The key difference between slip and twinning is that during a slip, all atoms in a block move the same distance whereas, in twinning, the atoms in each successive plane in a block move through different distances that are proportional to their distance from the twinning plane.
Slip and twinning are two terms useful in material science.
CONTENTS
1. Overview and Key Difference
2. What is Slip
3. What is Twinning
4. Side by Side Comparison – Slip vs Twinning in Tabular Form
5. Summary
What is a Slip?
Slip is the large displacement of one part of a crystal relative to another part along the crystallographic planes and directions. This term is used in the field of material science. A slip can occur by the passage of dislocations on close-packed planes. These are the planes containing the greatest number of atoms per area and in close-packed directions. Generally, we call the close-packed planes as slip or glide planes.
Figure 01: Slip System
Typically, an external force that is applied to a crystal lattice can cause the parts of the crystal lattice to glide along with each other, which can change the geometry of that material. We need a critical resolved shear stress in order to initiate a slip.
We can identify different slip systems, including the face-centred cubic system where the slip occurs along the close-packed plane, body centred cubic crystals where slip occurs along the plane of shortest Burgers Vector, hexagonal close-packed systems where the slip occurs along the densely packed plane, etc.
What is Twinning?
Crystal twinning is the event where two different crystals share some of the same crystal lattices that are pointing in a symmetric manner. This term is mainly used in the field of material science. The result of the twinning in a crystal lattice is an intergrowth of two separate crystals in a variety of specific configurations. In this phenomenon, the surface along which the crystal lattice points are shared in a twinned crystal is known as “composition surface or twin plane”. Twinning is often a problem in X-ray crystallography because the twinned crystals do not produce a simple diffraction pattern.
There are twin laws in material chemistry. We can define a twin law either using their own planes or using the direction of the twin axes. The most common twin laws in the isometric system include Spinel law (the twin axis is perpendicular to an octahedral face), and Iron cross (the interpretation of two pyritohedrons which is a subtype of the dodecahedron).
Figure 02: A Cross Twinned Material
Further, there are different types of twinning such as contact twins (simple twinned crystals), merohedral twinning (occurs when the lattices of the contact twins superimpose in 3Ds), penetration twin (where the individual crystals have the appearance of passing through each other in a symmetric manner), multiple or repeated twins, etc.
What is the Difference Between Slip and Twinning?
The key difference between slip and twinning is that during a slip, all atoms in a block move the same distance whereas in twinning the atoms in each successive plane in a block move through different distances that are proportional to their distance from the twinning plane.
Below infographic summarizes the differences between slip and twinning in tabular form.
Summary – Slip vs Twinning
The key difference between slip and twinning is that during a slip, all atoms in a block move the same distance whereas, in twinning, the atoms in each successive plane in a block move through different distances that are proportional to their distance from the twinning plane.
Reference:
1. “Slip system” By Sietske~commonswiki assumed (based on copyright claims). – No machine-readable source provided. Own work assumed (based on copyright claims). (Public Domain) via Commons Wikimedia
2. “Pyrite-254452” By Rob Lavinsky, iRocks.com (CC-BY-SA-3.0) via Commons Wikimedia