Papers by Author: Shigeru Suzuki

Abstract: The saturation value of the magnetostriction curve in the [100] direction of a Fe-Ga alloy single crystal was decreased from 226 to 55 ppm by applying the tensile strain of 533 ppm to the measured direction. By magnetic domain observation using a magneto-optic Kerr effect microscope, a complex structure composed of various magnetic domains was observed under zero applied strain. On the other hand, a stripe structure composed of magnetic domains with the magnetization direction in two kinds of <100> magnetic easy directions parallel to the tensile direction, which were separated by straight 180° domain walls, was observed under the tensile strain of 533 ppm. The characteristic magnetic domain structure due to the tensile strain was successfully observed as a cause of the significant decrease of the saturation value of the magnetostriction curve.

Abstract: Fe-Ga alloys are used for practical magnetostrictive materials. The understanding of mechanical properties under magnetic field is very important for their application to vibration power generation devices. Especially, Poisson’s ratio is one of key parameters because it is strongly related with the volume change of materials. In the study, we investigated actual behavior of Poisson’s ratio due to magnetostriction, instead of just the mechanical one. The sample was cube-oriented Fe-18mol%Ga single crystal disc. Static magnetic fields were applied in various directions parallel to (001). Strain values in various directions were measured by strain gauge and X-ray diffraction. And then, tri-axial strain analysis for single crystal was carried out. Theoretically, Poisson’s ratio due to magnetostriction is known to be 0.5, (volume conservative), value is close to the mechanical one, 0.45. On the other hand, we found that Poisson’s ratio exhibits anisotropic behavior despite the elastic constants are equivalent in [010] and [001]. This phenomenon is attributable to the magnetic domain structure under zero magnetic field. In this presentation, we will discuss this apparent Poisson’s ratio considering the volume strain and magnetostriction comprehensively.

Abstract: We carried out in situ tri-axial magnetostriction analysis for cube-oriented Fe-18%Ga single crystal by X-ray diffraction measurement under magnetic field. Periodic change in tri-axial magnetostriction with applied magnetic field direction was clearly observed. However, those values in [100] and [010] directions were not equivalent. Theoretical calculation of magnetostriction considering domain structure revealed this is caused by the non-equivalent volume fraction of initial magnetic domains.

Abstract: Fe-Ga alloys show large magnetostriction, which strongly depends on crystal orientation. This phenomenon is associated to some degree with large elastic anisotropy. In this study, white X-ray diffraction with micro-beam synchrotron radiation was used to evaluate the microscopic stresses evolved in a polycrystalline Fe-Ga alloy under tensile loading. In the analysis, the large elastic anisotropy of the Fe-Ga alloy was focused. The stress distribution in the alloy microstructure under tensile loading was estimated using a finite element method (FEM) simulation that considered the dependence of the elasticity on the crystal orientation. The crystal orientation of grains in the polycrystalline Fe-Ga alloy was measured using electron backscatter diffraction. The FEM simulation showed that the stress distribution in the microstructure depended on the crystal orientation. The X-ray diffraction stress analysis indicated that under tensile loading, the stresses in the alloy depended on the crystal orientation. This finding is similar to the results obtained from the FEM simulation, although the absolute values of the stresses may have reflected the effects of heterogeneous deformation on the stress distribution.

Abstract: X-ray diffraction (XRD) and electron microprobe microanalysis (EPMA) were used for characterizing the structure and composition of surface layers formed on austenitic Fe-Mn-Si shape memory alloys under vacuum. The XRD results demonstrated that during annealing, face centered cubic austenite is transformed to hexagonal close packed martensite on the alloy surface. The EMPA results revealed that manganese in the surface layer was depleted during annealing. Further, this analysis determined that the thickness of the surface layer of the alloy annealed at 1173 K for 1 h was approximately 20 μm and that value is consistent with the depth detected by XRD. The compositional changes of the surface layers such as manganese depletion by annealing were discussed based on the ternary Fe-Mn-Si phase diagram. Although the formation of body centered cubic ferrite is detrimental to shape memory alloys, the amount of manganese was also observed to change during processing and strongly influence the stability of the shape memory alloys.

Abstract: Dislocation-induced relaxations in different molybdenum single crystals were investigated by means of low-frequency internal friction measurements in the temperature range of 20–600 K. The results indicated that the appearance of the dislocation-induced relaxations strongly depends on the purity of the molybdenum, although the intrinsic dislocation relaxations appeared at about 100 K and 450 K in the high-purity molybdenum. The molybdenum containing a small amount of carbon did not exhibit the intrinsic dislocation relaxations but rather revealed a modulus increase due to the dislocation pinning caused by the dissolved carbon. When the molybdenum containing a small amount of carbon was annealed up to 700 K, a new relaxation peak appeared at about 450 K. The activation process for this relaxation indicated that it could be attributed to the relaxation due to a carbon-dislocation interaction. In addition, it was shown that the dislocation-induced relaxations in medium-purity molybdenum were small, which was attributed to the residual substitutional impurities in the molybdenum.

Abstract: White X-ray diffraction with micro-beam synchrotron radiation was used to analyze microscopic stress evolved in coarse grains of a twinning-induced plasticity Fe-Mn-C steel under tensile loading. In addition, electron backscatter diffraction (EBSD) was used to determine the crystal orientation of grains in the polycrystalline Fe-Mn-C steel. Based on these orientation data, the stress and strain distribution in the microstructure of the steel under tensile loading was estimated using FEM simulation where the elastic anisotropy or the crystal orientation dependence of the elasticity was taken into account. The FEM simulation showed that the strain distribution in the microstructure depends on the crystal orientation of each grain. The stress analysis by the white X-ray diffraction indicated that the direction of the maximum principal stresses at measured points in the steel under tensile loading are mostly oriented toward the tensile direction. This is qualitatively consistent with the results of by the FEM simulation, although absolute values of the principal stresses may contain the effect of heterogeneous plastic deformation on the stress distribution.

Abstract: Since the matrix phase is transformed to martensitic phase in shape memory alloys (SMAs) during plastic deformation, complicated residual stresses may arise during deformation, and they may affect the shape recovery ability of the alloys. Thus, it is important to be able to characterize the residual stresses formed in SMAs during plastic deformation and annealing. In this study, X-ray diffraction was used to characterize the residual stress formed in a Fe-Mn-Si-Cr SMA, which was deformed in the tensile direction and subsequently annealed. The results showed that the compressive stress persisted in the tensile direction of the face-centered cubic (fcc) matrix upon tensile deformation and unloading. Compressive stress is believed to result from the hexagonal close-packed (hcp) phase formed during stress-induced martensitic transformation. After the deformed samples were annealed to recover their shapes, the residual stress was considerably reduced. This is believed to be due to the decrease in the formation of the hcp phase or to the recovery of their shapes during annealing. Our results indicated that residual stress in the fcc matrix phase is associated with the shape recovery characteristics of the alloys after martensitic and reverse martensitic transformations.

Abstract: In this Study, Influences of P on the Microstructure, Mechanical Properties, and Retained Austenite Characteristics in Transformation Induced Plasticity (TRIP) Steels Were Investigated. Microstructure of 0.2mass%P Containing TRIP Steel Was Inhomogeneous and it Resulted in Deterioration of the Mechanical Properties. Retained Austenite Characteristics such as Volume Fraction and Carbon Concentration Were Also Affected by P. The Stability of Retained Austenite in P Containing TRIP Steel Was Different from that in P-Free TRIP Steel. Such Difference in the Stability of Retained Austenite Was Attributed to the Effect of the Carbon Concentration in Retained Austenite as Well as their Different Microstructure.

Abstract: Alloying elements added to steel for improving surface properties such as corrosion resistance are often enriched to the surface of the alloys during annealing at high temperatures. Their behavior depends on difference in their chemical characters and the condition of annealing. In this study, nondestructive depth-resolved analysis of amount distribution and chemical state of alloying element by using X-ray fluorescence analysis (XRF) technique in combination with X-ray absorption spectroscopy (XAS) in order to characterize the enrichment and oxidation of manganese on the surface layers of an Fe-Mn alloy annealed under low oxygen partial pressure. The experiments were carried out using a two-dimensional detector with geometrical arrangement of grazing exit in detection of fluorescence X-ray emitted from sample surface. The results showed that manganese was enriched to surface layers of the Fe-Mn alloys during annealing at high temperatures and formed as manganese oxide. The preferential oxidation of manganese by annealing under low oxygen partial pressure is considered the driving force for their enrichment on the alloy surface.