Two-stage B2 Research Articles

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.

Improved functional stability of Ti-rich TiNi shape memory ribbon prepared by melt-spinning

The great functional stability of Ti-rich Ti50.9Ni48.9Si0.2 ribbon was investigated in this study. The as-spun ribbon underwent a two-stage B2 ⇄ R ⇄ B19′ transformation in both forward and reverse. The grain size of the as-spun ribbon was moderately refined to about 500 nm to 1 μm, which contributed to grain refinement strengthening. The as-spun ribbon was in an early precipitation stage of G.P. zones and was strengthened to a hardness of 5.7 ± 0.2 GPa, which was significantly higher than the 4.7 ± 0.3 GPa of the overaged ribbon. The G.P. zones formed when the ribbon was aged at 500 °C for 15 min. The strengthening effect increased the critical stress for dislocation slip and thus prevented the formation of dislocations during martensitic transformation. During 1500 unconstrained thermal cycles, the R → B19′ and B19’ → R transformation peak temperatures of the as-spun ribbon decreased by only 6.8 °C and 0.6 °C, respectively. In shape memory effect tests for 20 cycles under 139 MPa and 324 MPa, the as-spun ribbon showed excellent stability, with recoverable strains of 4.7% and 5.3%, respectively. The Ti50.9Ni48.9Si0.2 ribbon exhibited a combination of large recoverable strain and high functional stability. These experimental results provide a novel method to improve the functional stability of Ti-rich TiNi shape memory alloys by the melt-spinning process, which results in promising functional properties for engineering applications.

Large superelastic recovery and elastocaloric effect in as-deposited additive manufactured Ni50.8Ti49.2 alloy

A Ni50.8Ti49.2 alloy is fabricated by the Laser Solid Forming (LSF) technique by using prealloyed powder in this work. Characterization of the microstructure, phase transformation, and shape recovery of the as-deposited alloy are explored. Columnar grains with ⟨110⟩ preferred orientation are well aligned with the building direction, and the martensitic transformation (MT) behavior shows spatial distribution characteristics. With the increase in the deposited height, MT varies from the two-stage B2 →R→ B19′ transformation to the one-stage B2 → B19′ transformation. Shape recovery properties are studied by compression tests. The as-deposited material without any further heat treatment shows a remarkably large superelastic recovery strain of 9.2% at room temperature, which is the largest value in the additive manufactured NiTi alloy. A negative adiabatic temperature change of –18.6 K induced by the elastocaloric effect during the unloading process is obtained. The shape memory effect (SME) of the as-deposited LSF Ni50.8Ti49.2 alloy is not only dependent on the detwinning of martensite but also associated with the partial superelasticity of the parent phase in the present work, demonstrating a unique two-stage yielding behavior. A residual strain up to 6% is recovered while heating, indicating an excellent SME.

TiNiCuAg shape memory alloy films for biomedical applications

TiNi-based shape memory alloys (SMAs) have been largely utilized in stents, orthopedic endo-prostheses and orthodontic implants. While bacterial infection in medical implants is the most common complication, development of antibacterial TiNi-based SMAs is essential. In the present study, a novel antibacterial TiNiCuAg shape memory alloy film (SMAF) was successfully developed by adding silver into TiNiCu matrix. The structural and biological properties of TiNiCuAg SMAF, including crystal structures, phase transformation temperatures, shape memory effect, antibacterial ability and cell cytotoxicity, were systematically investigated. The results showed that the phase transformation temperature gradually decreased and silver started to precipitate with the increasing silver content. The single-stage B2 ↔ B19′ transformation occurred in TiNiCu and Ti51Ni43.5Cu4.5Ag1 films, while the two-stage B2 ↔ R ↔ B19′ transformation occurred in Ti49.6Ni42Cu6.9Ag1.5, Ti47.4Ni41.2Cu6.4Ag5.6 and Ti47.7Ni41.2Cu4.1Ag7 films. Moreover, compared to TiNiCu films, TiNiCuAg films performed the better shape memory effect with the increasing silver content. For antibacterial tests, the TiNiCuAg films with the Ag content of 5.6 at.% had the best antibacterial ability against S. aureus and E. coli compared to those of TiNi and TiNiCu films. For cytotoxicity assay, all films maintained good biocompatibility and revealed no cell toxicity for L929 cells. As a result, excellent antibacterial ability and good biocompatibility suggested that TiNiCuAg SMAFs had potential applications for medical implant to avoid bacterial infection.

Study of structural phase transitions in quinary TiNi(MoFeAg)-based alloys

The structural phase transitions of quinary Ti50Ni49.5−XMo0.3Fe0.2AgX (X = 0, 0.1, 0.2, 0.5, 1.0, and 1.5 at%) alloys prepared by vacuum-induction melting were studied by means of the four-point-probe, x-ray diffraction (XRD), and optical microscopy methods after thermal cycling. The two-stage B2 ↔ R ↔ B19′ reversible martensitic transformation (MT) took place in all the investigated samples. It is found that the substitution of Ag for Ni in the studied alloy when CAg = 0–0.5 at%, reduces the TR, Ms, and Mf characteristic points by 20–30 K, whereas they increase by 15–35 K when the Ag content was varied from 1.0 to 1.5 at% and the B2 ↔ R ↔ B19′ MT was realized in the high-temperature area. XRD patterns of the studied alloys recorded at the ambient temperature detected the pure Ag phase as well as Ti2Ni precipitate with a small volume fraction (up to 5%) alongside with structural lines of B2, R, and B19′ phases.

Martensitic transformation and pseudoelasticity of aged Ti50.1Ni49.7Si0.2 shape memory ribbon

The effect of aging on transformation sequence, microstructure evolution, hardness change and pseudoelasticity (PE) of Ti50.1Ni49.7Si0.2 ribbon is investigated. Ti50.1Ni49.7Si0.2 ribbon aged at 500°C for ≧1h undergoes a two-stage B2→R→B19′ transformation during cooling. The microstructure and hardness of the aged ribbon are closely related to its composition. The matrix of the aged ribbon becomes Ni-rich because of its high Si content and low Ti/Ni ratio. A very high density Ti3Ni4 precipitate is observed in Ti50.1Ni49.7Si0.2 ribbons aged at 500°C for ≧1h, which gives the aged ribbons high hardness. Nanoindentation tests indicate that Ti50.1Ni49.7Si0.2 ribbon aged at 500°C for 15min has a maximal hardness of 5.5GPa and that aged for 336h decreases to 4.0GPa. The PE recovery of the ribbon is improved from 55% to 75% under a 100mN loading after aging at 500°C for 15min. The experimental results show that adjusting the composition of TiNi ribbon can effectively modify the microstructure, increase the hardness and the ribbon still retains excellent PE recovery.

Phase transformation behavior of Ti49Ni49.5Fe1V0.5 and Ti48Ni48.5Fe1V2.5 alloys after different heat treatments

The effects of heat treatments on the phase transformation behavior of Ti49Ni49.5Fe1V0.5 and Ti48Ni48.5Fe1V2.5 alloys were investigated. The results indicate that the alloys subjected to different heat treatments have B2 structure at room temperature. All the specimens exhibit a two-stage B2 → R → B19′ martensitic transformation on cooling, but a B19′ → B2 one-stage reverse martensitic transformation on heating except aged A1 alloy, which undergoes an abnormal two-stage transformation upon heating. The phase transformation temperatures are affected by heat treatments and V content, which can be attributed to the variation of the second-phase particles content in the matrix.

Microstructure and martensitic transformation in Si-coated TiNi powders prepared by ball milling

Si was coated on the surface of Ti–49Ni (at%) alloy powders by ball milling in order to improve the electrochemical properties of the Si electrodes of secondary Li ion batteries and then the microstructure and martensitic transformation behavior were investigated by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Ti–Ni powders coated with Si were fabricated successfully by ball milling. As-milled powders consisted of highly deformed Ti–Ni powders with the B2 phase and amorphous Si layers. The thickness of the Si layer coated on the surface of the Ti–Ni powders increased from 3–5μm to 10–15μm by extending the milling time from 3h to 48h. However, severe contamination from the grinding media, ZrO2 occurred when the ball milling time was as long as 48h. By heating as-milled powders to various temperatures in the range of 673–873K, the highly deformed Ti–Ni powders were recovered and Ti4Ni4Si7 was formed. Two-stage B2–R–B19′ transformation occurred when as-milled Si-coated Ti–49Ni alloy powders were heated to temperatures below 873K, above this temperature one-stage B2–B19′ transformation occurred.

Characterization of the martensitic transformation in Ni 50− xTi 50Cu x alloys through pure thermal measurements

The addition of a third element to the Ni–Ti system often changes the product and the path of the martensitic transformation of the alloy, which is a direct B2–B19′ transformation for the NiTi alloy in the fully annealed state. In this study we investigate the martensitic transformation of fully annealed Ni 50− x Ti 50Cu x ( x = 3–10 at%) shape memory alloy (SMA) samples using differential scanning calorimetry (DSC) and the four-probe electrical resistance (ER) measurements under stress-free conditions. DSC and ER data show that the ternary alloy goes through a direct B2–B19′ transformation for Cu content between 3 and 7 at% and through the two-stage B2–B19–B19′ transformation for Cu content between 8 and 10 at%. We find good agreement between the two techniques as regards the detection of the phase transformation temperatures. B19′ starting and finishing temperatures decreases with the increases of Cu content and show a significant reduction starting from 7 at%; the range of temperatures in which B19 is stable increases with increasing Cu content.

Isothermal and athermal martensitic transformations in the B2–R–B19′ sequence in Ni–Ti shape memory alloys

The kinetics of accumulation of martensite/austenite during the direct/reverse two-stage B2–R–B19′ martensitic transformation in NiTi has been studied by means of resistance measurements during interruptions of cooling/heating scans. Experimental results indicate that the R–B19′ transformation is isothermal, whereas both direct and reverse B2–R and R–B2 transformations are perfectly athermal. The isothermal accumulation of the R-phase is also detected during B19′–R transformation. A good correspondence exists between the rate of the isothermal transformation and the transformation rate in an uninterrupted cooling cycle.

Shape memory characteristics of as-spun and annealed Ti 51Ni 49 crystalline ribbons

The as-spun Ti 51Ni 49 ribbon is essentially crystalline and shows a two-stage B2 ⇒ R and R ⇒ B19′ martensitic transformation with a significant transformation enthalpy. The as-spun ribbon already has plate-like Guinier-Preston (GP) zones which are proposed to induce the R-phase in martensitic transformation. After being annealed at 200 °C for 1 h, the size of the GP zones increases but their number decreases simultaneously. The maximal recoverable strain of the as-spun and 200 °C annealed Ti 51Ni 49 ribbons reaches 6.2% and 6.4%, respectively. With the increase of the annealing temperature up to 500 °C and 600 °C, only Ti 2Ni precipitates can be observed in the ribbons and the two-stage martensitic transformation tends to coalesce. Experimental results reveal that the ribbons with GP zones show a higher recoverable strain, a lower permanent strain and a lower Ms temperature than those with Ti 2Ni precipitates. The reason why the GP zones are formed in as-spun Ti 51Ni 49 ribbon during melt-spinning procedure is also discussed.

A study of TiNiCr ternary shape memory alloys

Effects of Cr on transformation sequence and wear characteristic in Ti 50− X Ni 50Cr X , Ti 50Ni 50− X Cr X and Ti 51Ni 49− X Cr X alloys with X = 0.5–1.5 at.% are investigated. The lattice parameters of Ti 51Ni 48.5Cr 0.5 martensite are calculated from the XRD spectra as a = 0.2894 nm, b = 0.4109 nm, c = 0.4639 nm and β = 97.49° and the parameters a, c, β are slightly smaller, but b nearly the same, than those in binary Ti 50.8Ni 49.2 alloy. Experimental results indicate that the Cr atoms replace Ni atoms, instead of Ti ones in these alloys. In addition, in Ti 50Ni 49.5Cr 0.5 and Ti 51Ni 48.5Cr 0.5 alloys show a two-stage B2 → R → B19′ transformation upon cooling and a one-stage B19′ → B2 transformation on heating. Both Ti 50Ni 48.5Cr 1.5 and Ti 51Ni 47.5Cr 1.5 alloys exhibit a two-stage B2 ↔ R ↔ B19′ transformation; Ti 49.5Ni 50Cr 0.5 alloy has a one-stage B2 ↔ B19′ transformation and Ti 48.5Ni 50Cr 1.5 alloy shows a B2 ↔ R transformation. The shape recovery of these alloys can be improved by Cr solid-solution hardening. With higher matrix hardness in these alloys, there is better shape recovery. The wear mechanisms of TiNiCr ternary alloys are found to be similar to those of binary TiNi alloys. The Ti 48.5Ni 50Cr 1.5, Ti 51Ni 47.5Cr 1.5, Ti 50Ni 48.5Cr 1.5 and Ti 49.5Ni 50Cr 0.5 alloys have a better wear resistance than the Ti 50Ni 49.5Cr 0.5 and Ti 51Ni 48.5Cr 0.5 alloys due to their higher hardness, high cyclic hardening and/or pseudoelastic behaviors of B2 structure.

Ni sulfide/Ti 50Ni 50 electrode with the superelasticity

Ni sulfides layers were formed on the surface of a Ti 50Ni 50 alloy by reacting sulfur with a Ni film deposited on the alloy, and then microstructures, transformation behavior, shape memory characteristics, superelasticity and electrochemical properties were investigated. When a Ni film deposited on a Ti 50Ni 50 alloy was annealed under the sulfur pressure of 100 kPa at 623 K, sulfides layers consisted of NiS and NiS 2 were formed. When annealing was made at 648 K with annealing time less than 0.9 ks, sulfides layers with a mixture of NiS and NiS 2 were formed, while only NiS 2 was formed when it was made for 1.8 ks. When annealing was made at 673 K with annealing time longer than 0.9 ks, only NiS 2 was formed. With raising annealing temperature and prolong annealing time, NiS changed into NiS 2 accompanied with a morphological change from a particulate-like to a dense film-like. A Ti 50Ni 50 alloy with surface NiS 2 layer showed the two-stage B2–R–B19′ transformation behavior, the shape memory effect and a partial superelasticity with a superelastic recovery ratio of 78%. NiS 2 cathode showed a clear discharge behavior with multivoltage plateaus induced by intermediate reaction products; NiS and Ni 3S 2. The initial discharge capacity was 743 mA h g −1 corresponding to 85% of theoretical capacity and 65% of capacity duration is obtained at 20th discharge.

Fabrication of superelastic NiS/TiNi electrode/current collector materials

Ni sulfides were formed on a Ni-deposited Ti–50.0Ni(at) alloy by annealing under a sulfur pressure of 100 kPa at 673 K for 0.36 ks. Martensitic transformation, superelasticity and electrochemical properties were characterized using different techniques. Ni sulfides consisting of NiS and NiS2 of thickness 0.8 μ m were formed by a reaction between Ni and S. Some spherical particles and pores were found in the sulfides. A Ti–50.0Ni alloy with Ni sulfide surface layers showed a two-stage B2–R–B19′ transformation behavior. A partial superelasticity with a superelastic recovery ratio of 78% found in a current collector showed a clear discharge behavior with a voltage plateau between 1.3–1.6 V. Discharge capacities of the Ni sulfide cathode decreased largely from 700 to 510 mAh(g-NiS2)−1 with increasing number of cycles up to 3, above which it decreased gradually.

Combinatorial study of phase transformation characteristics of a Ti–Ni–Pd shape memory thin film composition spread in view of microactuator applications

Phase transformation characteristics of a Ti–Ni–Pd shape memory thin film composition spread have been investigated. The thin film composition spread was fabricated from elemental targets using an ultra-high vacuum combinatorial magnetron sputter-deposition system and subsequent annealing at 500 °C for 1 h in situ. Automated temperature-dependent resistance measurements ( R( T)), energy dispersive X-ray analysis (EDX) and X-ray diffraction measurements (XRD) have been applied for the high-throughput characterization of the composition spread. Reversible phase transformations within the measurement range of −40 to 250 °C within the Ti–Ni–Pd system were observed for compositions with Ti content between 50 and ∼59 at.%. For Ti-richer films, Ti 2Ni and Ti 2Pd precipitates are inhibiting reversible phase transformations. The transformation temperatures and the thermal hysteresis were determined from R( T) measurements. Rising transformation temperatures with increasing Pd content and significantly lower thermal hysteresis for the B2–B19, compared to the B2–R–B19′ transformations were found in good agreement with published data. For low Pd contents (<7–12 at.%, depending on the Ti content) two-stage B2–R–B19′ transformations were observed. Compositions with higher Pd contents showed a single-stage B2–B19 transformation. Increasing Ti content within the B2–B19 transformation region results in a linear increase of the thermal hysteresis and decreasing transformation temperatures.

Combinatorial fabrication and high-throughput characterization of a Ti–Ni–Cu shape memory thin film composition spread

The phase transformation properties of Ti–Ni–Cu shape memory thin films prepared in the form of a continuous composition spread were investigated. The thin film materials library was fabricated from elemental targets using an ultra-high vacuum combinatorial magnetron sputter-deposition system. Alternating wedge-type layers of Ti, Ni, and Cu were deposited on a thermally oxidized Si wafer and subsequently annealed at 500 °C for 1 h in situ. Automated temperature-dependent resistance measurements ( R( T)), energy dispersive X-ray analysis and X-ray diffraction measurements revealed the compositional region in the ternary phase diagram where thermoelastic transformations occur. The transformation temperatures and the thermal hysteresis were determined from R( T) measurements. Within the composition spread an extended transformation region was found. For Ni-rich compositions, a two-stage B2 → R → B19′ phase transformation was observed. Among others, compositions of Ti 51Ni 49− x Cu x (at.%) show a single-stage transformation for a Cu-content <6.5 at.% (B2 → B19′) and >14 at.% (B2 → B19). In the intermediate composition range two-stage transformations (B2 → B19 → B19′) were found. The dependency of the thermal hysteresis on the Cu-content as described in the literature was confirmed.

Stability of the B19 martensite in rapidly solidified Ti–Ni–Cu alloys

Stabilization of the B19 martensite by rapid solidification in a Ti–45Ni–5Cu (at.%) alloy has been investigated by means of differential scanning calorimetry and transmission electron microscopy. Rapid solidification process induced the B2–B19 transformation prior to formation of the B19′ martensite, and thus the two-stage B2–B19–B19′ transformation occurred in as-spun Ti–45Ni–5Cu alloy ribbons. Inducement of the B19 martensite was ascribed to the formation of Ti 2Ni particles coherent with the B2 matrix. With increasing Ti content from 49 to 51 at.%, the temperature range where the B19 martensite exists was expanded from 7 to 25 K, which was attributed to an increase in volume fraction of Ti 2Ni particles from 10 to 55% and a decrease in the size from 16 to 5 nm.

In situ observations of martensitic transformations in Ti 50Ni 34Cu 16 alloy by synchrotron radiation

The martensitic transformation in a Ti 50Ni 34Cu 16 alloy was studied by in situ X-ray diffraction experiment upon cooling, utilizing synchrotron radiation beams with great intensity. As a result the two-stage B2–B19–B19′ martensitic transformation was directly confirmed, and the lattice parameters of both B19 and B19′ martensites were accurately determined as a function of temperature. Based on the results, the origin of the broad internal friction peak was discussed.

Transformation sequence and second phases in ternary Ti–Ni–W shape memory alloys with less than 2 at.% W

Transformation sequence and second phases in Ti 50− X Ni 50W X , Ti 50Ni 50− X W X and Ti 51Ni 49− X W X alloys with X = 1–2 at.% are investigated. Two different second-phase particles located at grain boundaries are observed. The lattice parameters of Ti 50Ni 49W 1 martensite are calculated from the selected area diffraction patterns (SADP) and all of them are larger than those of Ti 50Ni 50 alloy. Experimental results indicate that the W atoms replace Ni atoms, instead of Ti ones in these alloys. In addition, Ti 50− X Ni 50W X and Ti 51Ni 49− X W X alloys have a two-stage B2 → R → B19′ transformation, but the Ti 50Ni 50− X W X alloy has a one-stage B2 → B19′ transformation because the former Ms temperatures are depressed significantly. The shape recovery of these alloys can be improved by the W solid-solution hardening. With higher matrix hardness in these alloys, there is better shape recovery.

A study on martensitic transformation in Ti50−x/2Ni50−x/2Cux alloys with X≤10 at%

The effect of Cu additions on the martensitic transformation sequence and temperature in Ti50−x/2Ni50−x/2Cuxalloys with x: 1–10 at% are investigated by ER, DSC, X-ray and IF measurements. Experimental results show that the transformation sequence of Ti50−x/2Ni50−x/2Cux alloys with x: 1–4 at% proceeding as two-stage B2→R→B19′ transformation on cooling and Ti50−x/2Ni50−x/2Cux alloys with x=5, 10 at% have no martensitic transformation. The addition of Cu in Ti50−x/2Ni50−x/2Cux alloys assists the formation of R-phase, a behaviour which is quite different from that in Ti50Ni50−xCux alloys. Both the Ms and T R temperatures decrease rapidly with increasing Cu addition in Ti50−x/2Ni50−x/2Cux alloys with x: 1–4 at%. It is proposed that the Cu+Ni effects on the Ms temperature in Ti50−x/2Ni50−x/2Cux alloys is similar as Cu +Ni effects in Ti50Ni50−xCux alloys and as Ni effects in as-quenched Ni-rich TiNi alloys.