Nickel and Nickel alloys
Metallographic Preparation Solutions by Material
Use this guide for advice on where to start when performing metallographic preparation and analysis on nickel and nickel alloys. Included are tips for achieving the best results for your metallographic application.
Things to consider when working with nickel and nickel alloys
Varying compositions and thermal treatments of nickel alloys may require slight adjustments to preparation processes. Some of these alloys are resistant to scratch removal, additional polishing may be in order.
Nickel and Nickel Alloys Cutting Tips
All sectioning should be performed wet, with an ample flow of coolant directed into the cut. Wet cutting will produce a smoother surface finish than a dry cut. Using coolant will also guard against surface damage caused by overheating and mechanical strains. Reducing damage while sectioning is important. Mechanical damage, such as fractures can penetrate deep into the structure and make later preparation steps longer. There are two main types of cutting tools offered: Abrasive and Precision.
Abrasive Cutting Tips
Abrasive blades have different bond strengths and are recommended based on the efficiency of their bond strength and abrasive type with different materials. Choosing an incorrect blade can lead to an improper wear rate and possibly alter the existing microstructure. An abrasive blade is entirely made up of abrasive that is bound together by a resin. When used, blades get smaller in size until they can no longer cut. A new blade can then be put on the sectioning piece of equipment to make additional cuts.
The size of the abrasive blade also affects cutting parameters. Larger, thicker blades remove more material but also generate more heat than a thinner blade. To reduce the heat produced by blades the feed rates should be lowered.
Nickel is an abrasion-resistant material that can be difficult to section efficiently while minimizing subsurface damage. Buehler’s superalloy blades are specially formulated to sectioning these alloys efficiently while minimizing deformation.
| Recommended abrasive blade for nickel and nickel alloys | |||
|---|---|---|---|
| 10in (254mm) | 12in (305mm) | 14in (356mm) | 16in (406mm) |
| 12-4205-010 0.051in [1.3mm] | 10-31205-010 0.08in [2mm] | 10-31405-010 0.10in [2.5mm] | 10-31605-010 0.125in [3mm] |
Precision Cutting Tips
Precision cutters are used in metallographic preparation of nickel when cutting samples that are small or delicate. Precision blades don’t break down as they are being used. On the outer edge of the blade there is a section where abrasive has been bonded with metal alloy and if taken care of one wafering blade can outlast a box of abrasive blades. Wafering blades can be used with nickel for more difficult cuts including making a precise cut close to a feature of interest, or when it is critical to reduce the width of a cut (kerf loss).
Additional benefits of using a precision cutter are a lower load is applied and consequently, less heat is generated, reducing the amount of damage.
Abrasive blades are also made for precision cutters. They can be used when potential for blades breaking is high. This could be if you have a difficult to clamp part, have a part that might shift during sectioning or you have more variability in the process (ex: multiple operators).
Cubic boron nitride (CBN) wafering blades are an effective way to section nickel alloy components with a precision saw. Abrasive blades can be an alternative to the CBN wafering blades. When using abrasive blades there are some things to keep in mind. Cleaning out the recirculation tank will have to be done more frequently. The external recirculation tank is recommended when using the IsoMet High Speed. The diameter of abrasive blades is reduced as the blade wears while cutting. When too small to cut a new blade can be put on the saw.
| Recommended Precision blades for nickel and nickel alloys | |||
|---|---|---|---|
| 5in (127mm) | 7in (178mm) | 8in (203mm) | Dressing Stick |
| 11-5265 0.006in [0.15mm] | 11-5267 0.012in [0.3mm] | 11-5268 0.035in [0.9mm] |
11-1190
11-2490 |
Nickel and Nickel Alloys Mounting Tips
Mounting samples also protects and preserves edges and surface defects during metallographic preparation. The method of mounting should in no way alter the microstructure of the specimen. Pressure and heat are the most common sources of injurious effects. Temperatures around 350oF and pressures around 4200 PSI are common for hot compression mounting.
Sample geometry and properties can determine the appropriate method for mounting aluminum samples. Use castable mounting when samples are thin or deformed easily. Samples such as these tend to be sensitive to high pressures. If samples are not sensitive to pressure hot compression mounting can be a time and money saver.
For electron microscopy ProbeMet® might be the conductive media you are looking for. If copper content is of interest in the sample KonductoMet® is a conductive alternative to ProbeMet.
Careful selection can ensure media meets mounting needs regarding timing, viscosity, temperature, and shrinkage. Acrylic systems can increase throughput with quick cure times between 5 and 30 minutes. Though quicker in curing, sample adhesion and viscosity of acrylics can sometimes be lacking especially with more intricate shapes. This is where epoxy systems can help. With longer working times mixed epoxy can be used with a vacuum system to help penetration levels in intricate geometries.
Nickel and Nickel Alloys Grinding & Polishing Tips for All Methods
For Ni-Cu, Ni-Cr-Fe, Ni-Fe alloys result in better surface finishes when the five-step preparation method is used.
For age-hardened superalloys, instead of Apex® Hercules S discs, Apex Hercules H discs should be used for the second step in the four-step process suggested. If polishing results with the second method are not yielding desired results, add a 1 micron step with a trident cloth after the 3 micron step.
A vibratory polishing step can be added at the end of the polishing procedure if scratches persist. The same oxide suspension used in the last mechanical step is commonly used in vibratory polishing. For pure nickel try using MasterPrep® final suspension. For nickel alloys, a vibratory polishing step with MaterMet® can help create a scratch-free surface.
Using Buehler’s Burst dispensing system can conserve diamonds and improve consistency.
The ideal rate for the Burst dispensing system changes with the size of the platen and the polishing cloth that is being used. As a general guideline, for platen size of 8” a burst setting of 3 is a good starting point, for sizes of 10″ and 12″ set the burst system to 4 and adjust as needed. Some experimentation may be required to determine optimal settings to ensure sufficient abrasive and wetting of the cloth. Burst dispensers are also capable of dispensing extenders simultaneously with diamond, if desired.
Loads listed in grinding and polishing methods are recommendations for one 1.25″ mounted specimen. If using central force during preparation the force listed should be multiplied by the number of samples being polished. For different sample sizes, use our load conversion calculator to determine the correct load for your application.
| Procedure for Grinding and Polishing Ni, Ni-Cu and Ni-Fe alloys | ||||
|---|---|---|---|---|
| Surface | Loads [N] | Base Speed [rpm] | Relative Rotation | Time |
| CarbiMet® 240 grit | 6 [27] | 300 rpm |
|
Until Plane |
| UltraPad® with 9um MetaDi® Supreme Diamond | 6 [27] | 150 rpm |
|
5:00 |
| Trident® with 3um MetaDi Supreme Diamond | 6 [27] | 150 rpm |
|
3:00 |
| Trident with 1um MetaDi Supreme Diamond | 6 [27] | 150 rpm |
|
2:00 |
| ChemoMet® with MasterMet® Colloidal Silica* | 6 [27] | 150 rpm |
|
2:00 |
 = Platen  = Specimen Holder *Plus MetaDi Fluid Extender as desired
|
||||
| *Attack polish may be used, 1 part ammonium persulfate solution (10 g ammonium persulfate per 100 mL distilled water) or 30% hydrogen peroxide to 5 parts silica. | ||||
| Procedure for Grinding and Polishing Ni-Based Superalloys | ||||
|---|---|---|---|---|
| Surface | Loads [N] | Base Speed [rpm] | Relative Rotation | Time |
| CarbiMet® 320 grit | 6 [27] | 300 rpm |
|
Until Plane |
| Apex® Hercules H or S with 9um MetaDi Supreme Polycrystalline | 6 [27] | 150 rpm |
|
5:00 |
| Trident® with 3um MetaDi Supreme Diamond | 6 [27] | 150 rpm |
|
5:00 |
| ChemoMet with MasterMet Colloidal Silica* | 6 [27] | 150 rpm |
|
2:00 |
|
= Platen = Specimen Holder *Plus MetaDi Fluid Extender as desired
|
||||
| *Attack polish may be used, 1 part ammonium persulfate solution (10 g ammonium persulfate per 100 mL distilled water) or 30% hydrogen peroxide to 5 parts silica. | ||||
Nickel and Nickel Alloys Etching Tips
| Table 20.7: Nickel and Alloys | |
|---|---|
| Composition | Comments |
| 5g FeCl3 2mℓ HCl 99mℓ ethanol | Carapella’s etch for Ni and Ni-Cu (Monel) alloys. Use by immersion or swabbing. |
| 40-80mℓ ethanol 40mℓ HCl 2g CuCl2 | Kalling’s no. 2 etch (“waterless” Kalling’s) for Ni-Cu alloys and superalloys. Immerse or swab specimen up to a few minutes. |
| 50mℓ water 50mℓ HCl 10g CuSO4 | Marble’s reagent for Ni, Ni-Cu and Ni-Fe alloys and superalloys. Immerse or swab sample 5-60 seconds. Reveals grain structure of superalloys. |
| 15mℓ HCl 10mℓ glycerol 5mℓ HNO3 | "Glyceregia", for superalloys and Ni-Cr alloys. Swab specimen for 5-60 seconds. Mix fresh. Do not store. Use under hood. |
| 60mℓ glycerol 50mℓ HCl 10mℓ HNO3 | Modified Glyceregia for superalloys. Reveals preciptates. Use under hood; do not store. Add HNO3 last. Discard when dark yellow. Immerse or swab specime 10-60 seconds. |
Nickel and Nickel Alloys Imaging Tips
Depending on the alloy and application the goal of analysis can differ. For nickel alloys grain size and phase percent are common analysis types. OmniMet® software also incorporates scripts specifically designed to measure these according to the ASTM standards.
Nickel and Nickel Alloys Hardness Testing Tips
| Reference Materials for Hardness Testing | ||
|---|---|---|
| ASTM | ISO | |
| Brinell | E10 | 6506 |
| Rockwell | E18 | 6508 |
| Vickers | E92,E384 | 6507 |
| Knoop | E384 | 4545 |
| Instrumented | E2546 | 14577 |
| Conversions | E140 | 18265 |
| ASM Handbook Volume 8: Mechanical Testing and Evaluation | Hardness Testing Section | |
At Buehler, the equipment offered is made to perform at least one of the following scales: Brinell, Rockwell, Vickers and Knoop. If utilizing a scale previously mentioned there are standards for the method of testing. Standards such as the ones listed in the table are good places to reference the proper method of testing for the scales. Requirements for equipment, samples, testing methods and more are described within that can help determine the correct scale.
Equipment capabilities vary in levels of automation and documentation. Some pieces are integrated with hardness testing software. Software such as Buehler’s DiaMet can assist with testing parts more efficiently, accuracy and result documentation. This can be especially helpful when a large amount of indents must be done on one part.
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