Mohamad Kobeissi – 307 170 136
AMME2302 Materials 1
Assignment 2
7.22
Plastic deformation is a function of dislocation mobility. Three different methods are used to restrict dislocation motion which results in the increase of hardness and strength. These methods are:

Grain size reduction:
This is simply the reduction of grain size of a polycrystalline material. Since grain boundaries serve as barriers to dislocation motion, reducing the grain size will reduce this motion.

Solid-solution strengthening:
This involves alloying a metal with impurity atoms that impose lattice strains on the surrounding host atoms. The lattice strain field interactions between dislocations and these impurity atoms restrict the dislocation movement.

Strain hardening:
This is when a ductile metal becomes harder and stronger as it is plastically deformed.
This results in an increase in dislocation density and the repulsive dislocation- dislocation strain field interactions.
11.D14 a) The range of compositions over which these alloys may be precipitation hardened is approximately between 0.3-2.7 wt% Be. b) For example a 1 wt% Be - 99 wt% Cu alloy, the solution heat treating temperature must be about 500°C-980°C. It is then quenched to room temperature and finally the precipitation heat treatment would be done at a temperature below 500°C.

Mohamad Kobeissi – 307 170 136
Essay
Much has been learnt in this semester on the structure, properties, processing and applications of materials. Although it is impossible to discuss all areas in such a short essay, as much as possible will be touched on.
Some of the important properties of solids materials depend on the geometrical atomic arrangements and the interactions that exist among nearby atoms. This geometrical atomic arrangement is known as the atomic structure and the interactions that occur within these structures depend on the bonding forces and energies.
The three types of primary bond in solids are ionic, covalent and metallic. For ionic bonds, electrically charged ions are formed by the transference of valence electrons from one atom type to another. Covalent bonding is the sharing of valence electrons between adjacent atoms.
In metallic bonding, the metallic material has one, two or at most three valence electrons that are free to move throughout the entire metal. The remaining non-valence electrons and nuclei form what are called ion cores. The free electrons shield the ion cores from mutually repulsive electrostatic forces and hold the ion cores together.
Atoms in crystalline solids are positioned in orderly and repeated patterns. Atoms may be represented as solid spheres and the spatial arrangement of these spheres is known as the crystal structure. Most common metals exist in at least one of three crystal structures: bodycentred cubic (BBC), face-centred cubic (FCC) and hexagonal close packed (HCP). Not all materials are as perfect as they sound though; solid materials contain large numbers of imperfections which are categorized on the basis of their geometry and size. Point defects are those associated with one or two atomic positions, including vacancies, self-interstitials and impurity atoms.
A solid solution may form when impurity atoms are added to a solid, in which case the original crystal structure is retained and no new phases are formed. For substitutional solid solutions, impurity atoms substitute for host atoms, and appreciable solubility is possible only when atomic diameters and electronegativities for both atom types are similar, also when both elements have the same crystal structure, and when the impurity atoms have a valence that is the same as or less than the host material. Interstitial solid solutions form for relatively small impurity atoms that occupy interstitial sites among the host atoms.
Dislocations are one-dimensional crystalline defects of which there are two pure types; edge and screw. An edge may be thought of in terms of the lattice distortion along the end of an extra half-place of atoms and a screw as a helical planar ramp. For mixed dislocations, components of both edge and screw are found. The magnitude and direction of lattice distortion associated with a dislocation are specified by what is known as the Burgers vector.
The directions of the Burgers vector and dislocation line are different in all three cases. They are perpendicular for edge, parallel for screw, and neither perpendicular nor parallel for mixed.

Mohamad Kobeissi – 307 170 136
Other imperfections include interfacial defects such as external surfaces, grain boundaries and twin boundaries. Also there are volume defects like cracks and pores, and atomic vibrations. Each type of imperfection has some influence on the properties of a material.
It is always important for engineers to be familiar with the mechanical characteristics and behaviour of different materials. Some of these characteristics can be determined by using stress-strain tests. Stress is a measure of an applied load over a cross-sectional area while strain represents the amount of deformation caused by this stress. There are four types of stress-strain tests: tension test, compression test, torsion test and shear test. The tensile test is the most common whereby a material is stretched and strain is recorded and compared with the applied stress.
One characteristic that is determined during the different tests is the modulus of elasticity for tension and compression. The value is calculated by using the stress value over the strain value in the elastic region of the test. After this region the deformation becomes plastic and permanent. Values of the yield strength and tensile strength then become important; were yield strength is the highest value of stress before plastic deformation occurs and tensile strength is the maximum possible stress a material can take. From the stress-strain diagrams we obtain, we can determine how ductile or brittle a material is. A material that is very ductile has greater strain values than a brittle material after plastic deformation and thus a larger elongation at fracture.
Another characteristic that is important is hardness; which is a measure of a materials resistance to localized plastic deformation. Hardness of a material can also be tested using popular hardness testing techniques such as the Rockwell test and Brinell test. In these tests a small indenter is forced into the surface of the material and an index number is determined on the basis of the size or depth of the resulting indentation.
To form metallic materials in the desired shape and size, different fabrication processing techniques may be utilized. Forming operations are those in which a metal piece is shaped by plastic deformation. If deformation is carried out above the recrystallization temperature, it is known as hot working, otherwise if below this temperature it is cold working. Casting, forging, rolling, extrusion and drawing are the most common forming techniques. Casting may be the most desirable and economical fabrication process, depending on the properties and shape of the finished piece.
The processing technique of materials is a function of the desired properties required for any given material. At the same time the desired properties of a material is related to the structure of the material. Learning and understanding these main concepts and how they are related to one another is very important for engineers when deciding which material best satisfies their requirements in different areas.