Crystal Structure & Mechanical Properties
The mechanical behavior of a metal — whether it's ductile or brittle, strong or soft, easy to form or resistant to deformation — is fundamentally determined by its atomic arrangement. Understanding crystal structures helps explain why aluminum is easy to form, why titanium is hard to machine, and why steel can be made incredibly strong through heat treatment.
Crystal Structures in Engineering Metals
Metals are crystalline — their atoms are arranged in repeating three-dimensional patterns called lattices. Three structures dominate engineering metals:
Body-Centered Cubic (BCC)
Atoms at each corner of a cube plus one atom in the center. BCC metals have 48 slip systems but only 12 are easily activated, making them moderately ductile at room temperature but brittle at low temperatures (ductile-to-brittle transition).
BCC metals: Iron (below 912°C), chromium, tungsten, molybdenum, vanadium, niobium Engineering significance: Ferritic and martensitic steels are BCC. The ductile-to-brittle transition temperature (DBTT) is why Charpy impact testing is critical for steels used in cold environments (ships, pipelines, aircraft at altitude).Face-Centered Cubic (FCC)
Atoms at each corner plus one atom centered on each face. FCC metals have 12 slip systems, all easily activated, making them highly ductile and formable with no DBTT.
FCC metals: Aluminum, copper, nickel, gold, silver, lead, austenitic stainless steels (304, 316) Engineering significance: FCC metals are excellent for forming operations (deep drawing, bending). Austenite (FCC iron) is the high-temperature phase used in heat treatment before quenching to martensite.Hexagonal Close-Packed (HCP)
Atoms arranged in hexagonal layers. HCP metals have only 3 primary slip systems, making them less ductile and more anisotropic (properties depend on direction).
HCP metals: Titanium (alpha phase), magnesium, zinc, zirconium, cobalt Engineering significance: HCP explains why magnesium sheets are hard to form at room temperature (must be warm-formed), and why titanium is difficult and expensive to machine.The Stress-Strain Curve
The tensile test is the most fundamental materials test. A dog-bone shaped specimen is pulled in a universal testing machine (UTM) until it breaks, producing a stress-strain curve.
Key Regions
Elastic Region — The linear portion where stress is proportional to strain (Hooke's Law: σ = Eε). Remove the load and the material returns to its original shape. The slope is Young's modulus (E). Yield Point — The stress at which permanent (plastic) deformation begins. For most engineering metals, this is defined by the 0.2% offset method: draw a line parallel to the elastic slope, offset by 0.2% strain. Where it intersects the curve is the yield strength (σy). Strain Hardening — After yielding, the stress continues to rise as dislocations multiply and tangle, making further deformation progressively harder. Ultimate Tensile Strength (UTS) — The maximum engineering stress on the curve. Beyond this point, necking begins — the cross-section narrows locally. Fracture — The specimen breaks. Ductile materials show significant elongation before fracture; brittle materials fracture with little warning.Mechanical Properties — Defined with Values
Young's Modulus (E) — Stiffness
Resistance to elastic deformation. A material property — cannot be changed by heat treatment or processing.
| Material | E (GPa) | Notes |
|---|---|---|
| Steel (all grades) | ~200 | All steels have essentially the same E |
| Aluminum alloys | ~70 | 1/3 of steel |
| Titanium alloys | ~110 | Between Al and steel |
| Magnesium alloys | ~45 | Lowest of structural metals |
| CFRP (quasi-isotropic) | ~70–150 | Depends on layup |
Yield Strength (σy) — When Permanent Deformation Starts
| Material | σy (MPa) | Application |
|---|---|---|
| Mild steel (1018) | ~250 | General fabrication |
| HSLA (DP590) | ~340 | Automotive structural |
| HSLA (DP980) | ~600 | B-pillar reinforcement |
| Hot-stamped boron steel | ~1,100 | A/B pillars, rocker panels |
| Al 6061-T6 | ~275 | Structural extrusions |
| Al 7075-T6 | ~505 | Wing spars, landing gear |
| Ti-6Al-4V (annealed) | ~880 | Aerospace structural |
| Inconel 718 | ~1,035 | Turbine discs |
Elongation — Ductility
How much a material stretches before breaking, measured as percentage increase in gauge length.
| Material | Elongation (%) |
|---|---|
| Mild steel | ~25 |
| DP590 | ~20 |
| Hot-stamped boron steel | ~5–7 |
| Al 6061-T6 | ~12 |
| Ti-6Al-4V | ~14 |
| CFRP | ~1–2 (brittle) |
Hardness
Resistance to surface indentation. Different scales for different materials:
- Rockwell C (HRC): Hardened steels (HRC 20–65). Uses a diamond cone indenter.
- Rockwell B (HRB): Softer metals — aluminum, brass, soft steels. Uses a ball indenter.
- Brinell (HB): Large ball indenter, good for castings with coarse microstructure.
- Vickers (HV): Diamond pyramid, works for any material. Most versatile scale.
Approximate conversions around HRC 30: HRC 30 ≈ HV 295 ≈ HB 285.
Rule of thumb: For steels, UTS (MPa) ≈ 3.45 × HB.Strengthening Mechanisms
Pure metals are soft. Engineers use four mechanisms to increase strength:
1. Grain Refinement (Hall-Petch)
Smaller grains mean more grain boundaries, which block dislocation motion. The Hall-Petch relationship: σy = σ₀ + k/√d, where d is grain diameter.
Example: HSLA steels use microalloying elements (Nb, V, Ti at 0.05–0.10%) to pin grain boundaries during hot rolling, achieving fine grains and higher strength without sacrificing weldability.2. Solid Solution Strengthening
Dissolving foreign atoms in the crystal lattice creates local strain fields that impede dislocation motion. Substitutional atoms (Mn, Ni, Cr replace Fe) or interstitial atoms (C, N sit between Fe atoms).
Example: Carbon-manganese (CMn) steels used in automotive body-in-white. Manganese substitutes for iron in the BCC lattice, increasing yield strength to ~280 MPa.3. Precipitation (Age) Hardening
Tiny precipitate particles form within the matrix during aging heat treatment, blocking dislocations. The most important strengthening mechanism in aluminum and nickel alloys.
Example: Al 6061-T6 is solution treated (dissolved Mg₂Si into solid solution), quenched, then artificially aged at ~175°C to precipitate fine Mg₂Si particles. Yield strength increases from ~55 MPa (O temper) to ~275 MPa (T6).4. Work Hardening (Strain Hardening)
Cold deformation increases dislocation density. More dislocations tangle and block each other, raising strength but reducing ductility.
Example: Cold-rolled sheet steel. A strip rolled from 3mm to 1.5mm thickness will be significantly harder and stronger than the hot-rolled starting material, but less formable. Annealing can restore ductility.Specific Strength and Specific Stiffness
In weight-critical applications, absolute strength matters less than strength-per-unit-weight:
| Material | Density (g/cm³) | σy (MPa) | Specific Strength (σy/ρ) (kN·m/kg) |
|---|---|---|---|
| Mild steel | 7.85 | 250 | 32 |
| Al 7075-T6 | 2.81 | 505 | 180 |
| Ti-6Al-4V | 4.43 | 880 | 199 |
| CFRP (quasi-iso) | 1.55 | 800 | 516 |
CFRP has ~16× the specific strength of mild steel. This is why composites dominate aerospace despite their high cost.
Temperature Effects
Creep — Time-dependent deformation under constant load at elevated temperature. Critical for jet engine components. Nickel superalloys like Inconel 718 are designed to resist creep up to ~650°C. Single-crystal turbine blades (René N5, CMSX-4) operate at 1,000°C+ with thermal barrier coatings. Ductile-to-Brittle Transition — BCC metals (steels) become brittle below their DBTT. The Titanic's hull steel had a DBTT around 0°C, contributing to brittle fracture in the icy North Atlantic. Glass Transition (Tg) — Polymers transition from hard/glassy to soft/rubbery at Tg. For epoxy matrices in composites, Tg is typically 120–180°C, setting the maximum service temperature.Key Takeaways
- Crystal structure (BCC, FCC, HCP) determines fundamental behavior: ductility, formability, and temperature sensitivity
- The stress-strain curve provides yield strength, UTS, modulus, and elongation — the core data for any structural design
- Four strengthening mechanisms (grain refinement, solid solution, precipitation hardening, work hardening) are used alone or in combination to engineer the required strength
- Specific strength (σ/ρ) is more important than absolute strength for weight-critical applications
- Temperature effects (creep, DBTT, Tg) set hard limits on where materials can be used
