Papers by Author: Takashi Fukuda

Abstract: We have investigated superelastic behavior of an Fe-30.8Pd (at.%) alloy. This alloy transforms from a FCC parent phase to a FCT martensite phase at 255 K with a latent heat of 38 J/mol. When a compressive stress is applied in the [001] direction above the transformation temperature, the specimen exhibits a large elastic-like strain of more than 3% under relatively small stress of 100 MPa. The large elastic-like strain mainly comes from the elastic strain of the parent and martensite phases. The transformation strain also contributes to the strain, but it decreases linearly with increasing temperature and stress. The transformation strain || is extrapolated to be about 1.4 % under zero stress, and 0 % under 100 MPa. Probably a critical point, at which first order nature of the transformation disappears, exists for the FCC to FCT transformation in Fe-Pd alloys.

Abstract: The ferromagnetic shape memory alloy Ni2MnGa exhibits a successive martensitic transformation from the L21-type structure to the so-called intermediate phase and then to the martensite phase with an incommensurate structure during cooling under zero stress. In addition to these phases, a new phase, which we call the X-phase, appears when Ni2MnGa is cooled under compressive stress applied in the [001] direction. In this paper, we discuss the structural relations between the X-phase and the other phases on the basis of experimental results of compressive tests, transmission electron microscope observations and neutron diffraction patterns. It is likely that a multicritical point exists in the stress-temperature phase diagram of Ni2MnGa.

Abstract: We have investigated effect of magnetic field at cryogenic temperature on the stability of austenitic phase in a sensitized SUS304 stainless steel. The sensitization of the steel was made by heat-treatment in vacuum at 973 K for 100h after the solution treatment at 1323 K for 0.5 h. Isothermal holding of the sensitized specimens was made under a static magnetic field of 5 T in the temperature range between 60 and 260 K for various holding times. As a result, the TTT diagram under the magnetic field corresponding to the formation of 0.5 vol. % of -martensite shows a double-C curve with two noses. We found that by the application of magnetic field, the nose temperature and the incubation time of upper part of the C-curve, that is due to the    transformation, become lower and shorter, respectively. On the other hand, the nose temperature of lower part of the C-curve, that is due to the      transformation, does not change under the magnetic field although the incubation time shortens by the application of magnetic field.

Abstract: We have investigated stress and temperature dependences of the structure of the X-phase in Ni2MnGa to understand structural relation between the X-phase and other phases. Position and intensity of satellites of the X-phase are different from those of the intermediate (I-) phase under compressive stress, but they approach those of the I-phase with decreasing stress. That is, the structure change associated with the I → X transformation is discontinuous under a compressive stress, while it is continuous under zero stress. In addition, the transformation from the X-phase to the L21-type parent phase is continuous regardless of applied stress. These results strongly suggest the existence of multi-critical point in Ni2MnGa. On the other hand, the transformation from the X-phase to the martensite phase is discontinuous regardless of applied stress.

Abstract: We have investigated the martensitic transformation behavior in a single crystal of Ni2MnGa under various magnetic field. The single crystal used in the present study exhibits an intermediate phase (I-phase) transformation at TI = 250 K and a martensitic transformation at TM = 202 K. Since the martensite phase (M-phase) of Ni2MnGa has a large magnetocrystalline anisotropy, the effect of magnetic field depends significantly on the direction of magnetic field. We have measured the reverse (i.e., M-phase to I-phase) transformation start temperature As from a single variant state to examine the effect of magnetic field because the forward (I-phase to M-phase) transformation usually forms a multivariant state of the M-phase. When the magnetic field is applied parallel to the easy axis, As increases linearly with increasing magnetic field. On the other hand, when the magnetic field direction is not parallel to the easy axis, As decreases in a low field region and then increases on further increasing the magnetic field. Such behavior of magnetic field dependencies of As are quantitatively explained by the Clausius-Clapeyron equation, where we have assumed that the magnetic field dependence of As agrees with the magnetic field dependence of the equilibrium temperature.

Abstract: The magnetic field strength, Hs, at which rearrangement of martensite variants initiates has been investigated in Ni2MnGa ferromagnetic shape memory alloy by magnetization measurements in the [001]P direction ("P" stands for the parent phase). We have also calculated Hs from the magnetocrystalline anisotropy constant Ku, spontaneous magnetization Ms, twinning shear s and twinning stress τreq by considering the condition for the rearrangement of martensite variants reported previously [Int. J. Appl. Electromagnetics and Mechanics, 23 (2006) 45]. The calculated value of Hs is in good agreement with the experimental value for all the examined temperatures. The agreement confirms the applicability of the reported condition.

Abstract: We have studied diffuse scatterings appearing in Ti-(50-x)Ni-xFe (x = 6, 7, 8, 10) alloy, which shows negative temperature coefficient in electrical resistivity below a temperature Tmin. Electron diffraction pattern of each alloy exhibits diffuse scatterings below Tmin on cooling. The intensity maximum of the diffuse scatterings is located at an incommensurate position of g+<ζζ0>, where g represents reciprocal lattice points of the B2-phase. The value ζ is slightly smaller than 1/3 and decreases with increasing Fe content. The diffuse scatterings of the present alloys are well explained by the nesting effect of the Fermi surface in the B2-type structure, i.e., ζ of each alloy agrees with the length of nesting vector in the [110] direction, which is evaluated by a first-principle calculation.

Abstract: We have found that solution-treated or sensitized SUS304L stainless steel transforms isothermally from the γ (fcc)-phase to the α' (bcc) martensite via the ε' (hcp) martensite, that is, the amount of the α'-phase increases with increasing holding time. By applying magnetic field, the α' martensite can be induced in the ε'-plate when the steel has a sufficiently large ε' plate formed beforehand by isothermal holding. However, the α' martensite cannot be induced by magnetic field in the ε'-plate formed beforehand by a deformation-induced transformation.

Abstract: We have studied effect of magnetic field on the martensite (10M) to intermediate phase transformation temperature (As) of Ni2MnGa in order to understand the influence of magnetocrystalline anisotropy on the transformation temperature under a magnetic field. In case of the transformation from multi-variant 10M phase to the intermediate phase, As decreases with increasing magnetic field H for H < 0.8 MA/m, and then it increases on further increasing H. On the other hand, in case of the transformation from the single-variant 10M to the intermediate phase, As increases monotonically with increasing H, where the easy axis of the single variant is parallel to the field direction. The difference between the multi-variant and single variant state can be explained by considering the high magnetocrystalline anisotropy of the martensite phase.

Abstract: Change in the R-phase transformation behavior with the increase of Fe/Co content in Ti-(50-x)Ni-xFe and Ti-(50-x)Ni-xCo alloys has been investigated through thermal property measurements and microscope observations in order to clarify the relation among the stability of the B2-type structure, premartensitic phenomena, and the R-phase transformation. It was found that with increasing Fe/Co content, the R-phase transformation behavior approaches a second-order like one. The transformation product formed very close to the second-order is composed of nanoscale domains, and its Debye temperature is low compared with the R-phase formed by obvious first-order transformation.