Question 2 (a) State the main types of imperfections commonly observed within cr

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Question 2 (a) State the main types of imperfections commonly observed within crystalline metallic structures 3 marks (b) Why does plastic deformation occur at lower stress than that predicted for single crystal materials? 4 markS (c) Consider a metal single crystal orientated such that the angle normal to the slip plane and the angle of slip direction are at 45° and 55 to the tensile axis respectively. If the critical resolved shear stress is 50 MPa, will an applied stress of 0.065 GPa cause the single crystal to yield? If not, what stress will be necessary? R COSCOS t, Resolved sheer stress and Tensile stress 9 marks (d) Describe the differences in the aluminium grain structure shown in the micrographs below and suggest what process has been used. Comment on the expected mechanical properties of the final product and describe the related underpinning theory Before processing After processing 9 marks (TOTAL 25 marks)

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Answers:

(a)Types of imperfections in crystalline metallic structures:

1.Point defects:

Point defects are defects that occur only at or around a single lattice point. They are not extended in space in any dimension. These defects typically involve at most a few extra or missing atoms.

1.1Vacancy defects are lattice sites which would be occupied in a perfect crystal, but are vacant. If a neighboring atom moves to occupy the vacant site, the vacancy moves in the opposite direction to the site which used to be occupied by the moving atom. The stability of the surrounding crystal structure guarantees that the neighboring atoms will not simply collapse around the vacancy. In some materials, neighboring atoms actually move away from a vacancy, because they experience attraction from atoms in the surroundings. A vacancy (or pair of vacancies in an ionic solid) is sometimes called a Schottky defect.

1.2Interstitial defects are atoms that occupy a site in the crystal structure at which there is usually not an atom. They are generally high energy configurations. Small atoms in some crystals can occupy interstices without high energy, such as hydrogen in palladium.

1.3A nearby pair of a vacancy and an interstitial is often called a Frenkel defect or Frenkel pair. This is caused when an ion moves into an interstitial site and creates a vacancy.

2. Line defects:

Dislocations are linear defects, around which (the dislocation line) some of the atoms of the crystal lattice are misaligned.

There are two basic types of dislocations, the edge dislocation and the screw dislocation. "Mixed" dislocations, combining aspects of both types, are also common.

Edge dislocations are caused by the termination of a plane of atoms in the middle of a crystal. In such a case, the adjacent planes are not straight, but instead bend around the edge of the terminating plane so that the crystal structure is perfectly ordered on either side.

The screw dislocation is more difficult to visualize, but basically comprises a structure in which a helical path is traced around the linear defect (dislocation line) by the atomic planes of atoms in the crystal lattice.

3.Planar defects:

Grain boundaries occur where the crystallographic direction of the lattice abruptly changes. This usually occurs when two crystals begin growing separately and then meet.

Antiphase boundaries occur in ordered alloys: in this case, the crystallographic direction remains the same, but each side of the boundary has an opposite phase: For example, if the ordering is usually ABABABAB (hexagonal close-packed crystal), an antiphase boundary takes the form of ABABBABA.

Stacking faults occur in a number of crystal structures, but the common example is in close-packed structures. They are formed by a local deviation of the stacking sequence of layers in a crystal. An example would be the ABABCABAB stacking sequence.

A twin boundary is a defect that introduces a plane of mirror symmetry in the ordering of a crystal. For example, in cubic close-packed crystals, the stacking sequence of a twin boundary would be ABCABCBACBA.

4.Bulk defects:

Three-dimensional macroscopic or bulk defects, such as pores, cracks, or inclusions.

Voids — small regions where there are no atoms, and which can be thought of as clusters of vacancies

(b) Plastic deformation occurs at lower stress than that predicted for single crystal materials.

Reason: Slip does not require all bonds across the slip line to break simultaneously, but only small fraction of the bonds are broken at any given time.

For example, if the top half of a crystal is slipping one plane at a time then only a small fraction of the bonds are broken at any given time and this would require a much smaller force. The propagation of one dislocation across the plane causes the top half of the crystal to move (to slip) with respect to the bottom half but we do not have to break all the bonds across the middle plane simultaneously (which would require a very large force). Thus, a lower force and lower stress is required.

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