The effect of antiphase boundaries on the elastic properties of Ni–Mn–Ga austenite and premartensite

Abstract

The evolution of elastic properties with temperature and magnetic field was studied in two differently heat-treated single crystals of the Ni-Mn-Ga magnetic shape memory alloy using resonant ultrasound spectroscopy. Quenching and slow furnace cooling were used to obtain different densities of antiphase boundaries. We found that the crystals exhibited pronounced differences in the c' elastic coefficient and related shear damping in high-temperature ferromagnetic phases (austenite and premartensite). The difference can be ascribed to the formation of fine magnetic domain patterns and pinning of the magnetic domain walls on antiphase boundaries in the material with a high density of antiphase boundaries due to quenching. The fine domain pattern arising from mutual interactions between antiphase boundaries and ferromagnetic domain walls effectively reduces the magnetocrystalline anisotropy and amplifies the contribution of magnetostriction to the elastic response of the material. As a result, the anomalous elastic softening prior to martensite transformation is significantly enhanced in the quenched sample. Thus, for any comparison of experimental data and theoretical calculations the microstructural changes induced by specific heat treatment must be taken into account.

Elasticity and magnetism of Ni2 MnGa premartensitic tweed

“…In the saturation field (0.4 T), again the coefficient is strongly affected by the premartensitic transition. The sudden decrease above the temperature is the result of vanishing ‐effect (); in the premartensitic phase, however, the value of appears to be just constantly shifted compared to the zero field data. There is again no direct measurable effect of the premartensitic transition on the coefficient, except of that this temperature approximately corresponds the start of a smooth change of the slope.…”

“…The and constants were measured by a conventional pulse‐echo method, i.e., from times‐of‐flight of ultrasonic longitudinal and shear waves, respectively, between the largest faces of the sample. As the pulse‐echo method is less suitable for the determination of the coefficient particularly for material with large elastic anisotropy, this shear modulus was measured by resonant ultrasound spectroscopy (RUS), see for more details on these measurements. To determine the effect of the magnetic field, the soft (i.e., in demagnetized state) and hard elastic modulus (i.e., in magnetic saturation ()) were measured at zero field and at field of 0.4 T, which is above magnetic saturation of the sample ().…”

“…Large softening in austenite is due to vanishing magnetic anisotropy and relatively large magnetostriction. Moreover, the field dependence of is also strongly affected by heat treatment, which was explained by the high concentration of antiphase boundaries ().…”

Martensitic transformation and mechanical properties of Ni49+xMn36–xIn15 (x=0, 0.5, 1.0, 1.5 and 2.0) alloys

“…Such phonon softening phenomena is also common in Heusler alloys, which has been investigated by TEM, resonant ultrasound spectroscopy and neutron scattering for NiMnGa alloys during the pre-martensitic transformation [25, [28][29][30][31][32]. It has been shown before that the phonon dispersion curve of the TA 2 (transverse acoustic) mode softens toward the martensitic transformation temperature [32].…”

“…In shape memory alloys, there are three orientation variants if the austenitic phase transforms to tetragonal martensite or six orientation variants if the austenitic phase transforms to orthorhombic martensite [38][39][40][41][42]. In addition, the Heusler alloys are strongly anisotropic materials [30,31], and therefore, the larger scatter of the reduced elastic modulus for the martensites can be attributed to the orientation differences among different martensitic variants.…”