Stacking fault energy
The stacking-fault energy (SFE) is a materials property on a very small scale. It is noted as γSFE in units of energy per area. Stacking-fault energy is a primary factor in determining the wear resistance of a metal and, primarily, its resistance to galling.
A stacking fault is an interruption of the normal stacking sequence of atomic planes in a crystal structure. These interruptions carry a certain stacking-fault energy. The width of stacking fault is a consequence of the balance between the repulsive force between two partial dislocations on one hand and the attractive force due to the surface tension of the stacking fault on the other hand. The equilibrium width is thus partially determined by the stacking-fault energy. When the SFE is high the dissociation of a full dislocation into two partials is energetically unfavorable, and the material deforms only by dislocation glide. Lower SFE materials display wider stacking faults and have more difficulties for cross-slip and climb. The SFE modifies the ability of a dislocation in a crystal to glide onto an intersecting slip plane. When the SFE is low, the mobility of dislocations in a material decreases.
A stacking fault is an irregularity in the planar stacking sequence of atoms in a crystal – in FCC metals the normal stacking sequence is ABCABC etc., but if a stacking fault is introduced it may introduce an irregularity such as ABCBCABC into the normal stacking sequence. These irregularities carry a certain energy which is called stacking-fault energy.
Stacking fault energy is heavily influenced by a few major factors, specifically base metal, alloying metals, percent of alloy metals, and valence-electron to atom ratio.
It has long been established that the addition of alloying elements significantly lowers the SFE of most metals. Which element and how much is added dramatically affects the SFE of a material. The figures on the right show how the SFE of copper lowers with the addition of two different alloying elements; zinc and aluminum. In both cases, the SFE of the brass decreases with increasing alloy content. However, the SFE of the Cu-Al alloy decreases faster and reaches a lower minimum.
Another factor that has a significant effect on the SFE of a material and is very interrelated with alloy content is the e/a ratio, or the ratio of valence electrons to atoms. Thornton showed this in 1962 by plotting the e/a ratio vs SFE for a few Cu based alloys. He found that the valence-electron to atom ratio is a good predictor of stacking fault energy, even when the alloying element is changed. This directly supports the graphs on the right. Zinc is a heavier element and only has two valence electrons, whereas aluminum is lighter and has three valence electrons. Thus each weight percent of aluminum has a much greater impact on the SFE of the Cu-based alloy than does zinc.
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