Phase Transformations and Material Properties in Metallurgy

Posted on Aug 22, 2024 in Physics

An Equilibrium Reaction: Monotectic

A monotectic reaction is one in which a molten solution, upon cooling, becomes immiscible. This process can lead to material defects like cracking.

Portevin Rules

1. In each balancing zone, only two phases can exist at most.
2. Except for the liquid phase region, all other single-phase regions are occupied by solid phases.
3. When crossing a line in the phase diagram, the number of phases varies by one.
4. Any region bounded by a horizontal line contains two phases.
5. Two-phase regions bounded by the same horizontal line contain no more than three associated phases.
6. Any phase contained in a two-phase region is a pure metal or a defined composite.
7. The region of phase equilibrium can be zoned by vertical lines stemming from a singular point on a horizontal line in the phase diagram.

Supercritical Annealing

When steel is heated to temperatures above its critical point (the maximum temperature at which it remains solid), it undergoes supercritical annealing.

Systems, Phases, and Components

A system is a collection of chemicals isolated for study. It comprises simple and composite substances in various proportions, capable of undergoing physical, chemical, or physicochemical interactions.

A phase is a part of a system with distinct physical properties.

A component is a pure substance or a composite material initially distinguishable within the system. In equilibrium, these components may form solutions or mixtures.

Solid Solutions and Their Classes

A solid solution forms when solute atoms are incorporated into the solvent’s crystal lattice. There are two primary types:

• Interstitial: Solute atoms occupy the spaces between solvent atoms in the lattice.
• Substitutional: Solute atoms replace solvent atoms within the lattice.

Common Material Defects

• Blowholes: Cavities formed by trapped gases during solidification.

System Stability

• Stable System: Remains unchanged under normal conditions.
• Metastable System: Remains unchanged unless specific external conditions trigger a change.
• Unstable System: Changes with variations in external conditions.

Metallurgical Processes

• Sintering: Bonding fine particles by applying pressure and heat below the melting point of the main component. This process relies on atomic diffusion.
• Segregation: Uneven distribution of elements during crystallization or solidification.

Crystal Lattice Systems

Crystal lattices are classified based on their symmetry:

• Cubic
• Tetragonal
• Orthorhombic
• Rhombohedral
• Hexagonal
• Monoclinic
• Triclinic

Important Temperatures in Material Science

• Recrystallization Temperature: The temperature at which atoms rearrange within the lattice, relieving internal stresses.
• Melting Temperature: The temperature at which a solid transforms into a liquid.

Bond Strength and Hardenability

Bond strength is directly proportional to the force holding atoms together. Hardenability refers to a material’s ability to be hardened through heat treatment.

Critical Temperatures in Steel

• Ac₀: 210 °C, Curie temperature of cementite (magnetic transformation).
• Ac₁: 723 °C, lower critical temperature for pearlite formation.
• Ac₂: 768 °C, Curie temperature of ferrite (magnetic transformation).
• Ac₃: 723 °C to 910 °C, upper critical temperature for ferrite formation.
• Ac₄: 1492 °C, upper critical temperature for austenite formation.

Atomic Structure and Properties

Valence: The number of electrons in an atom’s outermost shell, responsible for chemical bonding.

Quantum Numbers

Quantum numbers describe the properties of electrons:

• n (Principal Quantum Number): Energy level (1, 2, 3…).
• l (Angular Momentum Quantum Number): Shape of the orbital (0 to n-1).
• m_l (Magnetic Quantum Number): Orientation of the orbital in space (-l to +l).
• m_s (Spin Quantum Number): Spin of the electron (+1/2 or -1/2).

Atomic Mass and Atomic Number

• Mass Number: The sum of protons and neutrons in an atom’s nucleus.
• Atomic Number: The number of protons in an atom’s nucleus.

Phase Rule and Maximum Coexisting Phases

The Gibbs Phase Rule determines the maximum number of phases that can coexist in equilibrium:

F + V = C + 2

Where:

• F = Number of phases
• V = Degrees of freedom
• C = Number of components

Ternary System (C=3):

V = 3 + 1 – F = 4 – F

For maximum phases (V=0), F = 4. Therefore, a maximum of 4 phases can coexist in a ternary system.

Quaternary System (C=4):

V = 4 + 1 – F = 5 – F

For maximum phases (V=0), F = 5. Therefore, a maximum of 5 phases can coexist in a quaternary system.

Radioactive Decay

When an atom emits an alpha (α) particle, it releases a helium nucleus (2 protons and 2 neutrons), resulting in a decrease in atomic mass and atomic number. Alpha radiation is highly ionizing but has low penetration power.

When an atom emits a beta (β) particle, a neutron transforms into a proton and an electron. The electron (beta particle) is ejected, while the proton remains in the nucleus. This process increases the atomic number by one but doesn’t change the atomic mass. Beta radiation is less ionizing than alpha radiation but has higher penetration power.

Mechanical Treatments

Mechanical treatments modify a material’s microstructure and properties.

Cold Working

Cold working enhances hardness, yield strength, tensile strength, and fatigue resistance without affecting grain size.

Hot Working

Hot working alters grain size and can improve ductility.

Ledeburite

4’3% austenite + cementiteWhat we pursue mechanical treatments cold and hot? Cold: improving the hardness, yield strength, yield strength and fatigue resistance. It does not affect grain size. Warm: changes the grain size. 17 .- Determination of the solid solution. Interstición: solute atoms occupy the holes of the solvent; Replacement: solute atoms replace the solvent. Varieties allotropic allotropic iron We refer to those varieties that have the same composition but different crystallization. Nucleus properties and the bark of The properties of atoms atom’s nucleus are: a) mass number, classification of nuclides. b) loading and size of the nucleus. c) binding energy. d) the nature of nuclear forces.