METALLOGRAPHIC TECHNIQUES FOR SUPERALLOYS

George F. Vander Voort1, Elena P. Manilova2, Gabriel M. Lucas1

1Buehler Ltd., 41 Waukegan Road, Lake Bluff, IL 60044 USA 2Polzunov Central Boiler and Turbine Institute, Politechnicheskays str., 24, 194021 St. Petersburg, Russia

Keywords: Metallography, Specimen Preparation, Etching, Microstructure

Abstract

Superalloys are complex alloys of Fe-Ni, Ni, or Co-base compositions. Their microstructure can be quite complex due to the potential for a variety of phases that can be formed by heat treatment or service exposure conditions. The paper presents the use of new metallographic materials to prepare these alloys with emphasis on modern, four- and five-step practices. Different etchants are required to reveal the structure of these alloys properly. Examples will be presented showing the use of different etchants as a function of alloy composition, heat treatment, and microstructural phases.

Introduction

Preparation of superalloys for microstructural examination is not exceptionally difficult. The procedures are similar to those used to prepare stainless steels. Because they are face-centered cubic “austenitic” alloys with exceptionally good toughness, machinability is poorer than for steels and the age hardened alloys, especially the cast alloys, can be more difficult to section than most steels when they have a very high ' content. FCC metals readily deform and work harden, consequently aggressive sectioning methods (e.g., power-hacksawing or band sawing) will introduce considerable damage which can be very difficult to remove in the subsequent preparation steps. If these procedures must be used, it is advisable to re-section the material with the correct abrasive cutoff wheel (consumable type) with abundant cooling. These newly prepared surfaces will exhibit less damage and should be used as the starting surface for metallographic preparation. Many preparation problems can be traced to the generation of excessive cutting damage. Always use an abrasive cut-off blade design for metallographic work with superalloys. Otherwise, the plane-of-polish will probably exhibit remnants of the damage created during sectioning.

Preparation Methods

Mounting

Mounting of specimens may be performed, if desired, depending upon the nature of the analysis. Mounting is advisable for edge preservation whenever an exterior surface is to be examined. Compression-mounting thermosetting epoxy resins, such as Epomet

thermosetting resin, provide the best edge retention. Mounting presses that cool the specimen back to ambient temperature under pressure

reduce the occurrence and degree of shrinkage gaps. Shrinkage gaps degrade edge retention and lead to bleed-out problems of solvents or etchants and stains that obscure the edge inhibiting examination. Mounting may be merely a convenience factor as the back of the mount is an ideal surface for recording specimen details, or it may be useful to facilitate use of automated preparation devices.

Grinding

Historically, grinding performed using a series of water-cooled SiC papers of varying grit sizes (120, 240, 320, 400, and 600 grit) generally at 240 - 300 rpm on a rotating wheel. The specimen is held against the SiC paper by hand or through the use of a fixture. Typically, grinding with each paper is conducted for 60 - 120 seconds. If done manually, the specimen is rotated 45 - 90

between papers so that the grinding direction is not constant. Semiautomatic or automatic grinding machines produce omni-directional grinding patterns and yield superior flatness and edge preservation. Unless specimens are porous or contain cracks, a simple washing operation between steps is adequate. Otherwise, ultrasonic cleaning with a suitable solvent is required.

With today’s contemporary preparation methods, only one grinding step is used and this may be performed using SiC paper, or some other abrasive depending upon laboratory through put or personal preference. To make this approach work, sectioning must be performed with the best quality metallographic cut-off wheels so that sectioning damage is minimized. Then, the finest possible SiC grit size that removes the sectioning damage in a reasonable time can be used. Usually, 180- or 240-grit SiC paper can be used, or products with an equivalent abrasive particle size. As the size of the grinding abrasive increases, the depth of damage created by the grinding step increases. Consequently, to maximize the efficiency of the preparation method, and the results, always use the finest possible abrasive size that will remove the sectioning damage (which is minimized by the use of the best possible abrasive wheel) and get all of the specimens in the holder to the same common plane.

There are several alternatives to the use of SiC paper, and these work well on superalloys. One can use a rigid grinding disk (RGD) and a coarse size diamond abrasive, e.g., 45-µm, added to the RGD. The RGD has a very high removal rate and imparts exceptional flatness to the specimen. However, if you are preparing solution annealed specimens, or any non-age hardened specimen, the RGD chosen may be too aggressive as residual deformation of the matrix may be

hard to control. The Apex Hercules S rigid grinding disk has been specially formulated to avoid this problem. Another grinding option is a Diamond Grinding Disk. These have the diamond abrasive of various sizes, such as 70- and 45-µm, incorporated into the surface.

Polishing

In the traditional preparation method, grinding is followed by two to four polishing steps, depending upon the quality level needed. Polishing usually commences with either 6-

m or 3- m diamond abrasive, as a paste, slurry, or aerosol with the appropriate liquid extender/lubricant on a cloth pad. Historically, canvas, billiard, felt, nylon, or synthetic napless or low-nap cloths were used for these abrasive sizes. Now, rough polishing is conducted on napless cloths, such as UltraPol™, UltraPad™, TexMet® 1000, or TriDent™ cloths. The choice is often a matter of personal preference or a matter of cost. Wheel speeds are lower for polishing, generally 120 - 150 rpm. In the traditional method, some operators followed the first step with a second diamond abrasive step, generally 3- or 1- m in size, or they used one or two steps with aqueous aluminum oxide slurries. Cloths and wheel speeds are similar. The time for each step is 60 - 120 seconds. Cleaning between polishing steps must be performed carefully.

The most common fine polishing alumina abrasive sizes are 0.3- m (alpha alumina) and 0.05- m (gamma alumina). Recently, acidic alumina slurries and basic colloidal silica slurries have been employed for final polishing of superalloys. These can be highly effective. Cleaning after use of these abrasives requires careful scrubbing with cotton soaked in a liquid detergent solution. Colloidal silica does have some problems in its use. To clean the specimen properly, and the cloth, stop adding abrasive with about 20 s left in the cycle. With about 10 s left, direct the water jet onto the cloth. When the machine stops, wash the specimens and dry them with hot air. Despite such precautions, specimens polished with colloidal silica can exhibit an etching problem called “flashing.” For example, if an etchant containing Cl- ions (such as glyceregia) is used, it generally takes about a minute of swabbing to bring up the structure. However, sometimes when swabbing a specimen polished with colloidal silica, the surface darkens immediately upon etching. Examination shows a craze-crack pattern of deep scratches, Figure 1, while the structure is poorly revealed. This scratch pattern cannot be removed by repeating the final polishing step as the scratches are very deep. This problem appears to be due to surface Passivation.

MasterPrep® alumina suspension is a good alternative to the use of colloidal silica as it is totally free of this problem, as well as staining. Other alumina polishing abrasives are made by the calcinations process and they always contain agglomerates. MasterPrep alumina is made by the sol-gel process where the abrasive particles are precipitated from a solution. Vibratory polishers are commonly used for final polishing of superalloys. Cloths for final polishing, such as MicroCloth®, often have a nap that is good for scratch

Figure 1. Example of “flashing” that can occur when a specimen polished with colloidal silica is etched with a reagent that contains chlorine ions (glyceregia).

control. There are napless polishing cloths, such as the synthetic polyurethane Chemomet® pad, that can be effectively used for final polishing.

The contemporary practices utilize flat, napless cloths and pads, except for the final step. But, even in the final step, synthetic, napless polyurethane pads can be used. The contemporary procedures rely upon use of automated devices to improve specimen flatness and reproducibility of the procedures. A five-step method is shown in Table 1.

For this method, use pressure-sensitive adhesive (psa) backed SiC paper and cloths. A magnetic disk system, such as the Apex system, can be used effectively. Charge the choths with diamond in paste form and add MetaDi fluid lubricant. During the cycle, add small amounts of MetaDi Supreme diamond in suspension form to keep the cutting rate high. If you have a grinder/polisher with a head speed <100 rpm, you can use contra rotation (the head and platen rotate in opposite directions) and the abrasive consumption rate will be lower than if complementary rotation (head and platen rotate in the same, counterclockwise, direction). Use 6 pounds of load per specimen (27 N) with step 1 at 240-300 rpm, steps 2 to 4 at 120-150 rpm and step 5 at 80-150 rpm.

The five-step method is conservative and will yield perfect microstructures for Fe-Ni, Ni- and Co-base superalloys. If the specimens are relatively easy to prepare, and most age-hardened specimens are, step 4 can be eliminated. It is also possible to substitute other materials for step 1. For example, the ApexHercules S rigid grinding disk can be used with an addition of 45-µm diamond slurry for 2-3 minutes at 150 rpm, 6 lbs. load and contra or complementary rotation. Or, a 45-µm Diamond Grinding Disk can be used with similar settings.

Table 1. Contemporary Procedure for Ni-Based Superalloys

Surface Abrasive/Size Load Lb (N)/ Specimen

Platen Speed (rpm)/Direction

Time (minutes)

Carbimet® waterproof discs (psa)

220-240 (P240-P280) grit SiC water cooled 6

(27) 240-300 Comp.

Until Plane

Ultra-Pol Silk Cloth (psa)

9-µm Metadi® Supreme Diamond Suspension 6

(27) 100-150 Comp.

5

Trident cloth (psa) 3-µm Metadi® Supreme Diamond Suspension 6

(27) 100-150 Comp.

4

Trident cloth (psa) 1-µm Metadi® Supreme Diamond Suspension 6

(27) 100-150 Comp.

3

Microcloth pad (psa) 0.05-µm Masterprep™ alumina slurry 6 (27)

80-150 Contra

2

Table 2. Etchants for Superalloys

1. 15 ml HCl 10 ml glycerol 5 ml HNO3

Glyceregia. Mix fresh; do not store. Good for about 20 minutes. Use by swabbing. General purpose etch.

2. 15 ml HCl 10 ml Acetic acid 5 ml HNO3

1 - 2 drops glycerol

Acetic glyceregia, stronger than glyceregia. Mix fresh; do not store. Good for about 20 minutes. General purpose etch.

3. 15 ml HCl 10 ml Acetic acid 10 ml HNO3

Mixed acids, for the most corrosion-resistance grades; e.g., Alloy 625. Mix fresh; do not store. Good for 30 minutes. General purpose etch.

4. 5 g CuCl2

100 ml HCl 100 ml ethanol

Waterless Kalling’s reagent (number 2). This reagent can be made as a stock solution. Good grain boundary etch.

5. 5 ml HF 10 ml glycerol 85 ml ethanol

Electrolytic etch at 0.04 - 0.15 A/ m2, 6 - 12 V dc for Ni-based alloys. is in relief. Stop etching when edges get brownish in color.

6. 5 ml H2SO4

3 ml HNO3

92 ml HCl

Add sulfuric to HCl, stir, allow to cool; add nitric. Discard when etch turns orange. Use under hood. Do not store.

7. 50 ml HCl 1 - 2 ml H2O2 (30%)

Attacks in Ni-base alloys. Immerse for 10 - 15 seconds.

8. 10 g K3Fe (CN)6

10 g KOH 100 ml water

Murakami’s reagent. Mix fresh; do not store. Carbide darkened at room temperature; sigma darkened when used hot (75 C).

9. 50 ml lactic acid 150 ml HCl 3 g oxalic acid

Lucas’s reagent for Fe-Ni, Ni- and Co-base superalloys. Use electrolytically at 1-2 V dc, for 10-20 s. Electrolyte can be made as a stock solution.

10. 100 ml water 100 ml HCl 100 ml HNO3

3 g molybdic acid

Mix and allow the solution to age for at least 1 h before use. Immerse specimen for a few seconds to reveal as-cast structure. Reagent can be made as a stock solution.

Etching

Numerous etchants are used to reveal the structure of superalloys. However, as for most metals, it is wise to examine the polished surface carefully before etching. Some minor second-phase particles can be easily observed in the as-polished condition using bright field illumination. Obviously, nonmetallic inclusions can be best observed in the as-polished condition. Most wrought superalloys are double vacuum-melted (VIM/VAR) and have extremely low sulfur and oxygen contents and, thus, very low inclusion contents. The sulfur content in many alloys is well below 0.001 wt. %, and no sulfides are observed. Cast superalloys are generally made by vacuum induction melting (VIM) and contain higher inclusion contents (and more nitrides and carbides) than wrought alloys.

Several etchants are commonly used to reveal the general structure of superalloys. Table 2 lists some of the most commonly used etchants. Due to their excellent corrosion resistance, most etchants work best by swabbing with cotton soaked in the etchant. Immersion etching often results in a more irregular etch response. Many of the etchants must be mixed fresh and used within a short time span. Glyceregia is the mildest of the first three etchants. If etching occurs too slowly, use etchant 2 or 3.

Examples of Superalloy Microstructures

Cast Alloys

Metallographers are often requested to reveal the dendritic structure of cast specimens and perform measurements of the secondary dendrite arm spacing. Figure 2 shows the microstructure of as-cast CNK7, a Russian cast alloy turbine blade that was tested in creep for 20,100 hours at 700 ºC at a load of 60 kg/mm2. The dendrites are shown after etching with glycregia, waterless Kalling’s, and the Lucas electrolytic reagent.

Figure 2 a

Figure 2b

Figure 2c

Figure 2. Dendritic solidification structure in Russian alloy CNK7 creep-tested turbine blade revealed using a) glyceregia, b) waterless Kalling’s; and c) the Lucas electrolytic reagent (2 V dc, 10 s).

Figure 3 shows the dendrites in IN738 etched with a variety of reagents. Note that these dendrites are much finer than those in the CNK7 creep-tested turbine blade. Waterless Kalling’s reagent (Fig. 3a) and reagent 3 (the “15-10-10” reagent, Fig. 3b) revealed the dendrites with less contrast than the Lucas electrolytic etch.

Figure 3a

Figure 3b

Figure 3c

Figure 3. Dendritic solidification structure of IN738 revealed by a) waterless Kalling’s reagent, b) the 15-10-10 reagent (No. 3, Table 1); and c) the Lucas reagent (2 V dc, 10s).

Figure 4a

Figure 4b

Figure 4c

Figure 4. Dendritic solidification structure in MAR-M247 revealed using a) glyceregia, b) Lucas reagent (2 V dc, 10 s); and c) the molybdic acid reagent.

Figure 5. Dendritic solidification structure of MAR-M509 etched with the Lucas reagent (2 V dc, 10 s).

Figure 4 shows the as-cast dendritic structure in MAR-M247 after etching with glyceregia (Fig. 4a), the Lucas electrolytic etch (Fig. 4b) and the molybdic acid etch. The molybdic acid reagent produced the strongest contrast.

The dendritic structure of cast MAR-M509, a Co-based superalloy, is shown in comparison in Figure 5 etched with the Lucas electrolytic reagent.

Wrought Alloys

Wrought alloys are evaluated for grain size and the presence of second-phase precipitates. To illustrate the effectiveness of the preparation method, and the use of different etchants, several examples will be shown below. Figure 6 shows the microstructure of Alloy 718, the most commonly produced superalloy. This specimen is a non-rotating grade with a fine grain size and considerable delta phase and large MC-type primary carbides.

Figure 6a

Figure 6b

Figure 6c

Figure 6. Microstructure of Alloy 718 (non-rotating grade) with a fine grain size, considerable delta and large primary MC carbides revealed using a) glyceregia, b) the “15-10-10” etch (No. 3); and c) the Lucas reagent (2 V dc, 20 s).

In comparison to Figure 6, Figure 7 shows the microstructure of premium quality, rotating grade Alloy 718. It actually contained less primary MC carbides, although Figures 7a and b illustrate examples of the stringers present, and some delta is present but less than in the previous example (non-rotating grade). Etching with glyceregia only outlined the carbides after about one minute of swabbing. The “15-10-10” reagent produced a grain contrast etch. The Lucas reagent revealed the fine grain structure and the delta phase properly.

Figure 7a

Figure 7b

Figure 7c

Figure 7. Microstructure of premium quality Alloy 718 revealed using a) glyceregia, b) the “15-10-10” reagent; and c) Lucas reagent (2 V dc, 20 s).

In some cases metallographers examine the microstructure of specimens after forging and the grain structure is not fully recrystallized as demonstrated in Figure 8 that shows the structure of as-forged Carpenter Custom Age 625 Plus. This alloy is too corrosion resistant to use glyceregia. Figure 8 a and b show the results of etching with the “15-10-10” reagent, developed for this grade, and with the Lucas reagent. Both revealed the structure well.

Figure 8a

Figure 8b

Figure 8. Partially recrystallized grain structure of as-forged Custom Age 625 PLUS revealing a “necklace” type duplex grain structure after etching with a) the “15-10-10” reagent and b) the Lucas reagent (2 V dc, 20 s).

The final example shows the microstructure of a wrought, fully annealed Co-based alloy, Haynes 188. Figure 9 shows the specimen etched with a) the “15-10-10” reagent and b) with the Lucas reagent (2 V dc, 20 s). Note that both reagents reveal some dislocation etch pitting.

Figure 9a

Figure 9b

Figure 9. Microstructure of wrought, annealed Haynes 188 etched with a) the “15-10-10” reagent and with b) the Lucas reagent (2 V dc, 20 s).

Conclusions

A modern preparation cycle with five steps was presented in this paper. This procedure works perfectly for all Fe-Ni, Ni- and Co-based superalloys. For age-hardened alloys, step 4 can be eliminated to save time. Other products can be substituted for SiC in the initial grinding step. Always section specimens with an abrasive cut-off wheel designed for the metallographic preparation of superalloys. Lesser quality (“production” or shop wheels) will impart far more damage that may be impossible to remove. After proper sectioning, commence preparation with the finest possible abrasive size. Modern napless woven or pressed cloths and pads produce far better results, controlling flatness and reducing artifacts. MasterPrep alumina suspension, made by the sol-gel process, eliminates etching problems when chlorine ion containing reagents are used. A new reagent, the Lucas electrolytic reagent was illustrated. It is a versatile reagent for superalloys.