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

Shape-memory hooks were fabricated in a single processing step (one-time procedure) by a combination of tensile and bending deformation of a martensitic NiTi-wire. The hooks 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 hooks to a beam of high-energy synchrotron x-rays at room temperature ( A f ). This technique demonstrated that the hooks were martensitic at room temperature but contained both austenite and martensite at 150°C. The presence of martensite above A f is 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 from the inner to the outer curvature of the hook. On heating, the inner part of the hook transforms to austenite, while the stabilized martensite in the outer part is deformed pseudo-elastically by twinning. On cooling, the back-transformation of the inner part is biased by the pseudo-elastic recovery of the outer part, thus forcing the hook back into its initial shape.