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The effects of different forming process on the microstructure, martensitic transformation, and mechanical properties of the Ni50Mn34In14Co2 (numbers indicate at.%) alloys were investigated in detail. The results show that the microstructure of Ni50Mn34In14Co2 alloy consists of the matrix and the second phase by powder hot forging technology under the forces in different directions and spark plasma sintering (SPS), and the prepared sample has obvious preferred orientation by applied force in the three directions. The grain and the second phase of prepared sample were obviously refined by the hot forging technology. The martensitic transformation behavior was almost unchanged by different forming process. In addition, the shape memory effect of prepared Ni50Mn34In14Co2 alloy was significantly enhanced by different forming processes. [ABSTRACT FROM AUTHOR]

The active ω phase formation decreases the martensitic transformation temperatures sharply and can even suppress the martensitic transformation during thermal cycling in Ti-Nb-based alloys. No ω phase is observed in the present Ti-15.8Nb-4.94Al-0.06Sc alloy when it is annealed at 650 °C for 4 h or subsequently aged at 200 and 250 °C for 2 h. The exclusion of ω phase remarkably improves the stability of phase transformation temperatures during thermal cycling. The martensitic transformation start temperature of the present alloy decreases by 30 °C but tends to be stable gradually during the 20 thermal cycles, owing to the saturation of lattice defects. [ABSTRACT FROM AUTHOR]

Ni-Mn-Ga Heusler-type ferromagnetic shape memory alloys are attractive materials for micro-actuator, but the relatively poor ductility and low strength of Ni-Mn-Ga alloys have triggered a great deal of interest. In this study, we attempt to introduce some ductile second phase in the alloy by partially substituting Ti for Ga and constraint aging treatment. The results show that the martensitic transformation temperature first decreases and then increases slightly with the increasing of constraint-aging temperature, which can be attributed to the decrease of Ni content in the matrix and strengthening effect of the second particles. It is found that the amount of the Ni-rich precipitates by constraint-aged samples is more and the size of the second phase particle is smaller than that of the free-aged samples. The compressive stress and ductility can be significantly improved by the constraint-aging treatment, and the maximum compressive stress for constraint-aging alloy is about 1400 MPa, which is the highest value up to date compared with the 400 MPa in solution-treated Ni-Mn-Ga-Ti alloy and about 900 MPa in Ni-Mn-Ga-Ti alloy free-aged at 1073 K for 3 h. Scanning electron microscopy observations of fracture surfaces confirm that the Ni-rich second phase play a key role in improving the compression stress and ductility of Ni-Mn-Ga-Ti alloy. [ABSTRACT FROM AUTHOR]

The Ni53Mn23.5Ga18.5Ti5 ferromagnetic shape memory alloy has been aged under 2% constant-strain at various temperatures for 3 h, and the microstructure and martensitic transformation behaviors have been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). It was found that after constant-strain aging, the amount of the Ni-rich precipitates with lenticular morphology is higher and the size of the second-phase particle is smaller when compared to that of the conventional aged samples. The martensitic transformation temperatures first decrease remarkably with the increase of aging temperature, and then increase when the aging temperature exceeds 973 K, which can be attributed to the change of the Ni-content in the matrix as well as the strengthening effect by fine Ni3Ti precipitates. [ABSTRACT FROM AUTHOR]

The effects of aging on martensitic transformation and microhardness were investigated for Ni53Mn23·5Ga18·5Ti5 ferromagnetic shape memory alloy. The results show that precipitation has obvious effect on martensitic transformation temperature and the microhardness increases with increasing aging temperature up to 973 K, and then decreases gradually with the further increase in aging temperature. [ABSTRACT FROM AUTHOR]

A Heusler Ni50Mn29Ga16Gd5 alloy with a high transformation temperature has been obtained by substituting 5 at% Gd for Ga in a ternary Ni50Mn29Ga21 ferromagnetic shape memory alloy. The microstructure and phase transformations in the Ni50Mn29Ga16Gd5 alloy have been investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. It is shown that the microstructure of the Ni50Mn29Ga16Gd5 alloy consists of matrix and hexagonal Gd (Ni,Mn)4Ga phase, which indicates a eutectic structure composed of these two phases. One-step thermoelastic martensitic transformation occurs in this quaternary alloy. Ni50Mn29Ga16Gd5 alloy exhibits a martensite transformation start temperature up to 524 K, approximately 200 K higher than that of Ni50Mn29Ga21 alloy. At room temperature, non-modulated martensite with twin substructure is observed in Ni50Mn29Ga16Gd5 alloy. [ABSTRACT FROM AUTHOR]