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Shape-memory alloys, such as Ni-Ti and Cu-Zn-Al, show a large reversible strain of more than several percent due to superelasticity. In particular, the Ni-Ti-based alloy, which exhibits some ductility and excellent superelastic strain, is the only superelastic material available for practical applications at present. We herein describe a ferrous polycrystalline, high-strength, shape-memory alloy exhibiting a superelastic strain of more than 13%, with a tensile strength above 1 gigapascal, which is almost twice the maximum superelastic strain obtained in the Ni-Ti alloys. Furthermore, this ferrous alloy has a very large damping capacity and exhibits a large reversible change in magnetization during loading and unloading. This ferrous shape-memory alloy has great potential as a high-damping and sensor material.

In superelastic alloys, large deformation can revert to a memorized shape after removing the stress. However, the stress increases with increasing temperature, which limits the practical use over a wide temperature range. Polycrystalline Fe-Mn-Al-Ni shape memory alloys show a small temperature dependence of the superelastic stress because of a small transformation entropy change brought about by a magnetic contribution to the Gibbs energies. For one alloy composition, the superelastic stress varies by 0.53 megapascal/°C over a temperature range from −196 to 240°C.

We have identified cobalt-base superalloys showing a high-temperature strength greater than those of conventional nickel-base superalloys. The cobalt-base alloys are strengthened by a ternary compound with the L1₂ structure, [Formula: see text] Co₃(Al,W), which precipitates in the disordered [gamma] face-centered cubic cobalt matrix with high coherency and with high melting points. We also identified a ternary compound, [Formula: see text] Ir₃(Al,W), with the L1₂ structure, which suggests that the Co-Ir-Al-W-base systems with [Formula: see text] (Co,Ir)₃(Al,W) structures offer great promise as candidates for next-generation high-temperature materials.

Large magnetic-field-induced strains 1 have been observed in Heusler alloys with a body-centred cubic ordered structure and have been explained by the rearrangement of martensite structural variants due to an external magnetic field 1 , 2 , 3 . These materials have attracted considerable attention as potential magnetic actuator materials. Here we report the magnetic-field-induced shape recovery of a compressively deformed NiCoMnIn alloy. Stresses of over 100 MPa are generated in the material on the application of a magnetic field of 70 kOe; such stress levels are approximately 50 times larger than that generated in a previous ferromagnetic shape-memory alloy 4 . We observed 3 per cent deformation and almost full recovery of the original shape of the alloy. We attribute this deformation behaviour to a reverse transformation from the antiferromagnetic (or paramagnetic) martensitic to the ferromagnetic parent phase at 298 K in the Ni 45 Co 5 Mn 36.7 In 13.3 single crystal.

Direct magnetization measurements from narrow, complex-shaped antiphase boundaries (APBs; that is, planar defect produced in any ordered crystals) are vitally important for advances in materials science and engineering. However, in-depth examination of APBs has been hampered by the lack of experimental tools. Here, based on electron microscopy observations, we report the unusual relationship between APBs and ferromagnetic spin order in Fe 70 Al 30 . Thermally induced APBs show a finite width (2–3 nm), within which significant atomic disordering occurs. Electron holography studies revealed an unexpectedly large magnetic flux density at the APBs, amplified by approximately 60% (at 293 K) compared with the matrix value. At elevated temperatures, the specimens showed a peculiar spin texture wherein the ferromagnetic phase was confined within the APB region. These observations demonstrate ferromagnetism stabilized by structural disorder within APBs, which is in direct contrast to the traditional understanding. The results accordingly provide rich conceptual insights for engineering APB-induced phenomena. Atomic disordering in antiphase boundary regions is believed to deteriorate ferromagnetic spin order in many alloys and compounds. Here, using electron microscopy, Murakami et al. report the unusual relationship between thermal antiphase boundaries and ferromagnetic spin order in Fe 70 Al 30 .

Martensitic and magnetic transformation behaviors of Ni^sub 50^MnIn, Ni^sub 45^Co^sub 5^MnIn, and Ni^sub 42.5^ Co^sub 7.5^MnIn Heusler alloys were investigated by differential scanning calorimetry (DSC), vibrating sample magnetometry (VSM), and transmission electron microscopy (TEM). The martensitic transformation starting temperature (M^sub s^) decreases with increasing In composition, while the Curie temperatures (T^sub c^) of the parent phase are almost independent in each alloy series. On the other hand, the addition of Co resulted in a decrease of the M^sub s^ and an increase of the T^sub c^, and the degree of the decline of M^sub s^ was accelerated by magnetic transformation of the parent phase. The M^sub s^ temperature change induced by the magnetic field was also confirmed. It was found that the degree of M^sub s^ change is strongly related to the entropy change by the martensitic transformation, which shows a correlation with T^sub c^-M^sub s^. These behaviors can be qualitatively explained on the basis of thermodynamic considerations. [PUBLICATION ABSTRACT]

Polycrystalline Cu-Al-Mn shape memory alloys (SMAs) with a low degree of order of the β (L2 1 ) phase show excellent ductility and exhibit shape memory (SM) properties such as superelasticity, the one way memory effect and the two way memory effect based on martensitic transformation. These SM properties can be greatly enhanced by controlling microstructural factors such as grain size and texture by thermomechanical treatments. In the present paper, the SM properties of ductile Cu-Al-Mn based SMAs and microstructure control to obtain excellent SM characteristics are reviewed. Furthermore, an example of the application of Cu-Al-Mn based SMAs to a guidewire for medical use is also presented.

Phase equilibria including L 2 1 full- and C 1 b half-Heusler phases at 1373 K in the Ni-rich portion of the Ni-Ti-Sb system were determined by electron probe microanalysis, x-ray diffraction and differential scanning calorimetry. It was confirmed that both Heusler phases coexist at 1373 K, separated by C 1 b  +  L 2 1   two-phase region. The single-phase region of the C 1 b phase deviates slightly from the stoichiometric composition of NiTiSb to NiSb-rich direction. On the other hand, while existing in the vicinity of the stoichiometric composition of Ni 2 TiSb, the  L 2 1 phase is not stable at temperatures below about 1270 K, disappearing by an eutectoid reaction, L 2 1  →  C 1 b  + Ni 3 Ti- D 0 24 .

Interdiffusion in the face-centered cubic (fcc) Co-W binary alloys was investigated by the diffusion-couple technique between 1273 K and 1573 K (1000 °C and 1300 °C), on which interdiffusion coefficients of the binary alloys up to 12 at. pct W were evaluated by using the Sauer–Freise method. The interdiffusion data were assessed to develop the atomic mobility for the fcc Co-W alloys and its validity was tested by simulating the diffusion-couple experiments.

The phase equilibria between the γ (A1) and liquid phases and those between the γ, ε (A3), σ (D8 b ) and α (A2) phases at temperatures between 750 and 1300 °C in the Co-Cr-Ni system were determined by differential scanning calorimetry and electron probe microanalysis. It was found that the γ solidus and liquidus temperatures decrease with increasing Cr or Ni content and that the difference between them increases with increasing Cr content. The phase equilibria at 800, 900, 1000, 1100, 1200 and 1300 °C were determined using two- or three-phase alloys, and the phase boundaries due to the magnetically induced phase separation in the γ and ε phases were also observed at 850, 800 and 750 °C by the diffusion couple method.