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Loading-unloading cycles were applied in order to induce forward and reverse martensitic transformations B2 <-> (R) <-> B19’ in ultra-fine grained NiTi (50.9 at% Ni) wires. Stress-strain hysteresis loops change with increasing number of cycles. It has been observed that the upper plateau stress, which corresponds to the forward transformation induced in the loading part of the cycle, decreases considerably during mechanical cycling. In contrast, an only moderate drop of the lower plateau stresses characterizes the reverse transformation before the hysteresis loops reach their saturated shape. Moreover, a permanent plastic strain of the order of 1% accumulates before the hysteresis loops saturate. An extensive transmission electron microscopy investigation on the nano-grain scale has been performed in the present study in order to account for the observed hysteresis loop evolution. A representative sample of the selected area diffraction (SAD) rings was evaluated for both, the as received state of the wire before the fatigue experiment and the mechanically cycled material. Results of the SAD analysis suggest that the loading cycles stabilize the R phase. However, no evidence was found for retained B19’ phase after cyclic loading. Some consequences of these microstructural findings are discussed.

R-phase stabilization in ultra-fine grain NiTi wires after mechanical cycling