Equiatomic NiTi Shape Memory Alloy Research Articles

The effectiveness of reference-free modified embedded atom method potentials demonstrated for NiTi and NbMoTaW

One of the effective potentials that has proven to be very versatile and useful for describing metals is the modified embedded atom method (MEAM) potential. The reference-free version of the MEAM (RF-MEAM) potential provides more flexibility for fitting than the 2NN-MEAM because it also describes the pair potential as an explicit function. In this work, we present a methodology to fit RF-MEAM potentials to DFT data. We then evaluate the performance of the fitted potential by comparing MD simulations with experimental and DFT data. As an example, the methodology is applied to a binary and a quaternary alloy, namely NiTi and NbMoTaW. In the case of the equi-atomic NiTi shape memory alloy, our attention focuses on designing a potential that properly captures its mechanical behavior, given that the existing potentials fail to predict elastic constants in agreement with experiments. To reach our aim, we included the stress tensors of different high temperature NiTi configurations in the fitting database. The obtained RF-MEAM potential outperforms existing EAM and MEAM potentials in predicting the lattice and elastic constants of austenitic and martensitic phases as well as the corresponding transformation temperatures. To demonstrate the suitability of this methodology also for more complex systems, a RF-MEAM potential is fitted to model the multi-component NbMoTaW high-entropy alloy. Validation is achieved through comparison between observables obtained through the MD output and ab initio data. The article also reports key improvements to the optimization code MEAMfit v2 and the freely-available LAMMPS implementation of the RF-MEAM formalism. Most notably, resorting to analytic derivatives of the objective function with respect to the potential parameters rather than derivatives through finite differences, the time necessary for fitting has decreased by an order of magnitude.

Improved in vitro bioactivity and electrochemical behavior of hydroxyapatite-coated NiTi shape memory alloy

Nickel titanium (NiTi) shape memory alloys have a vigorous prospective for biomedical solicitations. In this work, electrochemical behavior of hydroxyapatite (HA) coated and thermally oxidized near equiatomic NiTi shape memory alloys was efficaciously investigated for biomedical applications. NiTi samples were spin-coated with laboratory-synthesized HA followed by drying and calcination at 120 ± 5 °C and 450 ± 5 °C, respectively. Similarly, uncoated NiTi samples were thermally oxidized by heating at 450 ± 5 °C in air. The comparison was made among HA-coated, thermally oxidized and uncoated samples. Scanning electron microscope (SEM), atomic force microscope and X-ray diffraction confirm the presence of Titania and HA coating on the NiTi substrates. Electrochemical testing results showed the better resistance against corrosion for the HA-coated samples because of the protective coatings of HA and Titania, as compared to the rest. Moreover, Fourier transform infrared spectroscopy and SEM reveal that the HA-coated sample will provide bioactive cushion to the host structure for better adhesion during the implanted period, while the implant will do its job.

Mechanical Behaviour and Phase Transition Mechanisms of a Shape Memory Alloy by Means of a Novel Analytical Model

Abstract The aim of the present paper is to examine both the fatigue behaviour and the phase transition mechanisms of an equiatomic pseudo-elastic NiTi Shape Memory Alloy through cyclic tests (up to 100 loading cycles). More precisely, miniaturised dog-bone specimens are tested by using a customised testing machine and the contents of both austenite and martensite phase are experimentally measured by means of X-Ray diffraction (XRD) analyses. On the basis of such experimental results in terms of martensite content, an analytical model is here formulated to correlate the stress-strain relationship to the phase transition mechanisms. Finally, a validation of the present model by means of experimental data pertaining the stress-strain relationship is performed.

Role of hydrostatic pressure on the phase stability, the ground state, and the transformation pathways of NiTi alloy

Equiatomic NiTi shape memory alloys exhibit multiple martensitic transformations from a parent B2 phase. However, there is an apparent discrepancy between theoretical predictions and experimental observations of the transformation pathways and final products. We studied the phase stabilities of NiTi martensites with respect to the effect of hydrostatic pressure using a generalised solid-state nudge elastic band method implemented in density functional theory to simulate the minimum energy pathways of transformations. The analysis demonstrates that base-centred orthorhombic (BCO) phase is unstable, thus will undergo the BCO→B19′ phase transformation at above 9GPa pressure, as confirmed by phonon calculations.

Effect of cobalt addition on the corrosion behavior of near equiatomic NiTi shape memory alloy in normal saline solution: electrochemical and XPS studies.

The electrochemical and corrosion (uniform and localized) behavior of a binary Ni52Ti48 shape memory alloy (SMA) and two ternary Ni52Ti48−xCox (x = 1.5 and 4.0 wt%) SMAs were studied. Measurements were conducted in 0.9% NaCl solution at 37 °C employing various electrochemical methods. These include: linear polarization resistance (LPR), linear sweep voltammetry (LSV), chronoamperometry and dynamic electrochemical impedance spectroscopy (DEIS). Such measurements were complemented with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analysis. Results revealed that the addition of alloyed Co to NiTi significantly reduced the uniform corrosion rate of the studied SMA and greatly enhanced its pitting corrosion resistance. XPS measurements evidenced high stability of the passive layer and limited adsorption of chloride ions. Additionally, it was found that the passive layer remained primarily composed of titanium oxides. Microstructure changes accompanying the addition of Co were also used to account for its role in improving the corrosion resistance of these materials.

Atomistic study on the super-elasticity of single crystal bulk NiTi shape memory alloy under adiabatic condition

The temperature-induced phase transition and the super-elasticity (from the stress-induced phase transition) of equiatomic single crystal bulk NiTi shape memory alloys are investigated by the molecular dynamics method. By the simulation to the thermo-mechanical response of the single crystal NiTi alloy along the 〈001〉B2 under the compression/unloading and an adiabatic condition, the temperature change and the nucleation and growth of martensite transformation during the compression/unloading are discussed. The simulated results of molecular dynamics show that the single crystal bulk NiTi shape memory alloy exhibits a significant temperature change during the martensite transformation and its reverse under an adiabatic condition; moreover, a localized instability occurs apparently in the process of martensite transformation, which is closely related to the nucleation and growth rates of martensite phase; finally the effect of model size and strain rate on the thermo-mechanical response of the single crystal bulk NiTi alloy is also discussed, and no instability is observed in the simulated stress-strain curves if the model size is relatively larger, e.g., 8V0 and 13.824V0.

Influence of Fe Addition on Phase Transformation, Microstructure and Mechanical Property of Equiatomic NiTi Shape Memory Alloy

Based on equiatomic nickel and titanium, three kinds of NiTiFe alloys with a nominal chemical composition of Ni49Ti49Fe2, Ni48Ti48Fe4 and Ni47Ti47Fe6 (at.%), respectively, have been designed to investigate the influence of the addition of Fe element on phase transformation, microstructure and mechanical property of equiatomic NiTi shape memory alloy. The microstructures of three kinds of NiTiFe alloys are characterized by the equiaxed grains instead of the dendrites. Consequently, some Ti2Ni precipitates are found to distribute in the grains interior and at the grain boundaries. The content of Fe element has an important influence on mechanical property of NiTiFe alloy. With increasing content of Fe element, the strength of NiTiFe alloy increases substantially, but the plasticity decreases sharply. It can be concluded that precipitation strengthening and solution strengthening play a significant role in enhancing the strength of NiTiFe alloy. In the case of three NiTiFe alloys, neither martensitic transformation nor reverse transformation can be observed in the range from −150 to 150 °C. On the one hand, the phase transformation temperature is probably out of the scope of the present experimental temperature. On the other hand, the addition of Fe element probably suppresses first-order martensitic transformation or reverse transformation, and consequently the second-order-like phase transformation from an incommensurate stage to a commensurate stage can probably take place.

Structural and functional properties of a semi equiatomic NiTi shape memory alloy processed by multi-axial forging

The capability of multi-axial forging (MAF) as an efficient severe plastic deformation (SPD) method to improve the shape memory responses of a semi equiatomic NiTi alloy was investigated. The MAF cycles were performed at temperatures in two different ranges, where the governing deformation mechanisms were thought to be different. This was confirmed through the material corresponding true stress-true strain flow behaviors. The presence of different phases was traced by means of X-ray analysis. The martensitic transformation was hindered in the specimens deformed at 150°C due to the presence of high dislocation density and in consequence the nano-crystalline austenite phase was the main phase after deformation. The specimens deformed at 400°C however, experienced restoration phenomena thereby brought the martensite beside the austenite phase in the microstructure. The latter was related to shifting the transformation point to lower temperatures. The processed material at 400°C exhibited both shape memory effect and super-elasticity due to the coexistence of restored and processed martensite and austenite phases. However the related recovery strain was reached to 3.2 pct. due to the corresponding shape memory effect. In addition, the results indicated that the multi-axial forging should be considered as a method to produce homogenous bulk NiTi alloy possessing an improved mechanical properties. To this end the microhardness, ultimate strength and elongation were improved from 220Hv, 920MPa and 18.5% in as-hot rolled condition to ~590Hv, ~1700MPa and 23% in optimum MAF condition scheme, respectively.

Molecular dynamics simulations to the pseudo-elasticity of NiTi shape memory alloy nano-pillar subjected to cyclic compression

The pseudo-elasticity of equiatomic NiTi shape memory alloy (SMA) nano-pillar subjected to a uniaxial cyclic compression is investigated by molecular dynamics (MD) simulation. By analyzing the local atomic structure, the nucleation sites and propagation paths of martensite transformation and its reverse, the degeneration of pseudo-elasticity and the interaction between martensitic transformation and defects at nano-scale during cyclic compressive deformation are discussed. It is indicated that the nucleation sites and propagation paths of the reverse transformation in the process of unloading are different from that observed in the process of loading (i.e., compression), and the dislocation and twinning are responsible for the changes in the atomic structure and mechanical response, the accumulated irreversible strain and the degeneration of pseudo-elasticity.

Mechanical and shape memory properties of porous Ni50.1Ti49.9 alloys manufactured by selective laser melting

Near equiatomic NiTi shape memory alloys were fabricated in dense and designed porous forms by Selective Laser Melting (SLM) and their mechanical and shape memory properties were systematically characterized. Particularly, the effects of pore morphology on their mechanical responses were investigated. Dense and porous NiTi alloys exhibited good shape memory effect with a recoverable strain of about 5% and functional stability after eight cycles of compression. The stiffness and residual plastic strain of porous NiTi were found to depend highly on the pore shape and the level of porosity. Since porous NiTi structures have lower elastic modulus and density than dense NiTi with still good shape memory properties, they are promising materials for lightweight structures, energy absorbers, and biomedical implants.

Atomistic Study on Size Effects in Thermally Induced Martensitic Phase Transformation of NiTi

The atomistic study shows strong size effects in thermally induced martensitic phase transformation evolution kinetics of equiatomic NiTi shape memory alloys (SMAs). It is shown that size effects are closely related to the presence of free surfaces; thus, NiTi thin films and nanopillars are studied. Quasi-static molecular dynamics simulations for several cell sizes at various (constant) temperatures are performed by employing well-established interatomic potentials for NiTi. The study shows that size plays a crucial role in the evolution of martensite phase fraction and, importantly, can significantly change the phase transformation temperatures, which can be used for the design of NiTi based sensors, actuators, or devices at nano- to microscales. Interestingly, it is found that, at the nanometer scale, Richard’s equation describes very well the martensite phase fraction evolution in NiTi thin films and nanopillars as a function of temperature.

Influence of Addition of Nb on Phase Transformation, Microstructure and Mechanical Properties of Equiatomic NiTi SMA

Three novel NiTiNb shape memory alloys, which possess a nominal chemical composition of Ni50−x/2-Ti50−x/2-Nbx (at.%) where x stands for 2, 4 and 6, respectively, were designed in order to investigate the influence of the addition of Nb on phase transformation, microstructure and mechanical properties of equiatomic NiTi shape memory alloy. All the three NiTiNb shape memory alloys contain B2 austenite phase, B19′ martensite phase and β-Nb precipitate phase. Martensite type II twin can be observed in the case of Ni49Ti49Nb2 alloy. In the case of Ni48Ti48Nb4 alloy, there exists a boundary between Ti2Ni precipitate phase and β-Nb precipitate phase. As for Ni47Ti47Nb6 alloy, it can be observed that there exists an orientation relationship of \( [01\bar{1}]_{{\upbeta{\text{ - Nb}}}} //[01\bar{1}]_{\text{B2}} \) between β-Nb precipitate phase and B2 austenite matrix. The increase in Nb content contributes to enhancing the yield stress of NiTiNb shape memory alloy, but it leads to the decrease in compression fracture stress. The addition of Nb to equiatomic NiTi shape memory alloy does not have a significant influence on the transformation hysteresis of the alloy, which is attributed to the fact that NiTiNb shape memory alloy is not subjected to plastic deformation and hence β-Nb precipitate phase is unable to relax the elastic strain in the martensite interface.

Constitutive equation and processing map of equiatomic NiTi shape memory alloy under hot plastic deformation

In order to describe the deformation behavior and the hot workability of equiatomic NiTi shape memory alloy (SMA) during hot deformation, Arrhenius-type constitutive equation and hot processing map of the alloy were developed by hot compression tests at temperatures ranging from 500 to 1100 °C and strain rates ranging from 0.0005 to 0.5 s−1. The results show that the instability region of the hot processing map increases with the increase of deformation extent. The instability occurs in the low and high temperature regions. The instability region presents the adiabatic shear bands at low temperatures, but it exhibits the abnormal growth of the grains at high temperatures. Consequently, it is necessary to avoid processing the equiatomic NiTi SMA in these regions. It is preferable to process the NiTi SMA at the temperatures ranging from 750 to 900 °C.

The Enhanced Shape Memory Effect and Mechanical Properties in Thermomechanically Processed Semi‐Equiatomic NiTi Shape Memory Alloy

The hot compression tests in a wide range of temperature were employed to produce the semi‐finished products of equiatomic NiTi shape memory alloy holding enhanced mechanical properties. The precise analyses of the compensated flow curves and the microstructures indicated the occurrence of dynamic recovery at lower temperatures and dynamic recrystallization at higher temperature regime. In addition a significant influence on the critical transformation temperatures was also realized. This could affect the martensite plates’ characteristics and the fraction of retained austenite phase. In consequence the room temperature mechanical properties and shape memory response of the alloy were influenced. In an elaborated optimized TMP scheme, the recovery ratio was increased from ≈30% to ≈80%.

Shape Memory NiTi and NiTiCu Alloys Obtained by Spark Plasma Sintering Process

The addition of Cu to near equiatomic NiTi shape memory alloys (SMAs) can provide some modifications of their shape memory properties by affecting their transformation behavior. The same effect was obtained in the case of Ni3Ti2 and Ni4Ti3 precipitates presence in the microstructure of NiTi. Also the substitution of Cu to NiTi alloys increases the hardness of the materials. This paper presents the microstructural and mechanical investigations of NiTi and NiTiCu alloys obtained by spark plasma sintering (SPS) process that represents a great potential for researchers as a new process for the fabrication of intermetallic compounds.

Tribological performance of near equiatomic and Ti-rich NiTi shape memory alloy thin films

Near equiatomic and Ti-rich NiTi shape memory alloy thin films were magnetron sputtered with the same processing parameters and thickness of 3μm. The microstructure, composition, shape memory behavior, mechanical and tribological properties of the deposited thin films were analyzed by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), differential scanning calorimetry (DSC), nanoindentation, ball-on-disc, scratch test, and three dimensional (3D) optical microscopy. The obtained results clearly show how the crystallization evolution and precipitation formation of these two sets of thin films can drastically influence their mechanical and tribological performances.

Effects of HSHPT on the martensitic transformation behaviour of an NiTi alloy

High speed high pressure torsion (HSHPT) is a novel severe plastic deformation technique that is used to produce bulk ultrafine-grained nickel-titanium shape memory alloys. In this study, the effect of grain refinement on phase transformation was investigated in a near equiatomic NiTi shape memory alloy subjected to processing by this technique. Phase transformations involving different degrees of deformation and stability of thermally-induced phase transformations were analyzed by differential scanning calorimetry (DSC). The measurements suggest that the martensitic transformation occurred even when the highest degree of deformation was applied. Optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations bring to light that the true strain applied controls the evolution of the microstructure. The results are presented and discussed in detail in this paper.

Work Production and Variation in Shape Memory Effects during Thermal Cycling of Equiatomic TiNi Alloy

The influence of thermal cycling under a symmetric scheme for work performance on the functional properties and work output of equiatomic TiNi shape memory alloys was studied. It was found that the Ti50Ni50 alloy produced an effective work output of 9.7 MJ/m3 and an efficiency of 1.47% in a symmetrical scheme for work production where the stress acting on cooling was 50 MPa, and the stress acting on heating was 200 MPa. It was observed that the Ti50Ni50 alloy demonstrated the monotonic dependence of plastic strain on the number cycle, and 3.1% of the residual strain was accumulated over 30 cycles. The data have shown that using a ‘symmetric’ scheme for work production allows one to reduce a plastic strain 13-fold in comparison with an ‘asymmetric’ scheme. Thus, the symmetric scheme for work production provides for the high stability of the functional properties and work output of TiNi-based shape memory alloys.

Formation of the Nanocrystalline Structure in an Equiatomic NiTi Shape-Memory Alloy by Thermomechanical Processing

The microstructural evolution during cold rolling followed by annealing of an equiatomic NiTi shape-memory alloy was investigated. The high purity Ni50Ti50 alloy was cast by a copper boat vacuum induction-melting technique. The as-cast ingots were then homogenized, hot rolled, and annealed to prepare the suitable initial microstructure. Thereafter, annealed specimens were cold rolled up to 70 % thickness reduction at room temperature. Post-deformation annealing was conducted at 400 °C for 1 h. The microstructure was characterized using scanning electron microscopy, transmission electron microscopy, x-ray diffraction, and differential scanning calorimetry techniques. The initial microstructure was free from segregation and Ti- or Ni-rich precipitates and was composed of coarse grains with an average size of 50 μm. The cold rolling of NiTi alloy resulted in a partial amorphization and the deformation-induced grain refinement. A nanocrystalline structure with the grain size of about 20-70 nm was formed during the post-deformation annealing.

Characterisation and modelling of behaviour of a shape memory alloys

Shape memory alloys (SMAs) provide an attractive solid-state actuation alternative to engineers in various fields due to their ability to exhibit recoverable deformations while under substantial loads. This feature is of particular importance when utilising the smart composite materials reinforced by SMA. Many constitutive models describing this repeatable phenomenon have been proposed, where some models also capture the effects of rate-independent irrecoverable deformations in SMAs. This paper presents experimental investigations and numerical simulations on shape memory alloys. First, by consisting in determining the transformations of equiatomic Ti–Ni shape memory alloys by differential scanning calorimeter. Then, in order to validate a 3D numerical model of the pseudoelastic behaviour of SMA allowing a finite strain analysis, a set of experimental tests at various initial temperatures is proposed. Finally, the numerical simulations of uniaxial tests performed on shape memory alloys are presented and compared with experimental data, permitting the validation of the proposed modelling. Reasonably good correlation is obtained between the experimental and model predictions.