Q. Describe the four main methods of strengthen crystalline materials against plastic yield.
Ans. (1) Work Hardening: Strain hardening or work hardening is a phenomenon which results in an increase in hardness and strength of a metal subjected to plastic deformation at temperatures lower than the Recrystallization range.
The increase in the stress required to cause slip because of previous plastic deformation is known as work hardening.
(2) Grain Refinement: Grain boundaries are region where atoms are at higher energy level and also where atomic orientation changes. Dislocation cannot glide past the grain boundaries easily. Hence, if there are more grain boundaries, there is more resistance to the movement of dislocations and hence an increase in strength. If there are more grains in a given amount of material, i.e., if the size of grains or crystals is smaller, there will is more grain boundaries compared to the case when the grains are larger. Hence, obviously the material with smaller grains or more grain boundaries will be stronger. Any process which tends to make the grains smaller will increase the strength of the material. This is the mechanism of strengthening by grain refinement.
(3) Solid-Solution Strengthening: Another technique to strengthen and harden metals is alloying with impurity atoms that go into either substitution or interstitial solid solution. Accordingly, it is called solid-solution strengthening.
Solid solution strengthen distorts the lattice, offers resistance to dislocation movement. Which is greater with interstitial elements which cause asymmetric lattice distortion e.g., carbon in steel.
Due to the difference in atomic, lattice distortion is produced when one element is added to the other. Smaller atoms will produce a local tensile stress field and larger solute atoms will produce a local tensile stress field and larger solute atoms will produce a local compressive field in the crystal. In both the cases, the stress field of a moving dislocation interacts with the stress field of the solute atom. This increase the stress required to move the dislocation through crystal.
(4) Dispersion Hardening: The resistance to motion of dislocation, in this strengthening mechanism, is increased by introducing finely divided hard particles of second phase in the soft matrix.
The increase in hardness and tensile strength is due to the interaction of the stress field around the particles with the stress field around the particles with the stress field of a moving distortion and also due to physical obstruction by the hard particles to the moving dislocation. The increase in yield strength due to very hard and inert particles is given by the relation.
τ=C b/l