Stability of mechanical behavior and work performance in TiNi-based alloys during thermal cycling

SAKUMA T., IWATA U., ARAI M.

In order to develop the utilization technology in which unexploited energy is retrieved and transformed into power, we have proposed a heat engine incorporating shape memory alloy wires. In this work, the output power characteristics are investigated experimentally from the viewpoints of maximization of the output power, stabilization of the power during operation and the amount of work to failure. Experiments are conducted at various heating and cooling temperatures, velocities of heating and cooling water and wire diameters. As a result, it is clarified that the output power increases with increases in both the heating temperature and the velocity of heating water. The output power also increases with decreasing cooling temperature. However, the velocity of cooling water at which the output power is maximum varies with the cooling temperature. Furthermore, the amount of work to failure does not always increase with increasing fatigue life and becomes maximum under the operating conditions at which the output power becomes maximum.

TOBUSHI H., DATE K., MIYAMOTO K.

Since a solid-state heat engine using a shape memory alloy (SMA) works at the temperature difference of several ten degrees, the development of the SMA-heat engine to use the low-temperature thermal energy around 373K is greatly expected from a view-point of global warming prevention. The working principle of the SMA-heat engine is discussed based on the recovery stress of the SMA. The output power characteristics of a tilt-disk offset crank SMA-heat engine and the basic working properties of a solar-powered car using an SMA-heat engine are investigated. The subjects for the development of the SMA-heat engine are also discussed.

Resnina N., Belyaev S.

Usually a multi-stage martensitic transformation is observed in Ni-rich TiNi alloys after heat treatment at 350–500 °C. It is due to the internal stresses created by the Ni 4 Ti 3 participate. In the present work it was found that the multi-stage martensitic transformation appeared in Ti–50.0 at.% Ni alloy after thermal cycles through the temperature range of the phase transitions. Annealed sample undergoing one-stage phase transition was subjected to 32 thermal cycles in the DSC apparatus. The results had shown that three-stage forward martensitic transformation observed after 32 thermal cycle was due to the B2 → R, B2 → B19′ and R → B19′ phase transitions. It was found that the B19′ phase obtained from the B2 phase underwent the reverse transformation at higher temperatures than the B19′ phase obtained from the R phase. After annealing the cycled sample at 400 °C the transformation behavior was similar to the non-thermal cycled alloy. It was concluded that the main reason for the multi-stage phase transition induced by the thermal cycles was the phase hardening.

Bertacchini O.W., Lagoudas D.C., Patoor E.

In this two-part paper, the thermomechanical fatigue of TiNiCu shape memory alloy (SMA) wire actuators undergoing thermally induced martensitic phase transformation in a corrosive environment is investigated. The main objective of this work is to evaluate the cyclic response and fatigue behavior of TiNiCu SMA wire under corrosive conditions and to compare it to results obtained for fatigue testing in a corrosion-free environment. Part I focuses on the various experimental aspects of this work, including the presentation of fatigue results as a function of various testing parameters. The variable test parameters are five applied stress levels from about 50 MPa to about 250 MPa, and two different actuation strains, one corresponding to full actuation or complete transformation and the other to partial transformation. The results from fatigue testing in a corrosive environment show a consistent reduction of the fatigue life compared to corrosion-free fatigue results, in both complete and partial transformation conditions. It is also observed that corrosion-assisted fatigue leads to more scattered fatigue data and this spread is mostly attributed to enhanced and accelerated damage mechanisms due to corrosion. From these conclusions, a microstructure evaluation is performed to understand the damage that contributes to lower fatigue limits under corrosion and is presented in Part II of this work. Fracture surfaces, development of fatigue cracks and effect of corrosion are presented and discussed. The conclusion from the microstructure analysis has led to the formulation of a damage accumulation model accounting for a cyclic corrosion mechanism. This modeling approach allows for determining the fatigue life reduction of SMA wire actuators in a corrosive environment. All results of the microstructure analysis and fatigue life modeling are presented in Part II.

Saikrishna C.N., Ramaiah K.V., Allam Prabhu S., Bhaumik S.K.

The use of NiTi wire as thermal actuator involves repeated thermal cycling through the transformation range under a constant or fluctuating load. The stability of the material under such conditions has been a concern for the past many years. Experimental results show that for a given alloy composition, the repetitive functional behaviour of NiTi wire is largely dependent on the processing schedule/parameters and the stress-strain regime of thermo-mechanical cycling (TMC). Among the various processing parameters, retained cold work in the material and the shape memory annealing temperature/time have significant influence. It has been shown in the present study that for a stable functional behaviour, the material needs to be tailored through judicious selection of these parameters. Study also shows that, after processing, the material requires an additional stabilization treatment for ensuring minimal variation in the repetitive functional response upon TMC.

Bombardelli C., Menon C.

A new concept of space power generation is presented that exploits thermal cycles of dilatation and contraction of a spinning tethered system exposed to solar radiation. The concept exploits a lightweight film made of shape memory material directly interacting with sunlight. A temperature variation cycle in the film is created by inducing periodic variation of the tether sun aspect angle. As the film length diminishes due to the increased temperature, the mechanical energy of the system increases and can be used to charge an energy storage device. A preliminary assessment of the system efficiency and power density is conducted. It is seen that in spite of the relatively low efficiency—for present shape memory alloys—of the phase transformation the system can benefit from a very high ratio between collecting surface and overall mass, which could make it an interesting alternative to photovoltaic systems for space power plans in the future. Current limitations appear to be in the design of an efficient power conversion and incident light modulation system.

Liu Y., Laeng J., Chin T.V., Nam T.

Effect of partial thermal cycling through incomplete B2 → B19′ martensitic transformation on the transformation behaviour of NiTi was investigated. This investigation was carried to assess the property stability of NiTi during operation in applications involving repeated transformation cycles. Partial thermal or mechanical cycling has been considered a remedy for more stable operation in such applications. It was observed that on equal transformation volume basis partial transformation cycling was less damaging compared to full transformation cycling. This observation is indicative that less structural damages are created during the early stages of the B2 → B19′ martensitic transformation and that incomplete transformation cycling has certain advantage in minimising the shift of shape memory properties of NiTi.

Van HUMBEECK J.

Pattabi M., Ramakrishna K., Mahesh K.K.

The results of studies carried out on the effect of thermal cycling on the phase transformation behavior of Ni-rich NiTi shape memory alloy (SMA), using powder X-ray diffraction (XRD) technique, are presented in this article. In Ni-rich NiTi alloy heat-treated at 660 °C, the integrated peak area (integrated intensity) corresponding to (1 1 0) A plane of the austenitic phase increases and the area corresponding to martensitic phase decreases with increasing temperature. Austenitic start and finish temperatures change during the first few thermal cycles. The former decreases by ∼15 °C and the latter decreases by ∼35 °C during the first cycle, whereas martensitic start and finish temperatures do not get affected by thermal cycling. The transformation characteristics determined through XRD are compared with those obtained from differential scanning calorimetry (DSC).

Liu Y., Laeng J., Chin T.V., Nam T.

The effect of partial thermal cycling through incomplete transformation cycles on the transformation behaviour of NiTi was investigated. This investigation was carried out with two purposes: (1) to establish an understanding of the evolution of internal plastic deformation during the process of thermally induced B2 ↔ B19′ martensitic transformation and (2) to provide experimental data for the application of NiTi alloys involving repeated transformation cycles. It was observed that on equal transformation volume basis partial reverse transformation caused the least changes to transformation behaviour, full transformation cycling caused the most severe changes whereas partial forward transformation cycling was in between. These experimental observations indicate that more structural damages are created towards the end of the forward transformation than at the early stages of the transformation and that more structural damages are created during the forward transformation than during the reverse transformation.

Gong C.W., Wang Y.N., Yang D.Z.

Ni 50 Ti 45 Ta 5 alloy is similar to equiatomic NiTi alloy with near equal transformation temperatures and having B 2 ↔  B 19′ one stage transformation. The main lattice twin is 〈0 1 1〉 type II twin. The lattice parameter of B 2 austenite is a  = 0.3025 nm, and the lattice parameters of B 19′ martensite is a  = 0.290 nm, b  = 0.412 nm, c  = 0.473 nm and β  = 98.2°. The transformation temperature depression of Ni 50 Ti 45 Ta 5 alloy is larger than that of NiTi alloy under the same number of thermal cycles due to the presence of more particles. The martensite stabilization occurs due to the deformation. It is closely related to the variation of the elastic energy and the irreversible energy during deformation. The Ni 50 Ti 45 Ta 5 alloy has a higher martensite stabilization than NiTi alloy under the same degree of deformation due to the presence of a soft β-Ta phase.

Wang Z.X., Dutta I., Majumdar B.S.

Shape memory alloy (SMA) discontinuous reinforcements in a solder matrix have the potential to reduce inelastic strains and improve thermo-mechanical fatigue (TMF) life of solder joints. In this work, concept viability was tested on a simpler configuration consisting of a single NiTi wire embedded in a cylindrical Sn–3.5Ag solder matrix. The composite samples were thermally cycled at a constant shear stress under double shear loading perpendicular to the fiber axis, and the local shear displacement was monitored. The local displacement plots indicate significant reduction in creep rate of the SMA reinforced composite versus a control Cu-wire reinforced composite, suggesting that SMA particulate reinforcements may be a feasible option. In order to understand and model the composite response, tests were also conducted on the bare NiTi wire. A preliminary model based on Eshelby's transformation strains provided reasonable agreement with the displacement response of the composites.

Otsuka K., Ren X.

Ti–Ni-based alloys are quite attractive functional materials not only as practical shape memory alloys with high strength and ductility but also as those exhibiting unique physical properties such as pre-transformation behaviors, which are enriched by various martensitic transformations. The paper starts from phase diagram, structures of martensites, mechanisms of martensitic transformations, premartensitic behavior, mechanism of shape memory and superelastic effects etc., and covers most of the fundamental issues related with the alloys, which include not only martensitic transformations but also diffusional transformations, since the latter greatly affect the former, and are useful to improve shape memory characteristics. Thus the alloy system will serve as an excellent case study of physical metallurgy, as is the case for steels where all kinds of phase transformations are utilized to improve the physical properties. In short this review is intended to give a self-consistent and logical account of key issues on Ti–Ni based alloys from physical metallurgy viewpoint on an up-to-date basis.

Eggeler G., Hornbogen E., Yawny A., Heckmann A., Wagner M.

Cyclic loading is one of the generic characteristic features of many of the present and potential future applications of NiTi shape memory alloys, no matter whether they exploit mechanical (pseudo-elasticity) or thermal shape memory (one and two way effect). Cyclic loading may well be associated with structural and functional fatigue, which both limit the service life of shape memory components. By “structural fatigue” we mean the microstructural damage that accumulates during cyclic loading and eventually leads to fatigue failure. There is a need to understand how microstructures can be optimized to provide good fatigue resistance. The term “functional fatigue” indicates that shape memory effects like the working displacement in a one way effect (1WE) actuator or the dissipated energy in a loading–unloading cycle of a pseudo-elastic (PE) damping application decrease with increasing cycle numbers. This is also due to a gradual change in microstructure. In both cases it is important to know how fatigue cycling affects shape memory properties. The present paper considers structural and functional fatigue of NiTi shape memory alloys. It discusses four cases of fatigue in NiTi shape memory alloys: (1) The evolution of the stress–strain hysteresis in low cycle pull–pull fatigue of pseudo-elastic NiTi wires. (2) Bending–rotation fatigue rupture of pseudo-elastic NiTi wires. (3) Strain localization during the stress induced formation of martensite. (4) Generic features of functional fatigue in NiTi shape memory actuator springs. The paper shows that fatigue of shape memory alloys is a fascinating research field and highlights the need for further work in this area.

Liu Y.

This paper is to evaluate the work production and thermal efficiency of shape memory alloy (SMA). NiTi wire under pure tension mode is chosen for the work production and several bias components are evaluated including a linear elastic component, a non-linear superelastic SMA wire and an SMA with well developed two-way memory effect (TWME). The calculated thermal efficiency shows good agreement with the reported experimental data. The estimation of the thermal efficiency is further discussed with respect to the latest results on SMAs, especially the transformation enthalpy change. Ways of increasing both the work production and the thermal efficiency of SMAs are further suggested.

Чуракова А.А., Исхакова Э.И.

В представленной статье проведены исследования влияния многократных мартенситных превращений В2-В19’ на структуру и температуры превращений в различных структурных состояниях сплава TiNi. Показано, что в крупнозернистом, ультрамелкозернистом и нанокристаллическом сплаве TiNi происходят последовательные изменения в микроструктуре и температурах фазовых переходов, при увеличении числа термоциклов до n=100 с быстрым нагревом и быстрым охлаждением до –196 °C. Температуры превращений в ультрамелкозернистом состоянии Ti49.15Ni50.85 более устойчивы к термоциклированию (ТЦ), чем в крупнозернистом состоянии. Обнаружено образование нанодвойников мартенсита в наноструктурном состоянии после многократных термоциклов.

Churakova A., Gunderov D.

The microstructure and mechanical properties of the ultrafine-grained Ti–50.8 at.% Ni alloy after thermal cycling treatment with the number of cycles up to 250 was investigated. A fractographic analysis of the samples after tensile tests was carried out. The fracture pattern of the alloy in the UFG state has a viscous character with microdepths on the fracture surface. The average size of microdepths decreases as the number of thermal cycles increases up to n= 250.

Churakova A.A., Gunderov D.V., Kayumova E.M.

Abstract The article were discussed the effect of multiple martensitic transformations with a different number of cycles on the functional characteristics, microstructure and mechanical properties of the TiNi alloys in different structural states and chemical compositions. As a result of multiple martensitic transformations, the accumulation of defects and a decrease in the size of structural elements are observed. The most effective phase hardening is in an equiatomic alloy. The Ti49.8Ni50.2 alloy demonstrates stability to phase hardening in all studied states.

Churakova A.A.

Abstract The influence of multiple martensitic transformations on the functional properties and microstructure of the equiatomic TiNi alloy is considered. An analysis of mechanical tests showed that in the CG and UFG state in the Ti50.0Ni50.0 alloy as a result of thermal cycling with an increase in the number of cycles, a monotonic increase in the yield strength is caused by phase hardening. Moreover, in the UFG state, the yield strength increases by 185 MPa, which is 67.5 MPa more than the increase in σYS in the CG state. In this case, ductility remains at the level of 40%, which is somewhat lower than ductility in the CG state (∼ 60%). The phase yield stress and the estimated reactive stress of the CG and UFG states as a result of the TC also increase, with σreac UFG reaching 830 MPa. The increase in the yield strength of the CG state in an overgrown alloy is 113 MPa, while in the UFG it is 155 MPa, in addition, an increase in the estimated reactive stress and the reversible deformation with an increase in the number of thermal cycling cycles.

Sibirev A., Belyaev S., Resnina N., Nikolaev V., Timashov R., Averkin A.

The work output of [001] Ni55Fe18Ga27 single crystals was studied under different loading conditions (cooling under 0, 30, 70 and 100 MPa and heating under 150 MPa) in torsion mode. It was shown that during thermal cycling, the functional properties, such as martensitic transformation temperatures, shape memory effect value and work output, of Ni55Fe18Ga27 single crystals stabilized after a few thermal cycles and then barely varied. A maximum work output of 1.15 MJ/m3 was found when the sample was cooled without stress or under a stress of 30 MPa and heated under a stress of 150 MPa. Simultaneously, it was observed that if the stress that was acted on cooling was 30 MPa, then less plastic strain appeared in the sample during thermal cycling.

Churakova, Gunderov

The influence of thermal cycling (TC) with a large number of cycles on the microstructure, the parameters of martensitic transformations (MTs), and the mechanical properties of a Ti-50.8 at.% Ni shape-memory alloy in coarse-grained (CG) and ultrafine-grained (UFG) states was investigated. The effect of microstructural and mechanical stability was found in both coarse-grained and ultrafine-grained states starting from the 100th cycle of martensitic transformations. In addition, an unusual temperature change was observed in martensitic transformations occurring with the formation of an intermediate R phase.

Churakova A.A., Gunderov D.V.

Abstract The influence of thermal cycling in the temperature range of B2-B19’ martensitic transformations on the TiNi alloy structure and properties are studied. The level of properties, obtained as a result of mechanical tests and measurements of microhardness, obtained in the process of thermal cycling, remains stable during soaking at room temperature in the TiNi alloy.

Churakova A.A., Kayumova E.M.

Abstract The mechanical behavior of the coarse-grained Ti49.14Ni50.86 alloy after multiple martensitic transformations B2-B19’ was investigated. A fractographic analysis of the samples after mechanical tests was carried out. The fracture pattern of the Ti49.14Ni50.86 alloy in the coarse-grained state has a viscous and heterogeneous character with microdepths on the fracture surface, the average size of which decreases as the number of thermal cycles increases.

Churakova A.

The effect of multiple martensitic transformations on the microstructure and thermodynamic parameters of the alloy of the TiNi system was investigated. It was shown that in the Ti50Ni50 alloy, with an increase in the number of thermal cycles with rapid heating and cooling up to n = 100, a consistent increase in the dislocation density occurs, and a decrease in the width of martensitic plates is observed. And also, that in TCs with fast heating and cooling of the Ti50Ni50 alloy in a coarse-grained state, a change in the trend in the temperatures of martensitic transformations is observed — with an increase in the number of thermal cycles to n = 50, they decrease, and at n> 50 the temperatures increase.

Churakova A.A., Gunderov D.V.

Abstract In this paper, we investigate the effect of preliminary thermal cycling in the temperature range of martensitic transformation on the aging processes in the ultrafine-grained Ti49.0Ni51.0 alloy, using as an example the examination of the microstructure, mechanical properties and fractography. Our studies show that preliminary thermal cycling leads to a significant increase in the density of dislocations that promote the intensification of the aging process and thereby enhance the strength and functional characteristics of the TiNi alloy in the ultrafine-grained state.

Sibirev A., Resnina N., Belyaev S.

AbstractThe aim of the present work is to find the relationship between a decrease in transformation temperature and an increase in dislocation density during thermal cycling of NiTi alloy. The resistivity is used as a measure of the dislocation density variation and it allows one to find a linear dependence of the increment in the transformation temperatures on the dislocation density variation starting with the fifth thermal cycle. It is found that the thermal cycling of NiTi alloy within a temperature range of 473 to 273 K is accompanied by less dislocation density variation than in a range of 413 to 273 K. The dependence of transformation temperatures on increase in dislocation density in the first five cycles is not linear, hence it is concluded that the dislocation density variation is not the unique reason for a decrease in transformation temperature during the thermal cycling of equiatomic NiTi alloy.

Churakova A.A., Gunderov D.V.

Abstract—This work considers the influence of thermal cycling on the microstructure and aging processes of a TiNi alloy with a high Ni content relative to the stoichiometry. Changes in the microstructure during thermal cycling in coarse-grained and ultrafine-grained states as well as the characteristic features of the aging process at 400°C in preliminarily thermally cycled samples are shown.

Churakova A.A., Gunderov D.V., Dmitriev S.V.

Khlopkov E., Volkov G., Vyunenko Y.

A study of strain recovery by ring-shaped bundle force elements used in fabrication systems has been carried out. After preliminary deformation the elements were connected to an elastic counter-body and heated. It was shown that the choice of the preliminary deformation temperature is a factor influencing the realization of the shape memory effect. This feature can be used to control the operation of a technological device.

Ostropiko E., Razov A., Cherniavsky A.

TiNi shape memory alloys are nowadays widely used in engineering and medicine. The unique capabilities of this material are particularly apparent in applications for space technology. One of the actual present problems is the design of thermosensitive devices for space applications. In this work, we investigated a TiNi alloy as a working element material for the thermosensitive wire drive as an actuator. An optimum thermomechanical treatment of NiTi alloys was selected based on the analysis of functional and mechanical properties. It consists of a multi-cycle implementation of transformation plasticity under 220 MPa constant stress and a shape memory effect in the free state.