Why do grain boundaries appear dark?

E.g High energy areas of grain boundaries are corroded fast. Under microscope, the ligh is not reflected back as an incident beam and scatters. Therefore they appear dark.

Why do grain boundaries appear dark under microscope?

The pearlitic grains reflect less light than the ferritic grains and thus appear darker. A matrix of this type has higher hardness, but lower ductility.

Why we see grain boundaries as dark lines within the structure of metals and alloys?

These areas tend to be under high local stress, such as at the grain boundaries, where there is imperfect packing of the atoms. In effect, a groove is produced that will scatter the incident light and therefore show up as a dark line.

What causes grain boundaries?

Grain boundaries are usually the result of uneven growth when the solid is crystallizing. Grain sizes vary from 1 µm to 1 mm. Most grain boundaries are preferred sites for the onset of corrosion and for the precipitation of new phases from the solid.

Why are grain boundaries visible in polished and etched metal?

Since the grain boundaries are attacked at a greater rate than the proper grain due to higher energy content of the grain boundaries. Etching occurs when the acid or base is contacted with specimen surface because of the difference in rate of attack of the various phases present and their orientation.

Why are grain boundaries visible in polished and etched samples?

Why do grain boundaries decrease conductivity?

The phonons are scattered at the grain boundaries (GBs) due to misalignment of lattice at the GBs. So, the thermal conductivity of a material decreases with increasing the GBs and this is one of the facts for low thermal conductivity in the nano structured material.

What is a diffusion boundary layer?

Boundary layer characterized by a transverse concentration gradient of a given component in the mixture; the effect of the gradient produces a transverse (mass) transfer of this component.

What is grain boundary sliding creep?

Grain Boundary Sliding (GBS) is a material deformation mechanism where grains slide against each other. This occurs in polycrystalline material under external stress at high homologous temperature (above ~0.4) and low strain rate and is intertwined with creep.

Are grain boundaries line defects?

Grain boundary dislocations are linear defects with all the characteristics of lattice dislocations, but with very specific Burgers vectors that can only occur in grain boundaries. The cut line, as before, will define the dislocation line vector l which by definition will be contained in the boundary.