Materials Science Forum Vols. 475-479

Abstract: We have suggested B2-(Pt, Ir)Ti as high temperature shape memory alloys. The phase transformation of (Pt, Ir)-50at% Ti from B2 to B19(2H) or 4H(4O) structures was investigated in our previous study. The microstructure suggested martensitic transformation. In this study, thermal expansion measurement and cyclic compression test were performed for (Pt, Ir)Ti to investigate if the shape memory effect appears. High temperature strength was also investigated because phase transformation temperature of the (Pt, Ir)Ti is above 1273 K and high strength is necessary as high temperature shape memory alloys in order to suppress dislocation motion and stabilize martenstic transformation. The potential of (Pt, Ir)Ti as high temperature shape memory alloys will be also discussed.

Abstract: The non-stoichiometric NiMnGa shape memory alloy with high Ni content has been developed as promising thermo-actuated materials applied at high temperature. A substitution of Al for Ga in the Ni54Mn25Ga21 alloys is performed. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) measurements have been carried out to study the effects of the phase transformations and microstructures of the Ni54Mn25Ga21-xAlx shape memory alloys. The results show that the martensitic transformation temperatures almost linearly decrease with the increase of Al substitution for Ga, which can be explained considering the effect of the size factor, i.e. the lattice parameter. A structural transition from a non-modulated tetragonal type to a seven-layered 14M one has been found during the increase of Al substitution for Ga.

Abstract: Proper amount of blend rare earth (RE), e.g. less than 0.2 wt%, was added into a Fe25Mn6Si5Cr alloy and showed beneficial effect on its shape memory property. On the other hand, measurements of internal friction revealed that RE-addition reduces the Ms temperature and the amount of thermal-induced martensite. Higher La-content in thin hcp (e) plates than that in fcc (g) matrix was observed by analytical TEM. Thermodynamic calculations also indicated a relatively high equilibrated concentration in e and a strong interaction between La atom and 1/6<112> Shockley dislocation. Except for the grain refinement and solid-solution strengthening effect, the influence of RE on the g ® e martensitic transformation is discussed in taking into account both the Suzuki Effect and Suzuki Lock. The former results in a reduced stacking fault energy, while the Suzuki Lock hinders the movement of Shockley partials and thus the extension of stacking faults.

Abstract: We have investigated the magnetic field-induced strain (MFIS) of the martensite and the parent phases in an Fe-31.2Pd(at.%) single crystal, which exhibits a martensitic transformation at TM = 230K. Below TM, a large MFIS of several percent appears due to rearrangement of martensite variants and this strain remains when a magnetic field is removed. Such rearrangement depends on magnetic field direction; Variants are perfectly rearranged into the variant, which lowers the magnetocrystalline anisotropy energy most, when a magnetic field is applied along [001]P, and partially when [011]P and hardly when [111]P (“P” represents “parent” phase). The dependence on the field direction can be explained by comparing the magnetic shear stress tmag with the shear stress t req required for rearrangement of variants. Above the temperature, TM, a relatively large MFIS appears and it increases up to about 10-3 with decreasing temperature from 280 K toward TM. This MFIS is probably caused by anomalies of some physical properties, such as elastic constant and dipole-dipole interaction coefficient in the parent phase.

Abstract: We have performed the various measurements of the transformation strain with and without the different external magnetic fields on the Ni51.6Mn23.4Ga25 single crystals. A stress-free and two-way thermoelastic shape memory, with -1.15% strain (negative sign represents the shrinkage) and 6 K temperature hysteresis, has been found in the single crystal. The deformation can be enhanced up to -2.35% with a bias field 1.2 T applied along the measurement direction of the parent phase [001] crystallographic axial direction. Turning the field laterally applied to [010] and [100] directions of the parent phase, however, the strain was suppressed by the field of 1.2 T to 0.56% and –0.55%, respectively, a different deformation scene. Moreover, it is found that even the field of 1.2 T does not have a significant influence on the phase transition temperature and the temperature hysteresis, which indicates that the mechanism of field-enhanced strain in this material is the twin boundary motion.

Abstract: A non-stoichiometric polycrystalline Ni50Mn27Ga23 magnetic shape memory alloy was prepared by melt-spinning technology. The effects of melt-spinning on the martensitic transformation and magnetic-field-induced strain (MFIS) of the melt-spun ribbon were investigated. The experimental results show that the melt-spun ribbon undergoes the thermal-elastic martensitic transformation and exhibits the thermo-elastic shape memory effect. But the martensitic transformation temperature decreases and Curie temperature remains unchanged. A particular internal stress induced by melt-spinning made a texture structure in the melt-spun ribbon, which made the melt-spun ribbon obtain larger transition-induced strain and MFIS. The internal stress was released under cycling of magnetic field. This resulted in a decrease of MFIS of the melt-spun ribbon.

Abstract: The behavior of twin variants reorientation in Ni50Mn27.5Ga22.5 single crystal under the applied mechanical stress and magnetic field was investigated. Through three times of compression, the twinning stress was reduced from 4.4MPa to 0.4MPa, and 2.5% stress induced strain was obtained. During the succedent MFIS measurement, with the increase of measurement number of times, the critical field (μ0H)c at which the MFIS jumps became lower, and the corresponding jumping magnetostrain ε j, and the maximal magnetostrain εmax ascended. At last, a quasi single-variant state was realized, and the sample showed repeatable 6.21% MFIS, which is close to the theoretical value (1-c/a)*100%. The results are discussed from the point of martensitic microstructure evolution.

Abstract: A Heusler-type Ni2.18Mn0.82Ga single crystal shows a shape memory effect. It makes a thermo-elastic martensitic transformation at around 340 K, which is coincident with a Curie temperature. We made a synchrotron radiation white X-ray diffraction of the single crystal by changing the temperature from 400 K to 103 K. We observed the change of Laue spots following the transformation. As a result of experiment, the single crystal shows one cubic Heusler structure at 400 K. The direction of the crystal growth is along cubic [010] direction. In the process of decreasing temperature, many tetragonal structures with small volume of different axial direction become to appear. At room temperature the transformation almost finishes and two tetragonal Heusler structures which are twin each other remain. One of them is nearly the same as the structure at the beginning room temperature. The direction of the crystal growth is [010] of this tetragonal structure. We found that the twinning plane of the tetragonal structure is (011) plane.

Abstract: The stability of magnetic field induced strain (MFIS) in Ni52Mn24Ga24 single crystal under temperature and magnetic field cycling is investigated and the corresponding micro-mechanism is also discussed. It shows that the saturated MFIS is very sensitive to temperature. Below martensitic transformation temperature(Tm), with increasing temperature, the saturated MFIS increases almost linearly. Besides, the saturated MFIS initially decreases with increasing the field cycling number less than four times, then does not change with further increasing the number of field cycles. The decrease of saturated MFIS can be attributed to the decrease of twin boundary mobility, which is related to the crystal defect introduced by immigration of twin boundary under field cycling.