George F. Vander Voort Buehler Ltd.
SPECIMEN PREPARATION is an extremely important precursor to image analysis work. In fact, more than 90% of the problems associated with image analysis work center on preparation. Once a well prepared specimen is obtained and the phase or constituent of interest is revealed selectively with adequate contrast, the actual image-analysis (IA) mea-surement is generally quite simple. Experience has demonstrated that getting the required image quality to the microscope is by far the biggest problem. Despite this, many treat the specimen preparation stage as a trivial exercise. However, the quality of the data is primarily a function of specimen preparation. This can be compared to the classic computer adage, “garbage in, garbage out.”
Sampling
The specimen or specimens being prepared must be representative of the material to be examined. Random sampling, as advocated by statisticians, rarely can be performed by metallographers. An exception is fastener testing where a production lot can be randomly sampled.
However, a large forging or casting, for example, cannot be sampled randomly because the component might be rendered useless commer-cially. Instead, systematically selected test locations are widely used, based on sampling convenience. Many material specifications dictate the sampling procedure. In failure studies, specimens usually are removed to study the origin of failure, examine highly stressed areas or secondary cracks, and so forth. This, of course, is not random sampling. It is rare to
encounter excessive sampling, because testing costs usually are closely controlled. Inadequate sampling is more likely to occur.
In the vast majority of cases, a specimen must be removed from a larger mass and then prepared for examination. This requires application of one or more sectioning methods. For example, in a manufacturing facility, a piece may be cut from incoming metal barstock using a power hacksaw or an abrasive cutter used without a coolant. This sample is sent to the laboratory where it must be cut smaller to obtain a size more convenient for preparation. All sectioning processes produce damage; some methods, such as flame cutting and dry abrasive cutting, produce extreme amounts of damage. Traditional laboratory sectioning procedures using abrasive cut-off saws introduce a minor amount of damage that varies with the material being cut and the thermal and mechanical history of the material.
Generally, it is unwise to use the sample face from the original cut made in the shop as the starting point for metallographic preparation because the depth of damage at this location can be quite extensive. This damage must be removed if the true structure is to be examined. However, the preparation sequence must be carefully planned and performed because abrasive grinding and polishing steps also produce damage (depth of damage decreases with decreasing abrasive size), and preparation-in-duced artifacts will be interpreted as structural elements.
The preparation method should be as simple as possible, yield consis-tent, high-quality results in a minimum of time and cost, and must be reproducible. The prepared specimen should have the following charac-teristics, which can be segmented and measured, to reveal the true structure:
O Deformation induced by sectioning, grinding, and polishing must be removed or be shallow enough to be removed by the etchant.
O Coarse grinding scratches must be removed, although very fine polishing scratches often do not interfere with image segmentation.
O Pullout, pitting, cracking of hard particles, smear, and so forth must be avoided.
O Relief (i.e., excessive surface height variations between structural features of different hardness) must be minimized.
O The surface must be flat, particularly at edges (if they are of interest).
O Coated or plated surfaces must be kept flat to be able to precisely measure width.
O Specimens must be cleaned adequately between preparation steps, after preparation, and after etching (avoid staining).
O The etchant chosen must be selective in its action (that is, it must reveal only the phase or constituent of interest, or at least produce strong contrast or color differences between two or more phases present), produce crisp, clear phase or grain boundaries, and produce strong contrast.
Many metallographic image analysis studies require more than one specimen. A classic case is evaluation of the inclusion content of steel.
One specimen is not representative of the entire lot of steel, so sampling becomes important. ASTM standards E 45, E 1122, and E 1245 give advice on sampling procedures for inclusion studies.
To study grain size, it is common to use a single specimen from a lot.
This may or may not be adequate, depending on the nature of the lot.
Good engineering judgment should dictate sampling. In many cases, a product specification may rigorously define the procedure. Because grain structure is not always equiaxed, it can be misleading to select only a plane oriented perpendicular to the deformation axis (transverse plane) for such a study. If grains are elongated due to processing, the transverse plane usually shows that the grains are equiaxed in shape and smaller in diameter than the true grain size. To study the effect of deformation on the grain shape of wrought metals, a minimum of two sections is required:
one perpendicular to, and the other parallel to, the direction of deforma-tion. Techniques used to study anisotropic structures in metals incoporate unique vertical sampling procedures, such as in the trisector method (Ref 1–5).
Preparation of metallographic specimens (Ref 6–8) generally requires five major operations: (1) sectioning, (2) mounting (optional), (3) grinding, (4) polishing, and (5) etching (optional).
Sectioning
Bulk samples for sectioning may be removed from larger pieces or parts using methods such as core drilling, band and hack sawing, flame cutting, and so forth. When these techniques must be used, the microstructure will be heavily altered in the area of the cut. It is necessary to resection the piece in the laboratory using an abrasive-wheel cutoff system to establish the location of the desired plane of polish. In the case of relatively brittle materials, sectioning may be accomplished by fracturing the specimen at the desired location.
Abrasive-Wheel Cutting. By far the most widely used sectioning devices in metallographic laboratories are abrasive cut-off machines (Fig.
1). All abrasive-wheel sectioning should be done wet; direct an ample flow of water containing a water-soluble oil additive for corrosion protection into the cut. Wet cutting produces a smooth surface finish and, most importantly, guards against excessive surface damage caused by overheating. Abrasive wheels should be selected according to the recom-mendations of the manufacturer. In general, the bond strength of the material that holds the abrasive together in the wheel must be decreased with increasing hardness of the workpiece to be cut, so the bond material can break down and release old dulled abrasive and introduce new sharp
Specimen Preparation for Image Analysis / 37
abrasive to the cut. If the bond strength is too high, burning results, which severely damages the underlying microstructure. The use of proper bond strength eliminates the production of burnt surfaces. Bonding material may be a polymeric resin, a rubber-based compound, or a mixture of the two. In general, rubber offers the lowest-bond-strength wheels used to cut the most difficult materials. Such cuts are characterized by an odor that can become rather strong. In such cases, there should be provisions to properly exhaust and ventilate the saw area. Specimens must be fixtured securely during cutting, and cutting pressure should be applied carefully to prevent wheel breakage. Some materials, such as commercial purity (CP) titanium (Fig. 2), are more prone to sectioning damage than many other materials.
Precision Saws. Precision saws (Fig. 3) commonly are used in metallographic preparation and may be used to section materials intended for IA. As the name implies, this type of saw is designed to make very precise cuts. They are smaller in size than the typical laboratory abrasive cut-off saw and use much smaller blades, typically from 8 to 20 mm (3 to 8 in.) in diameter. These blades are most commonly of the nonconsumable type, made of copper-base alloys and having diamond or cubic boron nitride abrasive bonded to the periphery of the blade. Consumable blades incorporate alumina or silicon carbide abrasives with a rubber bond and only work on a machine that operates at speeds higher than 1500 rpm.
These blades are much thinner than abrasive cutting wheels. The load applied during cutting is much less than that used for abrasive cutting,
Fig. 1 Abrasive cut-off machine used to section a specimen for metallo-graphic preparation
and, therefore, much less heat is generated during cutting, and depth of damage is very shallow.
While small section-size pieces that would normally be sectioned with an abrasive cutter can be cut with a precision saw, cutting time is appreciably greater, but the depth of damage is much less. These saws are widely used to section sintered carbides, ceramic materials, thermally-sprayed coatings, printed circuit boards, and electronic components.
Fig. 2 Damage to commercially pure titanium metallographic specimen resulting from sectioning using an abrasive cut-off wheel. The speci-men was etched using modified Weck’s reagent.
Fig. 3 A precision saw used for precise sectioning of metallographic speci-mens
Specimen Preparation for Image Analysis / 39