Dissemination of IT for the Promotion of Materials Science (DoITPoMS)


Further details of work hardening

Work hardening is the result of many contributing factors. It arises because of interactions between dislocations. As a material is plastically deformed, dislocations move extensively throughout the crystal, and in addition the dislocation density increases. The effect of this is to increase the number of entanglements - these are points where dislocations interact in such a way that their further motion is hindered.

Dislocations may combine with each other, but only if the reaction is energetically favourable. The energy (per unit length) of a dislocation, U, is ½Gb2, where b is the magnitude of the Burgers vector b and G is the shear modulus. For a dislocation reaction between two dislocations with Burgers vectors b1 and b2 to be energetically favourable, the energy of the combined dislocation (with Burgers vector [b1 + b2]) must be lower than the sum of the original dislocation energies, i.e. (b1 + b2)2 ≤ b12 + b22.

When dislocations on the same slip system meet, the reaction depends on their relative signs. If they both have the same sign, (b1 + b2)2 > b12 + b22, so they repel each other. This repulsion can inhibit further glide. Conversely, if they have opposite signs, they will attract each other, again reducing the mobility of the dislocations.

Dislocations on different slip systems may also react. If the "product" dislocation is not on a valid slip system, it is sessile, and blocks further slip on both systems. This is called a Lomer lock.

c.c.p crystals have many slip systems, so it is likely that many dislocations will interact to inhibit slip. This leads to the increased resistance to deformation in Stage II of the slip process.