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This study investigated the mechanisms and temperature dependence of the R phase variant reorientation deformation in a nanocrystalline NiTi wire by means of in-situ synchrotron high-energy X-ray diffraction technique during tensile deformation. The NiTi wire exhibited a single stage B2 → R transformation upon cooling with the R → B19′ martensitic transformation completely suppressed down to the liquid nitrogen temperature. The R phase variant reorientation deformation in tension was revealed to proceed in a Luders-manner, as evidenced by sudden changes of diffraction peak intensity and increases of lattice elastic strains of the R phase during the deformation. The variant reorientation obeys the principles to yield maximum crystallographic strain in the direction of the applied load. Thus, the variants with larger d-spacing values aligned to the axis of tension grew at the expense of those of lower d-spacing values. The increases of lattice elastic strains are attributed to the internal load transfer among the different crystallographic variants during the R phase reorientation. The crystallographic strain of R phase reorientation was found to increase with decreasing the deformation temperature. This corresponds directly to the continued evolution of the structure of the R phase of increased structural distortion relative the B2 phase with decreasing the temperature. This reflects the second order nature of the R phase.

VO2 exhibits metal-insulator transition (MIT) accompanied by structural phase transformation between rutile R phase and monoclinic M1 phase, and an intermediate monoclinic phase M2 may also emerge, stabilized by strain. The evolution of microstructures and properties across phase transition is not only critical for understanding the nature of MIT, but also important for numerous device applications, yet they are quite challenging to characterize. Utilizing advanced atomic force microscopy (AFM) techniques in combination with polarized light microscopy, X-ray diffraction, and Raman spectroscopy, we map the microstructure evolution of VO2 when it is heated from room temperature M1 phase to high temperature R phase through intermediate M2 phase, and acquire functional imaging of VO2 spanning these three phases as well. These result in quantitative mapping of electric conduction and Young's modulus of VO2 in one-to-one correspondence to its domain patterns, especially those with austenite-martensite interface between M2 and R phases. Young's modulus of M1, M2, and R phase of VO2 are determined to be 95 GPa, 65-117 GPa, and 98-100 GPa respectively, and significant anisotropy is observed in M2 phase. Rigorous continuum analysis has also been carried out to analyze the complicated domain pattern, validating our experimental observations that match theoretical expectation well.

A novel multiphase field theory for ferroelectric systems in the vicinity of a polymorphic phase boundary (PPB) is developed by coupling the Landau-Devonshire thermodynamic potentials of the individual phases. The model naturally predicts metastable coexistence of the rhombohedral (R) and tetragonal (T) phases near the PPB temperature, T P P B = 43 C ∘ , for the BZT-40BCT system, and provides a maximum temperature of coexistence, T C , 0 = 49.9 C ∘ , in agreement with experiments. For T > T P P B , results show that metastable coexistence of two ferroelectric phases is a result of a phase transformation-induced polarization rotation plus switching mechanism. Metastable domains of the low-temperature R phase coexist with the high-temperature, thermodynamically stable T phase for long periods of time, from minutes to hours. For T T P P B , the coexistence time is on the order of tens of seconds due to a decreased thermal energy that suppresses the polarization rotation plus switching mechanism. Further, the kinetics of macroscopic T → R phase transformation is accelerated by a large thermodynamic driving force and high mobility.

In this work, we designed a NiTiFe-Nb nanowire composite to achieve a high-stability and small-hysteresis R phase two-way shape memory effect (TWSME). The R-phase TWSME was achieved in the composite in an annealed state without thermomechanical training due to the inherent internal stresses associated with the Nb nanowires, as demonstrated by in situ synchrotron high-energy X-ray diffraction. Transmission electron microscopic analysis also revealed some details of the R-phase transformation process, which render an explanation of the high cyclic stability of the R-phase TWSME. The R-phase transformation in this NiTiFe-Nb nanowire composite was found to proceed over several stages, including the formation of precursor nanodomains of partial lattice distortion for the R phase, the formation of R phase particles of coordinated orientations, the coalescence of the R phase particles into selected plate variants of preferential orientations for TWSME, and the continued evolution of the R phase crystallographic structure for further TWSME with zero hysteresis. Such step-wise process of the R-phase transformation divides the total TWSME strain into segments of smaller magnitudes in the macroscopic behavior, and more importantly reduces the discrete lattice distortion mismatch at the transformation interface on the microscopic scale. This drastically reduces the chances of generating dislocations during the transformation process, thus rendering the high cyclic stability of the R-phase TWSME. Such intricate behavior may be closely related to the unique NiTiFe matrix – Nb nanowire microstructure and its unique inherent internal stress condition.

Precipitation of Ni4Ti3 is effective in tuning martensitic transformations (MTs) in NiTi shape memory alloys (SMAs). However, several fundamental issues concerning the influence of Ni4Ti3 on MTs remain unclear. In this study, the microstructure evolution process during precipitation and its influence on B2→R MT are investigated using computer simulations based on the phase field method. In particular, the roles played by Ni concentration gradient in the B2 matrix and internal coherency stress fields associated with different precipitate microstructures are analyzed in detail. It is found that Ni concentration gradient in the B2 matrix created by Ni4Ti3 precipitates alters significantly local martensitic start temperature and the overall MT behavior, while the internal coherency stress field associated with the Ni4Ti3 precipitates dictates the structure of martensitic domains via variant selection. The latter effect makes it promising to develop Invar alloys via stress-ageing to tailor the precipitate microstructure. These findings provide fundamental insights into the mechanism controlling the MT behavior in precipitate-bearing SMAs and provide guidance for the design of thermomechanical treatment for desired precipitate microstructures and the corresponding MT behavior.

By implementing a novel multiphase field model for ferroelectric systems, the phase coexistence of the tetragonal (T) and rhombohedral (R) phases in Pb-free BZT-40BCT was analyzed. Metastable coexistence of the T and R phases is predicted between a thermodynamic upper limit at T C , R = 49.90 ∘ C and a kinetic lower limit determined by the time-temperature-transformation behaviour. Predicted domain microstructures exhibit faceted domain walls and curved T-R phase interfaces that are consistent with recent TEM studies in the vicinity of the polymorphic phase boundary (PPB). Further, miniaturization of the domain structure near the PPB is a result of the relatively low interfacial energies and a pinning effect caused by the large metastable phase fraction that originates from the vanishing macroscopic driving force for phase transformation. Particularly, the vanishing of rhombohedral domain wall energies as T → T C , R enables a phase transformation-induced polarization rotation mechanism and predicts a hierarchical domain morphology for the R phase. These results are in agreement with the higher piezoelectric response reported near the maximum temperature for R+T coexistence and the observations of a miniaturized nanodomains structure within micron-sized, wedge-shaped domains in the R phase for the BZT- x BCT system.

The lack of information about point defects presents a big hurdle to a complete understanding of the premartensitic phenomena in shape memory alloys. Using first-principles density functional theory calculations, we have studied the behavior of substitutional Fe in both B2 and R phases of Ti 50 Ni 50− x Fe x , at different concentrations. We show that with no need of aggregation, an Fe atom can incur a drastic atomic scale local lattice distortion in the R phase and makes its nearest neighbor environment like an intermediate structure between B2 and R. In addition, its solution energy is lower in B2 than in R phase, indicating that Fe tends to stabilize the austenite against the martensite. This means that Fe exerts an anti-precursor effect in the B2 to R transformation, in contrast to the assumption that Fe fosters the growth of R-like nanodomains. The Fe–Fe interaction is very weakly attractive in both phases, suggesting that only short-range variation in concentration is possible and the formation of precipitates is unlikely. The difference in cohesive energy of B2 and R phases, and the martensitic transformation temperature as well, decreases with the increasing Fe concentration, until the Fe content exceeds a critical level at which point the R phase turns into B2 phase automatically and the martensitic transformation is suppressed altogether. Such a complete suppression is an excellent analogue to the concept of frozen strain glass. Since Fe atoms at different Ni sites in R phase have divergent solution energy, the anti-precursor effect of Fe may vary at different spot in the material, and so will be the local transformation temperature.

New data are presented on dynamic measurements of elasticity and anelasticity of several poly- and single crystalline Ni1+xTi1-x alloys for frequencies between 0.05 and 100 kHz over a wide temperature range below room temperature. Anelastic effects, previously attributed to "strain glass freezing", exist and are even intensified in alloys close to the equiatomic composition, which are not expected to demonstrate the "strain glass" state. We also compare the results of static zero-field-cooling and field-cooling experiments for normally transforming and non-transforming alloys and show that a strain maximum in static zero-field-cooling tests for high-Ni-content alloys attributed to "strain glass freezing" is due to elastic strain, which was not previously taken into account. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

After aging at intermediate temperatures (400–500 °C), Ni-rich Ti–Ni alloys undergo an abnormal 3-stage martensitic transformation behavior (1-stage R and 2-stage B19′), which stems from a preferential Ti 3 Ni 4 precipitation around grain boundary. On the other hand, if aged at low-temperatures (250–300 °C), they undergo 2-stage R-phase transformation, but the origin of this strange phenomenon is unclear. In the present study, we made a systematic study of this phenomenon by considering the grain boundary effect and composition effect. We found that all single crystals undergo 1-stage R-phase transformation; in contrast, the transformation behavior of polycrystals is dependent on Ni content: low-Ni (50.6Ni, 51Ni) polycrystals undergo 2-stage R-phase transformation while high-Ni (52Ni) polycrystals undergo 1-stage R-phase transformation. The abnormal 2-stage R-phase transformation is attributed to a large-scale compositional heterogeneity in B2 matrix between grain boundary region and grain interior, due to the heterogeneity in precipitate density between the grain boundary and grain interior. But for high-Ni polycrystals, precipitates are essentially homogeneously distributed across the whole grain and this leads to normal 1-stage R-phase transformation. The different transformation behavior of low-Ni and high-Ni polycrystals stems from a competition between two opposing tendencies: (1) for preferential precipitation in the grain boundary; (2) for homogeneous precipitation across the whole grain with high-Ni content. The difference between the effect of intermediate-temperature and low-temperature aging lies in the difference in the ability for long-range diffusion of Ni (from the grain interior to the grain boundary), which results in whether or not Ti 3 Ni 4 precipitates can form in the grain interior. Our results lead to a unified explanation for different transformation behaviors of both low-temperature and intermediate-temperature aged alloys in terms of the kinetics of precipitation in supersaturated polycrystals.

This study investigated the effects of low temperature ageing on the transformation behaviour of a Ti – 50.9at.%Ni alloy. It was found that ageing in the temperature range of 473–573 K induced a two-stage R-phase transformation, which was followed by a single-stage martensitic transformation at a lower temperature. The transformation behaviour was analyzed by means of differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and transmission electron microscopy (TEM). DSC measurements revealed two separate transformation heat peaks, each with a small thermal hysteresis. XRD measurements demonstrated a two-stage evolution of diffraction intensities from the B2 phase and from the R-phase. Transformation temperatures determined from these measurements exhibited good correlation. TEM examinations revealed the presence of fine coherent Ti3Ni4 precipitates. Based on these observations it is identified that the first R-phase transformation on cooling was associated with the formation of the precipitates whereas the second R-phase transformation at a lower temperature was from the matrix away from precipitates. The martensitic transformation was associated with the second R-phase transformation at the lower temperature, i.e., it was an incomplete transformation from the regions away from precipitates. The occurrence of the multiple-stage R-phase transformation is attributed to precipitation-induced inhomogeneity of the matrix, both in terms of composition and of internal stress fields.

A new type of the B19′ (monoclinic) Ni–Ti martensite—one which is internally twinned by the (0 0 1)m compound twinning mode, has been found in thermally cycled Ni–Ti shape memory alloys. This is an unexpected and remarkable martensite type on account of the fact that the (0 0 1)m compound twinning mode does not qualify to occur as the fine structure in the B2–B19′ martensite transformation. On the basis of the good concurrence of the observed crystallographic parameters with those predicted by the phenomenological theory of martensite transformations, it has been determined that this martensite is a product of the R phase-B19′ martensite transformation. However, the (0 0 1)m compound twinning mode can qualify to occur as the lattice invariant shear (LIS) only when the rhombohedral angle of the R phase is less than the critical value of 86.2°. The preference of this twinning mode as the LIS to the [0 1 1]m Type II twinning mode, which is the normally observed substructure of the Ni–Ti martensites, has been rationalized to be due to the closer proximity of the orientation relationships to the lattice correspondence and the lower magnitudes of twinning shear, shape strain shear, twin interface energy and nucleation strain energy.