Question

3. Describe the dependence of diffusivity on:
a. Temperature (2)
b. Concentration gradient (2)
c. Crystal structure / atom type (2)
d. List four strengthening mechanisms and describe how strengthening is achieved. (8)

Answer 1

a) Diffusion always increases with an increase in temperature. Diffusion is caused by the movement of atoms or molecules from an area of higher concentration to one of lower concentration.

b) The greater the difference in concentration, the more rapid the diffusion. The closer the distribution of the material gets to equilibrium, the slower the rate of diffusion becomes.

c) Smaller atoms diffuse more readily than big ones, and diffusion is faster in open lattices or in open directions.

d) Strengthening by Grain Size Reduction:By changing grain size one can influence the number of dislocations piled up at the grain boundary and yield strength.This can be explained by Hall-petch equation.

Strain Hardening: Ductile metals become stronger when they are deformed plastically at temperatures well below the melting point .The reason for strain hardening is that the dislocation density increases with plastic deformation due to multiplication.

Solid solution strengthening: Strengthen and Harden materials by alloying with impurity atoms that go either into substitutional or interstitial solid solution. Increasing concentration of alloy increases the strength.

Precipitation hardening: Age hardening, also known as precipitation hardening, is a type of heat treatment that is used to impart strength to metals and their alloys. It is called precipitation hardening as it makes use of solid impurities or precipitates for the strengthening process. The metal is aged by either heating it or keeping it stored at lower temperatures so that precipitates are formed.