Plastic Deformation

Plastic Deformation

When a material is stressed below its elastic limit, the resulting deformation or strain is temporary. Removal of stress results in a gradual return of the object to its original dimensions. When a material is stressed beyond its elastic limit, plastic or permanent deformation takes place, and it will not return to its original shape by the application of force alone. The ability of a metal to undergo plastic deformation is probably its most outstanding characteristic in comparison with other materials. All shaping operations such as stamping, pressing, spinning, rolling, forging, drawing, and extruding involve plastic deformation of metals. Various machining operations such as milling, turning, sawing, and punching also involve plastic deformation. Plastic deformation may take place by :

Slip

Twinning

Combination of slip and twinning

Deformation by Slip:

If a single crystal of a metal is stressed in tension beyond its elastic limit, it elongates slightly, a step appears on the surface indicating relative displacement of one part of the crystal with respect to the rest, and the elongation stops. Increasing the load will cause another step. It is as if neighboring thin sections of the crystal had slipped past one another like a sliding cards on a deck. Each successive elongation requires a higher stress and results in the appearance of another step, which is actually the intersection of a slip plane with the surface of the crystal. Progressive increase of the load eventually causes the material to fracture.

Slip occurs in directions in which the atoms are most closely packed, since this requires the least amount of energy.

Figure 1. The effect of slip on the lattice structure.

Figure 1 shows that when the plastic deformation is due to slip, the atoms move a whole interatomic space (moving from one corner to another corner of the unit cell). This means that overall lattice structure remains the same. Slip is observed as thin lines under the microscopes and these lines can be removed by polishing.

Figure 2. Slip appears as thin lines under the microscope.

Figure 3. Slip lines in copper.

Deformation by Twinning:

When mechanical deformation is created by twinning, the lattice structure changes. The atoms move only a fraction of an interatomic space and this leads to a rearrangement of the lattice structure. Twinning is observed as wide bands under the microscope. These wide bands can not be removed by polishing.

Two kinds of twins are of interest to the metallurgists:

1. Deformation or mechanical twins, most prevalent in close packed hexagonal metals (magnesium, zinc, iron with large amount of ferrite)

2. Annealing twins, most prevalent in F.C.C. (Face centered cubic) metals (aluminum, copper, brass, iron with austenite).These metals have been previously worked and heat treated. The twins are formed because of a change in the normal growth mechanism.

Figure 4. The effect of twinning on the lattice structure.

Figure 5. Twin bands

Figure 6. Twin bands in zinc.

Slip vs. Twinning:

	Slip	Twinning
Atomic movement	Atoms move a whole number of atomic spacing.	Atoms move fractional atomic spacing.
Microscopic appearance	Thin lines	Wide bands or broad lines
Lattice orientation	No change in lattice orientation. The steps are only visible on the surface of the crystal and can be removed by polishing. After polishing there is no evidence of slip.	Lattice orientation changes. Surface polishing will not destroy the evidence of twinning.

Material Science

Material Science

Term Papers (Fall 2004)

Activity	Due Date
Select three (3) topics and list them in priority order. The topics can be selected from the list of topics given in our course outline. You may select topics outside the list, that may be related to your job (subject to instructor's approval). Submit the topics along with your name. (You may submit this information by email no later than the due date)	October 7, 2004
Students will be informed about the approved topic of their interest.	October 14, 2004
Term Papers are due	December 2, 2004
Oral Presentations	December 2, 2004

The following is the format that should be used for the term paper :

1. Cover page

Should include : The topic, name of the student, date the paper is submitted, name of the instructor.

2. Explain the following:

• Scope of your paper (Focus on a specific area that is directly related to your topic. If you selected a topic such as ceramics, explain that you will be covering the properties of ceramics or the manufacturing processes of ceramics, etc. )
• Main topic may include graphs, photographs, tables, etc. (You may use Internet, Power Point, Overhead Projector, etc. for the oral presentation. )
• Conclusions

3. References (List the author, title, publisher of all the sources you used for the preparation of your term paper. If you used internet sites, you may include the URL of each site)

Term papers must be typed. Handwritten papers will not be accepted.

Oral presentation should be minimum 10 minutes and maximum 15 minutes.

There is no limit for the written part of the term paper. The important criteria is how well your topic is explained both in the written part and the oral presentation.

There are two types of bonds:

Primary

Secondary

Primary Bonds:

Primary bonds are the strongest bonds which hold atoms together. The three types of primary bonds are:

Metallic Bonds

Covalent Bonds

Ionic Bonds

Metallic Bonds

In a metal, the outer electrons are shared among all the atoms in the solid. Each atom gives up its outer electrons and becomes slightly positively charged. The negatively charged electrons hold the metal atoms together. Since the electrons are free to move, they lead to good thermal and electrical conductivity.

Figure 1. Metallic bond.

The lack of oppositely charged ions in the metallic structure and lack of sufficient valence electrons to form a true covalent bond necessitate the sharing of valence electrons by more than two atoms. Each of the atoms of the metal contributes its valence electrons to the formation of the negative " electron cloud". These electrons are not associated with a particular ion but are free to move among the positive metallic ions in definite energy levels. The metallic ions are held together by virtue of their mutual attraction for the negative electron cloud. This is illustrated schematically in Figure 2.The metallic bond may be thought of as an extension of the covalent bond to a large number of atoms.

Atomic Bonds

Figure 2. Metallic bond and electron cloud

Atoms like to have a filled outer shell of electrons. Sometimes, by transferring electrons from one atom to another, electron shells are filled. The donor atom will take a positive charge, and the acceptor will have a negative charge. The charged atoms or ions will be attracted to each other, and form bonds. The compound NaCl, or table salt, is the most common example.

Figure 3. Ionic bond

The electron structure of atoms is relatively stable when the outer shells contain eight electrons (or two in the case of the first shell). An element like sodium with one excess electron will give it up so that it has a completely filled outer shell. It will then have more protons than electrons and become a positive ion (charged atom) with a +1 charge. An atom of chlorine, on the other hand, with seven electrons in its outer shell would like to accept one electron. When it does, it will have one more electron than protons and become a negative ion with a -1 charge. When sodium and chlorine atoms are placed together, there is a transfer of electrons from the sodium to the chlorine atoms, resulting in a strong electrostatic attraction between the positive sodium ions and the negative chlorine ions. This explains the strong attraction between paired ions typical of the gas or liquid state.

Figure 4a. Formation of ionic bond in NaCl.

Figure 4b. Na+ and Cl- ions formed by ionic bonding mechanism.

Covalent Bonds

Some atoms like to share electrons to complete their outer shells. Each pair of shared atoms is called a covalent bond. Covalent bonds are called directional because the atoms tend to remain in fixed positions with respect to each other. Covalent bonds are also very strong. Examples include diamond, and the O-O and N-N bonds in oxygen and nitrogen gases.

Figure 5. Covalent bonding.