Metals & alloys — Al alloys (2024, 7075), Ti alloys (Ti-6Al-4V), Ni superalloys (Inconel, Hastelloy), stainless steels

1. The master variable: Specific strength

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

2. Why alloys are stronger than pure metals (first principles)

Formula Hall–Petch (derive the

form from first principles) A dislocation pile-up at a grain boundary concentrates stress. The number of dislocations $n$ that pile up over a grain of diameter $d$ scales with $d$ (bigger grain = more room = more dislocations = more stress concentration to break through). Working the pile-up stress concentration gives the extra yield stress $\propto d^{-1/2}$ . Hence: $\sigma_y = \sigma_0 + k\,d^{-1/2}$ WHY $d^{-1/2}$ and not $d^{-1}$ : stress at the pile-up tip grows as $\sqrt{\text{pile-up length}}\sim\sqrt{d}$ , and to break into the next grain you need a fixed critical tip stress — solving for the applied $\sigma$ flips it to $d^{-1/2}$ . Smaller grains → stronger.

Figure — Metals & alloys — Al alloys (2024, 7075), Ti alloys (Ti-6Al-4V), Ni superalloys (Inconel, Hastelloy), stainless steels

3. The four families in detail

3a. Aluminium alloys — 2024 & 7075

3b. Titanium — Ti-6Al-4V ("workhorse", α–β alloy)

3c. Nickel superalloys — Inconel, Hastelloy

3d. Stainless steels

Recall Feynman: explain to a 12-year-old

Imagine four backpacks for a school trip. One is super light for normal days (aluminium). One is light and tough for a long hike (titanium). One can sit next to a campfire without melting (nickel superalloy). One is cheap and won't rust in the rain (stainless steel). You don't bring one backpack — you grab the right one for the day. Metals are tougher when they're full of tiny lumps and walls inside, because the "cracks" (dislocations) trip over them like obstacles in an obstacle course. But near a campfire (turbine heat), the walls themselves get soft and slide — so the best hot parts are one giant smooth crystal with no walls.

Flashcards

What single quantity governs aerospace metal selection?

Mass-specific strength, σ/ρ (strength per unit mass); weight-specific is σ/(ρg) but g cancels in comparisons.

Main alloying element in 2xxx Al alloys (e.g. 2024)?

Copper (Cu).

Main alloying element in 7xxx Al alloys (e.g. 7075)?

Zinc (Zn), with Mg.

Why use 2024 over 7075 in fatigue-critical fuselage?

2024 has better fracture toughness and stress-corrosion resistance.

Composition meaning of Ti-6Al-4V?

~6 wt% Al + 4 wt% V; Al stabilizes α (HCP), V stabilizes β (BCC).

Approx max service temperature of Al alloys vs Ti vs stainless vs Ni superalloys?

~150 °C, ~400 °C, ~600 °C, ~1000 °C+.

Name the strengthening precipitate in Ni superalloys.

γ′ = Ni₃(Al,Ti), ordered, coherent with FCC γ matrix.

State the Hall–Petch relation.

σ_y = σ₀ + k·d^(−1/2); smaller grain → higher yield.

Why are top turbine blades single crystals?

At high T grain boundaries cause creep/sliding; removing them suppresses creep.

What makes steel "stainless"?

≥10.5% Cr forms a self-healing passive Cr₂O₃ film.

Difference between Inconel and Hastelloy?

Inconel (Ni-Cr-Fe) for oxidation/creep at high T; Hastelloy (Ni-Mo-Cr) for extreme corrosion.

What is the Orowan stress for bowing past precipitates?

τ = Gb/L; smaller spacing L → stronger.

What is the T6 temper?

Solution-treated + peak-aged for maximum strength.

Why is peak-aged strength a maximum, not monotonic?

Crossover between cutting (∝r) and bowing (∝1/L); over-aging coarsens particles, increasing L.

Connections

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

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, aerospace mein metal choose karna ek tradeoff game hai: strength chahiye, par weight bhi kam hona chahiye, aur kabhi-kabhi heat bhi bahut hoti hai. Isliye sabse important cheez hai specific strength yaani strength divided by density (σ/ρ) — yeh strength per unit mass hai. Plane mein har extra kilo ko poora flight bhar uthana aur kheechna padta hai, to "kitna strong" se zyada important hai "apne weight ke hisaab se kitna strong".

Char families yaad rakho ek temperature ladder ki tarah (sabse thanda se sabse garam): Aluminium (2024, 7075) halka aur sasta, par sirf thande hisson ke liye (~150°C tak) — 2024 mein Copper hota hai aur fatigue/toughness accha, 7075 mein Zinc hota hai aur strength sabse zyada. Titanium (Ti-6Al-4V) halka bhi aur strong bhi, ~400°C tak chalta hai, corrosion resistance top — par mehenga kyunki banana mushkil. Stainless steel sasta aur rust-proof (≥10.5% Cr), ~600°C tak, weight critical na ho to fasteners/exhaust ke liye. Aur sabse garam turbine ke liye Nickel superalloys (Inconel, Hastelloy) — yeh ~1000°C+ jhel lete hain jahan gas metal ke melting point se bhi garam hoti hai; inme γ′ Ni₃(Al,Ti) precipitate magic karta hai.

Strength ka asli raaz: metal tab bend hota hai jab andar dislocations slip karti hain. Inhe rokne ke liye hum obstacles daalte hain — chhote precipitates (age hardening), chhote grains (Hall–Petch, σ_y = σ₀ + k·d^(−1/2)), ya alag atoms. Peak strength wahan milti hai jahan precipitate na bahut chhote ho na bahut door (T6 temper). Ek interesting twist: room temperature pe grain boundaries strength badhaate hain, lekin turbine ki heat pe yahi boundaries creep se slide karke weak point ban jaate hain — isliye top turbine blades single crystal banaye jaate hain bina kisi boundary ke. Rule temperature ke saath ulta ho jaata hai!