Mechanisms of Strengthening in Metals

Solid Solution Strengthening

Alloy with impurities. Impurity atoms impose lattice strains on the surrounding host atoms. Lattice strain field interactions occur between dislocations and impurity atoms, thus altering strain fields and restricting dislocation movement.

Precipitation Strengthening

A solution is heated to a high temperature before rapid cooling, resulting in a single-phase highly saturated solution with very few dislocations, hence strengthening the metal. The impurities produced by the precipitation hardening are harder than the surrounding grains, thus disrupting the structure of the crystal lattice, thus inhibiting dislocation movement.

Grain Size Reduction

At a grain boundary, dislocations are blocked, thus causing the piling up of dislocations, creating a driving force. This pushes the dislocations to the nearby grains. The bigger the grain the bigger the pile up the bigger the force. By reducing grain size, there is a smaller pile up of dislocation resulting in a smaller driving force, and thus requiring a larger stress to move the dislocation to the next grain. A smaller grain is also harder and stronger because it has a greater total grain boundary area to impede dislocation motion. Hall-Petch Relation: Sigma(y)=Sigma(o)+k(y)/root(d) As grain diameter decreases, yield stress decreases. Grain boundaries are barriers to slip Barrier strength increases with increased angle of misorrientation

Strain Hardening (cold work)

Dislocation density increases with deformation (cold work). Consequently the average distance of separation between dislocations decrease (dislocations become entangled). On average, dislocation-dislocation strain interactions are repulsive. The net result is that the motion of a dislocation is hindered by the presence of other dislocation; as dislocation density increase, the resistance to dislocation motion is pronounced by other dislocations. Thus the imposed stress necessary to deform a metal increases with cold work.