Equiatomic NiTi Shape Memory Alloy Research Articles

Elastic and transformation behaviour of equiatomic NiTi shape memory alloys fabricated at different sintering temperatures

The growing demand for bulk Nitinol in ball and roller bearing applications is focused on in this article. The uniaxial compaction and sintering process is considered for the fabrication of the alloys where the sintering temperatures vary from 950 ℃ to 1150 ℃. Better homogenisation and diffusion are achievable except for the 950 ℃ sintered sample. The results of XRD of 950 ℃ and 1050 ℃ sintered samples show the presence of the Ni3Ti phase but in all the Samples sintered above 950 ℃, NiTi is the major phase, unlike Ni3Ti in 950 ℃. The TEM analysis shows a trace amount of TiO2 which comes from the impurity of the purged Ar gas. The O2 in the samples can be accommodated inside a few Ti2Ni phases to form Ni2Ti4O phases. Ni4Ti3 precipitates formed at higher temperatures help in precipitation strengthening. At room temperature, both austenitic B2 and martensitic B19′ coexist. The DSC analysis shows that an increase in the sintering temperature increases the response time by consuming more energy. The elastic modulus and hardness have been increased up to 1100 ℃ and then decreased. Above Af temperature successful transformation of B2 to B19′ takes place.

The effect of copper on phase transformation, microstructure and mechanical characterization of Ni50-xTi50Cux shape-memory alloy

This research is related to the investigation of the influence of copper additions on phase transformation, microstructure and mechanical characterization of equiatomic NiTi shape-memory alloys. A typical vacuum arc melting furnace was used in the preparation of Ni50-xTi50Cux (x = 2, 5, 10, and 15 at.%) shape-memory alloys. The crystalline phase formation and phase transformation temperature were studied using X-ray diffraction and a differential scanning calorimeter. The morphology and chemical composition of the annealed samples were investigated using a field emission scanning electron microscope and energy-dispersive X-ray spectrometer analysis. The Vickers microhardness test was utilized in the analysis of the homogeneity of the alloy. The experimental results obtained from X-ray diffraction confirmed the coexistence of austenite and martensite phases along with the presence of a Ti2Ni and minor rutile phases. The crystalline sizes of the alloys were in the range of 42 to 78nm. An energy-dispersive spectrometer validated the distinct phases and precipitates in the alloys. The size and dispersion of the secondary phase in the alloy showed a significant influence on the increased hardness from 407HV to 470HV. The following NiTiCu alloy compositions were employed for low-temperature applications since their transition temperatures were below 100°C.

Effect of volume energy density on selective laser melting NiTi shape memory alloys: microstructural evolution, mechanical and functional properties

Equi-atomic NiTi shape memory alloy (SMA) samples were manufactured by selective laser melting (SLM) with different laser volume energy densities via synchronously varying laser power and scanning speed. The processing quality, phase transformation, and microstructural evolution were investigated to explore the mechanisms that are responsible for mechanical and functional properties. The results showed scattered micro-sized spherical gaseous defects inside the deposit. One-stage phase transformation occurred without intermediate R-phase transition, and the temperature of both endothermic and exothermic peaks was lower than that of the ingot. The microstructure initially grew along the building direction, but displayed different crystal orientations between coarse columnar grains and refined irregular sub-grains. The micro-hardness mapping revealed that homogenous anti-indentation properties were acquired independent of location variations. While the anisotropic behavior in tensile properties occurred due to the passive effect of columnar B2 austenitic grains, a synergistic effect of strength and plasticity was acquired primarily due to the combined effect of sub-grain refinement strengthening and transformation-induced plasticity. The pre-deformed NiTi samples recovered to their original shape when heated above its critical phase transition point. This work demonstrated that the crack-free NiTi SMA with exceptional comprehensive performances could be fabricated by SLM through the control of energy density.

Large thermal hysteresis in a single-phase NiTiNb shape memory alloy

Large thermal hysteresis (Thys) is favorable in shape memory alloys (SMAs) because it renders wide service and storage temperature range of the SMA-made parts. In this letter, we show that large intrinsic Thys up to 100 K is achieved in a low Nb content (2 at.%), β-Nb-free and single-phase NiTiNb SMA fabricated by casting, forging, wire-drawing and annealing. Such a large Thys is comparable to that of the widely used dual-phase NiTiNb SMAs in which high Nb content and β-Nb are required. By exploiting the grain-size-dependence of Thys in our single-phase NiTiNb SMA and that in an equiatomic NiTi SMA, we attribute the large Thys to the enhanced kinetic resistance of transformation by the small grains. The connection between the Thys and the kinetic resistance is demonstrated by a modified dislocation-based kinetic model.

Mechanical and thermomechanical characterization of Ni50-x Ti50Cux(x = 2, 5, 10at%) with morphological studies

Shape memory alloys TiNi-xCux (x = 2, 5, 10 at%) were made in a traditional vacuum arc remelting furnace (VAM). The alloy was characterized after heat treatment in an inert atmosphere. X-ray diffraction (XRD), differential scanning calorimeter (DSC), Field emission scanning electron microscope (FESEM), and Energy Dispersive Spectrometer (EDS) were used to investigate the impact of replacing Ni with Cu in Equiatomic NiTi Shape memory alloy (SMA). The results obtained from XRD revealed the formation of monoclinic martensite and austenite phases and the presence of an intermetallic Ti2Ni phase. The EDS results revealed that there are some minor deviations in the compositions. The results of the differential scanning calorimeter revealed that the phase transformation sequence of all the samples are in a single- stage form B2 to B19ˈ and the transformation hysteresis is decreased as Cu content increases. The microhardness results confirmed that the Cu addition on equiatomic NiTi increases the hardness due to the formation of intermetallic secondary phases and its distribution on the matrix.

Mechanical behavior of cycled shape memory alloy

Shape memory alloys (SMA) are often used in many industrial, medical and automotive fields due to the ability to recover the initial shape caused by external loads. This aspect is due to the variations of the microstructure that may depend on thermal and/or mechanical causes. So far, many models have been developed in order to describe the mechanical behaviour of SMA but most of them do not take into account the real microstructure.In this work a conventional equiatomic NiTi SMA has been investigated in both terms of microstructure evaluation and mechanical cycling behaviour up to degradation the memory ability, which has obtained at around 100 cycles. The cycling behaviour has been investigated and a model has been proposed to predict the mechanical behaviour and the ability to recover the initial microstructure.The damaged alloy has been investigated in terms of fracture surface analysis obtained by conventional tensile tests after 1, 50 and 100 cycles.

Twin-based martensite stabilizing and improving the shape memory response of near equiatomic NiTi alloy through multi-axial forging

In the present study, a high cumulative true strain (∼4.5) was applied on a near equiatomic NiTi shape memory alloy through multi-axial forging in the course of “isothermal” and “continuous cooling” regimes. It was found that the processed sample under decreasing temperature condition (from 900 °C to 660 °C) represented a full shape memory response without any residual strain. This behavior was discussed considering the effects of the compound twins on stabilizing the pre-existed martensite structure and favoring the detwinning stage compared with type I and type II deformation twins.

A cyclic integrated microstructural-mechanical model for a shape memory alloy

Shape memory alloys are able to recover the initial shape even after a large deformation. The memory property is due to the ability to change the microstructure under a variation of the stress state or the temperature. The stress–strain curve is characterized by the presence of a stage where the slope of the curve sharply decreases and the initial microstructure transforms into a new one. In this work, an integrated microstructural-mechanical model is proposed for the equiatomic NiTi shape memory alloy, that is suitable to predict the cyclic behaviour of a pseudoelastic shape memory alloy starting from the microstructure composition.

Atomic layer deposited TiO2 films on an equiatomic NiTi shape memory alloy for biomedical applications

NiTi shape memory alloys (SMAs) have been widely applied in biomedical fields due to their excellent shape memory effect, super-elasticity, and biocompatibility. In this study, the plasma- enhanced atomic layer deposition was applied to deposit TiO2 film on the surface of Ni50Ti50 substrate. The characterization and surface properties of TiO2 film, bending strain test, corrosion resistance, nickel release and in vitro biocompatibility were studied to evaluate the performance of ALD-TiO2 film. The results show that the ALD-TiO2 film is uniform and exhibits an amorphous structure. The ALD-TiO2 films are less hydrophilic than the Ni50Ti50 substrate. As the deposition cycle increases, the water contact angle of ALD-TiO2 film only changes slightly. The ALD-TiO2 film exhibits an adequate adhesion to the Ni50Ti50 substrate and can withstand proper bending strain. ALD-TiO2 film can effectively inhibit the release of nickel ions. After being immersed in the simulated body fluid for 30 days, the amount of nickel ions released decreases from 60 ppb for Ni50Ti50 substrate to 20–20 ppb for Ni50Ti50 with 200-cycle ALD-TiO2 film. Meanwhile, Ni50Ti50 with ALD-TiO2 films have better corrosion resistance than the Ni50Ti50 substrate. In vitro biocompatibility reveals that the ALD-TiO2 film can enhance cell adhesion, promote cell proliferation and reduce cytotoxicity. After 5 days of culture, the 200-cycle ALD-TiO2 film can enhance the cell proliferation by 2.7 times and reduce the cytotoxicity from 48% for Ni50Ti50 substrate to 37% for Ni50Ti50 with 200-cycle ALD-TiO2 film. These results indicate that the TiO2 films deposited by PE-ALD technique can effectively improve the biocompatibility of NiTi SMAs.

Atomistic simulation of microstructure evolution of NiTi single crystals in bending deformation

In order to explore the microstructure evolution of the equiatomic NiTi shape memory alloy during the bending deformation, molecular dynamics simulations based on the 2NN-MEAM potential were respectively carried out in the compressive zone, tensile-compressive zone (identified as “combined zone”) and tensile zone, which were divided according to the force analysis. Positive and negative strain rates or velocities with linear variations were applied in different zones during the bending deformation. The results show that the simulated lath martensites and dislocations during the bending deformation are highly consistent with the results of bending experiment. In the bending simulation, the martensitic transformation occurs in different degrees in the above three zones. The microstructure evolution under compressive stress conditions is different from that of tensile stress conditions, including stress-induced martensitic transformation, dislocation density and distribution, the formation of highly stressed austenites and Frenkel defect pairs. Additionally, during the stress-induced martensitic transformation, the morphology and structure of martensites strongly depend on the deformation methods, strain rate and Ni/Ti atomic ratio. The current simulation work reveals lots of micro information that cannot be observed and captured by experiments on an atomic scale, which is expected to grasp the information of the stress-induced martensitic transformation and the resultant defect structures during the bending deformation.

Deformation Behavior, Structure and Properties of an Equiatomic Ti–Ni Shape Memory Alloy Compressed in a Wide Temperature Range

Deformation Behavior, Structure and Properties of an Equiatomic Ti–Ni Shape Memory Alloy Compressed in a Wide Temperature Range

Influence of copper addition on the properties of equiatomic NiTi shape memory alloy prepared by vacuum induction melting method

In this study the effect of Cu addition on the phase transformation behavior, microstructure, and micro hardness of equiatomic Ni-Ti shape memory alloy was investigated. NiTiCu SMA prepared with the composition (52.119 % at. Ni, 41.731% at. Ti and Cu 6.15 % at.) and compared with the properties of the equiatomic NiTi SMA with composition (50% at. Ni, 50% at. Ti). Vacuum induction melting method used in the preparation of Both SMAs. The Differential Scanning Calorimetry, Scanning Electron Microscope, X-ray Diffraction Analysis, optical microscope and vicker’s microhardness test was used to investigate the characteristics of the equiatomic NiTi and NiTiCu SMAs. The results revealed that when Cu element was added the phase transformation temperatures decreased below body temperature. NiTi matrix phase and Ti2Ni secondary phase exist in both SMA samples, also Cu-rich phase appeared in NiTiCu SMA and this is one of the reasons that lead to increasing the microhardness of alloy when Cu element was added. The value of equiatomic NiTi increases from 238.74 to 329 when Cu element was added (for NiTiCu alloy) after heat treatment.

Microstructures and Mechanical Properties of Equiatomic NiTi Shape Memory Alloy Undergoing Local Canning Compression and Subsequent Annealing

Local canning compression is imposed on equiatomic NiTi shape memory alloy (SMA) with complete B19′ martensite. A fraction of retained nanocrytalline grains are embedded in the dominant amorphous phase. The compressed NiTi samples are annealed for 2 h at various temperatures, including 300, 450 and 600 °C. Grain size increases with increasing crystallization temperature during heat treatment. Under annealing at 300 °C, nanocrystalline grains are dominant in NiTi SMA, where there exists local amorphous zone. Under annealing at 450 °C, almost complete nanocrystalline can be obtained. Under annealing at 600 °C, grain size increases substantially, but there still exists a small amount of nanocrystalline grains. Under annealing at 300 °C, NiTi SMA exhibits extremely high elastic limit, but its plasticity is poor. Under annealing at 450 °C, NiTi SMA exhibits very high yield stress and it simultaneously keeps high plasticity. Under annealing at 600 °C, NiTi SMA exhibits relatively low yield stress, but its yield stress is still higher than that of as-received NiTi SMA and its plasticity is also high.

Influence of annealing on incomplete detwinning and deformation twinning in equiatomic NiTi shape memory alloy undergoing severe plastic deformation

Equiatomic NiTi shape memory alloy (SMA) undergoing solution treatment, which contains {1¯11} type Ⅰ twin and 〈011〉 type Ⅱ twin, suffers from severe plastic deformation (SPD) using local canning compression, where nanocrystalline grains, amorphous phase, deformation twin and detwinning can be observed. SPD-processed NiTi alloy is subjected to subsequent annealing for 2 h at 300, 450 and 600 °C, respectively. 〈011〉 type Ⅱ twin still arises in NiTi sample annealed at 300 °C and it is attributed to incomplete detwinning of 〈011〉 type Ⅱ twin in the solution-treated NiTi sample. (001) compound twin appears in NiTi sample undergoing annealing at 450 °C and it results from the retained deformation twin due to plastic deformation of solution-treated NiTi sample. However, both (001) compound twin and 〈011〉 type Ⅱ twin occur in NiTi specimen annealed at 600 °C and they are generated in the new grains as a result of crystallization. The existence of the twins substantially influences phase transformation of NiTi SMA. Following annealing at 300 and 450 °C, NiTi SMA exhibits one-stage phase transformation during heating and cooling where the corresponding martensitic transformations of the involved NiTi samples are related closely to the retained twins or the retained coarse grains resulting from SPD. NiTi sample annealed at 600 °C exhibits two-stage transformation of B2-R-B19' on cooling, whereas it presents one-stage transformation from B19' to B2 phase on heating, where the corresponding martensitic transformation of the involved NiTi sample is completely dependent on the occurrence of the twins in the new crystallized grains.

The effect of cobalt element addition on the characteristics of equiatomic NiTi shape memory alloy

The influence of cobalt addition on the transformation temperatures, microstructure, and micro-hardness of equiatomic NiTi shape memory alloy was studied. The alloys composition was (50 % at. Ni, 50% at. Ti) and (43.45 % at. Ni, 54.728 % at. Ti and Co 1.815 % at.). Vacuum induction melting method is used to produce the shape memory alloys. The investigation of the characteristics of the samples was carried out using, scanning electron microscope conducted with energy dispersive X- ray spectrometer, differential scanning calorimeter, x-ray diffraction measurement and vickers micro-hardness testing. The results show the microstructure of two alloys contain Ti2Ni precipitate phase, the martensite phase layers increase with cobalt addition. The micro-hardness increased, the austenite starts temperature and austenite finish temperature decreases, and thermal hysteresis becomes narrower, after adding a small amount of cobalt.

Paramagnetism and martensite stabilization of tensile strained NiTi shape memory alloy

We present an experimental and theoretical study of Pauli paramagnetism and martensite stabilization in a near equiatomic NiTi shape memory alloy. We demonstrate a direct correlation between strain-induced shear of the B19′ NiTi lattice and its electronic and thermodynamical features. An increase in the monoclinic angle β from 97.4 to 98° induces a 7% decrease in the magnetic susceptibility because of a shift and deepening of a dip in B19′ density of states at the Fermi level. It also produces a decrease in the B19′ enthalpy, which translates into an increase in the martensite-to-austenite transition temperature by 60 K.

Effect of Sn Addition on Phase Transformation Behavior of Equiatomic Ni-Ti Shape Memory Alloy

Sn effect on the phase transformation behavior, microstructure, and micro hardness of equiatomic Ni-Ti shape memory alloy was studied. NiTi and NiTiSn alloys were produced using vacuum induction melting process with alloys composition (50% at. Ni, 50% at.Ti) and (Ni 48% at., Ti 50% at., Sn 2% at.). The characteristics of both alloys were investigated by utilizing Differential Scanning Calorimetry, X- ray Diffraction Analysis, Scanning Electron Microscope, optical microscope and vicker's micro hardness test. The results showed that adding Sn element leads to decrease the phase transformation temperatures evidently. Both alloy samples contain NiTi matrix phase and Ti2Ni secondary phase, but the Ti2Ni phase content decreases with Sn addition and this is one of the reasons that leads to decrease the micro hardness of alloy with adding Sn element in a noticeable manner. The micro hardness decreases from 238.74 for NiTi equiatomic alloy to 202 for NiTiSn alloy after heat treatment.

Tool wear progression of PCD and PCBN cutting tools in high speed machining of NiTi shape memory alloy under various cutting speeds

NiTi shape memory alloys are coming into prominence in various industrial applications owing to their unique shape memory effect, super-elasticity and mechanical properties. However, this material is characterized by its very poor machinability rates. One of the major reasons of this problem is the high tool wear rate, which primarily occurs due to nature of this material. The previous work reported in the literature has been limited to investigate low to medium speed machining characteristics of this material especially with carbide and coated carbide cutting tools. This paper presents the results of experimental investigation on the progressive wear behaviors of Polycrystalline Diamond (PCD) and Polycrystalline Cubic Boron Nitride (PCBN) cutting tools in turning of equi-atomic NiTi shape memory alloy. Effects of cutting speed, which varied between Vc = 70 and 250 m/min, were investigated. Periodical tool wear measurements were conducted in order to find tool life and evaluate wear mechanisms. The results show that tool wear mechanism is highly cutting speed dependent. PCD cutting tool performed better wear resistance at all of the investigated cutting speeds. The optimized cutting speed and cutting tool couple is suggested to increase productivity regarding longer tool life.

Preparation and characterization of equiatomic NiTi shape memory alloy

The present study investigated the vacuum induction melting of equiatomic NiTi (Ni 50 at. %, Ti 50at. %) shape memory alloy using graphite crucible. The cast alloy heat-treated at 865° C for 15 mints, followed by quenching the sample in icy water. The microstructure and micro hardness of alloy were investigated before and after heat treatment by using an optical microscope and Vickers micro hardness tester. Transformation temperatures upon heating/cooling were investigated by using differential scanning calorimeter (DSC). Scanning electron microscope (SEM) supported with energy dispersive X-ray (EDX) and X-ray diffraction analysis (XRD) were carried out to investigate the chemical composition and the phases present in the alloy after heat treatment. The results show that the vacuum induction melting process in a graphite crucible provides binary NiTi shape memory alloys with good chemical homogeneity. The microstructure becomes more homogenous and the Vickers’s micro hardness increases from 186.06 to 238.74 after heat treatment with highly controlling on the composition of the alloy. The DSC curve shows that the R- phase exists upon the heating and cooling process. In addition the precipitate phase Ti2Ni was also exist.

Stochastic multiscale modeling of the thermomechanical behavior of polycrystalline shape memory alloys

A multi-scale model simulating hysteretic thermomechanical behavior of polycrystalline shape memory alloys (SMA) is presented. Using a kinetic Monte-Carlo approach, the energy-based model estimates the stochastic average in terms of volume fraction for a phase or phase variant deriving from the Gibbs free energy density as a selection inside a given population. Indeed pseudo-elastic behavior for example is well-known to be associated with the nucleation of martensite plates inside the austenite parent phase. Associated variants are similar to sub-domains inside a thermodynamic system following the statistical definition of Sommerfeld et al. (1964). The germination process of a variant is on the other hand dictated by a germination potential barrier identified from a differential scanning calorimetry (DSC) measurement. The stochastic average for each type of variants inside a grain leads then to the numerical simulation of hysteresis phenomena at the single crystal scale. Homogenization operations allow finally macroscopic quantities (phases fraction, deformation and temperature) to be calculated at the polycrystalline scale. This procedure is applied to model the whole thermomechanical behavior of an equiatomic Ni-Ti SMA polycrystalline alloy considering the phase transformation between austenite, R-phase and martensite.