Titanium Shape Memory Alloy Research Articles

Abrasive aqua jet machining of NiTiV ternary smart alloy

ABSTRACT Nickel–titanium shape memory alloys (SMAs) are popular due to their usefulness and functional characteristics. The binary NiTi SMA was alloyed with vanadium to improve memory retention properties. In this study, the abrasive aqua jet machining method was adopted to machine the Ni50Ti48V2 ternary SMA to minimize the kerf taper angle (KTA), surface roughness (Ra) of machined Ni50Ti48V2 surface, and enhance the material removal rate (MRR). RSM-BBD optimization using the input variables such as abrasive flow rate (AFR), water jet pressure (PWJ), nozzle traverse speed (VTS), and standoff distance (SFD) predicted that PWJ and SFD highly contributed to MRR, Ra, and KTA responses. Small variation in variation after optimized AWJM conditions, and XRD analysis confirmed no phase changes after AWJM.

Cyclic and uniaxial tensile properties of superelastic niti shape memory alloy cables

This study investigates the impact of various factors, including annealing duration, strain amplitude, cyclic loading, loading rate, and pre-training, on the mechanical properties of Nickel–Titanium shape memory alloy (SMA) cable. The primary focus is on evaluating their recovery ability and energy dissipation capabilities. The tested SMA cable has an outer diameter of 9 mm and a 7 × 7 configuration. The variation of strength, stiffness, residual strain, hysteretic energy, and equivalent viscous damping ratio of SMA cable with the loading cycle is analyzed. Furthermore, the impact of various annealing durations on the tensile strength and elongation of both SMA cables and wires was examined through monotonic tensile tests. The results indicate that the annealing duration considerably affects the superelastic behavior of SMA cables by shifting the stress-strain loops down and widening them. The recovery ability of SMA cable degrades more progressively with increasing loading amplitude and the number of loading cycles. The mechanical properties gradually stabilized after 20 times of constant strain amplitude loading and unloading training. The strain selection for cyclic training should not make the SMA cable in the martensite hardening stage. The recovery ability and peak stress of SMA cable can be significantly improved by pre-training. With the increase of annealing duration, the tensile strength of the SMA cable decreases gradually. Compared with SMA wire, SMA cable has better ductility and robustness and provides sufficient restoring force under large deformation.

A study on Analysing the Selection of Materials for Knee Implant Femoral Components using the TOPSIS method

Specialists are creating more sophisticated and useful materials daily as technology advances. For orthopaedic implants such as "knee replacements, hip replacements, and orthopaedic accessories", biomaterials are employed to produce prosthetic organs. In patients with severe osteoarthritis (OA) of the knee, "total knee replacement (TKR)" is one of the most successful surgical procedures for pain management and functioning rehabilitation. Humans continue to experience issues with the kneecap, such as aseptic dislocation brought on by excessive wear across articular surfaces, stress-shielding of the bone by prostheses, and soft tissue formation at the junction of implanted bone. due to improper “TKR material selection”. Since selecting the best materials for the femoral component of TKR requires careful consideration, the "technique for order of preference by similarity to ideal solution (TOPSIS)" is used in this research paper. This technique uses an order of preference based on how closely the preferred option is to the ideal solution. The "equal weights method (EWM)" assigns various criteria distinct weights of importance. The rank of alternatives using the TOPSIS method for “Co-Cr-Mo is fourth, Co-Ni-Cr-Mo is third, NiTi SMA is first, Porous NiTi SMA is second, pure Ti is sixth and Ti-5Al-2.5Fe is fifth”. The result indicated that FC material using TKR Nickel Titanium Shape Memory Alloy (NiTi SMA) is at rank 1 with properties such as Tensile Strength 960 MPa, Density 6.45 g/cc, extremely high Corrosion resistance, and exceptionally high Wear resistance.

High-throughput membrane deflection characterization of shape memory alloy thin films

Nickel–Titanium (Ni–Ti) shape memory alloy thin films are currently gaining significant attention for small-scale systems, but device integration is still challenging due to its high compositional-dependent behavior. Therefore, it is crucial to identify the optimal parameters required to fabricate desirable samples. In this study, combinatorial high-throughput membrane deflection experiments were performed to investigate the mechanical properties of submicron-thick Ni–Ti thin films. The mechanical properties of freestanding Ni–Ti thin films, sputter-deposited with compositions and thicknesses ranging from 45.7 at. % to 52.7 at. % nickel and 270–530 nm were evaluated. With an increase in the Ni content, the crystallinity and existing phases of the films varied and exhibited an overall increasing trend of elastic modulus, transformation stress, percentage of recoverable strain and hysteresis area. Based on the obtained results, the compositional ratio of Ni–Ti films can be tailored accordingly to meet the different requirements of small-scale devices, and to then extend the employment of this high-throughput methodology in exploring advanced metallic alloys and ceramic composites.

Effect of robotic weaving motion on mechanical and microstructural characteristics of wire arc additively manufactured NiTi shape memory alloy

Abstract The effect of robotic weaving motion on the crystallographic orientation, texture and mechanical properties of thin-walled nickel titanium shape memory alloy fabricated via wire arc additive manufacturing is studied. Crystallographic orientation analysis reveals that the weaving motion alters the epitaxial growth of grains, which reduces the fraction of (001) oriented grains but increased the fraction of (110) and (111) oriented grains. Moreover, the weaving motion increases the fraction of equiaxed grains and homogeneously distributes Ni4Ti3 phases. These changes improve the hardness and strength of the build and also introduce isotropy in mechanical properties. Results of the study reveal that arc weaving strategy has a great potential in achieving near isotropic tensile characteristics and is beneficial in terms of tailoring texture in building nickel titanium thin-walled smart material structures.

Analysis of Shape-Memory-Alloy Fiber Interface Strength for Optimization of Self-Healing Composites

Self-healing materials, possessing an innate ability to mend damage and restore structural strength, have tremendous potential to improve safety and reliability, especially in space applications where recovery or manual performance of repairs may be prohibitive, dangerous, or impossible. Self-healing metallic materials developed with Nickel Titanium (NiTi) Shape Memory Alloy (SMA) reinforcement showed a promising prospect for recovering from larger scale damage. However, NiTi is known to form an inert oxide (TiO2) surface layer that is extremely hard to attach to. In this research, to promote better strength between the fiber and matrix, TiO2 surface layer has been removed by etching the NiTi wires in an inert environment and a pull test has been performed to evaluate the resulting change in adhesion strength between the fiber and metallic matrix. The experimental investigation of these interface fracture scenarios will enable the efficient design of composite materials.

On the Nature of Post‐Constrained Recovery Residual Stress of NiTi Shape Memory Alloys

In this article, the nature of the post‐constrained recovery residual stress (PCRRS) observed in nickel titanium (NiTi) shape memory alloys (SMAs) subjected to constrained recovery and returned to a low‐temperature state while remaining constrained is investigated. After performing the thermomechanical cycling to produce the PCRRS state, four experiments are performed with different deflection profiles applied from the PCRRS state, and the changes in the residual stress is observed. All tests produce a gradual decrease in stress after every load cycle, similar to cyclic softening. However, repetition of the process used to initially generate PCRRS restores the initial magnitude of the PCRRS, indicating that the decrease in residual stress is a stable, repeatable, and reversible phenomenon, and not true material degradation. To better understand the nature of PCRRS, an analysis is performed comparing the modulus relating stress and strain at different points along the thermomechanical loading path. In this analysis, it is shown that NiTi has a combination of twinned and detwinned martensite in the PCRRS state. Understanding the nature of PCRRS, properties and its stability have the potential for advanced engineering applications such as self‐healing and fatigue‐resistant materials by generating stresses without continuous actuation over a long period of time.

Investigation and optimization in electrochemical arc drilling of Ni55.7Ti nickel–titanium shape memory alloy with molybdenum electrode

Investigation and optimization in electrochemical arc drilling of Ni55.7Ti nickel–titanium shape memory alloy with molybdenum electrode

Study of ageing and size effects in Nickel–Titanium shape memory alloy using molecular dynamics simulations

ABSTRACT The effect of ageing process at 1073.15 K on the athermal phase transformation in different nanograins (4.5, 6 and 9 nm) of Nickel–Titanium shape memory alloy is studied using molecular dynamics simulations. The martensitic transformation start temperature and transformed martensite volume decrease with decreasing grain size. The two-stage phase transformation with intermediate phase formation is observed only in the smallest grain (4.5 nm) where a single martensite variant is formed. On the contrary, larger grains show a self-accommodated twinned martensite variants with one-stage transformation from austenite to martensite. The ageing process hardly influences the morphology of the martensite phase, however, it rises the martensite start temperature and reduces the austenite start temperature. The nano grain size models demonstrate the constrained transformation due to the transformation barrier and the accumulation of internal stress, while the ageing process shows the tendency for stress relaxation.

ELECTROCHEMICAL ARC DRILLING OF NICKEL–TITANIUM SHAPE MEMORY ALLOY USING MOLYBDENUM ELECTRODE: INVESTIGATION, MODELING AND OPTIMIZATION

In the present scenario, electrochemical arc machining (ECAM) (hybrid of electric discharge erosion and electrochemical dissolution) is an evolving procedure for difficulty in machining the materials due to constraints of existing processes. This research aims to investigate the machinability of Ni[Formula: see text]Ti alloy through electrochemical arc drilling using molybdenum electrode. Electrolyte concentration (ethanol with ethylene glycol and sodium chloride), supply voltage, and tool rotation are considered as the variable factors to evaluate the ECAM performance characteristics in drilling blind hole operation concerning overcut (OC), tool wear rate (TWR) and materials removal rate (MRR). Consequently, response surface methodology is implemented for predictive modeling of various performance characteristics. Finally, multi-objective optimization through desirability function approach (DFA) has produced a set of optimal parameters to improve the productivity along with the accuracy, which is the prime requirement for the industrial applicability of the ECAM process. Results demonstrated that supply voltage is the influential key factor for improvement of machining rate. Scanning electron microscope (SEM) photographs revealed the development of heat affected zone (HAZ), white layer, melted droplet, craters, re-solidified material, ridge-rich surface and voids as well as cavities around the end-boundary surfaces of a blind hole. Composition analysis through energy dispersive spectroscopy (EDS) indicated the oxygen content on the machined surface because electrolyte breakdown causes oxidation to take place at elevated temperatures across the machining zone. Moreover, carbide precipitation like TiC was found in the melting zone of the drilled hole, as revealed by X-ray diffraction (XRD) analyses, which has the affinity to reduce the SMA properties in HAZ.

Femtosecond Laser‐Induced Hydrophobic Surface of NiTi Shape Memory Alloy and its Blood Compatibility

The study of nickel–titanium shape memory alloy (NiTi SMA) surface wettability is important to improve the blood compatibility of medical implant materials. Herein, a femtosecond laser with a wavelength of 1030 nm is used to process the hydrophobic surface of NiTi SMA. A scanning electron microscope, an X‐ray photoelectron spectrometer, a surface profiler, and a contact angle meter are used to measure the microstructure, the chemical composition, the surface roughness, and the surface contact angle of NiTi SMA. The influence of the laser fluence on the NiTi SMA surface wettability is analyzed using the Wenzel–Cassie composite model. The results show that as the laser fluence increases, the surface roughness gradually increases and then fluctuates. Moreover, a rapid increase followed by a gradual rise is observed in the surface contact angle. The blood compatibility of NiTi SMA surface increases with the increase in the surface contact angle. Specifically, the superhydrophobic surface with high contact angle and low rolling angle can significantly improve hemolysis and coagulation while reducing platelet adhesion and deformation. The study of the superhydrophobic microstructure of NiTi SMA surface will provide a new approach for improving the blood compatibility of medical implant materials.

Blast reliability assessment and sensitivity analysis of steel MRFs equipped with NiTi SMA bolts

The probabilistic-based assessment of Moment Resisting Frame structure (MRFs) is significantly vital due to the high volatility of blast wave propagation parameters, material strength, and strain rate effect. In this study, the reliability and sensitivity of steel MRFs equipped with Nickel Titanium Shape Memory Alloy (NiTi SMA), and smart MRFs, subjected to Vehicle Borne Improvised Explosive Device (VBIED) are assessed. Since Eurocode does not address the design of a smart structure subjected to blast loading, the smart MRFs are designed based on the Eurocode-complying key design procedures proposed by the author’s previous works. It is always a question if the structure designed based on the proposed Eurocode-complying design methodology satisfies the safety level criteria. The newly developed reliability framework approach by the authors is employed to assess the safety level of the smart MRFs based on the criteria given in the international probabilistic design provisions. Performance functions are parameterized based on three failure modes: axial force and bending moments (global buckling) in the columns, maximum rotation demand at the connections, and system-level structural integrity through Inter-Story Drift Ratio (ISDR). The uncertainties of the charge weight, material strength, gravity loads, model uncertainties, connection behavior parameters, strain rate effects, and column cross-sectional dimensions are considered. The proposed reliability framework is applied to the 4-story, 7-story, 10-story, and 15-story smart MRFs. The results show that the developed smart MRFs satisfy the safety level criteria recommended in the international provisions’ probabilistic design requirement. Furthermore, the reliability-based outcome confirms the efficiency and applicability of the proposed Eurocode complying key design procedures and verifies the deterministic results of the smart MRFs. Finally, the sensitivity of the smart structures to various uncertain parameters is investigated. Among all the parameters, it is found that the Reliability Index (β) of the smart MRFs is more sensitive to the uncertainties of charge weight, column web length(s), connection behavior parameters, and material strength. The sensitivity analysis shows that the smart MRFs are safe under unknown extreme conditions, a very high Coefficient of Variation (COV).

Fitting the grain orientation distribution of a polycrystalline material conditioned on a Laguerre tessellation

The description of distributions related to grain microstructure helps physicists to understand the processes in materials and their properties. This paper presents a general statistical methodology for the analysis of crystallographic orientations of grains in a 3D Laguerre tessellation dataset which represents the microstructure of a polycrystalline material. We introduce complex stochastic models which may substitute expensive laboratory experiments: conditional on the Laguerre tessellation, we suggest interaction models for the distribution of cubic crystal lattice orientations, where the interaction is between pairs of orientations for neighbouring grains in the tessellation. We discuss parameter estimation and model comparison methods based on maximum pseudolikelihood as well as graphical procedures for model checking using simulations. Our methodology is applied for analysing a dataset representing a nickel–titanium shape memory alloy.

Design Recommendations for Columns Made of Ultra-High-Performance Concrete and NiTi SMA Bars

The use of new materials in construction endows structures with better mechanical characteristics. The combination of ultra-high-performance concrete (UHPC) and nickel and titanium (NiTi) shape memory alloy (SMA) improves the behavior of building structures by increasing both their ductility and dissipation energy due to the low-damage and self-centering properties of NiTi SMA. Since UHPC and NiTi SMA are expensive materials and still scarce in distribution channels, this article tries to offer design recommendations to reduce the length of the column-beam connection in which these new materials should be introduced, leaving the rest of the column with conventional materials. To achieve this, a nonlinear static pushover analysis of columns using finite element software, SeismoStruct, was performed. This model was calibrated using experimental results. Next, a parametric analysis was carried out to propose the design recommendations. Results indicated that an adequate design for the column–beam connection, considering both economy and performance, should include a main zone with UHPC and SMA reinforcements, a transition zone with UHPC and steel reinforcements, and another zone with conventional reinforced concrete. The transition zone improved the hybrid column’s performance without excessively raising the cost. The main zone length, the transition zone length, and the strength of the concrete in the rest of the column must be determined to ensure that the critical section of the column was in the main zone to develop the maximum strength and ductility. The length of the main zone depended on the compressive strength of the conventional concrete, the relative axial load of the column, and the required ductility.

Reliability Assessment Methodology of Blast Protective Steel Moment Resisting Frame Using NiTi SMA-based Connection

This paper proposes a methodology framework for the reliability assessment of smart steel Moment Resisting Frame structures (MRFs) equipped with Nickle Titanium Shape Memory Alloy (NiTi SMA) connections subjected to blast loading. The reliability assessment framework is formulated based on a two-step approach algorithm. In the 1st Step, the Monte Carlo Latin Hypercube Sampling Strategy simulation (MC-LHS) is adapted to generate the uncertain parameters sample points. Considering the numerical simulations, the 2nd Step employs simplified performance functions and the generated random outcomes from the 1st Step. The proposed reliability approach is verified against direct Monte Carlo Simulation (MCS) and First-Order Reliability Method (FORM). The performance functions are columns’ axial force and bending moments, rotation capacity at the connections, and Inter-Story Drift Ratio (ISDR). Throughout the development of the reliability assessment, the probabilistic models are parametrized on geometrical properties, material properties, vertical loads, model errors, and charge weights. The developed reliability framework is applied to a prototype 4-story smart MRF. The structural safety level is obtained in terms of the Reliability Index (β). The results show that the reliability framework provides an accurate and efficient structural collapse prediction of the MRFs equipped with NiTi SMA-based connections. Finally, sensitivity analysis is performed to indicate the sensitivity of building collapse to blast wave characteristics, material strength, vertical gravity loads, and column profile dimensions. The sensitivity analysis results also confirm the efficiency of the proposed reliability framework in observing the highly sensitive parameters, which is explosive charge weight.

Cryo treatment and corrosion studies of nickel-titanium shape-memory alloy

Nickel (Ni)-titanium (Ti) shape-memory alloy (SMA) is a functional material that has received a higher demand in aerospace industries, biomedical industries, actuator, and sensor industries owing to the property of superelasticity and shape memory effect. In addition, SMAs offer better mechanical and corrosion-resistant properties. In this current work, cryo treatment of NiTi alloy has been carried out to investigate the change in the properties. NiTi SMA prepared by powder metallurgy are cryo treated for 12 hours to 48 hours. Further, a corrosion study has been performed on the cryo-treated NiTi samples with the variation in corrosion time to know the effect on the morphology and the hardness. It is found that cryo-treated NiTi shows higher resistance to corrosion than the non-treated sample. The present work is an attempt to find out the effect of cryo treatment on the hardness and corrosion behavior of NiTi SMA. In the scanning electron microscope (SEM) analysis, it is observed that with the increase in cryo treatment time, NiTi (martensite) phase increases. Similar phases are present in both untreated and cryo-treated NiTi pellets. Out of all cryo-treated NiTi pellets, 24 hours cryo-treated NiTi has a lower percentage of corroded area with an increase in the corrosion exposure time. Corrosion preferentially starts at the pit area and progresses toward the Ti-rich area. The hardness value increased for 48 hours cryo-treated sample when compared with other cryo-treated and untreated NiTi pellets.

On the effect of scan strategies on the transformation behavior and mechanical properties of additively manufactured NiTi shape memory alloys

Laser-powder bed fusion (L-PBF) additive manufacturing (AM) presents excellent potential to fabricate geometrically complex structures with tailored microstructures and compositions from nickel titanium shape memory alloys (NiTi SMAs). The effect of common L-PBF process parameters, such as laser power, scanning speed, and hatch spacing, have been reported in many literature studies. However, one important factor that has not been investigated is the laser scan strategy, or the path that the laser follows within each layer. This is a particularly important factor to investigate in the case of NiTi SMAs that tend to be more sensitive to thermal histories than other commercial AM materials. For example, even slight variations in thermal history might exhibit notable influence on transformation behavior due to composition changes resulting from differential evaporation of nickel. The current work presents a first investigation on such effects of 12 laser scan strategy on the fabrication outcome of additively manufactured Ni-rich NiTi SMAs. The extent of warping deformation and surface morphology of fabricated parts with different scan strategies were found to show notable differences. Some phase transformation variations among different scan strategies were also identified in as-fabricated and solution heat treated conditions, although these variations were not as pronounced as variations in warping due to residual stresses and surface morphology. This study guides the selection of scan strategies such that build failures due to excessive warping and poor surface morphology are minimized. It also provides additional flexibility in controlling mechanical properties and phase transformation behavior of this relatively difficult-to-process class of materials.

Process parameter modeling and multi-response optimization of wire electrical discharge machining NiTi shape memory alloy

In the present work, the effects of different wire electrical discharge machining (WEDM) process parameters on nickel titanium shape memory alloy (NiTi-SMA) properties were investigated. Based on the orthogonal experimental design of the Taguchi method, the influence of five process parameters including peak current, discharge frequency, wire tension, flushing pressure, wire speed on the cutting speed (CS) and kerf width (KW) was analyzed. After 27 experimental runs, a multiple regression (MLR) model and a back-propagation neural network (BPNN) model were established to predict the CS and KW properties under different input conditions of process parameters. Utilize the programming language R for statistical testing of models and determination of important control factors. The Bat Algorithm (BA) searches for the best combination of process parameters. The experimental results and the proposed optimization method show that the prediction errors of the two combined optimization methods of MLR-BA and BPNN-BA are controlled by ± 2%. It has been proved that these two combined optimization methods are practical tools for optimizing the processing parameters of WEDM. Statistical analysis and observation results show that the peak current is the main parameter affecting the machining efficiency and surface quality of WEDM.

On the Fabrication of Defect-Free Nickel-Rich Nickel–Titanium Parts Using Laser Powder Bed Fusion

Abstract Laser powder bed fusion (L-PBF) additive manufacturing (AM) is an effective method of fabricating nickel–titanium (NiTi) shape memory alloys (SMAs) with complex geometries, unique functional properties, and tailored material compositions. However, with the increase of Ni content in NiTi powder feedstock, the ability to produce high-quality parts is notably reduced due to the emergence of macroscopic defects such as warpage, elevated edge/corner, delamination, and excessive surface roughness. This study explores the printability of a nickel-rich NiTi powder, where printability refers to the ability to fabricate macro-defect-free parts. Specifically, single track experiments were first conducted to select key processing parameter settings for cubic specimen fabrication. Machine learning classification techniques were implemented to predict the printable space. The reliability of the predicted printable space was verified by further cubic specimens fabrication, and the relationship between processing parameters and potential macro-defect modes was investigated. Results indicated that laser power was critical to the printability of high Ni content NiTi powder. In the low laser power setting (P < 100 W), the printable space was relatively wider with delamination as the main macro-defect mode. In the sub-high laser power condition (100 W ≤ P ≤ 200 W), the printable space was narrowed to a low hatch spacing region with macro-defects of warpage, elevated edge/corner, and delamination happened at different scanning speeds and hatch spacing combinations. The rough surface defect emerged when further increasing the laser power (P > 200 W), leading to a further narrowed printable space.

Drift limit state predictions of rectangular reinforced concrete columns with superelastic shape memory alloy rebars

This study derives empirical drift limit state expressions for rectangular concrete columns reinforced with nickel–titanium shape memory alloy (SMA) bars using numerical simulation results obtained from well-calibrated nonlinear finite element models. A parametric study is conducted to examine the effects of various design parameters, such as the axial load ratio, section information, and material properties of concrete and SMA on the drift limit states of the columns. For each limit state (LS), a multivariate stepwise regression analysis was performed. An expression was derived to estimate the associated drift capacity of the SMA-reinforced concrete columns. In addition, the precondition for the SMA-reinforced concrete columns to ensure ductile behavior was investigated. It turned out that the axial load and the amount of transverse reinforcement are the most influential parameters on the ductility of the columns. Finally, nonlinear static and dynamic analyses were conducted for an SMA-reinforced concrete building frame to demonstrate the application of the proposed drift LSs. Both analysis results revealed that the building model tends to experience core crushing or loss of lateral load capacity prior to SMA failure.