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So far shape memory alloys (SMA) are mostly characterized by their thermo‐mechanical behavior due to the underlying thermal effect. In technical applications however, where their benefits like low weight and compact design become relevant, they are activated electrically. This work presents methods for a thorough and systematic characterization of SMA wire samples under Joule heating with the focus on aspects relevant for applications. The goal is to achieve a precise understanding of the sensor and actuator properties of SMA wire samples with different trainings under varying loads. All experiments are conducted on a custom designed test bench with a commercially available NiTi wire with 72 μm diameter, which enables the direct comparisons of tensile tests to actuation tests. The characterization consists of tensile tests and actuator tests with varying load and heating power for differently trained wire samples. The results vividly represent the influence of heating power, training and changing loads on stroke output, working point and the functional stability of SMA actuator wires. Especially, the evolution of the resistance signal and the influence of the R‐phase on self‐sensing is discussed. The proposed method enables to compare and choose the best suitable alloy with a fitting training for a desired application. [ABSTRACT FROM AUTHOR]

One of the most important challenges in endodontics is to have files that have excellent flexibility, toughness, and high fatigue life. Superelastic NiTi alloys have been a breakthrough and the new R-phase NiTi alloys promise to further optimize the good properties of NiTi alloys. In this work, two austenitic phase endodontic files with superelastic properties (Protaper and F6) and two austenitic phase files with the R-phase (M-wire and Reciproc) have been studied. The transformation temperatures were studied by calorimetry. Molds reproducing root canals at different angles (30, 45, and 70°) were obtained with cooling and loads simulating those used in the clinic. Mechanical cycles of different files were realized to fracture. Transformation temperatures were determined at different number of cycles. The different files were heat treated at 300 and 500 °C as the aging process, and the transformation temperatures were also determined. Scanning and transmission electron microscopy was used to observe the fractography and precipitates of the files. The results show that files with the R-phase have higher fracture cycles than files with only the austenitic phase. The fracture cycles depend on the angle of insertion in the root canal, with the angle of 70° being the one with the lowest fracture cycles in all cases. The R-Phase transformation increases the energy absorbed by the NiTi to produce the austenitic to R-phase and to produce the martensitic transformation causing the increase in the fracture cycles. Mechanical cycling leads to significant increases in the transformation temperatures Ms and Af as well as Rs and Rf. No changes in the transformation temperatures were observed for aging at 300 °C, but the appearance of Ni4Ti3 precipitates was observed in the aging treatments to the Nickel-rich files that correspond to those with the R transition. These results should be considered by endodontists to optimize the type of files for clinical therapy. [ABSTRACT FROM AUTHOR]

Abstract So far shape memory alloys (SMA) are mostly characterized by their thermo‐mechanical behavior due to the underlying thermal effect. In technical applications however, where their benefits like low weight and compact design become relevant, they are activated electrically. This work presents methods for a thorough and systematic characterization of SMA wire samples under Joule heating with the focus on aspects relevant for applications. The goal is to achieve a precise understanding of the sensor and actuator properties of SMA wire samples with different trainings under varying loads. All experiments are conducted on a custom designed test bench with a commercially available NiTi wire with 72 μm diameter, which enables the direct comparisons of tensile tests to actuation tests. The characterization consists of tensile tests and actuator tests with varying load and heating power for differently trained wire samples. The results vividly represent the influence of heating power, training and changing loads on stroke output, working point and the functional stability of SMA actuator wires. Especially, the evolution of the resistance signal and the influence of the R‐phase on self‐sensing is discussed. The proposed method enables to compare and choose the best suitable alloy with a fitting training for a desired application.

This study aimed to evaluate and compare the effects of ambient temperature and post-manufacture heat-treatment on the mechanical behavior of nickel-titanium (NiTi) wires. Four types of commercial NiTi variants (Stock NiTi, heat treated "Blue", "Gold", "Superflex", all Dentsply Maillefer, Ballaigues, Switzerland) were stressed in a tensile testing machine in a temperature-controlled water bath at three different temperatures. Stress and strain values were extrapolated from the raw data, and 2-way ANOVA and Tukey's test for multiple comparisons were performed to compare the differences of the mechanical constants. Differential scanning calorimetry (DSC) tests established the martensitic transformation starting (Ms), finishing (Mf) and austenitic (reverse-martensitic) starting (As) and finishing (Af) points. Austenitic modulus of elasticity and transformation stress values increased with temperature for all NiTi groups. The martensitic modulus of elasticity, maximum transformation strain and ultimate tensile stress were not significantly affected by temperature changes. Stock NiTi and Gold wire samples presented with clearly delineated austenitic and martensitic transformations in the DSC experiments. Differences in manufacturing/heat treatment conditions and ambient temperature affect the mechanical behavior of nickel-titanium and may have clinical implications. Further improvements to the experimental setup could be considered to provide more accurate measurements of strain. [ABSTRACT FROM AUTHOR]

The ability to control the stress-induced phase transformation of the shape memory alloy, NiTi, is an important technological challenge that must be understood for their wide application in devices that can exploit their reversible strain properties. This study elucidates the direct relationship between dislocation density and the martensitic, B19' & R -phase transformations, including its formation temperature from interrupted annealing of rolled NiTi samples. Deformation is shown to determine the enthalpy change required for the B2→R→B19' transformation, with associated transformation temperatures being modifiable via dislocation density and recovery processes. Recovery is shown to be rapid, highly heterogeneous and sensitive to crystal orientation. Grains with a 〈100〉 direction close to the macroscopic rolling direction recover more rapidly than 〈110〉 and 〈111〉 orientated grains. Considered to be governed by processing induced residual stresses and resultant crystallographic dependent annihilation/slip pathways, there are opportunities to tune B2→R→B19' transformation on either a grain-averaged or an orientation dependant per-grain basis.   [ABSTRACT FROM AUTHOR]

Objectives: This study aims to evaluate the effects of cyclic loading on the bending moments and the developed stress state of austenitic and R-phase endodontic files through finite element analysis. Materials and methods: The mechanical properties of two groups of NiTi wires, austenite and R-phase, were measured in samples at two different conditions: uncycled and cycled. The cycled condition was achieved by subjecting samples of the two groups to 80% of the corresponding fatigue life under rotating bending efforts. The measured mechanical properties were then used in the finite element analysis, where the boundary and loading conditions were set to replicate a standard bending test. Results: The results showed that mechanical cycling leads to decreasing stress levels and bending moments in the simulated files, especially in the austenitic ones. In comparison with austenite, R-phase presented a more stable mechanical behavior during cycling. Conclusions: The results show that the moment and stress calculated for an instrument under bending can be considerably decreased after some cyclic work. Clinical relevance: The fatigue related to the clinical use of an endodontic file decreases the moment (as well as the forces) imposed by the instrument during the shaping of a curved root canal. This decrease is directly related to the type of atomic array present in the alloy. [ABSTRACT FROM AUTHOR]

An intermetallic phase in the Al–Mn–Ni system crystallizing in space group Cmcm (No. 63) and refined formula Al61.49Mn11.35Ni4 (called the R′ phase) has been synthesized by high-temperature sintering of a mixture with initial chemical composition Al60Mn7Ni3. In comparison with the structure model of the previously reported R phase with composition Al60Mn11Ni4 [Robinson (1954). Acta Cryst. 7, 494–497], there are two mutually exchanged Mn and Ni sites together with one positionally disordered Al site [occupancy ratio 0.811 (8):0.121 (7)] and one partially occupied Mn site [s.o.f. 0.677 (5)] in the current structure model of the R′ phase.

Abstract Background The aim of this study was to evaluate the mechanical behavior of M-wire, controlled memory (CM), and R-phase files under bending and torsion conditions and compare it to the mechanical response of NiTi file with the same geometry using finite element analysis. Methods A geometric model of HyFlex file size 25 and taper 0.06 was generated by micro-computed tomographic and stereomicroscope scanning. The file FE model was built using MATLAB and SolidWorks software. Four FE files models were constructed with different material properties, the data for the M-wire, CM, R-phase, and NiTi alloys were obtained from the literature. The mechanical behavior of the different models under bending and torsion was analyzed mathematically in SolidWorks software package. Results Under bending conditions, the maximum Von Misses stress value was related to NiTi file model (330 MPa), followed by M-wire (311 MPa), then CM file model (191 MPa), while the least amount of stress value was related to R-phase file model (169 MPa). When torsion test was performed the maximum stress value was also related to NiTi file model (270 MPa), followed by M-wire (261 MPa), then CM file model (191 MPa), while the least amount of stress value was related to R-phase file model (188 MPa). Conclusions Metallurgical improvement of rotary files resulted in increasing the flexibility and the torsional resistance of these files.

An important group of NiTi-based alloys employed in applications exhibits an intermediate R-phase transformation, which substantially modifies the material's thermomechanical response. Finite element analysis of products from such alloys requires a constitutive model covering the two-stage transformation sequence in its full thermodynamic nature. We propose an enhanced constitutive model for NiTi-based alloys, which incorporates R-phase as a distinctive phase with loading-dependent transformation strain. We validate the model on a newly acquired extensive experimental dataset on the thermomechanical response of Ni48Ti49Fe3 shape memory alloy and illustrate the capabilities of its implementation into finite element software on a computational structural analysis of a tightening ring.

Aim: The aim of this systematic analysis was to assess the prevalence of dentinal microcracks at various levels (3, 6, and 9 mm from the apex) after using instruments made with conventional, R-Phase, and M-Wire NiTi alloys and the SAF system. Materials and Methods: Electronic searches were conducted in the databases Embase, Cochrane Library, Scopus, PubMed, and Web of Science. To arrange search methods, "MeSH" terms and/or keywords typically associated with the subject were paired with the Boolean operators "AND" and "OR." Additional searches were conducted on the websites of four separate endodontic journals. After reading the titles and excluding duplicates, 1000 of the 1343 documents originally found were eliminated. Upon reviewing the abstracts, 310 of the remaining 343 experiments were also eliminated. Based on qualifying requirements, only 13 of the remaining 33 articles were included in the qualitative review. Results: All systems triggered dentinal microcracks; however, when chemo-mechanical preparation was performed using Self-Adjusting File (SAF) and systems manufactured with R-phase technology—K3XF and Twisted File Adaptive (TFA)—less of these defects were found when compared to those manufactured with traditional NiTi—ProTaper Universal and Mtwo—and with M-Wire—ProTaper Next, Reciproc, and WaveOne. Conclusions: A lower prevalence of dentinal microcracks was observed after using SAF and endodontic systems manufactured with R-phase. [ABSTRACT FROM AUTHOR]

Background/purpose: Heat treatment of nickel–titanium (NiTi) alloy produces a better arrangement of the crystal structure, thereby leading to increased flexibility and improved fatigue resistance or plastic behavior. This study aimed to assess the performance of various heat-treated NiTi rotary instruments in S-shaped resin canals. Materials and methods: Forty S-shaped resin canals were instrumented (10/group) with either Twisted Files (R-phase), WaveOne (M-wire), Hyflex CM, or V Taper 2H (CM-wire) with the same apical size and taper (25/0.08). Each S-shaped resin canal was scanned both before and after instrumentation with microcomputed tomography. Changes in canal volume and transportation were evaluated at regular intervals (0.5 mm). Differences between instruments at the apical curve, coronal curve, and straight portion of the canals were analyzed statistically. Results: All tested instruments caused more transportation at the coronal rather than apical curvatures, with the exception of Twisted Files for which apical transportation was the highest for any instrument or location (P0.05), whereas Hyflex CM created the most conservative preparations at the coronal curve (P

Aim: The aim of this systematic analysis was to assess the prevalence of dentinal microcracks at various levels (3, 6, and 9 mm from the apex) after using instruments made with conventional, R-Phase, and M-Wire NiTi alloys and the SAF system. Materials and Methods: Electronic searches were conducted in the databases Embase, Cochrane Library, Scopus, PubMed, and Web of Science. To arrange search methods, “MeSH” terms and/or keywords typically associated with the subject were paired with the Boolean operators “AND” and “OR.” Additional searches were conducted on the websites of four separate endodontic journals. After reading the titles and excluding duplicates, 1000 of the 1343 documents originally found were eliminated. Upon reviewing the abstracts, 310 of the remaining 343 experiments were also eliminated. Based on qualifying requirements, only 13 of the remaining 33 articles were included in the qualitative review. Results: All systems triggered dentinal microcracks; however, when chemo-mechanical preparation was performed using Self-Adjusting File (SAF) and systems manufactured with R-phase technology—K3XF and Twisted File Adaptive (TFA)—less of these defects were found when compared to those manufactured with traditional NiTi—ProTaper Universal and Mtwo—and with M-Wire—ProTaper Next, Reciproc, and WaveOne. Conclusions: A lower prevalence of dentinal microcracks was observed after using SAF and endodontic systems manufactured with R-phase.

This work deals with structural, electrical and mechanical characterization of Ti‒50.13Ni and Ti‒49.62Ni (at.%) shape memory alloys (SMAs) fabricated at different circumferential wheel velocities. The effect of wheel velocity, chemical composition and heat treatments are investigated. The characterization of crystallographic phases of the Ti‒Ni ribbons was carried out using X-ray diffraction. Electrical resistance variations as function of temperature (∆R/R %) were analyzed using a non-commercial technique, which consists in a thermal-adjustable bath apparatus revealing the temperatures of B2→R→B19´ two stage transformation, whereby the presence of R‒phase can be definitively confirmed. The Stress-Assisted Two-Way Memory Effect was measured by an own designed apparatus with an Linear Variable Differential Transformer captor and a current controlled heating, and results indicate that the as-spun condition, promotes the Stress-Assisted Two-Way Memory Effect. On the other hand, increments in Ni content tend to decrease transformation temperatures and high wheel velocities help to the R‒phase formation.

In this present study, the influence of isothermal aging temperature and duration on microstructural and mechanical properties of a hot-deformed UNS S32750 super duplex stainless steel (SDSS) alloy was investigated by SEM-EBSD (scanning electron microscopy-electron backscatter diffraction) and tensile testing techniques. An isothermal aging treatment, at temperatures between 400 and 600 °C and treatment duration between 3 and 120 h, was applied to a commercial UNS S32750 SDSS alloy. Microstructural characteristics of all thermomechanical (TM) processed states, such as distribution and morphology of constituent phases, grain’s modal orientation (MO), and obtained mechanical properties were analysed correlated with the TM processing conditions. The obtained experimental results show that the constituent phases, in all TM processed states, are represented by elongated γ-phase grains within the δ-phase matrix. The R-phase was observed in the case of aging treatment performed at 600 °C for 120 h. Within the δ-phase matrix, dynamically recrystallized grains were identified as a result of applying hot deformation and isothermal aging treatments. Also, it was observed that aging treatment parameters can significantly influence the mechanical behaviour exhibited by the UNS S32750 SDSS alloy, in terms of elongation to fracture and absorbed energy during impact testing.

A novel arc heat treatment technique was applied to design a uniquely graded super duplex stainless steel (SDSS), by subjecting a single sample to a steady state temperature gradient for 10 h. A new experimental approach was used to map precipitation in microstructure, covering aging temperatures of up to 1430 °C. The microstructure was characterized and functionality was evaluated via hardness mapping. Nitrogen depletion adjacent to the fusion boundary depressed the upper temperature limit for austenite formation and influenced the phase balance above 980 °C. Austenite/ferrite boundaries deviating from Kurdjumov–Sachs orientation relationship (OR) were preferred locations for precipitation of σ at 630–1000 °C, χ at 560–1000 °C, Cr 2 N at 600–900 °C and R between 550 °C and 700 °C. Precipitate morphology changed with decreasing temperature; from blocky to coral-shaped for σ, from discrete blocky to elongated particles for χ, and from polygonal to disc-shaped for R. Thermodynamic calculations of phase equilibria largely agreed with observations above 750 °C when considering nitrogen loss. Formation of intermetallic phases and 475 °C-embrittlement resulted in increased hardness. A schematic diagram, correlating information about phase contents, morphologies and hardness, as a function of exposure temperature, is introduced for evaluation of functionality of microstructures. [ABSTRACT FROM AUTHOR]

Introduction The purpose of this study was to assess the effects of sodium hypochlorite (NaOCl) immersion and sterilization on the cyclic fatigue resistance of heat-treated nickel-titanium (NiTi) rotary instruments. Methods Two hundred ten new 25/.06 Twisted Files (TFs; SybronEndo, Orange, CA) and Hyflex CM (Coltene Whaledent, Cuyahoga Falls, OH) files were divided into 7 groups ( n = 15) for each brand. Group 1 (control group) included new instruments that were not immersed in NaOCl or subjected to autoclave sterilization. Groups 2 and 3 were composed of instruments dynamically immersed for 3 minutes in 5% NaOCl solution 1 and 3 times, respectively. Groups 4 and 5 consisted of instruments only autoclaved 1 and 3 times, respectively. Groups 6 and 7 recruited instruments that received a cycle of both immersion in NaOCl and sterilization 1 and 3 times, respectively. Instruments were subsequently subjected to a fatigue test. The surface morphology of fractured instruments was studied by field-emission scanning electron microscopy and x-ray energy-dispersive spectrometric (EDS) analyses. The means and standard deviations of the number of cycles to failure (NCF) were calculated and statistically analyzed using 2-way analysis of variance ( P < .05). Results Comparison among groups indicated no significant difference of NCF ( P > .05) except for the groups of TFs sterilized 3 times without and with immersion in NaOCl ( P < .05). HyFlex CM files exhibited higher cyclic fatigue resistance than TFs when files were sterilized 3 times, independently from immersion in NaOCl ( P < .05). EDS analysis showed the presence of an oxide-rich layer on the Hyflex CM files’ external surface. No morphologic or chemical differences were found between files of the same brand subjected to different treatments. Conclusions Repeated cycles of sterilization did not influence the cyclic fatigue of NiTi files except for TFs, which showed a significant decrease of flexural resistance after 3 cycles of sterilization. Immersion in NaOCl did not reduce significantly the cyclic fatigue resistance of all heat-treated NiTi files tested. [ABSTRACT FROM AUTHOR]

NiTi precipitates are formed in Ni-rich NiTi shape memory alloys (SMAs) after a certain heat treatment. Such a treatment then results in the changed phase transformation behavior of the alloy switching from the one-step reversible phase change B2 ↔ B19′ to the two-step, B2 ↔ R ↔ B19′, phase change. The present study reports a two-way shape memory (TWSM) due to an all-round treatment followed by an R-phase constraint aging at room temperature. The enhanced TWSM behavior was observed upon temperature cycling between 273 K (R-phase) and 77 K (B19′ phase). The effect of various constraint strains in R-phase aging by employing different diameters of the constraint ring was studied. However, the TWSM effect due to constraining the R-phase will be eliminated after a temperature rise of specimen to the fully parent phase (373 K). [ABSTRACT FROM AUTHOR]

The shape memory mechanism associated with R-phase transformation was investigated using electromechanical tests on Ni-Ti shape memory alloy thin wires. To provide a more precise insight into the effects of R-phase on phase transformation of shape memory alloys, wires were prepared in three different initial states prior to conducting the experiments: pure martensite, mixture of R-phase and martensite, and pure R-phase. The electromechanical tests were done in two different loading states (under constant and variable stresses) using specially designed and manufactured apparatuses. Tests under different constant stresses were carried out for three cases of pure martensite, a mixture of martensite and rhombohedral, and pure rhombohedral phase, while those under variable stresses were done on pure martensite as well as mixed martensite and rhombohedral. For all the expressed cases, strain-time responses were investigated during electric heating-cooling cycles to interpret possible microstructural changes in the samples. According to the results, a phase diagram containing R-phase transition strip was proposed for Ni-Ti shape memory alloys. [ABSTRACT FROM AUTHOR]

Background/purpose Heat treatment of nickel–titanium (NiTi) alloy produces a better arrangement of the crystal structure, thereby leading to increased flexibility and improved fatigue resistance or plastic behavior. This study aimed to assess the performance of various heat-treated NiTi rotary instruments in S-shaped resin canals. Materials and methods Forty S-shaped resin canals were instrumented (10/group) with either Twisted Files (R-phase), WaveOne (M-wire), Hyflex CM, or V Taper 2H (CM-wire) with the same apical size and taper (25/0.08). Each S-shaped resin canal was scanned both before and after instrumentation with microcomputed tomography. Changes in canal volume and transportation were evaluated at regular intervals (0.5 mm). Differences between instruments at the apical curve, coronal curve, and straight portion of the canals were analyzed statistically. Results All tested instruments caused more transportation at the coronal rather than apical curvatures, with the exception of Twisted Files for which apical transportation was the highest for any instrument or location (P < 0.05). The transportation was mostly influenced by the alloy type rather than their cross-sectional characteristics (P < 0.05). The volumetric increase after instrumentation was similar for all tested instruments at the apical curve (P > 0.05), whereas Hyflex CM created the most conservative preparations at the coronal curve (P < 0.05). At the straight portion, volumetric changes were largest for Twisted Files and smallest for V Taper 2H (P < 0.05). Conclusion Amongst heat-treated NiTi instruments, the CM-wire based instruments created the most favorable preparations in S-shaped resin canals. [ABSTRACT FROM AUTHOR]

• Ni 50.2 Ti 49.8 wire with R-phase as the matrix was produced in the as-drawn state. • Cyclic phase transformation in Ni 50.9 Ti 49.1 initiates microcracks in phase boundaries. • The absence of R-phase to B19′ transformation in Ni 50.2 Ti 49.8 helps avoid microcracks. • The fatigue life of Ni 50.2 Ti 49.8 is approximately 12 times that of Ni 50.9 Ti 49.1. Ni 50.2 Ti 49.8 and Ni 50.9 Ti 49.1 wires in cold-drawn state were compared regarding tensile fatigue life and fatigue fracture mechanism. Through process control, Ni 50.2 Ti 49.8 with R-phase matrix at room temperature was achieved in as-drawn state without post heat treatment. The R-phase imparts exceptional fatigue performance to Ni 50.2 Ti 49.8 wire with fatigue life approximately twelve times of that for Ni 50.9 Ti 49.1 which comprises B2 phase and a small amount of B19′ under the same test conditions. The cyclic phase transformation under loading initiates microcracks in phase boundaries, leading to premature fracture of Ni 50.9 Ti 49.1. In contrast, the R-phase presents exceptional stability against fatigue failure upon cyclic loading. [ABSTRACT FROM AUTHOR]

This article studies changes observed on the R-phase thermoelastic behavior in a near-equiatomic Ti-Ni shape memory alloy. Three kinds of procedures have been performed: different treatments, thermomechanical cycling under constant loading in shape memory helical springs and thermal cycling in as-treated and trained samples. Several heat treatments were carried out to investigate evolution of the R-phase by differential scanning calorimetry (DSC). A heat treatment was chosen on which R-phase is absent. Shape memory springs were produced and submitted to a training process in an apparatus by tensioning the springs under constant loading. Thermal cycling in DSC was realized in as-treated and trained samples. Several aspects of one-step (B2→B19') and two-steps (B2→R→B19') martensitic transformations and R-phase formation and their evolution during tests were observed and discussed.

This work is concerned with different estimates of the quasiconvexification of multi-well energy landscapes of NiTi shape memory alloys, which models the overall behavior of the material. Within the setting of the geometrically linear theory of elasticity, we consider a formula of the quasiconvexification which involves the so-called energy of mixing.We are interested in lower and upper bounds on the energy of mixing in order to get a better understanding of the quasiconvexification. The lower bound on the energy of mixing is obtained by convexification; it is also called Sachs or Reuß lower bound. The upper bound on the energy of mixing is based on second-order lamination. In particular, we are interested in the difference between the lower and upper bounds. Our numerical simulations show that the difference is in fact of the order of 1% and less in martensitic NiTi, even though both bounds on the energy of mixing were rather expected to differ more significantly. Hence, in various circumstances it may be justified to simply work with the convexification of the multi-well energy, which is relatively easy to deal with, or with the lamination upper bound, which always corresponds to a physically realistic microstructure, as an estimate of the quasiconvexification. In order to obtain a potentially large difference between upper and lower bound, we consider the bounds along paths in strain space which involve incompatible strains. In monoclinic shape memory alloys, three-tuples of pairwise incompatible strains play a special role since they form so-called T -configurations, originally discussed in a stress-free setting. In this work, we therefore consider in particular numerical simulations along paths in strain space which are related to these T -configurations. Interestingly, we observe that the second-order lamination upper bound along such paths is related to the geometry of the T -configurations. In addition to the purely martensitic regime, we also consider the influence of adding R-phase variants to the microstructure. Adding single variants of R-phase is shown to be energetically favorable in a compatible martensitic setting. However, the combination of several R-phase variants with compatible or incompatible martensite yields significant differences between the bounds considered. [ABSTRACT FROM AUTHOR]

Textured polycrystals of NiTi-based shape memory alloys (SMA) exhibit pronounced anisotropic properties which significantly influence their response to mechanical and thermal loading. In this work, a constitutive model tailored for non-proportional multi-axial loading of NiTi SMA exhibiting two-stage phase transformation via R-phase is enhanced so that the anisotropy of martensitic structure is captured. Numerical simulations of the mechanical response of a NiTi SMA helical spring subjected to thermal cycling at a constant applied force are performed and compared with experimental data. Quantitative correspondence between experiments and simulations demonstrates the predictive potential of the model. Simulations also provide detailed information on the evolution of distributions of phase fractions and stress within a cross-section of the wire forming the spring. Because the loading is non-proportional, the evolution is rather complex and intriguing. [ABSTRACT FROM AUTHOR]

Ti–50.3Ni (at%) alloy film was prepared by vacuum magnetron sputtering, followed by heat treatment at 460 °C for 40 min. The microstructure of the Ni Ti alloy film was studied using transmission electron microscopy. A nanoscale interwoven structure of B2 and R-phase was observed. The lattice correspondences between B2 and R-phase were analyzed using an electron diffraction pattern. The R-phase can nucleate in the precipitate-free area of the Ni Ti alloy film at a rapid cooling condition. The full-field strain of B2 and R-phase was mapped through geometric phase analysis technique. The measured strains are consistent with the eigenstrains of the R-phase. [ABSTRACT FROM AUTHOR]