B2 Austenite Research Articles

Nitinol Reactive Oxygen Assist Gas Laser Micromachining: Demystifying the Implications Imposed Upon Pseudoelasticity and the Shape Memory Effect for Nickel-Titanium Alloys

Herein, a novel investigation examines a long-pulse microsecond laser cutting process with an oxygen assist gas and the subsequent effects on martensitic phase transformations for binary nickel-titanium alloys. To establish a comprehensive comparative analysis, a reactive oxygen assist gas process, a standard inert argon assist gas process, and athermal femtosecond laser micromachining are studied in order to form a complete analytical assessment of how laser processing can inflict implications upon pseudoelasticity and shape memory. In-situ thermography reveals an approximate 1.5-times increase in thermal area for an argon assist gas when compared to oxygen. In conjunction with thermal analysis, electron backscatter diffraction and optical microscopy demonstrates 1.7 to 25 μm of epitaxial grain recrystallization associated with the elevated level of thermal propagation. Low cycle fatigue testing of pseudoelastic performance realizes a 12.7 % decrease in the achievable upper plateau stress when comparing long-pulse cutting to femtosecond cutting. Furthermore, when the laser power exceeds 30 W, the ultimate tensile strength suffers notable reductions of 25 % and 40 % for the oxygen and argon processes respectively when compared to femtosecond micromachining. Bend and free recovery testing provides insights into shape memory performance of the endothermic phase transformation from B19’ martensite to B2 austenite, revealing marginal ∼1.5 ˚C reductions in the active austenite finish (Af) temperature as laser power was increased from 10 W to 60 W. Lastly, long-pulse laser processing results in notable ∼15 ˚C shifts in the austenite start (As) temperature during the reverse martensite phase transformation from B19’→R→B2.

Effect of Cu Alloying and Heat Treatment Parameters on NiTi Alloy Phase Stability and Constitutive Behavior

NiTiCu alloys with Cu-content up to 20 at.% were successfully made into fine wire with cold-drawn area reduction of 70%. These materials were heat treated and tested at different conditions to study phase transformation and constitutive behavior. It is found that: (1) At Cu-content less than 7.5 at.%, increasing Cu stabilizes the B2 austenite, suppresses R-phase, and slightly decreases B19’ transformation temperatures. At higher level, Cu suppresses B19’-phase, but promotes B19 martensitic phase. (2) Increasing Cu decreases thermal temperature hysteresis and lattice strains during phase transformation. (3) The effect of Cu on stress hysteresis depends on test conditions. A continuous increase in Cu does not guarantee a continuously decreased stress hysteresis. (4) Heat treatment affects Ti2(Ni,Cu)3 precipitation. The highest phase transformation temperatures were obtained after annealing at ~ 600 °C. (5) The 102B19 and 120B19 martensite texture comes from the 111B2 austenite texture after phase transformation through {111} and/or {011} martensite twins. These texture patterns explain the relatively short transformation strains and suggest potential for large improvement through processing-induced texture optimization. (6) The combined effects of Cu addition and cold work strengthening largely increase thermal cycling stabilities of NiTiCu alloys and make them promising candidates for actuator applications.

Microstructure, Mechanical Properties and Corrosion Performance of Laser-Welded NiTi Shape Memory Alloy in Simulated Body Fluid.

Laser-welding is a promising technique for welding NiTi shape memory alloys with acceptable tensile strength and comparable corrosion performance for biomedical applications. The microstructural characteristics and localized corrosion behavior of NiTi alloys in a simulated body fluid (SBF) environment are evaluated. A microstructural examination indicated the presence of fine and equiaxed grains with a B2 austenite phase in the base metal (BM), while the weld metal (WM) had a coarse dendritic microstructure with intermetallic precipitates including Ti2Ni and Ni4Ti3. The hardness decreased from the BM to the WM, and the average hardness for the BM was 352 ± 5 HV, while it ranged between 275 and 307 HV and 265 and 287 HV for the HAZ and WM, respectively. Uni-axial tensile tests revealed a substantial decrease in the tensile strength of NiTi WM (481 ± 19 MPa), with a reduced joint efficiency of 34%. The localized corrosion performance of NiTi BM was superior to the WM, with electrochemical test responses indicating a pitting potential and low corrosion rate in SBF environments. The corrosion rate of the NiTi BM and WM was 0.048 ± 0.0018 mils per year (mpy) and 0.41 ± 0.019 mpy, respectively. During welding, NiTi's strength and biocompatibility properties changed due to the alteration in microstructure and formation of intermetallic phases as a result of Ti enrichment. The performance and safety of welded medical devices may be impacted during welding, and it is essential to preserve the biocompatibility of NiTi components for biomedical applications.

Twin inheritance effect of thermomechanical-processing NiTiCu shape memory alloy

NiTiCu shape memory alloy undergoes thermomechanical processing which deals with local canning compression followed by annealing at 300, 450 and 600 °C, respectively. In particular, formation of twins is dependent upon size of grains. In the case of annealing at 300 °C, nanocrystalline NiTiCu specimen is fabricated, where a small number of nanotwins are observed and the smaller nanocrystalline grains contributes to suppressing the formation of twins. In addition, twin inheritance effect of NiTiCu shape memory alloy is revealed during thermomechanical processing which deals with local canning compression followed by annealing at 450 and 600 °C, respectively. {112} B2 austenite twin, (001) B19′ martensite twin and (100) B19′ martensite twin are found in NiTiCu specimen annealed at 450 °C. In addition to (001) B19′ martensite twin, {114} B2 austenite twin appears in NiTiCu sample annealed at 600 °C. Formation of {114} twin results from transformation of 201̄ B19′ martensite twin during reverse martensite transformation, whereas formation of {112} twin stems from transformation of {113} B19′ martensite twin during reverse martensite transformation. During subsequent martensite transformation, {114} and {112} twins are so stable that they are unable to be transformed into the corresponding 201̄ and {113} twins.

An Investigation on Microstructural Characteristics and Comprehensive Performances of Equiatomic Ratio NiTi Shape‐Memory Alloy Produced by Selective Laser Melting

The present study utilizes selective laser melting (SLM) technology to fabricate NiTi alloy and investigates the deposition quality, microstructure, mechanical performances, and functional properties of the as‐deposited parts. The NiTi component is fabricated by SLM at a suitable energy density (60.61 J cm−3), resulting in favorable surface forming quality and only micron‐sized porosity defects present in the interior. Dense and fine B19’ martensite is distributed within the epitaxial B2 austenite matrix that grows in the height direction through the melt pool boundary, along with the nanoscale Ti2Ni precipitates dispersedly distribute at B19’ grain interiors and grain boundaries. Both tensile and compressive results indicate that the as‐deposited NiTi components acquire slightly inferior ductility but superior strength than that of traditionally manufactured NiTi alloys. The shape‐memory effect of NiTi components is significantly influenced by holding temperature, exhibiting good and stable shape‐memory effect above a holding temperature of 75 °C. Simultaneously, the compressive superelastic behavior of the NiTi component exhibits favorable stability, with a recoverable strain of ≈5.5%. Overall, the NiTi component fabricated in the present study exhibits favorable mechanical properties and functional performance attributed to its excellent deposition quality and dense microstructure.

Hydrogen penetration into the NiTi superelastic alloy investigated in-situ by synchrotron diffraction experiments

Microstructural changes induced by a hydrogen permeation into the NiTi superelastic alloy were investigated in-situ using the X-ray synchrotron diffraction. A new design of an electrochemical cell enabled to uncover time and position dependent processes under a flat alloy surface exposed to the cathodic hydrogen. The diffraction data supported by thermo-elastic FEM calculations helped to quantify an evolution of compressive stresses in the B2 austenitic phase hosting hydrogen atoms. The compressive stress state initiates a formation of martensitic phases starting from the exposed surface layer and advancing into the alloy volume with increasing time of hydrogen charging. We have performed the ab-initio DFT study in order to rationalize volumetric changes associated with variations in the B2 austenite and B19′ martensite lattice parameters. The numerical results also contributed to the identification of a new hydride phase with orthorhombic crystal structure and lattice parameters a = 0.8505 nm, b = 0.7366 nm and c = 0.4722 nm.

Phase transformation behavior and shape memory effect of nickel-titanium shape memory alloy using the magnetic field assisted wire electrical discharge machining

Nickel-titanium shape memory alloy (NiTi-SMA) can restore its original shape after deformation, making it show great application in the fields of biomedicine, aerospace, etc. The wire electrical discharge machining (WEDM) has the advantage of machining micro and complex structures, which can greatly promote the application of NiTi-SMA. However, there are issues such as heat affected layer (HAL) on the surface of WEDM, which can affect the functionality of NiTi-SMA. Therefore, when using WEDM to NiTi-SMA, more attention should be paid to the functional changes after machining. This article primarily examine is the impact of peak current and pulse on time on the phase transformation behavior, shape memory effect, and mechanical properties of NiTi-SMA during WEDM, and introduces into the magnetic field assisted (MFA) process to improve the machining performance of NiTi-SMA. The results indicate that the HAL formed on the surface of NiTi-SMA is primarily composed of the irreversible phase transformation of B2 austenite. This layer induces a hardening effect, resulting in an increase in the phase transition temperature and a decrease in the phase transition energy (∆HM-A), as a result, the martensitic phase transformation behavior is suppressed. When the peak current and pulse on time increase, the surface quality of the NiTi-SMA processed decreases. The thickness of the HAL and microhardness increase, causing an increase in the phase transition temperature of the NiTi-SMA processed samples. Additionally, the transformation energy ∆HM-A, shape recovery rate, and mechanical properties decrease, and after recovery, more surface cracks appear and show larger extensions. MFA-WEDM can improve the surface quality of NiTi-SMA processing, reduce the thickness of the HAL, and effectively enhance the phase transformation temperature and energy ∆HM-A of the NiTi-SMA processed samples. With a magnetic field strength of 0.45 T, the tensile shape recovery rates of 98%, 94.4%, and 91.0% were achieved at 2%, 5%, and 8% strain, respectively. This represents an improvement of 8.9% compared to the maximum shape recovery rate with conventional WEDM. At the same time, it also inhibits the generation and propagation of surface cracks after deformation recovery. Additionally, MFA-WEDM improves the mechanical properties of NiTi-SMA, and reduces the thickness of the brittle fracture region by 49.7% compared to conventional WEDM. Therefore, MFA-WEDM can effectively enhance the shape memory effect of NiTi-SMA. However, it is important to avoid excessive peak currents and pulse on time to achieve optimal shape memory functionality.

Exploring the Friction and Wear Mechanisms of Nickel Titanium Alloy Fabricated by Laser Powder and Wire-Based Directed Energy Deposition

The NiTi alloy has attracted significant interest owing to its notable wear and corrosion resistance, suggesting the significant promise of utilizing additively manufactured NiTi alloys in tribological applications. In this study, dry sliding wear tests with sliding velocity of 0.1 m/s for total distance of 250 m at room temperature, 50 °C, 100 °C, and 200 °C were carried out on samples of NiTi alloy samples fabricated using laser powder and laser wire directed energy deposition (LP-DED and LW-DED) processes. Enhanced friction and wear behavior of LP-DED samples were captured at lower temperatures due to higher hardness and the stable dominating NiTi B2 austenite phase in the sample microstructure as confirmed from x-ray diffraction (XRD) analysis. Accumulated transferred material at lower temperatures played a crucial role in modifying the wear mechanisms. However, at higher temperatures both LP-DED and LW-DED samples presented similar wear mechanisms.

Microstructure and elastocaloric effect of NiTi shape memory alloy in-situ synthesized by laser directed energy deposition additive manufacturing

The NiTi SMAs were successfully in-situ synthesized by LDED additive manufacturing, and the microstructural evolution and elastocaloric effect of the NiTi SMAs along parallel and perpendicular to the build direction (BD) were investigated. The NiTi SMAs at room temperature consisted of B2 austenite, B19’ martensite and the precipitation phase Ti2Ni, which was diffusely distributed within the B2 grains and at grain boundaries. Grain morphology that crosses the molten pool boundary and elongates along the thermal gradient is found in the plane parallel to the BD direction. NiTi SMAs loaded perpendicular to the BD direction have less energy loss and fewer stress mechanical training times to obtain fully recoverable stress-strain behavior, which shows better stress-strain response behavior than that loaded parallel to the BD direction. The maximum strain and adiabatic temperature changes follow a monotonically increasing trend from the maximum stress when the compressive stress was applied along parallel and perpendicular to the BD direction and rapidly removed. When the 900 MPa stress was removed, the adiabatic temperature changes reached −5.2 K and −5.9 K, respectively. The degradation of the elastocaloric effect of loading parallel to the BD direction reaches 15% after 100 stress-strain cycles (800 MPa). In contrast, the degradation of loading perpendicular to the BD direction is negligible. This work provides a new idea to realize the preparation of NiTi SMAs and to study the elastocaloric effect of NiTi SMAs.

Porosity and phase composition effects on SHS-NiTi structure and mechanical properties

NiTi samples with 59–70% porosity were obtained by the self-propagating high-temperature synthesis (SHS) using Ti, Ni, and NiTi powders as inert. The maximum and average pore sizes increase, while the maximum and average dimensions of the bridges decrease with porosity, controlled by adjusting the amount of inert additives, based on the surface and cross-sectional microscopy studies. The phase composition of porous NiTi was determined by X-ray diffraction. The alloy contains an austenitic B2 TiNi phase, a martensitic B19′ TiNi phase, and a secondary Ti2Ni phase. SHS-NiTi samples with different porosity were tested by uniaxial compression, revealing mechanical properties’ dependence on porosity. It is shown that with an 11% increase in the NiTi alloy porosity, the yield and compressive strength are reduced by almost a factor of 2.

Near-spherical micron-porous NiTi alloys with high performances fabricated via metal injection molding

In this study, NiTi alloys with different porosity were prepared using metal injection molding combined with the powder space-holder method (MIM-PSH). The microstructures, transformation temperatures, compression and its recovery properties were investigated. The porous structure consisted of near-spherical pores (40.8 and 44.9 μm). The pore size was tied to the size of space-holder but not porosity, which was beneficial to controllability of pore diameter. The matrix was dominated by B2 austenite, with small amounts of B19′ martensite and Ti2Ni inclusions. Ni4Ti3 precipitates were also observed, not only in micrometer scale, but also as nanodomains. Highly broadened R peaks were observed in the differential scanning calorimetry thermograms, which were attributed to extensive, non-uniform, multi-scale precipitation. Overall, pores at this scale shown no influence on the transformation behaviors. Due to the low final oxygen content (0.19 % and 0.23 %) and homogeneous near-spherical pores, high compressive properties had been exhibited. The compressive recovery was almost complete at 3 %, 6 % and 8 % pre-strains. Irreversible strain increased with increasing pre-strain. The average volume variation of the pores under different deformations was analyzed in detail. In the future, MIM-PSH should be used to further enhance the customized fabrication of porous NiTi alloys.

Temperature and strain-rate dependence of the superelastic response in EBF3-fabricated and post-heat-treated NiTi shape memory alloy

In the present work, the temperature and strain-rate dependence of the superelastic response behavior of NiTi shape memory alloys prepared via electron beam freeform fabrication (EBF3) technique and post-heat-treated is systematically studied. It is demonstrated that the response hysteresis of the EBF3-fabricated NiTi alloys during superelastic cycling decreases with increasing strain-rate at the same test temperature. However, the superelastic response of material gradually alters to a nearly linear mode with the increase of test temperature under the same strain-rate. The corresponding stable superelasticity recovery ratio decreases from 62.6 % to 47.6 % when the test temperature improves from 100 °C to 150 °C. This is mainly due to the activation of dislocations in the {011}<001> slip system of B2 austenite, in which they become plugged and entangled during plastic deformation, deteriorating the material superelastic recovery ability. These findings provide a selection basis for the service conditions of the rapidly prepared NiTi alloys under EBF3-fabrication and post-heat-treatment technology routes.

Developing centimeter-sized CuZr-based metallic glass composites via multi-element microalloying

A multi-element microalloying strategy has been implemented to manipulate the microstructure of CuZr-based bulk metallic glass composites (BMGCs) and, in turn, to improve their mechanical properties. It is found that microalloying with a combination of Ti, Hf, Co, Ni and/or Nb/Sn can effectively refine the austenitic B2–CuZr phase, resulting in finely dispersed B2 crystallites embedded in the BMG matrix while enlarging the sizes of BMGCs. Particularly, an optimum BMGC, i.e., Cu40.75Zr42Al4Ag5Sn0.75Co0.5Ti5Hf1Ni1, can be made by copper-mold casting in the range of diameter from 6.5 to 13.5 mm, much wider than those of BMGCs obtained from microalloying with merely one or two elements. Meanwhile, this centimeter-sized BMGC possesses good mechanical properties with a fracture strength as high as 1960 MPa and a compressive plastic strain up to 6.8 %. Our results demonstrate that multi-element microalloying would reduce the driving force for nucleation of B2 by lowering the melting entropy, and slow down the B2-growth kinetics by promoting faceted crystal growth while retarding multi-component diffusion, both accounting for the making of centimeter-sized BMGCs that contain finely dispersed B2 crystallites in the glass matrix.

Elastic and transformation behaviour of equiatomic NiTi shape memory alloys fabricated at different sintering temperatures

The growing demand for bulk Nitinol in ball and roller bearing applications is focused on in this article. The uniaxial compaction and sintering process is considered for the fabrication of the alloys where the sintering temperatures vary from 950 ℃ to 1150 ℃. Better homogenisation and diffusion are achievable except for the 950 ℃ sintered sample. The results of XRD of 950 ℃ and 1050 ℃ sintered samples show the presence of the Ni3Ti phase but in all the Samples sintered above 950 ℃, NiTi is the major phase, unlike Ni3Ti in 950 ℃. The TEM analysis shows a trace amount of TiO2 which comes from the impurity of the purged Ar gas. The O2 in the samples can be accommodated inside a few Ti2Ni phases to form Ni2Ti4O phases. Ni4Ti3 precipitates formed at higher temperatures help in precipitation strengthening. At room temperature, both austenitic B2 and martensitic B19′ coexist. The DSC analysis shows that an increase in the sintering temperature increases the response time by consuming more energy. The elastic modulus and hardness have been increased up to 1100 ℃ and then decreased. Above Af temperature successful transformation of B2 to B19′ takes place.

Microstructure Characteristics, Mechanical Properties and Strain Hardening Behavior of B2 Intermetallic Compound-Strengthening Fe-16Mn-9Al-0.8C-3Ni Steel Fabricated by Twin-Roll Strip Casting, Cold Rolling and Annealing.

In this study, the Fe-16Mn-9Al-0.8C-3Ni (wt.%) lightweight steel was fabricated by novel twin-roll strip casting technology. The microstructure, tensile properties and strain-hardening behavior of the present steel have been investigated and compared to those of conventionally processed steels with similar chemical compositions. After annealing, a unique gradient microstructure of intermetallic compound (B2)-austenite was obtained along the thickness direction, consisting of granular B2 (average: 430 nm) and fine austenite (average: 1.82 μm) at the surface layer, blocky B2 (average: 1.03 μm) and medium austenite (average: 3.98 μm) at the quarter layer and polygonal B2 (average: 1.94 μm) and coarse austenite (average: 6.13 μm) at the center layer. The cooperative action of B2 pinning dislocation, plane slip and back stress led to stronger strain hardening, among which the strong back stress effect originated from the multistage discontinuous austenite deformation and the mechanical incompatibility between austenite and B2 is believed to be the most important reason, thereby achieving an excellent balance of strength (ultimate tensile strength: 1147 MPa) and ductility (total elongation: 43.2%). This work not only developed a new processing way to fabricate Ni-containing Fe-Mn-Al-C lightweight steel with outstanding mechanical properties, but also provided a potential solution for manufacturing some other metallic materials accompanied by brittle B2 intermetallic.

Nanoprecipitates enhanced the yield strength and output work of (TiHfZr)50(NiCu)50 high-entropy shape memory alloys

High entropy shape memory alloys (HESMAs) of (Ti40−xHf10Zrx)50(NiCu)50 (x = 0, 1, 5, 10, at%) were prepared by vacuum arc melting, solution treatment (800 °C for 30 min), and aging treatment (300 °C for 120 min for the solution-treated sample and 400 °C for 120 min for the 300 °C aged sample). The effects of Zr content and aging on the microstructure, mechanical performance, and superelasticity properties of HESMAs were investigated. The results show that uniform nanoscale phases are precipitated on the B2 austenite matrix of (TiHfZr)50(NiCu)50 HESMAs after solid solution treatment and aging treatment. Compared with the ternary TiNiCu and quaternary Zr0 alloys, the yield strength and critical strength of the stress-induced martensite transformation of the HESMAs improve significantly with increasing Zr content, and the nanoprecipitates further improve their mechanical performance. Due to the enhanced solid solution strengthening effect caused by the increase in Zr content and the dispersion strengthening effect of the nanoprecipitates, the (TiHfZr)50(NiCu)50 HESMAs show an excellent combination of high strength and large recoverable strain, which endows them with excellent output work. Among them, the Zr5 HESMAs have the best superelasticity properties, and their maximum recoverable strain and superelasticity strain are 8.8–9.7 % and 4.9–5.4 % in ten load cycles with a prestrain of 2–15 %, while the corresponding values of ternary TiNiCu alloys are only 2.6–4.7 % and 1.0–2.1 %, respectively. The output work of Zr5 HESMAs is 163.7–188.5 J/cm3, which is approximately 5–8 times larger than that of ternary TiNiCu alloys.

The atomic structure and mechanisms of formation of some geometrically incompatible interfaces within cubic B2 austenite – monoclinic B19′ martensite shape memory alloy microstructures

Microstructures of aged nickel‑titanium‑hafnium shape memory alloys (SMAs) Ni50.3Ti41.2Hf8.5 and Ni50.3Ti43.7Hf6 (at.%) are examined using high-resolution transmission electron microscopy (HRTEM) techniques. Characterizations of the structures of austenite-martensite, martensite-martensite, and martensite-precipitate interfaces provide new insights into cubic B2 austenite–monoclinic B19′ martensite phase transformation and martensite reorientation mechanisms. Specifically, in the absence of external load, theoretically unfavorable (001) compound martensite twins nucleate at H-phase precipitate interfaces. At the interface of these twins and the austenite matrix, a zig-zag-rise structure follows an extension of superdislocation theory previously developed to explain analogous zig-zag structured interfaces in non-shape-memory martensitic alloys. Additionally, 〈011〉 type II martensite twins that are theoretically favorable to form were also observed within the austenite matrices. However, instead of forming defect-free habit planes accommodated by elastic strains as predicted by theory, misfit dislocations worked in cooperation with elastic strain fields to accommodate atomic misfits between the austenite and martensite phases, and the elastic strain fields were smaller at the interfaces. Finally, the atomic scale thermoelastic mechanism of <011> type II twins reorienting to form (001) compound twins within the martensite is proposed in a manner consistent with direct observations of such interfaces. In total, connections with previous work on other binary and ternary NiTi base alloys show that these new mechanistic understandings generalize to other B2 - B19′ SMAs that should not perform well according to the elasticity-based Phenomenological Theory of Martensite Crystallography, yet still exhibit robust cyclic thermomechanical performances due to transformation-plasticity interactions.

A coherent atomic configuration model of Ni4Ti3 and its application to atomic-level characterization of precipitation induced strain fields in NiTi alloy

This paper proposes an innovative and practical modeling method to create a coherent atomic configuration of NiTi austenite with lenticular Ni4Ti3 based on molecular dynamics (MD) approach. The initial configuration of Ni4Ti3 is created by expanding lattice constants and then relaxed at specified temperature coupled with energy minimization scheme to optimize the configuration while maintaining coherence with B2 austenite. Ni4Ti3 precipitates in parallel, step-like and edge-to-edge models generate different strain fields by MD simulations; strain fields in the interior of Ni4Ti3 precipitates and at precipitate-matrix interfaces are more consistent with experiments in comparison to that yielded by the Eshelby solution (ES).

Significance of adiabatic heating on phase transformation in titanium-based alloys during severe plastic deformation

Experiments were conducted on Ti-based alloys including a commercially pure titanium (α phase), Ti-6Al-4 V (α + β phase) and TiNi (B19’-martensitic phase) alloys in order to investigate the phase transformation during high-pressure torsion (HPT) at room temperature. An allotropic α to ω phase transformation in the commercially pure titanium occurred during HPT processing but increasing the rotation speed led to a decrease in the volume fraction of the ω-phase. This procedure facilitates the loss of the β phase in the Ti-6Al-4 V alloy and promotes a B19’ to B2 (austenite) phase transformation in the TiNi alloy. The results show that the bulk temperature increment in the processed specimens was not sufficiently high to affect phase transformations. However, the calculations by assuming adiabatic conditions revealed that the local temperature increment is responsible to encourage or suppress the phase transformation in the Ti-based alloys. This fundamental understanding of phase transformations during severe plastic deformation is useful for conducting microstructure engineering of Ti-based alloys by tailoring the microstructures containing different phases and thereby producing different mechanical and functional behaviors.

Revealing the microstructure evolution of the deformed superelastic NiTi wire by EBSD

The generation of irreversible macroscopic strains in superelastic NiTi alloys is an important issue closely related to the crystallography of the transformation from B2 austenite to B19’ martensite under stress. Although widely investigated at micro and nano scales by transmission electron microscopy (TEM), the observation of this phase transformation at the mesoscale is still lacking. In the present study, a superelastic NiTi wire was maintained in bending condition and investigated by electron backscattered diffraction (EBSD) technique. A strong variant selection and a formation of martensite texture were observed and quantified using the interaction work (IW) based on the distortion matrix of a single variant. A new habit plane between martensite and parent B2, {114}B2 || (201¯)M, was experimentally measured. It was explained by the Phenomenological Theory of Martensite Crystallography (PTMC) using dislocation slip {110}<001>B2 as lattice invariant shear (LIS). Some new B2 domains were also found in {114}B2 twinning relation with primary B2 grain. Two scenarios are proposed to explain them: a) the direct formation of {114}B2 deformation twin in B2, and b) a more complex one in which a primary B2 grain is transformed into a martensite variant, a (201¯)M deformation twin is formed inside this variant; then the (201¯)M twinned martensite reversely transforms into a new {114}B2 twinned B2 domain. The scenario b) permits to explain some uncommon (130)M, (201)M, and (101¯)M twins observed by EBSD between the martensite variants of the two {114}B2 twin-related B2 phases.