Shape memory alloy (SMA) helical springs are special mechanical parts that require a previous evaluation of its behavior for application. Therefore, in this paper thermal and mechanical behaviour of superelastic Ni-Ti SMA helical extension springs manufactured by investment casting (IC) are evaluated. Phase transformation temperatures were measured by Electrical Resistance as a function of Temperature (ERT) and Differential Scanning Calorimetry (DSC). Tensile tests were carried out within strain and temperatures ranges. The pitch angle and stiffness of each spring were determined. Results demonstrated that Ni-Ti SMA helical springs produced by IC presented phase transformation corresponding to the superelastic effect (SE). The reversible deformations under tensile test were of the order of 70%. The mechanical behavior as function of temperature revealed a linear relationship between maximum force and spring temperature.

The objective of the present work was to evaluate welding of thin sheets (thickness < 1 mm) of similar and dissimilar NiTi alloys with AISI 304 stainless steel using the Gas Tungsten Arc Welding (GTAW) process and to study the mechanical and metallurgical properties of the joints with and without post-welding heat treatment (PWHT). The GTAW process was chosen because it is more economical than the usual welding processes for NiTi alloys, such as the Laser Beam Welding (LBW). The welded joints were characterized by the techniques of SEM, OM, electrical resistance in temperature (ERT), tensile test and Vickers microhardness. It was observed that the dissimilar joints presented a brittle behavior due to the formation of brittle elements along the weld metal by the excessive increase of the hardness in this region with peaks of hardness higher than 900HV. Similar joints presented superior mechanical behavior, with extensive plastic deformation before rupture and fracture surface with ductile appearance. The PWHT in the similar joints promoted a decrease in the rigidity of the material by minimizing the thermal stresses from the welding process, the PWHT did not influence the dissimilar joints mechanical behavior.

This paper presents a thermal and microstructural characterization of Cu87-xAl13Nbx (wt%) high temperature shape memory alloys (HTSMA), with Nb contents ranging from 1 to 3%. The alloys were obtained by arc melting under argon atmosphere. Thermal characterization was performed by differential scanning calorimetry (DSC), revealing that the phase martensitic transformation of all the studied alloys occurs in the range of 200 ºC to 450 ºC. Thermal cycling in this temperature range caused the martensitic transformation to disappear after approximately 6 cycles. However, a new heat treatment at 850 ºC followed by water quenching causes the martensitic transformation to be recovered. Microstructural characterization was performed using optical microscopy (OM) and scanning electron microscopy (SEM), revealing the presence of Nb precipitates in a Cu-Al martensitic matrix, mainly in the Nb-rich compositions. These different microstructures with Nb particles cause a hardness variation that initially decreases between 0.5 %Nb (275HV) and 1.0 %Nb (225HV), then increasing continuously up to 3.0% Nb (380HV).

Shape memory alloys (SMA) are attracting considerable attention owing to possible applications from biomedical to aerospace. In particular, CuAlNi alloys present significant advantages associated with low cost, easy processing and superior thermo-electric conductivity over other SMAs such as the NiTi alloys. Characterization of some properties and structural changes caused by martensitic transformation are still open to investigation. The present work evaluated these characteristics in an as-cast plasma processed shape memory Cu-14wt.%Al4wt.%Ni, which was compression tested until fracture. Experimental results showed that an as-cast ingot presents not only chemical and phase homogeneity, but also microstructures composed of grains with martensitic morphology. Martensites β'1 and γ'1, as well as intermediary martensitic R and high temperature β1 were identified by X-ray diffraction tests. It was found that the compressive deformation does not interfere in the phase composition and martensite morphology. However, compression changes the volumetric fractions and crystallographic orientation of the martensites. The mechanical behavior is characterized by an apparent elastic response until the fracture. The fractured surface exhibits brittle aspect like "river patterns" and evidence of intergranular rupture.

In copper-based shape memory alloys (SMAs), some exceptional phenomena, such as the shape memory effect (SME) or superelasticity (SE), are observable. However, commercial aluminum bronzes, Cu3Al-based alloys, do not present these functional properties (SME and/or SE) in their original state. Thus, since one of the main copper-based SMA systems is the Cu-Al-Ni alloy, this paper aims to analyze the modification of these commercial aluminum bronzes to SMA by the addition small amounts of Cu, Al and/or Ni. These modified bronzes were reprocessed by induction melting and injected by centrifugation into a ceramic coating mold. The modifications were made to determine the nominal composition for a Cu-13,0Al-4,0Ni (%wt) SMA. The effectiveness of the modifications was verified by differential scanning calorimetry (DSC) thermal analysis. All modified Cu-Al-Ni bronzes presented DSC peaks of the thermoelastic martensitic phase transformation, showing that SMA behavior was achieved, while the non-modified bronzes revealed no transformation. These results were supported by Vickers hardness (HV), X-ray diffraction (XRD), semi quantitative composition by EDS analysis and optical microscopy.

Ti‒50.13Ni and Ti‒49.62Ni (at.%) shape memory alloy ribbons were fabricated by melt-spinning method at different circumferential wheel velocities. The effects of wheel velocity, chemical composition and heat treatments on microstructure and Transformation temperature were investigated. Differences in wheel velocity led to differences in cooling rate and sample dimension, as well as in phase transformation temperatures. Two heat treatment conditions were studied, 350°C for 1h and 350°C for 5h. In the samples produced at high wheel velocity and heat-treated at 350°C for 5h, nanosized Ti-rich precipitates were observed in both chemical compositions. Cross-sectional microstructure was studied by optical microscopy; SEM was used to study the nanometric grains and nano precipitation. The transformation temperatures were analyzed by DSC.

This work deals with structural, electrical and mechanical characterization of Ti‒50.13Ni and Ti‒49.62Ni (at.%) shape memory alloys (SMAs) fabricated at different circumferential wheel velocities. The effect of wheel velocity, chemical composition and heat treatments are investigated. The characterization of crystallographic phases of the Ti‒Ni ribbons was carried out using X-ray diffraction. Electrical resistance variations as function of temperature (∆R/R %) were analyzed using a non-commercial technique, which consists in a thermal-adjustable bath apparatus revealing the temperatures of B2→R→B19´ two stage transformation, whereby the presence of R‒phase can be definitively confirmed. The Stress-Assisted Two-Way Memory Effect was measured by an own designed apparatus with an Linear Variable Differential Transformer captor and a current controlled heating, and results indicate that the as-spun condition, promotes the Stress-Assisted Two-Way Memory Effect. On the other hand, increments in Ni content tend to decrease transformation temperatures and high wheel velocities help to the R‒phase formation.

Resumo O processo de micro soldagem pode ser eficiente para a união de Ligas com Memória de Forma (LMF) do sistema NiTi devido à alta precisão e localização do calor, diminuindo os tamanhos da zona fundida (ZF) e da zona termicamente afetada (ZTA), promovendo melhorias nas propriedades mecânicas destes tipos de juntas. Nesse contexto, este estudo teve como objetivo determinar as variações das propriedades termomecânicas em juntas soldadas de fios de LMF NiTi. Para o trabalho, fios de uma LMF NiTi (ASTM F2063) com 0,9 mm de diâmetro, foram divididos em dois grupos: (a) fios sem tratamento térmico (NiTiA) e (b) fios com tratamento térmico a 400 °C durante 20 minutos (NiTi400). Em seguida estes fios foram soldados de topo e de forma autógena pelo processo micro TIG, por meio de pulsos controlados. A caracterização termomecânica dos fios sem solda e micro soldados foi realizada utilizando ensaios de calorimetria diferencial de varredura (DSC), análise dinâmico-mecânica (DMA), ensaios de tração uniaxial, microscopia óptica (MO), microscopia eletrônica de varredura (MEV) e microindentação Vickers. Os resultados mostraram a eficiência do processo TIG ao soldar os fios NiTi de ambos os grupos, mesmo com a redução das temperaturas de transformação de fase da junta com relação ao metal de base original. Os fios com as juntas soldadas apresentaram patamares de deformação superelástica máxima de aproximadamente 8%, com tensões de indução de martensita entre 400 e 500 MPa. Este desempenho das juntas soldadas permitiram a realização de ciclos superelásticos com 4% de deformação com segurança. A análise das superfícies de fratura das juntas soldadas permitiram constatar características dúcteis no processo de ruptura.

Abstract Micro welding can be an efficient way for joining shape memory alloys (SMA) due to high accuracy and location of heat, reducing the size of the molten zone and heat affected zone (HAZ) promoting improvements in mechanical properties in these types of joints. This study aimed to determinate variations in thermomechanical properties of welded joints of NiTi SMA wires. For this, NiTi wires (ASTM F2063) with 0.9 mm diameter were divided into two groups: (a) as received wires (NiTiA) and (b) heat treated wires at 400 °C for 20 min (NiTi400). These wires were welded by TIG process, using controlled pulses (spots). The thermomechanical characterization of micro welded wires was performed using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), uniaxial tensile tests, optical microscopy (OM), scanning electron microscopy (SEM), and Vickers microhardness. The results show the efficiency of TIG welding NiTi wires of both groups, even with the reduction of the phase transformation temperatures of the joint with respect to the original base metal. The wires with welded joints showed showed maximum superelastic strain levels of about 8% with martensite induction stresses between 400 and 500 MPa. This performance of welded joints allow the realization of superelastic cycles up to 4% deformation safely. The analysis of welded wire fracture surfaces allowed verifying ductile characteristics at the rupture process.

In this research, an ion milling equipment was used to elaborate nanoparticles from Cu-Zn-Al alloys with shape memory effect. Two different compositions were used, target A: 75.22Cu-17.12Zn-7.66Al at % with an Ms of -9 °C and target B: 76.18Cu-15.84Zn-7.98Al with an Ms of 20 °C. Nanoparticles were characterized by High Resolution Transmission Electron Microscopy, Electron Diffraction and Energy Dispersive X-ray Spectroscopy. The obtained nanoparticles showed a small dispersion, with a size range of 3.2-3.5 nm. Their crystal structure is in good agreement with the bulk martensitic structure of the targets. In this sense, results on morphology, composition and crystal structure have indicated that it is possible to produce nanoparticles of CuZnAl shape memory alloys with martensitic structure in a single process using Ion Milling.

In this paper the development of an experimental test bench to analyze and control the strain of a flexible aluminum beam subjected to external disturbances is described. In the proposed platform, strain-gauges are used to measure the strain of the beam in a single cantilever mode while Ni-Ti-Cu Shape Memory Alloy (SMA) wires are used as force actuators. Data acquisition and control system are implemented with an ADuC microcontroller. The response of the structure in open-loop was used to identify the mathematical model of the system. To find the appropriate controller and to reach the best performance of the system, it was used techniques of direct tuning (pole-zero cancellation) in the identified model. The PI controller has been used to control the strain of the beam for different types of reference signals, as square, sinusoidal and triangular. The frequencies of the reference signals have been varied to observe to which bandwidth the system can respond. Experimental results are used to demonstrate the usefulness of the PI controller in the proposed smart structure.

This article studies changes observed on the R-phase thermoelastic behavior in a near-equiatomic Ti-Ni shape memory alloy. Three kinds of procedures have been performed: different treatments, thermomechanical cycling under constant loading in shape memory helical springs and thermal cycling in as-treated and trained samples. Several heat treatments were carried out to investigate evolution of the R-phase by differential scanning calorimetry (DSC). A heat treatment was chosen on which R-phase is absent. Shape memory springs were produced and submitted to a training process in an apparatus by tensioning the springs under constant loading. Thermal cycling in DSC was realized in as-treated and trained samples. Several aspects of one-step (B2→B19') and two-steps (B2→R→B19') martensitic transformations and R-phase formation and their evolution during tests were observed and discussed.

This paper present a thermomechanical study of actuators in form of helical springs made from shape memory alloy wires that can work as actuator and/or as sensor. These abilities are due to the martensitic transformation. This transformation is a diffusionless phase transition that occurs by a cooperative atomic rearrange mechanism. In this work, helical spring actuators were manufactured from Cu-Zn-Al shape memory alloy wires. The springs were submitted to constant tensile loads and thermal cycles. This procedure allows to determine thermoelastic properties of the shape memory springs. Thermomechanical properties were analyzed during 50 thermal cycles in the temperature range from 20 to 130 °C. Results of variations in critical transformation temperatures, thermoelastic strain and thermal hysteresis are discussed based on defects rearrangement and martensitic transformation theory.

The use of shape memory alloys (SMA) as smart structures and other modern applications require a previous evaluation of its performance under load as well as a training procedure. In general, these requirements lead to the design and assembly of a specific test bench. In this work, an experimental set-up was specially designed to perform the electro-thermomechanical characterization of SMA wires. This apparatus was used to determine the strain-temperature (epsilon - T) and electrical resistance-temperature (R - T) hysteretic characteristics curves of a Ti-Ni shape memory wire (90 mm in length and 150 µm in diameter) under mechanical load. The SMA wire is loaded by means of constant weights and a controlled system for injection of electrical power allows performing the heating-cooling cycles. The obtained hysteretic epsilon - T and R - T characteristics curves for some levels of applied loads are used to determine important shape memory parameters, like martensitic transformation temperatures, temperature hysteresis, temperature slopes and shape memory effect under load. These parameters were in accord with the ones found in literature for the studied SMA wires.

Recently, electrical resistivity (ER) measurements have been done during some thermomechanical tests in copper based shape memory alloys (SMA's). In this work, single crystals of Cu-based SMA's have been studied at different temperatures to analyse the relationship between stress (s) and ER changes as a function of the strain (e). A good consistency between ER change values is observed in different experiments: thermal martensitic transformation, stress induced martensitic transformation and stress induced reorientation of martensite variants. During stress induced martensitic transformation (superelastic behaviour) and stress induced reorientation of martensite variants, a linear relationship is obtained between ER and strain as well as the absence of hys teresis. In conclusion, the present results show a direct evidence of martensite electrical resistivity anisotropy.

In this work, several tests of thermal cycling under constant load are carried out on Ti-45.0Ni-5.0Cu (at%) shape memory wires. The properties related to the Stress Assisted Two Way Memory Effect (SATWME) of the material are investigated as a function of the mechanical loading history for the same temperature range during cooling and heating. For this reason, two thermomechanical tests have been employed: tests I, where one sample is used for just one constant stress level test and tests II, where only one sample is employed for several constant stress level tests in sequence. The results obtained show that for loads applied below 150 MPa, the transformation temperatures and the thermal hysteresis associated with the transformation of the material are the same during the two tests. However, above 150 MPa in tests II transformation temperatures and thermal hysteresis are respectively higher and smaller than the ones obtained in tests I. On the other hand, transformation temperatures obtained from both tests are in good agreement with the ones measured by DSC and electrical resistance measurements. It is also observed that the SATWME obtained by tests II is smaller than the one measured during tests I. It is shown that these different behaviors are induced by accumulation of plastic strain in the sample during tests II.