Positive and negative two-way shape memory effect in [111]-oriented Ni51Ti49 single crystals

This study investigates the effect of precipitates and compressive loading at selected temperatures on the two-way shape memory effect (TWSME) properties of a [111]-oriented Ni51Ti49 single crystal shape memory alloy. The single crystals were either solution-treated or aged at 500°C for 1.5h to form Ni4Ti3 precipitates, and then deformed to obtain TWSME. When the solution-treated or aged specimens were loaded at low temperature in martensitic phase, positive (compressive) TWSME was observed. After compressive loading at high temperature in austenitic phase, the solution-treated specimen did not show TWSME while the aged specimen revealed negative (tensile) TWSME. TEM investigation revealed that the negative TWSME strain is produced by the selection of martensite variants due to the back stress created by the dislocation formation around Ni4Ti3 precipitates that affects the R-phase formation. A maximum negative TWSME strain of 1.9% was obtained in the aged specimen after compressive deformation of 6% at 200°C.