Thermomechanical Training Research Articles

Thermomechanical training and characterization of Ni–Ti–Hf and Ni–Ti–Hf–Cu high temperature shape memory alloys

Nickel–titanium (NiTi) is the most commonly used shape memory alloy (SMA) for actuator applications, though its usefulness is limited to temperature ranges below 100 °C. High temperature SMAs are formed by adding ternary elements to NiTi, but their usefulness as actuators is still in question. The purpose of this research was to characterize and train two high temperature SMAs, NiTi29.7Hf20 and NiCu5Ti29.7Hf20, to determine their effectiveness as linear actuators. Low temperature martensitic phase and high temperature austenitic phase stress–strain tests were performed to characterize the materials’ behavior followed by temperature cycling under constant stress. Temperature cycling under constant stress is known as thermomechanical training and resulted in small amounts of plastic strain growth and the development of two-way shape memory (TWSM). The results from these tests support the conclusion that hafnium distorts slip planes within the martensitic material phase, and that (Ti,Hf)2Ni and (Ti,Hf)3Ni4 particulates form during aging and annealing. The distorted slip planes cause slip and martensite reorientation to occur simultaneously, which develops a strong internal stress field during training within the first few cycles. The internal stress field develops TWSM, but limits further plastic growth. The particulate formation also embrittles the material. The transformation temperatures of both alloys were below creep and annealing temperatures making them ideally suited for high temperature actuators.

Microstructure, mechanical and functional properties of NiTi-based shape memory ribbons

The present work has been aimed to study the microstructures, functional properties and the influence of different thermomechanical training methods on the two-way shape memory effect in NiTi-based melt-spun ribbons. In order to get small-dimensioned shape memory alloys (SMAs) with good functional and mechanical properties, a rapid solidification technique was employed. Their fracture and elasticity characteristics have been determined, as well as shape memory properties by thermomechanical cycling. The ribbons were trained under tensile and bending deformation by thermal cycling through the phase transformation temperature range.The results displayed that all different training methods were effective in developing a two-way shape memory effect (TWSME). The influence of copper (5–25at.% Cu) and tungsten (2at.% W) on the microstructure, and the functional and mechanical behavior of NiTi thin ribbons was also investigated. All samples show a shape memory effect immediately after processing without further heat treatment. The melt-spun ribbons were trained under constant strain (bending and tensile deformation) by thermal cycling through the phase transformation temperature range. The addition of copper was effective to narrow the transformation hysteresis. The W addition has improved the TWSME stability of the NiTi alloys and mechanical properties. Results about microstructures, functional and mechanical properties will be presented.

Thermal Behavior of Mechanically Alloyed Powders Used for Producing an Fe-Mn-Si-Cr-Ni Shape Memory Alloy

In order to produce shape memory rings for constrained-recovery pipe couplings, from Fe-14 Mn-6 Si-9 Cr-5 Ni (mass%) powders, the main technological steps were (i) mechanical alloying, (ii) sintering, (iii) hot rolling, (iv) hot-shape setting, and (v) thermomechanical training. The article generally describes, within its experimental-procedure section, the last four technological steps of this process the primary purpose of which has been to accurately control both chemical composition and the grain size of shape memory rings. Details of the results obtained in the first technological step, on raw powders employed both in an initial commercial state and in a mixture state of commercial and mechanically alloyed (MA) powders, which were subjected to several heating-cooling cycles have been reported and discussed. By means of differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and X-ray diffraction (XRD), the thermal behaviors of the two sample powders have been analyzed. The effects of the heating-cooling cycles, on raw commercial powders and on 50% MA powders, respectively, were argued from the point of view of specific temperatures and heat variations, of elemental diffusion after thermal cycling and of crystallographic parameters, determined by DSC, SEM, and XRD, respectively.

Training Effects on Damping Capacity in Mn-22.5mass%Cu-5mass%Ni-2mass%Fe Alloy

The training treatments in the shape memory alloy are known as useful method to improve the shape memory effect. In our previous study, it was shown that the training treatments can also improve both the damping capacity and the hardness of the Fe–Mn alloy. In this study, training effects on damping capacity in solution treated Mn-22.5mass%Cu-5.08mass%Ni-1.96mass%Fe alloy have been investigated. As training treatments, the thermal training (only thermal cycling) and the thermo-mechanical training (thermal cycling with deformation) are carried out. Internal friction was measured at room temperature (R. T.) using a free-decay method. Although training effect cannot be found for the samples trained at higher annealing temperature (600 °C and 700 °C), damping capacity of the alloy is improved by thermal training annealed at 400 °C and 500 °C. The trade-off between the damping capacity and mechanical properties can be overcome by the training at lower temperature.

J044101 バリアント制御によるFe-Mn合金の制振能向上

In our previous study, it is reported that damping capacity as well as hardness of an Fe-20mass%Mn alloy can be improved by the thermo-mechanical training featured by rolling deformation. In this study, the thermo-mechanical training of an Fe-17mass%Mn alloy is carried out with bending mode, since vibration manner of the internal friction measurement refers to bending mode. An anisotropic damping capacity is observed for samples subjected to bending mode training. Moreover, the trade-off between the damping capacity and hardness can be overcome by thermo-mechanical training. To be concluded, the thermo-mechanical training is useful for enhancement of damping properties and hardness of Fe-Mn alloys.

NiTi shape memory alloy thin film micro-cantilevers array

An un-cooled far infrared (IR) imaging sensor micro-array was fabricated using Ni-rich NiTi shape memory alloy (SMAs) films. NiTi SMA films of ~ 5 μm thick were deposited on glass substrates by the direct current (DC) magnetron sputter deposition method. Micro-cantilever arrays were then patterned by the photolithographic technique. After crystallization aging treatments and thermo-mechanical training, the SMA films exhibit typical R-phase transition near room temperature. The thermo-mechanical sensitivity of the SMA film was found higher than that of bi-material films after the two way shape memory (TWSM) training. Illumination of the micro-arrays using IR light source had increased the temperature locally and caused curving up of the illuminated micro-cantilevers. This subsequently had led to a decrease in local reflectivity, which made digital IR imaging feasible.

Cyclic pseudoelastic behavior and energy dissipation in as-cast Cu-Zn-Al foams of different densities

This paper presents a study of the mechanical properties of pseudoelastic Cu-Zn-Al foams. Foams of different densities were cycled in compression. The paper shows the retained/dissipated energy by the material, i.e. the area of the compression cycle, as a function of the material density. The frequency and strain amplitude, usual parameters that may influence the damping properties, are studied by evaluating the dissipated energy and the specific damping capacity. Samples cyclically tested under constrained compression conditions show a remarkable mechanical stability and reproducibility. The material does not appreciably degrade after 1000 compression cycles at room temperature. We emphasize the material does not need any thermomechanical processing or training treatment in order to present the mechanical properties detailed in the present paper.

Comparative analysis of the effects of severe plastic deformation and thermomechanical training on the functional stability of Ti 50.5Ni 24.5Pd 25 high-temperature shape memory alloy

We compare the effectiveness of a conventional thermomechanical training procedure and severe plastic deformation via equal channel angular extrusion to achieve improved functional stability in a Ti 50.5 Ni 24.5 Pd 25 high-temperature shape memory alloy . Thermomechanical testing indicates that both methods result in enhanced shape memory characteristics, such as reduced irrecoverable strain and thermal hysteresis . The mechanisms responsible for the improvements are discussed in light of microstructural findings from transmission electron microscopy .

Transformation twinning of Ni–Mn–Ga characterized with temperature-controlled atomic force microscopy

The magnetomechanical properties of ferromagnetic shape memory alloy Ni-Mn-Ga single crystals depend strongly on the twin microstructure, which can be modified through thermomagnetomechanical training. Atomic force microscopy (AFM) and magnetic force microscopy (MFM) were used to characterize the evolution of twin microstructures during thermomechanical training of a Ni-Mn-Ga single crystal. Experiments were performed in the martensite phase at 25 degrees C and in the austenite phase at 55 degrees C. Two distinct twinning surface reliefs were observed at room temperature. At elevated temperature (55 degrees C), the surface relief of one twinning mode disappeared while the other relief remained unchanged. When cooled back to 25 degrees C, the twin surface relief recovered. The relief persisting at elevated temperature specifies the positions of twin boundaries that were present when the sample was polished prior to surface characterization. AFM and MFM following thermomechanical treatment provide a nondestructive method to identify the crystallographic orientation of each twin and of each twin boundary plane. Temperature dependent AFM and MFM experiments reveal the twinning history thereby establishing the technique as a unique predictive tool for revealing the path of the martensitic and reverse transformations of magnetic shape memory alloys.

Shape memory characteristics of the P/M-processed Ti–Ni–Cu alloys

Ti–Ni and Ti–Ni–Cu shape memory alloys (SMAs) were fabricated by the powder metallurgy (P/M) process using mechanical alloyed (MAed) powder. The phase transformation behavior shape memory characteristics and thermo-mechanical properties of the P/M alloys were investigated. The stress–strain curves of Ti–45.2 mol%Ni–5 mol%Cu and Ti–35.2 mol%Ni–15 mol%Cu alloys were stabilized by the thermo-mechanical training. The two-way shape memory effect appeared after the thermo-mechanical training. Only the Ti–30.2 mol%Ni–20 mol%Cu alloy showed clearly a two-step transformation. The transformation behavior of the P/M alloy was different than that of the wrought alloy because the Cu content of the P/M alloy was reduced by composition segregation. The narrowest hysteresis measured in Ti–35.2 mol%Ni–15 mol%Cu was 9 K, which is as narrow as that of the wrought alloy. The Ti–30.2 mol%Ni–20 mol%Cu alloy had the highest stress for slip deformation due to solid–solution hardening by Cu addition. The plastic strain of the alloy thermal-cycled at 175 MPa was 0.06%. The Ti–35.2 mol%Ni–15 mol%Cu alloy had the lowest value of d σ/d T and the alloy showed superelasticity in the austenite phase. Thus, it was found that the Ti–Ni and Ti–Ni–Cu SMAs with superior shape memory characteristics could be fabricated by the P/M process.

基于马氏体区域化形成的免训练铸造Fe-Mn-Si-Cr-Ni形状记忆合金 I. 构想与实现

Low cost Fe-Mn-Si based shape memory alloys(SMAs) has not got widely applications because of their poor shape memory effect(SME) and the need of thermo-mechanical training. so developing training-free Fe-Mn-Si based SMAs with high memory property is significant.In the present study,it was put forth that the formation of stress-inducedemartensite in a domain manner could improve the SME of Fe-Mn-Si based SMAs and it could be realized through subdividing austeniteγgrains into smaller domains using the residual lathyδferrite phase.According to Hammar's equivalents,a cast Fe-18Mn-5.5Si-9.5Cr-4Ni alloy with Cr/Ni equivalent ratio of 1.85 was prepared.OM and VSM(vibrating sample magnetometer) examination showed that the as cast microstructure consists ofγaustenite and lathyδferrite phase,and the lathyδferrite subdivided the austenite grains into smaller domains.which makes the stress-inducedemartensite bands form in a domain manner.Because the collisions between domain-like martensite bands were reduced,a high recovery strain of 4.9%was attained in the as-cast Fe-18Mn-5.5Si-9.5Cr-4Ni alloy.This result provides a novel way of developing training-free Fe-Mn-Si based SMAs.It can be expected that the SME of cast Fe-Mn-Si based SMAs will be further improved through modifying and optimizing alloy compositions, solidification parameters and heat treatment process.

Training Effects on Damping Capacity in Fe-Mn and Fe-Mn-Cr Alloys

Training effect in the Fe-Mn-Si shape memory alloy is known as useful method to improve the shape memory effect. In this study, the training effects on damping capacity in Fe-20mass%Mn and Fe-20.5mass%Mn-12.5mass%Cr alloys have been investigated. As training treatments, the thermal training (only thermal cycling) and the thermo-mechanical training (thermal cycling with rolling deformation) are carried out. Internal friction was measured at room temperature using a free-decay method. Moreover, the behavior of dislocations was observed by TEM. Both training treatments improve the damping capacity of the Fe-Mn alloys with increasing the number of treatment. Strong training effect was found for the specimens trained by the thermo-mechanical training. The main training effect by thermal cycles is concluded to be due to size effects, while the size effects and volume fractional effects of martensite phase affect the damping capacity of the thermo-mechanically trained alloys. These training methods can improve both damping capacity and strength of Fe-Mn alloys.

Recent Progress in FSMA Microactuator Developments

The giant magneto-strain effect is particularly attractive for actuator applications in micro- and nanometer dimensions as it enables contact-less control of large deformations, which can hardly be achieved by other actuation principles in small space. Two different approaches are being pursued to develop ferromagnetic shape memory (FSMA) microactuators based on the magnetically induced reorientation of martensite variants: (1) the fabrication of free-standing epitaxial Ni-Mn-Ga thin film actuators in a bottom-up manner by magnetron sputtering, substrate release and integration technologies and (2) the top-down approach of thickness reduction of bulk Ni-Mn-Ga single crystals to foil specimens of decreasing thicknesses (200 – 40 μm) and subsequent integration. This review describes the fabrication technologies, procedures for thermo-mechanical training adapted to the quasi-two-dimensional geometries of film and foil specimens as well as the performance characteristics of state-of-the art actuators after processing and training.

Characterization of phase transformations and shape memory behavior of Fe–27.79Mn–2.72Si (wt%) alloy by thermomechanical and thermal treatments

Magnetic properties and volume fraction of the phases of a Fe–27.79Mn–2.72Si (wt%) alloy have been investigated by Mossbauer spectroscopy. The microstructure has been investigated using transmission electron microscope, scattering electron microscope, and light optical microscope. The stress-induced martensitic transformation, shape memory behavior, and shape recovery processes have been studied through the differential scanning calorimetry (DSC), tensile, and bending tests. The specimen was treated by the repetition of small amount of tensile deformations at room temperature, followed by a subsequent annealing at 600 °C at every cycle for bending and tensile tests. During the tensile tests, Vickers hardness values were determined for every step of thermomechanical treatments. At low deformation rate, the shape memory effect is almost complete Bergeon et al. (A242:87, 1998), so low deformation rate was studied in this work. Bending test was another thermomechanical training procedure to see the recovery rate for this investigation.

Magneto-microstructural coupling during stress-induced phase transformation in Co49Ni21Ga30 ferromagnetic shape memory alloy single crystals

The present study reports on direct magneto-microstructural observations made during the stress-induced martensitic transformation in Co49Ni21Ga30 alloy single crystals with optical, scanning electron, and magnetic force microscopy (MFM). The evolution of the microstructure and the associated magnetic domain morphology as a function of applied strain were investigated in the as-grown condition and after thermo-mechanical training. The results demonstrated that the stress-induced martensite (SIM) evolves quite differently in the two conditions and depending on the martensite formation mechanisms, the magnetic domain configuration was dissimilar. In the as-grown crystals two twin-related martensite variants were formed and the growth of these twin variants resulted in large strain. After thermo-mechanical training a morphology similar to a self-accommodating martensite structure was present at the initial stages of the transformation and thereafter martensite reorientation (MR) was the main transformation mechanism. The magnetic domains were found to be superimposed on the nano-scaled martensite twins in the as-grown condition, whereas training brought about the formation of domains on the order of a few microns without showing the one-to-one correspondence between domains and the twin structure. After the thermo-mechanical training detwinning at high-strain levels led to the formation of stripe-like domain structures. The ramifications of the results with respect to the magneto-microstructural coupling that may cause the magnetic shape memory effect (MSME) in Co–Ni–Ga alloys under constant external stress is addressed.

A 1% magnetostrain in polycrystalline 5M Ni–Mn–Ga

Magnetostrain is achieved by twin boundary motion in magnetic shape memory materials. A 1% strain could be achieved in textured polycrystals of Ni–Mn–Ga at room temperature. Ni 50Mn 29Ga 21 was directionally solidified in order to obtain a texture parallel to the heat flow. The alloy was heat treated for chemical homogenization and stress relaxation in the austenite state. Thermomechanical training was employed to decrease the twinning stress and to enhance the strain by simplification of the variant microstructure. Investigations at slightly elevated temperatures were performed to increase the magnetostrain to 1%.

The effects of thermomechanical training treatment on the deformation characteristics of Fe–Mn–Si–Al alloys

We have studied the effects of the thermomechanical training treatment on the deformation mode for Fe–30Mn–(6 − x)Si– xAl ( x = 0, 1, 2 and 3 mass%) alloys, which exhibit complicated deformation processes due to coexistence of the stress-induced fcc to hcp martensitic transformation and the mechanical twinning of the fcc matrix in addition to slip deformation. The stress–strain curves at each training cycle were analyzed for the alloys, and the deformation microstructures were observed with atomic force microscopy on a Fe–30Mn–5Si–1Al alloy before and after the training treatment. It was found that the deformation mode shifts from the mechanical twinning to the stress-induced martensitic transformation, as the training proceeds.

Improvement of the Shape Memory Effect in Fe‐based Alloys by Training

Shape recovery in iron‐based shape memory alloys is known to be incomplete. Thermomechanical cycling or training is recognized to improve the shape memory effect. In the present work, different aspects of training are studied: the number of cycles, the amount of deformation and the annealing temperature. As the number of cycles increases, the ∊ martensite plates become thinner and within the different grains they tend to align in the same direction. Still, different variants are present. XRD measurements showed a strong increase in the ∊ martensite volume from cycle 1 to cycle 2 and a slight decrease for further cycles. There exists an optimum deformation for obtaining stress induced ∊ martensite which is reversibly transformable to austenite during annealing. When the amount of deformation is too low, the fraction of stress induced ∊ martensite is very low, therefore, the shape memory effect is small. If the deformation is too large, slip may occur and different ∊ martensite variants are formed, which hinders the reverse ∊→γ transformation. The recovery annealing temperature is critical for obtaining a good shape memory, especially during training. The recovery annealing temperature of 400°C is too low for completing the reverse transformation; annealing at 600°C is most commonly used.

Hysteresis modeling of two-way shape memory effect in NiTi alloys

In the present study the two-way shape memory effect (TWSME) of a Ni-51 at.% Ti alloy was investigated and a numerical model, able to simulate its hysteretic behaviour in the strain-temperature re- sponse, is proposed. In particular, the TWSME was induced through a proper thermo-mechanical training, carried out at increasing number of training cycles and for two values of training deformation, and the thermal hysteretic behaviour, between Mf (Martensite finish temperature) and Af (Austenite finish temperature), was recorded. The experimental measurements were used to develop a phenomenological model, based on the Prandtl-Ishlinksii hysteresis operator, which was implemented in a Matlab ® function and a Simulink ® model. A systematic comparison between experimen- tal results and numerical predictions is illustrated and a satisfactory accuracy and efficiency has been ob- served, therefore the method looks suitable for real- time control of NiTi based actuators.

Training effect on damping capacity in Fe–20 mass% Mn binary alloy

Training effects on damping capacity in Fe–20 mass% Mn alloy have been investigated. As training treatments, the thermal training (only thermal cycling) and the thermo-mechanical training (thermal cycling with deformation) are carried out. Both training treatments improve the damping capacity of the Fe–Mn alloy with increasing the number of training cycles. Comparing improvement ability of the damping capacity between both treatments, thermo-mechanical treatment can improve it more effective. It is found that these training treatments can improve both the damping capacity and the hardness of the Fe–Mn alloy.