NiTi Shape Memory Research Articles

Selective laser melting fabrication of body-temperature shape memory NiTi alloy for 4D-printed orthopedic implants

ABSTRACT SLM fabrication of NiTi alloy is receiving increasing attention. However, the relatively high austenite transformation finish (Af) temperature is the main problem for biomedical applications. In this study, it was found that phase transformation temperature (PTT) increased with increasing laser power, decreasing scanning speed, and decreasing hatch spacing. The combination of PPs with an energy density of less than 75 J/mm³ is expected to yield the Af temperature below 37 °C. P = 80 W, v = 600 mm/s, and h = 70 μm were identified as the optimal combination because of a good tensile strength of 612 MPa and a peak shape recovery rate of 96.94%. Biocompatibility assessment showed that SLM-NiTi porous scaffolds supported pre-osteoblast survival and proliferation. A patient-specific mesh-like supporting prosthesis was designed to show the prospect of 4D-printed orthopedics implant. In clinical application, SLM-NiTi prosthesis could reduce bony window and facilitate easier insertion through compressive deformation.

Calorimetric and Volumetric Studies of Dislocations During Martensitic Transformations in Shape Memory TiNi Alloy

Based on the use of appropriate approaches, methods and results of the analysis of a number of topical problems of physical materials science, an in-depth analysis has been done of calorimetric and volumetric data for direct, reverse and deformation martensitic transformations in a nanostructured Ti49.3Ni50.7 shape memory alloy obtained by cold rolling with simultaneous action of pulsed high-density current. For the first time, by applying a new technique for processing calorimetric spectra (peaks), the staging and “kinetics” of changes in heat content, as well as thermal effects (enthalpies of individual stages) during direct and reverse martensitic transformations during cooling or heating of samples at a constant rate (3 × K/ min) in the range 170–370 K has been done. It is shown, by processing volumetric data, using theoretical values of the dislocation density and elements of the classical theory of dislocations, that in the Ti49.3Ni50.7 shape memory alloy subjected to cold deformation accompanied by the action of a pulsed current, a deformation martensitic transformation occurs, leading to a positive volume effect (∆V/V) ≈ 3 × 10–3), which can be largely due to dislocations. It is shown, by applying the theoretical values of the dislocation density and elements of the classical theory of dislocations, that the possible contributions of dislocations to the enthalpies of direct and reverse martensitic transformations (in the Ti49.3Ni50.7 alloy) can and should be significantly lower in absolute value, but opposite in sign to the observed enthalpies of direct and reverse martensitic transformation in a given alloy. It is shown that the physics of the processes under consideration is contained to a certain extent in scientific discoveries No. 239 “The phenomenon of thermoelastic equilibrium during phase transformations of the martensitic type – the Kurdyumov effect” and No. 339 “The phenomenon of the formation of non-equilibrium grain boundaries in polycrystals when they absorb lattice dislocations”.

Transformation pattern of shape-memory NiTi alloy during stress-biased thermal cycling

Despite the superelastic deformation of NiTi has been documented and analyzed elaborately, its shape-memory behavior during stress-biased thermal cycling has not been thoroughly unveiled. This paper examines the evolution of transformation pattern in NiTi upon thermal cycling over a wide range of biasing stress. For the present shape-memory NiTi, both forward and reverse transformations proceed via the growth of localized deformation band (LDB) under low biasing stress (σbias ≤ 150 MPa), while LDB only appears during forward transformation and reverse transformation is uniform under high biasing stress (σbias ≥ 200 MPa). This is the first time that delocalization (conversion of deformation mode from localization to homogeneity) is observed during stress-biased thermal cycling. We clarify that the intrinsic undercooling/overheating of martensitic transformation results in unstable thermomechanical response, and it is the origin of localization in shape-memory NiTi. It is found that transformation-induced plasticity (TRIP), which is dominated by dislocation slip and deformation twining, not only causes irreversibility in the present shape-memory NiTi but also leads to delocalization through stabilization of intrinsic thermomechanical response of reverse transformation.

Hydroxyapatite coating of TiNi shape memory alloy via Electrophoretic Deposition (EPD)

ABSTRACT In this study, hydroxyl apatite (HA) as a bio-ceramic is coated on the TiNi shape memory. In this approach, Tri-ethanolamine and butanol were used as suspensions. To set up Electrophoretic Deposition (EPD) process, voltages of 20, 30 and 40 volts were applied on the cathode for 1-5 minutes. The sintering process was done in the Ar atmosphere for two hours at 800°C. SEM and XRD were used to investigate surface morphologies and phases. Obtained results showed that higher electrical currents led to generation of thicker coatings registering 105% and 12.3% improvement for weight and thickness after 6 minutes, respectively. Moreover, Transformation of HA to other calcium phosphate phases doesn’t happen at a sintering temperature of 800°C. Moreover, a uniform, homogeneous and crackles coating can be reached in the voltage of 30 volts at low sintering temperatures. This process can be utilized an alternative technique for bioactive coatings.

Thermodynamic analysis of production parameters and microstructural evolution in shape memory Ni(50-x)Ti(50)Fe(x) (x = 5, 10) alloy synthesized by combustion synthesis

This study represents an initial effort to produce NiTiFe shape memory alloys via the self-propagating high-temperature synthesis (SHS) process. The synthesis successfully yielded Ni45Ti50Fe5 and Ni40Ti50Fe10 alloys from elemental Ni, Ti, and Fe powders at three distinct preheating temperatures (240, 330, 420 °C). To support empirical findings, thermodynamic analysis using Factsage Thermodynamic Software was employed to correlate reaction propagation behavior with chemical composition. The calculation showed that the addition of 5 at.% Fe to the B2 phase did not hinder reaction self-propagation. This conclusion was supported with the ∆Hf/Cp ratio and transient liquid ratio, computed using the sub-lattice model, which closely resembled that of NiTi. Whereas 10 at.% Fe that synthesized at preheating temperature of 240 °C exhibits struggle, thus an increase in triggering time causes an effect on crystallite size and a decrease in porosity. Empirical results confirmed these findings, albeit influenced by ignition times. An increase in the liquid ratio due to the adiabatic temperature rise can also result in a reduction of NiTi content when the Fe ratio is increased, consequently diminishing the driving force for the reaction. In the sample containing – 5% Fe, the main phase is B2, and the R martensitic phase is also present. Ms is determined to be – 38.7 °C. SEM analyses revealed the presence of Ti2Ni and Ti2Ni3 phases in the upper and lower regions, while the distribution in the middle section is more homogeneous. No martensitic transformation was observed in 10% Fe. Additionally, nanocrystalline regions were detected within the samples by transmission electron microscopy, contributing to a nuanced understanding of their structural properties.Graphic abstract

Effects of laser welding on the martensitic transformation of a shape-memory Ti-Ni alloy

The objective of this study is to investigate the martensitic transformation behavior of a Ti-Ni alloy subjected to a laser welding process. Materials characterization techniques were used to analyze the influences of this welding procedure on the phase transformation behavior. This study can provide valuable insights for the improvement of Ti-Ni alloy welding processes used for technological applications, such as in the aerospace industry. In this study, samples were prepared under three conditions: thermal treatment at 400°C, thermal treatment at 500°C, and as-received. The characterization techniques used were: Optical Microscopy (OM), Differential Scanning Calorimetry (DSC), Electron Dispersion Spectroscopy (EDS), X-Ray Diffraction (XRD), and Vickers Microhardness (HV). The results allow for analysis of thermoelastic martensitic transformation and variations in the compositions of the alloy elements. The research, in general, aims to repurpose welding waste in Ti-Ni alloys for the production of mini sensors. The enthalpy was higher for the alloy treated at 500°C and welded. The research findings led to the observation of martensitic phase transformation in the laser-welded Ti-Ni alloy, which suggests that it can be used as a sensor/actuator.

Understanding Machining Process Parameters and Optimization of High-Speed Turning of NiTi SMA Using Response Surface Method (RSM) and Genetic Algorithm (GA).

This study aimed to optimize machining parameters to obtain better surface roughness and remnant depth ratio values under dry turning of NiTi-shape memory alloy (SMA). During the turning experiments, various machining parameters were used, including three different cutting speeds vc (105, 144, and 200 m/min), three different feed rates f (0.05, 0.1, and 0.15 mm/rev), and three different depths of cut ap (0.1, 0.15, and 0.2 mm). The effects of machining parameters in turning experiments were investigated on the response surface methodology (RSM) with Box-Behnken design (BBD) using the Design Expert 11; how the cutting parameters affect the surface quality is discussed in detail. In this context, the cutting parameters were successfully optimized using a genetic algorithm (GA). The optimized processing parameters are vc = 126 m/min, f = 0.11 mm/rev, ap = 0.14 mm, resulting in surface roughness and remnant depth ratio values of 0.489 μm and 64.13%, respectively.

Examination of the Corrosion Behavior of Shape Memory NiTi Material for Biomedical Applications.

In this study, corrosion and wear tests of NiTi alloy (Ni 55%-Ti 45%) samples, known as shape memory alloy, which offer a shape recovery memory effect between memory temperatures ranging from 25 to 35 °C, have been carried out. The standard metallographically prepared samples' microstructure images were obtained using an optical microscope device and SEM with an EDS analyzer. For the corrosion test, the samples are immersed with a net into the beaker of synthetic body fluid, whose contact with the standard air is cut off. Electrochemical corrosion analyses were performed after potentiodynamic testing in synthetic body fluid and at room temperature. The wear tests of the investigated NiTi superalloy were carried out by performing reciprocal wear tests under 20 N and 40 N loads in a dry environment and body fluid. During wear, a 100CR6-quality steel ball of the counter material was rubbed on the sample surface for a total of 300 m with a unit line length of 13 mm and a sliding speed of 0.04 m/s. As a result of both the potentiodynamic polarization and immersion corrosion tests in the body fluid, an average of 50% thickness reduction in the samples was observed in proportion to the change in the corrosion current values. In addition, the weight loss of the samples in corrosive wear is 20% less than that in dry wear. This can be attributed to the protective effect of the oxide film on the surface at high loads and the effect of reducing the friction coefficient of the body fluid.

Surface Deformation Recovery by Thermal Annealing of Thermal Plasma Sprayed Shape Memory NiTi Alloys

The shape memory effect is the most important attribute of shape memory alloys where material can recover to its initial shape after deformation by heating above its transformation temperature. In this article, the thermally induced recovery of well-defined microscopic deformation in a NiTi shape memory alloy was investigated. Surface deformation was performed by indenting the plasma sprayed NiTi shape memory alloy in a martensitic phase at room temperature using spherical indenters. In this article, a series of indentations, scratch lines and wear lines were made on the surface of two different NiTi shape memory alloys at the micrometre scale using two spherical indenters with different radii. Three-dimensional imaging of indentation topography using scanning confocal microscopy provided direct evidence of the thermally induced martensitic transformation of these plasma sprayed thick films allowing for partial recovery on the micro-scale. The partial recovery is achieved at various indentation depths and for different scratches and wear volumes.

Superelastic and shape memory equi-atomic nickel-titanium (Ni-Ti) alloy in dentistry: A systematic review

According to the Global Burden of Disease (GBD) 2015, around 3.5 billion people suffer from dental diseases. The treatment of these disease requires either a specific dental device or a material which are usually subjected to stress and corrosion of the oral environment due to varying temperature, pH, occlusion, and masticatory forces. Various smart materials have emerged in the biomedical field because of their superior properties compared to conventional alloys. However, Nickel-Titanium (Ni-Ti) alloys with nearly equi-atomic composition have a long-standing history of successful use in dentistry because of their shape memory (SM) and superelastic (SE) properties with excellent cytocompatibility and corrosion resistance. The SM property offers preferable control of the transition temperature range (TTR) near the oral temperature providing multiple actuation. The SE property has revolutionized orthodontic treatment by delivering light and continuous physiological forces to the teeth, thereby improving patient comfort. The SE property of Ni-Ti alloy has been extensively used to produce orthodontic archwires and endodontic files as they exhibit higher flexibility compared to conventional stainless-steel alloy. However, the duration and type of heat treatment performed on these devices have been reported to modify their thermo-mechanical properties, producing adverse effects. Besides this, the devices are also subjected to short-term cooling or heating, sterilization, and disinfection protocols that creates a corrosive environment. Thus, thermo-mechanical properties and the influence of oral environment on various commercially available Ni-Ti alloy dental devices have been briefly explored and discussed. Besides orthodontic archwire and endodontic files, other Ni-Ti alloy-based dental devices are also reviewed, such as Shape memory Ni-Ti alloy sleeve for Abutment System, SM Ni-Ti alloy palatal expander, and SM Ni-Ti alloy porous dental implant. Thus, this manuscript aims to provide a concise assessment of the properties of SE and SM Ni-Ti alloys and their evolution in the field of dentistry. • Large population suffers from dental diseases. • Among the smart materials, Ni-Ti alloys have been successfully used in dentistry. • Shape memory offers control of transition temperatures near to oral temperature. • Superelasticity delivers light and continuous physiological forces to the teeth. • Thermomechanical treatment and oral environment create undesirable device behavior.

In-vitro biocompatibility evaluation of cast Ni–Ti alloy produced by vacuum arc melting technique for biomedical and dental applications

The investigated cast Ni50–Ti50 shape memory alloy was prepared using a vacuum arc furnace. The cast samples were subjected to in-vitro biocompatibility studies according to ISO 10993-12:2004, and compared to other samples Ni–Ti orthodontic wires commercially available at the dental market. The cast samples were hydroxyapatite-coated using the electrodeposition technique. The effect of surface treatment on the coating quality was addressed. The hydroxyapatite-coated samples were investigated using electrochemical impedance (EIS) and potentiodynamic techniques. Coated samples were also examined using a scanning electron microscope to inspect the coating morphology. Cytotoxicity tests on MG63 and H9C2 cell lines showed the safety and biocompatibility of the cast NiTi alloy, with a direct relationship between the incubation period of the tested samples and cell viability. Well-adhered hydroxyapatite coating was obtained on the surface-treated NiTi samples using the electrodeposition technique. EDS analysis showed a hydroxyapatite coating having a calcium to phosphorus ratio close to that of the natural bone. Electrochemical tests indicated that the highest corrosion resistance was obtained for the uncoated samples followed by the anodized sample and finally the hydroxyapatite-coated samples due to their high porosity.

Multi-response optimisation of machining parameters to minimise the overcut and circularity error during micro-EDM of nickel-titanium shape memory alloy

ABSTRACT Micro-electrical discharge machining (µ-EDM) is one the most favourable process to machine difficult to cut materials in micro-scale with higher accuracy and precision. In this paper, two multi-response optimisation tools called Taguchi’s grey relation analysis (GRA) and response surface methodology (RSM)-based desirability function are used to evaluate optimised micro-machining parameters for minimal overcut and circularity error during µ-EDM of nickel-titanium (NiTi) shape memory alloy (SMA). Taguchi’s GRA revealed that electrode material has the strongest influence on both the responses followed by capacitance and discharge voltage. ANOVA analysis established that electrode material has the highest percentage contribution of 40.15% on both the responses. The optimisation result obtained by Taguchi’s GRA approach is validated by the RSM-desirability function-based technique. The final optimisation result (capacitance 155 pF, electrode material tungsten, discharge voltage 80 V) is a very close agreement with the literature model (capacitance 355 pF, electrode material tungsten, discharge voltage 80 V), with reference to the overcut and circularity error for the improvement of the dimensional accuracy of the micro-holes. Further, the response surface plots disclosed the variation of overcut and circularity error with the process parameters at various levels in a descriptive manner.

Design methodology of the Ni50Ti50 shape memory alloy beam actuator: Heat treatment, training and numerical simulation

Shape memory alloy (SMA) beam actuators are widely used in morphing structures, which leads to a surgent demand for a comprehensive design method. Previous methods have not considered the heat treatment and the evolution of actuation performance during training. In this study, an in-depth investigation was conducted into the Ni50Ti50 (at%) SMA beam actuator from the perspective of heat treatment, training, numerical simulation for developing a comprehensive and novel design methodology. The proper heat treatment conditions that result in the relatively high actuation performance for the Ni50Ti50 SMA beam were determined. The dependence of the thermomechanical behavior evolution on the training load was established. Moreover, a numerical analysis method for the SMA beam actuator based on a 3-D constitutive model with tension-compression asymmetry considered was detailed, followed by the calibration of material parameters. A forward design method for the SMA beam actuator was proposed with the shape and actuation performance evolution during training under various training loads fully considered in the design process. The proposed design method was applied for a design case and the accuracy of the design results demonstrated its feasibility.

Experimental Analysis on the Impact of Current on the Strength and Lifespan of a Ni-Ti Element

Intelligent materials, especially materials with shape memory, are an important discovery, with technical applications in the medical and aerospace field, among others, which led to the development of systems and applications with multiple advantages and disadvantages due to ignorance about their functionality. This paper presents an application developed in the research laboratory for determining and monitoring the behavior of a material element with Ni-Ti shape memory, and its lifespan. The application allows the stress level of the Ni-Ti element subjected to numerous repeated cycles of deformation to be determined by supplying it to a constant electric current. Thus, the results show the variation of the Ni-Ti element force, in the form of a spring, at the ambient temperature variations as well as force variations at different numbers of attempts. The Ni-Ti alloy has both shape retention and superelasticity properties, being the most common in the fields of applicability. Due to its unique properties, it can be used in the most demanding applications in the medical field, usually involving difficult conditions of resistance to fatigue.

Controlling microstructure evolution and phase transformation behavior in additive manufacturing of nitinol shape memory alloys by tuning hatch distance

Laser powder bed fusion (L-PBF), categorized as additive manufacturing technique, has a capability to fabricate NiTi (Nitinol) shape memory alloys with tailorable functional properties and complex geometries. An important processing parameter, hatch distance (h), is often related to macroscale structural defects; however, its role on controlling the microstructure and functional properties is usually underestimated in L-PBF of NiTi. In this work, equiatomic NiTi (50.0 at% Ni) parts were fabricated with various hatch distances to tailor the microstructure and their shape memory characteristics. Contrary to what is observed in Ni-rich NiTi alloys, in this work, we demonstrate that phase transformation temperatures of L-PBF equiatomic NiTi do not decrease proportionally with hatch distance but rather relate to a critical hatch distance value. This critical value (120 μm) is derived from the synergistic effect of thermal stress and in situ reheating. Below this value, epitaxial grain growth and in situ recrystallization are enhanced, while above, irregular grains are formed and dislocations induced by thermal stresses decrease. However, the critical value found herein is characterized by high dislocation density and fine grain size, resulting in a superior thermal cyclic stability. The proposed finite element model is proven to be an effective tool to understand and predict the effect of hatch distance on grain morphology and dislocation density evolutions in L-PBF NiTi SMAs. In the present study, we provide a comprehensive understanding for in situ controlling L-PBF NiTi microstructure and functional characteristics, which contributes to designing 4-dimensional shape memory alloys.

Process Modelling and Optimization using ANN and RSM during WSEM of Ni51.59Ti48.41 shape memory alloy

Shape memory Ti-Ni alloys have fascinated significantly in recent years since these types of material are intelligent, shape memory and functional materials. These materials find many applications in the engineering and medical fields. In the current study, ‘Ni-Ti’ shape memory alloy (SMA) has been processed by wire spark erosion machining (WSEM). This research article explores the effects of controllable machining variables such as spark on-time, spark off-time, wire feed and servo voltage on productivity, i.e., material removal rate (MRR) and surface integrity. To assess the impact of WSEM controllable factor, a statistical analysis is done. Mathematical modelling and ANOVA study have been conducted after performing 29 experiments based on Box-Behnken design (BBD) of response surface methodology (RSM) technique. Artificial neural network (ANN) modelling technique is used for further prediction of the response. The predicted values obtained from the ANN model shows excellent agreement with experimental outcomes. In this study, the correlation coefficient (R) value is 0.9919, which is very close to unity. It is clearly showing that the prediction accuracy of ANN model is high. Furthermore, scanning electron microscope (SEM) is utilized to assess the microstructure and composition of the WSEMed surfaces. After a critical study, it has been noticed that machined surfaces contain debris, cracks, craters and spherical droplets. In addition, energy dispersive spectroscopy (EDS) analysis has been performed to check the elemental composition on the machined surface.

Cold Rolling Electrostimulation of Hard-Deform Alloys

On the example of shape memory alloys in coarse-grained and ultrafine-grained states there is demonstrated an influence of pulse current on deformation behavior, microstructure and mechanical properties during cold rolling. The combination of cold rolling with the introduction of a pulsed current stimulates an increase in the deformation ability and strong structural refinement into the region of nanograins in a hard-deformed shape memory TiNi alloy. For both coarse-grained and ultrafine-grained structural states, the maximum achievable one-time and the total degree of deformation are higher in the case of using a pulsed current. Subsequent annealing improves the strength and functional properties. It has been shown that effect of pulsed current on deformability during cold rolling is higher in an alloy with a high content of impurities.

Magnetron Sputtering of Au-Based Alloys on NiTi Elements: Surface Investigation for New Products in SMA-Based Fashion and Luxury Accessories and Watchmaking

A novel approach for the deposition of Au-based coatings on NiTi components was proposed to give rise to innovative SMA-based products for the fashion, luxury, and watchmaking fields. Different Au-Cu and Au-Ag-Cu alloys (with confidential compositions within the color designations 2N, 4N, and 5N) were deposited by magnetron sputtering on superelastic and shape-memory NiTi ribbons. After preliminary morphological and microstructural characterizations, the influence of the film deposition on the functional, mechanical, and tribological behavior was deeply investigated. The macroscopic mechanical properties, including the damping, superelastic, and shape recovery characteristics, were not affected since the behavior upon both small and severe deformations was unchanged and the coatings were not damaged. Indeed, both the film adhesion and the precious aspect were maintained. Furthermore, a more detailed surface characterization, through nanoindentation, tribocorrosion, and scratch and wear tests, was performed. This experimental investigation evidenced the ductile behavior of the Au-based films and their good adhesion on NiTi substrates. Moreover, the coatings exhibited a good wear resistance, both in dry conditions and simulated body fluids, which proved to be suitable for fashion and watchmaking fields. Despite slight differences being observed within the films’ responses, all of them could be considered suitable and interesting for the design of smart luxury accessories, proving that the chosen deposition process is sound and reliable for these applications.

Investigation of the effect material responses parameter (alpha) on stress-strain behavior during loading and unloading of superelastic shape memory nitinol wire

Shape Memory Alloy (SMA) is able to recover big strains beyond an appropriate heating and/or mechanical loading procedure. Numerous uses being exploiting such notable characteristic and the NiTi wires are of particular attention to conduct the connections in various industrial states. The present research aims to achieve a numerical simulation of the superelastic conduct of the NiTi shape memory alloy wire, using Finite Element Method by employing commercial ANSYS 15.0 software. The influence of the material responses parameter (Alpha) on the stress–strain behavior during loading and unloading of superplastic shape memory NiTi wire has been investigated. Axisymmetric wire element is employed, and characteristic responses are analyzed by considering von Mises stress and von Mises strain curves at constant time for various values of Alpha parameter from during the bending process. The numerical simulations manifested the features of superelastic recovery, which is also in terms of time.

Thermo-mechanical coupling shock waves propagation with phase transition under impact loading

In the present study, a thermo-mechanical coupling model including constitutive relation and kinetics for phase transformation is developed to study stress wave propagation in shape memory TiNi rods. The front tracking/capturing method is adopted for numerical analysis and impact experiments are performed to verify theoretical model. It is demonstrated that, a phase transition shock wave will be induced during loading due to the mixed phase hardening effect originating from the temperature dependence of transformation stress, and for unloading, double wave profile (an elastic wave and a shock wave with reverse phase transition) will be excited since the change of unloading path caused by the transient temperature rise during loading. Furthermore, shock wave propagation speeds are increased with loading stress, while driving force acting on the shock wave is supplied by shock dissipation energy, in which the temperature gradient across the shock front plays an important role. SHPB experiments show that the phase change wavefront is also a moving temperature interface, which will have an important influence on the stress wave structure. Taylor impact experiments show that the proposed kinetic relation in this paper is in good agreement with the experimental results, which reflects the intrinsic characteristic of materials with strong nonlinear thermo-mechanical coupling behavior.