Martensite-austenite Transition Research Articles

Delocalization and hybridization enhance the magnetocaloric effect in Cu-dopedNi2MnGa

The structural and magnetic transitions in the shape-memory alloy ${\text{Ni}}_{2}\text{MnGa}$ can be tuned as a function of temperature by adding dopants. By altering the free energy such that the structural and magnetic transitions coincide, a giant magnetocaloric effect is created near room temperature. We show, using x-ray absorption spectroscopy and x-ray magnetic circular dichroism, how Cu, substituted for Mn, pulls the magnetic transition downward in temperature and also, counterintuitively, increases the delocalization of the Mn magnetism. At the same time, this reinforces the Ni-Ga chemical bond, raising the temperature of the martensite-austenite transition. At 25% doping, the two transitions coincide at 317 K.

DMA testing of Ni–Mn–Ga/polymer composites

Ni–Mn–Ga/polymer composites are an interesting group of materials with new potential for applications such as vibration damping. Polymer selection is critical for Ni–Mn–Ga/polymer composites. The transition temperatures of both components should be selected carefully considering the future application, the contact between Ni–Mn–Ga and polymer is important and stiffness of the polymer matrix is one of the influencing factors. The adhesion between Ni–Mn–Ga and the polymer matrix was studied in shear stress tests. Ni–Mn–Ga adhered strongly to the tested epoxies but considerably less so to silicones. The damping properties of Ni–Mn–Ga/polymer composites were studied by dynamic mechanical analysis (DMA). Compared to the pure polymer sample, DMA measurements confirmed extra damping in the martensite–austenite transition temperature region of the Ni–Mn–Ga after a soft epoxy matrix had been applied.

Thermal and magnetic field-induced martensite-austenite transition in Ni50.3Mn35.3Sn14.4 ribbons

Thermal and field-induced martensite-austenite transition was studied in melt spun Ni50.3Mn35.3Sn14.4 ribbons. Its distinct highly ordered columnarlike microstructure normal to ribbon plane allows the direct observation of critical fields at which field-induced and highly hysteretic reverse transformation starts (H=17kOe at 240K), and easy magnetization direction for austenite and martensite phases with respect to the rolling direction. Single phase L21 bcc austenite with TC of 313K transforms into a 7M orthorhombic martensite with thermal hysteresis of 21K and transformation temperatures of MS=226K, Mf=218K, AS=237K, and Af=244K.

Magnetocaloric effect in Heusler alloysNi50Mn34In16 and Ni50Mn34Sn16

We present results of detailed ac susceptibility, magnetization and specific heat measurements in Heusleralloys Ni50Mn34In16 and Ni50Mn34Sn16. These alloys undergo a paramagnetic to ferromagnetic transition around305 K, which is followed by a martensitic transition in the temperature regimearound 220 K. Inside the martensite phase both the alloys show signatures offield-induced transition from martensite to austenite phase. Both field- andtemperature-induced martensite–austenite transitions are relatively sharp inNi50Mn34In16. We estimate the isothermal magnetic entropy change and adiabatic temperature changeacross the various phase transitions in these alloys and investigate the possible influence ofthese transitions on the estimated magnetocaloric effect. The sharp martensitic transition inNi50Mn34In16 gives rise to a comparatively large inverse magnetocaloric effect across this transition. Onthe other hand the magnitudes of the conventional magnetocaloric effect associatedwith the paramagnetic to ferromagnetic transition are quite comparable in thesealloys.

Thermal and mechanical characteristics of stainless steel, titanium-molybdenum, and nickel-titanium archwires

In recent years, nickel-titanium (Ni-Ti) archwires have been developed that undergo thermal transitions. Before the practitioner can fully utilize these products, the effect of those transitions within the clinical application must be understood. The transitional temperatures and mechanical stiffnesses of 3 archwire alloys--stainless steel, beta-titanium, and Ni-Ti--were investigated were for 7 products. Among the nickel-titanium alloys, 2 were thought to represent classic Ni-Ti products and 3 copper (Cu)-Ni-Ti products. By using 2 techniques, differential scanning calorimetry to measure heat flow and dynamic mechanical analysis to measure storage modulus, transition temperatures were evaluated from -30 degrees C to +80 degrees C. With regard to the first technique, no transitions were observed for the stainless steel alloy, the beta-titanium alloy, and 1 of the 2 classic Ni-Ti products. For the other classic Ni-Ti product, however, a martensitic-austenitic transition was suggested on heating, and a reverse transformation was suggested on cooling. As expected, the Cu-Ni-Ti 27, 35, and 40 products manifested austenitic finish temperatures of 29.3 degrees C, 31.4 degrees C, and 37.3 degrees C, respectively, as the enthalpy increased from 2.47 to 3.18 calories per gram. With regard to the second technique, the storage modulus at a low frequency of 0.1 Hz paralleled static mechanical tests for the stainless steel alloy (183 gigapascal [GPa]), the beta-titanium alloy (64 GPa), and the Nitinol Classic (3M Unitek, Monrovia, Calif) product that represented a stable martensitic phase (41 GPa). The remaining 4 Ni-Ti products generally varied from 20 to 35 GPa when the low-temperature or martensitic phase was present and from 60 to 70 GPa after the high-temperature or austenitic phase had formed. From the clinical viewpoint, the Orthonol (Rocky Mountain Orthodontics, Denver, Colo), Cu-Ni-Ti 27, Cu-Ni-Ti 35, and Cu-Ni-Ti 40 (SDS/Ormco, Glendora, Calif) products increased at least twofold in stiffness as temperature increased, best emulating the stiffness of Nitinol Classic below the transformational temperature and the stiffness of TMA (SDS/Ormco, Glendora, Calif) above the transformational temperature. Of the 3 Cu-Ni-Ti products, the least differences were found between Cu-Ni-Ti 27 and Cu-Ni-Ti 35, thereby questioning the justification for 3 similar products.

Magnetically driven shape memory alloys

Magnetically driven shape memory alloys experience the martensitic transformation while being in a ferromagnetic state. Two-way shape memory can be achieved through the shift of martensite–austenite transition, and a giant magnetostriction could be obtained through the redistribution of martensitic variants under magnetic field. The realization of these effects can constitute grounds for the development of new technologies in medicine and industry.

Magnetic entropy change in Ni50.1Mn20.7Ga29.6 single crystal

Magnetic entropy change (ΔS) of a single crystal Ni50.1Mn20.7Ga29.6 alloy under different applied fields has been investigated near the martensitic–austenitic structural transition temperature of 219 K. With increasing applied field from 0.2 to 5 T, the sign of ΔS changes from positive to negative. At 0.8 T the positive ΔS reaches a maximum value of about 6.0 J/kg K, and above 0.8 T the increase of applied field results in a negative ΔS. At 5 T ΔS reaches −6.0 J/kg K. The interesting behavior of ΔS is attributed to the first-order martensitic–austenitic structural transition at 219 K on heating. The phenomenon of the considerable magnetic entropy change under the relatively low applied field of 0.8 T and the easy adjustment of the martensitic–austenitic transition temperature indicate that Ni–Mn–Ga single crystal materials may have a possibility for practical applications as magnetic refrigerants.

Ferromagnetic resonance in non-stoichiometric Ni 1− x− yMn xGa y

Non-stoichiometric alloys Ni 1− x− y Mn x Ga y characterised by different values of MSME (from 0.2% to 7.3%) were studied using ferromagnetic resonance (FMR). The angular dependence of the FMR signals was measured in the martensitic and austenitic states of the samples just before and after martensite–austenite transition. Experimental data were used for the determination of the magnetisation 4 πM s and anisotropy parameters K 1, K 2 for the martensitic state and K 1c for the austenitic state. All studied alloys were characterised by large values of the anisotropy parameters of the first and second orders. A special feature of the alloys possessing high MSME is a larger value of the coefficient K 2. The temperature dependence of the FMR signals was investigated in the temperature range from below M s to above T C, where FMR was replaced by conduction electron spin resonance (CESR). Magnetically induced strain in the martensitic phase was measured as a function of the applied magnetic field. The main difference between the alloys in the martensitic state revealing the large or small MSM strain is the behaviour of the electronic structure. In the alloys with the small MSM strain, all the electrons are involved in the ferromagnetic system. On the contrary, in the alloy with the large MSM strain, the narrow resonance line of one electron subsystem is present separately in the FMR spectra. An intensive signal of CESR is observed in the alloys with the large MSME, which is an evidence for a high concentration of free electrons. The suggestion made is that the high concentration of free electrons, i.e. enhanced metallic character of interatomic bonds, assists MSME.

Modification of the surface properties of materials by pulsed plasma beams

The paper describes various types of modifications of the surface properties of solids under the influence of short-duration (10 −6 s), high-power (several megawatts) pulses of dense (10 13–10 17 ions cm −2) plasma with various elemental compositions, generated by rod-plasma injector-type machines developed at SINS. Depending on the operational conditions, a wide range of elemental compositions in the plasma pulses can be obtained from an almost pure metallic plasma of the discharge electrode material through mixed gaseous–metallic plasmas to a pure plasma of the working gas. Such pulses, apart from depositing plasma material on the substrate surface carry sufficient energy to melt the surface layer. This leads to a number of interesting effects like diffusion in the molten phase, fast epitaxial re-crystallization, rapid re-solidification accompanied by the formation of new phases, mixing (alloying) of the surface layer with the bulk material and others. The peculiar features of pulsed plasma beam interactions with the substrate are discussed for a number of cases, in particular: • formation of photovoltaic p +–n and n +–p junctions and ohmic contacts in Si under the influence of BF 3 and PF 5 plasma pulses; • nitriding of steels and martensite–austenite transitions under the influence of N plasma pulses; • coating of steel with intermixed (alloyed) layers of Cu using N plasma pulses with controlled content of the appropriate metal; and • transition from deposition to ablation for Cu–N plasma pulses for varying proportions of constituents. The results presented in the paper indicate that intense pulse plasma beams may be an extremely valuable tool in the modification of the surface properties of materials of technological importance.

Martensite-austenite transition and phonon dispersion curves ofFe1−xNixstudied by molecular-dynamics simulations

We have done molecular-dynamics simulations of ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}$ employing a semiempirical model. We present a phase diagram of the martensite-austenite transition temperatures as a function of the Ni concentration which is in good agreement with experimental observations. In addition to this we have calculated the phonon dispersion curves of Fe and Ni from the model. Results show that the vibrational properties of the metals are well reproduced by the embedded-atom-method potentials. Finally, we have derived the phonon dispersion relations of bcc ${\mathrm{Fe}}_{80}{\mathrm{Ni}}_{20}.$ We find rather low energies of the $[110]\ensuremath{-}{\mathrm{TA}}_{1}$ phonons with a strong temperature dependence which we attribute to instabilities of Ni in the bcc phase. We do not find any indications of a soft mode at the martensite-austenite transition in ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}.$

Lattice Dynamics of Martensitic Transformations Examined by Atomistic Simulations

We have performed molecular dynamics simulations of Fe 80 Ni 20 alloys using an inter-atomic potential of the EAM-type which allows the simulation of the martensite-austenite transition. We present results, showing the development of an inhomogeneous shear system on a nanoscale during the thermally induced austenitic transformation. In addition to this we obtained the photon dispersion relations of the martensite phase by calculating the dynamical structure factor from out simulation results. On approaching the transition temperature the photon dispersion shows anomalies which might be connected with the formation of the microstructure during the austenitic transition.

Phase transformation in NiTi studied by isothermal mechanical spectrometry

NiTi samples were studied by internal friction measurements and differential scanning calorimetry (DSC). DSC experiments were performed using a Mettler TA30 DSC with a controlled cooling/heating rate of 1 K/min, to determine the transformation heats ({Delta}H) and the characteristic transformation temperatures. It was necessary to expect a very long time (about one day) after each temperature change to obtain a stabilized spectrum even for a small temperature step (10 K). This fact shows that the sample microstructure evolution occurring during this temperature change is in fact very slow. For higher frequency experiments the martensite-austenite transition corresponds to an internal friction decrease. This evolution exhibits in fact two stages: a first Q{sup {minus}1} diminution at 320 K and a second above 360 K corresponding to the A{sub S} temperature. Measurements performed at temperature higher than A{sub f} showed also a low frequency damping increasing. But in this case no thermally actived process was observed, experimental curves being exactly superimposed. The increasing internal friction at very low frequency was never observed before these experiments. As shown here, it disappears during the transition between the martensitic to austenitic phases and corresponds to mechanisms taking place in the martensite. This relaxation effect is thermallymore » actived. These relaxation effects observed in the martensitic phase can be attributed either to the motion of dislocations present in the plates or plate-boundaries or to the motion of the interface between martensite plate. So further experiments will be necessary to conclude about the origin of this thermally activated relaxation mechanism.« less