Breakdown of Shape Memory Effect in Bent Cu-Al-Ni Nanopillars: When Twin Boundaries Become Stacking Faults.

Abstract

Bent Cu-Al-Ni nano-pillars (diameters 90 nm - 750 nm) show the shape memory effect, SME, for diameters d > 300 nm. The SME and the associated twinning are due to a small deformed section of the nano-pillar. Thick nano-pillars (d>300nm) transform to austenite under heating, including the deformed region. Thin nano-pillars (d < 130 nm) do not twin but generate highly disordered sequences of stacking faults in the deformed region. No SME occurs and heating converts only the un-deformed regions into austenite. The defect-rich, deformed region remains in the martensite phase even after prolonged heating in the stability field of austenite. A complex mixture of twins and stacking faults was found for diameters 130 nm < d < 300 nm. The size effect of the SME in Cu-Al-Ni nano-pillars consists of an approximately linear reduction of the SME between 300nm and 130 nm when the SME completely vanishes for smaller diameters.