Metals, Alloys, and Allotropy Quiz

Metals and Alloys

• Metals are characterized by their ductility, malleability, high electrical and thermal conductivity, and high corrosion rate.
• Metallic bonding is responsible for the conductivity of heat and electricity due to the presence of free electrons.
• Alloys are composed of at least two different atoms, with at least one being a metal.
• Alloys are generally stronger, harder, more durable, and more corrosion-resistant than their pure metal forms.

Solid Solutions and Allotropy

• A solid solution is a homogeneous mixture of at least two different kinds of atoms in a solid state, with a single crystal structure.
• Allotropy refers to materials with different crystal structures in the solid state, depending on temperature and pressure.
• Iron (Fe), Titanium (Ti), and other materials exhibit allotropy.

Iron Allotropes

• Iron exists in different phases depending on temperature.
• The δ-BCC, γ-FCC, and α-BCC phases have varying lattice parameters and carbon solubility at different temperatures.

Main Transition Mechanisms in Fe-C Diagram

• Eutectic: A liquid phase transforms into two solid phases at constant temperature and concentration.
• Peritectic: One liquid and one solid phase transform into another solid phase at constant temperature and concentration.
• Eutectoid: One solid phase transforms into two different solid phases at constant temperature and concentration.

Fe-C System

• Three key reactions occur in the Fe-C system:
  • Eutectic: Liquid Fe → γ + Fe₃C
  • Peritectic: Liquid Fe + δ → γ
  • Eutectoid: γ → α + Fe₃C
• These reactions occur at specific temperatures and compositions.

Properties of Fe₃C

• Cementite (Fe₃C) is a hard and brittle compound.
• It has higher mechanical properties than pure iron due to its unique bonding characteristics.
• Fe₃C exhibits metallic, covalent, and ionic bonding.

Fe-C Diagram

• The Acm line represents the carbon solubility of iron as a function of temperature.
• The Led A and Led γ lines identify the transformation pathways of iron and cementite.

Composition 1 (eutectoid composition)

• At eutectoid composition (0.8%C), all carbon is initially dissolved in the γ phase.
• As temperature decreases, the γ phase transforms into pearlite (α + Fe₃C).
• At room temperature, the composition is divided between α (0.008%C) and Fe₃C (6.67%C).

Lever Rule Application

• The lever rule can be used to determine the phase ratios at room temperature.
• In this case, α accounts for 88.11% and Fe₃C accounts for 11.88% of the composition.

Composition 2 (0.3%C)

• At 0.3%C, the composition exists in the γ phase (FCC) with a maximum carbon solubility of 2.06%.

Composition 3 (1.2%C)

• At 1.2%C, the composition exists in the γ phase (FCC), forming a solid solution.

Acm Line and Carbon Solubility

• The Acm line indicates an increase in carbon solubility in the γ phase with increasing temperature.

TTT Diagram

• The TTT diagram (Time, Temperature, Transformation) depicts the microstructure evolution during cooling.
• The cooling rate affects the transformation process, influencing the microstructure or steady-state condition.

Diffusion-Based Transformation (α → BCC)

• The diffusion-based transformation from α to BCC involves short-range movements of iron atoms.
• The process leads to a decrease in lattice volume.

Non-Equilibrium Condition (Quenching)

• Quenching is a rapid cooling process that prevents diffusion-based transformations.
• Due to the rapid cooling, a non-equilibrium condition is achieved.

Martensite

• Martensite is a super-saturated structure of iron with carbon.
• The presence of carbon in the lattice causes lattice distortion, forming a tetragonal (BCT) structure.
• Martensite is characterized by high strength, hardness, and brittleness.

Tempering

• Tempering is a heat treatment applied after martensite formation to improve ductility.
• The tempering temperature depends on the carbon content and other alloying elements.

Mechanical Properties of Martensite

• The carbon content influences the formation of martensite.
• Lath martensite is known for its higher strength and hardness compared to plate martensite.
• Plate martensite exhibits a higher proportion and ductility than lath martensite.