TiNi-based Alloys Research Articles

Strong work hardening in a high niobium-containing TiNi-based bulk glassy alloy composite

In this study, a high niobium-containing bulk glassy alloy composite Ti41Ni39Nb17.5Al2.5 consisting of amorphous, B2-TiNi and β-Nb phases was fabricated through in situ precipitation strategy by utilizing copper-mold suction casting. Upon compressive loading, the composite alloy showed a pronounced average plastic strain of (17.6 ± 3.5)% and a high fracture strength of (2279 ± 33) MPa together with a strong work-hardening deformation behavior. The volume fraction of the amorphous phase in the composite was determined to be around 23 vol%. The work softening caused by the formation of shear bands can be compensated by the work hardening arising from the pile-ups of dislocation and severe lattice distortion in the precipitated crystals and thereby stabilize the plastic deformation of the composite material. The findings imply that the room-temperature mechanical properties of bulk glassy alloy composites can be well tailored by selecting suitable reinforcers.

Experimentally validated constitutive model for NiTi-based shape memory alloys featuring intermediate R-phase transformation: A case study of Ni48Ti49Fe3

An important group of NiTi-based alloys employed in applications exhibits an intermediate R-phase transformation, which substantially modifies the material's thermomechanical response. Finite element analysis of products from such alloys requires a constitutive model covering the two-stage transformation sequence in its full thermodynamic nature. We propose an enhanced constitutive model for NiTi-based alloys, which incorporates R-phase as a distinctive phase with loading-dependent transformation strain. We validate the model on a newly acquired extensive experimental dataset on the thermomechanical response of Ni48Ti49Fe3 shape memory alloy and illustrate the capabilities of its implementation into finite element software on a computational structural analysis of a tightening ring.

The prediction model for additively manufacturing of NiTiHf high-temperature shape memory alloy

NiTi-based alloys are one of the most well-known alloys among shape memory alloys having a wide range of applications from biomedical to aerospace areas. Adding a third element to the binary alloys of NiTi changes the thermomechanical properties of the material remarkably. Two unique features of stability and high transformation temperature have turned NiTiHf as a suitable ternary shape memory alloys in various applications. Selective laser melting (SLM) as a layer-based fabrication method addresses the difficulties and limitations of conventional methods. Process parameters of SLM play a prominent role in the properties of the final parts so that by using the different sets of process parameters, different thermomechanical responses can be achieved. In this study, different sets of process parameters (PPs) including laser power, hatch space, and scanning speed were defined to fabricate the NiTiHf samples. Changing the PPs is a powerful tool for tailoring the thermomechanical response of the fabricated parts such as transformation temperature (TTs), density, and mechanical response. In this work, an artificial neural network (ANN) was developed to achieve a prediction tool for finding the effect of the PPs on the TTs and the size deviation of the printed parts.

Experimentally Validated Constitutive Model for NiTi-based Shape Memory Alloys Featuring Intermediate R-phase Transformation: A Case Study of Ni <sub>48</sub> Ti <sub>49</sub> Fe <sub>3</sub>

An important group of NiTi-based alloys employed in applications exhibits an intermediate R-phase transformation, which substantially modifies the material’s thermomechanical response. Finite element analysis of products from such alloys requires a constitutive model covering the two-stage transformation sequence in its full thermodynamic nature. We propose an enhanced constitutive model for NiTi-based alloys, which incorporates R-phase as a distinctive phase with loading-dependent transformation strain. We validate the model on a newly acquired extensive experimental dataset on the thermomechanical response of Ni48 Ti49 Fe3 shape memory alloy and illustrate the capabilities of its implementation into finite element software on a computational structural analysis of a tightening ring.

Study on technique of TiNi-based shape memory alloy thin film coating on optical fibers

Study on technique of TiNi-based shape memory alloy thin film coating on optical fibers

The Features of Machining and Obtaining Products from TiNi-Based Alloys Undergoing Phase Transformations

This paper presents innovative technologies for processing and obtaining products made of an alloy based on TiNi, as well as studies concerning the features of machining of alloys that exhibit a shape memory effect (SME). Methods for severe plastic deformation and electroplastic deformation and the advantages and disadvantages thereof, as well as the conditions for using such alloys, are described. Technological operations applied to the formation of the final dimensions for obtaining finished parts such as cutting, drilling holes, milling, and abrasive processing are described.

The Influence of Isothermal Loading-Unloading Cycles on Inelastic Properties of a TiNi-Based Alloy in the B2-Phase State During Bending Deformation

Using Ti49.3Ni50.7 (at.%) alloy specimens, the influence of mechanical cycling by the bend method on the values of shape memory and superelasticity effects is investigated at the maximum given deformation in the interval from 4.0 to 22.8%. For every given value of the deformation, 11 mechanical cycles of loading and unloading are performed at T = 295 K, i.e., higher than Af, in a tool set with a V-shaped channel (angle between the channel walls 90°) during loading of the specimens by the cylinders of a given radius. After every loading-unloading cycle, the specimens are heated to 493 K and the plastic deformation component is recorded. It is found out that even a small deformation of 4.5%, set for the Ti49.3Ni50.7 (at.%) alloy specimens in the В2-phase, gives rise to the formation of a plastic deformation component. The results presented in this study have to be taken into consideration in the practical applications of this alloy composition in the fabrication and use of the medical products subjected to stress and bending conditions.

Exploring the role of surface modifications of TiNi-based alloys in evaluating in vitro cytocompatibility: a comparative study

The aim of this study was the comparative analysis of in vitro bio-testing of solid and porous TiNi samples with modified surfaces (intact, oxidated, and etched). Tests for cytocompatibility, hemolysis, and cytotoxicity (MTT) as well as visualization by confocal and scanning electron microscopy have shown that the chemically modified samples are the most cytocompatible. The intact and etched samples did not induce hemolysis greater than 2%, and thus they comply with the ISO 10993-4:2018 standard for hemolysis by blood-contacting biomaterials. Direct culture of etched samples with MCF-7 cells and human leukocytes showed low cytotoxicity. At the same time, the cytotoxicity of samples oxidated at 500 °C was significantly greater than that of the etched samples. Confocal and electron microscopy also confirmed the abovementioned quantitative data. The cells attached to the etched surface in numbers sufficient for them to be able to grow and proliferate on this substrate in vitro. These findings indicate that solid and porous TiNi alloy with surface modifications achieved by a cost-effective method is biotolerable and promising for clinical use and for tissue engineering.

Martensitic Transformation Behaviors of Compositionally Graded Ti–Ni-Based Shape Memory Alloys

In this study, we demonstrate a simple and effective way to fabricate functionally graded TiNi-based alloys with linear variations of composition and martensitic transformation behavior. Ti50Ni50 and Ti50Ni35Cu15 alloy strips were fabricated through a melt overflow process. The compositionally graded diffusion couple was fabricated by annealing two strips of different alloy compositions after being placed face to face in a pressing graphite mold. The mechanical properties and martensitic transformation behaviour of the diffusion couple were analysed by tensile test and DSC. The compositionally graded specimens exhibited unique superelastic property and wide martensitic formation temperature range. Such mechanical and thermal behaviors of the compositionally graded TiNi-based alloy offer good function and controllability for actuators.

Structural Defects in TiNi-Based Alloys after Warm ECAP

The microstructure, martensitic transformations and crystal structure defects in the Ti50Ni47.3Fe2.7 (at%) alloy after equal-channel angular pressing (ECAP, angle 90°, route BC, 1–3 passes at T = 723 K) have been investigated. A homogeneous submicrocrystalline (SMC) structure (grains/subgrains about 300 nm) is observed after 3 ECAP passes. Crystal structure defects in the Ti49.4Ni50.6 (at%) alloy (8 ECAP passes, angle 120°, BC route, T = 723 K, grains/subgrains about 300 nm) and Ti50Ni47.3Fe2.7 (at%) alloy with SMC B2 structures after ECAP were studied by positron lifetime spectroscopy at the room temperature. The single component with the positron lifetime τ1 = 132 ps and τ1 = 140 ps were observed for positron lifetime spectra (PLS) obtained from ternary and binary, correspondingly, annealed alloys with coarse-grained structures. This τ1 values correspond to the lifetime of delocalized positrons in defect-free B2 phase. The two component PLS were found for all samples exposed by ECAP. The component with τ2 = 160 ps (annihilation of positrons trapped by dislocations) is observed for all samples after 1–8 ECAP passes. The component with τ3 = 305 ps (annihilation of positrons trapped by vacancy nanoclusters) was detected only after the first ECAP pass. The component with τ3 = 200 ps (annihilation of positrons trapped by vacancies in the Ti sublattice of B2 structure) is observed for all samples after 3–8 ECAP passes.

Experimental investigations on the effect of copper on the microstructure and shape memory characteristics of NiTi alloys

Shape memory alloys are fascinating materials, which have exclusively been studied over the last three decades owing to their distinctive functional properties, such as shape memory effect (thermal memory) and superelasticity (mechanical memory). These materials have a unique capability to react to external stimuli, such as heat and stress, because of the reversible martensitic transformation, when subjected to appropriate thermomechanical processing. They find applications in various sectors and are in particular used as actuators and sensors. Among SMAs, NiTi-based alloys are more common and have proven their utility in many practical applications. However, there is still a scope for improvement of the alloys in terms of their shape memory characteristics if they are to be exploited in several other critical applications. In this context, addition of copper proves to be an appropriate element to enhance the transformation characteristics and biocompatibility of NiTi SMAs. Hence, in this work NiTiCu ternary alloys were synthesized by vacuum induction melting followed by subjecting them to suitable thermomechanical treatment. These alloys were then characterized by X-ray diffraction, differential scanning calorimetry and optical microscopy in order to study the influence of copper addition on phases present, transformation temperatures and microstructure of NiTi SMAs. The results are discussed in detail in the paper.

Development of Nickel-Rich Nickel–Titanium–Hafnium Alloys for Tribological Applications

Nickel-rich (54–56 at.% Ni) NiTi-based alloys have gained increased attention for their high hardness, corrosion resistance, strength, and wear resistance, leading to their development for high-performance bearings and other wear applications. An investigation of a broader compositional range of NiTiHf alloys, in terms of Ni and Hf content, is presented in this study. Their Vickers micro-hardness, 3 ball-on-rod rolling contact fatigue, and compression performances are benchmarked against early compositions identified for bearing applications, e.g., Ni55Ti45 and Ni54Ti45Hf1. The results show that by varying heat treatments and alloy composition, certain NiTiHf alloys can exhibit long life (~ 107 cycles) at 10–20% greater contact stress levels (up to 2.2–2.4 GPa), together with up to 20% greater hardness (up to 770 HV) and 30% larger compressive yield strengths (up to 3.4 GPa) than the original bearing compositions. These improvements are attributed to the ability to develop alloys with high-volume fractions of fine precipitate phases. Interestingly, a variety of combinations of different precipitate strengthening phases, depending on alloy composition and heat treatment, were able to achieve improvements relative to Ni55Ti45 and Ni54Ti45Hf1, demonstrating the versatility of the NiTiHf system.

The Surface Structure of a TiNi-Based Powder Alloy Manufactured by the Method of Diffusion Sintering

The structural features of the surface of a TiNi-based powder alloy produced by diffusion sintering are studied. Two-dimensional porous samples based on a TiNi powder alloy are manufactured for the experimental investigation of their structural features by the methods of non-destructive testing. It is determined that the phase composition of the TiNi-based powder alloy is represented by the TiNi (B2) austenitic phase and precipitates of the secondary phases enriched with nickel – Ti3Ni4 and titanium – Ti2Ni. The chemical compositions of the TiNi compound and the Ti2Ni secondary-phase particles are determined. A three-dimensional reconstruction of the surface of the TiNi-based powder alloy is performed by optical profilometry. It is shown that, depending on the sintering temperature regime and particle size distribution of the TiNi powder, it is possible to obtain a developed rough surface with a roughness coefficient within Ra = 64–87 μm. An analysis of the surface microstructure of the resulting material indicates the existence of several types of terraced relief — hexagonal islands of regular shape 5–10 μm and large terraced structures up to 30– 40 μm in size.

Variation in the structure of the amorphous NiTi-based alloys during mechanical vibrations

The aim of the present work is to study the influence of the vibration temperature, frequency and amplitude on the structure of amorphous NiTi-based shape memory alloys. The Ti40.7Hf9.5Ni44.8Cu5, Ti40.7Hf9.5Ni41.8Cu8 and Ti50Ni25Cu25 melt-spun amorphous thin ribbons were subjected to low-frequency mechanical vibrations in a Dynamical Mechanical Analyser or ultrasonic vibrations in anvil. The results of the study have shown that the mechanical vibrations lead to a structural relaxation and initiate the beginning stage of the crystallisation in the amorphous NiTi-based shape memory alloys. During the vibrations with an amplitude of 4 μm, crystalline clusters, with a size of 4–6 nm, appear in the amorphous matrix. On an increase in the vibration temperature or frequency, the size of the crystalline clusters hardly changes. The clusters that form during the vibrations do not influence the crystallisation temperature but decrease the crystallisation enthalpy.

Experimental and molecular dynamics studies of phase transformations during cryogenic thermal cycling in complex TiNi-based crystalline/amorphous alloys

In TiNi-based crystalline/amorphous alloys, superelasticity in crystalline phase coordinating the dislocation sinking in amorphous phase lead to a high ductility and outstanding anti-fatigue properties. We performed cryogenic thermal cycling, between 77 K and 303 K, on the complex TiNi-based alloys consisting of a major B2 austenite phase, an interdendritic amorphous phase, and a minor B19′ martensite phase in the as-cast state. The critical martensitic phase transformation stress (σm) increased with the number of thermal cycles, reaching a maximum at 10 cycles. The initial B19’ martensite which is confined in the amorphous phase transformed to B2 austenite due to thermal induced stable transformation. A lamellar structure of alternating amorphous and crystalline layers dominantly grew into the amorphous matrix as a consequence of the thermal fatigue during the cryogenic thermal cycling. Initial cell for the molecular dynamic simulations was carefully prepared to contain three different phases. Cyclic compressive loading and cryogenic thermal cycling simulations were consistent with the experimental results.

Мартенситные превращения сплавов на основе никелида титана с разными легирующими добавками

Investigations of the martensitic transformations of TiNi-based alloys grade TN-10, doped with 1 at.% V, Nb, Ag, were presented. It was shown that alloying elements (Ag, Nb, V) cause an increase in internal elastic stresses inside the crystalline structure of the B2 phase, which leads to an increase in the beginning temperature of direct martensitic transformation. Of the selected alloying elements, vanadium has a significant effect on this temperature, shifting it to the region of high values ​​by ~ 45 ° C. In alloys with vanadium and silver, a complex sequence of martensitic transformations B2-> R-> B19 'is realized, and in alloys with niobium, a simple B2-> B19' is realized.

Структура поверхности порошкового сплава на основе никелида титана, полученного методом диффузионного спекания

Структура поверхности порошкового сплава на основе никелида титана, полученного методом диффузионного спекания

Study of hydrogen redistribution in Ti49.1Ni50.9 alloy (at %) with nanocrystalline structure by resistometric and durometer methods

The electrolytical hydrogenation of TiNi-based alloys with shape memory and super elasticity results in a significant (by 1,5 times) increase in the microhardness of the surface layer. It is shown that the change of microhardness at long-term aging at room temperature in wire samples of Ti50,1Ni50,9 (at %) alloy with nanocrystalline structure can be used for estimation of diffusion mobility of hydrogen. On wire samples with different diameters it is shown that the increase in the relative electrical resistance at 83K at long-term aging at room temperature for this alloy is also associated with the diffusion mobility of hydrogen. Estimates of the hydrogen diffusion coefficient DH show that in nanocrystalline samples Ti50,1Ni50,9 (at %) the diffusion mobility of hydrogen is higher than in coarse-grained samples. This is connected with the increased contribution of grain boundary hydrogen diffusion in samples with nanocrystalline structure.

The Effect of Hydrogen on Martensite Transformations and the State of Hydrogen Atoms in Binary TiNi-Based Alloy with Different Grain Sizes.

The analysis presented here shows that in B2-phase of Ti49.1Ni50.9 (at%) alloy, hydrogenation with further aging at room temperature decreases the temperatures of martensite transformations and then causes their suppression, due to hydrogen diffusion from the surface layer of specimens deep into its bulk. When hydrogen is charged, it first suppresses the transformations B2↔B19′ and R↔B19′ in the surface layer, and when its distribution over the volume becomes uniform, such transformations are suppressed throughout the material. The kinetics of hydrogen redistribution is determined by the hydrogen diffusion coefficient DH, which depends on the grain size. In nanocrystalline Ti49.1Ni50.9 (at%) specimens, DH is three times greater than its value in coarse-grained ones, which is likely due to the larger free volume and larger contribution of hydrogen diffusion along grain boundaries in the nanocrystalline material. According to thermal desorption spectroscopy, two states of hydrogen atoms with low and high activation energies of desorption exist in freshly hydrogenated Ti49.1Ni50.9 (at%) alloy irrespective of the grain size. On aging at room temperature, the low-energy states disappear entirely. Estimates by the Kissinger method are presented for the binding energy of hydrogen in the two states, and the nature of these states in binary hydrogenated TiNi-based alloys is discussed.

Liquid-phase bonding of TiNi-based alloys

The paper considers the possibility of producing liquid-phase bonding of TiNi-based alloys containing a high volume fraction of Ti2Ni intermetallic particles and high nickel content in the alloy matrix. Using the methods of differential thermal analysis and scanning electron microscopy, the possibility of the eutectic reaction in the alloys upon heating has been determined as well as the precipitation of nickel-enriched particles in the bonding zone.