Difference Between Grain Boundary and Twin Boundary

The key difference between grain boundary and twin boundary is that grain boundary can occur between any type of crystal grains, whereas twin boundary can occur only between crystals having the same crystal lattice structure.

A boundary in the field of crystallography is the interface between two units of a crystal lattice. These unit structures are named grains or crystallites. We can observe these boundaries in a polycrystalline material, and these are known to be defects in the material’s structure.

CONTENTS

1. Overview and Key Difference
2. What is Grain Boundary 
3. What is Twin Boundary
4. Grain Boundary vs Twin Boundary in Tabular Form
5. Summary – Grain Boundary vs Twin Boundary

What is Grain Boundary?

Grain boundary is the interface between two grains in a polycrystalline material. Usually, the grain boundary in a polycrystalline material is a 2D defect in the crystal structure. This defect tends to decrease the electrical and thermal conductivity of the material. These grain boundaries are the preferred areas for the onset of corrosion. They are also the preferred areas for the precipitation of new phases from the solid. Grain boundary defects are important in many of the mechanisms of creep.

In addition, these grain boundaries tend to disrupt the motion of dislocations through the material, thereby reducing the crystallite size, which leads to improving the mechanical strength. This phenomenon is described by the Hall-Petch relationship. The study of grain boundaries, their mechanical and electrical properties are described under the field of material science.

We can describe a grain boundary through the orientation of the boundary to the two grains and the 3D rotation that we need to bring the grains into coincidence. Therefore, a grain boundary usually has 5 macroscopic degrees of freedom. However, commonly a grain boundary is only described as the orientation relationship of the neighboring grains.

Types of Grain Boundaries

There are two types of grain boundaries as high and low angle boundaries. This classification depends on the extent of misorientation between the two grains. Low angle boundaries are also known as subgrain boundaries, and these grain boundaries have a misorientation which is less than 15 degrees. In contrast, the high angle grain boundaries have their misorientations greater than 15 degrees. Normally, the high angle boundaries tend to be independent of the misorientation.

What is Twin Boundary?

Twin boundary is the interface between two separate crystals that are mirror images of each other. In other words, twin boundaries occur between separate crystal structures having the same crystal lattice. This type of crystal occurrence is also named crystal twinning. Usually, a twin boundary is symmetrical on both sides. The twin boundary is also named as composition surface or twin plane.

We can classify twin boundaries in different manners depending on the twin laws. Usually, the twin laws are specific for crystal systems. Moreover, the type of twin boundary can be useful for mineral identification.

Typically, a twin boundary can transform into a boundary of the common type. This happens due to the formation of walls of mismatch dislocations during the dislocation movement process, which occurs through these boundaries.

What is the Difference Between Grain Boundary and Twin Boundary?

Grain boundary is the interface between two grains in a polycrystalline material. Twin boundary is the interface between two separate crystals that are mirror images of each other.  Therefore, the key difference between grain boundary and twin boundary is that grain boundary can occur between any type of crystal grains, whereas twin boundary can occur only between crystals having the same crystal lattice structure.

The following infographic summarizes the difference between grain boundary and twin boundary in tabular form.

Summary – Grain Boundary vs Twin Boundary

A boundary in the field of crystallography is the interface between two units of a crystal lattice. Gain boundary is the common type of boundary, while twin boundary is a specific type of boundary. The key difference between grain boundary and twin boundary is that grain boundary can occur between any type of crystal grains, whereas twin boundary can occur only between crystals having the same crystal lattice structure.

Reference:

1. “Twin Boundary.” An Overview | ScienceDirect Topics.

Image Courtesy:

1. “TiltAndTwistBoundaries remade” By Slinky Puppet (CC BY-SA 3.0) via Commons Wikimedia

2. “Quartz-rhqtz-109b” Rob Lavinsky, iRocks.com – (CC-BY-SA-3.0) via Commons Wikimedia