Pure metals for specific needs: Al foil, Cu conductors, Ni/Cr plating, Au contacts.
An alloy = two or more elements, at least one a metal. Two forms: solid solutions and intermetallic compounds.

Solid Solutions

Solute (minor) dissolves in solvent (major); if the solvent’s crystal structure is retained, it’s a solid solution.
Substitutional (solute ≈ solvent size): e.g. brass (Zn in Cu). Requires similar crystal structures and atomic-radii difference < 15%.
Interstitial (solute much smaller): solute sits between host atoms. Requires solute radius < 59% of solvent radius. E.g. steel — C (0.071 nm) in Fe (0.124 nm).

Intermetallic Compounds

Two metals in fixed proportions, metallic or ionic bonding — e.g. Ti₃Al, Ni₃Al, Fe₃Al.

Iron–Carbon System

Carbon limits: pure iron ≤ 0.008% C; steels ≤ 2.11% C; cast irons ≤ 6.67% C (usually < 4.5%).

Phases (Fe–Fe₃C diagram):

Austenite (γ-iron): FCC, ductile at high T → good formability.
Ferrite (α-iron): BCC, soft and ductile.
Cementite (Fe₃C): 6.67% C, very hard, brittle intermetallic — strong influence on steel properties.

Special compositions:

Eutectic = 4.30% C — direct liquid→solid at a fixed 1148°C.
Eutectoid = 0.77% C — austenite → ferrite + cementite (pearlite) at 727°C.

Cast Irons

Fe + C (2.11–4.5%) + Si (up to ~3.5%), classified by graphite morphology:

Gray iron: graphite flakes (stress raisers) → negligible ductility, weak in tension but strong in compression; flakes damp vibration → machine-tool bases.
Ductile (nodular) iron: graphite spheroids (via Mg/Ce additions) → somewhat ductile, shock-resistant.
White iron: large amounts of iron carbide (not graphite) → very hard, wear-resistant, brittle; made by rapid cooling or low C/Si.

Heat Treatment of Ferrous Alloys

Controlled heating/cooling induces phase transformations affecting strength, hardness, ductility, toughness, wear resistance.

Quenching → Martensite

Rapidly cooling austenite transforms FCC → body-centered tetragonal (BCT) → martensite.
Fewer slip systems + interstitial carbon → extremely hard and brittle, low toughness (rarely used as-quenched).
Relative cooling capacity of quench media:

Medium	Relative capacity
Agitated brine	5
Still water	1
Still oil	0.3
Cold gas	0.1
Still air	0.02

Tempering (BCT → BCC)

Reheat martensite to ~150–650°C → decomposes to BCC ferrite + fine cementite → lower hardness, higher toughness.

Annealing & Normalizing

Annealing: restore a cold-worked/heat-treated alloy (↑ductility, ↓hardness/strength); also relieves residual stresses. Steps: heat → soak → air/furnace cool.
Full annealing (steels): heat above $A_1$ / $A_3$ , cool slowly (~10°C/h) in the furnace → coarse pearlite (soft, ductile).
Normalizing: cool in still air → higher strength/hardness than full annealing, more ductility than as-rolled.

Hardenability

The depth to which an alloy hardens on quenching (≠ hardness, which is indentation resistance).
Depends on carbon content, austenite grain size, alloying elements, cooling rate.
Jominy end-quench test: austenitize a 100-mm bar, water-quench one end (24°C), measure hardness along its length. Hardness falls away from the quenched end; deeper penetration = higher hardenability.
1040, 4140, 4340 share 0.40% C → same quenched-end hardness (~57 HRC) but differ in hardening depth (alloying).

Distortion

Irreversible dimensional change — size (no shape change) or shape (bending/twisting). Reduce via controlled heating/cooling, better design, localized treatment.

Case Hardening

Harden only the surface (gears, cams, shafts, bearings, fasteners) while the core stays tough — improves indentation, fatigue, and wear resistance, and induces a surface compressive residual stress that retards fatigue cracking. Surface is enriched with C, N, or B (high-C steels need none). Laser/electron beams can also surface-harden.

Furnaces & Design

Batch vs continuous furnaces; energy use, insulation, and cost matter.
Gas/oil introduces combustion products (disadvantage); electric heating is cleaner but slower to start and harder to control.
Design: avoid sharp corners and non-uniform thickness (→ thermal-gradient cracking and residual stresses).

Turkish Terms

English	Turkish
Annealing	tavlama
Tempering	menevişleme
Quenching	su verme