K. Oikawa

Promising ferromagnetic Ni–Co–Al shape memory alloy system

A system of ferromagnetic β phase Ni–Co–Al alloys with an ordered B2 structure that exhibits the shape memory effect has been developed. The alloys of this system within the composition range Ni (30–45 at.u200a%) Co–(27–32 at.u200a%) Al, undergo a paramagnetic/ferromagnetic transition as well as a thermoelastic martensitic transformation from the β to the β′(L10) phase. The Curie and the martensitic start temperatures in the β phase can be controlled independently to fall within the range of 120–420 K. The specimens from some of the alloys undergoing martensitic transformation from ferromagnetic β phase to ferromagnetic β′ phase are accompanied by the shape memory effect. These ferromagnetic shape memory alloys hold great promise as new smart materials.

Effects of B and C on the ordering of L10-CoPt thin films

We have investigated the effects of B and C on the ordering of L10-CoPt films and confirmed the following quite different role between B and C in CoPt. A small amount of B considerably decreases the temperature for ordering and is interstitially incorporated into L10-CoPt along its c axis due to the large negative heat of solutions with Co and Pt. X-ray photoelectron spectroscopy analyses have revealed that B atoms combine with Co and Pt. In contrast, C is not incorporated into the CoPt lattice due to its positive heat of solution, resulting in no decrease in the ordering temperature. The present results reveal that CoPt films containing a small amount of B are promising for high density recording media.

Thermodynamic calculations of Fe-Zr and Fe-Zr-C systems

Thermodynamic calculations of Fe-Zr and Fe-Zr-C systems have been performed using the Thermo-Calc software based on an extensive amount of experimental data, including the thermodynamic measurements and available phase diagram information. The calculated thermodynamic properties and phase diagram in the Fe-Zr system account for the experimental data reasonably, while the calculated solubility of ZrC in γFe in the Fe-Zr-C system is a little smaller than the only experimental data by Narrita; to clear up this discrepancy, further investigations on the Fe-Zr-C as well as the Zr-C systems are needed.

Thermodynamic calculations of phase equilibria of Co–Cr–Pt ternary system and magnetically induced phase separation in the FCC and HCP phases

Abstract The calculations of phase equilibria of the Co–Cr–Pt ternary system have been carried out based on the thermodynamic assessments of Co–Cr, Co–Pt and Cr–Pt binary systems by the calculation of phase diagram technique. The Gibbs energies of the liquid, FCC, BCC and HCP solution phases were approximated by a sub-regular solution model, while those of σ and Cr3Pt phases were approximated by a compound energy model. Almost all the experimental information on each sub-system has been well described by the present set of thermodynamic parameters. A critical calculation of the magnetically induced miscibility gap between the ferromagnetic HCP and the paramagnetic HCP phase has been conducted, where the two-phase separation has been found at the Curie temperature. The Cr content in the ferromagnetic HCP phase increases and the width of the two-phase separation becomes narrower with increasing Pt content. The present calculations would be useful for the design and development of the perpendicular magnetic recording media.

Prediction of effective elements for magnetically induced phase separation in Co–Cr-based magnetic recording media

Thermodynamic calculations of the magnetically induced phase separation of the hcp phase in Co–Cr-based alloy systems, which is the origin of the compositional modulation observed in magnetic recording media, have been carried out. The magnetic and nonmagnetic terms in the Gibbs energy are evaluated from available thermodynamic data and Miedema’s semiempirical values. It is demonstrated that the phase separation in Co–Cr–X ternary alloys can be classified into four types, depending on the values of interaction energies between Co and/or Cr and X atoms. Existing magnetic data on Co–Cr-based recording media are discussed in terms of the phase stability of the hcp phases. The present calculations would be useful for development of high-density magnetic recording media.

Optimization of the Fe-Rich Fe-Mn-S Ternary Phase Diagram

Phase equilibria in the Fe-rich side of the Fe-Mn-S ternary system were analyzed on the basis of the thermodynamic analysis of the Fe-FeS, Fe-Mn, Mn-MnS and Fe-FeS-MnS-Mn systems. The Gibbs energy of individual phases was approximated by the sublattice model, assuming S to be interstitial atoms. Most of the experimental information was well reproduced by the present set of thermodynamic descriptions. The present analysis is useful for understanding sulfide formation during solidification, precipitation ip solids, and the cause of the so-called hot shortness in mild steels.

Magnetically induced phase separation in the Co-Cr binary system

Direct evidence of phase separation in the Co-rich corner of the Co–Cr binary system, the transformation of high-temperature FCC α-Co into a ferromagnetic αf phase and a paramagnetic αp phase, has been experimentally obtained by using diffusion couple technique, scanning electron microscopy with energy dispersion analysis of X-ray (SEM-EDX), analytical transmission electron microscopy, optical microscopy and X-ray diffraction. Thermodynamic calculation, on the basis of the presently obtained phase equilibrium data, has verified that this phase separation arises from magnetic ordering, and that a similar phase separation appears in the HCP phase below 469°C which is transformed into a ferromagnetic ef phase and a paramagnetic ep phase. It is therefore concluded that magnetically induced phase separation must be responsible for the microscopic composition modulation of Cr in the CoCr thin film magnetic recording media.

Thermodynamic calculations of the effect of B and Ta on magnetically induced phase separation in Co-Cr-Pt alloys

In order to clarify the relation between the magnetically induced phase separation and the recording media characteristics, the thermodynamic calculations of Co–Cr–Pt–B and Co–Cr–Pt–Ta systems have been carried out by using the available binary assessment data and Miedema’s semiempirical values. B is segregated to the boundary in a similar manner as Cr, which makes the boundary region paramagnetic. This result is consistent with available data that B weakens the intergranular magnetic coupling and increases the magnetic anisotropy in Co–Cr–Pt recording media. By adding Ta, the Cr content in the paramagnetic phase is also increased, reducing the intergranular magnetic coupling. However, the Ta content in the ferromagnetic phase is higher than in the paramagnetic phase, decreasing the magnetic anisotropy. Accordingly, the thermodynamic calculations successfully explain experimental magnetic data for Co–Cr–Pt–B and Co–Cr–Pt–Ta recording media.

Magnetically induced two-phase separation in Co-Ge and Co-Si systems

Abstract Thermodynamic assessments of Co–Ge and Co–Si systems were carried out by the CALPHAD technique. Magnetic and non-magnetic terms were approximated by the Hillert and Jarl description and a sub-regular solution model, respectively. Almost all experimental data were well described by the present set of thermodynamic parameters. The calculated results predict that the magnetically induced two-phase separation occurs in the Co–Ge system.

Effects of alloying additions on the thermal stability of the lamellar structure of γ-TiAl-based cast alloys

Abstract The thermal stability of the as-cast lamellar structure of the Ti–48Al, Ti–45Al–3Ga, Ti–45Al–2Nb–2Cr, Ti–46Al and Ti–46Al–1.0Si alloys (in at.%) at 1150 and 1000°C was studied by optical microscopy. The addition of 1.0Si has been demonstrated to have a remarkable improvement on the thermal stability of the lamellar structure of TiAl alloy at high temperatures, but the precise mechanism for this improvement needs to be clarified.