Metallographic examination is one of the most important procedures used by metallurgists in failure analysis. Typically, the light microscope (LM) is used to assess the nature of the material microstructure and its influence on the failure mechanism. Microstructural examination can be performed with the scanning electron microscope (SEM) over the same magnification range as the LM, but examination with the latter is more efficient. This article describes the major operations in the preparation of metallographic specimens, namely sectioning, mounting, grinding, polishing, and etching. The influence of microstructures on the failure of a material is discussed and examples of such work are given to illustrate the value of light microscopy. In addition, information on heat-treatment-related failures, fabrication-/machining-related failures, and service failures is provided, with examples created using light microscopy.

After completing a fatigue test of an aluminum alloy component machined from a 7079-T6 forging, technicians noted a 5 in. crack which ran longitudinally above and through the flange. When the fracture face was examined by light microscopy, observers could not ascertain the exact mode of fracture. Electron fractography revealed that five different modes of crack growth were operative as the part failed. Region 1 was a shallow zone (about 0.002 in. at its deepest) of dimpled structure typical of an overload failure. Region 2 was a zone that grew by a stress corrosion mechanism. Through a fatigue mechanism was operative in Region 3, it was not the cause of the large crack. Region 4, which covered 50% of the fracture area, developed mainly by stress corrosion. This zone gradually changed into the combination of intergranular and transgranular overload in Region 5, which covered approximately the remaining 50% of the fracture. Apparently, after stress corrosion moved halfway through, the part failed by overload. This failure analysis proved that a crack, originally thought to be a fatigue failure, was actually a stress corrosion crack.

This article provides a discussion on the metallographic techniques used for failure analysis, and on fracture examination in materials, with illustrations. It discusses various metallographic specimen preparation techniques, namely, sectioning, mounting, grinding, polishing, and electrolytic polishing. The article also describes the microstructure examination of various materials, with emphasis on failure analysis, and concludes with information on the examination of replicas with light microscopy.

A cast silicon bronze (UNS C86700) impeller that had been severely corroded was submitted for failure analysis. The failed part was used to pump potable water, but service life and chlorine content of the water were unknown. The impeller displayed a Cu-rich red phase on its surfaces and showed a pattern very similar to dezincification. Further investigation to determine the cause of damage using light microscopy and SEM-EDS techniques revealed that the microstructure consisted of multiple phases and that a Si-rich phase was being preferentially attacked, leading to increased porosity. After a thorough examination, it was concluded that the part had failed due to dealloying via desiliconification.

Following the crash of a Mirage III-0 aircraft (apparently caused by engine failure), a small crack was detected in a bolt hole in the wing main spar (AU4SG aluminum alloy). Because this area was considered to be critical to aircraft safety and similar cracking was found in other spars in service, the Royal Australian Air Force requested that the crack growth rate during service be determined. The loading history of the aircraft was made available in the form of flight by-flight records of the counts from the vertical accelerometer sensors fitted to the airframe and a series of “overstress” events recorded during the life of the aircraft. The bolt hole was examined by eddy current testing, visual examination, high-powered light microscope, and scanning electron microscope. Simulation tests were also conducted. The use of simulation specimens permitted actual crack growth rate data to be determined for the configuration of interest.

A type 304 stainless steel tube that failed in a boiler stack economizer was analyzed to determine the cause. The investigation consisted of visual, SEM/EDS, and metallographic analysis. Several degradation mechanisms appeared to be at work, including pitting corrosion, chloride stress corrosion cracking, and fatigue fracture. Investigators concluded that the primary failure mechanism was fatigue fracture, although either of the other mechanisms may have eventually caused the tube to fail in the absence of fatigue.

A 1984 Chevrolet Blazer was being pushed by three youths after it ran out of gas when it was hit from behind by a 1979 Mercury Cougar. One of the youths was crushed between the two vehicles and killed. Optical microscopy was used to examine the tungsten filaments from the headlamps of the Cougar and from the four signal/emergency/parking lights of the Blazer to determine whether the vehicles lights were in use at the time of the accident. Based on degree of distension and brittle fragmentation, it was determined that the low-beam headlights of the Cougar and the parking lights of the Blazer were on at the time of the collision.

The US Army Research Laboratory performed a failure investigation on a broken main landing gear mount from an AH-64 Apache attack helicopter. A component had failed in flight, and initially prevented the helicopter from safely landing. In order to avoid a catastrophe, the pilot had to perform a low hover maneuver to the maintenance facility, where ground crews assembled concrete blocks at the appropriate height to allow the aircraft to safely touch down. The failed part was fabricated from maraging 300 grade steel (2,068 MPa [300 ksi] ultimate tensile strength), and was subjected to visual inspection/light optical microscopy, metallography, electron microscopy, energy dispersive spectroscopy, chemical analysis, and mechanical testing. It was observed that the vacuum cadmium coating adjacent to the fracture plane had worn off and corroded in service, thus allowing pitting corrosion to occur. The failure was hydrogen-assisted and was attributed to stress corrosion cracking (SCC) and/or corrosion fatigue (CF). Contributing to the failure was the fact that the material grain size was approximately double the required size, most likely caused from higher than nominal temperatures during thermal treatment. These large grains offered less resistance to fatigue and SCC. In addition, evidence of titanium-carbo-nitrides was detected at the grain boundaries of this material that was prohibited according to the governing specification. This phase is formed at higher thermal treatment temperatures (consistent with the large grains) and tends to embrittle the alloy. It is possible that this phase may have contributed to the intergranular attack. Recommendations were offered with respect to the use of a dry film lubricant over the cadmium coated region, and the possibility of choosing an alternative material with a lower notch sensitivity. In addition, the temperature at which this alloy is treated must be monitored to prevent coarse grain growth. As a result of this investigation and in an effort to eliminate future failures, ARL assisted in developing a cadmium brush plating procedure, and qualified two Army maintenance facilities for field repair of these components.

The shafts on two centrifugal pumps failed during use in a petroleum refinery. Light optical microscopy and scanning electron microscopy were used to analyze the damaged materials to determine the cause of failure. The results showed that one shaft, made of duplex stainless steel, failed by fatigue fracture, and the other, made of 316 austenitic stainless steel, experienced a similar fracture, which was promoted by the presence of nonmetallic inclusion particles.

A failed laser mirror and another complete mirror of the same construction were analyzed. The laser mirror consisted of three layers of material brazed together to form channels through which the cooling water flows. Samples were analyzed with light optical and scanning electron microscopy. The corrosion product contained molybdenum and copper with a trace of gold. The base material was analyzed as molybdenum with negligible alloying additions. The primary mode of corrosion attack on the base material appeared to be intergranular, although uniform corrosion was evident also. It was concluded that corrosion attack sufficiently weakened the base material and the brazed joints, allowing catastrophic failure of the mirror due to the pressure of the cooling water. It was recommended that the mirrors be cleaned of all corrosion products present as a result of past service conditions and proof tested. It was recommended that the water system consisting of deionized water and formaldehyde be replaced with water having a low oxygen content and a cathodic inhibitor (oxygen scavenger).

New aircraft wing panels extruded from 7075-T6 aluminum exhibited an unusual pattern of circular black interrupted lines, which could not be removed by scouring or light sanding. The panels, subsequent to profiling and machining, were required to be penetrated inspected, shot peened, H2SO4 anodized, and coated with MIL-C-27725 integral fuel tank coating on the rib side. Scanning electron microscopy and microprobe analysis (both conventional energy-dispersive and Auger analyzers) showed that the anodic coating was applied over an improperly cleaned and contaminated surface. The expanding corrosion product had cracked and, in some places, had flaked away the anodized coating. The corrodent had penetrated the base aluminum in the form of subsurface intergranular attack to a depth of 0.035 mm (0.0014 in.). It was recommended that a vapor degreaser be used during cleaning prior to anodizing. A hot inhibited alkaline cleaner was also recommended during cleaning prior to anodizing. The panels should be dichromate sealed after anodizing. The use of deionized water was also recommended during the dichromate sealing operation. In addition, the use of an epoxy primer prior to shipment of the panels was endorsed. Most importantly, surveillance of the anodizing process itself was emphasized.

This article focuses on the visual or macroscopic examination of damaged materials and interpretation of damage and fracture features. Analytical tools available for evaluations of corrosion and wear damage features include energy dispersive spectroscopy, electron probe microanalysis, Auger electron spectroscopy, secondary ion mass spectroscopy, and X-ray powder diffraction. The article discusses the analysis and interpretation of base material composition and microstructures. Preparation and examination of metallographic specimens in failure analysis are also discussed. The article concludes with a review of the evaluation of polymers and ceramic materials in failure analysis.

Examination of a damaged component involves a chain of activities that, first and foremost, requires good observation and documentation. Following receipt and documentation, the features of damage can be recorded and their cause(s) investigated, as this article briefly describes, for typical types of damage experienced for metallic components. This article discusses the processes involved in visual or macroscopic examination of damaged material; the interpretation of fracture features, corrosion, and wear damage features; and the analysis of base material composition. It covers the processes involved in the selection of metallurgical samples, the preparation and examination of metallographic specimens in failure analysis, and the analysis and interpretation of microstructures. Examination and evaluation of polymers and ceramic materials in failure analysis are also briefly discussed.

This article reviews photographic principles, namely, visual examination, field photographic documentation, and laboratory photographic documentation, as applied to failure analysis and the specific techniques employed in both the field and laboratory. It provides information on the photographic equipment used in failure analysis and on film and digital photography. The article describes the basics of photography and the uses of different types of lighting in photography of a fractured surface. The article also addresses the techniques involved in macrophotography and microscopic photography as well as other special techniques.

Accelerated aging tests on detonator assemblies, to verify the compatibility of gold bridgewire and Pd-In-Sn solder with the intended explosives, revealed an unusual form of corrosion. The tests, conducted at 74 deg C (165 deg F) and 54 deg C (130 deg F), indicated a preferential attack of the gold. To investigate the problem, a matrix of test units was produced and analyzed. Scanning electron microscopy, EDX analysis, and x-ray diffraction techniques were used to determine the extent of the corrosion and identify the corrosion products. The results indicated that the preferential attack of the gold was due to HCN formed by decomposition of the explosive powder at high temperatures. Other associated reactions were also observed including the subsequent attack of the solder by the gold corrosion product and degradation of the plastic header.

An investigation of a Stirling engine after an aborted test run revealed that the regenerator screens had suffered substantial damage. During the run, the individual screens oscillated as the helium working fluid was shuttled through the regenerator. In localized areas, the 41 mu m (1600 mu in.) diam type 304 stainless steel wire screening had been torn and pieces were missing. Scanning electron microscope revealed that the fracture had occurred at wire crossover locations by a fatigue mechanism. The problem was solved by sintering the individual screens into a single unit.

Fractography is the means and methods for characterizing a fractured specimen or component. This includes the examination of fracture-exposed surfaces and the interpretation of the fracture markings as well as the examination and interpretation of crack patterns. This article describes the former of these two parts of fractography. It presents the techniques of fractography and explains fracture markings using glass and ceramic examples. The article also discusses the fracture modes in ceramics and provides examples of fracture origins.