Papers by Author: Yoko Yamabe-Mitarai

Abstract: The effects of Fe addition on martensitic transformation and mechanical properties of AuTi were investigated in this study. It was found that B2 parent phase is stabilized by the Fe addition and that AuTi can contain at least 20mol%Fe. The lattice deformation strain evaluated from θ-2θ X-ray diffraction analysis (XRD) is not significantly changed by the Fe addition. The decrease in M_s evaluated by differential scanning calorimetry (DSC) is-40K/mol%Fe. Tensile tests revealed that, with increasing Fe content, the yield stress decreases up to about 13mol%Fe, largely increases up to 15mol%Fe and then decreases gradually. By taking into account XRD and DSC results, these behaviors are judged to correspond to reorientation of martensite variants, stress induced martensitic transformation and slip deformation of parent phase, respectively. The values of dσ_SIMT/dC_Fe and dσ_SIMT/dT are evaluated to be-170MPa/mol%Fe and-4.3MPa/K, respectively. The elongation is degraded with increasing Fe content from 8% in AuTi (0mol%Fe, martensite phase) to 2% in AuTi-20mol%Fe (parent phase) depending on the apparent phase.

Abstract: A Ti-50at%Pt alloy synthesized using the spark plasma sintering (SPS) technique has been characterized for phases’ identification. TiPt alloys have potential use as high temperature shape memory alloys(HTSMAs). Test specimens were prepared at SPS temperature of 1300°C. Sintering pressure and time were varied. The microstructural features of the specimens were investigated using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electron microscope used was equipped with an EDS detector, that, together with the XRD, were used for both the identification and analyses of the phases in the starting materials and the sintered alloys. High temperature XRD (800 -1300°C) as well as ambient temperature XRD analyses were done on the starting mechanically alloyed powders. All the samples tested at elevated temperatures were subsequently tested at room temperature after cooling. XRD analyses of the sintered samples were all done at room temperature. Analyses of the XRD results revealed new distinct phases from a temperature of 1000°C. A comparison of the room temperature XRD results for alloy powders and that of the sintered alloys was made. The following phases have been identified and studied TiPt B2, TiPt B19, Pt3Ti, Ti3Pt and Pt5Ti3. SPS pressure and sintering time did not show much effect on the phases detected. The alloy composition was found to be very inhomogeneous.

Abstract: Possibilities of heat resistant alloys based on a C15 Laves phase and an FCC phase have been examined in the Ir-Pt-Y ternary system. Although the Ir solid solution phase (A1) and the Ir2Y phase (C15) are not in equilibrium in the Ir-Y binary system, this equilibrium is attained by small Pt additions to the binary system. High temperature compressive strength of an A1/C15 monovariant eutectic alloy was found to be much lower than that of Ir-15Nb, an Ir based γ/γ' alloy. Low strength of the present alloys is attributed to the absence of effective strengthening mechanisms that operate in the A1 phase; for Y is hardly dissolved within the A1 phase, by which solution hardening and precipitation hardening are not available.

Abstract: A Laves phase Ir2Y is found to exhibit a temperature dependent solubility, which yields Ir3Y precipitation within the Ir2Y matrix of an arc-melted Ir-30at.%Y alloy. Microstructure observations have confirmed that the precipitation of Ir3Y has already started in the as-cast state. Upon annealing of an as-cast Ir-30at.%Y, the volume fraction of Ir3Y precipitates increases and finally reaches about 50%. The orientation relationship between Ir3Y and Ir2Y has been identified as: (0001)Ir3Y//(111)Ir2Y and [21 _ 1 _ 0]Ir3Y//[1 _ 10]Ir2Y. The orientation relationship is rationalized by the atom matching at the habit plane between the two phases.

Abstract: Shape recovery and superelasticity of Ti-50at%Pt and Ti-50at%(Pt, Ir), whose martensitic transformation temperature are above 1273 K, were investigated by thermal expansion measurement in dilatometer and loading-unloading compression test. The shape recovery was found in all compounds in at least one of the testing methods. The highest shape recovery, about 4% was found in Ti-25Pt-25Ir using loading-unloading compression test. On the other hand, superelasticity was found in only ternary compounds. Larger superelasticity was observed in ternary compounds with higher Ir contents. Potential of Ti-50Pt and Ti-50(Pt, Ir) as high-temperature shape memory alloys is discussed.

Abstract: The thermodynamic assessment of the Al-Ir binary system, one of the key sub-systems of the Ir-based alloys, was performed using the CALPHAD technique. The AlIr(B2) phase was described using the two sublattice model with the formula (Al,Ir)0.5(Ir,Va)0.5, while other intermetallic phases were treated as stoichiometric compounds. The calculated data of the phases in the Al-Ir system can be used to accurately reproduce experimental data, such as phase equilibria, invariant reactions, and formation enthalpies of the intermetallic phases.

Abstract: Severe plastic deformation (SPD) techniques are the best for producing of massive nanostructured materials. The methods of equal channel angular pressure (ECAP) and twist extrusion (TE) are realized by simple shear uniform deformation without change of cross-section sizes of sample. In the case of roll forming (RF) the shear strain is localized in the near-surface layer of metal. Intensity of shear strain in the near-surface layer depends on variation of parameters of deformation and conditions of friction in a contact. Steel 65G (0.65C, 0.3Si, 0.6Mn, 0.3Cr, and 0.3Ni) was deformed by roll forming. Transmission electron microscopy (TEM) of “cross-section” samples was used for studying of gradient structure of deformed material. TEM investigation shown that cell substructure in a near-surface layer have been formed. The depth of deformed layer is approximately 40 micrometers. Average cell size in cross-section direction is about 100 - 200 nm. Thin nanostructure layer with cell size about 20-30 nm was detected. In our opinion such substructure formed due to effect of “good” impurities.

Abstract: We have suggested B2-(Pt, Ir)Ti as high temperature shape memory alloys. The phase transformation of (Pt, Ir)-50at% Ti from B2 to B19(2H) or 4H(4O) structures was investigated in our previous study. The microstructure suggested martensitic transformation. In this study, thermal expansion measurement and cyclic compression test were performed for (Pt, Ir)Ti to investigate if the shape memory effect appears. High temperature strength was also investigated because phase transformation temperature of the (Pt, Ir)Ti is above 1273 K and high strength is necessary as high temperature shape memory alloys in order to suppress dislocation motion and stabilize martenstic transformation. The potential of (Pt, Ir)Ti as high temperature shape memory alloys will be also discussed.

Abstract: Solid solution hardening effects of Ir was investigated to develop high temperature materials at 2223 K. Pt, Rh, Hf, and Zr were chosen as second elements because their solubility into Ir at 2223 K is over 2at% and the melting temperatures of Ir solid solution are above 2273 K. Compressive strength of Ir solid solution at 2223K were investigated. Solid solution hardening effect of Ir is discussed in terms of lattice parameter change and solubility,