why grain boundaries grain size affects various properties c

why grain boundaries/ grain size affects various properties: conductivity, thermal, electrical, mechanical, magnetic, optical ? Please explain for all.

Solution

when two randomly growing dentries are meet they will form junction called as grain boundary. Grain boundary (G.B.) are highly impurity concentrated region, low melting region, high energy regions and regions of orientations mismatch. Because of these reasons Grain boundaries shows poor properties at high Temperature. Higher the number of G.B in a given volume, Coaser the Grain size. Since G. B are weaker at high Temperature so Fine Grains are used for high Temperature application and on the other hand low temperature application coarse gain material is used. And also as the finner the grain size , greater will be the hardness of material ( as wesee in case of harding and annealing)