Phase formation in Ti/Ni dissimilar welds

We explore phase formation in Ti/Ni dissimilar welds using a combination of microscopy and composition analysis (TEM, SEM and EDS). Main microstructural features are NiTi dendrites and Ti₂Ni grains in the inter-dendritic space. The high temperature B2 phase of NiTi is found to transform to trigonal ‘R’-phase, B19^′martensite, and rhombohedral Ni₄Ti₃ phase; these different transformation products highlight the composition inhomogeneity in the parent B2 phase and probable non-equilibrium solidification events during rapid cooling of the weld. Solidification sequence of NiTi and Ti₂Ni was found to vary depending on local conditions in the weld. Formation of impurity nitride phases of titanium is observed which signify incomplete shielding during welding.     

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.     

Porous NiTi shape memory alloys produced by SHS: microstructure and biocompatibility in comparison with Ti2Ni and TiNi3

Shape memory alloys based on NiTi have found their main applications in manufacturing of new biomedical devices mainly in surgery tools, stents and orthopedics. Porous NiTi can exhibit an engineering elastic modulus comparable to that of cortical bone (12–17 GPa). This condition, combined with proper pore size, allows good osteointegration. Open cells porous NiTi was produced by self propagating high temperature synthesis (SHS), starting from Ni and Ti mixed powders. The main NiTi phase is formed during SHS together with other Ni–Ti compounds. The biocompatibility of such material was investigated by single culture experiment and ionic release on small specimen. In particular, NiTi and porous NiTi were evaluated together with elemental Ti and Ni reference metals and the two intermetallic TiNi₃, Ti₂Ni phases. This approach permitted to clearly identify the influence of secondary phases in porous NiTi materials and relation with Ni-ion release. The results indicated, apart the well-known high toxicity of Ni, also toxicity of TiNi₃, whilst phases with higher Ti content showed high biocompatibility. A slightly reduced biocompatibility of porous NiTi was ascribed to combined effect of TiNi₃ presence and topography that requires higher effort for the cells to adapt to the surface.     

High damping NiTi/Ti3Sn in situ composite with transformation-mediated plasticity

The concept of transformation-induced plasticity effect is introduced in this work to improve the plasticity of brittle intermetallic compound Ti₃Sn, which is a potent high damping material. This concept is achieved in an in situ NiTi/Ti₃Sn composite. The composite is composed of primary Ti₃Sn phase and (NiTi + Ti₃Sn) eutectic structure formed via hypereutectic solidification. The composite exhibits a high damping capacity of 0.075 (indexed by tan δ), a high ultimate compressive strength of 1350 MPa, and a large plasticity of 27.5%. In situ synchrotron high-energy X-ray diffraction measurements revealed clear evidence of the stress-induced martensitic transformation (B2 → B19′) of the NiTi component during deformation. The strength of the composite mainly stems from the Ti₃Sn, whereas the NiTi component is responsible for the excellent plasticity of the composite.     

Porous high entropy (Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)B2: A novel strategy towards making ultrahigh temperature ceramics thermal insulating

Transition metal diborides based ultrahigh temperature ceramics (UHTCs) are characterized by high melting point, high strength and hardness, and high electrical and thermal conductivity. The high thermal conductivity arises from both electronic and phonon contributions. Thus electronic and phonon contributions must be controlled simultaneously in reducing the thermal conductivity of transition metal diborides. In high entropy (HE) materials, both electrons and phonons are scattered such that the thermal conductivity can significantly be reduced, which opens a new window to design novel insulating materials. Inspired by the high entropy effect, porous HE (Zr_0.2Hf_0.2Nb_0.2Ta_0.2Ti_0.2)B₂ is designed in this work as a new thermal insulting ultrahigh temperature material and is synthesized by an in-situ thermal borocarbon reduction/partial sintering process. The porous HE (Zr_0.2Hf_0.2Nb_0.2Ta_0.2Ti_0.2)B₂ possesses high porosity of 75.67%, pore size of 0.3–1.2 μm, homogeneous microstructure with small grain size of 400–800 nm, which results in low room temperature thermal diffusivity and thermal conductivity of 0.74 mm² s⁻¹ and 0.51 W m⁻¹ K⁻¹, respectively. In addition, it exhibits high compressive strength of 3.93 MPa. The combination of these properties indicates that exploring porous high entropy ceramics such as porous HE (Zr_0.2Hf_0.2Nb_0.2Ta_0.2Ti_0.2)B₂ is a novel strategy in making UHTCs thermal insulating.     

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.     

Effects of plate thickness on reverse martensitic transformation of prestrained NiTi/NiTi alloy

In this paper, differential scanning calorimeter (DSC) was used to study the effects of predeformation and plate thickness on the reverse martensitic transformation of explosively welded NiTi/NiTi alloy. Results showed that there was a constraint between Ni_50.4Ti (NiTi-1) and Ni_49.8Ti (NiTi-2), which led to that the thickness of NiTi-1 or NiTi-2 strongly affected the reverse martensitic transformation behavior because residual stress variations in thickness wound enable bias force to be built inside the composite. The DSC measurements showed that after deformation, the reverse martensitic transformation temperature of the composite was increased with the increasing thickness of NiTi-2. Also, the XRD results revealed that the microstructure of NiTi/NiTi alloy changed from B2 phase to B19’ phase along the thickness direction.     

Thermal Diffusivity of Metallic Thin Films: Au, Sn, Mo, and Al/Ti Alloy

The thermal diffusivity of Au, Sn, Mo, and Al_0.97Ti_0.03 alloy thin films, which are commonly used in microelectromechanical (MEMs) system applications, is measured by two independent methods — the ac calorimetric and photothermal mirage methods. Both methods yield similar results of the thin-film thermal conductivity, but the uncertainty of the mirage technique is found to be relatively large because of the large temperature increase during the measurement. The measured thermal diffusivities of the thin films are generally lower than those of the same bulk material. Especially, the Al_0.97Ti_0.03 thin film shows a pronounced thermal conductivity drop compared with bulk Al, which is believed to be mainly due to impurity scattering. Comparison of the thermal conductivity with the electrical conductivity measured by the standard four-probe technique indicates that the relation of thermal and electrical conductivities follows the Wiedemann–Franz law for the case of Au and Sn thin films. However, the Lorentz number is significantly larger than the theoretical prediction for the case of Al_0.97Ti_0.03 and Mo thin films.     

The effect of the combustion channels on the compressive strength of porous NiTi shape memory alloy fabricated by SHS as implant material

Porous NiTi shape memory alloy (SMA) with ideal porosity and high compressive strength as an implant material was fabricated by self-propagating high-temperature synthesis (SHS). In this study, a new ignition technique “high voltage electric arc” was used to ignite the green specimens and control the orientation of combustion channels which effect compressive strength. It was determined that the compressive strength of specimens was increased when the combustion channels were parallel along the specimen axis, and the compressive strength was decreased when the combustion channels were perpendicular to specimen axis. The desired phases such as B2(NiTi) and B19′ (NiTi) were dominant while the second phases (Ni₄Ti₃ and NiTi₂) in small amount. The undesired phases (such as pure Ni and Ni₃Ti) for biocompatibility are not found in the structure. The transformation temperatures were higher for medical applications by heat treatment and partly decreased at every next thermal cycle where the heating rate of the specimen was increased.     

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 heat treatment on the fatigue life of a laser-welded NiTi alloy wire

The influence of heat treatment on the rotating–bending fatigue of a laser-welded superelastic NiTi alloy wire was investigated. In comparison with the as-welded and annealed at 500 °C for 1 h specimens, the specimen annealed at 400 °C for 1 h shows the best fatigue resistance. The difference of fatigue life in the heat-treated materials is attributed to the different Ti₃Ni₄ precipitate size. Heat treatment to produce smaller coherent precipitates (10 nm) improves the fatigue resistance of the laser-welded NiTi alloy wire. When the Ti₃Ni₄ precipitates become coarse, the fatigue resistance decreases.     

Effect of passivation and surface modification on the dissolution behavior and nano-surface characteristics of Ti-6Al-4V in Hank/EDTA solution

The aim of the present study was to investigate the effects of passivation treatment (34% nitric acid passivation, 400 ^∘C heated in air, and aged in 100 ^∘C de-ionized water) and surface modification (2 hr and 8 hr vacuum-brazed treatments) on the ion dissolution and nano-surface characteristics of Ti-6Al-4V exposed in Hank's solution with 8.0 mM ethylene diamine tetra-acetic acid (EDTA) at 37 ^∘C. The results indicated that the original nano-surface characteristics and microstructure would influence the ion dissolution but not change the capability of the Ca and P adsorption upon immersion. Of the three passivated treatments, 400 ^∘C thermal treatment for both 2 hr brazed Ti-6Al-4V (B2) and 8 hr brazed Ti-6Al-4V (B8) exhibits a substantial reduction in the constituent release compared to the acid passivated and water aged treatment, because the thicker thickness and rutile structure of surface oxide could provide the better dissolution resistance for 400 ^∘C-treated specimens. Moreover, the reduced Ti₂Cu and increased α -titanium structure in B8 specimen could also improve ion dissolution resistance in comparison with B2 specimen. After soaking in Hank/EDTA solution, the adsorbed non-elemental Ca and P for all groups of specimens were observed by XPS analysis, and the AES depth-profile analysis indicate that the oxide films of all groups of specimens thicken with the longer immersion periods. The increasing oxide thickness may be the factor in the improved dissolution resistance at the longer immersion periods. The relation between lower dissolution rate and thicker oxide films were observed for all groups of specimens. The results suggest that the dissolution kinetics was governed by the metal ion transport through the oxide film in this study.     

Wettability of Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript> by Ag–Cu and Ag–Cu–Ti melts

Recently, the ternary carbide Ti₃SiC₂ has gained much attention due to its unique characteristics combining the properties of metals and ceramics (i.e., a low density, decent thermal and electrical conductivities, an excellent thermal shock resistance, a good machinability, damage tolerance, low friction, and so on). This study describes an investigation of the wettability in high vacuum of bulk Ti₃SiC₂ by a classical braze alloy based on the Ag–Cu–Ti system. Two techniques, i.e., the sessile drop and dispensed drop methods, were utilized. The results indicated that spreading kinetics is controlled by deoxidation kinetics of Ti₃SiC₂ surface under vacuum. The final contact angle on clean Ti₃SiC₂ is very small (~10°), testifying the development of strong, metallic interactions across the liquid–solid interface. The reactivity between the ternary carbide and the liquid phase during isothermal heating at 800 °C was also considered.     

High-porosity NiTi superelastic alloys fabricated by low-pressure sintering using titanium hydride as pore-forming agent

Porous NiTi shape memory alloys (SMAs) were successfully fabricated by low-pressure sintering (LPS), and the pore features have been controlled by adjusting the processing parameters. The porous NiTi SMAs with high porosity (45%) and large pore size (200–350 μm) can be prepared by LPS using TiH_1.5 as pore-forming agent. These alloys exhibit isotropic pore structure with three-dimensional interconnected pores. The porous NiTi SMA produced by LPS exhibits superelasticity and mechanical properties superior to that by conventional sintering.     

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.     

High porosity and low thermal conductivity high entropy (Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)C

Porous ultra-high temperature ceramics (UHTCs) are promising for ultrahigh-temperature thermal insulation applications. However, the main limitations for their applications are the high thermal conductivity and densification of porous structure at high temperatures. In order to overcome these obstacles, herein, porous high entropy (Zr_0.2Hf_0.2Ti_0.2Nb_0.2 Ta_0.2)C was prepared by a simple method combing in-situ reaction and partial sintering. Porous high entropy (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C possesses homogeneous microstructure with grain size in the range of 100–500 nm and pore size in the range of 0.2–1 μm, which exhibits high porosity of 80.99%, high compressive strength of 3.45 MPa, low room temperature thermal conductivity of 0.39 W·m⁻¹ K⁻¹, low thermal diffusivity of 0.74 mm²·s⁻¹ and good high temperature stability. The combination of these properties renders porous high entropy (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C promising as light-weight ultrahigh temperature thermal insulation materials.     

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.     

Processing of composites in the Ti-Si-C system

New composites utilizing Ti₃SiC₂ as the matrix phase have been synthesized. The thermal and kinetic aspects of the phase formation in the Ti-Si-C system were examined. Standard dry powder processing techniques, pressureless sintering and hot pressing (both under inert atmosphere) were employed to process the sample materials. The phase identification was performed by X-ray powder diffraction (XRPD) analysis. The ratio of the volume fractions of the reinforcement phases SiC and/or TiC _x to the matrix phase Ti₃SiC₂ have been measured and tailored over a wide range by means of controlling the stoichiometry of the starting elemental powders and varying the heat treatment. It was found that the Si tends to evaporate at elevated temperatures during sintering. The loss of Si is the likely reason why it is difficult to form the single phase Ti₃SiC₂. Incorporating this observation into the composite formation, a method to form in-situ composites is proposed. The volume fraction of the constituent phase in the composite can be controlled by varying the processing parameters.     

Preparation of K<Subscript>2</Subscript>Ti<Subscript>6</Subscript>O<Subscript>13</Subscript>/TiO<Subscript>2</Subscript> bio-ceramic on titanium substrate by micro-arc oxidation

K₂Ti₆O₁₃/TiO₂ bio-ceramic coatings are prepared successfully by micro-arc oxidation on titanium substrate in pure KOH electrolyte solution. The coating is prepared at various applied current density (150–500 mA/cm²) and in KOH electrolyte with different concentrations (0.5–1.2 mol/L). The composition and surface morphologies of coatings are strongly dependent on the applied current density and the electrolyte concentration. On the condition of lower current density and electrolyte concentration, K₂Ti₆O₁₃ phase almost cannot be formed. The phase is mainly composed of rutile and K₂Ti₆O₁₃ with increasing current density and electrolyte concentration. The surface morphologies are composed of whiskers and porous structures. The ability of K₂Ti₆O₁₃/TiO₂ bio-ceramic films inducing apatite deposition is evaluated by soaking it in biological model fluids. The results show the K₂Ti₆O₁₃/TiO₂ bio-ceramic coatings possess excellent capability of inducing bone-like apatite to deposit.     

Microstructure development during dissimilar welding: Case of laser welding of Ti with Ni involving intermetallic phase formation

Development of solidification microstructure in a laser welded Ti/Ni dissimilar binary couple is presented. At the fusion interfaces in both Ti and Ni, growth of the base metal grains into the weld pool is inhibited by the presence of composition gradients in the melt. Ti₂Ni dendrites grow toward the base metal at the Ti fusion interface. In the Ni side, appearance of a nickel solid solution phase is followed by layers of Ni₃Ti, Ni₃Ti+NiTi eutectic, and NiTi. NiTi dendrites and Ti₂Ni constitute the microstructure in the middle of the weld. Isolated titanium dendrites are observed throughout the weld, but their preferential occurrence toward the top surface of the weld is more prominent. Results are rationalised on the basis of interplay of the transport processes in the weld with the thermodynamics of the Ti-Ni system.