Ni Mn Ga Shape Memory Research Articles

Martensitic transformation in ferromagnets: experiment and theory

A phenomenological model relating the magnetization process in the twinned martensitic state of the Fe- and Ni-based alloys to the strain-induced magnetic anisotropy has been analyzed. The estimated values of the magnetic saturation fields, the magnetization jump at the martensitic transformation temperature and the magnetoelastic deformation of a NiMnGa shape memory alloy are in a good agreement with the experimental data.

Internal friction associated with the structural phase transformations in Ni-Mn-Ga alloys

A double peak in the temperature dependence of internal friction (IF) and elastic modulus in some off-stoichiometric Ni 2MnGa shape memory alloys has been observed both on cooling and heating between the parent and martensite phases. Transmission electron microscopy (TEM) observations have shown that these anomalies are related to structural transformations from the parent cubic phase (P) to an intermediate cubic modulated phase (I) and from the I phase to the martensitic one (M). As for the IF results, the I to M transformation has the characteristics of a first-order phase transition, whereas the P to I transformation shows some distinctive features, such as no temperature-rate dependence.

Phase Transformation and Phonon Anomalies in Ni2MnGa

Inelastic neutron scattering experiments and transmission electron microscopy have been used to study a single crystal of the Ni2MnGa shape memory Heusler alloy in a wide temperature range covering the parent phase (T > TI = 265 K), a recently discovered premartensitic (TI > T > TM) and martensitic (T < TM = 220 K) phase regions. A temperature dependent anomaly in the TA2 phonon dispersion in the parent phase was observed and related to the phase transformations. The premartensitic phase involves a transverse modulation of the parent cubic structure with a simple periodicity of 1/3 [110]. The approximately tetragonal lattice of the low-temperature martensite is distorted by transverse modulations with incommensurate wave vectors [ ζM ζM 0] and [ 2ζM 2ζM 0], ζM ≈0.43. The observed phenomena are attributed to electron-phonon interactions and anharmonic effects.