Yoshisato Kimura

Nanosized precipitates in half-Heusler TiNiSn alloy

The microstructure of half-Heusler TiNiSn alloy has been investigated in this study. A high density of coherent nanosized Heusler precipitates was found within the half-Heusler matrix. Formation of these Heusler precipitates occurs by a phase separation process, where a single phase solid solution (half-Heusler-Heusler) decomposes into an equilibrium two-phase mixture of half-Heusler and Heusler regions. These Heusler nanoprecipitates improve the thermoelectric properties of the alloy.

Thermoelectric properties of p-type half-Heusler compound HfPtSn and improvement for high-performance by Ir and Co additions

The authors found that the half-Heusler compound HfPtSn exhibits p-type thermoelectric behavior, contrary to the HfNiSn counterpart with the same valence electron count of 18 showing n-type behavior. Nearly single crystals of HfPtSn were fabricated using optical floating zone melting method. HfPtSn shows large thermoelectric power while its electrical resistivity and thermal conductivity are relatively high. They improved p-type thermoelectric properties of HfPtSn by the additions of Ir and Co for the Pt site. It aims not only to optimize carrier concentrations but also to suppress an increase in thermal conduction by reducing the lattice contribution through the solid solution effects.

Design of Laves phase strengthened ferritic heat resisting steels in the Fe-Cr-Nb(-Ni) system

Abstract A new class of heat resisting ferritic steels is investigated in the Fe–Cr–Nb and Fe–Cr–Nb–Ni systems, in which the major strengthener is the Fe2Nb Laves phase. Ferritic steels strengthened by Laves phase are expected to show excellent high temperature strength, while the brittleness of Laves phase may lower the toughness of the alloy. The α-Fe/Fe2Nb two-phase microstructure is selected to improve mechanical properties through changing volume fraction and morphology of Laves phase. Effects of Nb and Ni contents on the microstructure and mechanical properties of the alloys, fixed at 10 at.% Cr, have been systematically investigated. Mechanical properties were evaluated by tensile tests conducted at room temperature, 873 and 973 K. The tensile test revealed that the room temperature ductility decreases with increasing Nb content. The alloys with 1.0–1.5 at.% Nb are found to exhibit a good balance between room temperature ductility and high temperature strength.

Phase stability and relations of multi-phase alloys based on B2 CoAl and E21 Co2AlC

The CoAlC ternary phase diagram has been experimentally examined for the Co-corner with a particular interest in phase relations among cobalt primary solid solutions, denoted as (Co), the E21 type intermetallic compound Co3AlC, and the B2 type CoAl. Liquidus surface composed with monovariant and contour lines, and corresponding reaction scheme were determined by optical microscopy, differential thermal analysis, X-ray diffraction, and energy dispersive X-ray spectroscopy. Several isothermal sections and isoplethals with respect to a fixed concentration for one of the elements were also evaluated. It is shown that such multiphase alloys as B2/E21, B2/(Co), and B2/E21/(Co) can be produced in the ternary system by proper choices of alloy chemistry and heat treatment.

Thermoelectric properties of directionally solidified half-Heusler compound NbCoSn alloys

Single- and multiphase samples of the n-type half-Heusler NbCoSn were prepared by directional solidification using the optical floating zone melting method, and the thermoelectric properties of these samples were evaluated. NbCoSn has an excellent thermoelectric power which exceeds −250μVK−1 at around 900K and a relatively high carrier concentration, 4.82×1026m−3. A metalliclike temperature dependence of the electrical resistivity indicates that NbCoSn is a degenerate semiconductor. NbCoSn also shows an excellent power factor, 2.5mWm−1K−2 at about 650K, even without any tuning of the electrical properties which are susceptible to coexisting metallic phases.

Microstructural control for strengthening the γ-Fe/E21–(Fe, Mn)3AlCx alloys

Abstract We have proposed microstructural modification through lattice misfit control on the γ-Fe/E2 1 –(Fe, Mn) 3 AlC x two-phase alloys, aiming for the improvement of mechanical properties. Since it is quite difficult to independently control lattice parameters of both γ-Fe and E2 1 , we took the approach to educe the E2 1 lattice parameter based on two strategies, lowering carbon content in E2 1 by optimizing Al content of alloy composition, and the addition of Ni intending to reduce the phase stability of the E2 1 relative to the L1 2 . The former attempt effectively decreases the lattice parameter of E2 1 and increases that of γ-Fe, by which lattice misfit is reduced. The latter idea is effective for reducing the lattice parameter of γ-Fe, while the lattice parameter of E2 1 slightly decreases only with the first 2 at.% Ni addition and remains almost constant. Consequently, lattice misfit increases, against our expectation.

Improvement in room temperature ductility of intermetallic alloys through microstructural control

A summary is presented of the recent advances in the development of multiphase intermetallic alloys towards improved room temperature ductility investigated by the present authors. The alloy systems of interest are Ni-Al-Be, Co-Al-Ni-Ti, and Co-Al-C, in which two- or three-phase alloys can be designed involving the Ll2, B2 and the primary solid solution, hereby denoted as (Ni) or (Co,Ni) in the first two systems, while the B2, E21, and the primary solid solutions, denoted as (Co) in the latter system. It is shown that by choosing a proper combination of phases with particular morphologies, good room temperature ductility can be obtained not only by compression and bending tests but also by tensile tests. Such a strategy has been so far most successful in the Co-Al-Ni-Ti system, where room temperature ductility of near to 20% is achieved in a B2/Ll2/(Co,Ni) three-phase alloy. It is pointed out that keys to design ductile multi-phase intermetallic alloys would be: (1) refinement of the microstructure utilizing solid state phase transformations such as an invariant reaction: (2) adjustment of amounts and compositions of the constituent phases, which is not possible in the binary system, but is in a multi-component system, and (3) hot fabrication to reduce solidification defects.

Microstructure control and tensile properties of three-phase alloys based on the E21 Co3AlC and B2 CoAl

Abstract To investigate tensile mechanical behavior, tensile tests were conducted at room temperature and 1273 K on three-phase alloys consisting of the E21 Co3AlC, B2 CoAl and (Co) primary solid solution. The alloy containing a large volume fraction of E21 phase exhibits excellent ductility, exceeding 5% plastic strain at room temperature, while the alloy with a considerable amount of coarse B2 phase particles shows zero ductility. In this study, microstructure control was used to improve the ambient temperature ductility of the E21/B2/(Co) three-phase alloys. Hot forging and subsequent heat treatments were performed aiming at eliminating solidification defects and minimizing the heterogeneity of the as-cast microstructure. These thermomechanical treatments together with compositional control effectively improve the ductility of the E21/B2/(Co) three-phase alloys at ambient temperature and 1273 K.

Compressive mechanical properties of multi-phase alloys based on B2 CoAl and E21 Co3AlC

Abstract Mechanical behavior of multi-phase alloys, consisting of intermetallic compounds B2 type CoAl and E2 1 type Co 3 AlC with primary solid solution (Co), was investigated by compression tests over a wide temperature range from 77 to 1273 K. It has been revealed that E2 1 Co 3 AlC is a good candidate of coexisting phase with B2 CoAl to improve the alloys ambient temperature ductility and its elevated temperature strength. Most of the multi-phase Co–Al–C ternary alloys exhibit fairly good compressive ductility at whole test temperatures, even at 77 K. All the alloys show the anomalous positive temperature dependence of strength at intermediate temperatures. The deformation mechanism of E2 1 Co 3 AlC is expected to be the same as that of L1 2 intermetallic compounds because of the close resemblance of their crystal structures. The three-phase B2/E2 1 /(Co) alloy, 70Co20Al10C(at%), exhibits excellent ductility of virtually unlimited ambient temperature compressive plastic strain and the highest strength around 340 MPa at 1273 K.

Annealing behavior of a ferritic stainless steel subjected to large-strain cold working

Mechanisms of microstructure evolution during annealing after cold working were studied in an Fe-15%Cr ferritic stainless steel, which was processed by bar rolling/swaging to various total strains ranging from 1.0 to 7.3 at ambient temperature. Two types of recrystallization behavior were observed depending on the cold strain. An ordinary primary (discontinuous) recrystallization developed in the samples processed to conventional strains of 1.0–2.0. On the other hand, rapid recovery at early annealing resulted in ultrafine-grained microstructures in the larger strained samples that continuously coarsened on further annealing. Such annealing behavior was considered as continuous recrystallization.