Toshihiko Abe

Entropy change at the martensitic transformation in ferromagnetic shape memory alloys Ni2+xMn1−xGa

The entropy change ΔS between the high-temperature cubic phase and the low-temperature tetragonally based martensitic phase of Ni2+xMn1−xGa (x=0–0.20) alloys was studied. The experimental results obtained indicate that ΔS in the Ni2+xMn1−xGa alloys increases with the Ni excess x. The increase of ΔS is presumably accounted for by an increase of magnetic contribution to the entropy change. It is suggested that the change in modulation of the martensitic phase of Ni2+xMn1−xGa results in discontinuity of the composition dependence of ΔS.

Application of pulse discharge sintering (PDS) technique to rapid synthesis of Ti3SiC2 from Ti/Si/C powders

Abstract To synthesize Ti3SiC2 samples, pulse discharge sintering (PDS) technique was utilized to sinter elemental powders of Ti/Si/C with stoichiometric and off-stoichiometric ratios in a temperature range of 1200–1500xa0°C. The results showed that high purity Ti3SiC2 could not be obtained from the Ti/Si/C powder with molar ratio of 3:1:2, and Ti3SiC2 preferred to form at relatively low sintering temperature for a short time. When 5Ti/2Si/3C and 3Ti/1.5Si/2C powders were sintered for 15 min, the TiC content was respectively decreased to 6.4 and 10 wt.% at 1250–1300xa0°C. The corresponding relative density of the samples sintered from 5Ti/2Si/3C powder was calculated to be as high as 99% at the temperature above 1300xa0°C. It is suggested that low-temperature rapid synthesis of Ti3SiC2 would be possible through the PDS technique, provided that the composition of the starting powders should be adjusted to be off-stoichiometric ratio from 3:1:2.

A new synthesis reaction of Ti3SiC2 through pulse discharge sintering Ti/SiC/TiC powder

Abstract Ti 3 SiC 2 was synthesized by pulse discharge sintering 4Ti/2SiC/TiC mixture powder in a temperature range of 1250–1450 °C. The purity of Ti 3 SiC 2 was improved to 92 vol% at a sintering temperature of 1350 °C. The microstructure in the synthesized samples was controlled to be fine, coarse and duplex grains, depending on the sintering temperature and time.

Effects of sintering temperature and Si content on the purity of Ti3SiC2 synthesized from Ti/Si/TiC powders

Abstract To improve the purity of titanium silicon carbide (Ti3SiC2), Ti/Si/TiC powders with molar ratios of 1:1:2, 1:1.05:2, 1:1.1:2 and 1:1.15:2 were prepared by mixture method for 24 h in Ar atmosphere. The synthesis process was performed at 1200–1400xa0°C under a pressure of 50 MPa with the pulse discharge sintering (PDS) technique. The results showed that the purity of Ti3SiC2 was always higher than 92 wt.% in all the samples; however, the excess Si did not play a distinct role in improving the purity of Ti3SiC2. It was found that Ti3SiC2 with a purity of about 97 wt.% could be rapidly synthesized at 1250–1300xa0°C, which is 200–300xa0°C lower than those synthesized from Ti/Si/C and Ti/SiC/C powders. Meanwhile, the microstructure of Ti3SiC2 could be controlled to possess various grain sizes, such as fine, coarse or duplex grains. It is suggested that the synthesis reaction of Ti3SiC2 should occur between Ti–Si liquid phase and TiC, which could explain why the excess Si did not decrease the content of TiC in the synthesized products.

Synthesis of single-phase Ti3SiC2 powder

Abstract In this study, free 2Ti/2Si/3TiC powder mixture was heated at high temperatures in vacuum, in order to reveal the possibility for the synthesis of high Ti 3 SiC 2 content powder. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for the evaluation of phase identities and the morphology of the powder after different treatments. Results showed that almost single phase Ti 3 SiC 2 powder (99.3 wt.%) can be synthesized by heat treatment with free 2Ti/2Si/3TiC powders in vacuum at 1210°C for about 3 h. The nucleation and growth of Ti 3 SiC 2 within TiC particles was observed. The typical appearance of the formed Ti 3 SiC 2 is equiaxed with particle size of 2–4 μm. Effects of temperature and heating time on the morphology and the particle sizes of the synthesized Ti 3 SiC 2 powders are not obvious.

Influence of intermartensitic transitions on transport properties of Ni2.16Mn0.84Ga alloy

Magnetic, transport and x-ray diffraction measurements of the ferromagnetic shape memory alloy Ni2.16Mn0.84Ga have revealed that this alloy undergoes an intermartensitic transition upon cooling, whereas no such a transition is observed upon subsequent heating. The difference in the modulation of the martensite forming upon cooling from the high-temperature austenitic state (five-layered (5M) martensite), and the martensite forming upon the intermartensitic transition (seven-layered (7M) martensite) strongly affects the magnetic and transport properties of the alloy and results in a large thermal hysteresis of the resistivity ρ and magnetization M. The intermartensitic transition has an especially marked influence on the transport properties, as is evident from a large difference in the resistivity of the 5M and 7M martensites, , which is larger than the jump of resistivity at the martensitic transition from the cubic austenitic phase to the monoclinic 5M martensitic phase. We assume that this significant difference in ρ between the martensitic phases is accounted for by nesting features of the Fermi surface. It is also suggested that the nesting hypothesis can explain the uncommon behaviour of the resistivity at the martensitic transition, observed in stoichiometric and near-stoichiometric Ni–Mn–Ga alloys.

Ti3SiC2 powder synthesis from Ti/Si/TiC powder mixtures

Abstract Various heat treatments in vacuum were carried out on free Ti/Si/TiC powder mixtures with molar ratios of 2:2:3 and 1:1:2, in order to reveal the effect of original powder composition on the Ti 3 SiC 2 powder synthesis. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for the identification of phase constitutions and the characterization of the synthesized powder. During the synthesis process, Ti 5 Si 3 was found to be the only intermediate phase in both cases and part of which remained in the final synthesized powder together with Ti 3 SiC 2 . The optimal temperature range for the formation of Ti 3 SiC 2 was 1210–1300xa0°C; above 1300xa0°C the content of Ti 3 SiC 2 decreased significantly with temperature. At 1300xa0°C in the time range of 0.5–4 h, the effect of heating time on the Ti 3 SiC 2 content was not obvious.

An efficient control of Curie temperature TC in Ni–Mn–Ga alloys

Abstract We have studied the influence of alloying with a fourth element on the temperature of ferromagnetic ordering T C in Ni–Mn–Ga Heusler alloys. It is found that T C increases or decreases, depending on the substitution. The increase of T C is observed when Ni is substituted by either Fe or Co. On the contrary, the substitution of Mn for V or Ga for In strongly reduces T C .

Oxidation of Ti3SiC2 at 1000°C in Air

Oxidation of polycrystalline Ti3SiC2 was investigated at 1000°C by XRD and SEM. Results show that a thin SiO2-rich scale forms after short-term oxidation, but it cannot prevent further oxidation effectively. The TiO2 content increased with increasing time and covered the whole surface eventually. For the long-term oxidation, the oxide scales are layered, the outer layer is comprised of pure TiO2, while the inner layer is a mixture of TiO2 and SiO2. The position of the pre-formed SiO2 layer indicates that it can serve as an initial marker for the subsequent oxidation.

Synthesis and consolidation of TiAl by MA–PDS process from sponge-Ti and chip-Al

Abstract Low cost sponge-Ti and recycled chip-Al were used as starting materials for the synthesis and consolidation of TiAl alloy via a mechanical alloying (MA) and pulse discharge sintering (PDS) process. Amorphous structured TiAl powder was obtained by MA process from sponge-Ti/chip-Al and was sintered and concurrently consolidated with the PDS process. The obtained TiAl alloy consists of γ-TiAl and α2-Ti3Al and additional Ti2AlC phase. The microstructure of the alloy can be controlled by the sintering temperature. It was found that fine equiaxed grains were obtained by low temperature sintering while fine duplex structures formed at high temperature sintering. Compared with alloys fabricated from high cost elemental Ti/Al powder with a similar process, the alloys sintered at high temperature with this process possess much higher strength at both room temperature and elevated temperatures.