Materials Science and Metallurgy Unit 1

Materials Science and Engineering

• Investigates relationships between structure and properties of materials
• Designs or engineers material structure to produce predetermined properties

Structure of Materials

• Refers to arrangement of internal components
• Subatomic: electrons within individual atoms and interactions with nuclei
• Atomic: organization of atoms or molecules relative to each other
• Microscopic: large groups of atoms normally agglomerated together
• Macroscopic: viewable with the naked eye

Properties of Materials

• Material trait in terms of response to a specific imposed stimulus
• Independent of material shape and size
• Examples:
  • Deformation in response to forces
  • Reflection of light by a polished metal surface
• Properties of solid materials can be grouped into six categories:
  1. Mechanical
  2. Electrical
  3. Thermal
  4. Magnetic
  5. Optical
  6. Deteriorative

Classification of Materials

• Solid materials can be grouped into three basic classifications:
  1. Metals
  2. Ceramics
  3. Polymers

Metals

• Composed of one or more metallic elements (e.g., iron, aluminum, copper) and non-metallic elements (e.g., carbon, nitrogen, oxygen) in small amounts
• Atoms are arranged in an orderly manner
• Relatively dense
• Mechanical properties:
  • Relatively stiff and strong
  • Ductile (capable of large amounts of deformation without fracture)
  • Resistant to fracture
• Electrons are not bound to particular atoms, making metals good conductors of electricity and heat, and non-transparent to visible light

Ceramics

• Compounds between metallic and non-metallic elements (e.g., oxides, nitrides, carbides)
• Examples: aluminum oxide (alumina), silicon dioxide (silica), silicon carbide
• Properties:
  • Relatively stiff and strong
  • Very hard and brittle (lack ductility)
  • Highly susceptible to fracture
  • Insulative to the passage of heat and electricity
  • Can be transparent, translucent, or opaque

Polymers

• Carbon-based compounds
• Chain of H-C molecules
• Examples: polyethylene (PE), nylon, poly (vinyl chloride) (PVC), polycarbonate (PC), polystyrene (PS), silicone rubber
• Properties:
  • Low densities
  • Not as stiff nor as strong as ceramics and metals
  • Extremely ductile and pliable
  • Relatively inert chemically and unreactive in various environments
  • Limitations: tendency to soften and/or decompose at modest temperatures

Composites

• Composed of two or more individual materials (e.g., metals, ceramics, polymers)
• Objective: achieve a combination of properties not displayed by any single material
• Examples:
  • Cemented carbides (WC with Co binder)
  • Plastic molding compounds containing fillers
  • Rubber mixed with carbon black
  • Wood (natural composite)

Advanced Materials

• Used in high-technology applications
• Examples: electronic equipment, computers, fiber-optic systems, spacecraft, aircraft, military rocketry, liquid crystal displays (LCDs), fiber optics
• May be traditional materials with enhanced properties or newly developed high-performance materials
• Include semiconductors, biomaterials, and "materials of the future"

Biomaterials

• Used in components implanted into the human body for replacement of diseased or damaged body parts
• Must not produce toxic substances and be compatible with body tissues (i.e., not cause adverse biological reactions)
• Can include metals, ceramics, polymers, composites, and semiconductors

Semiconductors

• Have electrical properties intermediate between electrical conductors (metals and metal alloys) and insulators (ceramics and polymers)
• Extremely sensitive to the presence of minute concentrations of impurity atoms
• Enabled the advent of integrated circuitry and revolutionized electronics and computer industries

Materials Selection Process

• Determine required properties for a specific application
• Identify candidate materials based on properties
• Evaluate material structure and composition
• Determine required processing methods

Defects in Solids

• Deviations from the perfect periodic array of atoms in the crystal
• Can greatly affect material properties, such as mechanical strength, ductility, crystal growth, magnetic hysteresis, dielectric strength, and condition in semiconductors

Crystal Structures

• Crystalline materials have a long-range order of atomic arrangement
• Examples: metals, diamond, precious stones, ice, graphite
• Amorphous materials lack long-range order
• Examples: glass, amorphous carbon (a-C), amorphous Si, most plastics

Close Packing

• Metal atoms are typically packed closely together in a regular pattern
• Common arrangements: FCC (face-centered cubic), BCC (body-centered cubic), HCP (hexagonal close-packed), and SCC (simple cubic structure)

Formation of Alloys

• Involves mixing two or more elements, at least one of which is a metal, to create a new material with enhanced properties
• Steps:
  1. Selection of metals
  2. Melting and thorough mixing
  3. Cooling and solidification
  4. Heat treatment (optional)
  5. Characterization and testing
  6. Application

Solid Solutions

• Homogeneous mixtures of two or more substances in the solid state
• Types:
  • Substitutional solid solution (atoms of one element replace atoms of another element)
  • Interstitial solid solution (smaller atoms or ions occupy interstitial spaces between larger atoms or ions)
  • Phase-separated solid solution (components segregate into distinct regions or phases)
  • Ordered solid solution (atoms occupy specific positions within the crystal lattice according to a regular and predictable pattern)

Lever Rule for Phase Mixtures

• Applied to phase diagrams in materials science

• Principle: the proportion of each phase in a mixture is determined by the relative amounts of each component### Lever Rule

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

• It is particularly useful in understanding the composition of phases during phase transformations, such as solidification or solid-state reactions.

• The Lever Rule is based on the conservation of mass and the assumption of equilibrium conditions.

• It calculates the fractions or percentages of each phase present in the mixture using a phase diagram.

• The Lever Rule states that the ratio of the lengths of the segments of the tie line intercepted by each phase boundary is equal to the ratio of the masses of the respective phases in the mixture.

Nucleation and Growth

• Nucleation is 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.
• Nucleation can occur homogeneously throughout the bulk of the material or heterogeneously at impurities, surfaces, or interfaces.
• Homogeneous nucleation requires the formation of critical nuclei with a sufficient number of atoms or ions arranged in a stable configuration.
• Heterogeneous nucleation occurs at pre-existing surfaces or interfaces, where the formation of stable nuclei is facilitated due to the lower energy barrier.
• Growth occurs after nucleation, where the nuclei grow in size as more atoms, ions, or molecules are added to the solid phase.
• Growth can occur through various mechanisms, including atomic diffusion, surface attachment, and incorporation of solute atoms.
• The rate of growth depends on factors such as temperature, composition, crystallographic orientation, and presence of impurities.

Ingot Structure

• An ingot structure refers to the microstructural arrangement found in metallic ingots, which are large, typically rectangular or cylindrical blocks of metal produced by casting or solidification processes.
• The ingot structure is characterized by the arrangement of grains and dendrites formed during the solidification of the molten metal.
• Grain formation occurs during solidification, where individual crystalline grains develop within the solidified material.
• The size and shape of grains in the ingot depend on factors such as cooling rate, alloy composition, and processing conditions.
• Dendritic growth occurs during solidification, resulting in the formation of branched structures with a dendritic morphology.
• Segregation can occur during solidification, where certain elements or impurities are preferentially partitioned into specific regions of the ingot.
• Achieving uniformity and homogeneity in the ingot structure is essential for ensuring consistent mechanical and metallurgical properties throughout the material.

Dendritic Solidification

• Dendritic solidification begins with the nucleation of solid grains within the liquid metal.
• Nucleation can occur homogeneously throughout the bulk of the liquid or heterogeneously at impurities, surfaces, or container walls.
• Crystal growth occurs as the solidification progresses, where atoms from the surrounding liquid attach themselves to the solid nuclei.
• Dendritic growth occurs due to the anisotropic nature of crystal growth, where certain crystallographic directions exhibit faster growth rates than others.
• The morphology of dendrites is influenced by factors such as temperature gradient, solidification rate, alloy composition, and presence of impurities.
• Microstructure evolution occurs as dendritic solidification progresses, with dendrites growing and branching to form interconnected networks throughout the solidified material.