Materials Selection Criteria — Cross-Cutting Comparison
This note compares every materials-selection axis used across the Engineering library — Ashby’s method (Cambridge), GRANTA EduPack / Granta MI taxonomy, per-property selection (yield strength, fatigue, fracture toughness, creep, corrosion, thermal expansion, thermal conductivity, dielectric, magnetic permeability, density, cost, embodied CO₂, recyclability, biocompatibility ISO 10993, flammability UL94, machinability, weldability, manufacturability for AM/casting/forging/forming), Ashby charts (strength vs density, stiffness vs density, cost vs strength, etc.), application matrices (aerospace, automotive, structural, bottles, bearings, medical implants, turbine blades), and supply-chain / criticality (USGS Critical Minerals List 2024 — REs, Co, Li, Ni, Cu, Ga, In, PGMs). Decision tree at end picks by function + cost + scale + lifecycle + supply risk.
See also
- materials-selection
- materials-steel
- materials-aluminum
- materials-ceramics
- materials-composites
- materials-polymers
- mechanics-of-materials
- fatigue-analysis
- fracture-mechanics
- steel-grades
- aluminum-alloys
- titanium-alloys
- stainless-steels
- composites-taxonomy
- ceramics-taxonomy
- polymers-taxonomy
- copper-alloys
- battery-chemistries
- semiconductor-materials
1. Ashby’s method — the canonical framework
Ashby (Cambridge, 1980s, Materials Selection in Mechanical Design now 5th ed) frames every selection as a four-piece problem:
- Function — what the part does (carry a load, conduct heat, etc.).
- Constraints — what it must satisfy (no yield, ΔT < 50°C, mass < 5 kg, cost < $10).
- Objective — what to minimize / maximize (mass, cost, embodied CO₂, vibration).
- Free variables — what you can vary (material choice, cross-section, length).
The output is a material index: a single property combination that you maximize. Examples:
- Light, stiff beam in bending → maximize (where E = stiffness, ρ = density).
- Light, strong tie rod → maximize .
- Light, stiff panel under pressure → maximize .
- Light, strong panel under pressure → maximize .
- Heat sink (high thermal conductance, low mass) → maximize .
- Spring (max energy storage per mass) → maximize .
- Flywheel (max energy per mass) → maximize .
- Light pressure vessel (safe, fracture-tolerant) → maximize .
- Light pressure vessel (max yield) → maximize .
Plot these on Ashby charts (log-log property vs property with material families as bubbles); the index appears as a straight line of slope corresponding to the index exponent. Materials above the line are better.
2. Property dimensions
| Property | Units | What it governs | When it matters |
|---|---|---|---|
| Density ρ | kg/m³ | mass | every aerospace / automotive / weight-critical design |
| Young’s modulus E | GPa | stiffness, deflection, natural frequency | structural deflection, vibration |
| Yield strength σ_y | MPa | onset of plastic deformation | static load |
| Tensile strength σ_uts | MPa | ultimate failure | overload / safety factor |
| Fatigue strength σ_e (endurance limit) | MPa | cyclic failure | rotating machinery, bridges, aircraft |
| Fracture toughness K_1c | MPa·√m | crack-growth resistance | pressure vessel, aircraft skin |
| Hardness | HV, HRC, HRB | wear, indent resistance | bearings, gears, dies |
| Creep strength | MPa @ T | time-dependent deformation at high T | turbine blades, boilers |
| Thermal conductivity λ | W/(m·K) | heat transfer | heat exchangers, electronics |
| Thermal expansion α | µm/(m·K) | dimensional change w/ T | precision, bimetal, joining |
| Specific heat c_p | J/(kg·K) | thermal mass | thermal storage |
| Melting / max-use temperature | °C | thermal limit | high-T |
| Electrical conductivity σ | S/m | current carrying | conductors |
| Dielectric constant ε_r | dimensionless | capacitor, insulator | electronics |
| Magnetic permeability µ_r | dimensionless | transformer, motor core | magnetics |
| Coercivity H_c | A/m | magnetic memory | hard/soft magnets |
| Corrosion resistance | qualitative | chemical degradation | marine, chemical plant |
| Biocompatibility (ISO 10993) | pass/fail | non-toxic, non-inflammatory | medical implants |
| Flammability (UL94) | V-0 / V-1 / V-2 / HB | self-extinguishing | electronics, transit |
| Embodied CO₂ | kg CO₂eq / kg | lifecycle environmental | LCA, ESG, EU CSRD reporting |
| Embodied energy | MJ / kg | lifecycle energy | LCA |
| Recyclability | % closed-loop | end-of-life | circular economy |
| Cost | $/kg | manufacturing economics | every decision |
| Machinability | rating | machining ease | CNC parts |
| Weldability | rating | joining | structural |
| Castability | rating | net-shape | volume |
| Formability | rating | sheet / forging | volume |
| AM-printability | qualitative | additive manufacturing | new parts |
3. Material families on the Ashby canvas
STRENGTH (σ_y, MPa)
high |
| Ti6Al4V (900)
| AISI 4340 (1200)
| maraging steel (2000)
| CFRP (1500 fiber-direction)
|
medium | mild steel (250)
| 6061-T6 Al (270)
| brass (200)
| Al2O3 (250-400 flexural)
|
low | PE (20-30)
| PP (30-40)
| concrete (3-5 tension, 30 compression)
| wood (40-100 along grain)
|____________________________________
low medium high DENSITY (ρ)
polymer metal heavy metal
wood
foam
4. Per-property family ranking
| Property | Best family | Best representative |
|---|---|---|
| Density (low) | foams + polymers | PMI foam (50–80 kg/m³), balsa (160), HDPE (960), Mg alloy (1740) |
| Density (high) | tungsten + DU | W (19250), DU (19100), Pb (11340), Au (19320) |
| Stiffness | ceramics + composites + steels | diamond (1200 GPa), SiC (450), CFRP (300 fiber dir), steel (210) |
| Yield strength | maraging steel + composites | maraging 18Ni 350 (~2700 MPa), CFRP fiber (~1500 MPa), Ti6Al4V (~900 MPa) |
| Fatigue limit | steel + Ti | 4340 (700 MPa @ 10⁷ cycles), Ti6Al4V (500 MPa) |
| Fracture toughness | low-alloy steel + Al | 4340 (~50 MPa√m), 7075-T6 (~30), Al2O3 (~5), CFRP (~30 cross-ply) |
| Hardness | ceramics + carbides | diamond (~10,000 HV), WC (~2400 HV), B4C (~3200 HV), Si3N4 (~1800 HV) |
| Creep (1000°C, 100 MPa) | Ni superalloy + CMC | René N5 single-crystal Ni, CMSX-4, Inconel 718, SiC/SiC CMC |
| Thermal conductivity (high) | metals + diamond | diamond (2200 W/(m·K)), Cu (400), Al (240), Ag (430) |
| Thermal conductivity (low) | foams + aerogel | silica aerogel (0.013), polyurethane foam (~0.025), wood (0.1–0.3) |
| Thermal expansion (low) | Invar + Zerodur + CFRP | Invar 36 (1.2 µm/m·K), Zerodur (0.02), fused silica (0.5), CFRP (0–2) |
| Electrical conductivity (high) | Ag + Cu + Al + graphene | Ag (6.3×10⁷ S/m), Cu (5.96×10⁷), Al (3.5×10⁷) |
| Dielectric strength | mica + ceramic + polyimide | mica (~120 kV/mm), polyimide (~280), PTFE (60), Al2O3 (15) |
| Magnetic permeability (soft) | iron-nickel + amorphous | mu-metal (80,000–100,000), Metglas, electrical steel (~5000) |
| Coercivity (hard) | NdFeB > SmCo > AlNiCo > Ferrite | NdFeB N52 (1.4 T remanence), SmCo (~1.0 T), AlNiCo (~1.2 T), Ferrite (~0.4 T) |
| Corrosion (marine) | super-duplex SS + Ti + Inconel | 2507 super-duplex, Ti grade 5, Inconel 625 |
| Biocompatibility | Ti + CoCrMo + ceramic + UHMWPE + PEEK | Ti6Al4V, CoCrMo F75, Al2O3, ZrO2, PEEK Optima |
| Cost (low) | concrete + wood + steel | concrete ($0.1/kg), CDX plywood ($0.5/kg), mild steel ($0.6/kg) |
| Cost (high) | space alloys + diamond + Re | rhenium (~$3000/kg), Re-W alloy, single-crystal Ni superalloy ($200/kg parts), diamond ($1000–$10,000/g abrasive grade) |
5. Application matrices — what gets used where
| Application | Primary material | Secondary | Rationale |
|---|---|---|---|
| Aircraft primary structure (fuselage) | AA2024-T3 (skin); AA7075-T6 (wing spar) | Ti6Al4V (joints); CFRP (787, A350) | strength/density + fatigue + manufacturability |
| Aircraft engine fan blade | Ti6Al4V (cold section); single-crystal Ni René N5 (hot section); SiC/SiC CMC (post-2020 LEAP, GE9X) | Inconel 718 | strength × creep × density |
| Aircraft engine combustor / liner | Hastelloy X; CMC SiC/SiC | Inconel 718 | high-T creep + oxidation |
| Aircraft brake | C/C-SiC | sintered Fe-Cu | high heat, low mass |
| Spacecraft structure (cryotank, dewar) | Al-Li 2195 (SLS, Centaur); Ti grade 5; CFRP | none | density + low T toughness |
| Pressure vessel (gas cylinder) | T6 4130 steel; AA2024-T3 spun; Type IV CF-epoxy + HDPE liner | none | strength × fracture toughness |
| Pressure vessel (CNG tank, hydrogen tank) | CFRP + HDPE liner (Type IV) | none | high pressure (700 bar H₂) at low mass |
| Automotive body in white | mild steel (legacy), Boron / Press-hardened steel (modern), 5xxx Al, 6xxx Al, CFRP (BMW i3) | hot-stamped 22MnB5 | crashworthiness + cost + manufacturability |
| Automotive engine block | grey cast iron (legacy), AlSi9Cu3 (Ford EcoBoost), AlSi17 (BMW) | none | castability + machinability + thermal expansion |
| Automotive turbocharger turbine | Inconel 713C; Inconel 625; CMSX-4 | grey cast iron (cool side) | hot-T creep + oxidation |
| Bridge structural steel | A992 (US), S355 (EU), S460M, Cor-Ten weathering steel (no painting), HPS-100W high-perf | none | yield × cost × weldability |
| High-rise reinforced concrete | C30/C50 concrete + B500B rebar (EU), Grade 60 (US) | high-strength fibre-RC | compressive strength + cost |
| Beverage bottle | PET (drink); HDPE (milk); glass (premium); aluminum can (carbonated, recyclable) | PE/PA/PE multilayer (juice) | cost + barrier + recyclability |
| Bearings | 52100 chrome steel (radial ball); M50 (aerospace); silicon nitride (hybrid, high-speed); ZrO2 (corrosive); PEEK + carbon fiber (food / chemical) | bronze (plain) | hardness + fatigue + chemistry |
| Gears (auto) | 8620 case-hardened (carburized); 4140; 9310 (helicopter) | nylon (toy / low-load); brass | wear + fatigue |
| Medical hip stem | Ti6Al4V (cobalt-free, common); CoCrMo (heavy users); UHMWPE liner; ceramic head (Al2O3, ZrO2-toughened) | tantalum coating | biocompatibility + fatigue + low modulus matching bone |
| Medical hip head | Al2O3 (ceramic-on-poly); ZrO2 toughened alumina (ZTA); CoCrMo metal-on-poly | none | wear + biocompat |
| Medical knee tray | Ti6Al4V or CoCrMo + UHMWPE insert | PEEK Optima (research) | biocompat + wear |
| Dental implant | Ti grade 4 or grade 5; ZrO2 ceramic (Straumann ZLA, Zeramex) | none | osseointegration |
| Surgical scalpel | 440C stainless (reusable); carbon steel (cheap); ZrO2 ceramic (no metal contamination) | none | hardness + sterilization |
| Heat exchanger | Cu (water-water, refrigeration); Al + brazed plate (HVAC); stainless 316L (food/pharma); titanium grade 2 (seawater); SiC (corrosive); Inconel (high-T) | none | thermal conductivity + corrosion |
| Battery cell can | nickel-plated steel (cylindrical 18650, 21700, 4680); Al pouch foil + LDPE laminate (pouch); aluminum prismatic (LFP) | none | density + cost + processability |
| Battery cathode | NMC811 (passenger EV); NCA (Tesla); LFP (CATL, BYD); Li-S (R&D); solid-state Li (Toyota, Solid Power) | none | energy density + cost + supply (Co, Ni) |
| Battery anode | graphite (commodity); silicon (SiOx, blended); Li metal (solid-state) | none | capacity + cycle life |
| Permanent magnet | NdFeB N42-N52 (motors, MRI); SmCo (high-T, military); AlNiCo (industrial); ferrite (cheap) | none | energy product BHmax + temperature stability |
| Motor laminations | non-oriented electrical steel M250-35A; CRGO; amorphous Metglas; SiFe 6.5%; FeCo (high-perf aerospace) | none | low core loss |
| Transformer core | grain-oriented silicon steel; amorphous Metglas (efficient distribution); nanocrystalline (PFC) | none | low core loss + saturation flux |
| MEMS | single-crystal Si; polysilicon; quartz; SiN; metal piezo PZT | none | etching compatibility + crystal anisotropy |
| PCB substrate | FR-4 epoxy-glass; Rogers RO4350B/RO3003 (RF); polyimide (flex); ceramic LTCC/HTCC; PTFE (high-freq) | none | dielectric + thermal + cost |
| Window (aircraft) | stretched acrylic (cabin); chemically tempered glass + ion exchange (cockpit, 787); IR-coated multi-layer (heated) | polycarbonate (impact) | optical + fatigue + impact |
| Brake disc (auto) | grey cast iron (commodity); C/C-SiC (Ferrari, Porsche GT3 RS); FCD600 ductile iron (truck) | none | thermal mass + cost + wear |
| Tire | NR / SBR / BR blend + carbon black + silica + steel cord + nylon fabric | none | wear + grip + rolling resistance |
| Roof tiles | clay (terracotta); concrete; slate; metal (Zn, Cu, steel); composite | none | weatherability + cost |
| Window frames (architectural) | PVC (commodity); Al alloy 6063-T6; wood; FRP | none | thermal break + cost + maintenance |
6. Per-process material compatibility
| Process | Best material families |
|---|---|
| Forging | low-alloy steel, Al, Ti, Ni — all wrought |
| Casting (sand, investment, die) | grey iron, ductile iron, AlSi, brass, bronze, stainless, super alloy (investment) |
| Sheet forming / drawing | low-carbon steel, 5xxx Al, 1xxx Al, brass, soft Cu |
| Press hardening / hot stamping (22MnB5) | boron steel — automotive A-pillar, B-pillar |
| Extrusion | 6xxx Al (6063 the workhorse), Cu, brass, polymers (PE, PP, PVC) |
| Powder metallurgy / sinter | Fe-Cu-C (auto), CrCo, tungsten heavy alloy, hard-metal WC-Co (carbide tools) |
| Machining (CNC) | mild steel, 6061 Al, brass, plastics — all “free-machining” grades best |
| Welding (MIG/TIG/SAW) | mild steel, low-alloy steel, 6061/5xxx Al, austenitic stainless, brass (TIG) |
| Welding (laser) | thin steel, Al, Ti |
| Welding (friction stir) | Al (5xxx, 6xxx), Mg, Cu, Ti |
| Welding (electron beam, vacuum) | Ti, refractory, Ni superalloy |
| Brazing | Cu, brass, stainless, Inconel, Al (controlled atmosphere) |
| Adhesive bonding | composites, aluminum (anodized), polymer |
| Additive — laser powder bed fusion (LPBF, SLM) | AlSi10Mg, Ti6Al4V (most common), 316L, 17-4PH, Inconel 625/718, AlSi7Mg, H13 tool steel |
| Additive — electron beam PBF (EBM) | Ti6Al4V, Ti aluminide, CoCr, Inconel |
| Additive — directed energy deposition (DED) | repair / large-scale Ti, Inconel, low-alloy steel |
| Additive — binder jet | stainless 316L/420, brass, Inconel |
| Additive — FFF / FDM | PLA, PETG, ABS, nylon, PC, PEEK |
| Additive — vat photopolymerization (SLA, DLP) | acrylate UV, thiol-ene, ceramic-loaded resin |
| Additive — multijet (Carbon DLS) | engineering urethane, epoxy, silicone, EPU 41 |
| Injection molding | thermoplastics (commodity + engineering) |
| Compression molding | thermoset (epoxy, phenolic, BMC, SMC) |
| Pultrusion | continuous fiber + thermoset (PFR, epoxy) |
| Filament winding | continuous fiber + epoxy (pressure vessel, pipe) |
7. Cost, environmental + lifecycle layer
| Material | Cost range ($/kg, 2025) | Embodied CO₂ (kg/kg) | Recyclability |
|---|---|---|---|
| Concrete (ready-mix) | 0.05–0.1 | 0.13 (Portland cement is the driver: ~0.9) | crushable, downcycled to aggregate |
| Structural steel (A992 / S355) | 0.6–1.0 | 1.8 (BOF), 0.4 (EAF) | 100% closed-loop |
| Stainless 304 / 316 | 3–6 | 6.8 (304), 7.5 (316) | 100% closed-loop |
| Aluminum 6061-T6 | 3–4 | 8.2 (virgin smelt, BAYER + Hall-Héroult), 1.7 (recycled, secondary) | 90% closed-loop (cans 75%) |
| Aluminum-lithium 2195 | 50–100 | 12 | yes |
| Titanium grade 5 (Ti6Al4V) | 30–60 | 35 (sponge) | technical (rare in scrap stream) |
| Magnesium AZ91 | 4–8 | 40 (Pidgeon, Chinese — dropping w/ Mg-1 electrolytic) | poor (oxidation) |
| Copper (electrical) | 9–12 | 4.2 (primary), 1.2 (secondary) | 90% closed-loop |
| Brass C36000 | 7–10 | 5 | yes |
| Nickel superalloy (Inconel 718) | 40–80 | 12.4 | partial |
| Single-crystal Ni (René, CMSX) | 200+ (per-part) | high | very limited |
| HDPE | 1.5–2 | 1.8 | mechanical (HDPE bottles 30% rate) |
| PET | 1.0–1.5 | 2.2 | mechanical + chemical (Loop, Eastman) |
| PP | 1.0–1.5 | 1.9 | mechanical |
| Polycarbonate | 4–6 | 7.6 | mechanical + chemical (Covestro) |
| PEEK | 80–120 | 31 | mechanical (small scale) |
| Epoxy (CFRP matrix) | 5–12 | 3 (resin only) | thermal degradation; pyrolysis |
| Carbon fiber (T700) | 30–60 (precursor + carbonization energy) | 22–24 | thermal + pyrolysis (Carbon Conversions, ELG) |
| Glass fiber (E-glass) | 1.5–3 | 2.5 | poor |
| Glass (soda-lime) | 0.3–0.5 | 0.8 | 90% closed-loop (cullet) |
| Wood (sawn softwood) | 0.3–0.6 | -1.2 (sequesters; depends on LCA boundary) | reuse / biomass / compost |
| Bamboo | 0.3–0.5 | -0.8 (sequesters) | reuse / biomass |
| Lithium carbonate (battery) | 18–35 (volatile; ~80 peak 2022, ~9 trough 2024, ~17 mid-2025) | 5–18 | partial (Redwood, Li-Cycle, Ascend) |
| Cobalt (battery) | 30–80 (volatile) | 9.5 | partial |
| Nickel (sulfate, battery-grade) | 20–35 | 12 (HPAL pathway in Indonesia) | partial |
| Tungsten | 40–60 | 16 | yes |
| Rare earths (Nd-Pr oxide) | 60–120 (volatile) | very high (Chinese supply chain) | recovery from scrap magnets |
| Platinum | ~30,000 ($/kg, 2025) | very high | 95% closed-loop (catalytic converters) |
| Palladium | ~30,000 | very high | yes (cat conv) |
| Rhodium | ~150,000 | very high | yes (cat conv) |
8. Supply-chain criticality — USGS 2024 list and EU CRM 2023
The USGS 2024 Critical Minerals List (50 minerals) and EU Critical Raw Materials Act (CRMA) 2023 (34 strategic raw materials + 17 strategic technologies) flag materials whose supply could disrupt national economies. Both updated post-Russia-Ukraine and US-China tensions.
| Material | Why critical | Primary source |
|---|---|---|
| Rare earths (Nd, Dy, Tb, Pr, Sm) | wind turbine + EV motor magnets | China (~70% mine, ~85% refine) |
| Cobalt | Li-ion cathode | DRC (~70%) |
| Lithium | Li-ion battery | Australia, Chile, China (refine ~60%) |
| Nickel (class 1) | EV battery | Indonesia, Philippines, Russia |
| Graphite | anode | China (~70% natural + ~90% synthetic) |
| Manganese | EV battery + steel | South Africa, Gabon, Australia |
| Gallium | semiconductors, RF (GaAs, GaN), LEDs | China (~98% primary) |
| Germanium | optical fiber + IR optics | China (~60%) |
| Indium | ITO (touchscreens, solar) | China + Korea |
| Silicon (metallurgical) | solar, semiconductor precursor | China (~70%) |
| PGMs (Pt, Pd, Rh, Ir, Ru) | catalysts, fuel cells, electronics | S. Africa (~75%), Russia (~10%) |
| Tungsten | tool carbide, EV motor balance | China (~80%) |
| Niobium | HSLA steel, superalloy | Brazil (CBMM ~80%) |
| Tantalum | electronics capacitor | DRC, Rwanda |
| Titanium (sponge) | aerospace structure | Russia, Japan, China |
| Vanadium | structural + flow batteries | China, Russia, S. Africa |
| Antimony | flame retardant, ammunition | China (~55%) |
| Tin | solder | China, Indonesia, Myanmar |
| Bismuth | low-melt alloy, pharma | China (~85%) |
| Tellurium | CdTe solar (First Solar) | China, Sweden (Boliden), Canada |
| Zinc (high-grade) | galvanizing | China, Peru, Australia |
| Copper | electrification | Chile, Peru, DRC (~50% combined) |
| Helium | cryogenics, MRI | US, Qatar, Russia, Algeria |
| Uranium | nuclear | Kazakhstan, Canada, Australia, Niger |
| Fluorspar | HF, refrigerant, steel | China (~60%), Mexico |
| Magnesium (Pidgeon) | structural + steel | China (~85%) |
| Phosphate rock | fertilizer, LiFePO₄ | Morocco (~70% reserves) |
| Potash | fertilizer | Canada, Belarus, Russia |
| Strontium | ceramic ferrite | China, Iran, Spain |
Engineering implication: a 2025 selection that depends on Dy / Tb / Co / Nd is exposed to China supply policy. The mitigation strategies are:
- Substitution — Tesla’s switch from NMC811 to LFP for entry models; Toyota’s Mn-rich cathode; rare-earth-free magnets (TDK SmFeN, GM Magnex / Niron Magnetics iron nitride).
- Recycling — Redwood Materials, Li-Cycle, Ascend Elements; magnet recovery from EOL motors.
- Domestic supply — IRA tax credits for US-mined critical minerals; Australia’s Critical Minerals Strategy.
- Reformulation — high-Mn cathode, Na-ion batteries (CATL, HiNa, Northvolt), all-iron flow batteries (ESS Inc., RFC Power).
9. The CES EduPack / GRANTA MI taxonomy
GRANTA EduPack (Cambridge → Granta Design → ANSYS 2019) is the canonical commercial database with ~4000 materials and ~30 properties each. Its taxonomy:
Materials
├─ Metals + alloys
│ ├─ Ferrous (steel, stainless, cast iron, low-alloy, tool, maraging)
│ ├─ Aluminum + alloys (1xxx, 2xxx, 3xxx, 5xxx, 6xxx, 7xxx, 8xxx, Al-Li)
│ ├─ Magnesium + alloys
│ ├─ Titanium + alloys (α, β, α-β)
│ ├─ Copper + alloys (Cu, brass, bronze)
│ ├─ Nickel superalloys (Inconel, Hastelloy, René, CMSX)
│ ├─ Refractory (W, Mo, Ta, Nb, Re, Ir)
│ └─ Precious + others (Au, Ag, Pt, Pd)
├─ Polymers
│ ├─ Thermoplastics (commodity: PE, PP, PVC, PS, PET, PMMA)
│ ├─ Engineering thermoplastics (PA, PC, POM, PBT, PEEK, PEI, PPS, PSU, PEK)
│ ├─ Thermosets (epoxy, phenolic, polyester, polyurethane, melamine)
│ ├─ Elastomers (NR, SBR, BR, IIR, NBR, EPDM, silicone, fluoroelastomer)
│ └─ Bio-based (PLA, PHA, PHBV, starch blend)
├─ Ceramics + glass
│ ├─ Engineering ceramics (Al2O3, ZrO2, Si3N4, SiC, B4C, AlN)
│ ├─ Refractories (firebrick, MgO, CrO, SiC for kilns)
│ ├─ Cements + concrete (Portland, Roman, geopolymer)
│ ├─ Glass (soda-lime, borosilicate, fused silica, vycor, lead, optical)
│ └─ Glass-ceramics (Zerodur, Macor, Pyroceram)
├─ Composites
│ ├─ Polymer-matrix (GFRP, CFRP, AFRP, BFRP, hybrid)
│ ├─ Metal-matrix (Al/SiC, Mg/SiC, Ti/SiC)
│ ├─ Ceramic-matrix (C/C, C/SiC, SiC/SiC, oxide/oxide)
│ └─ Hybrid laminates (GLARE, ARALL, CARALL)
├─ Foams
│ ├─ Polymer foams (PU, polystyrene, PMI, PEI)
│ ├─ Metal foams (Al foam, Ni foam)
│ ├─ Aerogels (silica, carbon, polymer)
│ └─ Syntactic foams (microsphere-filled polymer)
└─ Natural materials (wood, bamboo, hemp, jute, wool, leather)
10. Decision tree — pick a material
What's the function?
├─ Light + stiff
│ → maximize E^a / ρ (a = 1 tie, 1/2 beam-bending, 1/3 panel)
│ → CFRP for ultimate; Mg + Al alloys for cost balance
├─ Light + strong
│ → maximize σ_y^a / ρ
│ → CFRP / Ti / 7xxx Al
├─ Light + tough
│ → maximize K_1c × σ_y / ρ
│ → low-alloy steel (4340, AerMet) > Ti > Al
├─ High-T creep
│ → Ni superalloy single-crystal > CMC SiC/SiC > polycrystalline Ni > steel
├─ Cryogenic toughness
│ → austenitic stainless (304L), Al-Li, Ti grade 5 ELI
├─ Wear-resistant
│ → hardened steel, WC-Co (carbide), Al2O3, Si3N4, PTFE (low friction)
├─ Corrosion-resistant (seawater)
│ → super-duplex stainless 2507 > Ti grade 5 > Inconel 625 > 316L
├─ Electrical conductor (current)
│ → Cu > Al > Ag (cost-prohibitive)
├─ Electrical insulator
│ → polymer (PE, PI, FR-4 epoxy) at low T; ceramic (Al2O3, BeO) at high T
├─ Thermal conductor
│ → Cu > Al > graphene > diamond (cost)
├─ Thermal insulator
│ → aerogel > PU foam > mineral wool > polystyrene
├─ Magnetic (soft)
│ → mu-metal, electrical steel, Metglas
├─ Magnetic (hard)
│ → NdFeB > SmCo > AlNiCo > ferrite
├─ Biocompatible implant
│ → Ti6Al4V (load-bearing), CoCrMo (high-wear), Al2O3 / ZrO2 (ceramic), UHMWPE / PEEK (polymer)
├─ Low cost / commodity
│ → mild steel, concrete, wood, PE, PP, glass
├─ Low embodied CO₂
│ → recycled steel (EAF), recycled aluminum, wood, glass-fiber (vs carbon), bamboo
├─ Supply-secure (post-2025)
│ → avoid Co (DRC), Dy/Tb (China); prefer LFP, Mn-rich, rare-earth-free magnets
└─ Process-compatible
├─ AM SLM → AlSi10Mg, Ti6Al4V, 316L, Inconel 625/718, 17-4PH
├─ Injection mold → all thermoplastics
├─ Forge → low-alloy steel, Al, Ti
├─ Cast → grey iron, ductile iron, AlSi9Cu3, brass, Inconel investment
└─ Composite layup → CFRP / GFRP w/ epoxy / BMI / PEEK matrix
11. Anti-patterns
- “Stronger = better” — strength without toughness is brittle; consider K_1c.
- Choosing material before process — process may not be available; design for both.
- Ignoring embodied CO₂ in 2026 (EU CSRD, EU Taxonomy, SBTi) — material LCA is now required in many sectors.
- Reusing the same alloy historically without re-selection — “we always use 6061” loses 20-50% mass / cost gain available from re-selection.
- Selecting Ti for cost-sensitive part — Ti is ~10× the cost of Al; only use when strength/density really matters.
- Selecting CFRP for high-volume automotive — cost + cycle time + repair / recycling all hurt vs. AHSS.
- Designing as if anisotropic CFRP were isotropic — fiber-direction strength is real but transverse strength is poor.
- Specifying mu-metal in a high-field environment — saturates at low B; use electrical steel.
- Welding 7xxx aluminum without considering HAZ softening — Al-Zn weldable but loses temper.
- Using NdFeB at 150°C without considering temperature derating — NdFeB demagnetizes at ~150°C; SmCo to 350°C.
12. The 2024–2026 frontier
- AI-assisted materials selection — Granta MI 2024 + ML-based property prediction (Citrine, Matgen-Bench, Materials Project).
- Inverse design — Bayesian / generative ML for “give me a material with these properties”: MIT Generative Models, Microsoft GNoME (2023).
- High-throughput experimentation + AI loops — Citrine, Kebotix, Atomic Machines for materials discovery.
- Carbon-aware design — EU Carbon Border Adjustment Mechanism (CBAM) phasing in 2026; Scope 3 reporting under EU CSRD.
- Rare-earth-free magnets (TDK SmFeN, Niron Magnetics iron nitride, GM Magnequench) — competitive with NdFeB without the supply risk.
- Solid-state batteries — Toyota, Solid Power, QuantumScape, Samsung SDI — change cathode/anode material selection.
- Low-carbon steel — H2 DRI-EAF (HYBRIT, H2 Green Steel, ArcelorMittal Innovation), CCS-blended BOF.
- Low-carbon aluminum — ELYSIS inert anode (Rio Tinto + Alcoa 2018+, commercializing 2026+), Hall-Héroult electrification.
- Low-carbon cement — supplementary cementitious materials (slag, fly ash, calcined clay LC3); novel binders (geopolymer, CO2-cured).
- Bio-based polymers — PLA (NatureWorks), PHA (Danimer), PEF (Avantium); replacing fossil-source plastics.
- Recycled composites — Carbon Conversions (formerly Boeing surplus); ELG Carbon Fibre; pyrolysis-recovered carbon fiber.
Adjacent
- Materials selection method — materials-selection for Ashby methodology in depth.
- Per-family — materials-steel, materials-aluminum, materials-ceramics, materials-composites, materials-polymers.
- Mechanics + failure — mechanics-of-materials, fatigue-analysis, fracture-mechanics.
- Tier-3 grade catalogs — steel-grades, aluminum-alloys, titanium-alloys, stainless-steels, composites-taxonomy, ceramics-taxonomy, polymers-taxonomy, copper-alloys.
- Process-specific — casting-forging-forming, joining-welding, machining, additive-manufacturing, additive-manufacturing-advanced.
- Application-specific — electric-motors, transformers-power-systems, battery-chemistries, semiconductor-materials.
- Sustainability — sustainable-engineering-and-circular-economy for LCA + circular-economy frame.
- Polymer chemistry — polymer-chemistry and _compare_polymerization_methods for the chemistry behind polymer selection.
- Material standards bodies — standards-bodies for ASTM, ISO, JIS, GB, EN.
When to pick what
The fastest narrowing: start from function + dominant constraint → derive Ashby index → screen on chart → shortlist by process compatibility → finalize on cost + supply + lifecycle. The single biggest practical lesson is never skip the formal selection — defaulting to the historical material loses 20-50% performance against the objective. By 2026 you also cannot skip the supply-chain layer (Critical Minerals) and the embodied-CO₂ layer (CSRD, CBAM) — both have moved from “nice to have” to “regulatory requirement”. The selection sequence: (1) Ashby formal screen, (2) process compatibility, (3) cost, (4) embodied CO₂ + recyclability, (5) supply-chain criticality — in that order, then iterate.