Can You Identify the Microstructure?
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MYSTERY MICRO
Can You Identify the Microstructure?
© ASM International 2020
This micrograph depicts a low carbon steel that has been quenched following a heat treatment, but the quench rate was inadequate to achieve full martensitic transformation. The sample was etched in 2% Nital and 10% potassium metabisulfite; these etchants were chosen to color the microstructural constituents distinctly. There are two constituents in the micrograph not colored by the etch—what are they? The winner’s name and answer will appear in an upcoming issue.
Winners are invited to submit their own “Mystery Micrograph.” Exceptional micrographs from winner submittals may be used on the journal cover. Send your clear, brief description by e-mail to the MMA editor at [email protected]. Entries should be titled: “MMA Mystery Micro 9(6).” The above micrograph was submitted by B. Aashranth, Indira Gandhi Centre for Atomic Research, Kalpakkam, India.
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Metallography, Microstructure, and Analysis
Answer for Volume 9, Issue 4 Mystery Micrograph
The lines that appear within the grains of this material are slip lines, caused by plastic deformation. The lines are not straight across because they encounter twins, where the crystal planes are at a different orientation compared to the surrounding material. The slip lines follow the crystal orientation, and so have the appearance shown here of changing angles along their path across the grain.
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B. Aashranth, Indira Gandhi Centre for Atomic Research, Kalpakkam, India, correctly responded as follows: “The parallel lines are slip bands, i.e., collections of parallel dislocation slip lines. These bands indicate slip of the crystal through dislocation glide. The bands are parallel, indicating slip on parallel planes. “These bands have appeared due to deformation of Astroloy, either through a metal forming process (e.g., cold rolling) or through fatigue. My guess is the latter, i.e., that these slip lines have formed after fatigue, considering that the grains themselves seem undeformed. “The bands are not straight across the grain for two reasons, both related to activation of multiple slip systems. The intersecting/hatched lines form due to dislocations on one slip system running into dislocations on another slip system. Since each slip system yields slip bands of different directions, these interactions lead to the hatched appearance of the lines. Additionally, some lines remain continuous even though they aren’t straight; this indicates cross-slip of (screw) dislocations from one slip plane to another slip plane containing the same slip direction.”
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