tribaloy 800 a

Metallography and Microstructuresof Cobalt and Cobalt AlloysRevised by D. Klarstrom, Haynes International, Inc.; P. Crook, Haynes International, Inc; and J. Wu, Deloro Stellite Group Limited

COBALT is used as an alloying element inalloys for various applications such as:

● Permanent and soft magnetic materials● Superalloys for creep resistance at high-tem-

perature● Hardfacing and wear-resistant alloys● Corrosion-resistant alloys● High-speed steels, tool steels, and other steels● Cobalt-base tool materials (e.g., the matrix of

cemented carbides)● Electrical-resistance alloys● High-temperature spring and bearing alloys● Magnetostrictive alloys● Special expansion and constant-modulus al-

loys

● Biocompatible materials for use as orthopedicimplants or dental materials

Historically, many of the commercial cobalt-base alloys are derived from the Co-Cr-W andCo-Cr-Mo ternaries first investigated in the earlytwentieth century by Elwood Haynes. He dis-covered the high strength and stainless nature ofbinary cobalt-chromium alloys and first patentedcobalt-chromium alloys in 1907. He later iden-tified tungsten and molybdenum as powerfulstrengthening agents within the cobalt-chro-mium system. These developments led to vari-ous cobalt-base alloys for corrosion and high-temperature applications in the 1930s and early1940s. Of the corrosion-resistant alloys, a Co-

Cr-Mo alloy with a moderately low carbon con-tent was developed to satisfy the need for a suit-able investment cast dental material. Thisbiocompatible material, which has the tradenameVitallium, is in use today for surgical implants.In the 1940s, this same alloy also underwent in-vestment casting trials for World War II aircraftturbocharger blades, and, with modifications toenhance structural stability, was used success-fully for many years in this and other elevated-temperature applications. This early high-tem-perature material, Stellite alloy 21, is still in usetoday, but predominantly as an alloy for wearresistance.

A current overview on cobalt-base alloys is inRef 1. This article describes the metallurgy, met-

Table 1(a) Compositions of various wear-resistant cobalt-base alloys

Alloy tradename(a)UNSNo.

Nominal composition, wt%

Co Cr W Mo C Fe Ni Si Mn Others

Cast, P/M, and weld overlay wear-resistant alloys

Stellite 1 R30001 bal 30 13 0.5 2.5 3 1.5 1.3 0.5 . . .Stellite 3 (P/M) R30103 bal 30.5 12.5 . . . 2.4 5 (max) 3.5 (max) 2 (max) 2 (max) 1 B (max)Stellite 4 R30404 bal 30 14 1 (max) 0.57 3 (max) 3 (max) 2 (max) 1 (max) . . .Stellite 6 R30006 bal 29 4.5 1.5 (max) 1.2 3 (max) 3 (max) 1.5 (max) 1 (max) . . .Stellite 6 (P/M) R30106 bal 28.5 4.5 1.5 (max) 1 5 (max) 3 (max) 2 (max) 2 (max) 1 B (max)Stellite 12 R30012 bal 30 8.3 . . . 1.4 3 (min) 1.5 0.7 2.5 . . .Stellite 21 R30021 bal 27 . . . 5.5 0.25 3 (max) 2.75 1 (max) 1 (max) 0.007 B (max)Stellite 98M2 (P/M) . . . bal 30 18.5 0.8 (max) 2 5 (max) 3.5 1 (max) 1 (max) 4.2 V. 1 B (max)Stellite 190 R30014 bal 27 14 1.0 (max) 3.5 3 (max) 4.0 (max) 2 (max) 1 (max) . . .Stellite 703 . . . bal 32 . . . 12 2.4 3 (max) 3 (max) 1.5 (max) 1.5 (max) . . .Stellite 704 . . . bal 30 . . . 14 1.0 2 (max) 3 (max) 1 (max) 0.5 (max) . . .Stellite 706 . . . bal 29 . . . 5 1.2 3 (max) 3 (max) 1.5 (max) 1.5 (max) . . .Stellite 712 . . . bal 29 . . . 8.5 2 3 (max) 3 (max) 1.5 (max) 1.5 (max) . . .Stellite 720 . . . bal 33 . . . 18 2.5 3 (max) 3 (max) 1.5 (max) 1.5 (max) 0.3 BStellite F R30002 bal 25 12.3 1 (max) 1.75 3 (max) 22 2 (max) 1 (max) . . .Stellite Star J (P/M) R30102 bal 32.5 17.5 . . . 2.5 3 (max) 2.5 (max) 2 (max) 2 (max) 1 B (max)Stellite Star J R31001 bal 32.5 17.5 . . . 2.5 3 (max) 2.5 (max) 2 (max) 2 (max) . . .Tantung G . . . bal 29.5 16.5 . . . 3 3.5 7 (max) . . . 2 (max) 4.5 Ta/NbTantung 144 . . . bal 27.5 18.5 . . . 3 3.5 7 (max) . . . 2 (max) 5.5 Ta/Nb

Laves-phase wear-resistant alloys

Tribaloy T-400 R30400 bal 9 . . . 28 . . . . . . . . . 2.6 . . . . . .Tribaloy T-400C . . . bal 14 . . . 27 . . . . . . . . . 2.6 . . . . . .Tribaloy T-800 . . . bal 18 . . . 28 . . . . . . . . . 3.9 . . . . . .Tribaloy T-900 . . . bal 31 . . . 4.3 1.6 3.0 (max) 3.0 (max) 2.0 (max) 2.0 (max) . . .

Wrought wear-resistant alloys

Stellite 6B R30016 bal 30 4 1.5 max 1 3 (max) 2.5 0.7 1.4 . . .Stellite 6K . . . bal 30 4.5 1.5 max 1.6 3 (max) 3 (max) 2 (max) 2 (max) . . .Stellite 706K . . . bal 31 . . . 4.3 1.6 3 (max) 3 (max) 2 (max) 2 (max) . . .

(a) Stellite and Tribaloy are registered trademarks of Deloro Stellite, Inc.; Tantung is a registered trademark of Asteg Sales Pty Ltd.

ASM Handbook, Volume 9: Metallography and Microstructures G.F. Vander Voort, editor, p762–774 DOI: 10.1361/asmhba0003771

Copyright © 2004 ASM International® All rights reserved. www.asminternational.org

allography, and microstructures of cobalt alloysused for applications that require wear resis-tance, high-temperature strength, or corrosionresistance. Cobalt-base alloys designed for wearservice typically have higher carbon content thancobalt alloys designed for high-temperaturestrength and/or corrosion resistance. The basicmetallurgy and representative microstructuresfor these three major categories of cobalt-basealloys are described in this article. Typical com-positions of present-day cobalt-base alloys arelisted in Tables 1(a) and (b) for these three ap-plication areas. This article does not addresscobalt-base materials in applications such as ce-mented carbides, biomedical devices, or mag-netic/electrical devices.

Cobalt Alloy Metallurgy

Many of the properties of the alloys arisefrom the crystallographic nature of cobalt. Co-

balt provides a unique alloy base because of itsallotropic face-centered cubic (fcc) to hexagonalclose-packed (hcp) phase transformation, whichoccurs at a temperature of approximately 422�C (792 �F) (Ref 2). Alloying elements such as

iron, manganese, nickel, and carbon tend to sta-bilize the fcc structure and increase stacking-fault energy, whereas elements such as chro-mium, molybdenum, tungsten, and silicon tendto stabilize the hcp structure and decrease stack-

Table 1(b) Compositions of various heat-resistant cobalt-base alloys

Alloy tradename(a)UNSNo.

Nominal composition, wt%

Co Cr W Mo C Fe Ni Si Mn Others

Wrought heat resistant alloys

Haynes 25 (L605) R30605 bal 20 15 . . . 0.1 3 (max) 10 0.4 (max) 1.5 . . .Haynes 188 R30188 bal 22 14 . . . 0.1 3 (max) 22 0.35 1.25 0.03 LaInconel 783 R30783 bal 3 . . . . . . 0.03 (max) 25.5 28 0.5 (max) 0.5 (max) 5.5 Al, 3 Nb, 3.4 Ti (max)S-816 R30816 40 (min) 20 4 4 0.37 5 (max) 20 1 (max) 1.5 4 NbHaynes alloy 6B(c) . . . bal 30 4 1.5 (max) 1.0 3 (max) 2.5 0.7 1.4 . . .Haynes alloy 31 . . . bal 25.5 7.5 . . . 0.50 2 (max) 10.5 1 (max) 1 (max) . . .Haynes alloy 150 . . . bal 28 . . . . . . 0.05 (max) 2 (max) . . . 1 (max) 1 (max) . . .

Wrought corrosion-resistant alloys

MP35N� alloy R30035 35 20 . . . 10 . . . . . . 35 . . . . . . . . .MP159� alloy R30159 bal 19 . . . 7 . . . 9 25.5 . . . . . . 0.2 Al, 0.6 Nb, 3 Ti

Corrosion resistant alloys

Ultimet (1233) R31233 bal 26 2 5 0.06 3 9 0.3 0.8 0.08 NDuratherm 600 R30600 41.5 12 3.9 4 0.05 (max) 8.7 bal 0.4 0.75 2 Ti, 0.7 Al, 0.05 BeElgiloy R30003 40 20 . . . 7 0.15 (max) bal 15.5 . . . 2 1 Be (max)Havar R30004 42.5 20 2.8 2.4 0.2 bal 13 . . . 1.6 0.06 Be (max)

Cast superalloys

AiResist 13 . . . 62 21 11 . . . 0.45 . . . . . . . . . . . . 0.1 Y, 3.4 Al, 2 TaAiResist 213 . . . 64 20 4.5 . . . 0.20 0.5 0.5 . . . . . . 0.1 Y, 3.5 Al, 6.5 Ta, 0.1 ZrAiResist 215 . . . 63 19 4.5 . . . 0.35 0.5 0.5 . . . . . . 0.1 Y, 4.3 Al, 7.5 Ta, 0.1 ZrFSX-414 . . . 52.5 29 7.5 . . . 0.25 1 10 . . . . . . 0.010 BJ-1650 . . . 36 19 12 . . . 0.20 . . . 27 . . . . . . 0.02 B, 3.8 Ti, 2 TaMAR-M 302 . . . 58 21.5 10 . . . 0.85 0.5 . . . . . . . . . 0.005 B, 9 Ta, 0.2 ZrMAR-M 322 . . . 60.5 21.5 9 . . . 1.0 0.5 . . . . . . . . . 0.75 Ti, 4.5 Ta, 2 ZrMAR-M 509 . . . 54.5 23.5 7 . . . 0.6 . . . 10 . . . . . . 0.2 Ti, 3.5 Ta, 0.5 ZrNASA Co-W-Re . . . 67.5 3 25 . . . 0.40 . . . . . . . . . . . . 2 Re, 1 Ti, 1 ZrS-816 R30816 42 20 4 4 0.4 4 20 0.4 1.2 4 NbV-36 . . . 42 25 2 4 0.27 3 20 0.4 1 2 NbWI-52 . . . 63.5 21 11 . . . 0.45 2 . . . . . . . . . 2 Nb � TaStellite 23 R30023 65.5 24 5 . . . 0.40 1 2 0.6 0.3 . . .Stellite 27 R30027 35 25 . . . 5.5 0.40 1 32 0.6 0.3 . . .Stellite 30 R30030 50.5 26 . . . 6 0.45 1 15 0.6 0.6 . . .Stellite 31 (X-40) R30031 57.5 22 7.5 . . . 0.50 1.5 10 0.5 0.5 . . .

(a) Haynes is a registered trademark of Haynes International, Inc.; MP35N and MP159 are registered trademarks of SPS Technologies, Inc. Elgiloy is a registered trademark of Elgiloy Specialty Metals; MAR-M is a registeredtrademark of Martin Marietta Corp.; Stellite is a registered trademark of Deloro Stellite, Inc.; (b) Maximum. (c) Haynes 6B is currently known as Stellite 6B.

Table 2 Deleterious intermetalliccompounds in cobalt-base superalloys

Compound Structure

Co2 (Mo, W, Ta, Nb) Hexagonal Laves phaseCo2 (Mo, W)6 Rhombohedral, hexagonal l phaseCo2 (Ta, Nb, Ti) Cubic Laves phaseCo2 (Mo, W)3 r phase

Table 3 Phases present in Haynes alloy 25

Phase Crystal structure Lattice parameters, nm

M7C3 Hexagonal (trigonal) a � 1.398, c � 0.053, c/a � 0.0324M23C6 fcc a � 1.055 to 1.068M6C fcc a � 1.099 to 1.102Co2W Hexagonal a � 0.4730, c � 0.7700, c/a � 0.1628�-Co3W Ordered fcc a � 0.3569b-Co3W Ordered hexagonal a � 0.5569, c � 0.410, c/a � 0.0802Co7W6 Hexagonal (rhombohedral) a � 0.473, c � 2.55, c/a � 0.539Matrix fcc a � 0.3569

hcp a � 0.2524, c � 0.4099, c/a � 0.1624

Table 4 Five-step procedure for cobalt

Surface Abrasive/size

Load Base speed(rpm)/direction(b)

Time(min:s)N lb

Abrasive disks (waterproof paper) 220–320 (P240–P400) grit SiC watercooled

27 6 250–300 contra Until plane

Cloth or rigid grinding disk 9 lm diamond suspension(a) 27 6 100–150 contra 5:00Pad 3 lm diamond suspension(a) 27 6 100–150 contra 5:00

1 lm diamond suspension(a) 27 6 100–150 contra 3:00Cloth or pad �0.05 lm colloidal silica or alumina

suspensions27 6 80–120 contra 2:00–3:00

(a) Plus fluid extender as desired. (b) Contra: platen and specimen rotate in opposite directions

Metallography and Microstructures of Cobalt and Cobalt Alloys / 763

ing-fault energy (Ref 3). The fcc to hcp trans-formation reaction is quite sluggish even forpure cobalt. However, in metastable composi-tions, it can be promoted by cold work via amechanism involving the coalescence of stack-ing faults (Ref 4). This phenomenon provides apractical limit in the design of wrought cobalt-base alloys in terms of the manufacturing meth-ods that can be used to produce various productforms. Those involving hot-working operationssuch as plate, bar, and hot-rolled sheet do notpresent obstacles, because the working tempera-tures typically are well within the stable fccrange. However, those products that require ex-tensive cold-working sessions, such as cold-rolled sheet, cold-drawn tubulars, and cold-drawn bar and wire products, must possessadequate levels of matrix stability to be eco-

nomically viable. Such stability usually is im-parted through additions of nickel.

For imparting resistance to oxidizing and sul-fidizing types of environments, chromium is thepreferred alloying element. Attempts to incor-porate aluminum in amounts sufficient to pro-vide protective alumina scales have not beencommercially successful, because the formationof the brittle intermetallic compound b-CoAlsignificantly reduced fabricability. In addition tochromium, small amounts of elements such asmanganese, silicon, and rare-earth elements(e.g., lanthanum) can be used to enhance the for-mation of protective oxide scales at elevatedtemperatures.

Strengthening of cobalt-base alloys is accom-plished by solid-solution alloying (e.g., molyb-denum, tungsten, tantalum, and niobium) in

combination with carbon to promote carbide pre-cipitation. Compared to the wrought alloys, castcobalt-base superalloys are characterized byhigher contents of high-melting metals (chro-mium, tungsten, tantalum, titanium, and zirco-nium) and by higher carbon contents. The solid-solution alloying decreases stacking-faultenergy, thereby making the cross slip and climbof glide dislocations more difficult. Carbide pre-cipitation (especially M23C6 carbides) also canbe quite effective in pinning glide dislocations,and both wrought and cast alloys depend on thedispersion of complex carbides for strength.However, the use of high levels of carbon willlimit manufacturing operations to hot-workingprocesses. Compositions intended for cold-working processes usually contain carbon at lev-els of 0.15% or less. For service temperatures of700 �C (1300 �F) or less, special alloy compo-sitions have been developed that exploit the fccto hcp transformation in products that are me-chanically worked and aged (Ref 5).

The various carbide phases that form dependon chemical composition, heat treatment, andcooling. The carbides also may be different sizesand somewhat varying shapes, even for the samephase. In Co-Cr-C, M7C3 and M23C6 are com-mon. In the as-cast condition, the MC phase ispredominant, because it is the first formed uponcooling from the molten state. Cast alloys invar-iably have M7C3 carbides located within thegrains, although M7C3 may be found at grainboundaries as well. Subsequent heat treatments(intentional or from service exposure) modifythe morphology, amounts, and types of carbidesfound in the grains of superalloys. Some sec-ondary carbides within grains can be formed byprecipitation on dislocations located near largeprimary carbides.

The carbides are seldom binary compositions;chromium, tungsten, tantalum, silicon, zirco-nium, nickel and cobalt may all be present in asingle particle or carbide. In more complex alloysystems, cobalt, tungsten, and molybdenum re-

Table 5 Electrochemical polishing recipes for cobalt alloys

No. Recipe Conditions

Electrolytic polishing

EP1 600 mL methanol 99.8% 10–60 s330 mL nitric acid 65% 40–70 V dc

Stainless steel cathode. Do not store!EP2 Phosphoric acid 85% 3–5 min. 1.5 V dc

Stainless steel cathodeEP3 600 mL distilled water 1–15 min. 1–2 V dc

400 mL phosphoric acid 85% Stainless steel or Co cathode. Rinsing will remove black deposit.EP4 900 mL distilled water Seconds to minutes. 6 or 4 V dc

100 mL sulfuric acid 95–97% Stainless steel cathodeEP5 200 mL perchloric acid 60% Seconds to minutes. 30–60 V dc

700 mL ethanol 96%100 mL butyl glycol 99%

Stainless steel cathode

EP6 15 mL sulfuric acid 95–98%85 mL methanol

25 V dc, 3–10 s

Chemical polishing

CP1 40 mL lactic acid 90%30 mL hydrochloric acid 32%5 mL nitric acid 65%

Seconds to minutes

CP2 100 mL distilled water5 g chromium (VI) oxide

Seconds to minutes

CP3 50 mL glacial acetic acid50 mL nitric acid 65%

Seconds to minutes

Source: Ref 14

Table 6 Staining and heat tinting procedures for identification of sigma phase and carbides in Stellite 21, wrought cobalt alloy

Coloringprocess Method Purpose

First etchant Staining method Colors of minor phases obtained byprevious investigatorsSolution Method Solution Method

1 Etch To stain sigma phase 8% oxalic acid92% water

Electrolytic etch8–10 s6 VRoom temperature

5 g KMnO4

5 g NaOH90 mL water

Immerse 10–20 sRoom temperature

Sigma: bright green or red topurple

2 Etch To differentiatebetween carbidesand sigma phase

10% NaCN90% water

Electrolytic etch10–20 s1.5 V

Murakami’s 10 g K3Fe(CN)6

10 g KOH100 mL water

Immerse 2–4 sRoom temperature

Carbides: straw to yellow-brownor buff

Gamma (sigma): gray to blue orgreenish gray

3 Etch To identify carbides 2% chromic acid98% water

Light electrolytic etch 1 part (20% KMnO4, 80% water)1 part (8% NaOH, 92% water)

Immerse 7 sRoom temperature

Cr23C6: brownCr7C3: pale yellow to light tanM6C: red, green, yellow, blue

(also reticulation)4 Heat tint To identify carbides

and sigma phase5% HCl95% water

Light electrolytic etch1 s5 V

Heat polished specimento dull red

Held at temperatureuntil surface becomescolored

Air cooled or Hgquenched

Sigma: dark medium brownCr23C6: whiteM6C: darkNote M6C and sigma cannot be

differentiated in same structure

Source: From Ref 16

764 / Metallography and Microstructures of Nonferrous Alloys

place some of the chromium in the carbidephases. Niobium and tantalum (8 to 10%), andtitanium and zirconium (less than 0.5%) formcarbides of the MC type. Molybdenum and tung-sten form M6C in the Co-Cr-C alloys when thecontent of either element is great enough so thatit will no longer substitute for chromium inM23C6. Molybdenum, although used extensivelyin nickel-base superalloys, is used only sparinglyin cobalt-base superalloys. In cobalt-base super-alloys, tungsten is more effective and less det-rimental than molybdenum. However, corrosion-resistant grades of cobalt alloys rely onmolybdenum instead of tungsten for corrosionresistance. In addition, molybdenum has beenfound to enhance wear resistance in cobalt-basewear resistant alloys (Ref 6).

Another important aspect in the physical met-allurgy of cobalt-base alloys is the occurrence ofintermetallic compounds such as r, l, and Lavesphases. These phases are deleterious in high-temperature applications, but Laves-phase alloysare used for wear-resistance applications (see thesection “Laves-Phase Alloys” in this article).The r and l intermetallic compounds are clas-sified as electron compounds, and they are oftenreferred to as topologically closed-packed (tcp)phases. The Laves phase is formed mainly onthe basis of atomic size factors. Some examplesof these compounds in terms of general com-position and crystal structure are given in Table2. Specific data for Haynes alloy 25 are listed inTable 3 (Ref 7). In addition to the gross chem-istry factors responsible for the formation ofthese compounds, it has also been recognizedthat minor elements, notably silicon, can playimportant roles (Ref 8, 9). The precipitation ofthese intermetallic phases can cause embrittle-ment, especially at low temperatures. Some suc-cess in retarding the formation of the Lavesphase was achieved in the development ofHaynes alloy 188 by control of the chemistry(Ref 10, 11). On the other hand, precipitation ofLaves phase can impart wear resistance, espe-cially at high temperatures.

Metallographic Preparation

Cobalt-base alloys are relatively easy to polishand nearly any mounting procedure can be em-ployed unless edge retention is required. Polish-ing techniques may be similar to those fornickel-base alloys, but cobalt and its alloys aremore difficult to prepare than nickel and its al-loys. Cobalt and its alloys work harden rapidly(due to the fcc-to-hcp transformation), and sometwin readily. Consequently, grinding and polish-ing rates are lower for cobalt than for nickel,copper, or iron. Preparation of cobalt and its al-loys is somewhat similar to that of refractorymetals.

The following general practice (see Table 4,Ref 12) is for preparing cobalt and its alloys.Two steps of SiC paper may be needed to get thespecimens coplanar. If the cut surface is of goodquality, start with 320-grit paper. Cobalt and its

Table 7 Etchants for cobalt alloys

No. Material Etchant Condition and comments

General

m1 Pure FeCo-Fe alloys

100 mL methanol 99.8%10 mL HNO3 65%

For Co and Co-Fe alloys. Immersesample for up to 30 s. 1–50% nital hasbeen used with varying immersiontimes from seconds to minutes. Do notstore; neutralize after use.

m2 Pure and low-alloy CoCo-B, Co-Ti, and Co-Mn alloysWC-TiC-TaC-Co hardmetalsGrain-boundary etchant

15 mL distilled water15 mL glacial acetic acid60 mL HCl 32%15 mL HNO3 65%

Age 1 h before use. Immerse sample from5 s up to 30 s.

m3 Co and alloys, general etch 7.5 mL HF2.5 mL HNO3

200 mL methanol

Immerse sample for 2–4 min

m4 Co alloys 25 mL water25 mL acetic acid50 mL HNO3

m5 Co alloys (Battelle) 80 mL lactic acid10 mL H2O2 (30%)10 mL HNO3

m6 Pure CoColor etchant

98 mL distilled water2 mL hydrofluoric acid 40%,

boiling, add molybdic acid tosaturate

Seconds to minutesRoom temperature

m7 Co-base casting material androlling stock

StelliteColor etchant

100 mL stock solution Beraha III1 g potassium disulfite

30 s to 5 minWet etchingKeep only for 1–2 h.

m8 Stock solution, Beraha III 600 mL distilled water400 mL HCl 32%50 mL ammonium hydrogen

difluoride

Caution: stock solution must be stored inplastic bottle.

m9 Hardfacing alloys andsuperalloys

100 mL HCl5 mL H2O2 (30%)

Use under a hood. Use fresh. Immersesample a few seconds.

Etchants for specific alloys/phases

m10 Co superalloys 200 mL HCl 32%5 mL nitric acid 65%65 g FeCl3

Immerse sample a few seconds. Useunder a head.

m11 Co superalloys 50 mL distilled water50 mL HCl 32%10 g copper (II) sulfate (Marble’s

etch)

Seconds to minutesImmersion or swabbingAdd few drops of sulfuric acid to increase

activity.m12 Magnetic alloys

Co-Fe alloys100 mL distilled water100 mL HCl 32%200 mL methanol 99.8%5 mL HNO3 65%7 g FeCl32 g copper (II) chloride

10–15 sImmersion or swab etching

m13 Co-Sm alloys 100 mL distilled water1 mL glacial acetic acid1 mL nitric acid 65%

Immerse sample a few seconds.

m14 So-Sm alloysColor etchant

100 mL distilled water2 mL HCl 32%20 g potassium disulfite

10 s

m15 Co-Sm alloysGrain-boundary etchant

100 mL distilled water8 g chromium (VI) oxide2 g sodium sulfate

5–10 sRinse in hot water and remove smudge

layer with 20% aqueous chromium (VI)oxide solution

m16 Co-Ti alloys 100 mL water2 mL Hf5 mL H2O2 (30%)

m17 Co-Pt alloys (48–54 at.% Co) 3 parts HNO3

1 part HClm18 Co-Pt alloys

WC-TiC-NbC-Co hardmetals75 mL HCl 32%25 mL HNO3 65%

Up to 5 minUse fresh mixture only.

m19 Co borides 30 mL distilled water10 mL HCl 32%10 mL HNO3 65%

Seconds to minutes

m20 Co silicide 100 mL distilled water15 g CrO3

Seconds to minutesAdd few drops of hydrochloric acid

before use.m21 Co and Co-Al alloys 25 mL distilled water

50 mL HCl 32%15 g FeCl33 g ammonium tetrachlorocuprate

(II) (Adler’s reagent)

Seconds to minutes

(continued)

Adapted from Ref 14, 17


● Cobalt-rich alloys (�50 wt% Co) such as Co-Ni, Co-Fe, Co-Pt, Co-V, and Co-Sm alloys

● Alloys with less than 50 wt% Co such as Al-nico alloys (25 to 30 wt% Co with Al, Cu,Fe, Li, Ni) Reamalloy (12 wt% Co with Fe),Unico (29 wt% Co with Cu, Ni), and Fe-Ni-Co alloys

Preparation of these diverse alloys is not verydifficult, although preparation has to be adjustedaccordingly for the mechanical behavior thatmay range from ductile to hard brittle. Wetgrinding on SiC paper down to 1200 grit is done.Polishing techniques include:

● For hard and brittle alloys, rough polishingwith 6, 3, 1, and 0.25 lm diamond particlesize, and final polishing with 0.05 lm alu-mina slurry

● For ductile alloys, rough polishing with 6 to1 lm diamond particle size, and shock pol-ishing with the electrolyte recipes EP1 or EP5(Table 5)

● Electrolytic polishing of Alnico with recipeEP2; intermetallic alloy with EP5 (cobalt al-loys tarnish easily and should only be rinsedwith alcohol)

Cobalt-Base Superalloys. Preparation ofwrought and cast cobalt-base superalloys is simi-lar to nickel-base superalloys. Preparation rec-ommended in Ref 14 is:

● Wet grinding on SiC papers down to 1200 grit● Grinding on SiC papers down to 320 grit,

then lapping on a synthetic-bonded lappingdisk with 6 lm diamond particle size

● Polishing with 3 and 1 lm diamond particlesize on a hard synthetic cloth. Final polishingwith 0.05 lm alumina on a hard silk cloth

● Electrolytic polishing with EP4 (Table 5)

A more detailed description of metallographicpreparation of Hastelloy and Haynes superalloysis in the section “Heat-Resistant Cobalt Alloys”in this article.

Wear-resistance alloys such as the Stellite al-loys can be prepared as follows:

● Wet grinding on SiC papers down to 320 grit,then lapping on a synthetic-bonded lappingdisk with 6 lm diamond particle size

● Grinding on a diamond disk with 20 lm par-ticle size

● Polishing with 6 lm diamond particle size ona hard synthetic cloth. Final polishing with 1lm diamond particle size on a cotton cloth

Etching. Cobalt is attacked readily by dilutenitric acid and less rapidly by hydrochloric orsulfuric acids, but not caustic solutions. Pittingis commonly encountered with many etchants.Morral has compiled an extensive list of reagentsfor cobalt and its alloys (Ref 13). Young has de-scribed etching methods delineating oxide andsulfide phases in cobalt (Ref 15). These etchantscolor the matrix phases, but do not affect theoxides and sulfides. Weeton and Signorelli (Ref16) have described different procedures for iden-tifying sigma phases and carbides in wrought

Table 7 (continued)

No. Material Etchant Condition and comments

m22 Co-base superalloysColor etchant

50 mL distilled water50 mL HCl 32%2 g potassium disulfite

Seconds to minutes; carbides remainwhite

m23 Alloy C73 (Co, 40Cr, 2.4C) 95 mL water1 g KOH4 g KMnO4

Colors M7C3 gray and M23C6 black

m24 Co, cobalt oxides, and sulfides Solution 1:1 g mercuric chloride99 mL water

Solution 2:35 g sodium bisulfite100 mL water

Etch with solution 1 for 30 s or withsolution 2 for 2 min to distinguishbetween Co metal, cobalt oxide, andsulfides. Both stain the Co matrixbrown, oxide remains dark gray, cobaltsulfide remains yellowish, and MnSremains bluish gray.

Tint etch

m19 Beraha’s tint etch for Co alloys Solution 1:HCl and water (1:1) (stock

solution)Ingredient 2:

0.6–1 g potassiummetabisulfite

Ingredient 3:1–1.5 g FeCl3

Add ingredient 2 to 100 mL of stocksolution 1, then add ingredient 3.Immerse sample at 20 �C for 60–150 s,agitate sample. Matrix is colored,carbides and nitrides are unaffected.

Electrolytic etching

Em1 Pure Co and Co-Al alloys 100 mL distilled water5 mL HCl 32%10 g FeCl3

Several seconds6 V dcUse stainless steel cathode.

Em2 Stellites up to 70% Co and Co-base superalloys

100 mL distilled water5–10 mL HCl 32%2–10 g CrO3

2–20 s3 V dcStainless steel cathode

Em3 Pure Co and Co-base superalloys 100 mL distilled water5–10 mL HCl 32%

2–10 s3 V dcGraphite cathode; pitting sometimes

occursEm4 Co-base abrasion-resistant alloys

and tool materials, superalloys100 mL HCl 32%5 mL hydrogen peroxide 30%

3–5 s4 V dcStainless steel cathode

Em5 Co-base superalloys 95 mL HCl 32%5 g oxalic acid

2–3 V dc, pointed stainless steel contactanode, graphite cathode, 1–5 s

Em6 Hardfacing alloys andsuperalloys

Solution 1:100 mL water2 g CrO3

Solution 2:85 mL water4 g NaOH10 g KMnO4

Use solution 1 at 3 V dc for 2 s. Rinse inwater and immerse sample in solution2 for 5–10 s. Use solution 2 fresh.

Em7 Co superalloys 140 mL HCl1 g CrO3

Use at 3 V dc for 2–10 s.

Em8 Cobalt 940 mL water45 mL HNO3

15 mL HCl

Use stainless steel cathode, 3.5 V dc, 0.75A/cm2, 15 s, 20 �C

Adapted from Ref 14, 17

alloys are more difficult to cut than most steels,regardless of their hardness, and abrasive cuttingwheels are recommended. Attack polishing hasbeen used after mechanical polishing. Morral(Ref 13) has recommended two chemical polish-ing solutions: equal parts of acetic and nitric ac-ids (immerse) or 40 mL lactic acid, 30 mL hy-drochloric acid, and 5 mL nitric acid (immerse).A wide variety of cobalt-base alloys has beenprepared with the previously described methodwithout need for chemical polishing. The 1-lmdiamond step could be eliminated for routinework.

Polishing procedures for different categoriesof cobalt alloys are summarized below (Ref 14):

Pure Cobalt. Cold working from cutting andgrinding can be troublesome with purer metals.Pure cobalt tends to form smear and deformation

layers. Grinding is done wet on SiC paper (downto 1200 grit) with light pressure. Polishing meth-ods include:

● Initial polishing with 6, 3, and 1 lm diamondand final polishing with 0.3 and 0.05 lm alu-mina slurry, with if necessary, etching be-tween polishing steps with 3% nitric acid inalcohol

● Initial polishing with 6 lm diamond particlesize, then for 2 to 3 h on a vibratory polishercharged with 0.3 lm alumina slurry or chro-mium oxide. Final polishing in a 10% aque-ous SiO2 suspension

● Electrolytic polishing with EP2 recipe (seeTable 5).

Magnetic Alloys. Cobalt alloys for magneticapplications are diverse and include:


Stellite 21 (Table 6, Ref 16). Etchants for cobaltand cobalt alloys are listed in Table 7. Macro-etchants are listed in Table 8.

Wear-Resistant Cobalt Alloys

The Stellite alloys listed in Table 1(a) are mostcommonly used in the form of castings or weldoverlays (hardfacing alloys). Only a few of themcan be made into hot-worked plates, sheets, orbars. Powder metallurgy techniques as well ashot isostatic pressing can be used to consolidateStellite alloy powders into solid components.Some alloys (e.g., Stellite alloys 1, 6, and 12)are derivatives of the original Co-Cr-W alloysdeveloped by Haynes. These alloys are charac-terized by their carbon and tungsten/molybde-num contents (Ref 18). Their microstructures (inweld overlay form) are presented in Fig. 1(a) to(c) and illustrate the extent of carbide precipita-tion.

The most common carbide in the hypereutec-tic high-carbon alloys is a chromium-rich M7C3

type, although chromium-rich M23C6 carbidesare abundant in the hypoeutectic low-carbon al-loys such as Stellite alloy 21. In high-tungstenalloys (such as Stellite alloy 1) tungsten-richM6C carbides also are usually present. Tungstenand molybdenum participate in the formation ofM6C carbides during alloy solidification. Stellitealloy 21 employs molybdenum, rather than tung-sten, to strengthen the solid solution. Stellite al-loy 21 also contains considerably less carbon.This alloy is more resistant to corrosion thanStellite alloys 1, 6, and 12, because of the highmolybdenum content and because most of thechromium is in solution (rather being tied up incarbides). The most recently developed alloys inthe Stellite family are those in the 700 series (al-loys 703, 704, 706, 712, and 720 in Table 1a).As with Stellite alloy 21, the tungsten in thesealloys has been replaced by molybdenum (mo-lybdenum contents range from 5 to 18%).

The microstructures of these alloys are char-acterized by the separation of chromium-rich andmolybdenum-rich eutectic regions, as shown inthe scanning electron micrograph (SEM) of Stel-lite 712 in Fig. 2, where the light areas are mo-lybdenum rich and dark areas chromium rich. Itis likely that M23C6 carbides are in the chro-mium-rich areas and the M6C or M2C carbidesare in the molybdenum-rich areas.

The size and shape of the carbide particleswithin the Stellite alloys are strongly influencedby cooling rate and subtle chemistry changes.For example, typical overlay microstructures asapplied by different welding processes areshown in Fig. 3 and 4. Such changes markedlyaffect abrasion resistance, because there is a dis-tinct relationship among the size of abrading spe-cies, the size of the structural hard particles, andthe abrasive wear rate. The fraction of carbidesis also influenced by cooling rate, carbon con-tents, and alloying. For example, at a carbonlevel of 2.4 wt% (Stellite 3), the carbides con-stitute about 30 wt% of the material. These are

of the M7C3 (chromium-rich primary) and M6C(tungsten-rich eutectic) types. At 1 wt% carbon(Stellite 6B alloy), the carbides constitute ap-proximately 13 wt% of the material.

Powder metallurgy (P/M) versions of sev-eral Stellite alloys (typically containing low lev-els of boron—1.0% max as listed in Table 1(a)—to enhance sintering) are available for applica-tions where the P/M process is cost effective(e.g., high-volume production of small simpleshapes). The microstructure of P/M Stellite al-loys contains complex combinations of M7C3,M6C, and M23C6 carbides that are embedded ina Co-Cr-W matrix. Boron is expected to replacesome of the carbon atoms in these carbides toform borocarbides.

Laves-phase alloys include the Tribaloy fam-ily of wear-resistant materials. Four cobalt-baseLaves-type alloy compositions (T-400, T-400C,T-800, and T-900) are listed in Table 1(a). Inthese materials, molybdenum and silicon areadded at levels in excess of their solubility limitwith the objective of inducing the precipitationof the hard (and corrosion-resistant) Laves phase(CoMoSi or Co3Mo2Si). Carbon is held as lowas possible in these alloys to discourage carbideformation. An example is shown in Fig. 1(f ) andFig. 5.

Because the Laves intermetallic phase is soabundant in these alloys (35 to 70 vol%), itspresence governs all the material properties. Ac-cordingly, the effects of the matrix composition

in these alloys are less pronounced than in thecase for the cobalt-base carbide-type Stellite al-loys, for example. The Laves phase is specifi-cally responsible for outstanding abrasion resis-tance, but it severely limits the material ductilityand the impact strength. In fact, it is difficult toattain crack-free overlays on all but the smallestcomponents given adequate preheat. In recentyears, with the advanced manufacturing tech-nologies, crack-free cast or P/M componentshave been made and used widely in applicationswhere high performance is required.

Wrought Alloys. The high-carbon alloys,such as Stellite alloys 6B, 6K, and 706K in Table1(a), are essentially wrought versions of thehardfacing alloys described previously. Wroughtprocessing improves chemical homogeneity (im-portant in a corrosion sense), markedly increasesductility, and modifies substantially the geome-try of the carbide precipitates within the alloys(blocky carbides within the microstructure en-hance abrasion resistance). In terms of compo-sition, the alloys are essentially Co-Cr-W/Mo-Cquaternaries with chromium providing strengthand corrosion resistance to the solid solution inaddition to functioning as the chief carbide for-mer (during alloy solidification). Tungsten ormolybdenum provides additional solid-solutionstrength. Alloy 6B contains approximately 12.5wt% carbides of the M7C3 and M23C6 types inthe ratio 9 to 1. Alloys 6K and 706K exhibit aneven greater carbide volume fraction, again with

Table 8 Macroetchants for cobalt and cobalt alloys

Material Etch composition Remarks and conditions

49%Co-49%Fe-V alloy and someStellites

50 mL HCl (32%)50 mL H2O

Good for general structure. Immersesample in hot (60–80�C) solution for30 min. Rinse in hot water.

Co-25Cr-10Ni-8W,Co-21Cr-20Ni,Co-3Cr-3Mo-1Nb,Stellite

50 mL HCl (32%)10 mL HNO3 (65%)10 g FeCl3100 mL H2O

Good for general structure and grain size.Swab sample until desired contrast isobtained.

HA-36 2 g cupric ammonium chloride5 g FeCl35 mL HNO3

50 mL HCl80 mL H2O

Good for general structure and grain size.Swab sample until desired contrast isobtained.

Co,Co alloys

Solution 1Saturated solution of FeCl3 in HCl

Solution 2Add 5%HNO3 to solution 1 prior to use.

Use at room temperature. After etchingdip sample in 1:1 solution of HCl andH2O.

Co,Co alloys

Solution 121 mL H2SO4

15 mL HCl21 mL HNO3

21 mL HF22 mL H2O

Solution 240 mL of a solution of 1 g

CuCl2•2H2O and 5 mL H2O40 mL HCl20 mL HF

Etch sample 5 min in solution 1 and then5 min in solution 2.

Nimonic,Ni-Cr-Co alloys,Ni-Cr-Co-Mo alloys

5 mL HF3 mL HCl50 mL alcohol “pinch” of sodium

hyposulfite

Use boiling.

Co-Ni-Fe high-temperature alloys 25 mL H2O50 mL HCl (32%)25 mL HNO3 (65%)

Immerse sample at room temperature for10–30 min.

Source: Ref 14, 17


the M7C3 as the predominant type. Alloy 706Kmay also contain M6C or M2C due to the pres-ence of molybdenum.

Heat-Resistant Cobalt Alloys

Cobalt-base superalloys for aircraft engineswere among the first true members of the super-alloy family. An historical perspective of thesematerials is given in Table 9. The initial appli-cations involved cast compositions for use asblades in turbo superchargers for piston enginesin the 1930s, then as blades and vanes in the firstgas turbine engines of the 1940s. The use of castcobalt-base alloys continues in current gas tur-bine engines, mainly as nozzle guide vanes andstator blades. Wrought cobalt-base superalloysdid not enjoy extensive use until the 1950s forsuch components as forged turbine blades, com-bustor liners, and afterburner tailpipes. Their de-velopment and use has been greatly over-shad-

owed by the advent of nickel-base superalloys,but cobalt-base superalloys still play an impor-tant role, by virtue of their excellent resistanceto sulfidation and their strength at temperaturesexceeding those at which the gamma-prime (c�)and gamma-double-prime (c�) precipitates dis-solve in the nickel and nickel-iron alloys.

Since the early use of Stellite 21 (which is nowused primarily for wear resistance), cobalt-basesuperalloys have gone through various stages ofdevelopment to increase their high-temperaturecapability. The use of tungsten rather than mo-lybdenum, moderate nickel contents, lower car-bon contents, and rare-earth additions typify cur-rent cobalt-base superalloys. For many years, thepredominant user of heat-resistant alloys was thegas turbine industry. In the case of aircraft gasturbine power plants, the chief material require-ments were elevated-temperature strength, resis-tance to thermal fatigue, and oxidation resis-tance. For land-base gas turbines, whichtypically burn lower grade fuels and operate at

Fig. 1 Microstructures of various cobalt-base wear-resistant alloys. (a) Stellite 1, two-layer gas tungsten arc deposit. (b) Stellite 6, two-layer gas tungsten arc deposit. (c) Stellite 12,two-layer gas tungsten arc deposit. (d) Stellite 21, two-layer gas tungsten arc deposit. (e) Haynes alloy 6B, 13 mm (0.5 in.) plate. (f) Tribaloy alloy (T-800) showing the Laves

precipitates (the largest continuous precipitates some of which are indicated with arrows). All 500�

Fig. 2 Scanning electron micrograph from as-cast Stel-lite 712 alloy characterized by the separation of

chromium-rich regions (dark) and molybdenum-richeutec-tic regions (light). Original magnification at 1000�. Cour-tesy of J. Wu, Deloro Stellite Group Limited


matrix, while tungsten provides solid-solutionstrengthening and promotes carbide formation.For high-temperature applications, strengtheningrelies on the solid-solution alloying and carbideprecipitation typical of cobalt-base alloys. In ad-dition, the diffusion of substitutional alloyingelements tends to be slower in cobalt than innickel (Ref 20), which gives the cobalt base aninherent advantage in high-temperature creep.

Carbide precipitation, especially M23C6 car-bides, is the other important key to the strength-ening of cobalt-base alloys, as noted. Carbideprecipitation is quite effective in pinning glidedislocations, and both wrought and cast super-alloys alloys depend on the dispersion of com-plex carbides for strength. Carbides withingrains act to impede basic dislocation move-ment, with an attendant increase in strength. Car-

lower temperatures, sulfidation resistance wasthe major concern. Today, the use of heat-resis-tant alloys is more diversified, as more efficiencyis sought from the burning of fossil fuels andwaste, and as new chemical processing tech-niques are developed.

The roles of alloying elements in cobalt-basesuperalloys are summarized in Table 10. The ad-dition of nickel helps to stabilize the desired fcc

Fig. 3 Typical overlay microstructures of Stellite 1 alloy applied by different weld processes. (a) Three-layer gas tungsten arc. (b) Three-layer oxyacetylene. (c) Three-layer shieldedmetal arc. All 500�

Fig. 4 Typical overlay microstructures of Stellite 6 alloy applied by different weld processes. (a) Three-layer gas tungsten arc. (b) Three-layer oxyacetylene. (c) Three-layer shieldedmetal arc. See also Fig. 1. All 500�


bide formation is not uniform and regular, as isthat of c� precipitation. Thus, the strength in-crease obtained from carbide dispersion in grainsis less than that of the typical hardening causedby c� precipitation but still may be significant.

These alloys also contain significant levels ofboth nickel and tungsten. Other alloying ele-ments contributing to the solid-solution and/orcarbide formation are tantalum, niobium, zirco-nium, vanadium, and titanium. Yttrium is alsoadded to some alloys for improved oxidation re-sistance. Attempts have been made to developage-hardenable alloys based on Co3Ti- andCo3Ta-type precipitates. However, problemswith the thermal stability of the precipitates andthe fact that the strength levels attained were eas-ily matched or exceeded by the c�-strengthenednickel-base alloys precluded their commercialintroduction (Ref 21).

In the as-cast condition, MC carbides are par-ticularly evident in cast superalloys, as they formfirst from the molten state. Numerous MC car-bides may develop within the grains and at thegrain boundaries and may have a script-form ap-pearance, as shown Fig. 6. More complex car-bides may also be present in either eutectic orprecipitate form. For example, Fig. 6 and 7 areas-cast microstructures from two Co-Cr-W-Tasuperalloys with MC carbides and more complexcarbides such as M23C6 (Fig. 6) and M6C (Fig.7). As-cast microstructures of Haynes 31 alloywith and without subsequent aging are shown inFig. 8. Subsequent heat treatments and/orwrought processing will modify the morphology,amounts, and types of carbides found in thegrains of superalloys. Examples are shown inFig. 9 and 10 for Haynes 25 and 188 alloys, re-spectively. Carbide distributions in wrought al-loys result from the mill anneal after final work-ing. Properties are largely a result of grain size,refractory-metal content, and carbon level. Somesecondary carbides within grains can be formedby precipitation on dislocations located nearlarge primary carbides.

Complete solution treatments, in which all mi-nor constituents are dissolved, is not possible inmost cobalt-base superalloys, because melting

often occurs before all the carbides are dissolved.Some enhancement of creep-rupture behaviorhas been achieved by heat treatment where somecarbides are dissolved for reprecipitation. Al-though some aging of a cobalt-base superalloymay lead to strength improvement, solutiontreating and aging is not suitable for producingstable cobalt-base superalloys for use above 815�C (1500 �F) because of subsequent carbide dis-solution or overaging during service exposure.

Typical wrought and cast cobalt alloy com-positions developed for high-temperature use arepresented in Table 1(b). Haynes alloy 25 (alsoknown as L605) and 188 are wrought alloysavailable in the form of sheets, plates, bars,pipes, and tubes (together with a range of match-ing welding products for joining purposes). Al-

loys 25 and 188 are considerably more ductile,oxidation resistant, and microstructurally stablethan the wear-resistant wrought cobalt alloys.Both alloys contain approximately 0.1 wt% C(about one-tenth of that in wrought, wear-resis-tant alloy 6B), which is sufficient to provide car-bide strengthening, yet low enough to maintainductility. Carbide precipitation, which is pre-dominately of the M6C type, is important to thehigh-temperature properties of these materials,partially because it restricts grain growth duringheat treatment and service. Structural stability isenhanced in these alloys by nickel, which de-creases the fcc/hcp transformation temperaturein cobalt-base alloys.

Cast heat-resistant alloys, such as alloysMAR-509 and FSX-414, are designed around a

Table 9 Cobalt-base superalloys: historicalperspective

Co-Cr alloys first patented by Elwood Haynes 1907Haynes alloy 6B 1913Stellite alloy 21 (Vitallium supercharger

buckets)1936–1943

Stellite alloy 31 (X-40) buckets 1941–1943S816 alloy wrought buckets 1946–1953Haynes alloy 25 afterburner components 1948–1955MAR-M 302 vanes 1958Airesist vanes 1964Haynes alloy 188 combustor, afterburner

components1966–1968

Note: Haynes is a registered trademark of Haynes International, Inc.;Stellite is a registered trademark of Deloro Stellite, Inc.; MAR-M is aregistered trademark of Martin Marietta Corp.; Airesist is a registeredtrademark of Allied Signal Aerospace Co.; and Vitallium is a registeredtrademark of Pfizer Hospital Products Group, Inc. Source: Ref 19

Table 10 Role of various alloying elements in cobalt-base superalloys

Element Effect

Chromium Improves oxidation and hot corrosion resistance; produces strengthening by formation of M7C3 andM23C6 carbides

Nickel Stabilizes fcc form of matrix; produces strengthening by formation of intermetallic compoundNi3Ti; improves forgeability

Molybdenum, tungsten Solid-solution strengtheners; produces strengthening by formation of intermetallic compoundCo3M; formation of M6C carbide

Tantalum, niobium Solid-solution strengtheners; produces strengthening by formation of intermetallic compound Co3Mand MC carbide; formation of M6C carbide

Carbon Produces strengthening by formation of carbides MC, M7C3, M23C6, and possibly M6CAluminum Improves oxidation resistance; formation of intermetallic compound CoAlTitanium Produces strengthening by formation of MC carbide and intermetallic compound Co3Ti with

sufficient nickel produces strengthening by formation of intermetallic compound Ni3TiBoron, zirconium Produces strengthening by effect on grain boundaries and by precipitate formation; zirconium

produces strengthening by formation of MC carbidesYttrium, lanthanum Increase oxidation resistance

Fig. 5 Electron backscattered image of as-cast Tribaloy T-400C. The white areas are the Laves phase. Original mag-nification at 500�. Courtesy of J. Wu, Deloro Stellite Group Ltd.


cobalt-chromium matrix with chromium con-tents ranging from approximately 18 to 30%.The high chromium content contributes to oxi-dation resistance, hot corrosion resistance, andsulfidation resistance, but also participates incarbide formation (Cr7C3 and M23C6) and solid-solution strengthening. Carbon content generallyranges from 0.25 to 1.0%, with nitrogen occa-sionally substituting for carbon. MAR-M alloy509 is an alloy designed for vacuum investmentcasting. Compared to the wrought alloys, castcobalt-base superalloys are characterized byhigher contents of high-melting metals (chro-mium, tungsten, tantalum, titanium, and zirco-nium) and by higher carbon contents.

Metallographic Preparation of Hastelloyand Haynes Alloys (Ref 22). The following areguidelines for etching to produce satisfactorysamples. It is the responsibility of individuals tofollow safe practices and to use protective equip-ment in accordance with all appropriate local,municipal, state, and federal regulations cover-ing safety and health.

Sectioning. Cut the specimen to a convenientsize using any of various types of SiC cutoffblades. Deformation damage can be minimizedby using thin cutoff wheels (1⁄32 in. thick as op-posed to 1⁄16 in.). This is especially beneficial ifthe sectioning is to be done dry. Adequate watercoolant is desired to reduce the amount of dis-turbed metal created, in part, from frictional heatduring this phase of preparation. The originalmicrostructure of a specimen may also be radi-cally altered, at least superficially, on the cut sur-face due to metallurgical changes if an excessiveamount of frictional heat is generated.

Coarse Grinding. Use a 120 grit silicon car-bide (SiC), wet-belt grinder and light contactpressure to obtain a plane surface free from deepgrooves. In addition to producing a flat surface,this procedure removes burred edges or othermechanical abuses that may have occurred dur-ing sectioning.

Mounting. To ensure flatness and facilitatehandling, it is recommended that specimens bemounted in phenolic, acrylic, or cold-setting ep-oxy resins. Epoxy resins involve the blending ofa liquid or powder resin in a suitable hardener toinitiate an exothermic reaction to promote hard-ening and curing at room temperature. This usu-ally requires an overnight operation. However,an advantage of epoxy is that the mount is sem-itransparent and permits observation of all sidesof the specimen during each phase of the prep-aration.

Compression molding techniques may beused with phenolic powders to produce the stan-dard 1.25 in. (�32 mm) diam mounts. Phenolicmounts are convenient when time constraints donot permit an overnight cold-setting operation.

Fine Grinding. Rotating disks flushed withrunning water are recommended with succes-sively finer grit papers of 220, 320, 400, and 600grit SiC. (A light to medium amount of pressureis exerted on the specimen to minimize the depthof deformation). Best results are obtained on the600 SiC paper by grinding the specimen twice.

Specimens are rotated 90� after each step untilthe abrasive scratches from the preceding grithave been removed. In each step, the grindingtime should be increased to twice as long as thatrequired to remove previous scratches. This en-sures removal of disturbed metal from the pre-vious step. Considerable care should be used inthe fine grinding state to prevent the formationof artifacts.

Rough Polishing. The specimen should behand washed and, preferably, ultrasonicallycleaned to ensure the complete removal of SiCcarryover from the fine-grinding stage. A pellonpan-W type cloth should be charged with 9 lm

diamond paste, and water should be used as thelubricant. The specimen is moved counter to thedirection of the rotating polishing wheel from thecenter to the outer periphery around the entirelapping surface. Extremely heavy pressure isused with diamond abrasive techniques to gainthe maximum cutting rate. At the conclusion ofthis stage, the specimen should again be ultra-sonically cleaned to remove any diamond pol-ishing residue remaining in pinholes, cracks, andcavities.

Vibratory Polishing. Semifinal and final pol-ishing operations on a major portion of metal-lographic specimens can be completed on vibra-

Fig. 7 As-cast microstructure of Co-Cr-W-Ta superalloy (MAR-M 302). (a) Structure at 100� reveals primary or, eu-tectic, M6C carbides (dark gray) and MC particles (small white crystals in the solid-solution matrix). (b) At higher

magnification (500�), the mottled gray islands are primary eutectic carbide; the light crystals are MC particles; thepepperyconstitutes within grains is M23C6. Kalling’s reagent

Fig. 6 As-cast structure of Co-Cr-W-Ta superalloy (MAR-M 509). (a) The structure consists of metal carbide (MC)particles in script form and M23C6 particles in eutectic form (gray areas) and precipitate form in the dendritic

alpha solid-solution matrix. (b) Higher magnification reveals morphology of the MC script particles, primary eutecticparticles, and precipitated M23C6 (shadowy constituent). (a) Kalling’s reagent. 1000�. (b) Electrolytic: 5% phosphoricacid, 500�


tory polishing units such as the Syntron units. Anylon polishing cloth using a slurry of 30 g ofLinde type “A” alumina polishing abrasive and500 mL of distilled water are recommended forthis operation. Additional weight in the form ofa stainless steel cap must be placed on the spec-imen. The suggested weight to achieve a satis-factory polish in 20 to 30 min on a standard 1.25in. (�32 mm) diam mount is 350 g.

Samples should be cleaned with a cotton swabunder running water to remove type “A” aluminafilm, placed on a short-nap microcloth with aslurry of 30 g of Linde type “B” alumina abra-sive and 500 mL of distilled water and polisheduntil a scratch-free surface is obtained. Again, a350 g weight is used to augment polishing. Spec-imens usually require 25 to 30 min to produce asatisfactory final polish. The specimen can usu-ally be polished an additional 10 to 15 min with-out producing harmful overpolishing effects, buttoo much time may create relief on thin samples.

Surface Preparation. The surface, prior toetching, should:

● Be free from scratches, stains, and other im-perfections that mar the surface

● Contain all nonmetallic inclusions intact● Not exhibit any appreciable relief effect be-

tween microconstituents

Immediately prior to etching, specimens shouldbe lightly polished (Linde type “B” wheel) andswabbed with cotton under running water to re-move any air-formed oxide film to reducechances of staining.

Electrolytic Etching. Place the specimen faceup in the etching reagent. The cathode is placedapproximately 25 mm (1 in.) from the specimen,and the anode is put in contact with the sample.During etching, the cathode is moved to ensurea uniform action of the etching reagent on thespecimen. The sample is then washed and re-polished lightly, if needed, to remove any traces

of disturbed metal on the surface and then re-etched.

The following etchant is used for most Has-telloy and Haynes alloys, with the exception ofHastelloy alloys B, B-2, N, and W (see the fol-lowing section “Immersion Etching”):

● 5 g oxalic acid mixed with 95 mL HCl (re-agent grade)

● Electrolytic—6 V dc● Carbon cathode● Stainless anode probe● 1 to 5 s, depending on heat treated condition

and size of samples

The procedure is:

● Sample must have a fresh polish. If surfacehas been dry, even for a few seconds, givesample 6 to 10 laps on final 0.05 lm alumina(Linde “B”) cloth then directly under runningwater and swab with a cotton pad. It is im-portant that the sample surface be kept wet.

● Put sample face up in etchant. Then, withgood overhead light to visually see samplesurface, make contact at end or corner of sam-ple with anode probe, dip carbon cathode intoetchant, watch to see any surface change,break contact.

● Before removing sample from etchant, it isimportant to agitate to remove any film onsurface, then pull sample and put under run-ning water, next rinse with methanol, thenplace sample under hair dryer until it is thor-oughly dry.

● If etch is too light and needs to be heavier, donot take sample back to running water andthen into etchant. Instead, it must go back tothe final cloth for 6 to 10 laps, making surethat no part of surface dries; failure to do thiscan, and most likely will, result in staining. Ifthe sample does stain, do not try to removestain on final cloth. Rather, go back to thepapers, at least to the 400 and 600 grit, then

Fig. 8 Haynes 31 casting microstructures. (a) As-cast structure consisting of large primary M7C3 particles and grain-boundary M23C6 in an fcc matrix. 400�. (b) As-cast thin section,aged 22 h at 730 �C (1350 �F) with precipitated M23C6 at grain boundaries and adjacent to primary (M7C3) particles. 400�. (c) As-cast thick section aged 22 h at 730 �C

(1350 �F); large particles are M7C3; grain boundary and mottled dispersions are M23C6 in fcc matrix. Electrolytic: 2% chromic acid. 500�

Fig. 9 Haynes 25 solution annealed at 1205 �C (2200 �F) and aged. (a) Aged at 650 �C (1200 �F) for 3400 h; constituentsare M6C and M23C6 in a mixed hcp and fcc matrix. (b) Aged at 870 �C (1600 �F) for 3400 h; structure consists

of precipitates of M6C and “Co2W” intermetallic compound in fcc matrix. Electrolytic: HCl. H2O2. Both 500�


9 lm diamond and then to 0.05 alumina, andagain, keeping sample surface wet, repeat asdescribed before.

Immersion etching techniques are usuallyused for the high-molybdenum alloys, namelyHastelloy alloys B, B-2, N, and W. The preferredetchant for this family of alloys is chrome-regia(one part chromic acid to three parts reagentgrade HCl). Stock chromic acid is made by mix-ing 300 g chromic acid with 300 mL of hot wa-ter.

For immersion etching, it is equally importantto work with a wet, freshly polished surface (i.e.,follow procedure steps above for electrolytic

etching). The wet sample is then immersed faceup into the chrome regia for 1 to 3 s, dependingon heat treated condition and sample size. Thenpull sample and put under running water, rinsewith methanol, and blow dry. If etch is too light,follow the last step of electrolytic procedures.

Cobalt-BaseCorrosion Resistant Alloys

Several low-carbon, wrought cobalt alloys areused to satisfy applications that require corrosionresistance with the attributes of cobalt as an alloy

base (resistance to various forms of wear andhigh strength over a wide range of temperatures).The cobalt-base wear-resistant alloys possesssome resistance to aqueous corrosion, but theyare limited by grain-boundary carbide precipi-tation. Wrought cobalt superalloys (which typi-cally contain tungsten rather than molybdenum)are even more resistant to aqueous corrosionthan the Stellite alloys, but they still fall wellshort of the Ni-Cr-Mo alloys in corrosion per-formance.

Compositions of several cobalt alloys for cor-rosion-resistant applications are listed in Table1(b). They include implant Co-Ni-Cr-Mo alloys(such as MP35N) and the Co-Cr-Mo alloys (such

Fig. 10 Microstructures of aged Haynes 188 alloys. (a) Cold-rolled 20% and fully annealed structure with M6C particles in an fcc matrix; solution anneal was 1177 �C (2150 �F) for10 min, water quenched. (b) Solution anneal (at 1177 �C) with aging at 650 �C (1200 �F) for 3400 h. (c) Solution annealed (at 1177 �C) and aged at 870 �C (1600 �F) for

6244 h. Structure consists of M23C6, Laves phase, and probably M6C in fcc matrix. Electrolytic HCl, H2O2. All 500�

Fig. 11 Microstructures of Co-Cr-Mo alloys. (a) Investment cast with relatively coarse carbides and the large grain size (ASTM macrograin size 7.5). (b) High-strength fine-grainforging from nitrogen-strengthened bar stock. (c) Hot isostatically pressed from powder


as Vitallium) for prosthetic devices and implantson account of their excellent compatibility withbody fluids and tissues. Cobalt-chromium-mo-lybdenum alloys are still used for biomedical ap-plications, while the use of MP35N alloy hasdeclined partially due to concerns about nickelrelease from the alloys, which can cause metal-sensitivity reactions in some patients. The Ulti-met alloy (UNS R31233) combines excellentcorrosion resistance to pitting (especially in ox-idizing acids) with very high wear resistance(cavitation erosion, galling, and abrasion).

These alloys are provided in various wroughtforms, in the work-hardened or work-plus-age-hardened condition. The alloys work harden rap-idly due to strain-induced transformation, whichprovide a dispersion of fine hcp platelets.Higher-strength, fine-grained Co-Cr-Mo alloysfor implant devices also have been made by forg-ing of nitrogen-strengthened bar stock or by hotisostatic pressing (HIP) of carbide-strengthenedpowders. Microstructures of these materials arecompared with a cast microstructure (Fig. 11).

REFERENCES

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tribaloy 800 a

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