Superalloys

Introduction and Overview

General Characteristics. Superalloys are nickel-, iron-nickel-, and

cobalt-base alloys generally used at temperatures above approximately

540 °C (1000 °F). They have a face-centered cubic (fcc, austenitic) struc-

ture. Iron, cobalt, and nickel are transition metals with consecutive posi-

tions in the periodic table of elements. The iron-nickel-base superalloys

are an extension of stainless steel technology and generally are wrought,

whereas cobalt- and nickel-base superalloys may be wrought or cast,

depending on the application/composition involved. The more highly

alloyed compositions are normally processed as castings. Cast alloys may

have equiaxed grain structures or directionally solidified columnar grains;

they may also be cast as single crystals (more accurately, a single grain or

primary dendrite). Some highly alloyed nickel-base compositions are also

processed by powder metallurgy (P/M) techniques. Fabricated structures

can be built up by welding or brazing, but many highly alloyed composi-

tions containing a high volume fraction (Vf) of hardening phase are diffi-

cult to weld.

A noteworthy feature of nickel-base alloys is their use in load-bearing

applications at temperatures in excess of 80% of their incipient melting

temperatures (0.85 Tm), a fraction that is higher than for any other class of

engineering alloys. Superalloys exhibit some combination of high

strength at temperature; resistance to environmental attack (including

nitridation, carbonization, oxidation, and sulfidation); excellent creep

resistance, stress-rupture strength, toughness, and metallurgical stability;

useful thermal expansion characteristics; and resistance to thermal fatigue

and corrosion. The aforementioned properties can be controlled by adjust-

ments in composition and by processing (including heat treatment and

thermomechanical processing). Figure 1 compares stress-rupture behavior

of the three alloy classes.

Superalloys were initially developed for use in aircraft piston engine

turbosuperchargers, and their development over the last 60 years has been

Superalloys / 291

paced by the demands of advancing gas turbine engine technology. The

vast majority of use by tonnage of nickel-base superalloys is found in tur-

bines, both for aerospace applications and for land-based power genera-

tion. These applications require a material with high strength, good creep

and fatigue resistance, good corrosion resistance, and the ability to be

operated continuously at elevated temperatures. Figure 2 traces the histor-

ical development of superalloys and lists some of the important alloying

additions that increased temperature capability.

Applications. Superalloys have been used in cast, rolled, extruded,

forged, and powder-processed forms. Sheet, bar, plate, tubing, shafts, air-

foils, disks, and pressure vessels (cases) are some of the shapes that have

been produced. These metals have been used in aircraft, industrial, and

marine gas turbines; nuclear reactors; aircraft skins; spacecraft structures;

petrochemical production; orthopedic and dental prostheses; and environ-

mental protection applications. Although developed for high-temperature

use, some are used at cryogenic temperatures and others at body temper-

ature. Applications continue to expand, but at lower rates than in previous

decades. Aerospace usage remains the predominant application on a vol-

ume basis.

Temperature, °F

100 h

ruptu

re s

trength

, M

Pa

100 h

ruptu

re s

trength

, ksi

Temperature, °C

100

12001000

649538

1400

760

1600

871

1800

982

2000

1093

2200 2400

1204 1316

689

827

80 552

60 414

40 276

20 138

00

120

Carbide-phase-

strengthened cobalt

alloys

Solid-solution-

strengthened iron,

nickel, and cobalt

alloys

Precipitation (γ ′ or γ ′′ )

strengthened nickel and

iron-nickel alloys

Fig. 1 General stress rupture behavior of superalloys

292

/ Stainless Steels an

d H

eat-Resistan

t Allo

ys

1100

1200

1000

900

800

7001950

N80

1960

Tem

pera

ture

(100 h

at 140 M

Pa),

°C

1970 1980 1990s1940

HS21

Hastelloy B

+Ti

+Al

X40

X750

N90

Waspaloy

Wrought

alloys

+Mo

+Co

Vacuum

melting

Cast alloys

IN100

713CUdimet 700

N100W152

N81

N91

Ni-base

Co-base

Ni-base

Co-base

DS and SC

ODS Ni-baseP/MCast

Wrought

Cast

N101

IN 738

IN 792

TRW VIA

R80

Udimet 720

IN6203

IN6202

IN 6201

+Re

IN 939

IN 935

MAR-M-509

N115

R77

Udimet 500

N105

MAR-M-200

+/W and Nb

+Ta

IN591

+Hf

MAR-M-22

M246M21

N80A

B1900

Directional

structures

DS eutectics

MA6000CMSX-4

CMSX-10

Single

crystals

SRR99

CMSX-2MAR-M-002 DS

MAR-M-200 + HfDS

Mechanical

alloying

SB16

Fig. 2 Temperature capability of superalloys with approximate year of introduction. DS, directionally solidified; SC, single-crystal; P/M, powder metal-lurgy; ODS, oxide-dispersion-strengthened

Superalloys / 293

Phases, Structures, and Alloying Elements Associated with Superalloys

Superalloys consist of the austenitic fcc matrix phase γ plus a variety of

secondary phases. Secondary phases of value in controlling properties are

the carbides MC, M23C6, M6C, and M7C3 (rare) in all superalloy types;

the γ ′ fcc ordered Ni3(Al, Ti), γ ″ bct (body-centered tetragonal) ordered

Ni3Nb, η hexagonal ordered Ni3Ti, and δ orthorhombic Ni3Nb inter-

metallic compounds in nickel- and iron-nickel-base superalloys. The

superalloys derive their strength from solid-solution hardeners and pre-

cipitated phases. Principal strengthening precipitate phases are γ ′ and γ ″.

Carbides may provide limited strengthening directly (e.g., through disper-

sion hardening) or, more commonly, indirectly (e.g., by stabilizing grain

boundaries against excessive shear). The δ and η phases are useful (along

with γ ′) in controlling the structure of wrought superalloys during proc-

essing. The extent to which they directly contribute to strengthening

depends on the alloy and its processing.

In addition to those elements that produce solid-solution hardening

and/or promote carbide and γ ′ formation, other elements (e.g., boron, zir-

conium, and hafnium) are added to enhance mechanical or chemical prop-

erties. Some carbide- and γ ′-forming elements may contribute

significantly to chemical properties as well. Tables 1(a) and (b), respec-

tively, give a generalized list of the ranges of alloying elements and their

effects in superalloys. Similar information is provided in Fig. 3.

Li Be

Na

K Sc V MnTi

Cr

Joint base

element

FeCo

Ni

Rb Sr NbMo

Tc

ReWTaHfCs Ba Os Ir Pt Au Hg Ti Pb Bi Po At Rn

Ru Rh Pd Ag Cd In Sn Sb Te I Xe

Cu Zn Ga Ge As Se Br Kr

Si

Al

P S Cl Ar

N O F Ne

He

Precipitation

modificationPrecipitate

formersSurface

protection

Grain-

boundary

phases

Grain-boundary

strengthening

Solid-solution

strengthening

B C

Mg

Ca

Y Zr

Ce,

La,

etc.

Fig. 3 Alloying elements used in nickel-base superalloys. The height of the element blocks indicatesthe amount that may be present. Beneficial trace elements are marked with cross hatching and

harmful trace elements are marked with horizontal line hatching.

294 / Stainless Steels and Heat-Resistant Alloys

Superalloy Systems

The three types of superalloys—iron-nickel-, nickel-, and cobalt-base—

may be further subdivided into cast and wrought. A large number of alloys

have been invented and studied; many have been patented. However, the

many alloys have been winnowed down over the years, and only a few are

extensively used. Alloy usage is a function of industry (gas turbines, steam

turbines, etc.). Not all alloys can be mentioned; examples of older and

newer alloys will be used to demonstrate the physical metallurgy response

of superalloy systems. Representative superalloys and compositions

emphasizing alloys developed in the United States are listed in Tables 2

and 3.

Table 1(b) Role of alloying elements in superalloys

Effect(a) Iron-base Cobalt-base Nickel-base

Solid-solution strengtheners Cr, Mo Nb, Cr, Mo, Ni, W, Ta Co, Cr, Fe, Mo, W, Ta, Re

fcc matrix stabilizers C, W, Ni Ni …Carbide form:

MC Ti Ti W, Ta, Ti, Mo, Nb, Hf

M7C3 … Cr Cr

M23C6 Cr Cr Cr, Mo, W

M6C Mo Mo, W Mo, W, Nb

Carbonitrides: M(CN) C, N C, N C, N

Promotes general precipitation of carbides P … …Forms γ ′ Ni3(Al, Ti) Al, Ni, Ti … Al, Ti

Retards formation of hexagonal η (Ni3Ti) Al, Zr … …Raises solvus temperature of γ ′ … … Co

Hardening precipitates and/or intermetallics Al, Ti, Nb Al, Mo, Ti(b), W, Ta Al, Ti, Nb

Oxidation resistance Cr Al, Cr Al, Cr, Y, La, Ce

Improve hot corrosion resistance La, Y La, Y, Th La, Th

Sulfidation resistance Cr Cr Cr, Co, Si

Improves creep properties B … B, Ta

Increases rupture strength B B, Zr B(c)

Grain-boundary refiners … … B, C, Zr, Hf

Facilitates working … Ni3Ti …Retard γ ′ coarsening … … Re

(a) Not all these effects necessarily occur in a given alloy. (b) Hardening by precipitation of Ni3Ti also occurs if sufficient Ni is present. (c) If present

in large amounts, borides are formed

Table 1(a) Common ranges of major alloyingadditions in superalloys

Range, %

Element Fe-Ni- and Ni-base Co-base

Chromium 5–25 19–30

Molybdenum, tungsten 0–12 0–11

Aluminum 0–6 0–4.5

Titanium 0–6 0–4

Cobalt 0–20 …Nickel … 0–22

Niobium 0–5 0–4

Tantalum 0–12 0–9

Rhenium 0–6 0–2

Superalloys / 295

Table 2 Nominal compositions of wrought superalloys

Composition, %

UNS

Alloy No. Cr Ni Co Mo W Nb Ti Al Fe C Other

Solid-solution alloys

Iron-nickel-base

Alloy N-155 (Multimet) R30155 21.0 20.0 20.0 3.00 2.5 1.0 … … 32.2 0.15 0.15 N, 0.2 La, 0.02 Zr

Haynes 556 R30556 22.0 21.0 20.0 3.0 2.5 0.1 … 0.3 29.0 0.10 0.50 Ta, 0.02 La,

0.002 Zr

19-9 DL S63198 19.0 9.0 … 1.25 1.25 0.4 0.3 … 66.8 0.30 1.10 Mn, 0.60 Si

Incoloy 800 N08800 21.0 32.5 … … … … 0.38 0.38 45.7 0.05 …Incoloy 800H N08810 21.0 33.0 … … … … … … 45.8 0.08 …Incoloy 800HT N08811 21.0 32.5 … … … … 0.4 0.4 46.0 0.08 0.8 Mn, 0.5 Si, 0.4 Cu

Incoloy 801 N08801 20.5 32.0 … … … … 1.13 … 46.3 0.05 …Incoloy 802 N08802 21.0 32.5 … … … … 0.75 0.58 44.8 0.35 …

Nickel-base

Haynes 214 N07214 16.0 76.5 … … … … … 4.5 3.0 0.03

Haynes 230 N06230 22.0 55.0 5.0 max 2.0 14.0 … … 0.35 3.0 max 0.10 0.015 max B, 0.02 La

Inconel 600 N06600 15.5 76.0 … … … … … … 8.0 0.08 0.25 Cu

Inconel 601 N06601 23.0 60.5 … … … … … 1.35 14.1 0.05 0.5 Cu

Inconel 617 N06617 22.0 55.0 12.5 9.0 … … … 1.0 … 0.07 …Inconel 625 N06625 21.5 61.0 … 9.0 … 3.6 0.2 0.2 2.5 0.05 …RA 333 N06333 25.0 45.0 3.0 3.0 3.0 … … … 18.0 0.05 …Hastelloy B N10001 1.0 max 63.0 2.5 max 28.0 … … … … 5.0 0.05 max 0.03 V

Hastelloy N N10003 7.0 72.0 … 16.0 … … 0.5 max … 5.0 max 0.06

Hastelloy S N06635 15.5 67.0 … 15.5 … … … 0.2 1.0 0.02 max 0.02 La

Hastelloy W N10004 5.0 61.0 2.5 max 24.5 … … … … 5.5 0.12 max 0.6 V

Hastelloy X N06002 22.0 49.0 1.5 max 9.0 0.6 … … 2.0 15.8 0.15 …Hastelloy C-276 N10276 15.5 59.0 … 16.0 3.7 … … … 5.0 0.02 max …Haynes HR-120 N08120 25.0 37.0 3.0 2.5 2.5 0.7 … 0.1 33.0 0.05 0.7 Mn, 0.6 Si, 0.2 N,

0.004 B

Haynes HR-160 N12160 28.0 37.0 29.0 … … … … … 2.0 0.05 2.75 Si, 0.5 Mn

Nimonic 75 N06075 19.5 75.0 … … … … 0.4 0.15 2.5 0.12 0.25 max Cu

Nimonic 86 … 25.0 65.0 … 10.0 … … … … … 0.05 0.03 Ce, 0.015 Mg

Cobalt-base

Haynes 25 (L605) R30605 20.0 10.0 50.0 … 15.0 … … … 3.0 0.10 1.5 Mn

Haynes 188 R30188 22.0 22.0 37.0 … 14.5 … … … 3.0 max 0.10 0.90 La

Alloy S-816 R30816 20.0 20.0 42.0 4.0 4.0 4.0 … … 4.0 0.38 …MP35-N R30035 20.0 35.0 35.0 10.0 … … … … … … …MP159 R30159 19.0 25.0 36.0 7.0 … 0.6 3.0 0.2 9.0 … …Stellite B N07718 30.0 1.0 61.5 … 4.5 … … … 1.0 1.0 …UMCo-50 … 28.0 … 49.0 … … … … … 21.0 0.12 …

Precipitation-hardening alloys

Iron-nickel-base

A-286 S66286 15.0 26.0 … 1.25 … … 2.0 0.2 55.2 0.04 0.005B, 0.3 V

Discaloy S66220 14.0 26.0 … 3.0 … … 1.7 0.25 55.0 0.06 …Incoloy 903 N19903 0.1 max 38.0 15.0 0.1 … 3.0 1.4 0.7 41.0 0.04 …Pyromet CTX-1 … 0.1 max 37.7 16.0 0.1 … 3.0 1.7 1.0 39.0 0.03 …Incoloy 907 N19907 … 38.4 13.0 … … 4.7 1.5 0.03 42.0 0.01 0.15 Si

Incoloy 909 N19909 … 38.0 13.0 … … 4.7 1.5 0.03 42.0 0.01 0.4 Si

Incoloy 925 N09925 20.5 44.0 … 2.8 … … 2.1 0.2 29 0.01 1.8 Cu

V-57 … 14.8 27.0 … 1.25 … … 3.0 0.25 48.6 0.08 max 0.01 B, 0.5 max V

W-545 S66545 13.5 26.0 … 1.5 … … 2.85 0.2 55.8 0.08 max 0.05 B

Nickel-base

Astroloy N13017 15.0 56.5 15.0 5.25 … … 3.5 4.4 <0.3 0.06 0.03 B, 0.06 Zr

Custom Age 625 PLUS N07716 21.0 61.0 … 8.0 … 3.4 1.3 0.2 5.0 0.01 …Haynes 242 … 8.0 62.5 2.5 max 25.0 … … … 0.5 max 2.0 max 0.10 max 0.006 max B

Haynes 263 N07263 20.0 52.0 … 6.0 … … 2.4 0.6 0.7 0.06 0.6 Mn, 0.4 Si, 0.2 Cu

Haynes R-41 N07041 19.0 52.0 11.0 10.0 … … 3.1 1.5 5.0 0.09 0.5 Si, 0.1 Mn,

0.006 B

Inconel 100 N13100 10.0 60.0 15.0 3.0 … … 4.7 5.5 <0.6 0.15 1.0 V, 0.06 Zr, 0.015 B

Inconel 102 N06102 15.0 67.0 … 2.9 3.0 2.9 0.5 0.5 7.0 0.06 0.005 B, 0.02 Mg,

0.03 Zr

(continued)

Iron-Nickel-Base. The most important class of iron-nickel-base super-

alloys includes those strengthened by intermetallic compound precipitation

in an fcc matrix. The most common precipitate is γ ′, typified by A-286, V-

57, or Incoloy 901. Some alloys, typified by Inconel (IN) 718, which pre-

cipitate γ ″, were formerly classed as iron-nickel-base but now are

considered to be nickel-base. Other iron-nickel-base superalloys consist

of modified stainless steels primarily strengthened by solid-solution hard-

ening. Alloys in this last category vary from 19-9DL (18-8 stainless with

slight chromium and nickel adjustments, additional solution hardeners,

and higher carbon) to Incoloy 800H (21% chromium, high nickel with

small additions of titanium and aluminium, which yields some γ ′ phase).

Nickel-Base. The most important class of nickel-base superalloys is

that strengthened by intermetallic-compound precipitation in an fcc

matrix. For nickel-titanium/aluminum alloys, the strengthening precipitate

is γ ′. Such alloys are typified by the wrought alloys Waspaloy and Udimet

(U) 720, or by the cast alloys René 80 and IN 713. For nickel-

niobium alloys, the strengthening precipitate is γ ″. These alloys are typified

by IN 718. Some nickel-base alloys may contain both niobium plus

296 / Stainless Steels and Heat-Resistant Alloys

Table 2 (continued)

Composition, %

UNS

Alloy No. Cr Ni Co Mo W Nb Ti Al Fe C Other

Precipitation-hardening alloys (continued)

Nickel-base (continued)

Incoloy 901 N09901 12.5 42.5 … 6.0 … … 2.7 … 36.2 0.10 max …Inconel 702 N07702 15.5 79.5 … … … … 0.6 3.2 1.0 0.05 0.5 Mn, 0.2 Cu, 0.4 Si

Inconel 706 N09706 16.0 41.5 … … … … 1.75 0.2 37.5 0.03 2.9 (Nb + Ta), 0.15

max Cu

Inconel 718 N07718 19.0 52.5 … 3.0 … 5.1 0.9 0.5 18.5 0.08 max 0.15 max Cu

Inconel 721 N07721 16.0 71.0 … … … … 3.0 … 6.5 0.04 2.2 Mn, 0.1 Cu

Inconel 722 N07722 15.5 75.0 … … … … 2.4 0.7 7.0 0.04 0.5 Mn, 0.2 Cu, 0.4 Si

Inconel 725 N07725 21.0 57.0 … 8.0 … 3.5 1.5 0.35 max 9.0 0.03 max

Inconel 751 N07751 15.5 72.5 … … … 1.0 2.3 1.2 7.0 0.05 0.25 max Cu

Inconel X-750 N07750 15.5 73.0 … … … 1.0 2.5 0.7 7.0 0.04 0.25 max Cu

M-252 N07252 19.0 56.5 10.0 10.0 … … 2.6 1.0 <0.75 0.15 0.005 B

Nimonic 80A N07080 19.5 73.0 1.0 … … … 2.25 1.4 1.5 0.05 0.10 max Cu

Nimonic 90 N07090 19.5 55.5 18.0 … … … 2.4 1.4 1.5 0.06 …Nimonic 95 … 19.5 53.5 18.0 … … … 2.9 2.0 5.0 max 0.15 max +B, +Zr

Nimonic 100 … 11.0 56.0 20.0 5.0 … … 1.5 5.0 2.0 max 0.30 max +B, +Zr

Nimonic 105 … 15.0 54.0 20.0 5.0 … … 1.2 4.7 … 0.08 0.005 B

Nimonic 115 … 15.0 55.0 15.0 4.0 … … 4.0 5.0 1.0 0.20 0.04 Zr

C-263 N07263 20.0 51.0 20.0 5.9 … … 2.1 0.45 0.7 max 0.06 …Pyromet 860 … 13.0 44.0 4.0 6.0 … … 3.0 1.0 28.9 0.05 0.01 B

Pyromet 31 N07031 22.7 55.5 … 2.0 … 1.1 2.5 1.5 14.5 0.04 0.005 B

Refractaloy 26 … 18.0 38.0 20.0 3.2 … … 2.6 0.2 16.0 0.03 0.015 B

René 41 N07041 19.0 55.0 11.0 10.0 … … 3.1 1.5 <0.3 0.09 0.01B

René 95 … 14.0 61.0 8.0 3.5 3.5 3.5 2.5 3.5 <0.3 0.16 0.01 B, 0.05 Zr

René 100 … 9.5 61.0 15.0 3.0 … … 4.2 5.5 1.0 max 0.16 0.015 B, 0.06 Zr, 1.0 V

Udimet 500 N07500 19.0 48.0 19.0 4.0 … … 3.0 3.0 4.0 max 0.08 0.005 B

Udimet 520 … 19.0 57.0 12.0 6.0 1.0 … 3.0 2.0 … 0.08 0.005 B

Udimet 630 … 17.0 50.0 … 3.0 3.0 6.5 1.0 0.7 18.0 0.04 0.004 B

Udimet 700 … 15.0 53.0 18.5 5.0 … … 3.4 4.3 <1.0 0.07 0.03 B

Udimet 710 … 18.0 55.0 14.8 3.0 1.5 … 5.0 2.5 … 0.07 0.01 B

Unitemp AF2-1DA N07012 12.0 59.0 10.0 3.0 6.0 … 3.0 4.6 <0.5 0.35 1.5 Ta, 0.015 B, 0.1 Zr

Waspaloy N07001 19.5 57.0 13.5 4.3 … … 3.0 1.4 2.0 max 0.07 0.006 B, 0.09 Zr

Superalloys / 297

Table 3 Nominal compositions of cast superalloys

Nominal composition, %

Alloy designation C Ni Cr Co Mo Fe Al B Ti Ta W Zr Other

Nickel base

B-1900 0.1 64 8 10 6 … 6 0.015 1 4(a) … 0.10 …CMSX-2 … 66.2 8 4.6 0.6 … 56 … 1 6 8 6 …Hastelloy X 0.1 50 21 1 9 18 … … … … 1 … …Inconel 100 0.18 60.5 10 15 3 … 5.5 0.01 5 … … 0.06 1 V

Inconel 713C 0.12 74 12.5 … 4.2 … 6 0.012 0.8 1.75 … 0.1 0.9 Nb

Inconel 713LC 0.05 75 12 … 4.5 … 6 0.01 0.6 4 … 0.1 …Inconel 738 0.17 61.5 16 8.5 1.75 … 3.4 0.01 3.4 … 2.6 0.1 2 Nb

Inconel 792 0.2 60 13 9 2.0 … 3.2 0.02 4.2 … 4 0.1 2 Nb

Inconel 718 0.04 53 19 … 3 18 0.5 … 0.9 … … … 0.1 Cu, 5 Nb

X-750 0.04 73 15 … … 7 0.7 … 2.5 … … … 0.25 Cu, 0.9 Nb

M-252 0.15 56 20 10 10 … 1 0.005 2.6 … … … …MAR-M 200 0.15 59 9 10 … 1 5 0.015 2 … 12.5 0.05 1 Nb(b)

MAR-M 246 0.15 60 9 10 2.5 … 5.5 0.015 1.5 1.5 10 0.05 …MAR-M 247 0.15 59 8.25 10 0.7 0.5 5.5 0.015 1 3 10 0.05 1.5 Hf

PWA 1480 … bal 10 5.0 … … 5.0 … 1.5 12 4.0 … …René 41 0.09 55 19 11.0 10.0 … 1.5 0.01 3.1 … … … …René 77 0.07 58 15 15 4.2 … 4.3 0.015 3.3 … … 0.04 …René 80 0.17 60 14 9.5 4 … 3 0.015 5 … 4 0.03 …René 80 Hf 0.08 60 14 9.5 4 … 3 0.015 4.8 … 4 0.02 0.75 HfRené 100 0.18 61 9.5 15 3 … 5.5 0.015 4.2 … … 0.06 1 VRené N4 0.06 62 9.8 7.5 1.5 … 4.2 0.004 3.5 4.8 6 … 0.5 Nb, 0.15 HfUdimet 500 0.1 53 18 17 4 2 3 … 3 … … … …Udimet 700 0.1 53.5 15 18.5 5.25 … 4.25 0.03 3.5 … … … …Udimet 710 0.13 55 18 15 3 … 2.5 … 5 … 1.5 0.08 …Waspaloy 0.07 57.5 19.5 13.5 4.2 1 1.2 0.005 3 … … 0.09 …WAX-20 (DS) 0.20 72 … … … … 6.5 … … … 20 1.5 …

Cobalt-base

AiResist 13 0.45 … 21 62 … … 3.4 … … 2 11 … 0.1 Y

AiResist 213 0.20 0.5 20 64 … 0.5 3.5 … … 6.5 4.5 0.1 0.1 Y

AiResist 215 0.35 0.5 19 63 … 0.5 4.3 … … 7.5 4.5 0.1 0.1 Y

FSX-414 0.25 10 29 52.5 … 1 … 0.010 … … 7.5 … …Haynes 21 0.25 3 27 64 … 1 … … … … … … 5 Mo

Haynes 25; L-605 0.1 10 20 54 … 1 … … … … 15 … …J-1650 0.20 27 19 36 … … … 0.02 3.8 2 12 … …MAR-M 302 0.85 … 21.5 58 … 0.5 … 0.005 … 9 10 0.2 …MAR-M 322 1.0 … 21.5 60.5 … 0.5 … … 0.75 4.5 9 2 …MAR-M 509 0.6 10 23.5 54.5 … … … … 0.2 3.5 7 0.5 …MAR-M 918 0.05 20 20 52 … … … … … 7.5 … 0.1 …NASA Co-W-Re 0.40 … 3 67.5 … … … … 1 … 25 1 2 Re

S-816 0.4 20 20 42 … 4 … … … … 4 … 4 Mo, 4 Nb,

1.2 Mn, 0.4 Si

V-36 0.27 20 25 42 … 3 … … … … 2 … 4 Mo, 2 Nb,

1 Mn, 0.4 Si

WI-52 0.45 … 21 63.5 … 2 … … … … 11 … 2 Nb + Ta

X-40 (Stellite alloy 31) 0.50 10 22 57.5 … 1.5 … … … … 7.5 … 0.5 Mn, 0.5 Si

(a) B-1900 + Hf also contains 1.5% Hf. (b) MAR-M 200 + Hf also contains 1.5% Hf.

titanium and/or aluminum and utilize both γ ′ and γ ″ precipitates in

strengthening. Alloys of this type are IN 706 and IN 909. Another class

of nickel-base superalloys is essentially solid-solution strengthened.

Such alloys are Hastelloy X and IN 625. The solid-solution strengthened

nickel-base alloys may derive some additional strengthening from carbide

and/or intermetallic-compound precipitation. A third class includes oxide-

dispersion strengthened (ODS) alloys such as IN MA-754 and IN MA-

6000, which are strengthened by dispersion of inert particles such as

yttria, coupled in some cases with γ ′ precipitation (MA 6000).

298 / Stainless Steels and Heat-Resistant Alloys

Nickel-base superalloys are utilized in both cast and wrought forms,

although special processing (powder metallurgy/isothermal forging) is

frequently used to produce wrought versions of the more highly alloyed

compositions (René 95, Astroloy, IN 100). An additional dimension of

nickel-base superalloys has been the introduction of grain-aspect ratio and

orientation as a means of controlling properties. In some instances, in fact,

grain boundaries have been removed (e.g., investment cast single crystal

alloys). Wrought P/M alloys of the ODS class and cast alloys such as

MAR-M 247 have demonstrated property improvements owing to control

of grain morphology by directional recrystallization or solidification.

Cobalt-Base. The cobalt-base superalloys are invariably strengthened

by a combination of carbides and solid-solution hardeners. The essential

distinction in these alloys is between cast and wrought structures. Cast

alloys are typified by X-40 and wrought alloys by alloy 25 (also known as

L605). No intermetallic compound possessing the same degree of utility

as the γ ′ precipitate in nickel- or iron-nickel-base superalloys has been

found to be operative in cobalt-base systems.

Properties and Microstructure

The principal microstructural variables of superalloys are the precipitate

amount and morphology, grain size and shape, and carbide distribution.

Nickel and iron-nickel-base alloys of the titanium/aluminum type have

their properties controlled by all three variables; nickel-niobium alloys

have the additional variable of δ-phase distribution; cobalt-base super-

alloys are not affected by the first variable. Structure control is achieved

through composition selection/modification and by processing. For a

given nominal composition, there are property advantages and disadvan-

tages of the structures produced by deformation processing and by casting.

Cast superalloys generally have coarser grain sizes, more alloy segrega-

tion, and improved creep and rupture characteristics. Wrought superalloys

generally have more uniform, and usually finer, grain sizes and improved

tensile and fatigue properties.

Nickel- and iron-nickel-base superalloys of the Ni-Ti/Al type typically

consist of γ ′ dispersed in a γ matrix, and the strength increases with increas-

ing Vf γ ′. The lowest Vf amounts of γ ′ are found in iron-nickel-base and

first-generation nickel-base superalloys, where Vf γ ′ is generally less than

about 0.25 (25 vol%). The γ ′ is commonly spheroidal in lower Vf γ ′ alloys

but often cuboidal in higher Vf γ ′ (≥0.35) nickel-base superalloys. The

nickel-niobium-type superalloys typically consist of γ ″ dispersed in a γmatrix, with some γ ′ present as well. The inherent strength capability of the

γ ′-and γ ″-hardened superalloys is controlled by the intragranular distribu-

tion of the hardening phases; however, the usable strength in polycrystalline

Superalloys / 299

alloys is determined by the condition of the grain boundaries, particularly

as affected by the carbide-phase morphology and distribution, and in the

case of nickel-niobium alloys, additionally by the distribution of the δphase.

Satisfactory properties in Ni-Ti/Al alloys are achieved by optimizing the

γ ′ Vf and morphology (not necessarily independent characteristics) in con-

junction with securing a dispersion of discrete globular carbides along the

grain boundaries. Discontinuous (cellular) carbide or γ ′ at grain bound-

aries increases surface area and drastically reduces rupture life, even

though tensile and creep strength may be relatively unaffected.

Wrought nickel- and iron-nickel-base superalloys generally are

processed to have optimal tensile and fatigue properties. At one time,

when wrought alloys were used for creep-limited applications, such as gas

turbine high-pressure turbine blades, heat treatments different from those

used for tensile-limited uses were applied to the same nominal alloy com-

position to maximize creep-rupture life. Occasionally, the nominal com-

position of an alloy such as IN-100 or U-700/Astroloy varies according to

whether it is to be used in the cast or the wrought condition.

Effects of Alloying Elements. Superalloys contain a variety of ele-

ments in a large number of combinations to produce desired effects. Some

elements go into solid solution to provide one or more of the following:

strength (molybdenum, tantalum, tungsten, rhenium); oxidation resistance

(chromium, aluminum); phase stability (nickel); and increased volume

fractions of favorable secondary precipitates (cobalt). Other elements are

added to form hardening precipitates such as γ ′ (aluminum, titanium) and

γ ″ (niobium). Minor elements (carbon, boron) are added to form carbides

and borides; these and other elements (magnesium) are added for purposes

of tramp-element control. Some elements (boron, zirconium, hafnium)

also are added to promote grain-boundary effects other than precipitation

or carbide formation. Lanthanum has been added to some alloys to pro-

mote oxidation resistance, and yttrium has been added to coatings to

enhance oxidation resistance. A major addition to nickel-base superalloy

chemistry in recent years has been the element rhenium, which has

extended the temperature capability of the directionally solidified, colum-

nar grain and single crystal alloys. Rhenium appears to produce these

improvements by significantly reducing the coarsening rate for γ ′. Many

elements (cobalt, molybdenum, tungsten, rhenium, chromium, etc.),

although added for their favorable alloying qualities, can participate, in

some circumstances, in undesirable phase formation (σ, µ, Laves, etc.).

Some of these elements produce readily discernible changes in

microstructure; others produce more subtle microstructural effects. The

precise microstructural effects produced are functions of processing and

heat treatment. The most obvious microstructural effects involve precipi-

tation of geometrically close-packed (gcp) phases such as γ ′, formation of

carbides, and formation of topologically close-packed (tcp) phases such as σ.

300 / Stainless Steels and Heat-Resistant Alloys

20

10

0Ch

rom

ium

, %

Fig. 4 Qualitative description of the evolution of microstructure and chromium content ofnickel-base superalloys

Even when the type of phase is specified, microstructure morphology can

vary widely—for example, script versus blocky carbides, cuboidal versus

spheroidal γ ′, cellular versus uniform precipitation, acicular versus blocky

σ, and discrete γ ′ versus γ ′ envelopes. Typical nickel-base superalloy

microstructures, as they evolved from spheroidal to cuboidal γ ′, are

depicted in Fig. 4.

The γ ′ phase is an ordered (L12) intermetallic fcc phase having the basic

composition Ni3(Al,Ti). Alloying elements affect γ ′ mismatch with the

matrix γ phase, γ ′ antiphase-domain-boundary (APB) energy, γ ′ mor-

phology, and γ ′ stability. A related phase, η, is an ordered (D024) hexago-

nal phase of composition Ni3Ti that may exist in a metastable form as γ ′before transforming to η. Other types of intermetallic phases, such as δ,

orthorhombic Ni3Nb, or γ ″, bct ordered (D022) Ni3Nb strengthening pre-

cipitate, have been observed.

Carbides also are an important constituent of superalloys. They are par-

ticularly essential in the grain boundaries of cast polycrystalline alloys for

production of desired strength and ductility characteristics. Carbide levels in

wrought alloys always have been below those in cast alloys, but some car-

bide has been deemed desirable for achieving optimal strength properties.

As cleanliness of superalloys has increased, the carbide levels in wrought

alloys have been lowered. Carbides, at least large ones, become the limiting

fracture mechanics criteria for modern wrought superalloy application.

Carbides may provide some degree of matrix strengthening, particularly

in cobalt-base alloys, and are necessary for grain-size control in some

wrought alloys. Some carbides are virtually unaffected by heat treatment,

while others require such a step to be present. Various types of carbides

are possible depending on alloy composition and processing. Some of the

Superalloys / 301

important types are MC, M6C, M23C6, and M7C3, where M stands for one

or more types of metal atom. In many cases, the carbides exist jointly;

however, they usually are formed by sequential reactions in the solid state

following break-down of the MC that normally is formed in the molten

state. The common carbide-reaction sequence for many superalloys is MC

to M23C6, and the important carbide-forming elements are chromium

(M23C6, M7C3); titanium, tantalum, niobium, and hafnium (MC); and

molybdenum and tungsten (M6C). Boron may participate somewhat inter-

changeably with carbon and produces such phases as MB12, M3B2, and

others. One claim made for boron is that primary borides formed by

adjustment of boron/carbon ratio are more amenable to morphological

modification through heat treatment.

Alloying Elements to Improve Oxidation Resistance. All super-

alloys contain some chromium plus other elements to promote resistance

to environmental degradation. The role of chromium is to promote Cr2O3

formation on the external surface of an alloy. When sufficient aluminum

is present, formation of the more protective oxide Al2O3 is promoted when

oxidation occurs. A chromium content of 6 to 22 wt% generally is com-

mon in nickel-base superalloys, whereas a level of 20 to 30 wt% Cr is

characteristic of cobalt-base superalloys, and a level of 15 to 25 wt% Cr

is found in iron-nickel-base superalloys. Amounts of aluminum up to

approximately 6 wt% can be present in nickel-base superalloys. High tan-

talum (>3%) and low titanium (<1%) contents are also recommended for

oxidation-resistant nickel-base alloys.

Effect of Boron, Zirconium, and Hafnium. Within limits, significant

improvements in mechanical properties can be achieved by additions of

boron, zirconium, and hafnium. However, only limited microstructural

correlations can be made. The presence of these elements may modify the

initial grain-boundary carbides or tie up deleterious elements such as sul-

fur and lead. Reduced grain-boundary diffusion rates may be obtained,

with consequent suppression of carbide agglomeration and creep crack-

ing. Hafnium contributes to the formation of more γ-γ ′ eutectic in cast

alloys; the eutectic at grain boundaries is thought (in modest quantities) to

contribute to alloy ductility. The effects of these elements are limited to

nickel- and iron-nickel-base alloys; virtually no cobalt-base alloys contain

them. Hafnium in particular contributes strongly to improved ductility in

transverse boundaries in directionally solidified, columnar grain alloys.

Effect of Trace Elements. The main trace elements present in superal-

loys are shown in Figure 3. A simple classification of the trace and minor

element additions in superalloys is given in Table 4.

The presence of gases can give rise to void formation via migration and

coalescence with the subsequent weakening of the alloy. For example,

oxygen has been shown to react with carbides to form either CO or CO2

302 / Stainless Steels and Heat-Resistant Alloys

Table 4 Classification of trace and minor elements found insuperalloys

Category Elements

Detrimental elements

Residual gases O, H, N, Ar, He

Nonmetallic impurities S, P

Metallic or metalloid impurities Pb, Bi, Sb, As, Se, Ag, Cu, Tl, Te

Beneficial elements

Refining aids Ca, Mg, Ce, La

Minor and ppm alloying additions B, Zr, Hf, Mg, C

Alloying addition up to 1.5% Hf, Zr

bubbles at grain boundaries. Of the residual gases, oxygen introduced dur-

ing heat treatment, high-temperature testing, or service, is known to cause

reductions in ductility while oxides picked up from the melting crucible

can limit the useful strength of alloys subjected to cyclic stressing.

Most superalloys contain strong nitride formers such as titanium; the

effects of small amounts of TiN can be beneficial in cobalt-base alloys (e.g.,

acting as nuclei for the formation of MC type carbides or inducing strain

aging to increase the strength of the alloy, although at the expense of duc-

tility). Nickel-base superalloys that contain large amounts of nitrogen are,

however, prone to excessive microporosity, which leads to reduced ductili-

ty and rupture life. This problem is most frequently associated with revert

alloys, that is, alloys made from superalloy scrap. A study of MAR-M002

doped with 24 and 50 ppm of nitrogen showed the increased porosity and a

change in the carbide morphology from an acicular to a blocky form. Many

of these blocky carbides have cored structures with a titanium-rich central

region, which means that the reaction between titanium and nitrogen to

form TiN particles is the first stage leading to high-porosity castings. These

nitride particles act as profuse nucleants for carbide formation and influence

the final solidification behavior by inhibiting the liquid flow.

Both sulfur and phosphorous form eutectics with nickel. Sulfur in par-

ticular has long been known to adversely affect ductility. Elements such as

titanium, zirconium and hafnium, which are commonly present in super-

alloys, reduce the effects of sulfur by forming carbosulfides of the type

M2SC, which crystallize as flakes or plates away from the grain bound-

aries. However, excessive additions of zirconium and hafnium can also

lead to embrittlement.

The effects of the harmful metallic and metalloid elements listed in

Table 4 have been studied extensively and with the aim of minimizing

their concentrations in nickel-base alloys. The stress-rupture properties of

alloys IN-100 and MAR-M002 have been investigated after deliberate

contamination with bismuth, lead, silver, tin, and tellurium. By far the

most damaging element was found to be bismuth, followed by tellurium,

selenium, lead, and silver; the presence of tin had almost no harmful

effects. The presence of only 0.2 ppm bismuth in IN-100 was shown to

reduce its creep life by 20%.

Superalloys / 303

Elements such as calcium, magnesium, cerium, and lanthanum have

been added to superalloy melts as deoxidizers or desulfurizers, although

large residual amounts of calcium and magnesium may cause problems of

their own. Magnesium is beneficial in terms of castability, but detrimental

to the high-temperature creep-properties. As mentioned earlier, minor

additions of lanthanum or yttrium improve the oxidation resistance of

superalloys.

Mechanical Alloying

Mechanical alloying (MA) is a dry, high-energy ball-milling process for

producing composite metallic powders with a controlled, fine microstruc-

ture. It is carried out in a highly agitated ball charge by repeated cold

welding and fracturing of a mixture of metal powders to which some non-

metal powders may be added. Its widest use has been in the production of

ODS nickel- and iron-base superalloys for service at 1000 °C (1830 °F)

and above.

Unlike mechanical mixing processes, MA produces a material whose

internal homogeneity is independent of starting powder particle size.

Thus, ultrafine dispersions (<1 µm interparticle spacing) can be obtained

with relatively coarse initial powder (50–100 µm average diameter).

The Processing Path. Figure 5 is a schematic showing the path of raw

materials using the MA process. The raw materials, the type of mill used,

the process of consolidation, and the details of heat treatment differ

depending on the type of product desired, but the processing route remains

essentially the same. It is possible that some minor steps are either added

or deleted in some special circumstances. The actual process of MA starts

with mixing of powders in the right proportion and loading the powder

into the high-energy ball mill, along with the grinding medium (generally

steel balls). This mix is then milled for the desired length of time until a

Fig. 5 Processing path in producing a product from powders by mechanical alloying

304 / Stainless Steels and Heat-Resistant Alloys

steady state is reached. A steady state occurs when the composition of

every powder particle is the same as the proportion of the elements in the

starting powder mix. Sometimes, the powder is milled to an intermediate

state either to form metastable phases or to achieve certain desired prop-

erties. The milled powder is then consolidated into a bulk shape and heat

treated to obtain the desired microstructure and properties.

The raw materials used for mechanically alloyed dispersion-

strengthened superalloys are widely available commercially pure powders

that have particle sizes that vary from about 1 to 200 µm. These powders

fall into the broad categories of pure metals, master alloys, and refractory

compounds. The pure metals include nickel, chromium, iron, cobalt, tung-

sten, molybdenum, and niobium. The master alloys include nickel-base

alloys with relatively large amounts of combinations of aluminum, titanium,

zirconium, or hafnium.

These master alloys are relatively brittle when cast and easily milled

into powder. In addition, because they consist of relatively exothermic

intermetallic compounds, the thermodynamic activity of the reactive

alloying elements, such as aluminum and titanium, is considerably

reduced compared to that of the pure metals.

A typical powder mixture may consist of fine (4–7 µm) nickel powder,

−150 µm chromium, and −150 µm master alloy. The master alloy may

contain a wide range of elements selected for their role as alloying con-

stituents or for gettering of contaminants. About 2 vol% of very fine yttria,

Y2O3 (25 nm, or 250 Å) is added to form the dispersoid. The yttriabecomes entrapped along the weld interfaces between fragments in thecomposite metal powders. After completion of the power milling, a uni-form interparticle spacing of about 0.5 µm is achieved.

The oxygen contents of the commercially pure metal powders and themaster alloys range from 0.05 to 0.2 wt%. The refractory compounds thatcan be added include carbides, nitrides, and oxides. For the production ofdispersion-strengthened materials, such additions are limited to very stableoxides, such as yttria, alumina, or, less frequently, thoria. These oxides,which are prepared by calcination of oxalate precipitates, consist of crys-tallites of about 50 nm agglomerated into pseudomorphs of about 1 µm.

The only restriction on the mixture of powder particles for mechanicalalloying (other than the particle size range mentioned above and the needto minimize excessive oxygen) is that at least 15 vol% of the mix shouldconsist of a compressibly deformable metal powder. The function of thiscomponent, which can consist of any one or all of the pure metals, is toact as a host or binder for the other constituents during the process.

Commercial Alloys. The most commercially important nickel-basemechanically alloyed ODS alloys include MA 754, MA 758, MA 6000,and, to a lesser extent, MA 760 (the latter alloy is in limited production).Compositions of these alloys are given in Table 5.

Superalloys / 305

The most significant advantage of ODS superalloys is the increasedstress rupture properties. Figure 6 compares the 1000 h specific rupturestrength (strength/density) for two ODS alloys (MA 6000 and thoria-dispersed nickel) and two cast alloys (directionally solidified Mar-M200+ Hf) and single-crystal PWA 454). It is clear from this figure that the MA6000 alloy can maintain a given stress for a much longer time than a cast-ing alloy for similar vane applications. This is mainly due to the benefitsof the combined strengthening modes (γ ′ and ODS) in the mechanicallyalloyed material.

Effects of Alloying Elements and Intermetallic Phases on Welding

Nickel- and iron-nickel-base superalloys are considerably less weldablethan cobalt-base superalloys. Because of the presence of the γ ′ strengthening

Table 5 Nominal compositions (wt%) of mechanically alloyed nickel-base superalloys

Alloy Ni Cr Al Ti Mo W Y2O3 Ta

MA 754 bal 20 0.3 0.5 … … 0.6 …MA 757 bal 16 4.0 0.5 … … 0.6 …MA 758 bal 30 0.3 0.5 … … 0.6 …MA 760 bal 20 6.0 … 2.0 3.5 0.95 …MA 6000 bal 15 4.5 0.5 2.0 4.0 1.1 2.0TMO-2(a) bal 6 4.2 0.8 2.0 12.4 1.1 4.7

(a) This alloy additionally contains 9.7 wt% cobalt.

Fig. 6 Comparison of 1000 h specific rupture strength of MA 6000 withdirectionally solidified Mar-M200 + Hf, TD-Ni, and single-crystal

PWA 454

306 / Stainless Steels and Heat-Resistant Alloys

René 62

Inconel X-750

Mar-M200

Udimet 700

Astroloy

AF2-1DA

Udimet 600

GMR 235 Inconel 700

Unitemp 1753René 41

Waspaloy

M-252

Inconel XInconel 718

Difficult to weld:weld and strain-age cracking

Udimet 500

Readilyweldable

0 1 2 3 4 5 6

Titanium, wt%

6

5

4

3

2

1

0

Alu

min

um

, w

t%

713C IN-100

Fig. 7 Weldability diagram for some γ ′-strengthened iron-nickel- and nickel-base superalloys, showing influence of total aluminum + titanium

hardeners

phase, the alloys tend to be susceptible to hot cracking (weld cracking) andpostweld heat treatment (PWHT) cracking (strain age or delay cracking).The susceptibility to hot cracking is directly related to the aluminum andtitanium contents (γ ′ formers). Hot cracking occurs in the weld heat-affected zone (HAZ), and the extent of cracking varies with alloy compo-sition and weldment restraint. Welding is normally restricted to the lowerVf γ ′ (≤ 0.35) alloys generally in the wrought condition.

Figure 7 shows a plot of weldability as a function of (Al + Ti) content.Those alloys with low aluminum and titanium contents, shown below thediagonal line, are more readily weldable and less susceptible to strain-agecracking. However, as combined aluminum and titanium is increased,welding becomes more difficult, and strain-age cracking is more likely.Alloys like René 41 and Waspaloy are borderline; they weld with rela-

tively little difficulty but sometimes crack during postweld heat treating.

Casting alloys with high aluminum and titanium, like 713C and IN-100,

have low ductility at all temperatures and usually crack during welding,

although repair welding (e.g., turbine-blade and vane-tip restoration) can

be carried out if nonhardenable filler metal is used.

Because of their γ ′ strengthening mechanism and capability, many nickel-

and iron-nickel-base superalloys are welded in the solution heat treated

condition. Special preweld heat treatments have been used for some

alloys. Nickel-niobium alloys, as typified by IN-718, have unique welding

characteristics. The hardening phase, γ ″, is precipitated more sluggishly

at a lower temperature than is γ ′ so that the attendant welding-associated

strains that must be redistributed are more readily accommodated in the

weld metal and HAZ. The alloy is welded in the solution-treated con-

dition and then given a postweld stress-relief-and-aging treatment that

Superalloys / 307

causes γ ′ precipitation. The superior weldability of IN-718 has largelycontributed to the popularity of the alloy. It is one of the most widely usedsuperalloys.

SELECTED REFERENCES

• M.J. Donachie and S.J. Donachie, Superalloys, Metals Handbook

Desk Edition, 2nd ed., J.R. Davis, Ed., ASM International, 1998,p 394–413 (Note: this article contains an extensive bibliography onsuperalloy processing, microstructures, and properties.)

• M.J. Donachie and S.J. Donachie, Ed., Superalloys: A Technical

Guide, 2nd ed., ASM International, to be published in 2002• J.R. Davis, Ed., ASM Specialty Handbook: Heat-Resistant Materials,

ASM International, 1997