How do grain boundaries appear under a microscope?

How do grain boundaries appear under a microscope?

• A. Dark lines separating each grain
• B. Bright lines outlining crystals
• C. Dotted boundaries between grains
• D. Uniform shading across the sample

Answer: (A)

Grain boundaries become visible after preparing a metal sample for microscopy by etching. Boundaries are regions where crystal orientation changes and they have higher energy than the interior of a grain, so an etchant preferentially attacks them. This makes the boundaries slightly grooved and creates a contrast difference at those locations. In standard optical microscopy, that contrast shows up as dark lines that separate one grain from another, since the boundary areas reflect and scatter light differently than the uniform grain interiors. The rest of the surface inside each grain tends to appear lighter and uniform, leading to clearly defined, dark boundaries. Dotted boundaries, uniform shading, or bright lines don’t match the common etched-micrograph contrast produced by grain boundaries.