Reverse Martensitic Transformation Temperature Research Articles

Effect of internal stress on sructure, martensitic transformation and magnetic properties of ferromagnetic shape memory alloy Mn2NiGa

The structures, the martensitic transformations and the magnetic properties of ferromagnetic shape memory alloy Mn2NiGa with the loading and the unloading of the external compressive stress are investigated. The plastic deformation occurring during the stressing causes a high level of dislocation defects existing in the sample. The residual internal stress results in a significant increase of the reversed transformation temperature. The threshold value of compressive pressure for causing the completely martensitic transformation at room temperature is identified to be 1.0 GPa based on the experimental measurements of XRD and magnetic coercivity. The coercivity of the martensitic phase increases from 50 Oe to 350 Oe due to the application of the external stress up to 2.0 GPa. It is also observed that the reversed martensitic transformation occurrs, when the sample is treated by heating up to about 730 K. This is attributed to the elimination of the dislocation by the annealing effect. Such a high reverse martensitic transformation temperature allows the measurement of the Curie temperature of martensitic phase and the obtained value is 530 K.

Magnetic field-induced reverse martensitic transformation in NiMnGaCu alloy

The reverse martensitic transformation was monitored by testing the x-ray diffraction patterns with increasing temperatures in an Ni46Mn33Ga17Cu4 alloy. A large magnetization change from a weak-magnetic martensite phase to a ferromagnetic austenite phase has been found on the M–T curves of heating cycle in this alloy. The reverse martensitic transformation temperatures have been lowered by 7 K under the magnetic field of 90 kOe. The magnetic field-induced reverse martensitic transformation has been confirmed at a fixed temperature in this alloy, demonstrating that another alloy performing the MFIRMT is discovered: NiMnGaCu alloy.

Novel Thermographic Method for Characterizing Transformation Temperatures of Thin-Film Shape Memory Alloys Aimed at Combinatorial Approach

A novel characterization method for the two-way martensitic transformation temperatures of thin-film shape memory alloys (SMAs) is proposed. The method uses thermography to detect the transformation as a change in emissivity. The proposed method was demonstrated for a Ti–Pd–Ni thin-film SMA along with electrical resistivity monitoring. Definite emissivity changes were observed during both heating and cooling. Furthermore, the changes in emissivity corresponded to changes in the electrical resistivity. To confirm the validity of the proposed method, the results were compared with results obtained using conventional differential scanning calorimetry (DSC). The reverse-martensitic transformation temperature was found to agree with the results of the proposed method. However, the martensitic transformation temperature did not agree well. The reason for this disagreement is the differences in the definition of the transformation temperature and stress conditions used. We expect that the proposed method will enable high-throughput characterization of SMAs by a combinatorial approach in the future.

Thermal Expansion and Magnetization Studies of Novel Ferromagnetic Shape Memory Alloys Ni<SUB>52</SUB>Mn<SUB>12.5</SUB>Fe<SUB>12.5</SUB>Ga<SUB>23</SUB> and Ni<SUB>2</SUB>Mn<SUB>0.75</SUB>Cu<SUB>0.25</SUB>Ga

Thermal expansion, permeability, and magnetization measurements of ferromagnetic shape memory alloys, Ni52Mn12.5Fe12.5Ga23 and Ni2Mn0.75Cu0.25Ga, were performed across the martensitic transformation temperature TM and the reverse martensitic transformation temperature TR. When cooling from austenite phase, a steep decrease in the thermal expansion due to the martensitic transformation was found for both alloys. Considering the permeability and magnetization results of Ni2Mn0.75Cu0.25Ga, the region above TM or TR is the paramagnetic-austenite (Para-A) phase and the region below TM or TR is the ferromagnetic-martensite (Ferro-M) phase. Magnetic phase diagrams were constructed based on the results of the temperature dependence of thermal expansion. TM and TR increased gradually with increasing magnetic field. For Ni52Mn12.5Fe12.5Ga23 and Ni2Mn0.75Cu0.25Ga, the shifts of TM in magnetic fields (B) were estimated as dTM⁄dB≈0.5 K/T and 1.2 T/K, respectively. The shifts of TM indicate that the that magnetization influences martensitic transition and the increase of TM in accordance with the magnetic fields is proportional to the difference between the magnetization of austenite phase with that of martensitic phase.

J044072 Ti-Ni-Zr薄膜金属ガラスのガラス転移特性と結晶化後の形状記憶特性評価([J04407]知的材料・構造システム(7))

We searched for Ti-Ni-Zr thin film metallic glasses (TFMGs) exhibiting a shape memory effect after crystallization. At first, we searched for alloy compositions of the Ti-Ni-Zr TFMGs. Thus, Ni-rich Ti-Ni-Zr thin film amorphous alloys (Ni-content of more than 50at.%) with Zr content ranged from 11 to 17 at.% showed glass transition during crystallization process, and they are TFMGs. On the other hand, the Ni-poor samples (Ni-content of less than 50at.%) did not exhibit glass transition, irrespective of Zr content. The Ti-Ni-Zr TFMGs after crystallization showed the martensitic transformation of B2 B19' phase upon cooling and heating. In the all crystallized Ti-Ni-Zr TFMGs, the martensitic transformation temperature and reverse martensitic transformation temperature increased with increasing annealing temperature. Ti39Ni50Zr11 sample annealed at 973 K for 3.6 ks showed the superior shape memory behavior, achieving a maximum recovery strain of about 2.5%, critical slip stress of approximately 550 MPa and fractured stress of 600 MPa. However, crystallized Ti-Ni-Zr TFMGs became brittle with increasing Zr content.

Kinetic arrest behavior in martensitic transformation of NiCoMnSn metamagnetic shape memory alloy

High-field magnetic measurements were carried out in order to investigate behaviors of field-induced reverse martensitic transformation and kinetic arrest of NiCoMnSn metamagnetic shape memory alloy. In the thermomagnetization curves, it was confirmed that the reverse martensitic transformation temperature decreases 67 K by applying magnetic field of 5 T, while in the magnetic field cooling process under 5 T, martensitic transformation does not occur down to low temperatures. Equilibrium magnetic field, defined from the critical magnetic fields of the metamagnetic evidence in the magnetization curves, exhibits almost constant below about 100 K, suggesting that the entropy change becomes zero, which is considered to cause kinetic arrest behavior.

Effects of pre-deformation on the martensitic transformation and magnetocaloric property in Ni-Mn-Co-Sn ribbons

This paper investigates the martensitic transformation and magnetocaloric effect in pre-deformed Ni-Mn-Co-Sn ribbons. The experimental results show that the reverse martensitic transformation temperature TM increases with the increasing pre-pressure, suggesting that pre-deformation is another effective way to adjust TM in ferromagnetic shape memory alloys. Large magnetic entropy changes and refrigerant capacities are obtained in these ribbons as well. It also discusses the origin of the enhanced martensitic transformation temperature and magnetocaloric property in pre-deformed Ni-Mn-Co-Sn ribbons.

Strong thermal-history-dependent magnetoresistance behavior in Ni49.5Mn34.5In16

The thermal-history-dependent (THD) isothermal magnetoresistance (MR) behavior of Ni49.5Mn34.5In16 metamagnetic shape memory alloy is investigated. Irreversibility in isothermal MR is observed in reverse martensitic transformation temperature range, while an intriguing “overshooting” phenomenon is observed in forward martensitic transformation temperature range, showing a strong thermal-history dependence of MR behavior. Such a THD MR behavior can be explained in terms of phase coexistence, THD metastable phase, and magnetic-field-induced phase transition.

Microstructure and shape memory effect of Ti–20Zr–10Nb alloy

The microstructure, martensitic transformation behavior and shape memory effect of Ti–20Zr–10Nb shape memory alloy have been studied by X-ray diffraction (XRD), optical microscopy (OM) and transmission electron microscopy (TEM) observation, differential scanning calorimetry (DSC) and tensile stress–strain measurements. The results show that the recrystallization occurs in the cold rolled Ti–20Zr–10Nb alloy by annealing at 600 °C and the grain size goes up with the increasing annealing temperature up to 800 °C. The Ti–20Zr–10Nb alloy is primarily composed of α″ martensite and a small amount of ω phase which appears after annealing. A reverse martensite transformation temperature higher than 500 °C upon heating has been detected for the Ti–20Zr–10Nb alloy annealed at 600 °C, but no obvious exothermic behavior can be found upon cooling. The tensile strength and the failure strain of the alloy are measured to be 542 MPa and 13.1%, respectively, associated with a maximum shape recovery strain of about 2.5%.

Magnetocaloric effect in preferentially textured Mn50Ni40In10 melt spun ribbons

Inverse and direct magnetocaloric properties were evaluated in preferentially textured Mn50Ni40In10 ribbons applying the magnetic field H∥ along the ribbon length and perpendicular H⊥ to the ribbon plane (ΔH=30 kOe). Maximum magnetic entropy change, hysteretic losses, and refrigerant capacity were not significantly affected by crystallographic texture. Refrigeration capacity around structural transition is strongly reduced by the large hysteretic losses associated to the metamagnetic field-induced reverse martensitic transformation and narrower working temperature range making the interval around the magnetic transition more efficient for a refrigerant cycle (RCstruct=71 J kg−1 versus RCstructeff≈60 J kg−1, and RCmagn=89–86 J kg−1, for H∥ and H⊥, respectively).

Effects of quenched-in vacancies on the reverse martensitic phase transformation temperature of the Cu-11.9Al-2.5Mn(wt%) shape memory alloy studied by internal friction

By internal friction technique，the behaviors of quenched-in vacancies and their influence on the reverse martensitic phase transformation (MT) temperature of the Cu-11.9Al-2.5Mn (wt%) shape memory alloy have been studied. Investigations on the specimens subjected to various cooling regimes indicate that rapid cooling rate leads to relatively high reverse MT temperature. For the water-quenched specimens，the reverse MT temperature bears a non-monotonic relationship to quenching temperature. This phenomenon may be related to the different formation energies of vacancies in ordered and disordered phases.

The influence of laser welding parameters on the microstructure and mechanical property of the as-jointed NiTi alloy wires

The Nd:YAG laser welding was used to join the binary NiTi alloy wires with different compositions(Ti–50.0 at.%Ni and Ti–50.9 at.%Ni) which had the same diameter of 1 mm. The wires were welded with different parameters, including impulse width and welding current. The aim was to assess the influence of the laser-welding process on the microstructure and mechanical properties of the welded joint of binary NiTi wires. The optical microscopy (OM) and the metallographic microscopy (MM) were used to analyze the microstructure of the welded joints. The tensile test and the differential scanning calorimetry (DSC) were carried out to examine the ultimate tensile strength and the reverse martensitic transformation temperatures of the welded joints. It was found that the welding current and the impulse width had great influence on the quality of the welded joints, an optimal parameter combination would remove the pores and micro-cracks appeared in the fusion zone, and result in good mechanical properties such as higher fracture strength and elongation. The laser welding had a few effect on the reverse martensitic transformation temperatures of the welded joints.

Effect of L2<sub>1</sub> Ordering on the Martensitic and Intermartensitic Transformations in a Ni-Mn-Ga Shape Memory Alloy

The present work analyses the influence of austenite ordering on a single crystal Ni-Mn- Ga alloy which displays, on cooling, a sequence of martensitic (MT) and intermartensitic (IMT) transformations. The MT and IMT show distinct behaviour after ageing in austenite: while the MT temperatures are not affected by the performed heat treatments, the IMT shifts toward lower temperatures after quenching from increasing temperatures, progressive recovery occurring upon ageing in parent phase. Such evolution can be related to changes in the L21 order degree, in the sense that ordering favours the occurrence of the intermartensitic transformation, while it does not affect noticeably the forward and reverse martensitic transformation temperatures. The closeness of the free energies of the different martensite structures allows to explain this behaviour.

Magnetization anomalies in melt-spun Ni–Mn–Ga ribbons

Temperature and field dependencies of the magnetization of melt-spun ribbons of Ni 45.7Mn 37.2Ga 17.1 ( T C< T A) and Ni 54.3Mn 20.5Ga 25.2 ( T C> T A), where T A and T C are the reverse martensitic transformation and Curie temperatures, respectively, have been studied in broad ranges of magnetic fields (±30 kOe) and temperatures (10–350 K). It was found that the magnetic and thermomagnetic properties have a number of peculiarities and anomalies. Particularly, the magnetization values measured at both low and high magnetic fields increase significantly after annealing. Low-temperature anomalies of magnetization suggest the presence of the superparamagnetic behavior and/or phenomena related to the concurrent exchange interactions in the as-spun state. The obtained results can be explained by the influence of concentration inhomogeneities and atomic disordering in the rapidly quenched ribbons, which can be reduced by annealing.

Effects of plate thickness on reverse martensitic transformation of prestrained NiTi/NiTi alloy

In this paper, differential scanning calorimeter (DSC) was used to study the effects of predeformation and plate thickness on the reverse martensitic transformation of explosively welded NiTi/NiTi alloy. Results showed that there was a constraint between Ni50.4Ti (NiTi-1) and Ni49.8Ti (NiTi-2), which led to that the thickness of NiTi-1 or NiTi-2 strongly affected the reverse martensitic transformation behavior because residual stress variations in thickness wound enable bias force to be built inside the composite. The DSC measurements showed that after deformation, the reverse martensitic transformation temperature of the composite was increased with the increasing thickness of NiTi-2. Also, the XRD results revealed that the microstructure of NiTi/NiTi alloy changed from B2 phase to B19’ phase along the thickness direction.

Martensitic transformation and shape memory effect in Ni 3Ta: A novel high-temperature shape memory alloy

This paper is dedicated to the first observations of the martensitic transformation (MT) and shape memory effect in Ni 3Ta intermetallic compound. It was found that Ni 3Ta undergoes MT with 50 K temperature hysteresis at elevated temperatures ( M s = 530 K, M f = 500 K, A s = 550 K, A f = 580 K). Thermal cycling over the temperature interval of MT does not affect MT temperatures except for a slight shift of the reverse MT temperatures during the first cycle. The crystal structure parameters of the austenite and martensite are determined. The volume change during forward MT deduced from the austenite and martensite lattice parameters relation is −0.45%. The shape recovery after 10% compression at room temperature is almost complete although it takes place in a wide temperature interval—about 300 K. This alloy might open new perspectives for the application of high-temperature shape memory alloys (HTSMA).

Influence of rare earth Gd addition on the structural and magnetic transitions of Ni–Mn–Ga alloys

Influence of rare earth Gd addition on the structural and magnetic transitions of polycrystalline Ni 50Mn 29Ga 21 ferromagnetic-shape memory alloy (FSMA) are investigated. The results show that different martensitic phase transformation appears in different Gd content. Martensite structure changes from 5 M with 0.1 at% Gd to 7 M when the content of Gd is more than 0.1 at%. The martensitic and reverse martensitic transformation temperatures increase remarkably with increasing Gd content, whereas the Curie temperature almost keeps unchanged at low Gd content and subsequently decreases. Three kinds of Ni–Mn–Ga–Gd alloys are observed according to the sequence of the structural and magnetic transition, whose structural transition temperatures are lower, equal to or higher than the magnetic transition temperature.

Pressure effect on transformation temperatures of ferromagnetic shape memory alloy Ni 50Mn 36Sn 14

The pressure derivative of the reverse martensitic transformation temperature d T R/d p of Ni 50Mn 36Sn 14 has been obtained from the results of temperature dependence of initial permeability under various pressures. The value of d T R/d p is found to be +2.4 K/kbar. The pressure change of the Curie temperature is also studied.

Internal friction study on fcc/hcp martensitic transformations in thermomechanically treated Fe–28Mn–6Si–5Cr–0.53Nb–0.06C (mass%) alloys

We have studied the temperature dependence of the internal friction and Young's modulus of the Fe–28Mn–6Si–5Cr–0.53Nb–0.06C (mass%) alloys with or without NbC precipitates produced by three thermomechanical treatments: (i) solution treatment, (ii) rolling at 870 K and subsequent aging at 1070 K, and (iii) rolling at 1070 K, in the vicinity of the forward ( M s) and the reverse ( A s) martensitic transformation temperatures using a dynamic mechanical analyzer (DMA). The specimen subjected to rolling at 870 K and subsequent aging showed the highest M s temperature and the largest amount of the hcp martensite. The solution treated specimen showed the lowest M s temperature and the minimum amount of the martensite. The magnetic susceptibility measurements made by means of a superconducting quantum interference device (SQUID) revealed that the martensitic transformation is hindered by the preceding magnetic transition from the paramagnetic (cubic) phase to the antiferromagnetic (tetragonal) phase.

Effect of pre-deformation on martensitic transformation behavior and the microstructure of a Ni–Mn–Ga alloy

The influence of pre-deformation on the martensitic transformation behavior and its microstructure has been put forward in a Ni 55Mn 20Ga 25 (at.%) ferromagnetic shape memory alloy. It is found that the reverse martensitic transformation temperatures ( A s and A f) increase linearly with increasing amount of deformation, while the martensitic transformation temperatures ( M s and M f) are not changed even when the deformation is up to 6%. However, the effect of the pre-deformation on the reverse martensitic transformation disappears in the following heating process. Transmission electron microscopy shows well-accommodated martensite variants in the undeformed specimen and interfacial boundaries are straight and well-defined. After deformation, the reorientation of martensite variants occurs at the expense of the unfavorable martensite variants, and the interfacial boundary changes from a sharp and straight morphology to a curved and irregular one. Meanwhile, some lattice defects are formed inside the variants even in slightly compressed samples. With increasing deformation, more lattice defects are generated inside the variants as well as in the interfacial boundaries between variants. Based on the analysis of the experimental results, the micromechanism of the effect of pre-deformation on martensitic transformation behavior has been clarified.