Engineering Materials Science Test 1

Concept:

For monoclinic system

a ≠ b ≠ c

α = γ = 90° ≠ β

Concept:

Line defects, or dislocations, are lines along which whole rows of atoms in a solid are arranged anomalously. The resulting irregularity in spacing is most severe along a line called the line of dislocation. Line defects can weaken or strengthen solids.

Dislocation: It is the boundary between slipped and unslipped portions of a crystal.

Edge dislocation: It is a linear defect that centers on the line that is defined along the end of the extra half-plane of atoms. There is distortion around the distortion line. Atoms above the plane are squeezed together and atoms below the plane are pulled apart.

Screw dislocation: In screw dislocation, the dislocations are formed as the shear stress is applied to produce the distortion. The upper front region of the crystal is shifted from one atomic distance to the right relative to the bottom portion. Dislocation lines are parallel to the applied shear stress.

Dislocation movement:

Edge dislocation moves || to the shear direction

Screw dislocation moves ⊥ to the shear direction

Classification of structural imperfection

Explanation

In the crystal structure, the atomic packing factor (APF) or packing efficiency or packing fraction is the volume of atoms in a unit cell divided by the volume of the unit cell. It is a dimensionless quantity and always less than unity

The atomic packing factor of different crystal structures is given in the table below:

Concept:

FCC has 4 effective atoms / unit cell. With APF of 74%

For FCC,

\(a = \frac{{4r}}{{\sqrt 2 }}\)

Where a is plane length [100]

Number of atoms in 100 \(= 1 + 4 \times \frac{1}{4} = 2\)

Calculation:

2r = 3° A ⇒ r = 1.50° A

\(a = \frac{{4r}}{{\sqrt 2 }} = 2\sqrt 2 \;r\)

∴ a = 4.2426 × 10^-7 mm

Now,

Atom/square mm \( = \frac{{2\; \times \;1}}{{{a^2}}} = \frac{{2\; \times \;{{10}^{14}}}}{{{{\left( {4.2426} \right)}^2}}}\)

∴ Atom/square mm = 1.11 × 10¹³ atoms.

Concept:

• The slow and continuous elongation of a material with time at constant stress and high temperature below the elastic limit is called creep.
• At high temperatures, stresses even below the elastic limit can cause some permanent deformation.
• The critical applications involving high operating temperatures and stresses include parts of internal – combustion engine and jet engine, high-pressure boilers ((the superheater and reheater tubes and headers) and steam turbines, blades of the turbine rotor.
• The increase of deformation with time is known as the creep rate. If the material under stress is heated, the creep rate increases with time and eventually results in rupture of the material.

Creep is of great importance in the following cases:

• Soft metals used at about room temperature, such as lead pipes, white metal bearings, etc.
• Steam and chemical plants operating at 450 – 550°C
• Gas turbines working higher temperatures

Fatigue

• Materials subjected to repetitive or fluctuating stress will fail at a stress much lower than that required to cause failure under steady loads. This behavior is called fatigue.
• The stress at which a material fails by fatigue is known as fatigue strength.

Impact strength

• It is the capability of the material to withstand a suddenly applied load and is expressed in terms of energy.
• Often measured with the Izod impact strength test or Charpy impact test, both of which measure the impact energy required to fracture a sample.
• Thus, the impact strength of a material is an index of its toughness.

Malleability 

• It is the property by virtue of which a material may be hammered or rolled into thin sheets without rupture.
• This property generally increases with the increase of temperature.
• Malleability is the ability of a metal to exhibit large deformation or plastic response when being subjected to a compressive force.