Plastic Deformation Mechanisms

PLASTIC DEFORMATION MECHANISMS

Plastic deformation of a single crystal occurs in two ways.

They are

(a) Slip

(b) Twinning

Slip

The deformation by slip takes place when one part of the crystal moves or glides over another part along certain planes.

The planes over which slip takes place are the planes of greatest atomic density and they are called slip planes. Slip directions are the closest packed directions within the slip planes.

Mechanism of slip

There are 4 stages during plastic deformation of single crystal by the slip. (Fig. 1.47).

Stage 1 → Perfect crystal without slip

Stage 2 → Deforming shear force is applied to the crystal

Stage 3 → Slip taking place along the slip plane

Stage 4 → Permanent deformation when deforming forces are removed.

The slip represents a displacement of a part of the crystal relative to another along certain crystallographic planes and in certain crystallographic directions. (Fig. 1.48)

The particular crystallographic planes where slip occurs are called slip planes. Similarly, the preferable direction along which slip occurs is called the slip direction.

Twinning

Another important plastic deformation mechanism which occurs in some metal is known as 'twinning'. The mechanism of twinning is shown in fig.1.49.

The shear forces produce atomic displacements such that the deformed lattice forms a mirror image of the undeformed lattice.

That is, the atoms on one side of the plane location is in mirror image position of the atoms on the other side.

The crystallographic plane of symmetry between the deformed and undeformed parts of the metal lattice is called the twinning plane.

Twinning is a major deformation notably seen in hexagonal close packed (HCP) metals like zinc and tin. Twinning also occurs in definite crystallographic planes and direction that depends on the crystal structure.

Types of Twins

1. Mechanical twins

2. Annealing twins.

1. Mechanical twins

Twins which are produced by mechanical deformation are called mechanical twins. These twins are produced in BCC and HCP metals under the conditions of decreased temperature and shock loading. Mechanical twins are not formed in FCC metals.

These twins can form in a few micro second time while slip takes several seconds to occur.

2. Annealing twins

The twins which are produced by annealing are called annealing twins. These are usually broader. Most of the FCC metals form annealing twins. They are formed because of a change in normal growth mechanism.

Table 1.4

Comparison between slip and twinning