Origin of two way shape memory effect in Cu-based shape memory alloys

Two-way shape memory effect (TWSME) should be trained by thermomechanical cycling or constrained aging. After constrained aging, oriented stress fields around precipitates and stabilized stress-induced martensite were found in Cu-Zn-Al alloys. The effect of these phases on TWSME has been studied by in-situ observations in TEM. For thermomechanical cycling, paired dislocation bands and oriented microstrain in re-oriented martensite were studied by molecular dynamics simulation. It was found that the oriented stress fields of the precipitates, pair dislocation bands and re-oriented martensite also have two-way memory effect during forward and reverse transformation. These stress fields can be termed two-way memory stress fields (TWMSF). The TWMSF contribute their stress fields to the formation of the preferential martensite variants that give raise the TWSME.