Plastic Deformation in B.C.C. and H.C.P. Metals

Introduction
Plastic deformation refers to the permanent change in shape or size of a material without fracturing. It occurs when the applied stress exceeds the elastic limit of the material. In the case of B.C.C. (Body-Centered Cubic) and H.C.P. (Hexagonal Close-Packed) metals, plastic deformation primarily occurs through twinning and slip.

1. Slip
Slip is the most common mechanism of plastic deformation in metals. It involves the movement of dislocations within the crystal lattice. Dislocations are line defects in the crystal structure where atoms are misplaced. Slip occurs when dislocations move along crystallographic planes, causing the atoms to slide past each other. This movement leads to plastic deformation as the crystal structure is permanently altered. In B.C.C. and H.C.P. metals, slip occurs predominantly along specific crystallographic planes and directions.

2. Twinning
Twinning is another mechanism of plastic deformation that occurs in certain crystal structures, such as H.C.P. metals. It involves the formation of a mirror-image twin boundary within the crystal. Twinning can occur when the crystal is subjected to a stress that is not easily accommodated by slip. In H.C.P. metals, twinning occurs along specific crystallographic planes and directions, leading to a change in the crystal structure. This change allows the material to undergo plastic deformation.

3. Edge Dislocation
Edge dislocations are another type of line defect in the crystal structure. They occur when an extra half-plane of atoms is inserted into the crystal lattice. Edge dislocations can contribute to plastic deformation by moving along specific crystallographic planes, leading to slip. While edge dislocations are not the primary mechanism of plastic deformation in B.C.C. and H.C.P. metals, they can interact with twinning and slip to facilitate plasticity.

4. Twinning in Combination with Slip
The correct answer, option D, states that plastic deformation in B.C.C. and H.C.P. metals occurs through twinning in combination with slip. This means that both mechanisms contribute to the overall plasticity of these materials. Twinning allows for plastic deformation in H.C.P. metals when slip alone is not sufficient to accommodate the applied stress. Slip, on the other hand, is the primary mechanism of plastic deformation in both B.C.C. and H.C.P. metals. The interplay between twinning and slip enables these metals to undergo plastic deformation and exhibit ductility.

In summary, plastic deformation in B.C.C. and H.C.P. metals occurs through twinning and slip. While slip is the primary mechanism, twinning plays a significant role in enabling plasticity, especially in H.C.P. metals. The combination of these mechanisms allows for the permanent change in shape or size of these materials under applied stress.