Metallurgy Unit 1: Introduction to Materials Science

Introduction to Metallurgy and Materials Science

• Materials science investigates the relationships between a material's structure and its properties.
• Materials engineering involves designing or engineering a material's structure to produce a predetermined set of properties.

Structure of Materials

• Structure refers to the arrangement of a material's internal components.
• Structure can be classified into:
  • Subatomic: electrons within individual atoms and their interactions with nuclei.
  • Atomic: organization of atoms or molecules relative to each other.
  • Microscopic: large groups of atoms agglomerated together.
  • Macroscopic: viewable with the naked eye.

Properties of Materials

• A property is a material's trait in response to a specific stimulus.
• Properties are independent of material shape and size.
• Examples of properties:
  • Mechanical (deformation, strength)
  • Electrical (conductivity)
  • Thermal (heat transfer)
  • Magnetic (magnetic fields)
  • Optical (reflection, transmission)
  • Deteriorative (corrosion, decay)

Classification of Materials

• Solid materials can be classified into three basic categories:
  1. Metals
     • Composed of one or more metallic elements (e.g., iron, aluminum, copper)
     • Atoms arranged in an orderly manner
     • Relatively dense
     • Mechanical properties: stiff, strong, ductile, resistant to fracture
     • Electrical properties: good conductors of electricity and heat
     • Examples: iron, copper, gold
  2. Ceramics
     • Compounds between metallic and non-metallic elements (e.g., oxides, nitrides, carbides)
     • Examples: aluminum oxide (alumina), silicon dioxide (silica), silicon carbide
     • Properties:
       • Stiff and strong
       • Hard and brittle
       • Insulators (low electrical conductivity)
       • May be transparent, translucent, or opaque
  3. Polymers
     • Carbon-based compounds (e.g., polyethylene, nylon, polyvinyl chloride)
     • Large molecular structures with a backbone of carbon atoms
     • Properties:
       • Low density
       • Not as stiff nor as strong as ceramics and metals
       • Ductile and pliable
       • Relatively inert chemically

Composites

• Composites are composed of two or more individual materials from different categories.
• Examples:
  • Cemented carbides (WC with Co binder)
  • Plastic molding compounds containing fillers
  • Rubber mixed with carbon black
  • Wood (a natural composite)

Advanced Materials

• Materials used in high-technology applications (e.g., electronic equipment, fiber optics, spacecraft)
• Examples:
  • Semiconductors
  • Biomaterials
  • Materials of the future (e.g., nanomaterials, metamaterials)

Biomaterials

• Materials used in components implanted into the human body (e.g., joint replacements, surgical instruments)
• Requirements:
  • Biocompatibility (non-toxic, non-reactive)
  • Biostability (resistance to degradation)
  • Mechanical properties (strength, durability)

Semiconductors

• Materials with electrical conductivity intermediate between metals and insulators.
• Properties sensitive to impurity concentrations and spatial control.
• Applications: electronic devices, computers, solar panels

The Materials Selection Process

• Identify required properties (mechanical, electrical, thermal, magnetic, optical, deteriorative)
• Identify candidate materials
• Consider structure, composition, and processing techniques
• Examples:
  • Casting
  • Sintering
  • Vapor deposition
  • Doping
  • Forming
  • Joining
  • Annealing

Defects in Solids

• Deviations from the perfect periodic array of atoms in a crystal
• Types of defects:
  • Point defects (vacancies, interstitials)
  • Line defects (dislocations)
  • Planar defects (grain boundaries)
• Defects affect material properties (mechanical strength, electrical conductivity, optical properties)

Crystal Structures and Materials

• Crystal structures:
  • Face-centered cubic (FCC)
  • Body-centered cubic (BCC)
  • Hexagonal close-packed (HCP)
• Materials can be:
  • Crystalline (long-range order)
  • Amorphous (no long-range order)
• Examples:
  • Metals (crystalline)
  • Glass (amorphous)
  • Polymers (semi-crystalline)

Close Packing

• Metals are packed closely together in a regular pattern

• Examples:

  • Face-centered cubic (FCC)
  • Body-centered cubic (BCC)
  • Hexagonal close-packed (HCP)

• Coordination number: number of nearest neighbors

• Packing efficiency: percentage of available space occupied by atoms### Lever Rule

• A principle used in materials science and thermodynamics to determine the relative proportions of phases in a multi-phase mixture.

• Based on the conservation of mass and the assumption of equilibrium conditions.

• Calculates the fractions or percentages of each phase present in the mixture.

• Typically applied using a phase diagram, which graphically represents the phases that are stable under different temperature and composition conditions.

Nucleation and Growth

• Nucleation: the initial stage of phase transformation where small clusters of atoms, ions, or molecules form stable nuclei of the new phase within the parent phase.
• Homogeneous nucleation: occurs throughout the bulk of the material, often at higher temperatures and under controlled conditions.
• Heterogeneous nucleation: occurs at pre-existing surfaces or interfaces, such as container walls or impurity particles, typically at lower temperatures and more common in practical applications.
• Growth: the phase transformation proceeds through the growth of nuclei, where the solid phase expands as more atoms, ions, or molecules are added.

Ingot Structure

• Refers to the microstructural arrangement found in metallic ingots, characterized by the arrangement of grains and dendrites formed during solidification.
• Grain structure: individual crystalline grains develop within the solidified material, with varying orientations throughout the ingot.
• Dendritic growth: tree-like structures that form as the solid phase extends into the liquid, influenced by temperature gradient, solidification rate, and alloy composition.

Dendritic Solidification

• A type of solidification process commonly observed in metallic alloys during casting or solidification from the melt.
• Characterized by the growth of dendritic or tree-like structures as the molten metal transforms into a solid phase.
• Nucleation: the formation of solid grains within the liquid metal, which serve as sites for further crystal growth.
• Crystal growth: atoms from the surrounding liquid attach themselves to the solid nuclei, causing the solid phase to grow.
• Dendritic growth: highly branched, resulting in the formation of dendrites that extend outward from the solid nuclei into the surrounding liquid.