Materials Science II

Point defects – Zero-dimensional lattice defects such as vacancies, interstitials, and substitutional atoms.

Vacancy – A missing atom from a lattice site.

Interstitial defect – An atom positioned in the spaces between lattice sites.

Substitutional defect – A foreign atom replacing a host atom in the lattice.

Linear defect (dislocation) – One-dimensional defect in a crystal lattice responsible for plastic deformation.

Volume (bulk) defect – Three-dimensional defect such as pores, cracks, or inclusions.

Dislocation – A line defect that allows slip and plastic deformation in crystals.

Tension – Loading mode that stretches a material.

Compression – Loading mode that squeezes a material.

Shear – Loading where forces act parallel and opposite, causing sliding deformation.

Torsion – Twisting deformation caused by applied torque.

Bending – Deformation caused by moments producing curvature.

Stress – Force per unit area (σ = F/A).

Strain – Relative deformation (ε = ΔL/L₀).

Stiffness – Resistance to elastic deformation measured by Young’s modulus.

Strength – Maximum stress a material can withstand before failure.

Young’s modulus – Ratio of stress to strain in the elastic region.

Tensile test – Experiment measuring material response under uniaxial tension.

Elastic region – Region where deformation is reversible.

Plastic region – Region where deformation is permanent.

Yield point – Stress at which plastic deformation begins.

Fracture point – Point at which material breaks.

Young’s modulus (E) – Slope of elastic stress–strain curve.

Shear modulus (G) – Ratio of shear stress to shear strain.

Bulk modulus (K) – Resistance to uniform compression.

High Young’s modulus – Indicates a stiff material.

Elastic deformation – Reversible deformation after unloading.

Plastic deformation – Permanent deformation after unloading.

Proportional limit – End of linear stress–strain relationship.

Anelasticity – Time-dependent elastic deformation.

Elasticity – Instantaneous reversible deformation.

Cause of anelasticity – Time-dependent atomic or microscopic rearrangements.

Poisson’s ratio – Ratio of lateral strain to axial strain (with negative sign).

Typical Poisson’s ratio for metals – Approximately 0.25–0.35.

Maximum Poisson’s ratio – 0.5 (no volume change).

Plastic deformation – Permanent deformation after yield point.

Cause of plastic deformation – Dislocation motion in crystals.

Yield strength – Stress at which plastic deformation begins.

Tensile strength – Maximum stress before fracture.

Strain hardening effect – Increase in strength after plastic deformation.

Ductility – Ability to undergo plastic deformation before fracture.

Percent elongation – Measure of ductility based on change in length.

Percent reduction in area – Measure of ductility based on cross-sectional change.

Modulus of resilience – Elastic energy absorbed per unit volume.

Toughness – Total energy absorbed before fracture.

True stress – Stress based on instantaneous cross-sectional area.

True strain – Natural logarithmic strain (ln(L/L₀)).

Engineering stress – Stress based on original area.

Engineering strain – Strain based on original length.

Elastic recovery – Recovery of elastic strain after unloading.

Permanent strain – Remaining deformation after plastic deformation.

Hardness – Resistance to localized plastic deformation.

Hardness test advantage – Simple, fast, and nondestructive.

Hardness test limitation – Not precise for design calculations.

Ideal strength – Theoretical maximum strength of a perfect crystal.

Approximate ideal strength – σ ≈ E/15.

Reason real materials are weaker – Presence of defects, especially dislocations.

Dislocation – Line defect allowing plastic deformation.

Slip – Movement of atomic planes along a crystal.

Plasticity in metals – Caused primarily by dislocation motion.

Driving force for dislocation motion – Applied shear stress.

Burgers vector – Measure of lattice distortion caused by a dislocation.

Solubility limit – Maximum solute concentration in a solid solution.

Phase diagram – A graphical representation of phases present under equilibrium conditions as a function of temperature, pressure, and composition.

Unary phase diagram – Phase diagram for a single-component system (variables: temperature and pressure).

Binary phase diagram – Phase diagram for a two-component system at constant pressure showing temperature vs composition.

Equilibrium phase diagram – Diagram showing stable phases under equilibrium conditions.

Exceeding solubility limit – Leads to formation of a second phase.

Phase boundary – Line separating two phases in equilibrium.

Triple point – Condition where solid, liquid, and vapor phases coexist in equilibrium.

Melting point – Temperature where solid and liquid phases coexist at a given pressure.

Boiling point – Temperature where liquid and vapor phases coexist.

Binary isomorphous system – Binary alloy system with complete solid solubility in all proportions.

Solid solution (α phase) – Single solid phase containing both components in a substitutional lattice.

Liquidus line – Boundary above which only liquid exists.

Solidus line – Boundary below which only solid exists.

Two-phase region – Region where both solid and liquid phases coexist.

Tie line – Horizontal line used in a two-phase region to determine phase compositions.

Phase composition – Composition of each individual phase in equilibrium.

Rule for phase composition – Determined by intersections of tie line with phase boundaries.

Lever rule – Method used to calculate mass fractions of phases in a two-phase region.

Mass fraction of phase – Fraction of total alloy present as a given phase.

Lever rule principle – Based on conservation of mass using tie-line segment ratios.

Inverse lever rule – Alternative name emphasizing segment ratio interpretation.

Lever rule basis – Derived from conservation of total mass and component mass balance.

Phase fraction calculation – Ratio of opposite tie-line segment to total tie-line length.

Two-phase condition – System where lever rule is applicable.

Microstructure – Structural features of a material visible under microscope.

Grain – Individual crystal in a polycrystalline material.

Grain boundary – Interface between two differently oriented grains.

Microstructure dependence – Depends on composition, cooling rate, and heat treatment.

Phase equilibrium – Condition where phases coexist without change over time.

Free energy (Gibbs free energy) – Thermodynamic function determining phase stability.

Equilibrium condition – State of minimum Gibbs free energy.

Entropy – Measure of atomic disorder in a system.

Gibbs phase rule – Relationship between number of phases, components, and degrees of freedom: F = C − P + 1 (for constant pressure).

Degrees of freedom – Number of independent variables (T, composition) that can be changed without changing number of phases.

Component – Chemically independent species in a system.

Nucleation – Initial formation of a new phase within a parent phase.

Homogeneous nucleation – Nucleation occurring uniformly throughout the parent phase.

Heterogeneous nucleation – Nucleation occurring at surfaces, grain boundaries, or defects.

Gibbs free energy change – Driving force for phase transformation.

Volume free energy (ΔGv) – Energy change due to phase transformation per unit volume.

Surface energy (γ) – Energy required to create a new interface.

Critical nucleus – Minimum-sized stable nucleus that can grow.

Critical radius (r*) – Minimum radius a nucleus must reach to become stable and grow.

Critical free energy (ΔG*) – Energy barrier required for nucleation.

Subcritical nucleus – Nucleus that dissolves instead of growing.

Supercooling (ΔT) – Cooling below equilibrium transformation temperature without phase change.

Effect of supercooling – Increases nucleation rate.

Nucleation rate – Rate at which stable nuclei form.

Blast furnace – Industrial reactor used to produce iron from ore.

Charge materials – Iron ore, coke, and limestone.

Coke – Carbon source and reducing agent.

Limestone role – Removes impurities by forming slag.

Pig iron – Crude iron product with high carbon content (>4%).

Basic oxygen furnace (BOF) – Steelmaking process using oxygen to reduce carbon content in molten iron.

Electric arc furnace – Steelmaking process using electrical energy to melt scrap metal.

Decarburization – Removal of carbon from molten iron during steelmaking.

Slag – Impurities separated from molten metal.

Continuous casting – Process where molten steel is solidified continuously into semi-finished shapes.

Tundish – Intermediate reservoir controlling molten metal flow.

Water-cooled mold – Device initiating solidification of molten steel.

Austenite – High-temperature FCC phase of iron.

Martensite – Hard, metastable phase formed by diffusionless transformation.

Bainite – Microstructure formed at intermediate transformation temperatures.

Pearlite – Lamellar mixture of ferrite and cementite.

Diffusionless transformation – Phase change without atomic diffusion (e.g., martensite formation).

TTT diagram – Time-Temperature-Transformation diagram showing isothermal phase transformations.

CCT diagram – Continuous Cooling Transformation diagram showing transformations during continuous cooling.

Transformation start curve – Boundary where phase transformation begins.

Transformation finish curve – Boundary where transformation completes.

Strain hardening – Increase in strength due to plastic deformation.

Cold work – Plastic deformation at room temperature increasing strength.

Spheroidite – Soft steel microstructure with spheroid-shaped carbides.

Pearlite strengthening – Strength increases with decreasing lamellar spacing.