5.4.1
Intuition Big picture (WHY yeh subtopic exist karta hai)Aerospace structures ek brutal trade war ladte hain: strength vs weight vs temperature vs corrosion vs cost . Koi bhi ek akela metal sab jagah nahi jeetta, isliye engineers har kaam ke liye sahi family choose karte hain. Poora subtopic basically ek hi sawaal hai:
"Kis temperature par, kitne stress par, aur kitne weight par mujhe yeh part survive karwana hai?"
Cold + light → Aluminium alloys
Light + medium-hot + strong → Titanium alloys
Bahut hot (turbine) → Nickel superalloys
Cheap + corrosion-resistant + structural → Stainless steels
Definition Specific strengthMass-specific strength: ρ σ σ ρ strength per unit mass . (Sach mein weight -specific strength σ / ( ρ g ) g σ / ρ
Intuition WHY weight dominate karta hai1 kg ko cruise altitude tak uthana aur poori flight tak drag karna continuously fuel jalata hai. Toh designers nahi poochte "kya yeh strong hai?" — woh poochte hain "kya yeh apne weight ke liye strong hai?" Isliye titanium (ρ ≈ 4.5 ρ ≈ 7.9
Family
ρ (g/cm³)
Typical σ_y (MPa)
σ_y/ρ
Max service T
Al 2024/7075
~2.8
350–500
~150 ~150 °C
Ti-6Al-4V
~4.43
~880
~200 ~400 °C
Ni superalloy
~8.2
~1000 (hot)
~120
~1000+ °C
Stainless steel
~7.9
~250–1000
~50–125
~600 °C
Intuition WHAT metal ko "strong" banata haiMetals tab deform hote hain jab dislocations (line defects) crystal mein se glide karte hain. Strength = dislocations ko move karna kitna mushkil hai. Har strengthening trick bas yahi hai — "dislocation ke raaste mein obstacles daal do."
Definition Char strengthening mechanisms
Solid-solution — foreign atoms lattice ko distort karte hain (Mg, Cu in Al).Precipitation (age) hardening — fine second-phase particles dislocations ko pin karte hain.Grain refinement — grain boundaries slip ko block karte hain (Hall–Petch).Work hardening — dislocations ek doosre se tangle ho jaate hain.
4-digit number = main alloying element. 2xxx = Cu , 7xxx = Zn (+Mg) .
2024 (Al–Cu–Mg): tough, good fatigue → fuselage skins, lower wing .7075 (Al–Zn–Mg–Cu): highest-strength Al → upper wing, spars .
Intuition WHY yeh age-harden karte hainCu (ya Zn+Mg) high T par dissolve hota hai, phir trap karne ke liye quench kiya jaata hai (supersaturated), phir aged kiya jaata hai: fine GP zones / precipitates (2024 mein CuAl₂, 7075 mein MgZn₂) grow hote hain aur dislocations ko pin karte hain. Temper codes : T6 = peak-aged (max strength), T3 = solution + cold-worked.
Common mistake "7075 hamesha 2024 se better hai."Kyun sahi lagta hai: 7075 ki yield strength zyada hai (~500 vs ~350 MPa), toh "stronger = better."
Fix: 7075 ki fracture toughness aur stress-corrosion resistance worse hai. Fatigue-critical, damage-tolerant lower fuselage 2024 use karta hai; compression-loaded upper wing 7075 use karta hai. Sahi tool, sahi load.
Structural metals mein specific strength sabse best, ~400 °C tak strength banaaye rakhta hai, aur ek tough self-healing TiO₂ film banaata hai → outstanding corrosion resistance. Jahan Al bahut kamzor/heat-sensitive ho lekin steel ka weight afford na ho, wahan use hota hai: engine fan blades, landing gear, airframe near engines, fasteners .
α (HCP) : Al se stabilize hota hai → strong, creep-resistant, kam ductile.β (BCC) : V se stabilize hota hai → ductile, formable.Ti-6Al-4V = 6% Al + 4% V → mixed α+β, heat treatment se tunable. "6-4" literally composition hai.
Common mistake "Titanium aluminium jaisa heat-treat hota hai, toh yeh sasta hai."Kyun sahi lagta hai: dono age-harden karte hain.
Fix: Ti reactive hai (jalta hai, high T par O/N absorb karta hai), vacuum/inert processing chahiye, aur machine karna mushkil hai → bahut expensive . Yahi cost reason hai ki Al tab bhi use hota hai jab temperature allow kare.
Intuition WHY hot section ke liyeTurbine mein gas metal ke melting point se zyada ho jaati hai. Aapko >0.7× melting temperature par strength + creep resistance + oxidation resistance chahiye. Ni ka FCC lattice hot mein stable rehta hai, aur isme magic γ′ precipitate Ni₃(Al,Ti) hota hai jo temperature badhne par zyada strong ho jaata hai (anomalous yield), γ matrix mein dislocations ko pin karta hai.
Inconel (Ni–Cr–Fe): oxidation + creep resistance → turbine blades/discs, exhaust.Hastelloy (Ni–Mo–Cr): harsh chemical/reducing media mein supreme corrosion resistance.
Strengthening = solid-solution (Cr, Mo, Co) + γ′ precipitates + grain boundaries par carbides.
Common mistake "Single-crystal blades koi sense nahi banate — grain boundaries metals ko strong banate hain (Hall–Petch)!"Kyun sahi lagta hai: room T par, zyada grain boundaries = zyada strong.
Fix: turbine temperatures par, grain boundaries weak link hote hain — woh creep/diffusion se slide aur crack karte hain. Isliye top blades single crystals hote hain (koi boundaries nahi) taaki creep khatam ho sake. Temperature ke saath strengthening rules palat jaate hain.
Iron + ≥10.5% Cr → self-healing Cr₂O₃ passive film → "stainless." Ti se sasta aur stiffer, jahan weight utna critical nahi wahan use hota hai: fasteners, hydraulic lines, firewalls, exhaust, structural fittings .
Types: Austenitic (304/316, FCC) — corrosion + weld friendly; Martensitic/PH (17-4PH) — heat-treatable high strength.
Recall Feynman: 12-saal ke bachche ko samjhaoSocho school trip ke liye chaar backpacks hain. Ek normal dinon ke liye super light hai (aluminium). Ek lambi hike ke liye light aur tough hai (titanium). Ek campfire ke paas baith sakta hai bina pighle (nickel superalloy). Ek sasta hai aur baarish mein rust nahi karega (stainless steel). Tum ek backpack nahi laate — tum din ke hisaab se sahi wala uthate ho. Metals tab tough hote hain jab unke andar tiny lumps aur walls hote hain, kyunki "cracks" (dislocations) unpe trip karte hain jaise obstacle course mein. Lekin campfire ke paas (turbine heat), walls khud soft ho jaate hain aur slide karte hain — isliye best hot parts ek giant smooth crystal hote hain jisme koi walls nahi hoti.
Mnemonic Temperature ke hisaab se metal chunno (cold→hot)"All Tigers Snore Nightly" → Al (cold/light, ~150 °C) → Ti (warm/strong, ~400 °C) → Stainless (cheap/corrosion, ~600 °C) → Ni (hottest turbine, ~1000 °C+). Yeh ladder service temperature ke order mein hai, density ke nahi — stainless (~7.9) actually Ni superalloys (~8.2) se thoda kam dense hai.
Aur "2 Cu, 7 Zn" : 2024 = Copper, 7075 = Zinc.
Aerospace metal selection mein kaun si ek quantity govern karti hai? Mass-specific strength, σ/ρ (strength per unit mass); weight-specific σ/(ρg) hoti hai lekin comparisons mein g cancel ho jaata hai.
2xxx Al alloys (jaise 2024) mein main alloying element kya hai? Copper (Cu).
7xxx Al alloys (jaise 7075) mein main alloying element kya hai? Zinc (Zn), Mg ke saath.
Fatigue-critical fuselage mein 7075 ki jagah 2024 kyun use karte hain? 2024 ki fracture toughness aur stress-corrosion resistance better hai.
Ti-6Al-4V ki composition ka matlab kya hai? ~6 wt% Al + 4 wt% V; Al, α (HCP) ko stabilize karta hai, V, β (BCC) ko stabilize karta hai.
Al alloys vs Ti vs stainless vs Ni superalloys ki approximate max service temperature kya hai? ~150 °C, ~400 °C, ~600 °C, ~1000 °C+.
Ni superalloys mein strengthening precipitate ka naam batao. γ′ = Ni₃(Al,Ti), ordered, FCC γ matrix ke saath coherent.
Hall–Petch relation state karo. σ_y = σ₀ + k·d^(−1/2); chhota grain → zyada yield.
Top turbine blades single crystals kyun hote hain? High T par grain boundaries creep/sliding cause karti hain; unhe hatane se creep suppress hota hai.
Steel ko "stainless" kya banata hai? ≥10.5% Cr ek self-healing passive Cr₂O₃ film banata hai.
Inconel aur Hastelloy mein kya difference hai? Inconel (Ni-Cr-Fe) high T par oxidation/creep ke liye; Hastelloy (Ni-Mo-Cr) extreme corrosion ke liye.
Precipitates ke past bowing ke liye Orowan stress kya hai? τ = Gb/L; chhoti spacing L → zyada strong.
T6 temper kya hai? Solution-treated + maximum strength ke liye peak-aged.
Peak-aged strength maximum kyun hoti hai, monotonic kyun nahi? Cutting (∝r) aur bowing (∝1/L) ke beech crossover; over-aging particles ko coarsen karta hai, L badhata hai.
Dislocations and Slip Precipitation Hardening Hall–Petch Strengthening Creep and High-Temperature Deformation Phase Diagrams (α–β Titanium) Corrosion and Passivation Fatigue and Fracture Toughness Single-Crystal Turbine Blades
Specific strength sigma/rho
Strength = block dislocations
