Influence of the Structure and Orientation of the Parent Phase on the Hysteresis of Single-Crystal Shape Memory Alloys
TU Berlin, Str. des 17. Juni 135, 10623 Berlin, Germany
2 1st Memory Alloys GmbH, Hauptstr. 24, 61279 Grävenwiesbach 2, Germany
3 Institute of Metals and Metal Physics, Moscow, Russia
The results of high-resolution electron microscopy, calorimetry and tension-compression tests on the Cu-Zn-Al single-crystals, of calorimetry on the Mn-Cu, NiTi single-crystal and on the NiTi polycrystals are presented. The features of the pseudo-plastic (ferroelastic) hysteresis by the deformation of the single-crystals in the martensitic state are investigated in detail. The similarity of ferroelastic and ferromagnetic hysteresis, the strongly orientation dependence of the ferroelastic hysteresis and its independence on the temperature are shown experimentally, which are well known peculiarities of the ferroelastic hysteresis. The dissipated energy (luring a one cycle of the ferroelastic deformation has parabolic dependence on the strain with an proportionality coefficient which is determined as an effective modulus of the martensitic polydomain. The hysteresis of the thermal-induced, stress-tree transformation of all samples is also investigated. Two equilibrium lines bracketing the permanent hysteresis inside the major hysteresis loop are found during the partial cycling. The dissipated energy involves two terms: the one, describing the permanent hysteresis, depends linear on the martensite phase traction and the other, parabolic. The influence of the tramforrnation type, polycristallinity and scanning rate on the thermal hysteresis are discussed.
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