Low-temperature Martensitic Phase Research Articles

Entropy change at the martensitic transformation in ferromagnetic shape memory alloys Ni2+xMn1−xGa

The entropy change ΔS between the high-temperature cubic phase and the low-temperature tetragonally based martensitic phase of Ni2+xMn1−xGa (x=0–0.20) alloys was studied. The experimental results obtained indicate that ΔS in the Ni2+xMn1−xGa alloys increases with the Ni excess x. The increase of ΔS is presumably accounted for by an increase of magnetic contribution to the entropy change. It is suggested that the change in modulation of the martensitic phase of Ni2+xMn1−xGa results in discontinuity of the composition dependence of ΔS.

Shape memory alloys — characterization techniques

Shape memory alloys are the generic class of alloys that show both thermal and mechanical memory. The basic physics involved in the shape memory effect is the reversible thermoelastic martensitic transformation. In general, there exists two phases in shape memory alloys, viz., a high-temperature phase or austenitic phase (A) and a low-temperature phase or martensitic phase (M). In addition, an intermediate R phase exists in some special cases. The M↔A transformation is associated with a recoverable strain of about 6.5–8% and the R↔A transformation is associated with a recoverable strain of about 1%. The former transformation has been widely used in the applications like antenna deployment of satellite, aerospace couplings, orthodontic arch wires, medical guide wires for diagnostic and therapeutic catheters and other industrial applications. Our group has been giving emphasis to the characterization techniques for R phase, using differential scanning calorimetry (DSC), electrical resistivity probe (ER) and thermomechanical analyzer (TMA). R phase is found to have attractive features like stability against thermal cycling, a small thermal hysteresis and a negligible strain recovery fatigue. DSC has been used successfully to characterize the recoverable strain parameters, apart from the determination of transformation temperatures. ER is used, for the systematic study of the dependence of various phases on heat-treatment temperatures. TMA has been effectively employed for the study of the mixed phases. A space-rotating platform is designed and fabricated, using an actuator of shape memory spring, for obtaining controlled rotations. The efficiency and the reliability of this actuator has been tested, over a million thermal cycles.

Structure and ordering of the 18-carat Al–Au–Cu β-phase

Gold alloys with around 30 at% copper and 25 at% aluminium undergo a reversible displacive transformation with M s and A s temperatures of about 30 and 75°C respectively. However, the nature of the parent and product phases in this system has not previously been clear. It is confirmed here that the parent phase is a β electron compound. If it is annealed and quenched, it has ordering intermediate between the A2 and B2 states, and in this case the displacive transformation between the parent phase and the low temperature (martensite) phase is poorly developed, and has some second order characteristics. Ageing of the parent phase causes it to take up L2 1-type ordering after which the transformation becomes classically displacive and reversible, with many of the characteristics associated with the known shape memory systems. The site occupancies of the parent phase have been estimated.

Passive vibration damping in an alpine ski by integration of shape memory alloys

With exception of the wooden core, all the components of a modern alpine ski exhibit a decreasing vibration damping capacity with decreasing temperature. Although the damping effect of the wooden core is prevailing, the extreme hardness of snow under freezing conditions, at temperatures on low as -30°C, favours the appearance of vibrations which tend to make the skis more difficult to control. Therefore, the aim is to obtain a simple adaptive system to enhance vibrational damping at low temperatures, without spoiling the overall characteristics of the ski at higher temperatures. Shape memory alloys (SMA), in their low-temperature martensitic phase, exhibit a damping capacity which is orders of magnitude higher than in the standard materials of a ski. Transition from the SMA's high-temperature and low-damping austenitic phase to the high damping martensitic phase is achieved through a reversible thermally induced martensitic phase transformation (MPT). In this project, CuZnAl plates have been integrated into the complex sandwich structure of an alpine ski. Three point bending and differential scanning calorimetry (DSC) have been used to characterise the materials and their effects on the damping behauvour. In order to ensure the most efficient damping through shear deformations, an excellent adherence between the SMA plates and other materials has to be guaranteed by means of a surface treatment including sand blasting and chemical etching. The damping effect also depends on the location of the SMA plates within the ski. First tests on modified full-size alpine skis have already illustrated the improvements in damping capacity achieved with the integration of CuZnAl elements.

Magnetic and magnetomechanical properties of Ni 2MnGa

Magnetocrystalline anisotropy and magnetostrictive properties are reported for the austenitic and martensitic phases of ferromagnetic shape memory Heusler alloy Ni 2MnGa. In the low-temperature martensitic phase, the phenomenon of field-induced variant rearrangement provides a mechanism which can produce large strains, while at the same time causing anomalous effects in the apparent anisotropy. These anomalies and their effects on measured M– H and torque curves are clarified. Magnetomechanical experiments were performed in the martensitic phase to characterize the work output of a suitably oriented specimen at various stresses, and with proper stress biasing of the initial microstructure produced the largest magnetostrictive strains to date of 4.3%.

Evaluation of Prestressing with a Shape Memory Alloy

ABSTRACTThe paper addresses the possibility of using shape-memory effect in Nitinol (Nickel-Titanium) alloy to remedy prestress losses in aging concrete beams. Straight annealed NiTi in the low temperature martensite phase can undergo large plastic deformations. This deformation and strain energy is recoverable when the material transforms to the higher temperature (about 55° C) austentite phase. This phenomena can be exploited to induce curvature, hence prestressing, by embedding NiTi wires and ropes in a structural member. Depending on the type of wire used its actuation effect can be reversed as necessary, creating the possibility of a smart prestressed concrete bridge of the future.Materials aspect studied in this project include the bond between NiTi wires and the host composite material, through mechanical testing and microscopy. An evaluation of the phase transformation is conducted using Acoustic Emission (AE) technology. AE event rate, locations and characteristics were evaluated during the actuation process.

Influence of uniaxial deformation on the phase transformation in lithium.

The influence of a uniaxial elastic deformation on the low-temperature martensitic phase transformation in lithium metal is investigated by neutron-scattering techniques. The experiments performed indicate that a [001] compression of the bcc matrix promotes the phase transition. The low-temperature phase induced by elastic deformation is characterized by the 9R structure as in transitions induced by a temperature change. Furthermore, an investigation of the influence of plastic deformation on the phase transition is presented.

Tweed in martensites: a potential new spin glass

We've been studying the "tweed" precursors above the martensitic transition in shape-memory alloys. These characteristic cross-hatched modulations occur for hundreds of degrees above the first-order shape-changing transition. Our two-dimensional model for this transition, in the limit of infinite elastic anisotropy, can be mapped onto a spin-glass Hamiltonian in a random field. We suggest that the tweed precursors are a direct analogy of the spin-glass phase. The tweed is intermediate between the high-temperature cubic phase and the low-temperature martensitic phase in the same way as the spin-glass phase can be intermediate between ferromagnet and antiferromagnet.

Self-accommodation in martensite

The shape-memory effect is a phenomenon wherein an apparently plastically deformed specimen recovers all strain when heated above a critical temperature. This is observed in some crystalline solids that undergo martensitic phase transformation. The martensitic transformation is a temperature-induced, diffusionless solid-to-solid phase transformation involving a change in crystalline symmetry. Shape-memory materials are able to transform from the high-temperature austenite to the low-temperature martensite phase without any apparent change in shape. This is known as self-accommodation. Necessary and sufficient conditions that the lattice parameters of a material must satisfy for the material to form a self-accommodating microstructure are derived. The main result states that if the austenite is cubic, the material is self-accommodating if and only if the transformation is volume preserving. On the other hand, if the symmetry of the austenite is not cubic, it is not possible to construct any microstructure that is self-accommodating unless the transformation strain or the Bain strain satisfies additional, rather strict, conditions. These results show good agreement with the available experimental data. The analysis here is significantly different from previous studies because it makes no a priori assumption on the microstructure.

Neutron-scattering and electron-microscopy studies of the premartensitic phenomena in NixAl100-x alloys.

Inelastic-neutron-scattering and electron-microscopy studies were performed on single crystals of the cubic phase of ${\mathrm{Ni}}_{\mathit{x}}$${\mathrm{Al}}_{100\mathrm{\ensuremath{-}}\mathit{x}}$ alloys for x=50, 58, 62.5, and 63.9 at. %. The [110]-${\mathrm{TA}}_{2}$ phonon branch, corresponding to atomic displacements along the [1\ifmmode\bar\else\textasciimacron\fi{}10] direction, is shown to exhibit anomalous behavior in that the entire branch is very sensitive to composition and exhibits a ``kink'' whose position in q space is also sensitive to x. A peak in the elastic diffuse scattering is also present at the same q as the kink. Amplitude-contrast electron-microscope images display a ``tweed'' pattern characteristic of many martensitically transforming materials. The high-resolution (phase-contrast) electron image reveals microscopic deviations of the cubic structure and can be viewed as embryos of the low-temperature (5,2\ifmmode\bar\else\textasciimacron\fi{}) martensitic phase. The displacement fields associated with these distortions are shown to be the origin of the elastic central peak observed in the experiments. Temperature-dependence studies show a substantial softening of the phonon branch in the vicinity of the kink and the elastic diffuse scattering increases as T approaches ${\mathit{T}}_{\mathit{M}}$, the martensitic-transformation onset temperature. The mode never becomes completely soft. Furthermore, the softening is narrowly restricted to the [1\ifmmode\bar\else\textasciimacron\fi{}10] direction, which is perpendicular to [110]. Studies of the phonon dispersion curve under uniaxial stress show a softening of the phonon mode near the q value of the kink, which can be interpreted in terms of Clapp's proposed localized-soft-mode theory of nucleation of materials undergoing martensitic transformation.

Structure Analysis of NiAl Martensite

Neutron elastic scattering experiments were performed in order to investigate the structure of the low temperature martensitic phase of Ni/sub 62.5/Al/sub 37.5/ alloy. The average structure analyzed from the integrated intensity was approximately described by the (5,/minus/2) structure proposed by Martynov et al. Small deviation from the exact (5,/minus/2) model in the positional parameters and the anomalously large Debye-Waller factor were obtained. The observed satellite profiles show asymmetrical broadening, and the peak positions shift from the regular reciprocal lattice points. These anomalous features of scattering profiles were tentatively interpreted by introducing spatial modulation of the strain and order parameters. 12 refs., 2 figs., 1 tab.

Modulated lattice relaxation in β-based premartensitic phase

Recent X-ray studies on a TiNi(Fe) alloy by Shapiroet al. show that the incommensurability of satellite reflections changes from Brillouin zone to Brillouin zone, giving an overall swirl-like shift pattern in the reciprocal space, which precludes any simple incommensurate wave model. We have succeeded in reproducing the intriguing shift pattern observed, based on two simple assumptions: (i) existence of a dip in TA phonon dispersion atq = 1/3[110] and (ii) existence of embryos of low temperature martensite phase. The proposed features of the premartensitic phase suggest a possible microscopic mechanism of martensite transformation in general as well as of w-phase formation. fa]This paper is based on a presentation made in the symposium “Pretransformation Behavior Related to Displacive Transformations in Alloys” presented at the 1986 annual AIME meeting in New Orleans, March 2–6, 1986, under the auspices of the ASM-MSD Structures Committee.

Low-temperature martensitic phase transition of bcc lithium.

The low-temperature martensitic phase transition of bcc lithium has been investigated by means of neutron scattering techniques. A softening of the low-lying transverse-acoustic [110] phonon branch polarized along [11\ifmmode\bar\else\textasciimacron\fi{}0] is found when the transition temperature is approached. Moreover, a diffuse scattering unmodulated in reciprocal space is observed showing a hysteresis in the vicinity of the transition. The low-temperature phase appears at 74 K where an orientation relation (110)bcc\ensuremath{\parallel}(002)hcp is observed. Finally, connected with the appearance of the new phase a streaked intensity extending from the (110)bcc toward the (220)bcc reflection is found indicating the presence of stacking faults on (110)bcc planes. The results corroborate a model for the phase transformation recently put forward for the related transition in Na and describing the basic feature of the transition as a short-wavelength shear instability occurring on (110)bcc planes.

‘Modulated lattice relaxation’ in β-based premartensitic phase

Recent X-ray studies on TiNi(Fe) alloy by Shapiro et al. show that incommensurability of satellite reflections changes from Brillouin zone to Brillouin zone, giving overall ‘swirl’ like shift pattern in the reciprocal space which precludes any simple incommensurate wave model. We have succeeded to reproduce the intriguing shift pattern observed, based on two simple assumptions: (i) existence of soft phonons at q = 1 3 [110] (ii) existence of ‘embrios’ of low temperature martensite phase. The proposed features of premartensitic phase suggest a possible microscopic mechanism of martensite transformation in general as well as of ω-phase formation.

Low-temperature phase transition and isomer-shift systematics in intermediate phases of rare-earth—gold compounds

The temperature dependence of the $^{197}\mathrm{Au}$, $^{166}\mathrm{Er}$, and $^{169}\mathrm{Tm}$ M\ossbauer effect has been studied in intermediate phases of $R\mathrm{Au} (R=\mathrm{S}\mathrm{c},\phantom{\rule{0ex}{0ex}}\mathrm{H}\mathrm{o},\phantom{\rule{0ex}{0ex}}\mathrm{E}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{T}\mathrm{m},\phantom{\rule{0ex}{0ex}}\mathrm{Y}\mathrm{b},\phantom{\rule{0ex}{0ex}}\mathrm{L}\mathrm{u})$ with the CsCl structure. Our results indicate that the CsCl structure is metastable and undergoes a low-temperature martensitic phase transformation to a close-packed (FeB-like) structure. The ratio of the quantity of the two phases present at 4.2\ifmmode^\circ\else\textdegree\fi{}K for various compounds is obtained. M\ossbauer-absorption patterns for $^{166}\mathrm{Er}$ in ErAu have a temperature dependence which is characteristic of relaxation effects. Isomer-shift systematics for $^{197}\mathrm{Au}$ in $R\mathrm{Au}$ for the CsCl and the transformed close-packed phases are discussed.

高炭素マンガン鋼の低温焼入れマルテンサイト相のメスバウアー効果

高炭素マンガン鋼の低温焼入れマルテンサイト相のメスバウアー効果