Vibrational properties of Ni–Mn–Ga shape memory alloy in the martensite phases

Abstract

Studying the phonon dispersion of the ferromagnetic shape memory alloy system Ni-Mn-Ga gives insight into the mechanism of the martensite transition and the forces driving the transition. Transformation of austenite single crystals under uniaxial stress results in the coexistence of two martensitic variants with perpendicular modulation vector. Here we report on inelastic neutron scattering studies of martensite crystals with off-stoichiometric compositions, varying from non-modulated (NM) to five-(5M) and seven-(7M) layer modulated martensite phases. Both the 5M and 7M crystals show fully commensurate satellite peaks along [ξξ 0], corresponding to the five-and seven-layer modulation. These superstructure peaks become -points of the modulated structure. Due to the coexistence of two variants within the (001) plane, both new acoustic phonons reflecting the modulation vector [ξξ 0] and acoustic TA 2 [ξ ξ 0] phonons corresponding to the non-modulated direction are observed. The latter display a pronounced softening around ξ = 0.2-0.4 when approaching the martensite-austenite transition from above and below, i.e. this soft mode has lowest frequency at the transition temperature. Overall the phonon dispersion of the austenite and martensite phase resemble each other very much.

Localized deformation in Ni-Mn-Ga single crystals

“…Thus, a direct comparison of the phonon spectra with the measured reduced moduli is not possible. However, the variation of normal and shear stiffnesses on the order of 40% deduced from the neutron scattering results 28 agrees with the variation in reduced moduli between the two c-axis orientations of about 50% reported here. Pop-ins typically stem from bursts of dislocations or stress-induced displacive phase transformations.…”

“…Phonon dispersion curves measured with neutron scattering provide further insight into the elastic properties. 28 At small wave vector, the slopes of the [n00] and [00n] phonon branches are proportional to the corresponding sound wave velocities. Furthermore, the corresponding stiffness constants c 11 and c 33 are proportional to the square of the sound velocities.…”

Hysteresis Design of Magnetocaloric Materials—From Basic Mechanisms to Applications

“…This is expressed quantitatively in terms of the vibrational density of states (VDOS) g ( ϵ ), which can be measured by inelastic neutron scattering or nuclear resonant inelastic X‐ray scattering . VDOS and phonon dispersion relations have been calculated from first principles achieving excellent agreement with experimental data . The expression for S lat resembles S el , where the expression in the parentheses takes into account the Bosonic character of the phonons through the Bose–Einstein distribution function n ( ϵ , T )=(exp( ϵ / k B T )−1) −1 for the occupation numbers, which becomes large for high T or small phonon energies ϵ : …”

“…cellent agreementw ith experimental data. [23,25,33,[36][37][38][39] Thee xpression for S lat resembles S el ,where the expression in the parentheses takes into account the Bosonic character of the phononst hrough the Bose-Einstein distribution function n(e,T) = (exp(e/k B T)À1) À1 for the occupation numbers,w hich becomesl arge for high T or small phonone nergies e: [22,40]…”

Inelastic x-ray scattering measurements of phonon dispersion and lifetimes in PbTe1−xSexalloys

“…The softening may also relate to the slight distortion from the perfect crystalline structure suggested by XRD. Phonon softening was observed in metal alloys and was attributed to structural phase transition [26][27][28], yet there is no experimental evidence of phase transition in PbTe 1−x Se x alloys. A deep understanding of the [1 1 1] TA modes in PbTe 1−x Se x alloys requires future investigation.…”