K. Endo

Microstructure and martensitic transformation in the Fe-Mn-Al-Ni shape memory alloy with B2-type coherent fine particles

Microstructure and martensitic transformation yielding a magnetic change were investigated for Fe43.5Mn34Al15Ni7.5 alloy with B2-type fine precipitates. Thermoelastic martensitic transformation from the ferromagnetic parent phase to the weak magnetic martensite with a nano-twinned fcc structure was confirmed. High-angle annular dark-field scanning transmission electron microscopic observation revealed that a β particle of about 10 nm maintains coherency with the matrix martensite phase, even though distorted due to the martensitic transformation. The martensitic transformation temperatures decreased about 75 K by application of a magnetic field of 70 kOe and magnetic field-induced reverse martensitic transformation was confirmed.

Magnetic Phase Diagram of the Ferromagnetic Shape Memory Alloys Ni2MnGa1-xCux

X-ray powder diffraction, permeability, magnetization and differential scanning calorimetry measurements were carried out on the magnetic shape memory alloys Ni2MnGa1−xCux (0 ≤ x ≤ 0.25). On the basis of the experimental results, the phase diagram in the temperature– concentration plane was determined for this alloy system. The determined phase diagram is spanned by the paramagnetic austenite phase (Para-A), paramagnetic martensite phase (Para-M), ferromagnetic austenite phase (Ferro-A), ferromagnetic martensite phase (Ferro-M) and the premartensite phase. It was found that the magnetostructural transition between the phases Para-A and Ferro-M can occur in the concentration region 0.12 < x ≤ 0.14 and that Ni2MnGa1−xCux has the characteristics of the phase diagram similar to those of the phase diagrams of Ni2+xMn1−xGa and Ni2Mn1−xCuxGa. In order to understand the phase diagram, the phenomenological free energy as a function of the martensitic distortion and magnetization was constructed and analyzed.

Thermal expansion and magnetization studies of the novel ferromagnetic shape memory alloy Ni2MnGa0.88Cu0.12 in a magnetic field

Thermal expansion, permeability and magnetization measurements of the ferromagnetic shape memory alloys, Ni2MnGa0.88Cu0.12, were carried out across the martensitic transition temperature TM and the reverse martensitic transition temperature TR at atmospheric pressure. When cooling from the austenite phase, a steep decrease in the thermal expansion due to the martensitic transition was found. The permeability indicates a sharp peak around TM and ferromagnetism below TM. Considering the permeability and magnetization results of Ni2MnGa0.88Cu0.12, the region above TM or TR is the paramagnetic-austenite phase and the region below TM or TR is the ferromagnetic-martensite phase. Magnetic phase diagrams were constructed based on the results of the temperature dependence of thermal expansion. TM and TR increased gradually with increasing magnetic field. The shift of TM in magnetic fields (B) around zero magnetic fields was estimated as dTM/dB=1.3 K T−1. The shift of TM indicates that magnetization influences the martensitic transition, and the increase of TM with magnetic field is proportional to the difference in the magnetization between the austenite and martensite phases.