Introduction to Materials Science
Mixed Dendrites
If a pure metal is cooled uniformly, reaching the solidification temperature, germ nuclei arise. From these nuclei, crystals develop in the direction of crystal axes, giving rise to certain arborescent forms called dendrites.
Diffusion
Diffusion is the displacement of atoms disordered in the crystal lattice from their equilibrium position to another nearby.
Theoretical Density vs. Practical Density
Why is the actual density less than the theoretical density?
The actual density is less than the theoretical density because materials are not perfect in their atomic arrangement. Therefore, when we calculate the experimental density (true density), it will be less than the theoretical density due to these imperfections. The theoretical density assumes a perfect atomic arrangement.
Rockwell and Vickers Hardness Tests
The Rockwell test is done with a steel ball or cone. The Vickers test is done with a diamond indenter.
- Rockwell: Uses a hardened steel ball or a conical diamond indenter with an angle of 120° ± 30′ and a rounded apex forming a spherical cap of radius 0.20 mm.
- Vickers: Uses a diamond pyramid indenter with a base angle of 136°.
The Brinell test uses penetrators in the form of balls of different diameters. These balls can be made of hardened steel or tungsten carbide.
Aging
Aging is a process in which castings are maintained, or peeled, for a time, at room temperature, to increase dimensional stability or improve mechanical properties (hardness and yield strength) through structural changes. This phenomenon is also called maturation.
Covalent Bonds
Covalent bonds are formed by the reaction between two nonmetal atoms, where the electrons are shared by both atoms. This type of bonding occurs when there is polar electronegativity. It is formed when the electronegativity difference is not large enough for electron transfer to take place; instead, one or more atoms share electron pairs in a new type of molecular orbital called a covalent bond.
Steadite
Steadite is a type of eutectic, hard, and brittle, with a melting temperature of 960°C. In foundries, it is shown in gray.
Elasticity
A material is said to be elastic if deformations disappear when external stresses are removed.
Thermal Energy and Specific Heat of Solids
The ways in which a solid absorbs energy are:
- Increasing the kinetic energy of free electrons
- Stimulating atomic vibrations
- Increasing rotational energy
Internal energy is the sum of the thermal energy and any energy that may be present at absolute zero. Specific heat is the heat capacity of a unit mass and represents the amount of heat to be supplied to the unit mass of a substance to increase its temperature by one degree.
Rockwell, Vickers, and Brinell Hardness Tests
These tests measure the hardness of materials:
- Rockwell: Uses a hardened steel ball or a conical diamond indenter.
- Vickers: Uses a diamond pyramid indenter.
- Brinell: Uses balls of different diameters, made of hardened steel or tungsten carbide.
Fatigue
Fatigue is the tendency of a material to fracture under loads below the fracture strength, which act repeatedly over time.
Casting
Casting is a manufacturing process that consists of pouring molten metal into a mold with the desired shape and allowing it to solidify. Classes of casting include chilled casting, white iron, gray iron, malleable iron, and ductile iron foundry.
Creep
When a material is subjected to external stresses, there is a linear relationship between stresses and strains. This linear relationship is maintained until it reaches a particular yield point, where the deformations that occur in the material become irreversible. Therefore, the yield strength is the value of the load or stress from which the deformations become permanent.
Factors Influencing Grain Size
The grain structure of metals can vary widely, as the size and shape of grains depend on the solidification process or subsequent changes. Grain size varies with heat treatment, cold working, and crystallization.
Graphite
Graphite is a compound that contains iron (Fe) and carbon (C). It crystallizes in the hexagonal system. It is very soft, gray, and brittle. It does not form chips but rather dust.
Frequent Inclusions in Steels
Common inclusions in steels include silicon (Si), sulfur (S), manganese (Mn), chromium (Cr), and molybdenum (Mo).
Inoculation
Inoculation is the addition of core products to liquid metal that act as seeds for crystallization or to refine the grain.
Martempering
Martempering is a heat treatment process that involves:
- Heating steel to austenitizing temperature.
- Cooling it suddenly in a salt bath or hot oil to a temperature slightly above Ms (the temperature at which martensite starts to form).
- Holding the temperature constant to equalize the temperature of the piece (the isothermal treatment is stopped before the start of bainitic transformation).
- Cooling to room temperature to produce 100% martensite.
This treatment produces martensite with less risk of distortion and thermal shock fractures.
Thermocouple
A thermocouple is a device made by joining two dissimilar metals. It produces a voltage (Seebeck effect) that is a function of the temperature difference between the hot junction (measuring end) and the cold junction (reference end).
Pyrometers and Their Classes
Pyrometers are devices for measuring high temperatures exceeding 600°C. The most common types are optical pyrometers, thermoelectric pyrometers, and those that utilize the ability of certain materials to deform permanently under the action of external stresses.
Periods of Flow (Yield Point)
When a material is subjected to external stresses, there is initially a linear relationship between stresses and strains. This linear relationship holds until the yield point is reached. At the yield point, the deformations in the material become irreversible, marking the transition from elastic to plastic deformation. The yield stress is the stress value at which permanent deformations occur.
Peritectic Reaction
A peritectic reaction occurs when a liquid phase and a solid phase react to form a new solid phase.
Straight Part of a Cooling Curve
The straight part of a cooling curve represents the period during which a material transitions from a liquid to a solid state. As the material cools, it reaches its crystallization temperature. At this point, the temperature remains constant as the material releases latent heat of fusion while solidifying. The straight line on the cooling curve corresponds to this isothermal process.
Critical Radius of a Seed
The critical radius is the minimum size a seed crystal must reach to overcome the energy barrier for further growth. If a seed crystal is smaller than the critical radius, it will tend to dissolve back into the liquid phase. If it’s larger, it will continue to grow.
Resilience
Resilience is a material property that quantifies the amount of energy a material can absorb before fracturing under impact loading. It is typically expressed as energy absorbed per unit fracture surface area.
Recrystallization
Process by which a cold-worked metal is heated to a temperature high enough for a interval of time sufficient to form a new grain structure free of distortion.