TiNiCu Alloys Research Articles

Tribological and electrochemical behaviour study of porous TiNiCu alloys manufactured by the powder metallurgy process

ABSTRACT The experimental work aims to study the effect of the Copper (Cu) addition on the tribological and electrochemical behaviour of porous Ti50Ni50-x-Cux alloys. In the first step pure metal powders of titanium (Ti), nickel (Ni), and Cu were ball-milled during 8 h in a planetary ball mill, and then compacted under a 4 ton load to obtain pre-alloyed powders. The final step is sintering at 950°C for 7 h in a tubular furnace under a controlled atmosphere. X-ray diffraction measurements (XRD), scanning electron microscopy (SEM) analyses, microhardness measurements, friction tests and electrochemical were carried out to characterise the tribological and electrochemical performances of the elaborated specimens. DRX results reveal the formation of four major phases: TiNi, Ti2Ni, TiNi3 and TiNi0.8Cu0.2. Potentiodynamic polarisation tests show the positive effect of adding Cu on corrosion resistance, where the 6% Cu sample exhibits the best corrosion behaviour. Tribological results show a decrease in the coefficient of friction and wear rate with the addition of Cu, compared to the initial state, without Cu addition. The minimum coefficient of friction (0.585) was recorded for the 2% Cu sample, while the minimum wear rate (Ws = 1.06. E−4mm3/N/m) was achieved for the 4% Cu sample.

Nanoprecipitates enhanced the yield strength and output work of (TiHfZr)50(NiCu)50 high-entropy shape memory alloys

High entropy shape memory alloys (HESMAs) of (Ti40−xHf10Zrx)50(NiCu)50 (x = 0, 1, 5, 10, at%) were prepared by vacuum arc melting, solution treatment (800 °C for 30 min), and aging treatment (300 °C for 120 min for the solution-treated sample and 400 °C for 120 min for the 300 °C aged sample). The effects of Zr content and aging on the microstructure, mechanical performance, and superelasticity properties of HESMAs were investigated. The results show that uniform nanoscale phases are precipitated on the B2 austenite matrix of (TiHfZr)50(NiCu)50 HESMAs after solid solution treatment and aging treatment. Compared with the ternary TiNiCu and quaternary Zr0 alloys, the yield strength and critical strength of the stress-induced martensite transformation of the HESMAs improve significantly with increasing Zr content, and the nanoprecipitates further improve their mechanical performance. Due to the enhanced solid solution strengthening effect caused by the increase in Zr content and the dispersion strengthening effect of the nanoprecipitates, the (TiHfZr)50(NiCu)50 HESMAs show an excellent combination of high strength and large recoverable strain, which endows them with excellent output work. Among them, the Zr5 HESMAs have the best superelasticity properties, and their maximum recoverable strain and superelasticity strain are 8.8–9.7 % and 4.9–5.4 % in ten load cycles with a prestrain of 2–15 %, while the corresponding values of ternary TiNiCu alloys are only 2.6–4.7 % and 1.0–2.1 %, respectively. The output work of Zr5 HESMAs is 163.7–188.5 J/cm3, which is approximately 5–8 times larger than that of ternary TiNiCu alloys.

Study of Structure and Phase Transformations in Rejuvenated Rapidly Quenched TiNiCu Alloys

Alloys of the quasibinary TiNi-TiCu system manufactured by melt quenching in the form of thin 20–50 μm ribbons have proven to show good potential as materials for the fabrication of micromechanical devices. At high cooling rates (about 106 K/s), this method allows producing high-copper (more than 20 at.%) amorphous alloys which exhibit an excellent shape-memory effect after crystallization. Their properties are known to largely depend on the crystallization conditions and the structure of the initial amorphous material acting as a precursor for the formation of crystal phases. It has been shown recently that the rejuvenation procedure (cryogenic thermocycling) of metallic glasses is one of the most promising methods of improving their properties. In this study, we investigated for the first time the effect of cryogenic thermocycling of rapidly quenched amorphous TiNiCu on the initial state, as well as on structure formation and the phase transformation patterns of subsequent crystallization conducted using various methods. The effect was analyzed utilizing the methods of scanning and transmission electron microscopy, X-ray diffraction analysis, and differential scanning calorimetry. The results show that rejuvenation treatment slightly reduces the glass transition and crystallization onset temperatures and moderately changes the sizes of structural features (grains, martensite plates), the quantity of the martensite phase, and the characteristic temperatures and enthalpy of the martensitic transformation.

Coherent Precipitates as a Condition for Ultra-Low Fatigue in Cu-Rich Ti53.7Ni24.7Cu21.6 Shape Memory Alloys

Sputtered Ti–rich TiNiCu alloys are known to show excellent cyclic stability. Reversibility is mostly influenced by grain size, crystallographic compatibility and precipitates. Isolating their impact on cyclic stability is difficult. Ti2Cu precipitates for instance are believed to enhance reversibility by showing a dual epitaxy with the B2 and B19 lattice. Their influence on the functional fatigue, if they partly lose the coherency is still unknown. In this study, sputtered Ti53.7Ni24.7Cu21.6 films have been annealed at different temperatures leading to a similar compatibility (λ2 ~ 0.99), grain size and thermal cyclic stability. Films annealed at 550 °C exhibit a superior superelastic fatigue resistance but with reduced transformation temperatures and enthalpies. TEM investigations suggest the formation of Guinier–Preston (GP) zone-like plate precipitates and point towards a coherency relation of the B2 phase and finely distributed Ti2Cu precipitates (~ 60 nm). Films annealed at 700 °C result in the growth of Ti2Cu precipitates (~ 280 nm) with an irregular distribution and a partial loss of their coherency. Thus, GP zones are assumed to cause the reduction of transformation temperatures and enthalpies due to increased internal stresses, whereas the coherency relation of both, Ti2Cu and GP zones, help to increase the superelastic stability, well beyond 107 cycles.

Effect of External Impacts on the Structure and Martensitic Transformation of Rapidly Quenched TiNiCu Alloys

TiNi-TiCu quasibinary system alloys with a high Cu content produced by rapid quenching from liquid state in the form of thin amorphous ribbons exhibit pronounced shape memory effect after crystallization and are promising materials for miniaturized and fast operating devices. There is currently no complete clarity of the mechanisms of structure formation during crystallization from the amorphous state that determine the structure-sensitive properties of these alloys. This work deals with the effect of the initial amorphous state structure and crystallization method of the alloys on their structure and phase transformations. To this end the alloy containing 30 at.% Cu was subjected to thermal and mechanical impact in the amorphous state and crystallized using isothermal or electropulse treatment. We show that after all types of treatment in the amorphous state the structure of the alloy remains almost completely amorphous but the characteristic temperatures and enthalpy of crystallization become slightly lower. Isothermal crystallization of alloy specimens produces a submicrocrystalline structure with an average grain size in the 0.4–1.0 μm range whereas electropulse crystallization generates a bimorphic structure consisting of large 4–6 μm grains and 2–3 μm high columnar crystals in the vicinity of the surface. The grains have nanosized plate-like and subgrain structures. The largest grains are observed in thermally activated samples, meanwhile, mechanical impact in the amorphous state leads to the formation of equiaxed finer grains with a less defective subgrain structure and to the shift of the temperature range of the martensitic transformation toward lower temperatures.

Influence of copper content and thermal treatment on shape memory effect in rapidly quenched TiNiCu alloys

Amorphous alloys of the TiNi - TiCu system with a copper content of 25 to 40 at.% were prepared by planar flow casting at a melt cooling rate of 106 K/s. Crystallization of the alloys was carried out by isothermal annealing and by action of single electric pulse with a duration of 5 ms. Shape memory behavior and structure of the alloys was investigated by means of bending tests and X-ray phase analysis. It was found that increasing the copper content to above 30 at.% considerably reduces the plasticity and shape memory effect of the alloys. However, abrupt decreasing the annealing duration significantly improves the shape memory performance due to prevention of the formation of brittle Ti-Cu phases in the alloys structure.

Shape Memory Behavior of Rapidly Quenched High-copper TiNiCu Alloys

Rapidly quenched quasibinary TiNi–TiCu system alloys with high copper contents (above 20 at.%) exhibit excellent shape memory effect and have considerably narrower hysteresis as compared with the TiNi binary alloy, this advantage being of special importance for cyclic load applications, e.g. for microelectromechanics (MEMS). The aim of this work is to study the effect of annealing parameters and copper content on the shape memory effect in TiNiCu alloys. Thin amorphous ribbons of TiNi-TiCu alloys with copper contents of 25 to 40 at.% were produced by planar flow casting at a melt cooling rate of about 106 K/s. The alloys were crystallized by isothermal annealing with variable duration and by exposing specimens to a short (10 ms) electric pulse. Increasing the copper content to above 30 at.% considerably reduces the plasticity and shape memory effect of the alloys. However, significant reduction of annealing duration greatly improves the shape memory performance due to prevention of the formation of brittle Ti-Cu phases in the alloys structure.

Structure and functional properties of rapidly quenched TiNiCu alloys with high copper contents

Thin materials exhibiting the shape memory effect, with a narrow temperature hysteresis, are required to create miniature and high-speed devices. Quasi-binary intermetallic TiNi-TiCu alloys with high copper contents (more than 10 at %) demonstrate the reversible martensitic transformation with a small (4-6 K) hysteresis. Alloys of the TiNi–TiCu system with a copper content of 30–40 at.% were fabricated in an amorphous state by the planar flow casting technique at a melt cooling rate of 106 K/s in the form of ribbons 30–50 μm thick. The alloy samples were subjected to dynamic crystallization using a single electric current pulse with duration of 5 ms. X-ray diffraction studies revealed almost fully martensitic state of the alloys with B19 structure at room temperature. TEM examination showed their structure to contain typical B19-martensite plates with a mean size of 20–80 nm. At the same time, the alloys exhibit a one-stage phase martensitic transformation B2↔B19 in the temperature range of (55÷75)°C, as well as pronounced shape memory effect, whose properties are largely determined by the structural parameters of the alloys.

Formation of structure of TiNiCu alloys with high copper content upon producing by planar flow casting

The rapidly quenched alloys of the quasibinary intermetallic TiNi-TiCu system with a high copper content (more than 25 at.%) are of great interest as shape memory materials due to the possibility of a significant decrease in the temperature and deformation hysteresis in comparison with the binary TiNi alloy. To obtain alloys with a copper content of 25 to 40 at.%, the planar flow casting technique was used. The alloys were fabricated at a melt cooling rate of about 106 K/s in the form of ribbons 30-50 μm thick and wide in the range from 7 to 20 mm. The study of the structure of the alloys was carried out using X-ray diffraction analysis, scanning and transmission electron microscopy. It was shown that from the ribbon side, contacting the quenching wheel, all alloys are amorphous, while on the non-contact side of the ribbons of alloys with 25 and 30 at.% Cu, a thin surface crystalline layer with a B2 structure is observed. Using energy dispersive X-ray spectroscopy, it was found that the content of the alloy components in the amorphous and crystalline phases coincides.

Specifics of Crystallization of Amorphous TiNiCu Alloys with High Concentrations of Copper

Ribbons of alloys of the quasi-binary TiNi–TiCu system with 25, 30, 35, and 40 at % copper were fabricated by planar flow casting at a cooling rate of 106 K/s. The thickness of the ribbons was 30–50 μm, and their width was 10–20 mm. The structure and phase transformations of alloys were studied using electron microscopy, X-ray diffraction, and differential scanning calorimetry. It was found that the alloys with 25 and 30 at % Cu in the initial state have an amorphous-crystalline structure, which undergoes one-stage polymorphic crystallization of the amorphous state with the formation of austenitic phase B2 under heating in a calorimeter. When cooled to room temperature, phase B2 undergoes a martensitic transformation to orthorhombic phase B19. It was demonstrated that the alloys with 35 and 40 at % Cu become amorphous when quenched and undergo two-stage (primary and eutectic) crystallization under heating. The resulting structure is two-phase (tetragonal phase B11 (TiCu) with a small fraction of phase B2). An increase in the concentration of copper leads to a reduction in the onset temperature of crystallization.

Two-Way Shape Memory Effect in Rapidly-Quenched High-Copper TiNiCu Alloys Deformed in the Martensitic State

The effect of martensitic deformation on the two-way shape-memory effect (SME) of quasi-binary high-copper TiNi–TiCu alloys (with 25–40 at % Cu) is studied. The alloys are prepared by rapid quenching from the melt in an amorphous state in the form of ribbons 30–50 μm in thickness. The alloys crystallize during isothermal annealing, the time of which is varied from 100 to 300 s, and during a single current pulse 10 ms in duration. The relatively low bending strains (1.5–3%) are shown to cause two-way SME in TiNiCu alloys; as the strain of martensite increases, the reversible strain in the martensitic transformation range increases markedly. An increase in the copper content is found to cause a substantial decrease in the value of two-way SME, which is related to the formation of the brittle TiCu phase. At the same time, a decrease in the heat treatment time causes an increase in the reversible strain, which reaches the maximum value (2.35%) for the alloy with 25 at % Cu subjected to high-rate electric pulse treatment.

Crystallization Features of Amorphous Rapidly Quenched High Cu Content TiNiCu Alloys upon Severe Plastic Deformation

In recent years, the methods of severe plastic deformation and rapid melt quenching have proven to be an effective tool for the formation of the unique properties of materials. The effect of high-pressure torsion (HPT) on the structure of the amorphous alloys of the quasi-binary TiNi–TiCu system with a copper content of more than 30 at.% produced by melt spinning technique has been analyzed using the methods of scanning electron microscopy, X-ray diffraction analysis, and differential scanning calorimetry (DSC). The structure examinations have shown that the HPT of the alloys with a Cu content ranging from 30 to 40 at.% leads to nanocrystallization from the amorphous state. An increase in the degree of deformation leads to a substantial change in the character of the crystallization reflected by the DSC curves of the alloys under study. The alloys containing less than 34 at.% Cu exhibit crystallization peak splitting, whereas the alloys containing more than 34 at.% Cu exhibit a third peak at lower temperatures. The latter effect suggests the formation of regions of possible low-temperature crystallization. It has been established that the HPT causes a significant decrease in the thermal effect of crystallization upon heating of the alloys with a high copper content relative to that of the initial amorphous melt quenched state.

Impact of variation in wire electro discharge machining responses of homologous TiNiCu shape memory alloys for smart applications: an experimental investigation

In this study, three distinct TiNiCu alloys were processed using wire electro discharge machining. These alloys are potential materials for smart applications owing to their fast actuation response resulting from suitable martensitic transformation temperatures. Due to differences in their physical property (thermal conductivity in this case), these alloys exhibit variation in machining response during wire electro discharge machining. Based on wire electro discharge machining responses of these alloys, order of thermal conductivity of these alloys are concluded in this study. This investigation aims at addressing the variation in terms of kerf width, recast layer thickness and machined surface microhardness to form the basis for production of intricate components. Ti50Ni40Cu10 alloy exhibited diverging machining response compared to its counterparts. Influential parameter for a given machining response changes depending on thermal property of the workpiece which makes it vital to investigate and choose the parameters in a more sensible manner while machining such materials. It was found that kerf width decreases with increasing servo voltage whereas recast layer thickness (mostly influenced by pulse off duration) increases with increasing pulse off duration. Microhardness of the machined surface (mostly influenced by pulse on duration) increases at higher discharge energy levels owing to presence of oxides formed during spark erosion process. Microhardness of Ti40Ni50Cu10 was found to be high (approximately 560 Hv) compared to its counterparts which was due to presence of Ni rich and γ-CuTi precipitates which also affected its phase transformation temperatures.

Effect of the Conditions of Crystallization of Rapidly Quenched TiNiCu Amorphous Alloys with High Copper Contents on the Characteristics of Martensite Transformations and Shape Memory Effect

Amorphous TiNiCu alloys with copper contents of 28 to 38 at % are fabricated via melt spinning. The isothermal crystallization of alloys is conducted at 500°C with variable durations of the heat treatment (100 to 300 s). It is shown that shortening the duration of crystallization prevents the formation of brittle phases of the Ti–Cu system and contributes to the martensitic B2 ↔ B19 transformation, with the temperature intervals of transformation shifting to higher values and a marked increase in the enthalpy of the martensitic transitions and the magnitude of the shape memory effect.

Effect of Cu Content on Atomic Positions of Ti50Ni50−xCux Shape Memory Alloys Based on Density Functional Theory Calculations

The study of crystal structures in shape memory alloys is of fundamental importance for understanding the shape memory effect. In order to investigate the mechanism of how Cu content affects martensite crystal structures of TiNiCu alloys, the present research examines the atomic displacement of Ti50Ni50−xCux (x = 0, 5, 12.5, 15, 18.75, 20, 25) shape memory alloys using density functional theory (DFT). By the introduction of Cu atoms into TiNi martensite crystal to replace Ni, the displacements of Ti and Ni/Cu atoms along the x-axis are obvious, but they are minimal along the y- and z-axes. It is found that along the x-axis, the two Ti atoms in the unit cell move in opposite directions, and the same occurred with the two Ni/Cu atoms. With increasing Cu content, the distance between the two Ni/Cu atoms increases while the Ti atoms draw closer along the x-axis, leading to a rotation of the (100) plane, which is responsible for the decrease in the monoclinic angle. It is also found that the displacements of both Ti atoms and Ni/Cu atoms along the x-axis are progressive, which results in a gradual change of monoclinic angle and a transition to B19 martensite crystal structure.

Phase transformation and damping behavior of lightweight porous TiNiCu alloys fabricated by powder metallurgy process

Porous TiNiCu ternary shape memory alloys (SMAs) were successfully fabricated by powder metallurgy method. The microstructure, martensitic transformation behavior, damping performance and mechanical properties of the fabricated alloys were intensively studied. It is found that the apparent density of alloys decreases with increasing the Cu content, the porous Ti50Ni40Cu10 alloy exhibits wide endothermic and exothermic peaks arisen from the hysteresis of martensitic transformations, while the porous Ti50Ni30Cu20 alloy shows much stronger and narrower endothermic and exothermic peaks owing to the B2-B19 transformation taking place easily. Moreover, the porous Ti50Ni40Cu10 alloy shows a lower shape recovery rate than the porous Ti50Ni50 alloy, while the porous Ti50Ni30Cu20 alloy behaves reversely. In addition, the damping capacity (or internal friction, IF) of the porous TiNiCu alloys increases with increasing the Cu content. The porous Ti50Ni30Cu20 alloy has very high equivalent internal friction, with the maximum equivalent internal friction value five times higher than that of the porous Ti50Ni50 alloy.

Shape memory effect of TiNi-based springs trained by constraint annealing

This study investigated shape memory effect (SME) behaviors of TiNi-based shape memory alloys springs trained by constraint annealing treatments. The results indicated that SME behaviors were determined by constraint annealing sequences and the chemical composition of alloys. Springs made by Ni-rich TiNi binary alloys showed both single-way SME (SW-SME) and two-way SME (TW-SME) depending on the constraint annealing sequences, while those made by TiNiCu alloys only showed SW-SME. The mechanism and stability of TW-SME induced by constraint annealing were discussed. The occurrence of TW-SME was found to be associated with oriented internal coherent strain caused by preferential Ti3Ni4 precipitates. A special method, which could achieve a stable post-inherent TW-SME and size selectivity, was proposed to provide an effective approach for devising actuators in Ni-rich alloys.

XPS and NEXAFS Investigation of Electronic Energy Structure of Ti-Ni and TiNi-Cu Alloys

The electronic energy structure of Ti-Ni and TiNi-Cu alloys has been studied experimentally by XPS and NEXAFS and theoretically by the full-potential local-orbital minimum-basis code. The study has revealed formation in the valence band of the Ti50Ni50 alloy of a common d-band through hybridization of the d-states of Ti with those of Ni, which is localized within a narrow binding-energy interval. This brings about an intra-atomic redistribution of Ti electron density in the Ti50Ni50 alloy compared to the pure metal: decrease of the occupancy of the Ti d-shell is countered by an increase in the number of Ti p-electrons. The occupancy of the Ti d-shell in the TiNi-Cu alloys, where Ni atoms are partially substituted by Cu, and in the Ti-Ni alloys with an excess of the Ni is more than in the equiatomic Ti50Ni50 alloy. The occupancy of the Ni and Cu d-shells in the Ti-Ni and TiNi-Cu alloys is independent of the atomic composition. The Ti d-shell sensitivity to variation of the nearest atomic environment is apparently due to their relatively itinerant behavior in the studied alloys. The results obtained suggest that the increase of the stiffness of an interatomic bond in the TiNi-Cu alloys with increasing Cu content and in the Ti-Ni alloys with increasing Ni content is possibly associated with the filling of Ti bonding d-states.

Experimental valence-band study of Ti(NiCu) alloys with different compositions and crystal structures

The density of valence-band electronic states of Ti(NiCu) alloys with different crystal structures and elemental compositions has been studied by X-ray photoelectron spectroscopy. It has been established that the change in the crystal state initiated by a martensitic transformation or a transition from the amorphous state to the crystal state does not affect the valence-band electronic state density distribution of the Ti50Ni50 and Ti50Ni25Cu25 alloys. It has been shown that a change in the elemental composition leads to a noticeable redistribution of the electronic density in alloys of the Ti50Ni50 − xCux system (x = 0, 10, 15, 25, 30, 38, 50 at. %). As the copper concentration in the Ti(NiCu) alloys increases, the contribution of the Ni d states in the vicinity of the Fermi level decreases, with the d band of nickel shifting toward higher binding energies, and that of copper, toward lower binding energies.

Property Improvement of TiNi by Cu Addition for Orthodontics Applications

This study aims to investigate mechanical properties and transformation behavior of TiNiCu shape memory alloys to obtain optimal conditions for utilizing as orthodontic wires. TiNi binary alloys with Ni-content 50.6 at.%, TiNiCu alloys with Cu-content ranging from 5 to 10 at.% were prepared. The alloys were melted by electrical arc-melting method and then homogenized at 800°C for 3600 s. The alloys were subsequently sliced into thin plates (1.5 mm) by EDM wire cutting machine. To evaluate mechanical properties, the specimens were cold-rolled with 10, 20 and 30%, followed by heat treatment at 400°C and 600°C for 3600 s, respectively. A Differential Scanning Calorimeter (DSC) was used to detect transformation temperatures. Mechanical properties were evaluated by micro hardness and three-point bending tests. The results showed that transformation temperatures were strongly increased with increasing Ni-content. Moreover, the decrease in transformation temperature after increasing level of cold-rolling reduction ratio suggests that internal stress can depress transformation. However, internal stress seemed to support the introduction of superelasticity for each specimen. In addition, specimens heat treated at 400°C have, more appropriate properties as orthodontic wires than those heat-treated at 600°C due to the remaining effect of cold-working. These results can be take into consideration for optimizing alloy composition and mechanical properties of TiNiCu shape memory alloys for orthodontics wires purposes.