A new type of intrinsic two-way shape-memory effect in hacks of NiTi-wires

Shape-memory hacks were fabricated in a single processing step ("one-time procedure") by a combination of tensile and bending deformation of a martensitic NiTi-wire. The hacks reversibly changed curvature on heating and cooling in a reproducible manner over 50 thermal cycles. This new type of intrinsic two-way shape memory effect was studied by subjecting the hacks to a beam of high-energy synchrotron x-rays at room temperature «Mr) and at 150°C (>Ar). This technique demonstrated thaI the hacks were martensitic at room temperature hut contained both austenite and martensite at 150°c. The presence of martensite above Ar is due to a high dislocation density, which stabilizes martensite and inhibits its transformation. The amount of stabilized martensite scales with the extent of plastic deformation and thus increases trom the inner to the euter curvature of the hock. On heating, the inner part of the hock transforms to austenite, while the stabilized martensite in the euter part is deformed pseudo-elastically by twinning. On cooling, the back-transformation of the inner part is biased by the pseudo-elastic recovery ofthe euter part, thus forcing the hock back into its initial shape.