Variation In Transformation Temperatures Research Articles

Understanding the role of laser processing parameters and position-dependent heterogeneous elastocaloric effect in laser powder bed fused NiTi thin-walled structures

To reduce the driving load and enhance the heat exchange capacity and elastocaloric refrigeration efficiency, increasing interests in porous structure design and laser-based additive manufacturing (LAM) of NiTi materials with a large specific surface area have been emerging. As a type of characteristic unit of porous components, we mainly focused on the LAM process optimization and elastocaloric effect of NiTi-based thin-walled structures (TWSs) in this work. Firstly, we systemically studied the influence of laser processing parameter on the forming quality and phase transformation behavior of NiTi-based TWS samples. Results showed that high relative density (>99.0%) was inclined to be obtained in a range of 67–133 J mm−3 (laser energy density). Besides, the transformation temperatures (TTs) and enthalpy change roughly showed a positive linear relationship with the applied laser energy density. At an optimized parameter (P = 100 W and v = 1000 mm s−1), the sample exhibited a high relative density (99.88%), good dimensional accuracy, and the lowest TTs. Then, this work emphatically unveiled the position-dependence of phase transformation behavior and elastocaloric effect (eCE) in a NiTi-based TWS sample. It was found that both the TTs and enthalpy change monotonously decreased along the building direction, while the transformation strain kept an increase trend. As a result, the middle portion of the sample exhibited the largest adiabatic temperature change which reached 6.5 K at the applied stain of 4%. The variation in TTs and eCE could be attributed to the heterogeneous solidification microstructure induced by the thermal cycle nature of LAM process.

Effect of energy density on the superelastic property of Ni-rich NiTi alloy fabricated by laser powder bed fusion

Laser powder bed fusion (LPBF) has proved to be a promising process for tuning the microstructure and mechanical response in NiTi-shaped memory alloys using a varied energy density input by modifying the process parameters such as laser power or scanning speed. The microstructure, precipitation, chemical composition, and their relationship with austenite–martensite transformation temperatures (TTs) and superelastic properties have been extensively investigated in Ni-rich NiTi alloys processed at varying volume energy densities (52–110 J/mm3) through LPBF. A specific amount of Ni3Ti precipitation was observed and primarily located along the boundary and at the center of the melt pool, and nanoscale spherical NiTi2/Ni2Ti4OX precipitation was homogeneously dispersed in the B2 matrix and B19’ martensite, but no Ni4Ti3 was observed as frequently reported in the literature. Decreasing the volume energy density (VED) led to lower TTs and a higher recovery strain ratio. However, there is an exception for the sample built with the highest laser power of 250 W that possesses the largest Ni/Ti atomic ratio and therefore the highest superelastic property despite the mediated VED (∼87 J/mm3) employed. We concluded that the variation in TTs and superelastic properties among the samples is caused by the varying amounts of Ni, which causes competition among alloying elements in evaporation, condensation, and precipitation during LPBF.

Relationship between the variation in transformation temperatures, resistivity and dislocation density during thermal cycling of Ni50Ti50 shape memory alloy

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.

On the Transformation Behavior of NiTi Shape-Memory Alloy Produced by SLM

Selective laser melting has been applied as a production technique of nickel titanium (NiTi) parts. In this study, the scanning parameters and atmosphere control used during production were varied to assess the effects on the final component transformation criteria. Two production runs were completed: one in a high (~1800 ppm O2) and one in a low-oxygen (~220 ppm O2) environment. Further solution treatment was applied to analyze precipitation effects. It was found that the transformation temperature varies greatly even at identical energy densities highlighting the need for further in-depth investigations. In this respect, it was observed that oxidation was the dominating factor, increased with higher laser power adapted to higher scanning velocity. Once the atmospheric oxygen content was lowered from 1800 to about 220 ppm, a much smaller variation of transformation temperatures was obtained. In addition to oxidation, other contributing factors, such as nickel depletion (via evaporation during processing) as well as thermal stresses and textures, are further discussed and/or postulated. These results demonstrated the importance of processing and material conditions such as O2 content, powder composition, and laser scanning parameters. These parameters should be precisely controlled to reach desired transformation criteria for functional components made by SLM.

熱エンジン用形状記憶合金テープ素子の形状記憶・機械的特性に及ぼす加熱冷却サイクルの影響

Since Ti-Ni shape memory alloys(SMAs) work well at temperature less than 373K ,SMAs are expect to apply the heat-engine which recovers low temperature thermal energy .The pulley-type SMA heat-engine, which is one of the SMA heat-engine, is driven by heating and cooling of the liner or coil shape SMA element. We fabricated tape-shaped SMA element for the purpose of improving the product's life span. However, shape memory and mechanical characteristics of tape-shaped SMA is deteriorated with increasing number of heating and cooling cycle. This function deterioration leads to variation of transformation temperatures and shape memory characteristics. The output power of SMA heat-engine depends on the transformation temperature and shape memory characteristics of SMA element. Owing to this, the improvement of function deterioration characteristics of SMA element leads to the output stabilization of SMA heat-engine. In this research, the effects of heating and cooling cycle on the shape memory and mechanical characteristics of tape-shaped SMA element is investigated.

Martensitic Transformation and Shape Memory Effect in TiNi-Based Alloys during Neutron Irradiation

The chapter is devoted to a study of the influence of neutron irradiation on the martensitic transformations and shape memory effects in TiNi-based shape memory alloys. Irradiation of the samples was carried out in the low-temperature helium loop of a WWR-M fusion reactor at Petersburg Nuclear Physics Institute in Gatchina (Russia). The experimental data showed that the variation in transformation temperatures depended on the irradiation temperature. The main factors influencing the variation in transformation temperatures during irradiation were disordering of the solid solution at low temperatures, radiation ordering at high temperatures, and thermally activated annealing of radiation damage. All of these mechanisms were taken into account in the differential equation given in the present work for description of the transformation temperature variation during irradiation at different temperatures. It was found that irradiation up to a fluence of 7⋅1018 cm-2 did not suppress the transformation plasticity and shape memory effects in TiNi alloy in spite of the variation in transformation temperatures. It was observed that the shape memory effect may be initiated by irradiation up to a fluence of 5⋅1020 cm-2 at a constant temperature (under isothermal conditions) due to a decrease in transformation temperatures.

Effect of Variation in Applied Force on Transformation Temperatures of NiTinol SMAs

Shape Memory Alloys (SMA) are promising materials for actuation in space applications, because of the relatively large deformations and forces that they offer. However, their complex behaviour and interaction of several physical domains (Electrical, Thermal and Mechanical), the study of SMA behaviour is a challenging field. Phase transformation temperature is one of the most important parameters for the shape memory alloys (SMAs). In this present work, a novel method of determining transformation temperatures is carried out which uses dead weight. The SMA NiTinol wire was applied with force and maintained constant throughout the experiment and at the same time it was heated up slowly until a temperature of approximately 80°C with direct current. The direct current was then slowly decreased to cool down the temperature of the NiTinol wire. All the phase transformation temperatures for NiTinol SMA wire were obtained. A variation in transformation temperatures are recorded with variation in applied force, which is an important aspect calls for attention before using the material in any application. In the present work, it has been investigated and reported in detail.

Ni24.7Ti50.3Pd25.0 high temperature shape memory alloy with narrow thermal hysteresis and high thermal stability

High temperature shape memory alloys with operating temperatures above 100°C are in demand for use as solid-state thermal actuators in aerospace, automobile and other engineering applications. The present study deals with transformation behaviour and thermal stability of Ni24.7Ti50.3Pd25.0(at.%) high temperature shape memory alloy, in cast and homogenized condition. The martensite finish temperature and transformation hysteresis of the alloy were determined to be 181.0°C and ∼8.5°C respectively. The alloy showed high stability upon stress-free thermal cycling, variation in transformation temperatures being ±1°C. The narrow thermal hysteresis and high thermal stability of the alloy upon transformation cycling has been discussed and correlated with its microstructural features, activation energy and elastic strain energy of thermoelastic martensitic transformation. The alloy exhibited modulus of ∼82GPa and hardness of ∼4.7GPa in martensite phase.

Influence of Sc addition on microstructure and transformation behaviour of Ni24.7Ti50.3Pd25.0 high temperature shape memory alloy

Vacuum-arc melted Ni24.7Ti50.3Pd25.0 and Ni24.7Ti49.3Pd25.0Sc1.0 (at.%) alloys were investigated to study effect of Sc micro-addition on microstructure and transformation behaviour of NiTiPd alloy. Study showed that microstructure of homogenized NiTiPd alloy consisted of NiTiPd matrix interspersed with Ti2(Ni,Pd) precipitates. In contrast, NiTiPdSc alloy showed a single phase NiTiPdSc matrix with a few scandium oxide particles at isolated places. TEM and X-ray diffraction studies confirmed matrix phase of the alloys to be of orthorhombic B19 structure. TEM observations showed that NiTiPdSc alloy had relatively larger martensite plates with a smaller twin ratio compared to that of NiTiPd alloy. Also, APB (anti-phase boundary) like regions with twinless martensites was observed in both the alloys, area fraction of APB-like regions being more in NiTiPdSc alloy. Thermal analysis showed that transformation temperatures (TTs) of NiTiPd alloy decreased significantly with addition of Sc. The martensite finish temperature (Mf) of 181 °C for NiTiPd alloy lowered to 139 °C upon 1.0 at.% Sc addition. The transformation hysteresis of Ni24.7Ti49.3Pd25.0Sc1.0 (at.%) alloy was measured to be 7 °C, significantly lower than that of 15 °C for Ni24.5Ti50.0Pd25.0Sc0.5 alloy, reported in literature. Alloy purity, lower volume fraction of second phase and presence of twinless/small twin ratio martensite in microstructure is believed to be the reasons for such low transformation hysteresis. The transformation behaviour of the alloys upon stress-free thermal cycling was found stable, variation in TTs being within 1–2 °C.

Effect of La2O3 on the physical and crystallization properties of Co2+-doped MgO–Al2O3–SiO2 glass

The physical and crystallization properties of MgO–Al2O3–SiO2 glasses with various La2O3 additions (0–10wt.%) were studied by means of density, hardness and coefficient of thermal expansion (CTE), differential thermal analysis (DTA), in situ high temperature XRD, room temperature XRD, and absorption spectrum. Density, hardness and CTE were found to increase with adding La2O3 content. The slight variation of transformation temperature (Tg) observed between different LMAS glass compositions can be attributed to the relatively strong LaO bonds. La2O3 was found to hardly influence the nucleation and crystallization processes of Co2+-doped MgO–Al2O3–SiO2 glasses when lower than 5wt.%, while the nucleation of the glass was hindered and MgAl2O4 spinel cannot be formed with the La2O3 content greater than 5wt.%. Transparent glass-ceramics were synthesized by heat-treating glasses with 0–5wt.% La2O3 at 760°C/12h+930°C/4h; the changes in absorption spectra reflected that Co2+ ions entered into MgAl2O4 nanocrystals precipitated in the glass-ceramics and occupied tetrahedral coordinated site.

Microstructural Evolution in CuAlNi Alloy with Ageing

This research has been performed to investigate the effect of ageing onto the grain refinement of 82wt%Cu-13.5wt%Al-4.5wt%Ni-0.1wt% misch metal alloy with ageing temperature and time. The variation of transformation temperature with ageing has been found to be heavily dependent on ageing time and temperature. In the second reversed transformation cycle, the temperature of Ms point decreases with ageing time at 300, but no variation at 100. The temperature of As point increases with ageing time at both temperatures. After the second reversed transformation cycle, the transformation temperature does not show any significant changes in martensitic phase with ageing, the temperature of Ms point being the same afterwards. Parent phase after ageing appears to show M18R and N2H martensitic phases depending on the ageing time and temperature. It is also found that α and γ2 phases are formed only at the condition of 300 and 24h.

Effects of Pre-Strain and Nb Content on Transformation Temperatures in Ti-Ni-Nb Shape Memory Alloy

Ti-Ni-Nb shape memory alloy has a wide transformation hysteresis, and has been used as coupling devices and so on. However systematic researches of the influence of the amount of Nb addition on transformation temperatures are few. The purpose of this work is to clarify the influences of pre-strain and Nb content on transformation temperatures in Ti-Ni-Nb shape memory alloys. The specimens were Ti-Ni-Nb alloys (Ni/Ti=1.0, Nb=0-15at%), annealed at 1223 K for 0.3 ks. The variation of transformation temperatures with pre-strain and Nb content were investigated experimentally. The variation of As, Ms and transformation temperature hysteresis with pre-strain and Nb content will be discussed in relation to the elastic strain energy and the volume fraction of slip-deformed martensite.

Novel micro-thermal characterisation of thin film NiTi shape memory alloys

Abstract A novel approach in determining the transition temperatures of thin films of NiTi shape memory alloys was investigated and compared with conventional techniques. The technique is based on micro-thermal analysis utilising a scanning thermal microscope (SThM). This method has the potential to allow the transformation temperatures of thin films to be investigated in situ. In this study, thermal micro-analysis using an SThM was investigated on thin film samples of 47.5 at.% Ni and 49.6 at.% Ni in Ti. Thin amorphous NiTi films were deposited on silicon substrates using DC plasma assisted sputtering. The thin films were recrystallised and subsequently characterised by SThM. Results were then compared with DSC, XRD and resistivity measurements. The SThM sensor variation with temperature showed clear onset and completion of transformation from martensite to austenite during a heating cycle. The small probe size of 1 μm2 allowed variations in transformation temperatures across the NiTi thin film to be investigated. A range of transition temperatures was observed, which was consistent with the difference between AS and AF as determined by resistivity and bulk DSC measurements. The linear contraction between martensite and austenite was also measured by this technique and was found to be similar to the theoretical transformation of 0.45%. This novel method will provide a route to investigate both the shape memory transformation temperatures of NiTi thin films and their spatial variation across devices such as micro-actuators.

Thermal and electrical characterization of R-phase dependence on heat-treat temperature in Nitinol

The variation of transformation temperatures with heat treatment has been studied for the near equiatomic, 40% prior cold worked Nitinol by the DSC measurements. The austenitic and martensitic start and finish temperatures are found to be almost independent of heat-treat temperature in the range 525–680°C. Intermediate R phase is found to be stable only in the range 340–410°C. The Electrical resistivity probe is found to be better than the DSC probe in the study of hysteresis associated with R-phase transformation. Large and sharp changes in electrical resistivity in R↔A transformation is expected to be useful in applications.

Effects of Recrystallization and Grain Size on the Martensitic Transformation in Fe-31%Mn-5%Si Alloy

The effects of recrystallization and grain size have been studied in an Fe-31%Mn-5%Si shape memory alloy. The amount of martensite formed in samples which have been hot rolled at 1373 K and subsequently homogenized for 24 h at 1273 K followed by a final 30 min anneal at 1323 K is twice as large as in samples which have been hot rolled and homogenized in the same way as the former followed by a compression and a subsequent recrystallization at 1323 K for 30 min. The only difference between the samples is the plastic deformation induced by the compression. We thus conclude that some effect of the compressive deformation remains despite the recrystallization and suppresses the subsequent martensite formation. However, we have not yet been able to identify this effect. This matter will be subject to further investigations. No significant change is found in the M(S) and A(S) temperatures for the gamma epsilon transformations determined by DSC. When the grain size varies between 40 and 160 mu m in completely recrystallized samples the variation in transformation temperatures is small, less than 10 K.

Fatigue of the Shape Memory Effect in thin Wires-Comparison between TiNi and CuZnAl

Composites containing shape memory alloys in the form of fibres are well-known for their use as actuators in adaptive structures. However orientation ofthe research work concerning these materials has been mainly directed to qualitative analysis, thus a lack of data on the mechanical properties of these alloys, especially on fatigue behaviour, necessitates futher research. The aim of the presesnt research work is to study the shape-memory effect on TiNi and CuZnAl wires. A tensile testing machine has thus been developped specialy adapted to such kind of investigations. The fatigue analysis of the shape memory was performed using wire specimens. The specimens were subjected to ranges of stresses under thermal cycles, thus the variation of transformation temperatures as well as the variation of strain between the high and low temperature phases was recorded. Finally, a comparative study is presented in case of TiNi and CuZnAl wire specimens of same diameters

Homogeneous martensitic nucleation in FeCo precipitates formed in a Cu matrix

The martensitic transformation on subambient cooling has been monitored in defect-free nanocrystalline f.c.c. FeCo particles that have been coherently precipitated in a Cu matrix; such a CuFeCo system was chosen for study because the f.c.c. → b.c.c. transformational driving force in FeCo alloys is exceptionally large. Transformation is found to occur at a driving force of ∼ 10kJ/mol, a factor of 7 higher than the known critical driving forces for heterogeneous nucleation in bulk alloys. The observed transformation characteristics are entirely consistent with classical homogeneous coherent nucleation, whereas heterogeneous nucleation and homogeneous semicoherent nucleation can be ruled out in this case. An observed variation in transformation temperatures is explained by the experimentally-determined differences in Co content (and, hence, in transformational driving force) among the FeCo precipitates in relation to their distribution of particle sizes. The role of thermal activation in the homogeneous nucleation process is demonstrated by applying a high magnetic field to impose an increment of driving force at low temperatures.

THERMAL CYCLIC DEFORMATION AND DEGRADATION OF SHAPE MEMORY EFFECT IN Ti-Ni ALLOY

Abstract The most basic single axial cyclic motion (i.e. fatigue) tests by the alternative heating up and cooling down of shape memory Ti-Ni50.2at%wires were performed. Heating up was done by direct current method or temperature controllable water-bath and the characteristics on the cyclic motion and its degradation in shape memory effect (SME) are discussed. The degradation of SME can be divided into two factors; the cyclic deformation by thermoelastic transformation ΔD and the gradual inelasatic elongation U of Ti-Ni wire. The stress dependency of each SME degradation factor, ΔD or ΔU, and the variation of transformation temperature with thermal cycles are discussed. Lastly, nondestructive estimation of total thermal fatigue life of wire is tried from the view point of the initial changing rate of these degradation parameters.

Displacive transformations in near-equiatomic Nb-Ru alloys—II. Energetics and mechanism

The transformation distortions during the β→β′ and β′→β′′ transformations in near eluiatomic Nb-Ru allols produce phases that have progressivell loler slmmetrl than the CsCl tlpe β phase and approach the packing of the hexagonal close packed structure. Bl the use of a simple approximation—the superposition approximation—a model is developed lhich explains all of the experimental measurements on this allol slstem. The driving force for these transformations is electronic in nature. The Fermi energl μ of the β phase allols is near a sharp peak in the densitl of status. ρ( E). The transformations broaden this peak, lolering ρ(μ) and the energl of the phase. The significant experimental measurements accounted for bl this model are the variation of magnetic susceptibilitl lith phase and Ru content, the variation of transformation temperatures and transformation distortion lith Ru composition, and distortion in the β phase lith temperature and Ru composition.

Effects of the Residual Water on the Transformation Temperature and the Structure of BaO-P<sub>2</sub>O<sub>5</sub> Glasses

Water content of BaO-P2O5 glasses prepared by various preparation conditions was determined by chemical analysis and the effects of the residual water on the transformation temperature and the structure of these glasses were investigated. Following conclusions were obtained.(1) The transformation temperature of the glasses is exceedingly influenced by preparation condition.(2) The cause of such a variation of transformation temperature is attributed to the change of water content in the glasses.(3) Water content in the glasses is 1-3 wt% and varies according to preparation condition and composition.(4) The cause why such a considerable amount of water is contained in the glasses is attributed to the nature of the chemical bond of PO4 group; that is, the branching point P*O4 group is unstable according to Van Wazer's “Antibranching Rule of PO4 group”. Therefore, it is assumed that the residual water in the glasses exists as OH group as in the following.-O-O|P|O|-*O-O|P|O|-*O-+H2O→←-H2O2[-O-O|P|OH-O-](5) Considering “Antibranching Rule” and high volatility of P2O5 component, the preparation of water free BaO-P2O5 glasses is impossible.(6) Since the water in the glasses can be regarded as the network modifier in the glass structure, residual water has a great influence on the transformation temperature of the glasses.