Symmetry-conforming Short-range Order Research Articles

Ferroelectric and electromechanical performance of diverse engineered states of Mn-doped 0.75Pb(Mg1/3Nb2/3)O3-0.25PbTiO3 ceramics

The materials processing history has a great influence on their properties and finally determines their application effect. In this paper, the ferroelectric, polarization-switching current, and strain properties of Mn-doped 0.75Pb(Mg1/3Nb2/3)O3-0.25PbTiO3 ceramics were studied in fresh state, aged state, and poled state, respectively. Compared with the symmetric polarization-electric-field (P-E) hysteresis loops, current-density-electric-field (J-E) curves, and bipolar electric-field-induced strain (S-E) curves in fresh state samples, asymmetric P-E loops, J-E curves, and bipolar S-E curves were obtained in poled state samples. Well-aged-state samples exhibit double hysteresis P-E loop, four peaks J-E curves, and symmetric S-E curves without negative strain. The symmetry-conforming short-range order (SC-SRO) principle of point defects and internal electric field Ei is employed to clarify the different phenomenon of three states. Results indicated that randomly oriented defect polarization PD in aged samples can reverse the spontaneous polarization PS back and result in the double hysteresis P-E loop and four peaks J-E curves. The oriented PD and resulting Ei in poled-state samples will lead to the asymmetric loops and strain memory effect.

Martensite stabilization in shape memory alloys – Experimental evidence for short-range ordering

Thermal stabilization of martensite in shape memory alloys is known to strongly affect functional properties due to changes in transformation temperatures. As martensite stabilization in many alloys proceeds in an uncontrollable fashion, it has been treated as a detrimental mechanism in the past. In a recent study it was found that martensite stabilization can be controlled by aging of stress-induced martensite, allowing development of a new class of high-temperature shape memory alloys. Symmetry-conforming adaptation of short-range order during thermal treatment has been stated to be the mechanism responsible for this phenomenon. However, direct experimental evidence for changes in short-range ordering has not been presented. The current study has been conducted in order to fill this gap. A Co–Ni–Ga shape memory alloy has been studied by neutron diffraction in different conditions, i.e. as-grown austenite, quenched martensite, heat-treated austenite and stabilized stress-induced martensite. The results obtained unequivocally reveal that martensite stabilization is triggered by a chemical disordering mechanism. Thus, the concept of symmetry-conforming short-range order proposed 1997 by Ren and Otsuka has finally found experimental verification for a Co–Ni–Ga alloy.

Martensite aging – Avenue to new high temperature shape memory alloys

High-temperature shape memory alloys are attractive for efficient solid state actuation. A key criterion for shape memory alloys is the martensite start temperature. The current study introduces a concept for increasing this temperature of alloys initially not suited for high-temperature actuation. Aging of stress-induced martensite, referred to as SIM-aging in the current work, is able to increase the martensite start temperature by about 130°C as demonstrated in the present study for a Co–Ni–Ga shape memory alloy. The increase of transformation temperatures can be explained based on the concept of symmetry-conforming short-range order. Following SIM-aging the Co–Ni–Ga alloy shows cyclic actuation stability at elevated temperatures. While martensite aging has always been viewed as detrimental in the past, it can actually be exploited to design new classes of high-temperature shape memory alloys with excellent properties.

Impact of the volume change on the ageing effects in Cu–Al–Ni martensite: experiment and theory

The time evolution of the physical properties of martensite during martensite ageing is traditionally explained by the symmetry-conforming short-range order (SC-SRO) principle, which requires the spatial configuration of crystal defects to follow the symmetry change of the host lattice. In the present study, we show that the volume change of the host lattice also contributes to the ageing effects in Cu–Al–Ni shape memory alloy besides the symmetry change. To substantiate this statement the gradual increase of the storage modulus with time at constant temperature was measured by dynamic mechanical analysis (DMA) and the experimental results were quantitatively described in the framework of the symmetry-conforming Landau theory of martensitic transformations in a crystal with defects. The comparison of experimental and theoretical results confirmed that the time dependence of the storage modulus is caused by two different physical mechanisms. Evaluations showing that the first mechanism is driven by the spontaneous symmetry change and the second mechanism is caused by the volume change after the martensitic transformation was carried out.

Intrinsic origin of rubber-like behavior in thermoelastic alloys

The evolution of polytwinned martensitic variants, which accompanies changes in various energies under applied stress, is investigated using a 3-dimensional phase-field simulation. The intrinsic origin of the rubber-like behavior (RLB) in a thermoelastic alloy is revealed as the disappearance of one variant during loading and its reappearance during unloading, whereas long “stress aging” enhances martensite stabilization. The applied stress and elastic strain energies drive the respective microstructural evolution and its reversal. This intrinsic origin of the RLB cannot be excluded in thermoelastic alloys for which the RLB can be explained by “symmetry conforming-short range order” principle.

Martensite destabilization in Au 7Cu 5Al 4 shape-memory alloy

Aging-induced changes in the austenite peak ( A P) temperature of Au 7Cu 5Al 4 shape-memory alloy are investigated. Whereas heat treating the parent phase at temperatures >140 °C or aging the martensite for long times at room temperature both stabilized the A P to ∼80 °C, low-temperature excursions into the parent phase caused the subsequent A P to drop to ∼60 °C and the transformation hysteresis to decrease. The evidence indicates that this destabilization of the martensite is caused by time-dependent relaxation of elastic constraint due to parent-phase lath migration during the preceding low-temperature austenitizing treatment. This mechanism of aging is different from that of the better-known symmetry-conforming short-range order phenomenon.

Microscopic mechanism of martensitic stabilization in shape-memory alloys: Atomic-level processes

Aging in martensite, which is accompanied by a gradual change in physical properties, has been observed in most shape-memory alloys for more than half a century. However, its microscopic mechanism has remained controversial due to a lack of experiments that can probe the atomic-level processes. By using a method which combines molecular-dynamics and Monte Carlo simulations, we clarify the atomic mechanism for one of the well-observed martensitic aging effects, martensitic stabilization. We successfully reproduce the observed effects using our method. Quantitative analysis of the atomic configurations during aging reveals that martensite stabilization is not associated with a change in the average martensite structure. It involves instead a gradual change in the short-range order of point defects so that the defect short-range order tends to adopt the same ``symmetry'' as the crystal symmetry of the host martensite lattice. Our simulation results are consistent with the symmetry-conforming short-range order model [X. Ren and K. Otsuka, Nature (London) 389, 579 (1997)].

AGING-INDUCED NOVEL MULTI-SCALE EFFECT IN FERROELECTRICS AND ITS UNDERLYING MECHANISM

Aging, the gradual changing of materials properties with time, is a well observed but puzzling phenomenon in ferroelectrics. It lowers the reliability of ferroelectric devices, and is thus usually undesirable. However, based on a defect mediated reversible domain switching mechanism, we obtained large recoverable electro-strain in aged BaTiO 3-based ferroelectric crystals, which indicates aging is not always a detrimental effect; it might be beneficial for applications. Moreover, direct evidence for the reversible domain switching process was given by in situ polarizing microscope observation. The underlying microscopic origin was discussed in light of a symmetry-conforming short-range-order (SC-SRO) principle of point defects and electron paramagnetic resonance spectroscopy.

Sharp low frequency dissipative effects in tetragonal BaTiO3 ceramics

Mechanical anomalies (damping peaks sharper than Debye peaks, in contrast to a broad relaxation peak) were observed in tetragonal barium titanate ceramic via broadband viscoelastic spectroscopy at low frequencies (<10 Hz) at ambient temperature after aging at 90 °C for 15 h. The sharp peaks disappear after aging above the Curie point (150 °C for 10 h). Mechanical anomalies are tentatively attributed to negative stiffness heterogeneity, the mechanism of which is proposed based upon the theory of symmetry-conforming short-range order property of point defects.

Electron microscopic studies of the antiferroelectric phase in Sr 0.60Ca 0.40TiO 3 ceramic

The structural variants and their coexistence across the antiferroelectric phase transition in Sr 0.60Ca 0.40TiO 3 ceramic has been studied through transmission electron microscopy (TEM) at room temperature and ∼100 °C. A clear evidence of the presence of superlattice reflections, corresponding to the cell doubling along the c-axis of Pbnm (or b-axis along Pnma), occurring during paraelectric to antiferroelectric transition, has been obtained through selected area electron diffraction, convergent beam electron diffraction and lattice-resolution imaging. Coexistence of the Pbnm and Pbcm phases at room temperature has been observed and attributed to the strain/disorder-induced broadening of the first-order antiferroelectric phase transition. Drastic changes in the domain structure during Pbnm to Pbcm transformation have been observed. This clearly indicates that the antiferrodistortive transition responsible for the occurrence of the antiferroelectric phase is of completely different origin and it is not just an additional follow-up of the already-existing ordering due to a − a − c + tilt schemes in the Pbnm domain. Thermal cycling studies on microstructural changes indicate some kind of memory mechanism, which retains the memory of the original a − a − c + tilt schemes in the Pbnm phase. This has been attributed to the symmetry conforming short-range order (SC-SRO) of the point defects.

Migration and redistribution of oxygen vacancy in barium titanate ceramics

Degradation of barium titanate based multilayer capacitor mainly results from migration and redistribution of oxygen vacancy. For barium titanate ceramics, the authors observe an internal friction relaxation peak around 70°C due to oxygen vacancy, and its relaxation strength differs greatly for specimen aged at 85°C for 120h and at 150°C for 5h. Two possible explanations are proposed, one based on symmetry-conforming short-range order while the other on the interaction between oxygen vacancy and domain wall during aging process. In any case, relaxation thermodynamics is a powerful tool to investigate the migration and redistribution of oxygen vacancy in barium titanate ceramics.

The aging effect in Au-Cd alloys: A Mössbauer spectroscopy study

The martensite aging effect, observed in a number of alloy systems and through which fundamental properties for particular materials are altered by aging in the martensite state, has attracted considerable interest in recent years. It is important to understand and control the effect, particularly in shape-memory alloys for which stability of properties is fundamental to applications. The most likely model, at least for the Au-Cd system in which it has been most critically tested, is the symmetry conforming short-range order (SC-SRO) model first proposed by Ren and Otsika. 197 Au Mossbauer spectroscopy offers the possibility to observe the nature of defects associated with the gold sites. The spectra for single crystal and polycrystalline Au-Cd in both the aged martensite and quenched austenite conditions are presented and discussed. While some uncertainty in interpretation of the spectra remains, particularly in relation to any quadrupole splitting to be expected from either Au atoms on Cd-nearest-neighbour sites or the martensite lattice distortion, the best single-line fits provide evidence for two defect types.

Twin/domain pattern memory in aged Au-49.5Cd martensite

We performed in-situ optical microscopy observations on a Au-9.5Cd single crystal during heating/cooling and tensile loading/unloading runs. We found that for aged martensite domain pattern can be reproduced or memorized during heating/cooling or stress loading/unloading runs. This mesoscopic memory effect contradicts the basic notion of martensitic transformation that variants/domains are crystallographically and energetically equivalent It points to the interaction of point defects with martensitic transformation. We show that the interesting mesoscopic memory effect can be fully explained by a general symmetry property of point defects, the symmetry-conforming short-range order property (Ren and Otsuka, Nature, 1997).

Ageing in Parent Phase and Martensite Stabilization in a Ni<SUB>50</SUB>Ti<SUB>30</SUB>Hf<SUB>20</SUB> Alloy

The effect of ageing in parent phase and martensite stabilization on melt spun Ni 50 Ti 30 Hf 20 ribbons were investigated by Differential Scanning Calorimetry and X Ray Diffraction. Thermal treatments in parent phase generally shift the martensitic transformation range as far as an equilibrium state is reached. The equilibrium state, here found for a thermal treatment at 450°C (723 K), was adopted as start state to investigate ageing in martensite (commonly indicated as martensile stabilization). Martensite stabilization involves diffusion processes with an activation energy in the order of 1.2 eV: results add a piece of evidence to the general principle of Symmetry-Conforming Short-Range Order advanced by Ren and Otsuka.

Mechanism of martensite aging effects and new aspects

We reviewed recent progress in studying the long-standing puzzle of the martensite aging effect, which includes the rubber-like behavior of martensite and the martensite stabilization effect. Emphasis was laid on a recently proposed general principle called symmetry-conforming short-range order (SC-SRO), which states that there exists interaction between crystal symmetry and distribution of point defects such that the SRO tries to follow the crystal symmetry, if in equilibrium. In fresh (unaged) martensite SRO symmetry inherits the parent symmetry due to the diffusionless nature of the martensitic transformation, thus it does not match the crystal symmetry of martensite right after the transformation. This is an unstable state; as a result atomic diffusion occurs so as to correct the ‘incorrect’ SRO symmetry during the subsequent aging. This is what occurs during martensite aging. This mechanism fully explained all the known facts about martensite stabilization and rubber-like behavior. We further provided TEM evidence for the SC-SRO principle, and predicted two new effects, which are expected from the SC-SRO principle: (i) microstructure memory, and (ii) two-way shape memory (due to aging). Both were verified experimentally. The symmetry-conforming SRO property, which is a general property of crystalline materials, may also be applied to a wide range of phenomena concerning symmetry change.