Surface defects are the boundaries, or planes, that separate a material into regions, each region having the same crystal structure but different orientations. There are different kinds of surface defects, some of which are described:
Material Surface The exterior dimensions of the material represent surfaces at which the lattice abruptly ends. Each atom at the surface no longer has the proper coordination number and atomic bonding is disrupted. The exterior surface may also be very rough, may contain tiny notches, and may be much more reactive than the bulk of the material.
Grain Boundaries The microstructure of most materials consists of many grains. As said above in the introduction, the orientation of the atom arrangement, or crystal structure, is different for each adjoining grain. Three grains are shown schematically in the following figure:
The lattice in each grain is identical but the lattices are oriented differently. The atoms are so close at some locations in the grain boundary that they cause a region of compression, and in other areas they are so far apart that they cause a region of tension.
One method of controlling the properties of a material is by controlling the grain size. By reducing the grain size, the number of grains is increased and, hence, the amount of grain boundary is increased. Any dislocation moves only a short distance before encountering a grain boundary, and the strength of the metal is increased. The Hall-Petch equation relates the grain size to the yield strength:
Small Angle Grain Boundaries A small angle grain boundary is an array of dislocations that produces a small misorientation between the adjoining lattices as shown in the following figure:
Because the energy of the surface is less than that at a regular grain boundary, the small angle grain boundaries are not as effective in blocking slip.
Stacking Faults These occur in FCC metals, and represent an error in the stacking sequence of close-packed planes. Normally, a stacking sequence of ABCABCABC is produced in a perfect FCC lattice. But, suppose the sequence ABCABABCABC occurs. In the portion ABAB, a type A plane is shown where a type C plane should be located. This small region, which has a HCP stacking sequence instead of the FCC stacking sequence, represents a stacking fault. Stacking faults interfere with the slip process.
Twin Boundaries A twin boundary is a plane across which there is a special mirror image misorientation of the lattice structure as shown:
Twins can be produced when a shear force, acting along the twin boundary, causes the atoms to shift out of position. Twinning occurs during deformation or heat treatment of certain metals. The twin boundaries interfere with the slip process and increase the strength of a metal. Movement of twin boundaries can also cause a metal to deform. The following figure shows that the formation of a twin has changed the shape of the metal.
The effectiveness of the surface defects in interfering with the slip process can be judged from the surface energies. The high-energy grain boundaries are much more effective in blocking dislocations than either stacking faults or twin boundaries.