superalloy-processing

name	superalloy-processing
description	Superalloy processing — investment casting (wax/ceramic process, directional solidification, single crystal growth), γ/γ' microstructure (γ' solvus, anti-phase boundary energy, coherency strain), heat treatment sequences (solution, primary/secondary age), hot isostatic pressing (HIP) for castings, powder metallurgy superalloys (disc alloys), welding difficulties (strain-age cracking), wrought vs. cast alloys, and AMS/ASTM standards for superalloy products.
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Superalloy Processing — Complete Skill

Superalloy Classification

Alloy Systems

Nickel-base superalloys: γ (austenite FCC matrix) + γ' precipitates (Ni₃(Al,Ti) ordered L1₂ structure; coherent with γ) γ' volume fraction: 40–70% in advanced alloys (IN738, René N5, CMSX-4) Temperature capability: to 1100°C with film cooling; single crystal to 1150°C material temperature

Cobalt-base superalloys (MAR-M509, X-40): Strengthened by solid solution (W, Mo, Cr) + carbides; no γ' precipitation Better hot corrosion resistance than Ni-base; lower strength at high T Applications: vane segments, combustor liners, stationary hot section

Iron-nickel superalloys (A286, IN718): More weldable; lower cost; T_max ≈ 700°C IN718: γ'' (Ni₃Nb, BCT) + δ (orthorhombic) precipitation; metastable γ'' → δ above 650°C limits temperature Most widely used superalloy (quantity) for discs, fasteners, shaft hardware

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γ' Precipitation Strengthening

Microstructure and Strengthening

γ' characteristics: Composition: Ni₃(Al,Ti,Ta); ordered L1₂ superlattice; same FCC-type as γ → coherent Coherency strain: (a_γ − a_γ')/a_γ × 100% = −0.1 to +0.3% (slight; maintained for fine precipitates) Anti-phase boundary (APB) energy: γ_APB ≈ 100–250 mJ/m²; dislocation must create APB or pair with superdislocation → strong obstacle

Strengthening mechanisms:

1. Order hardening: dislocations enter γ' creating APB → paired superdislocations; shear stress τ ∝ γ_APB × f^(1/2)
2. Coherency hardening: lattice mismatch strains oppose dislocation glide
3. Modulus hardening: E_γ' > E_γ in some alloys → additional resistance

γ' anomalous yield behavior: Yield strength of γ' increases with temperature up to ~800°C (anomalous peak) → superalloys stronger at temperature than at room temperature in some respects Mechanism: cross-slip of <112> dislocations from {111} to {001} at elevated T → Kear-Wilsdorf lock

γ' solvus temperature (T_γ'): Temperature at which γ' dissolves completely; critical for heat treatment CMSX-4: T_γ' ≈ 1320°C (high → leaves narrow solutioning window above T_γ') IN738LC: T_γ' ≈ 1175°C (more processable) Solution heat treatment: above T_γ' → dissolve all γ' → homogenize Aging: below T_γ' → controlled re-precipitation of bimodal (primary + secondary) γ' size distribution

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Investment Casting

Lost-Wax Process

Investment casting steps:

1. Wax pattern injection: wax injected into die; ceramic core for internal cooling passages
2. Assembly: wax patterns attached to runner/gate system ("tree")
3. Shell building: dip in ceramic slurry (silica, alumina, zircon) → stucco → dry; repeat 8–15 times
4. Dewax: steam autoclave at 150°C removes wax
5. Burnout: 1000°C to sinter ceramic shell; remove wax residual
6. Metal pour: superalloy melted under vacuum; poured into preheated mold (1100°C)
7. Shell removal: mechanical vibration, high-pressure water, caustic leach
8. Core removal: NaOH leach at 180°C/300 psi (dissolves silica core without attacking metal)

Solidification control: Conventional equiaxed: random grain orientation; weakest creep (grain boundaries perpendicular to stress are weak) Directionally solidified (DS): Bridgman furnace; crystal solidifies from bottom → all grains aligned with [001] || blade axis; eliminates transverse grain boundaries → 2–3× life vs. equiaxed in creep Single crystal (SX): select crystal from DS specimen; single grain propagates → no grain boundaries → highest creep life (René N5, CMSX-4, PWA 1484 for 1st/2nd stage turbine blades)

DS and SX Growth

Bridgman directional solidification: Mold heated above liquidus; lowered out of furnace zone at controlled withdrawal rate Withdrawal rate: 100–500 mm/hr (DS); 100–200 mm/hr (SX) — slower → fewer dendrite spacings → lower porosity Thermal gradient G × velocity V → microstructure: G > G_crit × V → planar front; G < G_crit × V → cellular/dendritic Target: maximize G/√V for finest dendrite arm spacing (DAS ≈ 150–500 μm)

Crystal selector (pig-tail): Helical section at base of mold → only one grain navigates spiral → exits as single crystal Primary dendritic axis aligned ≈ <001> (natural easy-growth direction in FCC) Misorientation tolerance: < 5–10° from [001] for turbine blades (assessed by Laue X-ray)

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Heat Treatment of Nickel Superalloys

Multi-Step Heat Treatment

Cast alloy (IN738LC) heat treatment sequence:

1. HIP: 1185°C / 103 MPa / 4 h → close casting porosity; >99% density
2. Solution anneal: 1120°C / 2 h / air cool → dissolve coarse γ' (must be below T_γ' = 1175°C to avoid grain growth); partial solution
3. Primary age: 1050°C / 4 h / air cool → re-precipitate bimodal large primary γ'
4. Secondary age: 850°C / 16 h / air cool → fine secondary γ' precipitation

SX alloy (CMSX-4) heat treatment:

1. Multi-step solution (ramp): 1296°C/1300°C/1308°C/1313°C/1316°C (step near T_γ' = 1320°C) → dissolve γ' without melting
2. Primary age: 1140°C / 6 h
3. Secondary age: 870°C / 20 h Result: cuboidal γ' precipitates 0.4–0.5 μm; V_f ≈ 65%; aligned in γ channels 50–100 nm wide

HIP for castings: Critical: closes microporosity (reduces fatigue initiation sites) After HIP: σ_fatigue increases 30–100%; creep properties improved; scatter reduced HIP before machining (HIP-machined vs. machined-HIP: no difference for solid blades; HIP before machining avoids distortion)

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Powder Metallurgy Superalloys (Disc Alloys)

PM Processing

Why PM for discs (vs. cast): High-alloy compositions (>50% γ'; IN100, René 95, ME3/René 104) → hot cracking in ingot casting; segregation PM: fine powder (d₅₀ = 50–100 μm) compacted → no segregation → uniform microstructure → higher γ' content possible

PM superalloy process:

1. Gas atomize alloy under vacuum/argon → spherical powder
2. Consolidate: extrude or HIP at 1100–1200°C; density > 99.9%
3. Isothermal forge: subsolvus (T < T_γ') → fine-grain necklace microstructure OR supersolvus (T > T_γ') → coarse grain for creep
4. Heat treat: solution + age

ME3 / René 104 (advanced disc alloy): σ_y = 1170 MPa at RT; σ_y at 760°C = 1000 MPa; sustained creep 140 MPa / 900°C life > 500 h Damage tolerant dual-microstructure disc: coarser grain in bore (better creep); finer grain in rim (better fatigue)

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Welding Challenges

Strain-Age Cracking

Strain-age cracking (SAC): weld heat-affected zone → sensitized by welding heat → rapid γ' reprecipitation on cooling → residual welding stresses → cracking Critical alloys: high γ' volume fraction (>40%); high Ti+Al content (>4 wt%) Safe for welding: IN625 (no γ'); IN718 (γ'' instead of γ' → slower precipitation → welding window) Cannot weld (or extremely difficult): Waspaloy, IN738, CMSX-4 (must use diffusion bonding or brazing instead)

PWHT mitigation: Controlled cooling after welding; avoid rapid temperature drop through γ' precipitation range Stress relief before solution (prevent stress + precipitation occurring simultaneously) Pre-weld solution: dissolve γ' → some residual strength lost but no cracking during weld cooling

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Standards and References

Standard	Scope
AMS 5397	IN738 casting specification
AMS 5369	René 77 (Udimet 700) wrought
AMS 5667	IN718 bar and forgings
ASTM B637	Ni alloys for corrosion and high-T service
AMS 2315	HIP for nickel superalloy castings
AMS 2806	Inspection of castings (Fluorescent penetrant)

Output

Provide: component (turbine blade/vane/disc/combustor; stage; inlet temperature [°C]; material temperature [°C] with cooling), alloy selection (equiaxed/DS/SX; conventional cast or PM disc; alloy: IN738/CMSX-4/René N5/IN718; T_γ' [°C]; γ' volume fraction [%]; composition highlight: Al+Ti+Ta content), casting process (investment cast; DS withdrawal rate [mm/hr]; G [K/cm] target; SX selector type; orientation tolerance [° from <001>]; core removal method), heat treatment sequence (HIP if cast: T/P/t; solution: T [°C] / time [h] / cool rate; primary age: T [°C] / t [h]; secondary age: T [°C] / t [h]), γ' microstructure (cuboidal precipitate size [nm]; volume fraction [%]; channel width [nm]; primary large γ' size [μm]; APB energy [mJ/m²]), mechanical properties (σ_y at RT [MPa]; σ_y at T_operating [°C] [MPa]; creep rupture life at design stress/temperature [h]; LCF initiation life [cycles]), welding assessment (γ' content: SAC risk? high/medium/low; recommended joining method: weld/braze/diffusion bond), and applicable standard (AMS 5397 for IN738 casting; AMS 2315 for HIP; AMS 5667 for IN718 forgings).