Yue Zheng

Giant magnetic-field-induced strains in Heusler alloy NiMnGa with modified composition

A giant magnetic-field-induced strain (MFIS) of −3100 ppm has been obtained in Heusler alloy Ni52Mn22.2Ga25.8 single crystal in the [001] direction at a temperature from 23 to 31u200a°C. This MFIS reaches saturation in an applied field about 6 kOe, and exhibits the same amplitude with an opposite sign while the field is perpendicular to the samples. According to a previous model, this MFIS associates with the twin boundary motion. The martensitic self-strain has been found to be 2%, implying a preferential orientation of martensite variants. Results related to the magnetic properties are discussed.

Effect of aging on the phase transformation and mechanical behavior of Ti36Ni49Hf15 high temperature shape memory alloy

The TiNiHf alloys are newly developed as high temperature shape memory alloys with the high transformation temperatures and with lower cost in comparison with TiNiX (X 5 Pd, Pt) alloys. Recently a lot of investigations were focused on the TiNiHf alloys, in which Hf contents are ranged from 1% to 30% [1–8]. Zhu et al. [9] proposed that the decrease of the transformation temperatures of the Ti51.5-xNi48.5Hfx (x 5 15, 30) alloys in the early period of high temperature duration at 723K was due to the precipitated second phase though no further experimental results were provided. In the Ti36.5Ni48.5Hf15 alloy aged at 873K for 150h, a new precipitate phase of the composition of (Ti0.4Hf0.6)Ni of a spindle-like shape with a habit plane of (100) P//(001)M and a long axis of [001]P//[1#10]M was observed by Han et al. [10]. It is well known that, in the Ni-rich binary TiNi SMAs, the precipitation sequences are as follows [11]:

Shape memory properties of the Ti36Ni49Hf15 high temperature shape memory alloy

Abstract The shape memory properties have been studied in a Ti 36 Ni 49 Hf 15 high temperature shape memory alloy (SMA) by bending tests. The shape memory effect (SME) of the alloy is closely related to the deformation condition. It shows about 3% completely reversible strain when the TiNiHf alloy is deformed at the room temperature. The shape recovery ratio is constant at 92% when the deformation temperature is below 457 K, then rapidly decreases to zero above 590 K for the specimen deformed to 4.5%. Obvious two-way shape memory effect (TWSME) is obtained in the Ti 36 Ni 49 Hf 15 alloy aged at 973 K for various hours. As the aging time further increases, the TWSME decreases. Moreover, TWSME in the aged Ti 36 Ni 49 Hf 15 alloy is unstable and decreases rapidly after several thermal cycles.

Stress-free two-way thermoelastic shape memory and field-enhanced strain in Ni52Mn24Ga24 single crystals

Stress-free and two-way thermoelastic shape memory, with 1.2% strain and 6 K temperature hysteresis, has been found in single crystalline Ni52Mn24Ga24. The deformation can be enhanced more than three times, up to 4.0% shrinkage with a bias field 1.2 T applied along the measurement direction, or changed to 1.5% expansion by the 1.2 T applied perpendicular to the measurement direction. For achieving a large deformation, the magnetic field exhibits a more evident contribution than an external stress on this material. These characteristics can be attributed to the low level of internal stress and the preferential orientation of the martensitic variants.

Stress-induced martensitic transformation behavior of a Ti-Ni-Hf high temperature shape memory alloy

The stress-induced martensitic transformation behavior and the microstructure of stress-induced martensite (SIM) in a Ti36Ni49Hf15 high temperature shape memory alloy (SMA) have been investigated using tensile tests and transition electronic microscopy (TEM) observations. It shows that, compared with the TiNi SMAs, there is no stress plateau in the stress-strain curve as the TiNiHf alloy deformed in an austenite. The martensitic transformation enthalpy is calculated to be � 106.84 cal/ mol. The martensite variants mainly show preferential oriented morphologies for the TiNiHf alloy deformed to 8% at 523 K. The substructure of SIM and deformed SIM in the present alloy are (001) compound twin. Martensite variants are (011) type I twin related. Further, increasing the deformation temperature and deformation strain, the preferential oriented SIM variants gradually develop to martensite variants with variant-crack/variant-intersect morphologies. The inexistence of stress plateau in the stress-strain curve of TiNiHf may mainly result from the dislocation slip during the stress-induced martensitic transformation. D 2002 Elsevier Science B.V. All rights reserved.

Simulation of interface dislocations effect on polarization distribution of ferroelectric thin films

Effects of interfacial dislocations on the properties of ferroelectric thin films are investigated, using the dynamic Ginzburg–Landau equation. Our results confirm the existence of a dead layer near the film/substrate interface. Due to the combined effects of the dislocations and the near-surface eigenstrain relaxation, the ferroelectric properties of about one-third of the film volume suffers.

Microstructure of stress-induced martensite in a Ti–Ni–Hf high temperature shape memory alloy

The microstructure of stress-induced martensite and its evolution in a Ti36Ni49Hf15 alloy have been investigated by TEM techniques. And the reasons for no stress plateau in the tensile stress–strain curve of the Ti36Ni49Hf15 alloy deformed in an austenite state have also been discussed. 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Effect of low dc magnetic field on the premartensitic phase transition temperature of ferromagnetic Ni2MnGa single crystals

The temperature and field dependence of alternating-current (ac) susceptibility in a Ni50Mn25Ga25 single crystal was measured using an ac susceptometer with a dc magnetic field oriented in the [100] and [110] directions, respectively. It is found that even at very low fields the premartensitic transition temperature decreases monotonically with the increase of the applied field, and it decreases more rapidly with field applied along the [110] direction than with field along the [100] direction. In accord with a previous model, present results confirm that the magnetoelastic interaction is responsible for the premartensitic transition, and the magnitude of the magnetoelastic interaction in the [110] direction is stronger than that in the [100] direction.

Critical thickness for dislocation generation during ferroelectric transition in thin film on a compliant substrate

The formation energy of misfit dislocations in a ferroelectric thin film grown on compliant substrate is calculated based on the Landau-Devonshire formalism and Timosheko’s method for thermal stresses. The critical thickness is shown to change significantly according to the polarization in the film, leading to serious concerns, particularly for thick substrates, in the device design stage.

Magnetic properties and structural phase transformations of NiMnGa alloys

The magnetic properties and structural phase transformations of Heusler alloys Ni/sub 2+x/Mn/sub 1-x/Ga and Ni/sub 2-x/Mn/sub 1+x/2/Ga/sub 1+x/2/ were investigated. It was found that the martensitic transformation temperature increases and the Curie temperature decreases with decreasing Mn concentration in the Ni/sub 2+x/Mn/sub 1-x/Ga system, but the magnetic properties decrease faster than those do in the Ni/sub 2-x/Mn/sub 1+x/2/Ga/sub 1+x/2/ system. This suggests that the influence of the conduction electron concentration and the distance between Mn atoms on the Curie temperature is stronger than the magnetic dilution effect. The martensitic transformation temperature shows a peak value at x=0.06 in the Ni/sub 2-x/Mn/sub 1+x/2/Ga/sub 1+x/2/ system. A large stress-free magnetic-field-induced strain (MFIS) of 1.8% has been obtained at room temperature in a single crystal sample with the composition of the first system.