As discussed in the previous section, the ability of a crystalline material to plastically deform largely depends on the ability for dislocation to move within a material. Therefore, impeding the movement of dislocations will result in the strengthening of the material. There are a number of ways to impede dislocation movement, which include:
- controlling the grain size (reducing continuity of atomic planes)
- strain hardening (creating and tangling dislocations)
- alloying (introducing point defects and more grains to pin dislocation)
Control of Grain Size
The size of the grains within a material also has an effect on the strength of the material. The boundary between grains acts as a barrier to dislocation movement and the resulting slip because adjacent grains have different orientations. Since the atom alignment is different and slip planes are discontinuous between grains. The smaller the grains, the shorter the distance atoms can move along a particular slip plane. Therefore, smaller grains improve the strength of a material. The size and number of grains within a material is controlled by the rate of solidification from the liquid phase.
Strain hardening (also called work-hardening or cold-working) is the process of making a metal harder and stronger through plastic deformation. When a metal is plastically deformed, dislocations move and additional dislocations are generated. The more dislocations within a material, the more they will interact and become pinned or tangled. This will result in a decrease in the mobility of the dislocations and a strengthening of the material. This type of strengthening is commonly called cold-working. It is called cold-working because the plastic deformation must occurs at a temperature low enough that atoms cannot rearrange themselves. When a metal is worked at higher temperatures (hot-working) the dislocations can rearrange and little strengthening is achieved.
Strain hardening can be easily demonstrated with piece of wire or a paper clip. Bend a straight section back and forth several times. Notice that it is more difficult to bend the metal at the same place. In the strain hardened area dislocations have formed and become tangled, increasing the strength of the material. Continued bending will eventually cause the wire to break at the bend due to fatigue cracking. (After a large number of bending cycles, dislocations form structures called Persistent Slip Bands (PSB). PSBs are basically tiny areas where the dislocations have piled up and moved the material surface out leave steps in the surface that act as stress risers or crack initiation points.)
It should be understood, however, that increasing the strength by cold-working will also result in a reduction in ductility. The graph to the right shows the yield strength and the percent elongation as a function of percent cold-work for a few example materials. Notice that for each material, a small amount of cold-working results in a significant reduction in ductility.
Effects of Elevated Temperature on Strain Hardened Materials
When strain hardened materials are exposed to elevated temperatures, the strengthening that resulted from the plastic deformation can be lost. This can be a bad thing if the strengthening is needed to support a load. However, strengthening due to strain hardening is not always desirable, especially if the material is being heavily formed since ductility will be lowered.
Heat treatment can be used to remove the effects of strain hardening. Three things can occur during heat treatment:
- Grain growth
When a stain hardened material is held at an elevated temperature an increase in atomic diffusion occurs that relieves some of the internal strain energy. Remember that atoms are not fixed in position but can move around when they have enough energy to break their bonds. Diffusion increases rapidly with rising temperature and this allows atoms in severely strained regions to move to unstrained positions. In other words, atoms are freer to move around and recover a normal position in the lattice structure. This is known as the recovery phase and it results in an adjustment of strain on a microscopic scale. Internal residual stresses are lowered due to a reduction in the dislocation density and a movement of dislocation to lower-energy positions. The tangles of dislocations condense into sharp two-dimensional boundaries and the dislocation density within these areas decrease. These areas are called subgrains. There is no appreciable reduction in the strength and hardness of the material but corrosion resistance often improves.
At a higher temperature, new, strain-free grains nucleate and grow inside the old distorted grains and at the grain boundaries. These new grains grow to replace the deformed grains produced by the strain hardening. With recrystallization, the mechanical properties return to their original weaker and more ductile states. Recrystallization depends on the temperature, the amount of time at this temperature and also the amount of strain hardening that the material experienced. The more strain hardening, the lower the temperature will be at which recrystallization occurs. Also, a minimum amount (typically 2-20%) of cold work is necessary for any amount of recrystallization to occur. The size the new grains is also partially dependant on the amount of strain hardening. The greater the stain hardening, the more nuclei for the new grains, and the resulting grain size will be smaller (at least initially).
If a specimen is left at the high temperature beyond the time needed for complete recrystallization, the grains begin to grow in size. This occurs because diffusion occurs across the grain boundaries and larger grains have less grain boundary surface area per unit of volume. Therefore, the larger grains lose fewer atoms and grow at the expense of the smaller grains. Larger grains will reduce the strength and toughness of the material.