Volume 09 - Metallography and Microstructures Part 12 doc

Fig. 39 Zn-12Al-0.75Cu-0.02Mg alloy, as die cast in a cold- chamber machine. Structure is fine primary crystallites consisting of aluminum-rich solid solution and eutectoid in a matrix of eutectic. Compare with Fig. 38. Etchant 2, Table 1. 250× Fig. 40 Same alloy as Fig. 39, except gravity cast in a permanent mold. With slower freezing than the die- cast specimen in Fig. 39 , primary crystals and lamellar matrix are coarser; properties of the alloy are not sensitive to the freezing rate, and the casting has good strength. Compare with Fig. 39. Etchant 2, Table 1. 1000× Fig. 41 Hypoeutectic alloy 3 (ASTM AG40A; 4.1Al-0.35Mg), gravity cast same as Fig. 40. Zinc- rich primary solid solution in a eutectic matrix. This alloy has excell ent mechanical properties when die cast with rapid freezing, but properties decrease with slow freezing. Etchant 2, Table 1. 1000× Fig. 42 Alloy 3 (ASTM AG40A) within specified composition limits, exposed 10 days to w et steam at 95 °C (205 °F). Specimen shows no intergranular corrosion. Compare with lead-contaminated alloy 3 in Fig. 44. As- polished. 100× Fig. 43 Fracture surface of the 10-mm (0.375-in.) diam end of a tension test bar d ie cast from alloy 3 to which 0.018% Pb was added (0.005% Pb is allowed). Exposed 10 days to wet steam at 95 °C (205 °F). Dark ring is intergranular corrosion. See also Fig. 44. Not polished, not etched. 6× Fig. 44 Micrograph of edge of fracture surface in Fig. 43 . Subsurface intergranular corrosion (top) causes swelling and decrease in mechanical properties. Deliberate addition of 0.018% Pb to the alloy approximates the contamination that might occur from the use of remelted scrap. As-polished. 100× Fig. 45 Hot-rolled brass special zinc [99% Zn (min), 0.6% Pb (max), 0.03% Fe (max), 0.50% Cd (max)], under polarized light; grains are clearly defined. Etchant 1, Table 1. 250× Fig. 46 Same alloy as Fig. 45, except cold rolled and photographed under polarized light. Note distortion of the grains caused by cold working. Etchant 1, Table 1. 250× Fig. 47 Zinc containing 1% Cu, hot rolled. Polarized light illumination clearly defines the zinc-copper ε phase at grain boundaries. Etchant 1, Table 1. 250× Fig. 48 Cold-rolled Zn- 1Cu alloy, photographed under polarized light. Note the severe distortion of grains caused by cold working (compare with Fig. 47), Etchant 1, Table 1. 250× Fig. 49 Hot-rolled Zn-O.6Cu-0.14Ti alloy, photographed under polarized l ight to define the grains between titanium-zinc stringers (parallel to the direction of rolling). Etchant 1, Table 1. 250× Fig. 50 Replica electron micrograph of the hot-rolled alloy in Fig. 49 , showing the particles (white) that comprise the Ti-Zn stringers in that micrograph. Etchant 2. Table 1.4400× Fig. 51 Zn-22Al alloy (eutectoid composition), showing superplastic, fine- grained structure obtained by annealing at 350 °C (660 °F) and water quenching. See also Fig. 52. Etchant 2, Table 1.2500× Fig. 52 Same alloy as Fig. 51, after being held 1 h at 350 °C (660 °F) and air cooled. Structure consists of lamellar and granular α and η both products of eutectoid transformation. Etchant 2, Table 1. 2500× Fig. 53 Steel coated with Galfan (Zn-5Al-mischmetal) alloy. White zinc- rich phase is surrounded by eutectic phase. Note absence of intermetallic between coating and steel. Etchant 5, Table 1. 500×. (F.E. Goodwin) Powder Metallurgy Materials: Metallographic Techniques and Microstructures Leander F. Pease III, President, Powder-Tech Associates, Inc. Introduction POWDER METALLURGY (P/M) MATERIALS encompass enough differences to necessitate describing specific specimen preparation procedures in addition to those provided in the Section "Metallographic Techniques" in this Volume. The major difference between parts made of metal powders and those made of wrought metal is the amount of porosity. Sintered materials generally exhibit 0 to 50% porosity, which affects mechanical properties and strongly interferes with metallographic preparation and interpretation of the structure. When examining photomicrographs, it is important to determine how the specimens were prepared. Careful metallographic preparation is significant in the analysis of sintered structures, because the shape of the porosity is as important as the amount in judging sintered strength and degree of sintering. In metallographic preparation of most sintered specimens, the pores are smeared during grinding and rough polishing. This occurs to some degree even in materials whose pores have been filled with plastic resins. Proper polishing should open the smeared pores, then reveal their true shapes and amounts. Routine metallography of the type used on a medium- carbon, ingot-base steel will not suffice. Showing the proper amount of porosity is necessary to facilitate measuring the density variation from point to point over short distances (0.25 to 6.25 mm, or 0.01 to 0.25 in.), because such measurements cannot be made accurately using ASTM Standard B 328 (Ref 1). When the specimen is properly prepared, the area fraction of porosity will equal the volume fraction of porosity, and these must equal the porosity calculated from the measured and theoretical densities of the part: TM V T ρρ ρ ρ − = where V p is the volume fraction porosity, Tρ is the theoretical density, and Mρ is the measured density. Detailed information on density and porosity measurements can be found in Powder Metal Technologies and Applications, Volume 7 of the ASM Handbook. In a properly prepared specimen that is 80% dense, 20% of the area should appear as porosity if the part is uniformly dense. The surface of cold-pressed and sintered parts will always be somewhat denser than the interior because of pressure losses due to interparticle friction. However, parts that are P/M forged in tools at approximately 370 °C (700 °F) can have a chilled surface lower in density than the hotter, softer interior. During sintering of cold-pressed compacts, the original particle boundaries disappear and result in a plane of fine pores, then larger pores. In as-pressed parts, particle boundaries appear as thin, gray lines. The progress of sintering can be judged by the disappearance of these boundaries. The original particle boundaries are similar to elongated, disk-shaped pores and have very sharp corners. These are extreme stress raisers. There is virtually no bonding across the original particle boundaries. Proper specimen preparation is required to distinguish residual original particle boundaries from the thin, gray boundaries that often appear at the edges of smeared pores. Therefore, an improperly prepared specimen with smeared porosity is often erroneously judged to be undersintered. If micro-hardness testing is performed, proper presentation of the porosity will result in fewer diamond indentations falling into hidden pores and thus fewer wasted or incorrect readings. For additional information on microhardness testing of P/M materials, see the article "Metallography of Powder Metallurgy Materials" in Powder Metal Technologies and Applications, Volume 7 of the ASM Handbook. The open porosity in a mounted sintered part can result in trapping of water (moisture). During etching, this water will bleed out, resulting in staining. Water will also corrode some sintered materials and can evaporate, then condense on the objective lens of the microscope, resulting in a foggy image. Etchants cause similar problems. Open porosity can trap abrasives and carry them onto subsequent cloths, which should hold only fine abrasives. The result is an increased tendency toward scratching of specimen surfaces. Filling the pores with epoxy resins alleviates these difficulties, but requires considerable technique. Many of the interesting structures seen in P/M parts are caused by porosity and by the blends of elemental powders that constitute many alloys. These blends do not always result in homogeneous, well-diffused structures. Such heterogeneity is not necessarily detrimental and, in certain nickel steels and diffusion-alloyed steels, may be advantageous. It is important to recognize when the observed heterogeneity is beneficial. The pores allow carburizing and nitriding gases to penetrate the interior of a sintered steel part, resulting in less well-defined cases on carbon steel and nitriding 300 series stainless steels. The P/M steels are generally low in manganese, and when the alloys are prepared as elemental blends, hardenability is lower than for fully dense, homogeneous low-alloy steels. This is not a problem in the fully dense low- alloyed steels fabricated by forging or injection molding. Reference 1. "Standard Test Method for Density and In terconnected Porosity of Sintered Powder Metal Structural Parts and Oil-Impregnated Bearings," B 328, Annual Book of ASTM Standards, Vol 02.05, ASTM, Philadelphia, 1984, p 162-163 Sample Preparation Specimen Selection and Sectioning. It is usually necessary to examine a section that extends from the surface to the interior of the part and from top to bottom. The surface is susceptible to changes from (1) the sintering atmosphere, such as decarburization in a steel, (2) being sealed over with pure copper, or (3) shallow hardenability in elemental blends of steels. Density can vary from point to point. During sintering, the protected bottom of the part "sees" a different atmosphere than the top, and the internal area can be more protected from the sintering atmosphere than an outer surface. Infiltrated parts have certain artifacts that appear where the infiltrant entered. Improper tool design or press setup can cause cracks or laminations at different levels in a part, such as between a hub and flange; low-density regions normally occur just below flanges. If a part is overpressed in certain regions or if the tooling does not have the correct exit taper, microcracks can occur upon ejection. These are usually parallel to a punch face (normal to the direction of applied pressure) and are best seen in sections that run parallel to the pressing direction. In a case-carburized gear, a section showing the plan view of the teeth would show the depth of case and whether or not the teeth were through-hardened. Therefore, likely potential defects in a P/M part affect selection of the planes of sectioning. Because of the difficulty in opening all the smeared pores on a P/M part, the use of smaller sections is recommended. An easily polished specimen will measure less than 12 by 12 mm ( 1 2 by 1 2 in.). A soft abrasive wheel that breaks down easily will control overheating at cutoff and will not glaze easily. Low-speed diamond wheels are very precise and damage free, but require much time. If the conventional abrasive wheel glazes because the workpiece is too large, four small sections (3 cm 2 , or 0.5 in. 2 ) can be removed using pliers at equal positions around the periphery of the wheel. Because this weakens the wheel somewhat, it should always be operated in a well-guarded enclosure. A substantial flow of water that correctly strikes the interface between the wheel and the part is also required. Rust inhibitor should be added to recirculated water. Sectioning an annularly shaped part with the wheel advancing from the outer diameter toward the inner diameter can result in the wheel being captured when it penetrates the inner diameter. This often occurs in heat-treated parts containing residual stresses and can cause wheel breakage. It can be avoided if the part is sectioned with the wheel moving along the axis of the part so that material on both sides of the inner diameter is cut at the same time. In general, it is preferable to secure in a vise the section of the specimen to be mounted. This section rarely contains burrs and can be mounted without further hand grinding. Fluid Removal and Washing. Entrapped oil from heat treating or machining as well as water and rust inhibitors from the cutoff wheel must be removed from the pores before the specimen is mounted. Other sources of contamination include the sizing lubricants used at repressing and rust inhibitors added during tumbling deburring of the parts. Failure to remove these contaminants will obscure the specimen surface under the microscope. Oils also seem to interfere with polishing. If the specimen is not heat treated, contains no substantial amount of oil, and can tolerate heating to 260 to 370 °C (500 to 700 °F) for approximately 1 min, then the fluids can be removed on a hot plate under a hood. The specimens are easily heated until straw-colored or light blue. Water or small amounts of oil will evaporate or burn off quickly. When the specimen cannot be heated, an extractor-condenser of the type shown in Fig. 1 or a Soxhlet apparatus may be used. The extractor-condenser consists of a flask, a siphon cup, and a condensing-coil unit that fits on the top of the flask. A solvent, such as toluene or acetone, is placed in the flask, and the specimens to be cleaned are placed in the siphon cup. Multiple specimens must first be coded for subsequent identification. Fig. 1 Extractor-condenser used for washing P/M specimens to remove contaminants from pores A cold-water line is connected to the condensing coil. The flask is heated to the boiling temperatures of the solvent. The solvent evaporates, and when the vapor contacts the cold condensing coil, it drips into the siphon cup and onto the parts. When the siphon cup is filled to the level determined by the upper bend in the exit tube, it empties, returning solvent and dissolved oil to the boiling flask. Recycling allows a subsequent flow of clean solvent over the specimens. The oil and foreign matter removed remain in the flask. Six cycles, requiring a total of approximately 1 h, will usually ensure removal of the oil. This method is also described in ASTM B 328 (Ref 1) and ISO 2738 (Ref 2). The latter test method includes the technique for removing oil when testing for total carbon. Because laboratory investigations often involve testing for carbon along with metallography, it may be efficient to use the more thorough ISO method. Following extraction with the solvents, it is necessary to dry the parts for approximately 1 h at 120 °C (250 °F) to remove the solvent. The ultrasonic cleaner used for washing P/M specimens consists of a power supply and a small tank, which holds a solvent bath. The power source produces high-frequency waves in the bath. The waves force the solvent into the pores of the specimen, removing foreign substances. The specimen is placed in the solvent bath; therefore, most of the washing takes place in contaminated solution. Because the specimens represent a small fraction of the bath volume, the amount of contamination is not significant. The use of 1-1-1 trichloroethane and a hot ultrasonic bath for 1 h has been recommended (Ref 3). The latter procedure should be carried out under a hood. Again, the residual entrapped solvent should be evaporated from the specimens. Wax Impregnation. After removal of fluid from the pores, subsequent abrasives, water, and etchants must be kept out of the pores. Wax impregnation may be used for specimens that are to be hand held or mounted in Bakelite. The specimens are soaked 2 to 4 h in a molten synthetic wax at 175 °C (350 °F). After cooling and removal of the surface wax, the specimens are ready to hand grind or to mount in Bakelite. Wax impregnation should ideally be carried out in a vacuum oven at the recommended temperature. The vacuum allows the air entrapped in the pores to bubble up through the molten wax. The atmospheric pressure is returned to the system with the specimens immersed in the molten wax. The air pressure then should be allowed to act on the molten wax for 30 min to force it into the pores. Subsequently, the specimens may be cooled, and the excess wax removed. Mounting of Compacted Specimens. Specimens that have been filled with wax may be mounted in thermosetting Bakelite resins, Lucite, or clear-liquid cold-mounting resins. Epoxy resins, rather than wax and Bakelite, are preferred for filling the pores and mounting the specimens, because the pores can be sealed as the multiple specimens are mounted. A convenient container for mounting is a length of 25-mm (1-in.) copper or aluminum tubing with an inner diameter of 32 mm (1 1 4 in.). This can be placed on a small, flat sheet of glass. The interior of the tube and the glass are coated with a mold-release agent. An alternate two-piece cup can be machined from low-carbon steel (Fig. 2). The steel base may be ground flat in the lab as needed, and excess epoxy may be removed by sintering or heat treating. The bases of the plastic cups commercially available become concave after a few uses. Fig. 2 Machined two-piece cup for mounting P/M specimens When more than one specimen is to be inserted in the same mount, each specimen must be identified for future reference. The use of an asymmetric arrangement of specimens or the mounting of a distinctive object, such as a small, twisted piece of a paper clip, will permit easy accurate reference. The epoxy resin should be selected for low vapor pressure of resin and hardener, and any new resin should be tested in the vacuum chamber to note the pressure at which it bubbles. For example, some epoxies should not be used below 75 torr (10 kPa). This limits the amount of air that can be removed and the volume of pores that can be filled. Epoxy resins should be placed in disposable cups for stirring and mixing of resin and hardener. Volumetric measurement of the components with plastic syringes works well. Epoxy resins should be selected for low viscosity. Rapid hardening is convenient, but must not be allowed to interfere with pore filling. Epoxy-Resin Impregnation. In one method, the resin is carefully poured over the specimens in their mounting cups to a depth of approximately 19 mm ( 3 4 in.). Any bubbles that form during air evacuation can be held in the remaining 6 mm ( 1 4 in.) of the cup. The cups should be placed in a container that will catch any minor epoxy spills, and the container should be placed in a vacuum chamber. The air is then evacuated to the lowest pressure tolerated by the resin (see example above). The specimens should bubble for approximately 10 min. Air pressure is then restored and allowed to impregnate the specimens for 15 min. The specimens can then be cured overnight (preferred), at an accelerated rate of 1 h at 50 °C (120 °F), or by following manufacturer's directions. Heat often causes cavities to form against the porous specimen. The cavities interfere with polishing and rarely form when the specimen cures slowly at room temperature. An alternate method for impregnation with epoxy resin involves suspending the specimen above the epoxy bath during air evacuation. This allows the air to exit the pores rapidly and cause no air bubbling in the epoxy. It is similar to the technique used to fill the pores with oil in a sintered bearing. Yet another method is to hold the specimen above the oil magnetically or with a vacuum feed-through manipulator. After 1 to 2 min at low pressure, no air will remain in the specimen, and it can be lowered into the epoxy bath, at which point the pressure is readmitted into the chamber. This method is limited by the vapor pressure of the epoxy resin and hardener. Commercial equipment with vacuum feed- through evacuates air from the specimens and directs a stream of epoxy from outside the chamber into the specimen cup located in the vacuum. Again, the vapor pressure of the resins determines the lowest usable pressure. Most mounts show evidence that resin enters the specimen from unmachined surfaces; this is never the surface against the bottom of the specimen cup. Therefore, most mounts show evidence of epoxy resin in the surface pores, but the interior pores are rarely full. This effect is apparent to the unaided eye during initial polishing, because the outer edges will appear more porous (like an orange peel) and the interior more mirrorlike. Edge retention, discussed in the article "Mounting of Specimens" in this Volume, is achieved by adding light or dark alumina (Al 2 O 3 ) granules to the epoxy resin (Fig. 3). Some of the ceramic may be blended with resin and poured around the specimens to form a 1.6-mm ( 1 16 -in.) thick layer. The rest of the mount is formed from clear resin poured on top of the mixture. Fig. 3 Edge-retention technique in which dark Al 2 O 3 granules (right) are added as a reinforcer to the epoxy resin. Not all of the pores are open, which indicates that Al 2 O 3 additions necessitate extended polishing times. Fe-0.8C specimen (7.0 g/cm 3 ) pressed at 550 MPa (40 tsi) and sintered 30 min in dissociated ammonia at 1120 °C (2050 °F). 2% nital. 95× Alternatively, loose Al 2 O 3 can be poured into the specimen cup to a depth of 1.6 mm ( 1 16 in.), surrounding the specimens, and the clear resin carefully poured on top. During vacuum evacuation, the resin flows in among the Al 2 O 3 particles. This ceramic reinforces the epoxy and results in very little rounding of the specimen edge during polishing. Because the oxide greatly slows the rate of grinding and polishing, the times recommended below must be adjusted. A third technique involves forming a thin oxide-reinforced layer around the specimen itself by applying a thick, pasty mixture of resin and oxide to the specimen surface. In this way, the entire surface of the mount is not hardened, just a layer approximately 0.5 mm (0.020 in.) adjacent to the specimen. Therefore, the polishing time is not unduly increased. [...]... stain the copper-cored areas for easier identification Iron-copper P/M structures are shown in Fig 58, 59, 60, 61, and 62 in the section "Atlas of Microstructures for Powder Metallurgy Materials" in this article Iron-Copper-Carbon Alloys The most common of the moderate-strength, as-s n-tered alloys is iron-copper-carbon with 0.8% C and 2 to 5% Cu (Fig 63, 64, 65, 66 in the section "Atlas of Microstructures. .. review copper-base, titanium-base, and aluminum-base P/M materials Additional microstructures of P/M materials can be found in the articles "Beryllium," "Titanium and Titanium Alloys," "Refractory Metals and Alloys," "Electrical Contact Materials," and "Magnetic and Electrical Materials" in this Volume Copper-base alloys include pure copper for high-density electrical applications; 90Cu-10Sn bronzes... demonstrates that it is possible to open pores and show the correct porosity area fraction using a method that requires approximately 12 min and does not use diamond It consists of 10 min of hand polishing using 1- m Al2O3 on a synthetic suede, short-nap cloth at 250 rpm on a 200-mm (8-in.) lap and 2 min of light hand polishing using 0.0 5- m Al2O3 at 125 rpm on the same type of cloth Fig 5 Fig 6 Fig... specimen surface is the change in length of the Knoop diagonal divided by 30.51 (for a standard indenter) For a 2 5- by 25-mm (1by 1-in.) specimen, polishing using a 250-rpm, 200-mm (8-in.) diam lap, 1- m Al2O3 on a synthetic suede, short-nap cloth, and moderate hand pressure will remove 0.4 μm/min A smaller specimen, such as 12 by 6 mm (0.5 by 0.25 mm) will polish at 1.45 μm/min Additional information on material... have a particle size of 2 to 5 μm and are often used in injection-molded P/M parts (see the article "Ultrafine and Nanophase Powders" in Powder Metal Technologies and Applications, Volume 7 of the ASM Handbook) Their high surface area and fine particle size allow the material to sinter to near full density with large associated shrinkages The resulting structure will be ferrite, with small rounded and. .. beveling Titanium alloys require a 4-min rough polishing using 1- m Al2O3 on felt cloth at 250 rpm on a 200-mm (8-in.) diam lap with moderate hand pressure This will open and slightly enlarge the pores The use of 1- m diamond or 0.0 5- m Al2O3 may cause polishing artifacts and is therefore not recommended References cited in this section 1 "Standard Test Method for Density and Interconnected Porosity of... should be performed using deagglomerated 0.0 5- m Al2O3 on a long-nap cloth for 30 s with light hand pressure on a 200-mm (8-in.) diam, 125 -rpm lap Prolonged polishing or heavy hand pressure during final polishing will round the particle surfaces Because some particles have internal pores that may have been smeared, it is important to examine some of the particles, unetched, at 500 or 1000× for the... 90Cu-10Sn bronzes for bearings and structural parts; brasses with 10, 20, and 30% Zn; and nickel silver (Cu-18Zn-18Ni) The brasses and nickel silvers are used for structural parts that require ductility, moderate strength, corrosion resistance, and decorative value Copper will exhibit a single-phase structure with some annealing twins The most significant feature will be the particle boundaries or their... 106, 107, 108, 109, 110, 111) Injection-molded parts made of fine powders tend to sinter to a closed-pore state with no original particle boundaries (Fig 131 and 132) Powder metallurgy forgings and hot isostatically pressed parts would not display such boundaries (Fig 133 and 134) In the etched condition, sintered steels may exhibit carburization or decarburization (Fig 135 and 136) If parts of nonuniform... may be vacuum hot-pressed or preformed, canned, and hot isostatically pressed to full density Titanium alloy P/M structures are shown in Fig 123 , 124 , and 125 in the section "Atlas of Microstructures for Powder Metallurgy Materials" in this article Aluminum P/M alloys are pressed and sintered to 90 to 95% density The common alloys are 201AB and 601AB The alloys are prepared using low-alloy aluminum . 10 min of hand polishing using 1- m Al 2 O 3 on a synthetic suede, short-nap cloth at 250 rpm on a 200-mm (8-in.) lap and 2 min of light hand polishing using 0.0 5- m Al 2 O 3 at 125 rpm on. polishing using a 250-rpm, 200-mm (8-in.) diam lap, 1- m Al 2 O 3 on a synthetic suede, short-nap cloth, and moderate hand pressure will remove 0.4 μm/min. A smaller specimen, such as 12 by 6 mm (0.5. Fig. 39 Zn-12Al-0.75Cu-0.02Mg alloy, as die cast in a cold- chamber machine. Structure is fine primary crystallites consisting of aluminum-rich solid solution and eutectoid in a matrix