Chapter 11-2triangle.kaist.ac.kr/lectures/MS371/2019 spring... · – Rene 77 alloy Effect of Heat Treatment on Stress-Rupture Properties /MS371/ Structure and Properties of Engineering

/MS371/ Structure and Properties of Engineering Alloys

Chapter 11-2

Nickel and Cobalt Alloys

/MS371/ Structure and Properties of Engineering Alloys/MS371/ Structure and Properties of Engineering Alloys

Nickel-Base Superalloys

• In general, Ni-base superalloys to be used at 760 to 980 °C

• MAR-M246 (cast) maintain strength at higher temp.

High-Temp Stress-Rupture Properties



• Wrought alloys– By adding intermediate temp , the

longtime rupture strength is increased.

→ MC + γ → M23C6 + γ’

GB of coarse particles in M23C6 carbides

to form in a layer of γ’

• Cast alloys– Rene 77 alloy

Effect of Heat Treatment on Stress-Rupture Properties



• Definition: accelerated surface of superalloy hot-gas-path component

• Prerequisite: presence of condensed alkali metal salts ( )

• Improvement of hot-corrosion resistance– Cr: protective surface oxides

– Ti or a high Ti-to-Al ratio: protective surface oxide

– Al: detrimental for hot-corrosion resistance

Hot Corrosion ( )


Nickel-Iron-Base Superalloys

• 25~45% Ni, 15~60% Fe, 15~28% Cr (oxidation resistance), 1~6% Mo

(solid-solution strengthening), Ti/Al/Nb (precipitate strengthening)

Chemical Comp and Typical Application



• The austenite matrix (FCC)

– stabilizers (Cr, Mo, wt% minimum of Ni to maintain fcc matrix)

– High-nickel contents: high temp, high stability, high cost

– High-iron contents: low cost, high malleability, low oxidation resistance

• Solid-solution strengthener– 10~25% Cr, 0~9% Mo, 0~5% Ti, 0~2% Al, 0~7% Nb, C, B

– Cr: oxidation resistance

– Mo: the most , to expand γ matrix, to enter carbides and γ’

• Precipitation strengthener– Ni-Fe alloys are all susceptible to of secondary phases (η, δ, μ, Laves).

– Ti: major γ’-forming element

– Al: oxidation resistance

– Nb: γ’’-forming element

Microstructure



• Inconel 901

– Strengthened by FCC γ’

– Solution-heat-treatment (2 h at 1066 °C) + water quenching + aging (2 h at 802 °C) + air-cooling + aging (24 h at 732 °C)

– A precipitate of γ’ is developed in the γ matrix

– Service exposure at 650 ~ 760 °C: needlelike precipitate of η HCP phase (Ni3Ti)

– High Ti-to-Al ratio: The antiphase boundary (dislocation movement ↓)

strength ↑



• Inconel 718

– Strengthened by Nb-rich γ’ ( , FCC) precipitates

– γ’ particles: 7.5 ~ 30 nm spherical and disk-like morphology

– After prolonged exposure at 650 ~ 700 °C

• spherical precipitates: FCC γ’

• small plates: BCT NixNb

• large plates: orthorhombic Ni3Nb



• Ni-Fe-base superalloys cannot be used at as high temp as Ni-base alloys.



Cobalt-Base Superalloys• Cobalt has many physical properties similar to such as atomic size, melting

point and density.

Chemical Comp and Typical Applications• 50~60% Co, 20~30% Cr, 5~10% W, 0.1~1% C

• Less subject to hot than the nickel-base alloys

• stress-rupture, time-temp properties

• Long-lived static parts which are at relatively low stresses and high temp

• Predominant in nozzle-guide vane-partition application


Cobalt-Base Superalloys

• Austenitic matrix (FCC)– ~ 50% Co, 25% Cr, Ni, W, Ta, Fe, Mo

– Ni, Fe, Zr and Ta, which increases the stacking-fault energy, stabilize the structure

– Cr, Mo and W, which decreases the stacking-fault energy, stabilize the structure

• Carbides– A fine dispersion of carbides contributes significantly to the of Co-base superalloys.

– M23C6 carbides: M = Cr, W, Mo

– MC carbides: M = Ta, Ti, Zr, Nb

– M6C carbides: M = W, Mo ( > 5%)

– Precipitate (M23C6) at GB decrease GB and prolong life.

– Precipitate in stacking faults impede dislocation (decrease in ductility).

Microstructure



• Effect of heat treatment on microstructure

(d) Blocky agglomerated M23C6 is developed. The fine background

precipitate of M23C6 is coarsened and more evenly distributed.

(c) Precipitation of M23C6 particle

- a finely dispersed semicoherent precipitate

- Widmanstatten plates on the {111} planes of the matrix.

Undissolved M23C6 is agglomerated.

(b) GBs are cleaned up. The principal residual carbide is M6C.

(a) As-cast structure of MC carbides and M23C6 colonies



• The lower strength of the cobalt alloys at temp is due to a lack

of γ’-type precipitates, which all nickel alloys have.



Single-Crystal Castings of Nickel-Base Superalloys

• Columnar-grained and single-crystal castings produced a major in

strength and temp capability of superalloy castings.

• addition improved the intermediate temp by greatly reducing the

tendency to form longitudinal cracks between directionally solidified grains.

Directionally Solidified Single-Crystal Castings of

Ni-Base Superalloys



• Spiral channel → single-crystal castings

• A finer γ’ dispersion for maximum strengthening can be precipitated and

grown at temp.



• Antiphase boundary (APB) region:

The region between the moving pair of dislocations

in the ordered gamma prime (γ’) precipitate (Ni3Al)

• APB energy:

The energy required to pass the initial dislocation

through the ordered γ’

• Before peak strength:

Glissile dislocations become by cross slipping from octahedral {111} planes

onto {010} cubic planes.

• Beyond peak strength:

The immobile cross-slipped dislocations act as nuclei for cubic glissile slip of dislocations

which move easier

Strengthening Mechanisms in Single-Crystal

Ni-Base Superalloys

𝜎𝑦 ∝1

𝑅+1

𝜆

𝜎𝑦: yield strength

𝑅: gamma prime size𝜆: cross-slip intercept

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Chapter 11-2triangle.kaist.ac.kr/lectures/MS371/2019 spring... · – Rene 77 alloy Effect of Heat Treatment on Stress-Rupture Properties /MS371/ Structure and Properties of Engineering