Increase Of Martensitic Transformation Temperatures Research Articles

Thermal stability and ordering effects in Ni–Fe–Ga ferromagnetic shape memory alloys

The martensitic transformation (MT) from L2 1 (Heusler) structure to layered martensites in three polycrystalline Ni 53.5+ x –Fe 19.5− x –Ga 27.0 alloys, with x = 0, 0.5, and 1.5 at.%, has been monitored by differential scanning calorimetry and transmission electron microscopy in order to follow the evolution under ageing in parent phase as well as the effect of atomic order on the MT temperatures. Ageing at 670 and 770 K produces degradation of the MT after 10 5–10 6 s, concomitant with the appearance of small (<1 μm), elongated γ particles. Water quenching from different temperatures (470–1070 K) causes an increase in MT temperatures compared to slow cooled samples. This effect is related to a decrease of the L2 1 order of the parent phase upon quenching, promoting an increase in the MT temperatures. The effect is maximum after quenching from 970 K; a temperature just above the B2–L2 1 order transition. As for the magnetic properties, order leads to larger saturation magnetization and higher Curie temperatures.

Martensitic transformation and shape memory effect of Ni-rich Ni 2MnGa sputtered films under magnetic field

The inference of a magnetic field on the martensitic transformation (MT) temperatures and shape memory effect (SME) was investigated for the Ni-rich Ni 2MnGa sputtered films. The compositions of the films were Ni 51.4Mn 28.3Ga 20.3, Ni 54.1Mn 23.0Ga 22.9 and Ni 54.4Mn 21.3Ga 24.3. The films were heat treated at 1073 K for 3.6 ks and aged at 673 K for 3.6 ks in a constrained condition. Hereafter, the constraint-aged films were described as N51.4(AG), N54.1(AG) and N54.4(AG) using nickel content, respectively. The reversible two-way SME by the thermal change was confirmed for the all constraint-aged films through the MT and its reversion. The gradient of the strain–temperature curve, the amount of strain accompanied by the two-way SME and the width of thermal hysteresis were dependent on the composition of the films. The strain–temperature curves shifted to a high temperature region and the martensitic phase was stabilized by the magnetic field. The strain at the outer surface was given by ε= d/2 r, where d was the thickness of the films and r was the radius of a curvature for the films. The increase of MT temperatures caused by a magnetic field was about 1 K/T. Furthermore, the two-way SME by the magnetic field was observed around the MT temperature on cooling for the N54.4(AG) film with the large gradient and small thermal hysteresis in the strain–temperature curve.

Effect of ageing in Ni–Fe–Ga ferromagnetic shape memory alloys

Three polycrystalline Ni 53.5+ x –Fe 19.5− x –Ga 27.0 alloys, with x = 0, 0.5, 1.5 (at.%), have been aged at 520 and 670 K. The initial increase of martensitic transformation temperatures upon ageing has been attributed to a decrease of L2 1 order. The alloys with lower Fe content show remarkable thermal stability for ageing times larger than 10 7 s at 520 K. In the case of ageing at 670 K, degradation of the alloys occurs after ∼10 6 s. Optical and transmission electron microscopy reveal the initial presence of γ′ particles mostly located at the grain boundaries, as well as the appearance of small (<1 μm) elongated precipitates after long ageing periods at 670 K, associated to a drop of transformation temperatures.

Martensitic transformation and magnetic properties of Heusler alloy Ni–Fe–Ga ribbon

The martensitic transformation and magnetic properties of ferromagnetic shape memory alloy Ni 50+ x Fe 25− x Ga 25 ( x = − 1 , 0, 1, 2, 3, 4) ribbons have been systematically studied. It has been found that with the increase of Ni concentration, the martensitic transformation temperature increases, but the Curie temperature decreases. Both the two-step thermally induced structural transformation and the one-step transition have been observed in NiFeGa alloys with different compositions. It is found that the two-step transition became the one-step transition after the ribbon being heat treated at 873 K or higher. X-ray diffraction patterns show that only L 2 1 → B 2 transition occurs in the samples treated at 873 K, while the γ phase will form in the samples treated at higher temperature. Transmission electron microscopy (TEM) studies show that the alloys with martensitic transformation temperature above the room temperature are non-modulated martensite with the large domain size, being different from the stoichiometric Ni 2FeGa alloy that is a modulated martensite with small domain size. The influences of Fe substitution for Ni in Ni 2FeGa on the saturation magnetization and exchange interaction are also discussed.

Phase Transformation of Ti-Ni Containing Platinum-Group Metals

ABSTRACTSince the maximum shape recovery temperature of the binary Ti-Ni alloys is limited to be around 400K, the increase in martensitic transformation temperature (Ms) of Ti-Ni should be done by alloying for the demand of high temperature applications. Although most of additional elements are known to decrease Ms of Ti-Ni, substitutional elements having large atomic size are expected to increase Ms. In this study, phase constitution, phase transformation temperature, lattice parameter of B2 phase and Vickers hardness were investigated for Ti-Ni alloys containing several platinum-group metals (PGM). The alloy systems investigated were the pseudobinary systems of TiNi-TiRh, TiNi-TiIr and TiNi-TiPt where the PGM atoms substitute for the Ni-sites of TiNi. The phase transformation and phase constitution were assessed by differential scanning calorimetry (DSC), X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM). It was found by XRD that TiNi can contain a large amount of the PGMs as Ti(Ni, Rh), Ti(Ni, Ir) and Ti(Ni, Pt). Lattice parameters monotonously increase with increasing amount of PGMs. With increasing Pt content, Ms slightly decreases when less than 10mol%Pt while continuously increases as the rate of 26K/mol%Pt when more than 10mol%Pt. On the other hand, Ms decreases and then disappears with increasing Rh or Ir content. Hardness ranges from HV180 to HV570 and the maximum values in the pseudobinary systems lie around 20–30mol%PGM, suggesting solid solution hardening caused by the substitution of PGMs.

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.

Evolution of martensitic transformation in Cu–Al–Ni shape memory alloys during low-temperature aging

The effect of thermal aging at 473 K on martensitic transformation characteristics have been studied in the Cu–Al–Ni shape memory alloys. An increase of martensitic transformation temperatures as a result of aging has been observed. Furthermore, the martensitic transformation shows an evolution with aging time from a single β3 ⇒ β′3 structural transformation to a β3 ⇒ γ′3 + β′3 transformation. Using internal friction and thermoelectric power measurements, as well as specific thermal treatments, it has been determined that the mechanism responsible for this evolution is an increase in the degree of order, thus rejecting the participation of a γ1 precipitation process. Finally, the increase of order at the next nearest neighbors has been established as the main factor responsible of the evolution of the martensitic transformation characteristics.