Ti50Ni25Cu25 Alloy Research Articles

Comparative analysis of the amorphous phases obtained in Ti50Ni25Cu25 alloy using different approaches

The structure of amorphous Ti50Ni25Cu25 alloy is investigated upon megaplastic deformation in a Bridgman chamber at room temperature. The dependences of average and local fractions of the crystalline phase along a sample’s radius on deformation are obtained. Radial distribution functions that allow calculation of the interatomic distances and coordinate numbers in several coordinate shells of investigated sample are found. It is concluded that the amorphous state in a Ti50Ni25Cu25 system fabricated by melt quenching is virtually the same as the one that arises through deformation-driven amorphization at room temperature in a Bridgman chamber.

Formation of amorphous states in Ti50Ni25Cu25 alloy subjected to severe plastic deformation: Nanoglass issue

Peculiarities of structure and mechanical behaviour of amorphous Ti50Ni25Cu25 alloy were the focus of this research. Amorphous melt-spun ribbons and bulk crystalline samples were subjected to high pressure torsion (HPT) in order to modify their structure and mechanical behaviour. Some properties of obtained SPD-processed samples were compared with initial state with the help of x-ray diffraction (XRD), differential scanning calorimetry (DSC) and nanohardness tests. It was shown that according to XRD no nanocrystallization occurs during HPT of melt-spun ribbons yet at the same time high density of shear bands might be formed in the alloy. Due to the formation of shear bands with the excess free volume the decrease of hardness and crystallization temperature were observed in the alloy. Analysis of structural data and mechanical behaviour allowed us to assume, that SPD processing of melt- spun Ti50Ni25Cu25 alloy might lead to the formation of structural state similar to the new kind of noncrystalline state – "nanoglass" state.

Structural and phase transitions in the amorphous and nanocrystalline Ti50Ni25Cu25 alloys upon high-pressure torsion

The evolution of the structure and phase composition upon room temperature high-pressure torsion of the amorphous and nanocrystalline Ti50Ni25Cu25 alloy has been studied. It is shown that the accumulation of deformation regardless of the initial state of the alloy leads to the occurrence of cyclic «amorphous state⇒crystal» and «crystal⇒amorphous state» phase transformations. After the application of only hydrostatic pressure, without torsion, to the initially amorphous Ti50Ni25Cu25 alloy, a small amount of crystalline phase is present in the structure. The observed features of the structural transformations are explained in terms of the dissipation of mechanical energy, which is continuously introduced into the solids upon high-pressure torsion.

Effect of deviations of composition from the quasi-binary section TiNi-TiCu on structural and phase transformations in rapidly quenched alloys

Methods of X-ray diffraction, transmission and scanning electron microscopy, and selected-area electron diffraction (SAED) have been used to study the phase and elemental composition and structure of alloys close to the stoichiometric Ti50Ni25Cu25 alloy. Based on the method of rapid quenching of the melt (free-jet melt spinning), alloys of the quasi-binary TiNi-TiCu section have been prepared, which in the initial as-cast state exhibited the thermoelastic martensitic transformations B 2 ↔ B 19 and related shape-memory effects. The chemical composition of the Ti50 + x Ni25Cu25 − x alloys was varied by changing titanium and copper concentrations within x ≤ ±1 at % (from Ti49Ni25Cu26 to Ti51Ni25Cu24). It has been established that quenching at a cooling rate equal to 106 K/s leads to the amorphization of all the alloys under consideration. Heating to 723 K and higher leads to the devitrification of the alloy with the formation of a nanocrystalline or submicrocrystalline structure of the B2 austenite. The mechanical properties of these alloys have been measured in the initial amorphous state and in the polycrystalline martensitic state. It has been shown that, depending on the extent of the deviations of the alloy composition from the stoichiometry, which cause the decomposition of the alloys in the process of nanocrystallization, regular changes are observed in their mechanical properties and in the shape-memory effects. The kinetics of the processes of the devitrification of the alloys, as well of the forward and reverse martensitic transformations, have been studied, their characteristic temperatures have been determined, and a diagram of the dependence of the characteristic temperatures on the chemical composition of the alloys has been constructed.

Modeling the amorphous structure of mechanically alloyed Ti50Ni25Cu25 using anomalous wide-angle x-ray scattering and reverse Monte Carlo simulation

An amorphous Ti50Ni25Cu25 alloy was produced by 19h of mechanical alloying. Anomalous wide angle x-ray scattering data were collected at six energies and six total scattering factors were obtained. By considering the data collected at two energies close to the Ni and Cu K edges, two differential anomalous scattering factors about the Ni and Cu atoms were obtained, showing that the chemical environments around these atoms are different. Eight factors were used as input data to the reverse Monte Carlo method used to compute the partial structure factors STi3Ti(K), STi–Cu(K), STi–Ni(K), SCu3Cu(K), SCu–Ni(K) and SNi–Ni(K) and the partial pair distribution functions GTi3Ti(r), GTi–Cu(r), GTi–Ni(r), GCu3Cu(r), GCu–Ni(r) and GNi–Ni(r). From the RMC final atomic configuration and Gij(r) functions, the coordination numbers and interatomic atomic distances for the first neighbors were determined.

Martensitic Transformation in Ti<sub>50</sub>Ni<sub>25</sub>Cu<sub>25</sub> Shape Memory Alloy Studied by EBSD

Presented work summarizes studies done on the course of the martensitic transformation in the Ti50Ni25Cu25 alloy. Studies were carried out using scanning electron microscopy JEM 6480 equipped with an electron back scattered detector (EBSD) as well as a heating attachment. As-received ribbon was mainly amorphous. At the free side of the ribbon some fine randomly oriented crystalline grains were formed. During heating followed by cooling the reversible martensitic transformation occurred with sequence: B2↔B19. The crystallographic correlation between the B2 phase and the B19 martensite were confirmed. It allowed finding transformation matrix between both phases.

Formation of nanocrystalline structure in the amorphous Ti50Ni25Cu25 alloy upon severe thermomechanical treatment and the size effect of the thermoelastic martensitic B2 ↔ B19 transformation

The results of the comparative analysis of the Ti50Ni25Cu25-alloy structures produced in the initial amorphous state by rapid quenching from the melt (RQM), after severe plastic deformation by torsion under high pressure (HPT), and postdeformation heat treatment (PHT) are presented. The study was carried out using neutron and X-ray diffraction, transmission and scanning electron microscopy, and measurements of electrical properties. The initially amorphous alloy has been established to nanocrystallize after torsion under a pressure of 7 GPa to 0.5 revolutions of the anvil. Then, after 1, 5, 10, and 15 rev, the alloy again undergoes the strain-induced amorphization even with the retention, even after 5–15 rev, of a large number of highly dispersed nanocrystals less than 3–4 nm in size with a distorted B2 lattice in the amorphous matrix. Their crucial role as nuclei of crystallization provides the total low-temperature nanocrystallization during subsequent annealing starting from 250–300°C. It is shown that PHT of the alloy amorphized by HPT makes it possible to produce extremely uniform nanocrystalline (NC), submicrocrystalline (SMC), or bimodal (NC + SMC) austenitic B2-type structures in it. A complete diagram of thermoelastic martensitic transformations in the region of B2-austenite states, from nanostructured state to conventional polycrystalline one, has been constructed. The size effect on the stabilization of martensitic transformation in nanocrystalline B2 alloy has been established.

Formation of the nanocrystalline structure in the Ti50Ni25Cu25 shape-memory alloy under severe thermomechanical treatment

Results of comparative studies of the structure of the cast martensitic Ti50Ni25Cu25 alloy in the initial state, after severe plastic deformation by high-pressure torsion (HPT), and after subsequent annealing are presented. The studies have been performed by X-ray diffraction, transmission and scanning electron microscopy, and measurements of electrical properties. It has been established that the alloy undergoes almost complete amorphization after torsion using 5 and 10 rev of anvils under a pressure of 7 GPa. This result can be explained by the large value of shear deformation (true strain from 6 to 7 units) and the retention of an extremely large quantity of highly dispersed (less than 3–4 nm in size) nanocrystals with a distorted B2 lattice in the amorphous matrix even at room temperature. Their determining role as nuclei of crystallization ensures the total process of low-temperature nanocrystallization upon subsequent annealing, beginning from 250–300°C. It is shown that the annealing of the alloy amorphized during HPT makes it possible to produce extremely uniform nanocrystalline (NC), submicrocrystalline (SMC), or bimodal (NC + SMC) structures of B2 austenite. For the first time, a complete diagram of thermoelastic martensitic transformations in the field of B2-austenite states, from nanostructured to usual polycrystalline, has been constructed for the Ti50Ni25Cu25 alloy. The size effect of stabilization of the martensite transformation has been found in the nanocrystalline B2 alloy.

The cyclic character of phase transformations of the crystal ⇔ amorphous state type during severe plastic deformation of the Ti50Ni25Cu25 alloy

The cyclic character of phase transformations of the crystal ⇔ amorphous state type during severe plastic deformation of the Ti50Ni25Cu25 alloy

Features of the surface layers of TiNi-based alloy thin ribbons

The chemical composition of the surface of TiNi-based alloys in the form of ribbons produced by rapid melt quenching in oxygen-containing environment has been investigated. It is shown that titanium oxide is formed on the Ti50Ni25Cu25 alloy surface, while on the Ti39.2Ni24.8Cu25Hf11 alloy surface titanium and hafnium oxides are formed. Atomic-force microscope images reveal crystalline structures of titanium oxide with sizes of up to 500 × 500 nm2 on the surface of the Ti50Ni25Cu25 sample. After removing the surface oxide layer by ion etching, the ratio of elements becomes close to the stoichiometric composition.

Nanostructured shape memory alloys of the TiNi-TiCu system

We study the shape memory Ti50Ni25Cu25 (at %) alloy fabricated by melt spinning. The techno-logical parameters were optimized to obtain the alloy in the amorphous state. The dynamic crystallization of the alloy by a single electric pulse with durations of 1 to 100 ms was used to form the nanostructural state. Transmission electron microscopy and differential scanning calorimetry study show that reducing the pulse duration to 2 ms resulted in considerable refinement of the alloy with the formation of nanosized martensitic plates (20–60 nm). It was established that nanostructurization of the alloy can lead to an increase in the value of the recovery strain when the shape memory effect is manifested.

Giant reversible deformations in a shape-memory composite material

New scheme of a functional composite material based on a shape memory alloy has been developed and experimentally tested. The proposed scheme ensures a giant reversible bending deformaion using only one-way shape memory. The scheme has been experimentally implemented on models manufactured by galvanic deposition of nickel onto preliminarily pseudoplastically deformed rapidly quenched Ti50Ni25Cu25 alloy ribbons. The proposed scheme is especially promising for applications in micro- and nanomechanics. In particular, prototype nanotweezers have been manufactured on this basis with record small dimensions of 12 × 3 × 1 μm and a 500-nm-thick shape-memory layer, which is capable of manipulating objects with sizes from 10 to 1000 nm. Controlled deformation of nanotweezers was achieved by heating them using semiconductor laser radiation in a vacuum chamber of scanning ion-probe microscope.

An enhanced composite scheme of shape memory actuator for smart systems

An enhanced scheme of a functional composite material has been proposed and tested in the model experiments. The composite consists of an element with shape memory effect (SME) in the form of ribbon, film or plate connected rigidly to an element of an elastic material. The novel feature in this scheme is that an SME element should be preliminary given the uniaxial pseudoplastic tensile straining. This operation promotes the martensite twins formation along the stretching axis and results in the maximal longitudinal deformation of SME element. Such scheme provides a well-controlled high reversible bending deformation of the composite, though only one-way SME of the material is used. It can be applied to the most conventional shape memory materials, such as polymers, ferromagnetic and non ferromagnetic alloys. The scheme is experimentally tested on the composites made by applying an elastic layer on the preliminary pseudoplastically stretched melt-spun ribbons of the Ti50Ni25Cu25 alloy (see video on the web: www.smwsm.org/ll/composites.html). The elastic layer was applied by three different techniques: gluing, electroplating and focused ion beam assisted chemical vapor deposition (FIB/CVD). Fatigue experiments showed that the composite made by electroplating of Ni layer demonstrates fairly stable actuation for at least 2000 cycles. FIB/CVD allowed a preparing of a composite actuator with the dimensions of about 25×1×1μm. The scheme is promising for the applications in the fields of MEMS and NEMS, microfluidics and biomedical technologies.

An observation of amorphous-crystalline phase transitions at severe plastic deformation of the Ti50Ni25Cu25 alloy

The features of structural and phase transitions during severe plastic deformation (in Bridgman anvils) of the amorphous Ti50Ni25Cu25 alloy have been studied by X-ray diffraction and transmission electron microscopy. Application of successively increasing deformation has revealed three cycles of successive phase transitions from amorphous to crystalline state and vice versa. The results obtained are explained in terms of the superposition of the different channels of elastic energy dissipation, which are activated during severe plastic deformation.

Shape memory effect in Ti-Ni-Cu alloy caused by Joule heating

The properties of melt-spun Ti50Ni25Cu25 alloy have been investigated by multiple thermocycling (up to 8000 cycles) within the martensitic transformation temperature range under constant mechanical stress from 5 to 200MPa, using electric current heating. A nearly linear relation is established between the alloy strain and electric resistance, which is independent of the applied stress and the number of thermal cycles.

Microstructure modifications of Ti–Ni–Cu shape memory alloy strips fabricated by arc melt overflow process

Six batches of Ti50Ni50-xCux alloy (x=0–25) were prepared by a melt overflow process and strips of about 550 μm thickness were fabricated by rapid solidification. Microstructures and shape memory characteristics were investigated by means of x-ray diffraction (XRD), optical microscopy, differential scanning calorimetry and thermal cycling tests under a constant load. The microstructures of the as-cast strips exhibited columnar grains normal to the strip surface. XRD showed B2–B19′ martensitic transformation in the Ti50Ni50 binary alloy strip, while B2–B19 martensitic transformation in all the ternary alloy strips. The transformation temperature (Ms) increased with Cu-content. During cycle deformation with an applied stress of 60 MPa, the smallest transformation hysteresis and the largest elongation associated with the B2–B19 transformation were observed to be 3.5% in Ti50Ni25Cu25 alloy strip and 3.02% in Ti50Ni40Cu10 alloy strip.

Effect of Hydrogen on the Crystallization of Amorphous Ti50Ni25Cu25 Titanium Alloy

We describe effects unknown earlier in the physics of interaction between hydrogen and amorphous metallic alloys: an increase in the crystallization temperature of hydrogen-containing Ti50Ni25Cu25 alloy in the course of its heating and suppression of the inverse martensitic transformation B2 → B19 after crystallization.

Effect of Hydrogen on Damping Capacity of Ti<SUB>50</SUB>Ni<SUB>25</SUB>Cu<SUB>25</SUB> Alloy

Ternary Ti–Ni–Cu alloy is known to exhibit higher damping capacity than binary Ti–Ni alloy. Hydrogen doping was used to further improve the damping capacity of this ternary alloy. Considering ease of manufacture, the Ti50Ni25Cu25 alloy was produced by a lamination process. As a result of hydrogen doping, a new damping peak appeared at around 260 K. The peak temperature shifted with changes in oscillation frequency. The activation energy (H) and the relaxation time constant (� 0) of the damping peak were calculated according to the Arrhenius plot as H ¼ 57:5 kJ/mol, and � 0 ¼ 2:2 � 10 � 13 s, respectively. The damping capacity due to hydrogen doping increased with increasing hydrogen concentration up to 0.45 at%, showing the maximum peak value of tan � ¼ 0:05, but thereafter decreased. On the other hand, the peak temperature increased monotonically with increasing hydrogen concentration, and the peak shape was broader than the single Debye relaxation peak. From these results, we concluded that this damping peak was not the Snoek peak but the Snoek-Koester peak caused by hydrogendislocation interaction. The decrease of the Snoek-Koester peak with increasing hydrogen concentration is explained by formation of hydrogen clusters or hydrides. Schoeck’s theory for the Snoek-Koester peak may be applicable. On further hydrogen doping, both the Snoek-Koester peak and the transformation peak were hardly visible above 5 at% hydrogen. This was found to be due to destruction of the martensitic phase by excess hydrogen doping.

Ｔｉ‐Ｎｉ‐Ｃｕ合金の減衰能に及ぼす水素の影響

Ti-Ni shape memory alloy is one of the most promising materials among high damping alloys. Ti-Ni alloys show a transient damping peak due to martensitic transformation and an almost temperature independent damping in the martensitic phase. Since the transient damping peak is useless due to many limitations, we can utilize only the damping capacity of martensitic phase. In the present work, we doped hydrogen into a ternary Ti-Ni-Cu alloy, which is known to exhibit higher damping capacity than binary Ti-Ni alloy, to improve further the damping capacity. Ti50Ni25Cu25 alloy was produced by a lamination process from a high workability point of view. As a result of hydrogen doping, a new damping peak occurred at around 260 K. The temperature of damping peak was shifted with change in oscillation frequency. The activation energy (H) and relaxation time constant (τ0) of the damping peak were calculated according to the Arrhenius plot as H=57.5 kJ/mol. τ0=2.2×10-13 s. The damping capacity due to hydrogen doping increased with increasing hydrogen concentration up to 0.45 at% hydrogen showing tan φ=0.05, but thereafter decreased. On the other hand, the peak temperature increased monotonously with increasing hydrogen concentration, and the peak shape was broader than single Debye relaxation peak. From these results, we conclude that this damping peak is not the Snoek peak but the Snoek-Koster peak possibly caused by hydrogen and dislocation interaction effect; G. Schoecks theory for Snoek-Koester peak may be applicable.

The effect of hydrogen on crystallization of a Ti50Ni25Cu25 amorphous alloy

Two effects not reported previously for the interaction of hydrogen with amorphous metal alloys were observed in a Ti50Ni25Cu25 alloy: (i) hydrogen-induced increase in the crystallization temperature and (ii) suppression of the reverse martensitic transition B19→B2 in the alloy samples hydrogenated upon crystallization.