Ni-rich NiTi Shape Memory Alloy Research Articles

Elastic and Irreversible Energies of a Two-Stage Martensitic Transformation in NiTi Utilizing Calorimetric Measurements

Elastic energy and irreversible energy are quantified based on calorimetric measurements. We analyze energetics for each stage of the stress-free, thermally induced two-stage phase transformation A ⇔ R ⇔ B19′ in an aged Ni-rich NiTi shape memory alloy. Heating/cooling rates are imposed from 1 K/min up to 100 K/min. We compare energetic analysis after multiple thermal cycles to virgin (i.e., first-cycle) material. Fundamental thermodynamic formulations are applied from two perspectives: the free energy change dG, and the rate of change of free energy expressed as dG/dfm. Two measures of irreversible contributions are defined: the difference between the forward and reverse transformation heats, and the product of the entropy and the thermal hysteresis. Higher values are determined for the former. For scan rates of 10 K/min and greater, the energetic values become relatively stable. Substantial variations are evident at 1, 5, and 10 K/min. The scan rate impacts the elastic strain energy and irreversible energy of the B19′ markedly compared with the R-phase transition. The findings are rationalized considering morphologic changes at the lower scan rates and the impacts on elastic and irreversible energies.

Micromechanics of precipitated near-equiatomic Ni-rich NiTi shape memory alloys

The specific thermo-mechanical behavior of precipitated, near-equiatomic Ni-rich NiTi shape memory alloys, i.e., thermal actuation under stress and pseudoelasticity, are investigated via the finite element method. The deformation response of the material-at-large is simulated using a representative volume element, taking into account the structural effect of the precipitates, as well as the effect of the Ni-concentration gradient in the matrix. An existing rate-independent constitutive model, similar to the one employed to describe the matrix behavior, is calibrated based on the deformation response of the representative volume elements. The actuation and pseudoelastic response of the homogenized material are found to be very close to those of the representative volume elements. The obtained results reproduce and provide important insight into several of the experimentally observed precipitation-induced changes on the transformation characteristics of these materials.

Effect of Ageing Temperatures on the Transformation Behaviour of Ti-50.7at.%Ni Shape Memory Alloy

Ti-50.7at.%Ni as one of Ni-rich TiNi shape memory alloy is very responsive to precipitation ageing treatment due to the formation of Ti3Ni4 precipitates. This study indicates that this alloy exhibits different transformation sequences under different ageing temperatures. Precipitation of Ti3Ni4 has introduced the formation of intermediate R-phase prior to martensitic transformation. The lack of 2-step R-phase transformation, B2→R1 and B2→R2 transformation in this alloy, suggested that Ti3Ni4 precipitation occurs homogeneously.

Effect of thermal and thermo-mechanical cycling on the microstructure of Ni-rich NiTi shape memory alloys

Microstructural changes of Ni-rich NiTi shape memory alloy during thermal and thermo-mechanical cycling have been investigated using Electron Back Scattered Diffraction. A strong dependence of the orientation of the prior austenite grain on the misorientation development has been observed during thermal cycling and thermo-mechanical cycling. This effect is more pronounced at the grain boundaries compared to grain interior. At a larger applied strain, the volume fraction of stabilized martensite phase increases with increase in the number of cycling. Deformation within the martensite leads to stabilization of martensitic phase even at temperatures slightly above the austenite finish temperature. Modulus variation with respect to temperature has been explained on the basis of martensitic transformation.

Microstructure and Strengthening Mechanism of Ni-rich TiNi Shape Memory Alloys via Powder Metallurgy Process

Microstructure and Strengthening Mechanism of Ni-rich TiNi Shape Memory Alloys via Powder Metallurgy Process

<i>In Situ</i> Structural Characterization of Laser Welded NiTi Shape Memory Alloys

The demand of emerging joining techniques for shape memory alloys (SMA) has become of great importance, as their functional properties, namely shape memory effect (SME) and superelasticity (SE) present unique solutions for state-of-the-art applications. Literature shows that significant efforts have been conducted on laser welding of these alloys, although very limited results concerning mechanical properties are repeatedly achieved. A better understanding of the mechanical behaviour of these welded joints may be got through a detailed analysis of the structural characteristics of the material from the base metal to the weld bead. Such studies have been carried out on a series of Ni-rich Ni-Ti SMA laser welded plates using synchrotron radiation.

Impact of hydrogen-assisted heat treatments on microstructure and transformation path in a Ni-rich NiTi shape memory alloy

Abstract We report results of differential scanning calorimetry and transmission electron microscopy experiments in which martensitic transformations and microstructure of Ti–50.9 at%Ni shape memory samples were investigated after heat treatments in controlled gaseous environments. The heat treatments were performed in mixtures consisting of hydrogen and helium with an overall pressure of 900 mbar. The cylindrical samples were heat treated either in a Regime I (850 °C/1800 s/WQ) or in a Regime II (the Regime I plus 450 °C/3600 s/WQ). DSC charts clearly document that the B2 → B19′ or R → B19′ martensitic phase transitions diminish after Regime I and Regime II treatments with increasing partial pressure of hydrogen. We show that the hydrogen rich environments do not cause variations in size and density of Ni4Ti3 particles. Conversely, the Regime I heat treatments assisted by hydrogen result in 1/2 〈1 1 1〉B2 diffuse intensity maxima in selected area electron diffraction patterns. These maxima were not observed after heat treatments in hydrogen deficient atmospheres. Therefore, we suggest that the gradual suppression of the martensitic transformation may be related to either chemical and/or strain modulations of the B2 lattice triggered by the presence of interstitial hydrogen atoms.

Leaf-like dislocation substructures and the decrease of martensitic start temperatures: A new explanation for functional fatigue during thermally induced martensitic transformations in coarse-grained Ni-rich Ti–Ni shape memory alloys

During repeatedly imposed thermally induced martensitic transformations in Ti–Ni shape memory alloys, the martensite start temperature Ms decreases. This has been rationalized on the basis of a scenario where an increasing dislocation density makes it more and more difficult for martensite to form. However, it is not clear why dislocations which form because they accommodate the growth of martensite during the first cooling cycle should act as obstacles during subsequent transformation cycles. In the present work we use diffraction contrast transmission electron microscopy to monitor the formation of unique leaf-like dislocation substructures which form as the martensite start temperature decreases during thermal cycling. We interpret our microstructural results on the basis of a microstructural scenario where dislocations play different roles with respect to the propagation of a big martensite needle in one transformation cycle and the nucleation and growth of new martensite needles in the following cycles. As a consequence, chestnut-leaf-like dislocation arrays spread out in different crystallographic directions.

On the Stress-Induced Formation of R-Phase in Ultra-Fine-Grained Ni-Rich NiTi Shape Memory Alloys

Phase transformations in binary ultra-fine-grained (UFG) pseudoelastic NiTi wires were studied in a wide temperature range using mechanical loading/unloading experiments, resistance measurements, differential scanning calorimetry (DSC), thermal infrared imaging, and transmission electron microscopy (TEM). The formation of R-phase can be detected in the mechanical experiments. It is shown that the stress-strain response of the R-phase can be isolated from the overall stress-strain data. The R-phase always forms prior to B19′ when good pseudoelastic properties are observed. The stress-induced B2 to R-phase transition occurs in a homogeneous manner, contrary to the localized character of the B2/R to B19′ transformations. The temperature dependence of the critical stress values for the formation of the martensitic phases shows a Clausius Clapeyron type of behavior with constants close to 6 MPa/K (B19′) and 18 MPa/K (R-phase). A stress-temperature map is suggested that summarizes the experimentally observed sequences of elementary transformation/deformation processes.

Effects of ECAE and Aging on Phase Transformations and Superelasticity of a Ni-Rich TiNi SMA

Effects of equal-channel angular extrusion (ECAE) process at 773K and heat treatment on phase transformations and superelasticity of a Ni-rich TiNi shape memory alloy were investigated by differential scanning calorimeter (DSC) measurement and cyclic tensile test. The R phase transformation of Ni-rich TiNi alloy was stimulated after ECAE processes and within a larger temperature range. The martensitic transformation start temperature Ms of Ni-rich TiNi alloy decreased sharply after ECAE processes at 773 K, then rapidly rose back after the specimen aging at 773K for 30min. Reasons for the changes of phase transformation behaviors have been discussed. ECAE processed TiNi samples exhibit better super-elasticity than solution treated one. With the increase passes of ECAE, the superelasticity becomes more stable, and a completely recoverable strain of 6% is obtained for TiNi sample after 8 passes ECAE.

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.

Microstructures evolution and phase transformation behaviors of Ni-rich TiNi shape memory alloys after equal channel angular extrusion

Ni-rich TiNi shape memory alloys were subjected to the effect of equal channel angular extrusion (ECAE) processes at 773 K by Bc path. The effects of ECAE processes on microstructures evolution and phase transformation behaviors were investigated. The initial 60–80 μm equiaxed coarse grains of samples were elongated along the shearing force direction of ECAE and refined to 300–400 nm after eight passes ECAE. The R phase transformation of Ni-rich TiNi shape memory alloys was stimulated by ECAE processes within a larger temperature range. The martensite transformation peak temperature (Mp) dropped in previous 1–3 ECAE treatments, but the dropped Mp increased gradually with the increase of ECAE processes. Ti 3Ni 4 phase was observed in the regions with high density of dislocations after ECAE treatment. Reasons for microstructures evolution and phase transformation changes were also discussed.

An Investigation on Phase Formations and Microstructures of Ni-rich Ni-Ti Shape Memory Alloy Thin Films

Ni-rich Ni-Ti alloy thin films were fabricated by radio frequency (RF)-direct-current (DC) magnetron sputtering using elemental Ni and Ti as sputter targets. Si (100) was chosen as a substrate that was either held at room temperature or at 573 K (300 °C) during the depositions. The 380-nm-thick films were characterized by field emission scanning electron microscopy, energy dispersive spectroscopy, grazing incidence X-ray diffraction, atomic force microscopy, high-resolution transmission electron microscopy, and Vickers microhardness tester. The results suggests that because of the lack of surface mobility of the adatoms at the room temperature, the deposited films were smooth and amorphous, with a crystallite size of 15 nm accompanied with a porous morphology. At higher substrate temperatures, an increase in the surface diffusion leads to the formation of partially crystalline, rougher films with a denser, compact, fibrous grain microstructure. The formation of Ni-rich precipitates such as Ni4Ti3, Ni2Ti, and Ni3Ti along with small amount of the NiTi phase were attributed to the localized heating and cooling within the grains. Few grains exhibited band structures that are believed to be a result of type II twins.

Influence of heat treatment and microstructure on the tensile pseudoelastic response of an Ni-rich NiTi shape memory alloy

Abstract The influence of microstructure on the stress–strain behavior of an Ni-rich NiTi shape memory alloy is examined. Specimens cut from a large-diameter bar of Ni50.7Ti49.3 shape memory alloy were analyzed in two states: (i) annealed and (ii) annealed and aged. The annealed state shows a fully austenitic structure with no precipitates and no distortions caused by residual stresses. The annealed and aged state has coherent Ni4Ti3 particles precipitated in the proximity of the austenite grain boundaries. The size of the precipitates increases moving away from the grain boundaries toward the grain interiors. The evolution of the two states in the stress–strain–temperature space has been analyzed using tensile specimens with special geometry. Due to the complex effects of the coherent precipitates, the specimens in the aged state exhibited lower stress plateaus in the tensile loading–unloading curves, which enabled the occurrence of transformation pseudoelasticity from room temperature to 333 K.

Effect of low-temperature precipitation on the transformation characteristics of Ni-rich NiTi shape memory alloys during thermal cycling

Thermal cycling of NiTi shape memory alloys is associated with functional fatigue: the characteristic phase transformation temperatures decrease with increasing number of cycles, and the transformation behavior changes from a single- to a two-stage martensitic transformation involving the intermediate R-phase. These effects are usually attributed to a gradual increase of dislocation density associated with micro-plasticity during repeated cycling through the transformation range. Here, these changes are shown to increase at a higher maximum temperature (in the fully austenitic state) during differential scanning calorimetric cycling of a Ni-rich alloy. Additional thermal cycling experiments without repeated phase transformations, and post-mortem microstructural observations by transmission electron microscopy, demonstrate that a relevant portion of functional fatigue is due to the formation of nano-scale Ni-rich precipitates of type Ni 4Ti 3 even at temperatures relatively close to the austenite finish temperature. These results show that both dislocation generation during the diffusion-less phase transformation, and diffusion-controlled nucleation and growth of Ni 4Ti 3 precipitates, can interact and contribute to the evolution of functional properties during thermal cycling of Ni-rich NiTi.

Superelastic cycling and room temperature recovery of Ti74Nb26 shape memory alloy

The superelastic cyclic response of Ti74Nb26 shape memory alloy (SMA), and the nature of cyclic evolution of its superelastic properties and their unexpected static recovery process after cycling, were investigated at room temperature. The critical stress for stress-induced martensitic transformation (σSIM) and stress hysteresis (Δσ) continuously decrease with increasing number of superelastic cycles. However, cumulative irrecoverable strain during cycling in samples of particular processing conditions increases only up to a certain number of cycles before decreasing with further cycling. Stress-free aging at room temperature after cycling was shown to increase σSIM and Δσ. The unexpected room temperature recovery is attributed to the recovery of retained martensite and point defects. Similar experiments conducted on conventional Ni-rich Ni–Ti SMAs also show static recovery at room temperature, indicating that the recovery process is not unique to Ti–Nb SMAs.

Elastic anisotropy of Ni 4Ti 3 from first principles

The elastic strain fields surrounding Ni 4 Ti 3 precipitates affect the transformation behavior of Ni-rich NiTi shape memory alloys. We present first-principles calculations of the Ni 4 Ti 3 elastic constants that allow for the first time to characterize the elastic anisotropy, the direction-dependent Young’s modulus and the macroscopic elastic moduli. A comparison of the direction-dependent moduli of Ni 4 Ti 3 and the austenitic NiTi matrix fully rationalizes the experimentally observed shapes of the precipitates.

Effect of cooling rate on the phase transformation behavior and mechanical properties of Ni-rich NiTi shape memory alloy

TiNi alloy is a well-known shape memory alloy and has been widely used for bio-medical, mechanical and electrical applications. In this study, a Ni-rich NiTi alloy was prepared by vacuum arc melting in a water-cooled copper crucible. Three samples of this alloy were heated to 1000 °C and cooled in three media: furnace, water, and dry-ice bath. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), hardness measurement and tensile test were carried out to investigate the effect of cooling rate on transformation temperature and mechanical properties. The results show that Ni3Ti intermetallic compounds have a great influence on martensitic phase transformation temperature. These tests clearly showed the correlation between cooling rate and properties of the alloy.

Cold Rolling-Induced Multistage Transformation in Ni-Rich NiTi Shape Memory Alloys

Cold Rolling-Induced Multistage Transformation in Ni-Rich NiTi Shape Memory Alloys

Cold Rolling-Induced Multistage Transformation in Ni-Rich NiTi Shape Memory Alloys

A multi-stage transformation (MST) has been observed in cold-worked Ti-50.3 at%Ni shape memory alloy by differential scanning calorimetry (DSC). When thickness reduction reaches about 5% strain, the phenomenon is able to occur in 50.3 at%Ni alloy. The multi-stage transformation behavior is due to interior microstructure variation, i.e., texture formation and defects aggregation which caused localized stress fields and affected the phase transformation behavior.