Q. Describe the structure of grain boundary.
Ans. One phase of metal contains many grains of various orientation, the individual crystals are called grains. Thus a grain is a crystal with almost any external shape but with an internal atomic structure based upon the space lattice, with which it was born.
The various mechanical properties of metal that obtain maximum strength are depend on the arrangement of grains, their shape and specially their size. The grain size is controlled by temperature and time of heating.
Grain boundaries are those surface imperfections which separate crystals or grains of different orientation in a polycrystalline aggregation during crystallization. The grain boundary area depends on the grain size of material and increase with decrease in grain size and vice-versa. When the orientation difference between two adjacent grain is less than 10 degree the boundary is termed as low angle grain boundary, and if it is more than 10 degree it is termed as high angle grain boundary.
In grain boundaries the atomic packing is imperfect. The grain boundary between two adjacent grains is a transition zone which is not aligned with either grain. Various degrees of crystallographic misalignment between adjacent grains are possible.
Effects of Grain boundary:
1. When a metal is subjected to corrosive medium, chemical attack is more rapid at grain boundary.
2. It has been found that high purity in; grain boundary melts at 0.5 degree C below the melting point of grain themselves. This is so because grain boundary atoms have high potential energy and they require less amount of thermal energy for melting. Thus melting always begins at grain boundary foreign atom.
3. Self diffusion of foreign is usually high at grain boundary.
4. In perfect crystal in which atom is at rest and other no resistance to the flow of electrical current. Thus electrical resistivity is the result of crystal imperfection hence grain boundary increase the resistivity of a conductor at any temperature.
5. Grain boundary influence recrystallization.
6. The grain boundaries increase the amount of dislocational energy stored in metal, most of the energy is stored in grain boundary. Hence nucleation begins first near the grain boundary.
7. Grain boundary impedes slip and the strain hardening rate because the dislocations are pitted up along the slip plane at the grain boundary.
The mismatch of the orientation of adjacent grain produces a less efficient packing of the atoms at boundary. Thus, the atoms along the boundary have a higher energy than those within the grain.
The boundary between two crystals which have different crystalline arrangement or different compositions is called an interphase or an interface. The fine grain structure has a small grain size whereas coarse grain structure has a large grain size.