Question

Grain boundaries are areas of a metal microstructure:

a. where high melting point constituents are concentrated
b. made up of rows of atoms arrayed in straight lines
c. that are not attacked by etchants and generally stand in relief during metallographic examination
d. where low melting impurities tend to concentrate

Answer 1

Grain boundaries in the microstructure of metals are areas where impurities, especially those with lower melting points, tend to concentrate. They have different properties than the crystalline grains and can be preferentially attacked by etchants in metallography.

Step-by-step explanation:

Grain boundaries in a metal's microstructure are regions where the crystalline grains or particles meet, which can heavily influence the properties of the metal. In metals, grain boundaries typically serve as locations for the concentration of impurities, especially those with lower melting points compared to the host metal. This is explained by the fact that the disordered atomic structure of grain boundaries can accommodate impurities more readily than the well-ordered lattice within the grains. Grain boundaries can also act as barriers to dislocation movement, which affects the mechanical properties of the metal. These areas often possess different chemical and structural characteristics compared to the grains they separate, making them more susceptible to certain types of corrosion and to becoming sites for crack initiation under stress. Chemical etchants commonly used in metallography preferentially attack these areas due to their structural disarray, often highlighting them as they stand in relief after etching.