In-Situ High-Energy X-Ray Diffuse-Scattering Study of the Phase Transition in a Ni2MnGa Ferromagnetic Shape-Memory Crystal

Abstract

The full information on the changes in many crystallographic aspects, including the structural and microstructural characterizations, during the phase transformation is essential for understanding the phase transition and ''memory'' behavior in the ferromagnetic shape-memory alloys. In the present article, the defects-related microstructural features connected to the premartensitic and martensitic transition of a Ni 2 MnGa single crystal under a uniaxial pressure of 50 MPa applied along the [110] crystallographic direction were studied by the in-situ highenergy X-ray diffuse-scattering experiments. The analysis of the characteristics of diffusescattering patterns around different sharp Bragg spots suggests that the influences of some defect clusters on the pressure-induced phase-transition sequences of Ni 2 MnGa are significant. Our experiments show that an intermediate phase is produced during the premartensitic transition in the Ni 2 MnGa single crystal, which is favorable for the nucleation of a martensitic phase. The compression stress along the [110] direction of the Heusler phase can promote the premartensitic and martensitic transition of the Ni 2 MnGa single crystal.

In-Situ High-Energy X-Ray Diffuse-Scattering Study of the Phase Transition of Ni2MnGa Single Crystal under High Magnetic Field

“…Note that the distance (D) of the spring actually controls the stress applied to the specimens. [13] It was very difficult to determine the T PM of the Ni 2 MnGa single crystal from its thermomagnetization curves under magnetic fields of 50 KOe, as the difference of the magnetization of its parent phase and premartensitic phase was hardly observed. However, it could be determined from the X-ray scattering results (Figures 3(c) and 5(c)) that PM transformation occurred at 250 K (À23°C).…”

“…When the sample was cooled from 250 and 172 K (À23 and À101°C) under 50 KOe magnetic field, diffuse maxima were formed in the streaks off the Bragg positions (Figures 5(c) and 6) and their intensities increased gradually, corresponding to the formation of the PM phase. [13] The characteristics of the cubic lattice and streaks of diffuse scattering remained until M s (about 172 K (À101°C)). When the sample was cooled below 172 K (À101°C), some new diffraction spots appeared at the position near the diffuse maxima of the streaks produced by the PM phase (Figures 5(d) and 6), while the intensity of the diffuse streaks decreased gradually.…”

“…In comparison with the magnetic-field-free condition, M s was lowered by about 3 K, while A s was raised by about 3 K. The T PM increased to 250 K (À23°C), which was similar with the effects of a compressive stress of 50 MPa along the [110] direction of the parent Heusler phase on T PM . [13] In the previous work, [25,26] the changes of the transition temperature under different magnetic fields were explained quantitatively using the Clausius-Clapeyron equation:…”

“…When the martensitic transition began under a magnetic field of 0 KOe, several diffraction spots of martensite phase always appeared together characterized by a perfect symmetry, [13] which could be thought of as the coexistence of the diffraction patterns of several martensite variants. However, it was observed that the symmetric characteristics of the diffraction spots of martensitic phase were destroyed under a magnetic field of 50 KOe (Figures 3(e) and 5(e)).…”

“…A lot of progress has been made in this kind of alloy through intensive studies toward many scientific aspects, including the composition dependence of phase transformation behaviors, [4] magnetic properties, [5] magnetocaloric effects, [6] martensite stabilization, [7] crystal structure and twinning crystallography, [8][9][10][11][12] and the effect of uniaxial pressure and magnetic fields on the premartensitic (PM) and martensitic transition of NiMnGa alloys. [12][13][14][15][16] An exploration on the phase transition process and the rearrangement of martensite variants in response to multiple external parameters (stress, temperature, and magnetic fields) is very important to enable the quick response and remote control of NiMnGa alloys used as actuator and sensor.…”