Grain size effect on the thermal-induced martensitic transformation in polycrystalline Cu-based shape memory alloys.

LA ROCA, PAULO; ISOLA, LUCIO M.; SOBRERO, CÉSAR E.; VERMAUT, PHILIPPE; MALARRÍA, JORGE A.

Bilbao

Congreso; International Conference on Martensitic Transformations (ICOMAT); 2014

The grain size (d) effect on shape memory behavior was investigated by differential scanning calorimetry, four probe electrical resistometry, and light and electron microscopy. Cu-based SMA specimens of various d ranging from about 500 nm (ribbons and tapes obtained by rapid solidification techniques), till single-crystals of 6 mm diameter (grown by the Bridgman method) were prepared by heat treatments in order to modify their grain size condition. The reduction of d shifts the forward transformation temperature downwards, and d effects on transformation temperatures and hysteresis are already observed in specimens with d below ~ 100 μm, and became more pronounced for d < ~ 20 μm. The present investigation analyzes the shift in the martensitic transformation temperature by a phenomenological model which takes into account mainly three issues: The contribution to the elastic energy from the constraint introduced by the grain boundaries, a dissipative effect produced by micropasticity and the intervariant boundaries energy.