Martensitic phase transformation and ferromagnetic shape memory effect in iron palladium single crystal.

摘要

In this thesis the ferromagnetic shape memory effect in Fe₇₀Pd ₃₀ alloys is studied in three stages. The first stage is to grow a single crystal of Fe₇₀Pd₃₀ using Bridgman method; the second stage focuses on the characterization of material properties. Both x-ray analysis and DSC measurements show that the FCC-FCT transformation is a weak first order thermoelastic transition. The average lattice parameters are a = 3.822 Å and c = 3.6298 Å for the FCT martensite, and a₀ = 3.7557 Å for the cubic austenite. The latent heat is 10.79 ± 11 J/cm3. The Curie temperature is 300°C. The saturation magnetization is m_s = 1217 emu/cm3 for the martensite and m_s = 1081 emu/cm 3 for the austenite in Fe₇₀Pd₃₀ alloy; the easy axis is in the [100] or [010] direction (the long axis of the FCT lattice). The magnetic anisotropy is −4.8 × 103 erg/cm 3 for the austenite at 60°C, and it is 3.46 × 10 5 erg/cm3 for the martensite at −20°C. In addition, the effect of heat treatment on the materials properties is investigated.; The third stage is to characterize the ferromagnetic shape memory effect of this alloy using magnetomechanical tests and the results are compared with micromagnetic theory. The sign of the field-induced strain agrees with the constrained theory. The maximum ferromagnetic strain in this material is about 0.9%, which is 1/4 of the theoretical prediction. The blocking stress is about −4 MPa, and the work output is about 20 × 103 J/m 3 per cycle at −12 MPa and 10°C. Furthermore, stress has large effect on the phase transformation temperature, for the compressive stress, it is 0.7°C/MPa. The phase transformation temperature can also be changed by applying a magnetic field during cooling or heating. The direction and the degree of changes depends on the direction of the field. The most significant change happens at a [001] field less than 1700 G, where [001] refers to the short axis of the FCT lattice.