and diffusion behavior. For instance, HAGBs can act as barriers to dislocation motion, impeding the plastic deformation of metals and thus influencing their mechanical strength. Conversely, they can also serve as pathways for atomic diffusion, impacting material stability and phase transformations.

In addition to their mechanical significance, HAGBs are essential in grain boundary engineering, where their properties are deliberately manipulated to enhance material performance. By controlling grain boundary characteristics such as structure, chemistry, and energy, engineers can tailor the properties of polycrystalline materials to meet specific application requirements.

Similarities Between Low-angle and High-angle Grain Boundary

1. Both types of grain boundaries involve a discontinuity in the atomic structure between adjacent grains.
2. These types of grain boundaries influence the mechanical properties of the material.

Difference Between Low-angle and High-angle Grain Boundary

Definition

• A low-angle grain boundary (LAGB) refers to the interface between two crystalline grains in a material that is misoriented by only a small angle relative to each other, while a high-angle grain boundary (HAGB) is a type of interface that separates adjacent crystalline grains in a material.

Misorientation Angle

• High-angle grain boundaries have a large misorientation angle between adjacent grains, typically greater than 10 degrees, while low-angle grain boundaries have a smaller misorientation angle, typically less than 10 degrees.

Disruption in the Lattice

• Furthermore, high-angle grain boundaries often result in significant disruptions in the crystal lattice structure, leading to abrupt changes in orientation and atomic arrangement. In contrast, low-angle grain boundaries involve less disruption to the crystal lattice structure.

Energy Levels

• High-angle grain boundaries tend to have higher energy levels compared to low-angle grain boundaries.

Conclusion

In conclusion, low-angle and high-angle grain boundaries play vital roles in determining the properties of crystalline materials. While low-angle boundaries involve slight misorientations between adjacent grains, high-angle boundaries exhibit more significant misorientations. Understanding the difference between low-angle and high angle grain boundary is essential for tailoring material properties to specific applications and advancing various fields such as materials science, engineering, and manufacturing.

FAQ: Low-angle and High-angle Grain Boundary

1. What are the different types of grain boundaries?

Grain boundaries can be categorized into two main types based on their orientation: low-angle grain boundaries and high-angle grain boundaries. Low-angle grain boundaries have a slight misorientation between adjacent grains, typically less than 15 degrees. In contrast, high-angle grain boundaries exhibit a significant misorientation, generally greater than 15 degrees.

2. What are the angles of grain boundaries?

In crystalline materials, grain boundaries can have various angles of misorientation, typically classified as low angle or high angle grain boundaries. Low angle grain boundaries have a slight misorientation between adjacent grains, with typical values falling below 15 degrees. High angle grain boundaries exhibit a significant misorientation, typically greater than 15 degrees.

3. What is the difference between grain and grain boundary?

A “grain” refers to a single crystal or crystalline region with a consistent atomic arrangement, while a “grain boundary” denotes the interface or boundary between adjacent grains where the atomic arrangement changes abruptly.

4. How do grain boundaries affect properties?

Grain boundaries impact material properties by controlling the movement of atoms, electrons, phonons, and photons. They can affect mechanical strength, electrical conductivity, and thermal behavior by impeding dislocation movement, altering electron mobility, and influencing phonon scattering.

5. How thick is a grain boundary?

Conventional grain boundaries typically have a thickness of 1–2 atomic layers. This thin interface separates adjacent crystalline grains within a material.

Reference:

1. “High Angle Grain Boundary.” Science Direct.
2. “Low Angle Boundary.” Science Direct.

Image Courtesy:

1. “CrystalGrain” By Edward Pleshakov – Own work (CC BY 3.0) via Commons Wikimedia