TiNi-based Alloys Research Articles

DSC and microstructure analysis of high temperature Ni-Ti-Hf, low hysteresis Ni-Ti-Cu and conventional super-elastic and shape memory Ni-Ti alloy ingots and wires

The progression of martensitic transformations in calorimetric experiments on NiTi-based shape memory alloys (SMA) depends significantly on the material composition, processing history and the thermo-mechanical treatment which is performed to prepare the material for use in application. The phase transformations in Ni-Ti alloys are first order transitions, i.e. the microstructure symmetry changes occur spontaneously and superheating and supercooling effects emerge. Thus, the thermal scanning rate in differential scanning calorimetry (DSC) affects the observation of important material parameters for the characterization of shape memory alloys like onset temperatures, hysteresis width and change in enthalpy of phase transformations. This work depicts how the transformation characteristics as observed by DSC measurements vary significantly by altering the alloy composition and processing conditions of the material. An in-depth calorimetric study on a wide range of NiTi-based alloys which were prepared as heat treated ingot and processed wire was performed. In order to evaluate also the rate effect on the transformation of distinct alloys, DSC rates from 1 to 30 °C/min were tested, highlighting the interplay between thermal scanning velocity and material parameters. Manifold results are presented on how chemistry, microstructure, DSC sample geometry and processing stresses in the materials contribute to award the alloys diverging sensitivity to the thermal scanning rate. Deviations in binary Ni-Ti composition alter the sensitivity to heating and cooling rate, e.g. martensitic transformation in annealed super-elastic (SE) wire is stronger affected by the rate than shape memory wire due to differences in the precipitation behavior. Ni-Ti-Hf high temperature shape memory alloys show a rather small enlargement of hysteresis, but a huge shift in transformation temperatures, while Cu addition to Ni-Ti causes very sensitive hysteresis changes up on modifying thermal rates which is assigned to B19 orthorhombic transformation. Transformation parameters of wires are less affected by DSC rates than ingots due to their high surface to volume ratio, while sample weight and surface finish is observed to have a minor impact. Small grains and residual strains owing to processing history of drawn wire tend to stabilize the transformation. In materials which exhibit R-phase transformation, the low heat capacity of this phase could be responsible for its minor sensitivity to variations in DSC rate.

Influence of the holding temperature on the kinetics of the isothermal B2 → B19′ transformation in TiNi-based shape memory alloy

The aim of the present paper was to study the kinetics of the isothermal B2 → B19′ transformation in the quaternary Ti40.7Hf9.5Ni44.8Cu5 shape memory alloy in a wide temperature range to find the common features of this phenomenon in NiTi-based alloys. It was found that the isothermal B2 → B19′ martensitic transformation occurred during holding at a constant temperature both within and outside of the temperature range of the forward transformation. The holding temperature of T* influenced the maximum value of the martensite phase (ΦM) that appeared under isothermal condition and the rate of the martensite volume fraction variation. The maximum ΦM value of 80% was found when holding was at T* = Ms (the start temperature of the forward martensitic transformation on cooling), and it was two times larger than in quenched Ni51Ti49 alloy. An increase in holding temperature larger than Ms decreased the maximum ΦM value and the slope of the ΦM(t) curve. A decrease in the holding temperature less than Ms decreased the ΦM value and hardly influenced the dΦM/dt value. It was found that the variation in volume fraction of the martensite phase that appeared under isothermal holding at different temperatures in the Ti40.7Hf9.5Ni44.8Cu5 shape memory alloy was close to the binary Ni51Ti49 alloy, and hence, it was concluded that the features of the kinetics of the isothermal B2 → B19′ transformation were common for this type of transformation in non-stoichiometric NiTi-based alloys.

A Perspective on Elastocaloric Effect in Ti–Ni-Based Shape Memory Alloys

Elastocaloric cooling utilizes in a cyclic manner the latent heat of the stress-induced transformation, which sets challenges for the fatigue endurance and performance of elastocaloric materials. Ti–Ni-based alloys are widely studied and presently the most frequently used shape-memory alloy group and they have attracted wide research interest in the elastocaloric aspect. The latent heat caused by stress-induced B2–B19′ martensite transformation has been frequently applied for the elastocaloric use, and large adiabatic temperature changes have been reported. Despite the high latent heat associated to B2–B19′ phase transformation, the fatigue properties have not been satisfactory. Stress-induced B2–R transformation of Ni–Ti-based alloys is interesting in this respect, since it has a low hysteresis but yet relatively high latent heat. In this short review, we give a brief insight to the Ti–Ni-based shape memory alloys from the elastocaloric point of view.

Thermal Cycling Effect on Transformation Temperatures of Different Transformation Sequences in TiNi-Based Shape Memory Alloys.

In TiNi-based shape memory alloys (SMAs), the effects of thermal cycling on the transformation peak temperatures of B2 ↔ B19′, B2 ↔ R, B2 ↔ B19, B2 ↔ R ↔ B19′, and B2 ↔ B19 ↔ B19′ one-stage and two-stage transformations have been investigated and compared. Experimental results of the differential scanning calorimeter and hardness tests indicate that the alloy’s intrinsic hardness and the shear strain, s, associated with martensitic transformation, are two important factors, due to their relation to the ease of introducing dislocations during cycling. The temperature decrease by cycling for one-stage transformation was in the order of B2 ↔ B19′ > B2 ↔ B19 > B2 ↔ R according to the orders of magnitude of their s values. This phenomenon also affected the suppression of B19 ↔ B19′ and R ↔ B19′ transformation peak temperatures in two-stage transformation. Both Ti50Ni48Fe2 and Ti48.7Ni51.3 SMAs aged at 450 °C for 4 h exhibited B2 ↔ R ↔ B19′ transformation, but the hardness of the latter was much higher than that of the former due to the precipitation hardening of the Ti3Ni4 precipitates. This causesd the decrease of the R ↔ B19′ transformation peak temperature in the Ti50Ni48Fe2 SMA to be much higher than that in Ti48.7Ni51.3 SMAs aged at 450 °C for 4 h, which directly affected the sequential B2 ↔ R transformation of Ti50Ni48Fe2 SMA in the next thermal cycle and decreased this transformation peak temperature. The Ti48Ni52 SMA aged at 600 °C for 150 h underwent B2 ↔ B19′ transformation and then B2 → R → B19′/B19′ → B2 transformation as the cycle number increased, in which the B2 ↔ R transformation peak temperature raised slightly by cycling. This characteristic is uncommon and may have resulted from the strain field around the thermal-cycled dislocations favoring the formation of the R-phase.

Aging Effect on the One-Way and Two-Way Shape Memory in TiNi-Based Alloys

The influence of aging of TiNi-based shape memory alloy (SMA) specimens on the one-way and two-way shape memory (TWSM) is studied experimentally and simulated with the use of a microstructural model. Experiments show that a long-time storage of preliminarily strained TiNi specimens does not change the strain recovery on heating. At the same time, the strain stroke due to the realization of the TWSM is bigger than before aging. Computer modeling of these phenomena is done within the frames of a microstructural model, in which the macroscopic strain is considered to be the average of the micro-strains–strains of micro-volumes. The irreversible strains are described from the standpoint of the plastic flow theory. Isotropic hardening and kinematic hardening are taken into account and are related to scattered and oriented deformation defects. To describe the effects of aging, special terms are introduced into the evolution equations for the defect densities. Thus, modified equations are able to account for a decrease of the yield limit due to a decrease of the deformation defect densities. The results of modeling show a good qualitative agreement with the experimental data, particularly an increase of the TWSM deformation after aging.

Strain variation during the isothermal martensitic transformation in Ti40.7Hf9.5Ni44.8Cu5 alloy

The aim of the paper was to study the strain variation during the isothermal B2 → B19′ transformation in NiTi-based alloy. It was found that isothermal holding of the Ti40.7Hf9.5Ni44.8Cu5 alloy under a stress of 235 MPa at constant temperatures, which were close to the start temperature of the B2 → B19′ transition, was accompanied by an additional strain variation. This strain was completely reversible on further heating and depended on the position of the holding temperature compared to the start temperature of the forward transformation. The maximum of isothermal strain of 2.9% was observed during holding at a temperature of 50 °C.

The Influence of Activating Co and Ni Additions on the Mechanisms of Reaction and Diffusion Interaction During Sintering of PV-N55Т45 TiNi-Based Powders

An investigation of the influence of reaction- and diffusion-induced interactions in the system of PV-N55Т45 TiNi-based powders and activating Co and Ni additions during sintering on special characteristics of microstructure and structural-phase compositions of porous TiNi-based alloys are investigated aimed at designing porous-monolithic structures. Using the method of liquid-phase sintering of TiNi-based powder, porous specimens with activating Co and Ni additions are manufactured with the concentrations of the latter being 0.5, 1.0, 1.5 and 2.0 at.%. It is found out that the activating action of Co is more moderate compared to that of Ni. A calculation of the heat released in the course of the chemical reaction as a result of introduction of activating additions demonstrates that the values of –∆H for manufacturing porous alloys with additions of Co and Ni are 28 and 52 kJ/mol, respectively. It is established that all specimens under study contain the following phases: austenitic B2, martensitic B19′, Ti2Ni, 2(О, N, С) and Ti3Ni4. Moreover, in the material with additions of Ni a TiNi3 phase is observed. Due to an introduction of a monolithic plate into the sintered powder mixture it is possible to compensate for the heat sink at the concentration of the activating Co addition 1.5 at.%. This allows manufacturing porous-monolithic structures based on the TiNi alloy with a high quality of fusion between the porous and monolithic parts.

The Influence of Surface Treatment on Wettability of TiNi-Based Alloy

The results of an investigation of surface wettability of TiNi-based alloys by water and DMEM-medium are presented. A comparative analysis of the wetting angle is performed after processing the specimens by grinding, sand blasting, oxidation in air, etching in a solution of acids, processing by vapor, hydrogen peroxide, and detergent solution in an ultrasonic bath. It is shown that oxidation in air and etching in acids increase the surface hydrophilic property of TiNi-based alloys. The highest hydrophilic value with the water wetting angle about 30° is achieved in the case of vapor treatment following sand blasting and treatment of the oxidized specimens with hydrogen peroxide. The differing influence of treatments on the wetting angle is attributed to variations in the surface energy of TiNi-based alloy specimens, which is affected by the microrelief and structural-chemical characteristics of the surface.

Phase equilibrium, structure, mechanical and biocompatible properties of TiNi-based alloy with silver

The article presents a comprehensive study of the effect of different silver contents on the cells viability, structure and mechanical properties of TiNi-based alloy. Silver, crystallized in the form of pure particles, has a positive effect on cell viability. This is evidenced by a greater amount of cell mass on the surface of the alloy and an increase in the cell viability level by more than 30% at the presence of silver. A phase diagram of the Ag–Ni–Ti ternary system was constructed based on the structure of the two-component systems Ag–Ni, Ag–Ti and Ti–Ni and atoms from these systems. Due to the strong difference in the electronic structure and size factor of the components of the Ti–Ni–Ag ternary system, the limited solubility of Ag (∼0.5 at% Ag) in the Ti–Ni binary compound was revealed. It has been established that the optimal combination of the physicomechanical properties of the alloys is achieved at silver concentrations, not exceeding 0.2 at% Ag.

Calculation of Third-Order Elastic Constants of the Crystals of a TiNi-Based Alloy with Shape Memory

Calculation of Third-Order Elastic Constants of the Crystals of a TiNi-Based Alloy with Shape Memory

Solution effect on improved structural compatibility of NiTi-based alloys by systematic first-principles calculations

The functional stability of a shape memory alloy (SMA) may be related to its structural compatibility between the parent-phase and the martensitic-phase structures. In this study, we perform systematic first-principles calculations for 276 Ni–Ti-based ternary alloys to investigate their energetic stability as well as their structural compatibility between the parent- and the martensitic-phase structures. We analyze in detail the dependences of the energetics and structural properties on the additional element X, on X concentration, and on the replaced chemical element. Some X are found to energetically stabilize the B19 structures more than the B19′ structures at X concentrations above 6.25 at. %. It is also found that the B19–B2 martensitic transformation shows better structural compatibility than the B19′–B2 transformation for most of the investigated ternary Ni–Ti–X alloys. These alloys also tend to have better structural compatibility than binary equiatomic NiTi. Moreover, we screen the investigated alloys on the basis of their energetic stability and structural compatibility, and we identify 26 Ni–Ti–X alloys as possible SMAs with good functional stability. In this study, we reveal a strong potential of the computational design for improving the functional stability of Ni–Ti SMAs by alloying additional elements.

Production of niobium vacuum-arc coating on a TiNi-based alloy

Production of niobium vacuum-arc coating on a TiNi-based alloy

Impact of infrared radiation on oxide layer of ultrathin TiNi-based alloy wire

Impact of infrared radiation on oxide layer of ultrathin TiNi-based alloy wire

The Influence of the Surface Layer on the Combination of Properties of Thin TiNi Alloy Wires

We have studied the influence of the surface layer on the parameters of the multiply repeated shape memory effect, developing stresses, characteristic temperatures, and intervals of martensitic transformations in thin (1 mm diameter) wires made of TiNi-based alloys. Examination of the surface layer structure showed that, in 1-mm-diameter TN-1V grade alloy wire, the oxide layer is about 15 μm thick and consists mostly of titanium, nickel, oxygen, and carbon. Removal of this surface layer leads to an increase in the maximum accumulated deformation, shift of the temperature interval of formation toward higher temperatures, and increase in the strength and plasticity characteristics.

Suspended droplet alloying: A new method for combinatorial alloy synthesis; nitinol-based alloys as an example

A new combinatorial alloy synthesis method (suspended droplet alloying) has been developed as a high-throughput approach for alloy discovery. The method is based on using a laser to melt elemental or alloyed wires fed at a controlled rate to achieve a specific chemistry. In this study, the metallurgical characteristics of alloy buttons created using this technique were assessed for TiNi-based shape memory alloy buttons deposited using pure Ni, Ti, and Cu wires. The microstructural and chemical inhomogeneity was assessed using quantitative electron microscopy and X-ray diffraction. Furthermore, the phase transformation temperatures of the coupons have been compared to cast and heat-treated (reference) samples. In general, the samples displayed a limited local deviation from the target chemistry (±1 wt%), while displaying a fairly homogeneous microstructure with the expected phase distribution. Post-process homogenisation heat treatments enhanced the phase transformation response, approaching the response obtained from the reference samples.

The Influence of Long-Term Storage on the Functional Properties of Shape Memory Alloys

The influence of long-term storage of TiNi-based and CuZnAl alloys on their functional properties was investigated. It was established that during 30 years’ storage of TiNiFe thermomechanical couplings, the stresses in them practically did not decrease. Couplings with CuZnAl alloy sleeves during storage weakened slightly. The one-way shape memory effect in equiatomic TiNi alloy did not change after 25 years. It was found that the value of two-way shape memory effect in the first cycle after storage at room temperature in martensitic state for more than 15 years was higher than before storage.

Evaluation of TiNi-based wire mesh implant for abdominal wall defect management

Purpose. Synthetic meshes are now widely used for a repair of various tissue defects. Polypropylene meshes (PPMs), the type most frequently used for repair of abdominal wall defects, have some disadvantages, therefore the development of new mesh materials for reconstructive surgery is an important topic. A TiNi-based alloy superelastic mesh implant has been compared with a PPM in the Wistar rat model. Experimental. A midline resection abdominal wall defect (2 cm × 3 cm) was treated with a mesh implant (2.5 cm × 3.5 cm). Three groups of 20 animals were assessed: (1) a TiNi-based alloy mesh (TNM) group with the defect managed by a TiNi-based wire woven implant; (2) a PPM group with the defect repaired by Optomesh®; and (3) a no-mesh (NM) group treated without the use of any prosthetic material for systemic reaction to implant control. Evaluation of both mesh implantations was carried out on days 14, 28, 56 and 90 after surgery. The following data were compared: general condition of the animals, macroscopic evaluation of the implant area and microscopic evaluation of the tissue surrounding the mesh with the use of scanning electron and light optical microscopy. Results. All the animals from NM group developed hernias. The PPM group showed satisfactory results, although some complications like implant dislocation (6/20) and hernia recurrence (5/20) were noted. In the TNM group neither shrinkage nor implant dislocation were observed. Compared with the PPM group, the TNM group was characterized by no inflammation reaction and better integration with tissues. Conclusions. Suppression of inflammation response together with a more physiological wall remodeling process than with PPM makes TNM an attractive concept for abdominal wall defect reconstruction. Since there was neither shrinkage nor dislocation of the TNM itself it may be suitable for intraperitoneal management. The essence of enhanced biocompatibility is due to stress–strain mechanical behavior of TNM and its surface oxycarbonitride layer, which facilitate its incorporation. Summarizing, TNM was found to be a very promising material for the repair of abdominal wall defects in clinical practice. Ideally TNM would be applied in circumstances when there are indications for the management of large hernias by PPM.

An Investigation of Porous Structure of TiNi-Based SHS-Materials Produced at Different Initial Synthesis Temperatures

An investigation of structural characteristics and behavior of TiNi-based pore-permeable materials manufactured by the methods of selfpropagating high-temperature synthesis (SHS) at the initial synthesis temperatures T = 400 and 600°С is performed. It is shown that depending on the temperature regime, the resulting structure and properties of the material can differ. It is found out that the SHS-material produced at the initial synthesis temperature T = 400°С possesses the largest number of micropores in the pore wall surface structure due to a high phase inhomogeneity of the alloy. The regime of structure optimization of the resulting materials is described and the main stages of formation of the pore wall microporous surfaces are revealed. It is demonstrated that after optimization of the surface structure of a TiNi-based fine-pore alloy by its chemical etching, the fraction of micropores measuring in size less than 50 nm increased from 59 to 68%, while the number of pores larger than 1 μm increased twofold from 11 to 22%. In addition, peculiar features of interaction between certain cell cultures with the surface of the SHS-material manufactured at different initial synthesis temperatures are revealed. It is found out that the dynamics of the cell material integration depends on the pore wall surface morphology and dimensions of macropores.

The effect of mechano-chemical treatment on structural properties of the drawn TiNi-based alloy wire

The rapid development of biomedical materials with the advanced functional characteristics is a challenging task because of the growing demands for better material properties in-clinically employed. Modern medical devices that can be implanted into humans have evolved steadily by replacing TiNi-based alloys for titanium and stainless steel. In this study, the effect of the mechano-chemical treatment on structural properties of the matrix and surface layer of the drawn TiNi-based alloy wire was assessed. A range of samples have been prepared using different drawing and etching procedures. It is clear from the results obtained that the fabricated samples show a composite structure comprising the complex matrix and textured oxycarbonitride spitted surface layer. The suggested method of surface treatment is a concept to increase the surface roughness for the enhanced bio-performance and better in vivo integration.

СОЧЕТАННАЯ ТРАНСПЛАНТАЦИЯ ГЕПАТОЦИТОВ С МЕЗЕНХИМАЛЬНЫМИ СТВОЛОВЫМИ КЛЕТКАМИ КОСТНОГО МОЗГА НА КЛЕТОЧНЫХ НОСИТЕЛЯХ ИЗ ПОРИСТО-ПРОНИЦАЕМОГО НИКЕЛИДА ТИТАНА

The aim of the research was to study of the therapeutic efficacy of combined transplantation of liver cells and mesenchymal stem cells of bone marrow (MSC) on TiNi-based alloy scaffolds of rats with chronic hepatitis. Materials and methods: Three groups of experiments were carried out on Wistar rats and intact rats (control) - I group; in the II group - chronic hepatitis; III group - cell-engineering designs with liver cells; in the IV group - cell-engineering designs with liver cells and BM MMSC. The activity of recovery processes was evaluated by using biochemical methods in dynamics on 10, 20, 30th days. Results. The optimal ratio of liver cells to MSC (5:1) was previously found in TiNi-based alloy scaffolds after transplantation and through co-cultivation of hepatocytes and MSC at different ratios. It was shown that recovery processes in the group with scaffolds transplanted into damaged livers were much more active if compared with those in the control group with chronic hepatitis. At the same time, in the group with combined transplantation on TiNi-based alloy scaffolds contributed to a more rapid normalization of liver enzyme indices compared with the group with transplantation of some hepatocytes on scaffolds. These data are confirmed by an increase in the life span of animals with combined hepatocytes and MSC transplantation on TiNi-based alloy scaffolds. Conclusions. A higher level and prolonged periods of liver regeneration during the transplantation of TiNi-based alloy scaffolds are due to the creation of biologically appropriate conditions for cells with certain ratio.