Deformation by Slip

In this tutorial, you will explore deformation by slip, covering its basic mechanisms and significance. You will learn about slip in metal crystals, how to remove slip lines from surfaces, and what constitutes a slip system. You will also examine slip systems in different crystal structures—hexagonal close-packed (hcp), face-centered cubic (fcc), and body-centered cubic (bcc)—and understand the factors influencing slip and its applications.

Contents:

1. What is Deformation by Slip?
2. Slip in Metal Crystal
3. Removal of Slip Lines from the Metal Surface
4. What Constitutes the Slip System?
5. Slip Systems in Hexagonal Close-Packed (hcp) Metals
6. Slip Systems in Face-Centered Cubic (fcc) Metals
7. Slip Systems in Body-Centered Cubic (bcc) Metals
8. Factors Affecting Slip
9. Applications and Implications

What is Deformation by Slip?

Deformation by slip occurs when internal forces within a material cause adjacent layers or grains to slide past each other. This type of deformation is often seen in crystalline materials and is a primary mechanism in plastic deformation. The key elements involved in slip deformation include:

• Slip Systems: The specific planes and directions along which slip occurs are known as slip systems. For crystalline materials, slip systems are defined by the crystal structure and include slip planes (the planes along which slip occurs) and slip directions (the directions within these planes).
• Dislocations: Dislocations are line defects within the crystal lattice that play a crucial role in slip deformation. They allow layers of atoms to slide past each other at lower stresses than would be required in a perfect crystal.
• Shear Stress: Slip deformation is driven by shear stress, which acts parallel to the slip plane. The magnitude and direction of shear stress determine how easily a material will deform by slip.

Slip in Metal Crystal

The usual method of plastic deformation in metals is by slip.

• Slip is the sliding of blocks of crystal over one another along definite crystallographic axes called slip planes. It can be considered analogous to the distortion produced in a deck of cards when it is pushed from one end.
• Slip occurs when the shear stress applied on a metal reaches a critical value. The atoms move an integral number of atomic distances along the slip plane.
• A step is produced in the polished surface of the metal. When this surface is viewed from above with a microscope, the step looks like a line, which is the slip line.

Removal of Slip Lines from the Metal Surface

The metal cube under observation must have a top polished surface.

• Slip lines occur when a step is produced in the crystal. After slip has occurred, if the metal surface is repolished so that the step is removed, the slip line will disappear.
• Because of the translational symmetry of the crystal lattice, a crystal structure is perfectly restored after slip has taken place, if the deformation was uniform. Each atom in the slipped part of the crystal moves forward the same integral number of lattice spacings.
• The slip lines are due to changes in surface elevation. The surface must be suitably prepared for microscopic observation before deformation if slip lines are to be observed.

What Constitutes the Slip System?

The slip plane together with the slip direction constitutes the slip system.

• A single crystal remains a single crystal after homogeneous plastic deformation. This fact imposes limitations on the ways in which plastic deformation may occur.
• Slip occurs most readily in specific directions on certain crystallographic planes.
• Generally, a slip plane is the plane of greatest atomic density. A slip direction is the closest-packed direction within the slip plane.
• The planes of greatest atomic density are also the most widely spaced planes in the crystal lattice. So the resistance to slip is generally less for these planes compared to any other set of planes.
• Therefore slip occurs along definite slip systems in the crystal.
• Certain metals show additional slip systems with increasing temperature. In these cases, slip direction remains the same while slip planes change with temperature.

Slip Systems in Hexagonal Close-Packed (hcp) Metals

Hcp structures generally have less slip systems compared to other close-packed structure.

• In the hexagonal close-packed metals, the only plane with high atomic density is the basal plane. It is denoted by (0001) according to Miller indices.
• The axes <1120> are close-packed directions. For metals like zinc, cadmium, magnesium, cobalt slip occurs on the (0001) plane in the \(<11\bar20>\) directions.
• Since there is one basal plane per unit cell and three \(<11\bar20>\) directions, hcp structure possessed three slip systems. Low number of slip systems is the reason for low ductility and extreme orientation dependence in hcp metals.

Slip Systems in Face-Centered Cubic (fcc) Metals

There are 12 slip stems in fcc lattices.

• In the face-centered cubic structure, the {111} octahedral planes and the <110> directions are the close-packed systems. In the fcc unit cell, there are eight {111} planes.
• The planes at the opposite sides of the octahedron are parallel to each other. This leaves with only four sets of octahedral planes.
• Each {111} plane contains three <110> directions. The reverse directions are neglected. Therefore the fcc lattice has 12 slip systems.

Slip Systems in Body-Centered Cubic (bcc) Metals

In contrast to fcc and hcp metals, body-centered cubic (bcc) metals do not have close-packed planes. The planes with the highest atomic density in bcc metals are the {110}, {112}, and {123} planes, but these are not as closely packed as the {111} planes in fcc metals.

Slip in bcc metals occurs along the close-packed <111> direction, but the slip can occur on multiple planes, which makes the slip process more complex. Despite having 48 possible slip systems, bcc metals require higher shear stress to initiate slip compared to fcc metals due to the lack of a well-defined close-packed plane. This leads to lower ductility and a wavy appearance of slip bands on the surface.

Factors Affecting Slip

Several factors influence slip in metal crystals, including:

• Temperature: In certain metals, the number of available slip systems increases with temperature. This enables metals to deform more easily at higher temperatures.
• Applied Stress: Slip occurs only when the applied stress exceeds a critical value known as the critical resolved shear stress.
• Crystal Orientation: The orientation of the crystal relative to the applied stress affects which slip systems are activated and the ease with which slip occurs.

Applications and Implications

• Material Strength: Understanding slip deformation helps in designing materials with desired strength and ductility. For example, alloying elements can be added to steel to improve its mechanical properties by influencing slip behavior.
• Manufacturing Processes: Processes such as forging, rolling, and extrusion rely on controlled slip deformation to shape materials. Knowledge of slip mechanisms allows for optimization of these processes.
• Geological Materials: In geology, slip deformation is relevant to the study of fault lines and earthquakes. The movement along faults involves slip on geological planes and has significant implications for understanding seismic activity.