Determination of corrosion rate of orthodontic wires based on nickel‐titanium alloy in artificial saliva

The goal of this study was to determine corrosion behavior of three orthodontic wires based on nickel‐titanium alloy (NiTi) in artificial saliva at temperature of 37 °C as function of immersion time. Following orthodontic wires were used: uncoated (NiTi), rhodium coated (Rh NiTi) and nitrified (N NiTi) orthodontic wires. Corrosion of investigated orthodontic wires were monitored by measuring of Ni²⁺ and Ti⁴⁺ ions released in artificial saliva by inductively coupled plasma‐optical emission spectroscopy (ICP‐OES) after 3, 7, 14, 21 and 28 days of immersion. Obtained results indicate that corrosion reaction of the NiTi wires in artificial saliva follows the parabolic rate law. According to the obtained values of parabolic corrosion rate constants, corrosion susceptibility of orthodontic wires decreases in the following order: Rh NiTi wire (K_p = 2.48 μg²/cm⁴ h) > NiTi wire (K_p = 1.6 × 10^–3 μg²/cm⁴ h) > N NiTi wire (K_p = 6.0 × 10^–4 μg²/cm⁴ h). These results indicate that in comparison with uncoated NiTi wire, rhodium coating significantly increases corrosion susceptibility, while nitrification effectively suppresses the release of Ni²⁺ and Ti⁴⁺ ions.     

Characterization and corrosion behavior of NiTi–Ti2Ni–Ni3Ti multiphase intermetallics produced by vacuum sintering

An in-situ synthesis method was employed to produce NiTi–Ti₂Ni–Ni₃Ti multiphase intermetallics. In this regard, the amorphous/nanocrystalline Ni–Ti powders were sintered at 1300 °C for 2 hrs to obtain Ni–Ti alloys with dense structure. Tafel polarization tests were employed to study corrosion behavior of multiphase materials in 3.5% NaCl and 0.1 M H₂SO₄ corrosive media. The results indicated that the microstructure of sintered samples consists of NiTi(Fe) and Ti₂Ni/Ti₄Ni₂O_x phases embedded in a Ni₃Ti matrix. The synthesized multiphase materials had microhardness up to 873 HV1 kg.Further investigations showed the corrosion performance of multiphase samples in 3.5% NaCl solution was inferior to that of wrought NiTi alloy. In contrast, the corrosion resistance of multiphase samples in 0.1 M H₂SO₄ solution was comparable to that of wrought NiTi alloy.     

Microstructure and martensitic transformation and mechanical properties of cast Ni rich NiTiCo shape memory alloys

Abstract

The influence of Co additions on the microstructure, second phase precipitates, phase transformation and mechanical properties of cast Ni_51?xTi₄₉Co_x (x?=?0, 0·5, 1·5 and 4 at-%) shape memory alloys was investigated. At the expense of Ni, Co added to NiTi alloy significantly increases the martensitic transformation temperature. The matrix phase in the microstructure of Ni₅₁Ti₄₉Co₀ alloy is the austenite phase (B2) in addition to martensite phase (B19′) and precipitates of NiTi intermetallic compounds. However, the parent phase in the other three alloys, Ni_50·5Ti₄₉Co_0·5, Ni_49·5Ti₄₉Co_1·5 and Ni₄₇Ti₄₉Co₄, is martensite. Ti₂Ni phase was found in the microstructures of the all investigated alloys; however, Ni₃Ti₂ phase precipitated only in the NiTi alloy with 0 at-%Co. The volume fraction of Ti₂Ni phase decreased by the additions of 0·5 and 1·5 at-%Co, while it is slightly increased with 4 at-%Co. The hardness value of NiTi alloy is affected by Co additions.     

Effect of heat treatment on microstructure,hardness and rollability of V55Ti30Ni15 alloy membranes

The effect of heat treatment on the microstructure, hardness and rollability of V₅₅Ti₃₀Ni₁₅ alloy membranes has been investigated in this study. The microstructure resulting from different heat treatment conditions has a great influence on hardness. Fine NiTi particles precipitate from the supersaturated V-matrix solid solution at temperatures above 600 °C, increase in quantity until 800 °C, then dissolve back into the V-matrix with a further increase in temperature up to 950 °C. The resultant hardness decreases with temperature until 800 °C, and then increases from 800 to 950 °C. In the present study, a comparison has been made between the rollability of the as-cast and the heat treated state selected for deformation at different rolling temperatures. The percent reduction in thickness of the heat-treated alloy (800 °C/18 h) has been found to be up to 30% higher than that of the as-cast alloy, even at room temperature (cold rolling).     

Precipitation of second phases in aged Ni rich NiTiRe shape memory alloy

Abstract

This study investigated the effect of aging on the structure and precipitation of second phases of Ni₅₂Ti_47·7Re_0·3 shape memory alloys. The alloy was solutionised at 1000°C for 24 h before aging at various temperatures ranging from 300 to 600°C for 3 h. The matrix phase in both solutionised and aged specimens was martensite. Ti₂Ni phase was also present in the microstructure of both solutionised and aged specimens and its volume fraction decreased as the aging temperature increased. Ni₄Ti₃ phase began in appearance by increasing aging temperature to 400°C. Ni₄Ti₃ precipitates had lenticular and non-geometry shapes. Aging at 600°C led to precipitation of Ni₃Ti phase in the microstructure. This precipitated phase formed in white blocky shapes. Ti/Ni ratio increased and/or Ni content decreased in the matrix with increasing in aging temperature.     

Effects of high temperature treatment on microstructure and mechanical properties of laser-clad NiCrBSi/WC coatings on titanium alloy substrate

Laser-clad composite coatings on the Ti6Al4V substrate were heat-treated at 700, 800, and 900 °C for 1 h. The effects of post-heat treatment on the microstructure, microhardness, and fracture toughness of the coatings were investigated by scanning electron microscopy, X-ray diffractometry, energy dispersive spectroscopy, and optical microscopy. The wear resistance of the coatings was evaluated under dry reciprocating sliding friction at room temperature. The coatings mainly comprised some coarse gray blocky (W,Ti)C particles accompanied by the fine white WC particles, a large number of black TiC cellular/dendrites, and the matrix composed of NiTi and Ni₃Ti; some unknown rich Ni- and Ti-rich particles with sizes ranging from 10 nm to 50 nm were precipitated and uniformly distributed in the Ni₃Ti phase to form a thin granular layer after heat treatment at 700 °C. The granular layer spread from the edge toward the center of the Ni₃Ti phase with increasing temperature. A large number of fine equiaxed Cr₂₃C₆ particles with 0.2–0.5 μm sizes were observed around the edges of the NiTi supersaturated solid solution when the temperature was further increased to 900 °C. The microhardness and fracture toughness of the coatings were improved with increased temperature due to the dispersion-strengthening effect of the precipitates. Dominant wear mechanisms for all the coatings included abrasive and delamination wear. The post-heat treatment not only reduced wear volume and friction coefficient, but also decreased cracking susceptibility during sliding friction. Comparatively speaking, the heat-treated coating at 900 °C presented the most excellent wear resistance.     

Characterization and corrosion behavior of NiTi-Ti2Ni-Ni3Ti multiphase intermetallics produced by vacuum sintering

An in-situ synthesis method was employed to produce NiTi-Ti₂Ni-Ni₃Ti multiphase intermetallics. In this regard, the amorphous/nanocrystalline Ni-Ti powders were sintered at 1300 °C for 2 hrs to obtain Ni-Ti alloys with dense structure. Tafel polarization tests were employed to study corrosion behavior of multiphase materials in 3.5% NaCl and 0.1 M H₂SO₄ corrosive media. The results indicated that the microstructure of sintered samples consists of NiTi(Fe) and Ti₂Ni/Ti₄Ni₂O_x phases embedded in a Ni₃Ti matrix. The synthesized multiphase materials had microhardness up to 873 HV1 kg.Further investigations showed the corrosion performance of multiphase samples in 3.5% NaCl solution was inferior to that of wrought NiTi alloy. In contrast, the corrosion resistance of multiphase samples in 0.1 M H₂SO₄ solution was comparable to that of wrought NiTi alloy.     

Microstructure stability and mechanical properties of an age-hardenable Ni–Mo–Cr alloy subjected to long-term exposure to elevated temperature

This paper characterizes the microstructure and mechanical properties of a nickel-based superalloy with a nominal composition of Ni–25Mo–8Cr (wt.%) after long-term exposures to elevated temperatures. The alloy is strengthened by long-range-ordered precipitates of an oI6 metastable phase with the Ni₂(Mo,Cr) stoichiometry. The alloy was annealed at 650 °C for 1000, 2000 and 4000 h, after it had been plastically deformed in order to accelerate diffusion processes occurring at elevated temperature and consequently to ease the formation of stable phases. The microstructure was characterized using TEM, SEM and X-ray phase analyses; mechanical properties were measured in tensile tests.It has been determined that the alloy loses its phase stability upon plastic deformation and subsequent long-term annealing at 650 °C. The microstructure, composed initially of a dispersed Ni₂(Mo,Cr) strengthening phase in a Ni-based solid solution, decomposes during annealing into a mixture of Ni₃Mo- and Ni₄Mo-type phases, Mo-lean Ni-based solid solution and a complex intermetallic P phase. The dominant new phase is a plate-shaped Ni₃Mo-type phase while the P phase appears as singular small precipitates. The Ni₃Mo phase is formed mainly in regions of highly localized deformation, e.g., in shear bands, and only occasionally nucleates in regions where the deformation was relatively uniform (dislocations or twins in one system). Regions adjacent to the plates of the Ni₃Mo phase are recrystallized and free from an Ni₂(Mo,Cr) strengthening phase. Changes in microstructure of the deformed alloy during long time annealing at 650 °C result in the decrease in the yield strength as well as tensile elongation at both room temperature and 650 °C. A significant decrease in elongation at 650 °C occurs only in specimens tested in air but not those tested in vacuum.     

Multiple-step martensitic transformations in the Ni51Ti49 single crystal

Multiple-step martensitic transformations of an aged Ni₅₁Ti₄₉ single crystal using calorimetric method were investigated. Results show that for short aging times (10–45 min) multiple-step martensitic transformations on cooling occur in two steps. Applying intermediate aging times (1.25–4 h) results in three steps and long aging times (more than 8 h) lead to two-step martensitic transformations again. This behavior has not been recognized in NiTi single crystals in literatures. It can be related to the heterogeneity of composition and stress fields around Ni₄Ti₃ precipitates.     

Diffusion bonding of titanium alloy to micro-duplex stainless steel using a nickel alloy interlayer: Interface microstructure and strength properties

In the present study, diffusion bonding of titanium alloy and micro-duplex stainless steel with a nickel alloy interlayer was carried out in the temperature range of 800–950 °C for 45 min under the compressive stress of 4 MPa in a vacuum. The bond interfaces were characterised by scanning electron microscopy, electron probe microanalyzer and X-ray diffraction analysis. The layer wise Ni₃Ti, NiTi and NiTi₂ intermetallics were observed at the nickel alloy/titanium alloy interface and irregular shaped particles of Fe₂₂Mo₂₀Ni₄₅Ti₁₃ was observed in the Ni₃Ti intermetallic layer. At 950 °C processing temperature, black island of β-Ti phase has been observed in the NiTi₂ intermetallics. However, the stainless steel/nickel alloy interface indicates the free of intermetallics phase. Fracture surface observed that, failure takes place through the NiTi₂ phase at the NiA–TiA interface when bonding was processed up to 900 °C, however, failure takes place through NiTi₂ and β-Ti phase mixture for the diffusion joints processed at 950 °C. Joint strength was evaluated and maximum tensile strength of ∼560 MPa and shear strength of ∼415 MPa along with ∼8.3% ductility were obtained for the diffusion couple processed at 900 °C for 45 min.     

Influence of compression aging on phase transition temperatures in single crystalline Ni‐rich NiTi shape memory alloys

The present paper considers the phase transition behavior of a single crystal Ni‐rich NiTi alloy which was compression aged to produce one single family of Ni₄Ti₃ precipitates. The single crystal material was produced in a two stage process. Polycrystalline material was first melted under an inert atmosphere and remelted when single crystals were produced. Compression aging treatments in <111>‐orientation were carried out in order to suppress all but one family of Ni₄Ti₃‐precipitates which nucleate and grow on {111}‐planes of the B2 matrix. The objective of this study is to investigate the influence of Ni₄Ti₃‐precipitates on the martensitic transformation behavior. It was previously shown that grain boundaries provide heterogeneous nucleation sites for the formation of Ni₄Ti₃; this results in heterogeneous microstructures which undergo multiple step martensitic transformations. Single crystals avoid grain boundaries and the present study aims at clarifying how homogeneously precipitated particles affect martensitic transformations.     

Influence of second phases on fatigue crack growth behavior of nickel aluminum bronze

The influence of phases with different morphology and mechanical properties on fatigue crack growth behavior in nickel aluminum bronze (NAB) has been investigated. Annealing at 675 °C and normalizing at 920 °C heat treatments were used to produce different morphologies and fractions of second phases. This analysis shows that the coarse dendritic κ_II particles and κ_III lamellae as hard brittle phases in as-cast and annealed NAB have an accelerative effect on the fatigue crack propagation where by cracks propagate through α and κ_II/κ_III interface. Fatigue cracks in normalized NAB prefer to propagate through the ductile α grains, form fatigue striations and have the lowest crack growth rate. The uniformly distributed, fine κ_IV precipitates in the α grains improves fatigue crack growth resistance. This work identifies the role of NAB second phases on propagation of fatigue cracks, and provides suitable heat treatment for improving fatigue crack resistance in terms of controlling second phase type, distribution and percentage.     

Effect of Solution Treatment on Thermal Conductivity of Porous NiTi Shape Memory Alloy

In this study, a new solution treatment “solution treatment under loading” was applied to a porous Ni–50 at.%Ti shape memory alloy (SMA), which was fabricated by self-propagating high-temperature synthesis (SHS), to explore the microstructural improvement regarding single-phase NiTi. The effects of solution treatment under loading and without loading on the phase constituent and thermal conductivity were investigated and discussed. The phase constituent and thermal conductivity of the specimens considerably changed with solution treatment under loading, but they were not affected significantly with solution treatment without loading. Intermetallic phases such as B19′(NiTi), Ti₂Ni, and Ni₄Ti₃ disappeared, the density of the B2(NiTi) phase increased with solution treatment under loading, and thus the thermal conductivity was increased. It was also seen that the thermal conductivity of porous NiTi was less than that of solid NiTi.     

Effect of Ba deoxidation on oxygen content in NiTi alloys and non-metallic inclusions

The effect of Ba deoxidation on the oxygen content and non-metallic inclusions in NiTi alloys was investigated. The NiTi alloy melt was held in a CaO crucible at 1673 K under Ar gas flow for 0–600 s. Metallic Ba (0.5–1.5 mass% with respect to NiTi alloy melt) was added to the melt. After melting, oxygen content of 710 and 330 mass ppm decreased to 210 and 130 mass ppm, respectively, after a holding time of 300 s after the addition of Ba. The oxygen in the melts was reduced to as low as 64 mass ppm after a second Ba deoxidation. The non-metallic inclusions observed in the NiTi alloys were the Ti₄Ni₂O _X and Ti(C,N,O) _X types. Decreased oxygen content through Ba deoxidation caused a change in the main phase of the non-metallic inclusions from the Ti₄Ni₂O _X type to the Ti(C,N,O) _X type, and a decrease in the area percents of the non-metallic inclusions.     

Structural and chemical characterization of precipitates in Al-2024/TiC composites

Structural characterizations based on transmission electron microscopy observations were carried out on as-fabricated and heat-treated Al-2024/TiC composites. These composites types reinforced with TiC particles were produced with a pressureless melt infiltration route at 1200 °C for 2 h under argon atmosphere. The composites were heat-treated at 530 °C during 150 min, cold-water quenched and subsequently artificial and natural aged at 190 °C for 12 h in an argon environment and at room temperature for 96 h, respectively. Different precipitate types were obtained and they were identified as CuAl₂, Al₃Ti, Ti₃AlC and Ti₃Al. Most of the precipitates were found to be uniformly distributed in the matrix and some regions show precipitates which have a cubic morphology (Ti₃Cu). High-resolution electron microscopy images were partially used for the characterization of the precipitates in these composites.     

Effect of aging on martensitic transformation behavior of Ti48.8Ni50.8V0.4 alloy

In this article, the effect of aging on martensitic transformation of Ti_48.8Ni_50.8V_0.4 alloy was investigated. The results show that the martensitic transformation of the solution-treated shape memory alloy is a typical single-stage transformation process. The transformation temperatures of the samples aged at different temperatures for 0.5 h were lower as compared to that of the solution-treated alloy. With the increase of aging temperature, the transformation temperatures increase. After aging at 500 °C, the samples exhibit a multiple-stage transformation. The samples after aging at 500 °C for more than 5 h resulted in the transformation sequence of A → R→M1 and A → M2 upon cooling and M2 → A and M1 → A upon heating.     

Fracture of precipitated NiTi shape memory alloys

The fracture mechanisms in single crystal and polycrystalline Ti-50.8at%Ni shape memory alloys containing Ti₃Ni₄ precipitates are studied using the scanning electron microscope (SEM). Aged materials with three different precipitate sizes (50 nm, 150 nm, and 400 nm), which have interfaces ranging from semi-coherent to incoherent, are considered. The mechanisms of material fracture identified in the single crystal NiTi are: 1. Nucleation, growth, and coalescence of voids from the Ti₃Ni₄ precipitates, 2. Cleavage fracture on {100} and {110} crystallographic planes, 3. Nucleation, growth, and coalescence of voids from fractured Ti-C inclusions. Cleavage and ductile tearing mechanisms also operate in polycrystalline NiTi, however, since the Ti-C inclusions are an artifact of single crystal growth processes, mechanism 3 was not discovered in the polycrystalline materials. Cleavage fracture and ductile tearing are found to act in conjunction, with the relative dominance of one over the other depending on the local precipitate size and concentration. As the Ti₃Ni₄ precipitate size increases to about 400 nm, the overall fracture is dominated by failure mechanism 1, and the cleavage markings become diffuse. Finally, we assert that the high tensile ductility of drawn NiTi polycrystals is due partially to the fact that drawn bar and wire stock usually have a strong {111} fiber texture. Such a texture promotes the initiation of the transformation at low stresses and concurrently prevents primary cleavage on the {100} or {110} planes.     

Phase transformation and microstructure and mechanical properties of as cast NiTiRe shape memory alloys

Abstract

The microstructure, martensitic transformation and mechanical properties of as cast Ni₅₂Ti_48?xRe_x shape memory alloys (SMAs) were investigated. The microstructure of these alloys consists of B19′ martensite phase as a matrix and B2 austenite in small percentages in addition to some precipitations of NiTi intermetallic compounds. There are two types of NiTi precipitates: the first one is Ti₂Ni, which can be seen in the all microstructures of the three alloys, and the other is Ni₂Ti, which is found only in the microstructure of Ni₅₂Ti_47·7Re_0·3 alloy. According to differential scanning calorimetry measurements, one stage of transformation reaction B2 to B19′ accompanied with forward and backward martensitic transformation was observed. The martensitic peak and the austenitic peak were increased with the addition of rhenium. Both are increased as the number of valence electron per atom increase and the valence electron concentration decrease. Hardness measurements of Ni₅₂Ti_48?xRe_x SMAs are improved by the Re additions.     

Effect of precipitation on two-way shape memory effect of melt-spun Ti50Ni25Cu25 ribbon

In the present work, effect of precipitation on two-way shape memory effect and deformation induced martensite stabilization of Ti₅₀Ni₂₅Cu₂₅ ribbon was investigated. The two-way shape memory effect developed as a result of martensite deformation is affected by formation of precipitates. After straining martensite to 5.5%, a two-way shape memory strain of 1.25% was obtained in a sample annealed at 800 °C for 300 s that does not introduce B11 precipitates. Results also show that the deformed Ti₅₀Ni₂₅Cu₂₅ ribbon containing precipitates has multiple-stage transformation characteristics during the first heating.     

Preparation of NixTiy compound via Ni plating followed by thermal treatment of Ti powder

In this research, Ni_xTi_y compound was prepared by thermal treatment of Ni-plated Ti powder. For this purpose, Ti powder was plated in an electroless Ni bath for various times (120, 225, 300, and 720?min). Hydrazine hydrate was used as a reductant for the deposition of pure Ni on the Ti particles. The plated powder (225?min) was heat treated under argon atmosphere to achieve Ni_xTi_y powder. Finally, the heated/plated powder was pressed by CIP followed by sintering at 980°C for prepare the Ni_xTi_y bulk sample. The plated powders as well as sintered one were characterized using scanning electron microscopy, energy dispersive spectrometer, X-ray fluorescence, X-ray diffraction and differential scanning calorimetric. The NiTi₂, NiTi, and Ni₃Ti phases were detected in the XRD patterns of heated/plated Ti powder. According to DSC data, the heated/plated Ti powder showed reversible martensitic transformation at temperature range of ?38.0°C to +38.1°C, while sintered/heated/plated Ti powder displayed reversible transformation at temperature range of 16.0°C–15.4°C.