Binary NiTi Alloy Research Articles

Functional properties of nanocrystalline, submicrocrystalline and polygonized Ti–Ni alloys processed by cold rolling and post-deformation annealing

Thermomechanical processing consisting of cold rolling ( e = 0.3–2.0) and post-deformation annealing (300–450 °C, 1 h) was applied to binary Ti–Ni alloys to produce nanocrystalline structures (NS) or polygonized dislocation substructures (PDS), or their mixture. The evolution of the material structure and properties was studied using TEM, X-ray, microhardness, calorimetry and tensile testing techniques. Recovery stress and strain of the Ti–50.26 at.%Ni alloy and superelastic strain of the Ti–50.6 at.%Ni alloy were measured under static and fatigue conditions. It was found that higher true yield stress of NS alloys not only increases the recovery stress potential, but, since it is combined with a relatively low transformation yield stress; it increases the completely recoverable strain. NS alloys generate recovery stresses that are twice as high as those of PDS alloys (1200 MPa), completely recoverable strains that are 10% greater (up to 6% in tension), and they demonstrate a higher cyclic stability of shape memory and superelastic properties. This improvement comes with the cost of a lower NS alloy fatigue damage tolerance, aggravated by the presence of microcracks caused by cold working. Binary Ti–Ni alloys, processed by annealing of an intermediately cold-worked ( e = 0.75…1) alloy and containing mixed nanocrystalline structure and polygonized dislocation substructure, allow a high fatigue life combined with relatively high and cyclically stable functional properties.

Refinement of Master Densification Curves for Sintering of Titanium

An approximate master curve for the densification of cold-pressed titanium powder during vacuum sintering was published previously. It was based on the combined results for three different titanium powders. The master densification curve model incorporates the effects of particle size, compaction pressure, sintering time, and sintering temperature on densification. The collection of a large amount of additional data now allows refinement of the model. Distinct curves are presented for three different titanium powders, prealloyed Ti6Al4V, and Ti-Ni binary alloys. The master densification curve is sigmoidal, but deviates from the ideal form at high sintered density; the relative sintered density saturates at 90 to 100 pct, depending on the particle size of the titanium powder, and to a lesser extent the compaction pressure. The master densification curve below the saturation level is slightly dependent on the compaction pressure.

Effect of Cu on Structural and FF-Behavior of NiTi Shape Memory Alloy

The functional fatigue behavior of Ti50Ni30Cu20 (at. %) shape memory alloy was investigated after subjecting to cold working and heat-treatment. Copper addition modified the phase transformation behavior with the introduction of B19-phase in the binary NiTi alloy. It was observed that aging after annealing and thermal cycling (-60 to 100)°C significantly effect the transformation temperatures. Observations in optical microscope and scanning electron microscope reveal inhomogeneity in the composition in the form of coarse Cu+Ti-rich precipitates. Investigations under transmission electron microscope showed growth of internally twined martensitic plates in solution treated sample. The phase transformation temperatures were determined with differential scanning calorimeter. The transformation temperatures were shifted towards lower side. Dislocations introduced during cold working and fine precipitation after aging, may be responsible for this change in the transformation characteristics of the material.

A study of TiNiCr ternary shape memory alloys

Effects of Cr on transformation sequence and wear characteristic in Ti 50− X Ni 50Cr X , Ti 50Ni 50− X Cr X and Ti 51Ni 49− X Cr X alloys with X = 0.5–1.5 at.% are investigated. The lattice parameters of Ti 51Ni 48.5Cr 0.5 martensite are calculated from the XRD spectra as a = 0.2894 nm, b = 0.4109 nm, c = 0.4639 nm and β = 97.49° and the parameters a, c, β are slightly smaller, but b nearly the same, than those in binary Ti 50.8Ni 49.2 alloy. Experimental results indicate that the Cr atoms replace Ni atoms, instead of Ti ones in these alloys. In addition, in Ti 50Ni 49.5Cr 0.5 and Ti 51Ni 48.5Cr 0.5 alloys show a two-stage B2 → R → B19′ transformation upon cooling and a one-stage B19′ → B2 transformation on heating. Both Ti 50Ni 48.5Cr 1.5 and Ti 51Ni 47.5Cr 1.5 alloys exhibit a two-stage B2 ↔ R ↔ B19′ transformation; Ti 49.5Ni 50Cr 0.5 alloy has a one-stage B2 ↔ B19′ transformation and Ti 48.5Ni 50Cr 1.5 alloy shows a B2 ↔ R transformation. The shape recovery of these alloys can be improved by Cr solid-solution hardening. With higher matrix hardness in these alloys, there is better shape recovery. The wear mechanisms of TiNiCr ternary alloys are found to be similar to those of binary TiNi alloys. The Ti 48.5Ni 50Cr 1.5, Ti 51Ni 47.5Cr 1.5, Ti 50Ni 48.5Cr 1.5 and Ti 49.5Ni 50Cr 0.5 alloys have a better wear resistance than the Ti 50Ni 49.5Cr 0.5 and Ti 51Ni 48.5Cr 0.5 alloys due to their higher hardness, high cyclic hardening and/or pseudoelastic behaviors of B2 structure.

The effect of chemical composition on enthalpy and entropy changes of martensitic transformations in binary NiTi shape memory alloys

In the present research work the binary NiTi alloys with various compositions in the range of 50.3–51 at.% Ni were used. Samples have been annealed at 850 °C for 15 min and then quenched in water. In order to characterize transformation temperatures and enthalpy changes of the forward and the reverse martensitic transformation, Differential Scanning Calorimetric (DSC) experiments were performed. The enthalpy and entropy changes as a function of Ni atomic content have been thermodynamically investigated. Results show that enthalpy and entropy changes of martensitic transformation decrease when Ni atomic content increases. The variation of enthalpy and entropy of martensitic transformation with Ni content in binary NiTi alloys were explained by thermodynamic parameters and electron concentration of alloy ( e/ a) respectively.

Internal Defects of B19’ Martensite via R-Phase in Ti-Ni-Fe and Thermally Cycled Ti-Ni Alloys

Crystallography and morphology of B19’ martensite via R phase transformation in Ti-Ni-Fe alloy have been investigated by X-ray diffraction and transmission electron microscopy. The microstructure aspects of the B19’ martensite in the solution treated Ti-Ni-Fe alloy was the same as those in the solution treated binary Ti-Ni alloy. In the thermal cycled Ti-Ni-Fe alloy, the (001)B19’ compound twins frequently observed were the same as those in the thermal cycled binary Ti-Ni alloy. It is considered that the (001)B19’ compound twin was a kind of deformation twin to reduce the internal stress due to dislocations introduced during the thermal cycle, rather than a lattice invariant shear of the R to B19’ transformation.

Optimization of a FIB/SEM slice‐and‐view study of the 3D distribution of Ni4Ti3 precipitates in Ni–Ti

The 3D morphology and distribution of lenticular Ni(4)Ti(3) precipitates in the austenitic B2 matrix of a binary Ni(51)Ti(49) alloy has been investigated by a slice-and-view procedure in a dual-beam focused ion beam/scanning electron microscope system. Due to the weak contrast of the precipitates, proper imaging conditions need to be selected first to allow for semi-automated image treatment. Knowledgeable imaging is further needed to ensure that all variants of the precipitates are observed with equal probability, regardless of sample orientation. Finally, a volume ratio of 10.2% for the Ni(4)Ti(3) precipitates could be calculated, summed over all variants, which yields a net composition of Ni(50.27)Ti(49.73) for the matrix, leading to an increase of 125 degrees for the martensitic start temperature. Also, the expected relative orientation of the different variants of the precipitates could be confirmed.

Electropolishing of a Nickel–Titanium–Copper Shape Memory Alloy in Methanolic Sulfuric Acid

Electropolishing of NiTiCu shape memory alloy was performed in methanolic sulfuric acid as a surface processing method. The current investigation, using a rotating disk electrode, aims at identifying appropriate electropolishing conditions for this alloy based on understanding the underlying mechanism. The influence of temperature, rotation rate, and sulfuric acid concentration ( or ) was quantified. Electropolishing in methanolic is solely mass-transport-controlled, while a mixed control is found for the methanolic . This behavior is different from that of the binary NiTi alloy. From the electrochemical perspective, a methanolic solution is more suitable for electropolishing NiTiCu. The activation energy for electrodissolution is .

Melting and solidification of TiNi alloys by cold crucible levitation method and evaluation of their characteristics

The addition of Re, Fe and Cr into Ti–50 mol.-%Ni has been carried out to improve the oxidation and mechanical properties. The mono phase consisting of TiNi with the B2 type structure was identified in micro-alloyed materials proposed on the basis of the d-electrons concept. Experimentally, TiNi alloys were melted and solidified by the cold crucible levitation melting (CCLM) method. The TiNi–(Cr, Fe, Re) alloys with high purity and without contamination from a crucible were prepared, and the homogeneous microstructure was achieved by the diffusion mixing effect of CCLM even in the as-cast alloys which contained Re and Cr with higher melting temperatures and different specific gravities. The transformation from austenite to martensite phases occurred in all alloys below or above room temperature. Some alloys had the ability of shape memory even at room temperature. Ternary alloys showed a higher flow stress level compared with the binary TiNi alloy. On the other hand, the oxidation at 1273 K was promoted by the formation of titanium oxides (TiO2) on the alloy surfaces. The oxidation resistance was improved by the formation of the continuous Cr2O3 film in TiNi–Cr alloys. The alloying effects by ternary elements (Re, Fe, Cr) in the intermetallic TiNi as well as metallic materials were explained well using two parameters used in the d-electrons concept.

The electrochemical behavior and surface analysis of Ti 50Ni 47.2Co 2.8 alloy for orthodontic use

Objectives The aim of this study was to investigate the electrochemical behavior of Ti 50Ni 47.2Co 2.8 alloy in deaerated artificial saliva solutions with binary NiTi alloy as reference and to characterize the composition and structure of the passive film after polarization tests. Methods The corrosion behavior of NiTiCo alloy was systematically studied by open circuit potential, potentiodynamic and potentiostatic techniques. The surface characterization after anodic polarization tests were demonstrated by XPS. The electrolytes were maintained at 37 °C by immersing the test cell in water bath. 500 mV vs. SCE was applied on the samples for potentiostatic tests. The metal ions concentration was measured by ICP/OES. Results Potentiodynamic and potentiostatic tests results showed that the corrosion behavior of NiTiCo was similar to that of NiTi alloy. With the increase of pH value of the electrolytes, both corrosion potential ( E corr) and pitting corrosion potential ( E b) decreased. XPS results revealed that the outmost passive film consisted mainly of TiO 2 which were identical with that of NiTi alloy. The Ni ion release amount of NiTiCo was very close to that of NiTi alloy as examined by ICP/OES. Neither Ti nor Co ion was detected due to the detection limitation. Significance The corresponding relationship between corrosion behavior and pH value of the artificial saliva should be taken into consideration during the application of orthodontic wires. The addition of Co had little effect on the corrosion behavior of NiTi as well as the formation of the passive film.

Microwave brazing of NiTi shape memory alloy with Ag–Ti and Ag–Cu–Ti alloys

The present investigation was undertaken to study brazing of the commercially available binary NiTi alloy, using Ag–Ti and Ag–Cu–Ti filler metals in a microwave furnace with Ar–H2 atmosphere. Both filler alloys are successful in joining the shape memory alloy NiTi to itself and to other metals such as nickel-based superalloy and stainless steel. However, interface cracks, pores and oxides were observed due to insufficient protection during the heating and cooling stages.

The microstructure and properties of rapidly quenched Ti-rich Ti-Ni binary alloys with shape memory effects

The structure, phase composition, and martensitic transformations in binary titanium-rich Ti-Ni alloys with shape memory effects, produced by ultrarapid quenching using melt jet spinning, have been studied using electron microscopy, X-ray diffraction, and measurements of some physicomechanical properties in a wide temperature range. The alloys with a Ti content that exceeded the stoichiometric composition by 5% and more can be produced in an amorphous state. The alloys with a smaller deviation from the stoichiometry, as well as the Ti50Ni50 alloy, are crystallized in a submicrocrystalline state and undergo a B2 → B19’ martensitic transformation at temperatures above room temperature. They have high strength and plastic properties and demonstrate narrow-hysteresis shape-memory effects.

Optimal glass-forming composition and its correlation with eutectic reaction in the Ti–Ni–Al ternary system

The composition dependence of glass-forming ability was investigated for the Ti-Ni-Al ternary alloys. Glassy ribbons were fabricated via single-roller melt spinning. The optimal glass-forming composition was located at Ti(54)Ni(32)Al(14), with a critical thickness of about 90 mu m for complete glass formation. With respect to the Ti-Ni binary alloy, Al addition in the system only marginally improves the glass-forming ability. It is attributed to a small atomic size difference between Ti and Al. For the Ti(54)Ni(32)Al(14) alloy, together with the binary eutectic Ti(76)Ni(24) and hypereutectic Ti(68)Ni(32) alloys for comparison, phase selection in the arc-melted ingots was investigated. The optimal glass-forming composition, Ti(54)Ni(32)Al(14), is near the univariant eutectic groove of [L --> Ti(Al)(2)Ni + beta-Ti(Al)] in the phase diagram. It is suggested that if the primary crystallization of the Ti(2)Ni intermetallic can be suppressed, solidification of deeply undercooled melt will undergo such a eutectic reaction prior to glass formation. (C) 2008 Elsevier B.V. All rights reserved.

The influence of laser welding parameters on the microstructure and mechanical property of the as-jointed NiTi alloy wires

The Nd:YAG laser welding was used to join the binary NiTi alloy wires with different compositions(Ti–50.0 at.%Ni and Ti–50.9 at.%Ni) which had the same diameter of 1 mm. The wires were welded with different parameters, including impulse width and welding current. The aim was to assess the influence of the laser-welding process on the microstructure and mechanical properties of the welded joint of binary NiTi wires. The optical microscopy (OM) and the metallographic microscopy (MM) were used to analyze the microstructure of the welded joints. The tensile test and the differential scanning calorimetry (DSC) were carried out to examine the ultimate tensile strength and the reverse martensitic transformation temperatures of the welded joints. It was found that the welding current and the impulse width had great influence on the quality of the welded joints, an optimal parameter combination would remove the pores and micro-cracks appeared in the fusion zone, and result in good mechanical properties such as higher fracture strength and elongation. The laser welding had a few effect on the reverse martensitic transformation temperatures of the welded joints.

Powder metallurgical production of TiNiNb and TiNiCu shape memory alloys by combination of pre-alloyed and elemental powders

Abstract The common Ti 44 Ni 47 Nb 9 and Ti 50 Ni 40 Cu 10 ternary shape memory alloys were produced by sintering techniques and the microstructure, phase structure and phase transformation behaviour were investigated. A combination of pre-alloyed binary TiNi powder and elemental Nb, Ni and Cu, Ti powders, respectively, were used. In contrast to the use of pre-alloyed ternary powders, which have to be produced in each new composition, a higher flexibility in the alloy composition becomes possible. In case of the Ti 44 Ni 47 Nb 9 alloy, liquid phase sintering was done to obtain the eutectic phase structure known from cast material. In case of the Ti 50 Ni 40 Cu 10 alloy, the pore size and porosity can be improved by choosing a two-step sintering process, as a eutectic melt between Ti and Cu is formed at low temperatures which influences the sintering behaviour. Controlling the impurity contents and the resulting secondary phases is necessary for both alloys in the same way as for binary TiNi alloys.

Effect of heat treatment on the structure and properties of rapidly quenched binary titanium-enriched Ti-Ni alloys with a shape-memory effect

The effect of heat treatment on the structure, martensitic transformations, and properties of rapidly quenched binary titanium-enriched TiNi alloys with a shape-memory effect, which were produced by melt-spinning, has been studied using transmission and scanning electron microscopy, X-ray diffraction, and measurements of mechanical properties and electrical resistivity over a wide temperature range. Annealing of initial amorphous alloys was found to ensure their nanocrystallization with the formation of a two-phase (TiNi + Ti2Ni) nanocomposite structure. The annealing of initial submicrocrystalline alloys initiates their decomposition with the heterogeneous formation of Ti2Ni disperse particles in the supersaturated B2 austenite matrix. All alloys under study undergo a single B2 ↔ B19′ martensitic transition occurring above room temperature, exhibit high strength and plasticity, and demonstrate a narrow-hysteresis shape-memory effect.

Microstructure of rapidly quenched nickel-enriched TiNi binary alloys in the initial state and after annealing

The microstructure of shape-memory melt-spun TiNi binary alloys of nonstoichiometric composition in the initial state and after annealing has been studied for the first time by transmission electron microscopy and X-ray diffraction in a wide temperature range.

Low-temperature X-ray diffraction study of martensite lattice parameters in binary Ti–Ni alloys

Concentration and temperature dependency of the B19′-martensite lattice parameters (MLP) in binary Ti–Ni alloys as well as the effect of the structural state of a parent austenite on these parameters were studied using low-temperature X-ray diffraction analysis. The existence of a concentration dependence of the MLP found for the hyper-equiatomic nickel concentration range is proved for cryogenic temperature range and extended, at least, up to 51.2 at.% of nickel. Temperature dependency of MLP is observed in the studied nickel concentration range, and they are approximately the same for different alloys in the temperature range of stable martensite. During heating in the temperature range of the martensite existence, all parameters of the B19′-martensite monoclinic cell change towards the values of corresponding parameters of the austenite tetragonal cell with which they have “genetic” relations. The maximum transformation lattice strain calculated at the martensite-start temperature of each alloy, and, hence, the resource of the recoverable strain, are higher for pre-equiatomic and equiatomic alloys than that for alloys in hyper-equiatomic nickel concentration range. For Ti–50.7 at.% Ni alloy, the lattice parameters of martensite formed from the austenite containing a well-developed dislocation substructure deviate from the corresponding lattice parameters of the quenched martensite formed from a low-dislocated recrystallized austenite. This distinction is a general feature for the alloys undergoing B2 → B19′ and B2 → R → B19′ martensitic transformations.

Mechanical Properties and Shape Memory Effects of TiNiNb Shape Memory Alloys with Low Niobium Content

Microstructures evolution, mechanical properties, transformation behaviors and shape memory effects of low niobium content (5at.%) NiTiNb shape memory alloys were investigated. It was found that the amount of Nb resolved in the NiTi matrix is closely related to the Ni/Ti ratio and has significant influence on the mechanical properties, especially the yield stress of the alloys. Ti 45.2 Ni 49.8 Nb 5 and Ti 45 Ni 50 Nb 5 alloys exhibits as excellent shape memory effect as the equiatomic binary NiTi alloys.

Effect of Dy addition on the microstructure and martensitic transformation of a Ni-rich TiNi shape memory alloy

The effect of rare earth element Dy addition on the microstructure and martensitic transformation behavior of Ti 49.3Ni 50.7 shape memory alloy was investigated by optical microscope (OM), scanning electronic microscope (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results show that the microstructure of TiNiDy ternary alloy consists of Ti 2Ni phase, DyNi phase and the matrix. One-step martensitic transformation is observed in quenched TiNiDy ternary alloys, which is the same as that in quenched TiNi binary alloys. The martensitic transformation temperatures increase evidently with Dy addition, and the maximum increase of M s is about 110 °C.