Speaker
Description
In the past two decades Ni-Mn-Ga Heusler alloys have received great attention as these
exhibit magnetic shape memory effects, first described in 1999. Modulated Ni 2 MnGa
martensite has exceptionally low stress for pseudoplastic deformation or structural
reorientation by twinning mediated by highly mobile twinning boundaries [1]. Owing to large
magnetic anisotropy and low twinning stress the large pseudoplastic deformation can be
induced by a magnetic field. Thus, these materials can replace giant magnetostrictive
materials with fast external actuation control and strain exceeding 10 % [1].
In this study, the energy needed for the deformation by a magnetic field and by mechanical
force was measured and compared in monocrystalline Ni 50 Mn 28 Ga 22 modulated martensite.
Measurements were conducted on five samples by vibration sample magnetometer and stress-
strain device to compare directly different modes of loading. In contrast with the
phenomenological model [2] the calculated energy of magnetically induced reorientation or
pseudoplastic deformation were considerably higher compared to the energy needed using
mechanical force. The switching field of samples with nucleated twinning boundaries was
also measured and compared to single variant crystals. Results only partly support modelling
of the twinning stress and switching field [1-3]. The model-data discrepancies are an
unresolved issue suggesting a need for modification of the model.
