Papers by Author: Elena Villa

Abstract: Magnetic and structural properties in multifunctional FSMA (Ferromagnetic Shape Memory Alloys) belonging to Heusler family are frequently related to the occurrence of structural modulation in martensitic phases. The highest MFIS (Magnetic Field Induced Strain) effect has been observed in Ni-Mn-Ga alloys showing martensitic modulated structures. Depending on the composition, pressure and temperature conditions, this periodic structural distortion, consisting of shuffling of atomic layers along specific crystallographic directions, accompanies the martensitic transformation. Over the years, different modulated martensitic structures have been observed and classified depending upon the periodicity of corresponding superstructure (nM with n=3, 5, 6, 7, 12 etc). On the other hand, it has been demonstrated that in most cases such structural modulation is incommensurate and the crystal structure can be solved by applying superspace approach. The crystallographic representation of different modulated structures, obtained by structure refinement on powder diffraction data, suggests a unified description where every different “nM” periodicity can be straightforwardly represented. It will be presented an overview illustrating structural features of several displacive modulated martensitic lattices. For a specific Ni-Mn-Ga composition, the evolution of structural modulation upon temperature change will be illustrated.

Abstract: The internal friction (IF) and the Young’s modulus (E) of NiTi based alloys have been investigated at 1 Hz and 1 kHz frequencies after various sequences of thermo-mechanical treatments and hydrogen-doping given to the materials. Differential scanning calorimetry (DSC) has also been used as a complementary investigation tool. Apart from the transient effects, only occurring at 1 Hz frequencies, the data indicate a substantial insensitivity of damping to frequency. The results show that the H-Snoek and the H-twin boundary relaxations get their maximum height for H contents nH (nH=H/Me) equal to about 0.025 and 0.008, respectively. At kHz frequencies the IF peaks associated with these relaxations occur at around room temperature in the Ni49Ti51 and Ni30Ti50Cu20 alloys. Thus, these appear to be the most promising materials for applications aimed at the reduction of vibrations.