Metastable effects on martensitic transformation in SMA

The efficiency of shape memory alloy (SMA) as damper and/or standard actuator is truly enhanced when the material can be cycled without any relevant accumulation of the permanent deformation (i.e. under 0.5% for several hundreds of cycles). The particular properties of the CuAlBe alloy permit relevant grain growth with reasonable reduction of mechanical properties (from 300–350 to 250–300 MPa at fracture). Samples prepared with an appropriate heat thermal treatment (HTT) and relevant mean diameter of grain avoids accumulative deformation for series of cycles (near 500) up to 3.5% of deformation. The analysis of different wires of CuAlBe alloy shows, in the first part of HTT, a proportionality between the grain surface and the time at 1123 K. In the last part of the HTT the grain growth shows an increased complexity related with interactions between the grain boundaries and the external surface of the samples.     

Metastable effects on martensitic transformation in SMA

The applicability of shape memory alloys (SMA) for dampers in civil engineering requires guaranteed behavior with well controlled or irrelevant changes after several years of inactivity and/or under the action of daily or seasonal temperature effects. The long time analysis of the aging temperature effects on a NiTi alloy shows similar behavior to other SMA but with higher time constants at temperatures near 373 K: 1.9 days at 410 K and near 100 days at 363 K with an activation energy value near 12500 K (RT=104 kJ mol⁻¹). At the present state of the art, the measurements show that the change of transformation temperatures under the action of the external temperature (or ‘room temperature’ change) is close to 15 K. The direct use of the as furnished material can suffer relevant changes of working properties in devices under the action of direct sunlight or high room temperature working conditions.     

Metastable effects on martensitic transformation in SMA

The Clausius-Clapeyron coefficient (CCC or α_CC) or relation of the stress to transform a thermoelastic SMA sample and temperature, is revisited for 1-dimensional stressed SMA when polycrystalline materials are used. The experimental method is discussed in the frame of equilibrium thermodynamics for polycrystalline materials. Suggestions for evaluation of the experimental value in polycrystalline NiTi and CuAlBe specimens are proposed. The analysis for NiTi wire gives a CCC of α_CC=6.3±0.3 MPa K⁻¹. On the other hand CuAlBe provides a value of α_CC=2.2±0.4 MPa K⁻¹ for tensile stress.     

Metastable effects on martensitic transformation in SMA

The use of Shape Memory Alloys (SMA) in technical applications as damping in civil engineering structures requires the characterization of the alloy for each specific application. This involves the evolution of the mechanical properties and damping capacity with the number of cycles, frequency, maximum deformation, applied stresses, and the evolution of the alloy with aging time and temperature. In particular, the temperature effects associated to self-heating need to be evaluated. In continuous cycling the effects of latent heat, the associated dissipation induced by the hysteresis, the heat flow to surroundings and the cycling frequency induce different states of temperature in the specimen, which in turn produces changes in the transformation-retransformation stresses. In this article, the temperature effects associated to cycling are outlined for different cycling frequencies. The results show that, for relatively faster frequency the temperature arrives at an oscillatory state superimposed to an exponential increase. For lower frequencies, some parts of the sample attain temperatures below room temperature. The experimental results are represented with an elementary model (the 1-body model or the Tian equation used in calorimetric representation) of heat transfer. For the higher fracture where life requirements are associated to damping in stayed cables for bridges, the results show (for the NiTi alloy) a reduction of the hysteresis width as the frequency increases for deformations up to 8%. For reduced deformation, under 2% appears an asymptotic behavior where the frictional area is practically independent of the cycling frequency (up to 20 Hz). In addition, it is shown that more than 4 million of working cycles can be attained if the maximum applied stress is kept below a threshold of about 200 MPa. Although under this condition the deformation must remain lower than 2% a reasonable damping capacity can still be obtained.     

Metastable effects on martensitic transformation in SMA part VII. Aging problems in NiTi

Shape memory alloys (SMA) are interesting for applications in damping of civil engineering structures. To achieve the SMA as a guaranteed material for damping of structures requires a reliability study of the static and dynamic properties adapted to each type of application. Here we present static (temperature and time effects) and dynamic actions in pseudo-elastic NiTi SMA. We concern with long time effects of temperature and time mainly in beta phase. NiTi results are presented, including measurements of electrical resistance as a function of temperature and time, DSC to −80°C (TA Instruments), non-conventional conduction calorimetry (to 80 K), and several X-ray diffraction spectra at different temperatures. Diffusion effects are present, and all the measurements show that transformation temperatures change with time of aging even at moderate temperatures (i.e., near 100°C), depending on time and temperature. It is possible to visualize the diffusion change in the R phase transformation via classical X-ray characterization. We include some results of pre-stressing effects applying the stress at different zones of the hysteresis cycle. The experimental analysis shows that coupling between stress and temperature enhances the changes. For long times (10, 20 years) and direct sunny actions, more deep analysis is required. Finally, some dynamic effects in cycling affecting the SMA creep are outlined.     

Metastable effects onmartensitic transformation in SMA

The behavior of shape memory alloys (SMA) allows their use as a passive smart material. In particular, the existence of a hysteretic cycle in the domain of the elementary coordinates strain-stress-temperature (σ, ε, T) suggests its application for damping in mechanical and/or in civil engineering. We are working in the application of SMA as dampers for earthquakes in small houses as family homes. For dampers installed in the inner porticos of the house, the suggested SMA is the CuAlBe and, eventually, the NiTi. At room temperature the used SMA wires induces forces situated between 2–3kN/wire. The properties related with the damping applications for CuAlBe and NiTi, i.e., the SMA creep and the self-heating will be presented, together with some other minor stress and temperature effects on NiTi modifying the hysteretic behavior.     

Metastable effects on martensitic transformation in SMA (I) recoverable effects by the action of thermodynamic forces in parent phase

Summary The applicability of SMA requires a matching of the properties with the technological needs. The metastability effects and the phase coexistence can produce, via atomic diffusion, some changes in the expected properties against time. The careful analysis of the external thermodynamic forces (stress) on the parent phase of a Cu-Al-Zn single crystal alloy establishes a proportional minor change in critical transformation stress (near 1 per cent of the external stress). Via the Clausius-Clapeyron coefficient, the stress effects show a similar behaviour but faster than the room temperature effects on the transformation temperature M_S. In parent phase, the effect of the thermodynamic force regarded as an M_S shift lies between 10 to 15 percent of temperature change.     

Metastable effects on martensitic transformation in SMA part V. fatigue-life and detailed hysteresis behavior in NiTi and Cu-based alloys

The fatigue-life (traction-traction only) is experimentally studied mainly for pseudoelastic NiTi wire of 2.46 mm of diameter for eventual application in damping of structures under wind or rain. Thermal effects are highly relevant in determining the fatigue-life. The results shows that the fracture level overcomes 130000 cycles when the moving air is used for cooling. When the number of working cycles overcomes 30000, the frictional energy decreases 40% vs. N roughly with an exponential behavior.     

Influence of chemical composition on martensitic transformation of MnNiIn shape memory alloys

The most extensively studied Heusler alloys are those based on the Ni–Mn–Ga system. However, to overcome the high cost of Gallium and the generally low martensitic transformation temperature, the search for Ga-free alloys has been recently attempted, particularly, by introducing In, Sn or Sb. In this work, two shape memory alloys, Mn₅₀Ni_50?xIn_x (x = 7.5 and 10), were obtained by rapid solidification. We outline their structural and thermal behaviour. The structural austenite–martensite transformation was checked by calorimetry. The transformation temperatures decrease as In content increases. The same pattern is reflected in entropy and enthalpy changes linked to transformation. The control of the valence electron by atom (e/a) determines the transformation temperatures range in this kind of alloys, and it is possible to develop alloys that can be candidates in applications such as sensors and actuators. In addition, X-ray diffraction was performed to verify the crystalline structure at room temperature.

     

On the “martensitic” transformation in the A-15 superconductors compounds

Although a great number of studies have been reported on the “martensitic” transformation in A-15 superconductors, there are still fundamental questions concerning the nature of the transformation which remain unsolved. After a critical analysis of previous experimental work on this transformation in V₃Si and Nb₃Sn, including our own results, we propose a model for the origin and mechanism of the tetragonal distortion based on the consequences of the strong structural relation between the A-15 cubic structure and the W₅Si₃ tetragonal one. As a matter of fact since the analysis of the previous results leads to the conclusion that the transforming A₃B sample has a composition richer in B than stoichiometric, we propose that the distortion is due to a number of defects (antisite type which is larger in transforming than in nontransforming crystals). The presence of these point defects leads to the formation of local strain of tetragonal symmetrical resulting from repulsive interactions between the B atoms, in equilibrium with the cubic symmetry of the three-dimensional network of strongly bonded A atoms. At low temperature, as soon as the lattice softening becomes important enough, a new equilibrium is reached, namely the distortion of the lattice.     

Effect of Ag additions on the reverse martensitic transformation in the Cu-10 mass% al alloy

The effect of Ag additions on the reverse martensitic transformation in the Cu-10 mass% Al alloy was studied using differential thermal analysis (DTA), optical (OM) and scanning electron microscopies (SEM) and X-ray diffractometry. The results indicated that Ag additions to the Cu-10 mass% Al alloy shift the equilibrium concentration to higher Al contents, allow to obtain both β'₁ and β' martensitic phases in equilibrium and that Ag precipitation is a process associated with the perlitic phase formation.     

The martensitic transformation entropy values of thermal and mechanical origin in shape memory Cu-Zn-Al single crystals

By using a calorimetric technique, the entropy values of the martensitic transformation of thermal origin have been determined and compared with those obtained by mechanical compression tests for the stress-induced martensitic transformation.     

Micromechanism of Cu and Fe alloying process on the martensitic phase transformation of NiTi-based alloys: First-principles calculation

Using first-principles pseudo-potential plane wave method, the formation enthalpy ΔH, binding energy ΔE, elastic constants, and electronic structure were calculated and analyzed carefully for NiTiX (X = Cu, Fe) shape memory alloy. The results show that the Cu or Fe element prefers to occupy the Ni site in the NiTi matrix phase respectively. Compared with the NiTi matrix phase, the ΔH, ΔE, c ₄₄ and c′ of NiTi (Cu) are similar to each other. However, the structural stability of the NiTi phase is improved obviously by the Fe alloying process. Simultaneously, the shear modulus c ₄₄ and c′ of NiTi (Fe) are larger than those of the NiTi matrix phase. Furthermore, Milliken population results indicate that Q _Cu–Ti is smaller than Q _Ni–Ti after the Cu alloying process, but Q _Fe–Ti is larger than Q _Ni–Ti. The electron density difference shows that some covalent bonding exists between Fe and Ti elements. Based on the upward analysis, the difference in the phase stability and elastic constants of NiTiX (X = Cu, Fe) is the substantial mechanism for the different M _s of NiTiX (X = Cu, Fe) although Cu or Fe substitutes for the same atom Ni elements in the NiTi matrix phase.     

Metastable ions in chemical ionization

The ion [C₃H₅]⁺ generated in a chemical ionization source by a variety of methods, including protonation and charge exchange, exhibits a metastable peak for H₂ loss which is two orders of magnitude weaker than that formed in an electron impact source. The stable [C₃H₅]⁺ ions generated by electron impact and chemical ionization undergo collision-induced dissociation to a comparable extent, both losing H₂ by only one of the two competitive mechanisms observed for metastable ions. In contrast to the behavior of [C₃H₅]⁺, the molecular ions of p-substituted nitrobenzene, generated by charge exchange at high source pressure, yield composite metastable peaks for NO loss which are very similar in shape and intensity to those generated by electron impact. The contrasting behavior of the metastable ions extracted from high pressure ion sources in the two systems may be due to differences in the efficiencies of quenching of the ionic states responsible for fragmentation as metastable ions. It is noteworthy that the NO loss reactions require considerably lower activation energies than does the H₂ loss reaction.     

Metastable ion fragmentation processes in n-alkanes

The n-alkanes, propane to heptane, have been studied in a modified MS-9 mass spectrometer under conditions that enabled the daughter ions, resulting from metastable fragmentation processes occurring in the first field free region, to be observed with high sensitivity. The precursor ions in these fragmentations have been identified unambiguously and a general fragmentation scheme for the n-alkanes is proposed. The mass spectrometer incorporated a variable width monitor slit and narrow settings of this slit improved the energy resolution so that fine structure could be observed in some of the ‘metastable peaks’. The implications of this fine structure are discussed.     

Metastable peak study in methylactophenone isomers

The relative intensity of the metastable peaks at 70 eV appearing at m/z 105.7 and 69.6 in the mass spectra of methylacetophenone isomers, as well as the kinetic energy (T_1/2) released with the peaks at 14 and 70 eV respectively, are reported. The profile of all the metastable peaks studied has been found to be approximately flat-topped. For the same transition it is found that the kinetic energy released with the metastable peaks formed by unimolecular decomposition of the three isomers decreases from the ortho to the para isomer. In addition, the kinetic energy released during the same transition is found to be roughly directly proportional to their corresponding intensity ratios. This result, together with the observation that the energy released with each metastable peak decreases by lowering the electron energy, may reveal the role of the internal energy of the reacting ions in producing the kinetic energy associated with the metastable peaks produced from methylacetophenone isomers.     

Metastable transitions in n-butane and n-butane-d7

Metastable ions have been investigated for n-butane d₇ molecular ions using a tandem mass spectrometer which samples unimolecular decay processes occuring during the time interval of c.2 μs to 4 ms after ion formation. Some 37% of ions formed by 70 eV electron impact decay on this time scale. The competing unimolecular processes observed, in order of relative importance, are methance, methyl radical and hydrogen atom elimination. The slow metastables sample the threshold energy regime of unimolecular reactions responsible for forming the ordinary mass spectrum of butance and very large isotope effect are noted for the deuterated molecule.     

Metastable states in antiferroelectric liquid crystalline mixtures

We report on the observation of relaxation phenomena with extremely long relaxation times, amounting to several hours. These effects take place in some liquid crystal mixtures exhibiting ferroelectric and antiferroelectric dipole order. The observed phenomena are connected with the transformation from the supercooled ferroelectric state to another, metastable state. This transition may be described using a Debye type relaxation formula. At low temperatures, a second slow transition takes place: from the metastable intermediate state to the antiferroelectric phase. This transition is characterized by unidimensional growth of the antiferroelectric domains with a constant velocity. Close to the lower temperature limit of existence of the ferroelectric phase, a direct transition from the ferroelectric to the antiferroelectric phase takes place. This transition is described by an Avrami model, hence it is governed by the creation and growth of nuclei of the antiferroelectric phase.     

Metastable states in antiferroelectric liquid crystalline mixtures

We report on the observation of relaxation phenomena with extremely long relaxation times, amounting to several hours. These effects take place in some liquid crystal mixtures exhibiting ferroelectric and antiferroelectric dipole order. The observed phenomena are connected with the transformation from the supercooled ferroelectric state to another, metastable state. This transition may be described using a Debye type relaxation formula. At low temperatures, a second slow transition takes place: from the metastable intermediate state to the antiferroelectric phase. This transition is characterized by unidimensional growth of the antiferroelectric domains with a constant velocity. Close to the lower temperature limit of existence of the ferroelectric phase, a direct transition from the ferroelectric to the antiferroelectric phase takes place. This transition is described by an Avrami model, hence it is governed by the creation and growth of nuclei of the antiferroelectric phase.