Metallography is the study of the structure of metals and alloys. It can be used as a tool to help identify a metal or alloy, to determine whether an alloy was processed correctly, to examine multiple phases within a material, to locate and characterize imperfections such as voids or impurities, or to observe damaged or degraded areas in failure analysis investigations. The method most often used in such evaluations is microscopy. Both optical and scanning electron microscopy (SEM) can be useful in metallographic analysis.
Samples prepared for metallographic microscopy are often first set into an epoxy potting compound, although this step may not be necessary if the specimen to be examined is large enough and sturdy enough to undergo grinding and polishing without a supporting material. The document ASTM E 3, “Standard Guide for Preparation of Metallographic Specimens,” describes recommended procedures for preparing a number of different metals and alloys. Once the specimen surface is polished to the desired level it may be examined immediately or it may be etched. Chemical or electrochemical etching is performed in cases where grain structure is of interest. Grain boundaries are preferentially attacked during the brief exposure to the etchant, leaving behind a surface wherein the two-dimensional grain structure can be clearly seen.
We examine polished metal samples with an Olympus PMG3 Inverted Research Metallurgical Microscope equipped with bright field, dark field, neutral density filters, polarizer with ¼ plate, and Nomarski differential interference contrast. This microscope is also excellent for examining microelectronics dies. Alternatively, we can examine metallographically etched specimens with our digital SEM.
Investigations may determine such issues as:
Grain size and growth
Grain structure resulting from processing
Intermetallic phase microstructures
Carbide formation at surfaces and grain boundaries
Equiaxed or columnar grains
Chemical microsegregation
Microshrinkage and porosity
Inclusions
Planar, cellular, and dendritic interfaces
Microstuctures as a function of cooling rate
Grain size as a function of work hardening
Graphite structures in cast irons
Silicon structures in Al-Si alloys
Copper and magnesium segregation to the surface in aluminum alloys
TiAl intermetallic structure in Ti-6Al-4V alloy
Some examples of metallographic investigations using samples prepared and evaluated at Anderson Materials Evaluation are shown below.
Case Study #1: Inclusion Content in Steel - Qualifying Material Lots
The specimen shown in this optical metallographic microscope image was similar to the one shown above in the epoxy potting compound. Each specimen was prepared and evaluated according to the requirements of ASTM Standard E 45, “Standard Test Methods for Determining the Inclusion Content of Steel,” Method A (Worst Fields). In this evaluation a specimen is ground and polished according to selected procedures outlined in ASTM Standard E 3. A visual survey over 160 mm2 (0.25 in.2) of the surface is then performed at 100x in fields that have an area of 0.50 mm2 each. The entire sample is rated by its worst field for a number of different potential inclusions, such as the “Type D Globular Oxide” heavy inclusion indicated by the circle. By comparing the worst fields to preset acceptance criteria, this type of evaluation can be used to qualify bulk material lots before introducing them into manufacturing processes.
Case Study #2: Graphitic Corrosion in Grey Cast Iron - Water Pipe Inspection
These SEM images show cross sections of a grey cast iron water pipe. The cross section surfaces were ground and polished to reveal the continuous network of flake-like graphite peculiar to this form of iron.
A grey cast iron pipe that has undergone graphitic corrosion often visually appears to be fine other then some general surface corrosion. However, due possible subsurface attack a substantial portion of a pipe’s wall thickness can be converted to a weak and brittle graphite network with dramatically reduced mechanical strength. Graphitic corrosion can lead to catastrophic failure in grey cast iron pipes carrying water at relatively high pressures.
The free surface at the left side of the first image was the outside surface of the previously buried pipe, which had been in contact with moist soil. A damaged area is plainly visible penetrating the pipe wall from the outer surface at the left. This form of attack, known as graphitic corrosion, is specific to grey cast iron. It occurs when the more noble graphite promotes the accelerated attack of the nearby iron metal through galvanic action in a corrosive environment such as a damp soil.
The free surface at the right side of the second image was the inner surface of the same pipe, which had been in contact with potable water. The inner surface clearly suffered corrosive attack resulting in roughening and loss of wall thickness. Additional evidence of graphitic corrosion is visible here. Metal loss due to galvanic attack is obvious around several of the graphite flake clusters visible in this cross section plane. This subsurface damage is possible because of the continuous graphite network and would not have been identified through a surface-based visual inspection.
