R-phase Transformation Research Articles

Relevant Aspects in the Mechanical and Aging Degradation of NiTi Alloy with R-Phase in Endodontic Files.

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.

R-Phase Transformation Evolution in NiTi SMA Wires Studied via the Internal Friction Technique

The specific damping capacity variation of heat-treated NiTi was observed during a pseudoelasticity test. The detailed B2 → R-phase transformation process in cold-drawn NiTi wires undergoing middle-temperature aging was studied via X-ray diffraction, transmission electron microscope, and internal friction technique. Results show that, as aging time increased at 450 °C, the dynamic phase transition splitting from B2 → R to B2 → R1 and B2 → R2 became evident. However, such a splitting process was not observed for the sample after aging at 400 °C. The reason for R-phase generation is attributed to non-uniformly distributed stress fields. The splitting of the internal friction peak, in conjunction with high-resolution transmission electron microscope and mechanic results, suggests a substantial occurrence of short-range segregation of Ni atoms in the B2-NiTi matrix. Furthermore, the specific damping capacity (SDC) exhibits a gradual increase with prolonged annealing time. Specifically, the sample with significant dynamic phase transition splitting reaches an SDC value of 0.60.

Effect of Fe Composition and Heat Treatment Temperature on the Transformation Temperature of R-Phase of Ti-Ni-Fe Alloy

R-phase transformations in TiNi shape memory alloys (Nitinol) have various applications because of their small thermal hysteresis and low fatigue resistance. Adding a third element such as Fe or Al is one of the useful ways to induce R-phase transformation. Controlling the R-phase transformation temperature is crucial for industrial and bio-field applications. However, the effects of adding Fe have rarely been reported. In this study, the effects of Fe addition and heat treatment temperature on the R-phase transformation temperature of TiNiFe shape memory alloys were systematically investigated. Results showed that increasing Fe composition decreases the R-phase and martensitic transformation temperatures of 19K/1at%Fe and 51 K/1at%Fe, respectively. Additionally, in the Ti–49Ni–1Fe alloy, rather than a martensite transformation, the R phase transformation temperature was constant irrespective of heat treatment temperature and increasing number of thermal cycles. This means that the R-phase transformation temperature is not affected by dislocation density resulting from the heat cycling or cold working processes. This allows practical applications of R-phase transformation to be easily realized. Furthermore, this means that the R-phase transformation temperature is only affected by the Ni content of the matrix.

High-strength porous NiTi shape memory alloys with stable cyclic recovery properties fabricated using elemental powders

Based on previous research regarding the high-vacuum sintering of elemental powder (EP) NiTi alloys, this study investigates the preparation of high-strength, highly cyclically stable porous NiTi shape memory alloys. The microstructures, martensitic transformation behaviors, compressive and compressive recovery properties, and cyclic stabilities of the sintered and aged porous NiTi alloys were compared and analyzed. The porosity and average pore size increased with the addition of NaCl, with the oxygen content increasing slightly. The sintered alloy displayed an inhomogeneous distribution of the Ni4Ti3 precipitate, which exhibited a dispersed and homogeneous distribution after aging. The sintered porous NiTi alloy exhibited a multistage phase transformation. The martensitic transformation was suppressed after aging owing to changes in the size and distribution of the precipitates, and only R-phase transformation was observed. Porosity and aging precipitation influenced the dual yield points and recovery properties of porous NiTi. The pinning effect of aging precipitation increased the yield point and reduced the residual strain in the recovery. However, at a high porosity, the volume fraction of NiTi matrix decreased, resulting in a small strengthening contribution of precipitation. The disruption of porous structure was the dominant causes of irreversible strain. As the porosity increased, the porous structure became increasingly susceptible to damage during cyclic loading. Overall, the EP porous NiTi alloy exhibited a higher compressive strength, enhanced compression recovery properties and cyclic stability compared to those reported in previous studies. This is because of the optimization of high-temperature homogenization via sintering at 1250 °C and enhancement effects of aging precipitation. This study provides a novel approach for developing porous EP NiTi alloys.

Phase transformation behavior and mechanical properties of HyFlex EDM nickel-titanium endodontic rotary instrument: Evaluation at body temperature

Background/purposeTemperature-dependent phase compositional changes influence the mechanical properties of heat-treated nickel-titanium (NiTi) rotary instruments. This study evaluated the phase composition, bending properties, and cyclic fatigue resistance of HyFlex EDM NiTi rotary instruments against differently heat-treated and non-heat-treated NiTi instruments at body temperature (BT). Materials and methodsHyFlex EDM OneFile (EDM) instruments, heat-treated HyFlex CM (CM) and Twisted File (TF) instruments, and non-heat-treated K3 instruments (size #25/.08) were subjected to differential scanning calorimetry, and the martensitic, R-phase, and reverse transformation starting and finishing temperatures were determined. A cantilever bending test and a cyclic fatigue test were conducted at BT (37 °C ± 1.0 °C), and the bending load and number of cycles to failure (NCF) were recorded. Statistical analysis was performed using Kruskal–Wallis and Mann–Whitney U tests (α = 0.05). ResultsTF and K3 had reverse transformation finishing temperatures lower than BT, while those for EDM and CM were higher than BT. The bending loads at a 0.5 mm deflection were in the order of EDM < TF < CM < K3 (P < 0.05), and those at a 2.0 mm deflection were EDM < CM and TF < K3 (P < 0.05). EDM had the highest NCF among the four instruments (P < 0.05). ConclusionThe EDM instrument had a reverse transformation finishing temperature higher than BT indicating its martensite/R-phase composition at BT. The EDM instrument had superior flexibility and greater resistance to cyclic fatigue than the CM, TF, and K3 instruments at BT.

A Thermal, Mechanical, and Materials Framework for a Shape Memory Alloy Heat Engine for Thermal Management.

Shape memory alloy (SMA) heat engines possess an inherent property of sensing a change in temperature, performing work, and rejecting heat through the shape memory effect resulting from a temperature-induced phase transformation. This work presents a framework for the design and implementation of an SMA-based Stirling heat engine for maximum torque or speed incorporating and combining mechanical, thermal, and material aspects. There is a growing need for such engines for reliable thermal management and energy recovery in both ground and space applications. Mechanical aspects were addressed from force balances in the SMA element and focused on the resulting stress distribution. Thermal aspects considered heat transfer between the SMA element and both the heat source and the heat sink. Materials aspects considered the chemical, elastic, and frictional contributions to the enthalpy of the transformation. The roles of nano- and microstructure through composition, precipitates, variant interfaces, training, cycling, texture, defects, nucleation sites (bulk vs. surface), and multi-step transformations (e.g., a trigonal R-phase transformation) in NiTi based-alloys are also emphasized. The aforementioned aspects were combined to present a figure of merit to aid in the design and implementation of a Nitinol Stirling heat engine operating to maximize torque or maximize speed.

Influence of the thermomechanical behavior of NiTi wires embedded in a damper on its damping capacity: Application to a bridge cable

&lt;abstract&gt;&lt;p&gt;Thanks to high dissipation properties, embedding NiTi Shape Memory Alloys in passive damping devices is effective to mitigate vibrations in building and cable structures. These devices can inconceivably be tested directly on full-scale experimental structures or on structures in service. To predict their effectiveness and optimize the set-up parameters, numerical tools are more and more developed. Most of them consist of Finite Element models representing the structure equipped with the damping device, embedding parts associated with a superelastic behavior. Generally, the implemented behavior laws do not include all the phenomena at the origin of strain energy dissipation, but stress-induced martensitic transformation only. This article presents a thermomechanical behavior law including the following phenomena: (i) intermediate R-phase transformation, (ii) thermal effects and (iii) strain localization. This law was implemented in a commercial Finite Element code to study the dynamic response of a bridge cable equipped with a NiTi wire-based damping device. The numerical results were compared to full-scale experimental ones, by considering the above-mentioned phenomena taken coupled or isolated: it has been shown that it is necessary to take all of these phenomena into account in order to successfully predict the damping capacity of the device.&lt;/p&gt;&lt;/abstract&gt;

Improvement of tensile superelasticity by aging treatment of NiTi shape memory alloys fabricated by electron beam wire-feed additive manufacturing

For the first time, this work comprehensively studied the effectiveness of precipitation hardening achieved by aging treatment in improving the tensile superelasticity of NiTi alloys fabricated by electron beam wire-feed additive manufacturing (EBAM), which possesses inherent advantages in producing dense and oxidation-free structures. Aging treatments under three temperatures (450, 350, and 250 °C) and different durations were conducted, and the resultant performance of tensile superelasticity, together with the corresponding evolution of precipitation and phase transformation behavior were investigated for the EBAM-fabricated NiTi alloys. Results showed that by appropriate aging treatment, EBAM fabricated NiTi alloys could achieve excellent recovery rates of approximately 95% and 90% after the 1st and 10th load/unload cycle for a maximum tensile strain of 6%, which were almost the highest achieved so far by AM processed NiTi alloys and close to those of some conventional NiTi alloys. The improvement of tensile superelasticity benefited from the fine and dispersive Ni4Ti3 precipitates, which could be introduced by aging at 350 °C for 4 h or at 250 °C for 200 h. Moreover, the large amount of Ni4Ti3 precipitates would promote the intermediate R-phase transformation and bring a two-stage or three-stage transformation sequence, which depended on whether the distribution of the precipitation was homogeneous or not. This work could provide guidance for the production of NiTi alloys with good tensile superelasticity by EBAM or other additive manufacturing processes.

Cyclic degeneration of elastocaloric effect for NiTi shape memory alloy: Experimental observation and constitutive model

Based on the cyclic tension-unloading tests with various peak strains (4%-12%), the effect of loading level on the cyclic deformation and evolution of elastocaloric effect (eCE) for NiTi shape memory alloy (SMA) wires was investigated. Experimental results show that the transformation ratchetting and degeneration of eCE simultaneously occur during the cyclic deformation, and these two phenomena aggravate with the increase of loading level. Although the refrigeration capacity (maximum temperature drop in the reverse phase transformation process) exhibits a positive relativity with loading level in the 1st loading cycle, it changes non-monotonically with the variation of loading level in the steady-state cycle, which can be attributed to the interaction between the phase transformation and plasticity deformation. Moreover, it is found that the phase transformation between austenite (A) and martensite (M) phases occurs as a two-step process involving an intermediate phase, rhombohedral (R) phase. Then, a thermo-mechanically coupled cyclic constitutive model is constructed based on the fundamental laws of irreversible thermodynamics. Besides the thermo-elastic deformation, four different inelastic deformation mechanisms, i.e., R phase transformation (RT), martensite transformation (MT), plasticity in A and R phases (ARP), transformation-induced plasticity (TRIP) and their interactions are considered in the proposed model. The volume fractions of R phase, M phase, martensite affecting zone (MAZ) and the free zone (FZ) of mixed A + R phases are introduced. The thermodynamic driving forces for the four considered inelastic deformation mechanisms, i.e., RT, MT, ARP and TRIP are derived by the energy dissipation inequality and a new proposed Helmholtz free energy. Internal heat generation caused by the mechanical dissipation and the latent heat of phase transformation, and the heat balance equation are obtained by applying the conservation law of energy. The forward and reverse inheritances of dislocation (i.e., plastic deformation) caused by the moving martensite interfaces during MT are addressed. Adopting a first-order approximation of the constitutive equations and lumped analysis of heat transfer, the proposed constitutive model for a material point is extended to describe the overall responses of the whole wire. Finally, the predicted results for the cyclic deformation of NiTi SMA and correspondent evolution of eCE are compared against the experimental ones to validate the capability of the proposed model.

Occurrence of the R-phase with increased stability induced by low temperature precipitate-free aging in a Ni50.9Ti49.1 alloy

The R-phase transformation presents low thermal hysteresis, high stability against thermal cycling and ultrahigh internal friction (IF). However, a clear explanation for the occurrence of the R-phase after low temperature aging and systematic characterization of the condition are still lacking. In this study, the evolution of the phase transformation behaviors and IF values after aging at 250 °C for different times are investigated in a Ni50.9Ti49.1 alloy. Direct experimental evidence for the microstructure evolution of the B2 phase and the corresponding R-phase is provided utilizing in-situ transmission electron microscopy (TEM) techniques, together with geometric phase analysis (GPA). The reason for the occurrence of a nanodomain-structured R-phase after low temperature precipitate-free aging and the mechanism of ultrahigh intrinsic IF are illustrated. The results show that Ni segregation indicated by the localized strain fields is responsible for the formation of the R-phase after low temperature aging. Longer aging time causes larger element heterogeneity in the matrix, as a result, a wider existing temperature window for the R-phase. The ultrahigh intrinsic IF plateaus (IFInt = 0.120 ∼ 0.183) found in NiTi shape memory alloys (SMAs) are predominantly determined by the volume fraction of nanodomain boundaries. These findings provide basic insights into the R-phase formation mechanism and provide a simple way to adjust ultrahigh IF performance suitable for different application scenarios.

Small-scale confined R-phase transformation in Ni47Ti49Fe2-Nb2 alloy

This study investigated the transformation behaviour of small-scale confined B2→R phase transformation in a Ni47Ti49Fe2-Nb2 alloy. Such confined transformation was achieved by introducing a high density of dislocations in the alloy through warm wire drawing. In contrast to the thermally induced B2→R transformation in a conventional polycrystalline matrix with a low dislocation density, where the twined R phase plates of 25 nm in width are formed, the R-phase transformation in the highly confined matrix was found to produce small twinning-free R phase particles of 5 nm in size and to form a reduced c/a ratio.

Step-wise R phase transformation rendering high-stability two-way shape memory effect of a NiTiFe-Nb nanowire composite

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.

Structure and Multistage Martensite Transformation in Nanocrystalline Ti-50.9Ni Alloy

An electron microscopic study of the evolution of the size, morphology, and spatial distribution of coherent Ti3Ni4 particles with a change in the aging temperature in a nanocrystalline (NC) Ti-50.9 at % Ni alloy with an inhomogeneous grain–subgrain B2-austenitic nanostructure has been carried out. It was found that with an increase in the aging temperature, along with a change in the size and shape of Ti3Ni4 nanoparticles, their spatial distribution changes from location at dislocations to precipitates at subboundaries. Research has shown that the presence of different types of internal interfaces in the nanostructure contributes to the heterogeneous distribution of coherent Ti3Ni4 nanoparticles in the volume of the B2 matrix, which is associated with the precipitation of particles in the region of low-angle subboundaries and the suppression of the Ti3Ni4 precipitation in nanograins with high-angle boundaries. The difference in the structural-phase state of nanograins and subgrains regions is the main reason for the implementation of the anomalous R-phase transformation effect in the sequence of multistage martensitic transformations B2↔R↔B19′.

High-Stability and Small-Hysteresis R Phase Two-Way Shape Memory Effect of a Nitife-Nb Nanowire Composite

In this work, we designed a NiTiFe-Nb nanowire composite to realize 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. Besides, in situ transmission electron microscopy analyses 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 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 stage-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.

Experimentally validated constitutive model for NiTi-based shape memory alloys featuring intermediate R-phase transformation: A case study of Ni48Ti49Fe3

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.

A one-dimensional constitutive model for NiTi shape memory alloys considering inelastic strains caused by the R-phase transformation

In some experiments, an intermediate phase, i.e., the rhombohedral phase (R-phase), can be observed in the phase transformation process between austenite and martensite for NiTi shape memory alloys. To predict phase transformations among these three phases, a one-dimensional constitutive model for shape memory alloys considering the inelastic strains caused by the R-phase transformation is proposed. Firstly, a temperature-stress phase diagram reflecting martensitic transformation as well as R-phase transformation is provided. On the basis of this phase diagram, the novel model for shape memory alloys is developed in the framework of thermo-mechanics taking into account various inelastic deformation mechanisms, involving the independent phase transformation (e.g., the phase transformation between pure austenite and pure R-phase or the one between pure R-phase and pure martensite) and the mixed phase transformation (e.g., the phase transformation between mixed phases of ‘austenite + R-phase’ and pure martensite). Through numerical examples, it is demonstrated that the proposed model is capable to predict the deformation properties of shape memory alloys associated with the R-phase transformation, including shape memory effect and pseudo-elasticity, as well as internal sub-loops due to the incomplete phase transformation. Furthermore, the numerical results are compared with experimental results and show good agreement with the experimental ones, which confirms the feasibility of the proposed model.

A study on two R-phase transformations in intermediate temperature aged Ni-rich TiNiFe-based shape memory alloys

The effects of intermediate temperature aging on two-stage R-phase transformation behavior exhibited in Ti50-xNi48+xFe2 (x = 0.5 and 1.5) SMAs were investigated. In Ti48.5Ni49.5Fe2 SMA, no transformation occurred in the as solution-treated state, and B2↔R1 transformation was induced by Ti3Ni4 precipitates when the specimens were aged at 350 °C. When the specimens were aged at 450 °C, a two-stage R-phase transformation, B2↔R1 and B2↔R2, appeared. The B2↔R1 transformation, induced by Ti3Ni4 precipitates, occurred at higher temperature, whereas the B2↔R2 transformation, related to the Fe content in the matrix, occurred at lower temperature. The Ti49.5Ni48.5Fe2 SMA possesses a two-stage B2↔R2↔B19′ transformation in the as solution-treated state. When Ti49.5Ni48.5Fe2 specimens are aged at 350 °C/450 °C, R2→B19′ is suppressed and B2↔R1 appears as the aging time increases with R2↔B19′ disappearing and B2↔R1 appearing earlier when the aging temperature is higher. The two-stage R-phase transformation behavior exhibited in these two aged TiNiFe-based SMAs is a compromise between Ti3Ni4 precipitation and the decrease in Ni content and increase in Fe content in the matrix during the aging process.

Experimentally Validated Constitutive Model for NiTi-based Shape Memory Alloys Featuring Intermediate R-phase Transformation: A Case Study of Ni &lt;sub&gt;48&lt;/sub&gt; Ti &lt;sub&gt;49&lt;/sub&gt; Fe &lt;sub&gt;3&lt;/sub&gt;

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 Ni48 Ti49 Fe3 shape memory alloy and illustrate the capabilities of its implementation into finite element software on a computational structural analysis of a tightening ring.

The higher compressive strength (TiB+La2O3)/Ti–Ni shape memory alloy composite with the larger recoverable strain

In the present study, in-situ TiB whiskers and La2O3 particles showing a network structure were designed and introduced into Ti–Ni composites. With LaB6 content increasing, the amount of in-situ TiB whiskers and La2O3 particles also increases. In addition to the in-situ reinforcements, Ti2Ni second phase also precipitates. The B19'⇌B2 martensitic transformation can be detected in all Ti–Ni composites, irrespective of LaB6 contents. It should be noted that R-phase transformation also appears for Ti–Ni composite added 0.5wt%LaB6. The martensitic transformation temperatures firstly decrease and then increase with the increased LaB6 content, which can be attributed to the changing of matrix composition. As LaB6 content increases from 0wt% to 0.97wt%, the evolution of network structure is as follows: quasi-continuous network structure → continuous network structure → quasi-continuous network structure. In proportion, the mechanical properties of Ti–Ni composites firstly decrease and then increase. The superior mechanical properties with the highest fracture strain of 35.9% and the largest fracture stress of 2476 MPa can be obtained in Ti–Ni composite with adding 0.97wt%LaB6, which is due to the larger capacity of bearing the loading for reinforcements showing a quasi-continuous network structure. Meanwhile, at the pre-strain of 7%, the maximum strain recovery ratio of Ti–Ni composite is 88.34%.

Quantitative FIB/SEM three-dimensional characterization of a unique Ni4Ti3 network in a porous Ni50.8Ti49.2 alloy undergoing a two-step martensitic transformation

The three-dimensional (3D) nanostructure of Ni4Ti3 precipitates in a porous Ni50.8Ti49.2 alloy has been reconstructed by “Slice-and-View” in a Focused Ion Beam/Scanning Electron Microscope (FIB/SEM). The 3D configuration of these precipitates forming a network structure in the B2 austenite matrix has been characterized via 3D visualization and quantitative analysis including volume fraction, skeleton, degree of anisotropy and local thickness. It is found that dense Ni4Ti3 precipitates occupy 54% of the volume in the B2 austenite matrix. Parallel Ni4Ti3 precipitates grow alongside the surface of a micro-pore, yielding an asymmetric structure, while nano-voids do not seem to affect the growth of Ni4Ti3 precipitates. The small average local thickness of the precipitates around 60 nm allows their coherency with the matrix, and further induces the R-phase transformation in the matrix. On the other hand, the B2 matrix exhibits a winding and narrow structure with a skeleton of 18.20 mm and a thickness similar to the precipitates. This discontinuous matrix segmented by the Ni4Ti3 network and pores is responsible for the gradual transformation by stalling the martensite propagation.