Stacking-fault energy

Lower SFE materials display wider stacking faults and have more difficulties for cross-slip.

[10] It has long been established that the addition of alloying elements significantly lowers the SFE of most metals.

The figures on the right show how the SFE of copper lowers with the addition of two different alloying elements; zinc and aluminum.

He found that the valence-electron to atom ratio is a good predictor of stacking fault energy, even when the alloying element is changed.

Thus each weight percent of aluminum has a much greater impact on the SFE of the Cu-based alloy than does zinc.

Twinning occurs when there are not enough slip systems to accommodate deformation and/or when the material has a very low SFE.

When cross-slip frequently occurs and certain other criteria are met, sometimes only three independent slip systems are needed for accommodating large deformations.

Smallman found that cross-slip happens under low stress for high SFE materials like aluminum (1964).

This gives a metal extra ductility because with cross-slip it needs only three other active slip systems to undergo large strains.

High SFE materials therefore do not need to change orientation in order to accommodate large deformations because of cross-slip.