Intermetallic compounds formed during brazing of titanium with aluminium filler metals

The effect of aluminium filler metal composition on the formation of AI-Ti intermetallic compounds was investigated in brazed aluminium-to-titanium (Al/Ti) joints and titanium-totitanium (Ti/Ti) joints. The clearance filling ability was also studied. In Ti/Ti joints, the thickness of the intermetallic compound layer was strongly dependent on the aluminium filler metal composition, whereas the clearance filling ability was independent of the composition. The maximum intermetallic compound layer thickness was observed in 99.99% highly pure aluminium filler metal; therefore all additional elements reduced the layer thickness. Above all, the addition of 0.8% Si greatly reduced the thickness. After brazing at 680° C for 3 min, the intermetallic compound formed by Al-0 to 0.8% Si filler metal was found to be of type Al₃Ti. Other compounds, of types Ti₉Al₂₃ and Ti₇Al₅Si₁₂, were also found in joints brazed by Al-3 to 10% Si filler metals. AI-0.8% Si filler metals maintained a higher joint strength than pure aluminium filler metal under brazing conditions of high temperature and long heating time. In Al/Ti joints, AI-Cu-Sn and AI-Cu-Ag filler metal mainly formed Al₃Ti, and Al-10Si-Mg filler metal mainly formed Ti₇Al₅Si₁₂ at the brazed interface of the titanium side after brazing at 600 to 620° C.     

Element distribution and phase constitution of Al2O3-TiC/W18Cr4V vacuum diffusion bonded joint

Vacuum diffusion bonding between Al₂O₃-TiC ceramic composites and W18Cr4V tungsten-based tool alloy has been carried out by using Ti/Cu/Ti multi-interlayer. Element distribution near the Al₂O₃-TiC/W18Cr4V interface was discussed and fracture morphology was analyzed using electron probe microanalysis. Additionally, phase constitutions of the Al₂O₃-TiC/W18Cr4V joint were determined by X-ray diffraction. The results indicate that Ti-rich layers are formed near both Al₂O₃-TiC and W18Cr4V. The Ti-rich layer near Al₂O₃-TiC helps to wet the Al₂O₃-TiC surface. The Ti-rich layer near W18Cr4V can restrain the formation of Fe-Ti intermetallic compounds in the diffusion transition zone. Residual Cu in the diffusion transition zone can act as a stress releasing zone. The structures of interfacial phases are identified as follows: Al₂O₃-TiC/TiO + Ti₃Al/Cu + CuTi/TiC layer/mixed layer of Fe₃W₃C, Cr₂₃C₆ and α-Fe/W18Cr4V. The fracture morphology of Al₂O₃-TiC/W18Cr4V joint appears brittle features and the failure occurs within the Al₂O₃-TiC ceramic.     

Reactivity and interface characteristics of titanium-alumina composites

The reaction kinetics between -Ti alloys and single crystal sapphire, the phase composition and morphology of the reaction-zone, and the phase compatibility in the system Ti-Al-O were investigated as part of a study to determine the feasibility of fabricating useful Al₂O₃-reinforced titanium matrix composites. In the temperature range 650 to 1000° C titanium reduces Al₂O₃ to form a complex reaction layer consisting of two distinct zones; an inner zone adjacent to the Al₂O₃ of a TiO phase containing isolated particles of (Ti, Al)₂O₃, presumably, and an outer zone of a Ti₃Al phase adjacent to the Ti matrix. The isothermal growth of the reaction layer follows a parabolic rate law. The temperature dependence of the rate constants fits an Arrhenius equation yielding activation energies of 50 to 52 kcal/mol. The high Al alloys, except Ti-6Al-2Sn-4Mo-2Zr, reacted more rapidly than pure Ti indicating that Al diffusion through the reaction zone may be the rate-limiting step.     

Interface microstructure and shear strength in the diffusion brazing joint of TiC–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ceramic composite with W18Cr4V tool steel

The diffusion brazing of TiC–Al₂O₃ ceramic composite with W18Cr4V tool steel is realized by using Ti and Cu filler metals at 1500°K for 1 h under a pressure of 15 MPa in a vacuum of 10^–5 Pa. Thus, a brazed TiC–Al₂O₃/W18Cr4V joint was obtained with an interface shear strength of up to 105 MPa. The microstructural characteristics of the TiC–Al₂O₃/ W18Cr4V joint are studied by using a scanning electron microscope (SEM) with energy-dispersion spectroscopy (EDS) and X-ray diffractometer (XRD). The results indicated that the interface layer with a thickness of 90 μm was formed between TiC–Al₂O₃ and W18Cr4V steel. The Ti and Cu filler metals were completely fused and diffused to react with Al and C from the substrates, whereas Ti₃Al, Ti₃AlC₂, TiC, and CuTi₂ were produced in the TiC–Al₂O₃/W18Cr4V joint.     

Interface compounds formed during the diffusion bonding of Al2O3 to Ti

The interfacial reaction products of Ti/Al₂O₃ joints obtained in the context of real diffusion bonding technology were investigated by means of X-ray diffraction analysis, X-ray photoelectron spectroscopy, and transmission electron microscopy. Some Ti reacted with Al₂O₃ giving titanium oxides, but the main mass transport occurred into the bulk Ti due to Al₂O₃ dissolution. The formation of a Ti[Al, O] solid solution followed by a order/disorder reaction yielded Ti₃Al. Further Al enrichment at the interface could lead to the formation of TiAl, which was not observed in the present work due to either the short residence time at the bonding temperatures or to its lower oxygen solubility. For joints obtained at 800°C and shear test fractured it was ascertained that the crack always propagated within the Ti₃Al layer.     

5A06/TA2 diffusion bonding with Nb diffusion-retarding layers

The structure and performance of 5A06/TA2 diffusion bonding joints with or without Nb diffusion-retarding layers were studied by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and shear strength measurement. The results showed that a diffusion reaction occurred and Al₁₈Ti₂Mg₃ was formed, which markedly decreased the joint strength, and the highest shear strength of 5A06/TA2 joint in direct bonding was 83 MPa. The Nb interlayer impeded the diffusion of Mg atoms from the Al side to the Ti side and also retarded the diffusion of Ti atoms from the Ti side to the Al side, which was acting as a diffusion-retarding layer. The joint strengths were increased by the Nb diffusion-retarding layers, and the highest shear strength reached 105 MPa. When Ti diffused across the Nb layer and achieved saturation nearby the interface with Al alloy, the diffusion reaction of Ti, Al and Mg occurred and Al₁₈Ti₂Mg₃ appeared which decreased the joint strength.     

Interfacial reactions between titanium film and single crystal α-Al2O3

Titanium is commonly used to join metals and ceramics by active metal brazing methods. In this work, titanium was sputter deposited on to single-crystal -Al₂O₃ substrates and the interfacial reactions between the titanium film and the Al₂O₃ substrate were studied. Al₂O₃ was reduced by titanium when samples were annealed at 973 and 1173 K for 300 s in an argon gas flow. Metallic aluminium was produced at the interface, and this diffused from the interface into the titanium film. At 1173 K, the intermetallic compound Ti₃Al and the intermediate titanium oxides, such as Ti₂O and TiO, were formed. The Al⁰ diffusion is important in stimulating interfacial reactions.     

Fabrication of multilayered titanium aluminide sheets by self-propagating high-temperature synthesis reaction using hot rolling and heat treatment

This study is concerned with the fabrication of multilayered and bulk Ti aluminide sheets by self-propagating high-temperature synthesis (SHS) reaction using hot rolling and heat treatment. A multilayered Ti/Al sheet was prepared by stacking thin Ti and Al sheets alternatively. When this sheet was hot-rolled and heat-treated at 1000°C, a multilayered sheet composed of Ti₃Al and TiAl was made through the process of formation and growth of intermetallic phases at Ti/Al interfaces and porosity reduction. A bulk Ti aluminide sheet having a lamellar structure of TiAl and Ti₃Al was also fabricated successfully by heat treatment at 1400°C.     

Quasiparticle Freeze-Out in Superconducting Tunnel Junction X-ray Detectors with Killed Base Electrode

The current–voltage characteristics of superconducting tunnel junction (STJ) X-ray detectors were measured in the temperature range from 4.2 to 0.1 K. The freeze-out of the thermal tunneling current was compared between an STJ detector with a traditional Nb/Al/Al \(_{2}\) O \(_{3}\) /Al/Nb layer structure and a Ti/Nb/Al/Al \(_{2}\) O \(_{3}\) /Al/Nb/NbN detector whose low-gap Ti film kills the X-ray response of the base electrode. The current decrease and the linear low-temperature I(V) characteristics for the detector with the killed electrode can be qualitatively explained by tunneling current contributions from the subgap states of the Ti film. The data are analyzed on the basis of the proximity theory in the dirty limit.     

Influence of sol–gel derived Al2O3 film on the oxidation behavior of a Ti3Al based alloy

Al₂O₃ thin films were deposited on a Ti₃Al based alloy (Ti–24Al–14Nb–3V–0.5Mo–0.3Si) by sol–gel processing. Isothermal oxidation at temperatures of 900–1000 °C and cyclic oxidation at 800–900 °C were performed to test their effect on the oxidation behavior of the alloy. Results of the oxidation tests show that the oxidation parabolic rate constants of the alloy were reduced due to the applied thin film. This beneficial effect became weaker after longer oxidation time at 1000 °C. TiO₂ and Al₂O₃ were the main phases formed on the alloy. The thin film could promote the growth of Al₂O₃, causing an increase of the Al₂O₃ content in the composite oxides, sequentially decreased the oxidation rate. Nb/Al enriched as a layer in the alloy adjacent to the oxide/alloy interface in both the coated and uncoated alloy. The coated thin film decreased the thickness of the Nb/Al enrichment layer by reducing the scale growth rate.     

Characterization of the interaction between molten titanium alloy and Al2O3

Pure titanium and Ti6Al4V alloys with single-crystal Al₂O₃ rod cores were prepared at 1740 °C and 1.2 atm Ar for 30 min. Aluminium diffusion takes place from Al₂O₃ into the titanium region for both Ti/Al₂O₃ and Ti6Al4V/Al₂O₃ interaction couples and results in the formation of an ordered ₂ phase (Ti3Al) in the titanium region adjacent to interfaces, even though there is no visible interaction product at interfaces.     

Effect of nano-CeO2 on microstructure properties of TiC/TiN+TiCN-reinforced composite coating

TiC/TiN+TiCN-reinforced composite coatings were fabricated on Ti–6Al–4V alloy by laser cladding, which improved surface performance of the substrate. Nano-CeO₂ was able to suppress crystallization and growth of crystals in the laser-cladded coating to a certain extent. With the addition of proper content of nano-CeO₂, this coating exhibited fine microstructure. In this study, Al₃Ti+TiC/TiN+nano-CeO₂ laser-cladded coatings have been studied by means of X-ray diffraction and scanning electron microscope. X-ray diffraction results indicated that Al₃Ti+TiC/TiN+nano-CeO₂ laser-cladded coating consisted of Ti₃Al, TiC, TiN, Ti₂Al₂₀Ce, TiC_0·3N_0·7, Ce(CN)₃ and CeO₂, this phase constituent was beneficial in increasing microhardness and wear resistance of Ti–6Al–6V alloy.     

Oxidation behavior of (Ti1 − xAlx)N films perpared by r.f. reactive sputtering

(Ti, Al)N films have drawn much attention as alternatives for TiN coatings, which are oxidized easily in air above 500 °C. We have investigated the effect of Al content on the oxidation resistance of (Ti_{1 − x}Al_x)N films prepared by r.f. reactive sputtering.(Ti_{1 − x}Al_xN films (O ≤ x ≤ 0.55) were deposited onto fused quartz substrates by r.f. reactive sputtering. Composite targets with five kinds of Al-to-Ti area ratio were used. The sputtering gas was Ar (purity, 5 N) and N₂ (5 N). The flow rate of Ar and N₂ gas was kept constant at 0.8 and 1.2 sccm, respectively, resulting in a sputtering pressure of 0.4 Pa. The r.f. power was 300 W for all experiments. Substrates were not intentionally heated during deposition. The deposited films (thickness, 300 nm) were annealed in air at 600 900 °C and then subjected to X-ray diffractometer and Auger depth profiling.The as-deposited (Ti_{1 − x}Al_x)N films had the same crystal structure as TiN (NaCl type). Al atoms seemed to substitute for Ti in lattice sites. The preferential orientation of the films changed with the Al content of the film, x. Oxide layers of the films grew during annealing and became thicker as the annealing temperature increased. The thickness of the oxide layer grown on the film surface decreased with increasing Al content in the film. For high Al content films an Al-rich oxide layer was grown on the surface, which seemed to prevent further oxidation. All of the films, however, were oxidized by 900 °C annealing, even if the Al content was increased up to 0.55.     

Rapidly quenched intermetallic compounds,TiAl and Al3Ti

Two types of molten intermetallic compounds, with stoichiometric compositions TiAl and Al₃Ti, are rapidly solidified at a cooling rate ranging from 10⁴ to 10⁵ Ksec^–1 using a melt-spinning method. Solidified specimens are analysed by X-ray diffractometry and observed by a high-voltage electron microscope and an analytical transmission electron microscope. Extra phases other than stoichiometric composition phases were not detected for either TiAl or Al₃Ti specimens by X-ray diffractometry. Both specimens have very small grains from 1 to 3 m in diameter. TEM observation reveals that very fine precipitates (100 to 300 nm), which are not detected by X-ray diffractometry, are present within grains. They are Ti₃Al in the TiAl specimens, and aluminium in the Al₃Ti specimens, respectively. Electron micrographs of the Al₃Ti specimens show the presence of pair dislocations (super dislocations) and anti-phase boundaries. They are believed to have been formed at high temperature.     

Solid-state bonding of alumina to austenitic stainless steel

A low temperature and low pressure bonding process for alumina and 316L austenitic stainless steel has been developed using a titanium/molybdenum laminated interlayer. The intermetallic compounds of Ti₃Al (or Ti₂/Al) and TiAl were formed at the alumina/titanium interface on bonding at above 1273 K. The activation energy of the layer growth was about 142 kJ mol^–1. The construction of Al₂O₃/Ti/Mo/steel gave the most stable joints. The highest tensile strength was above 60 MPa with a titanium 0.4 to 0.6mm thick/molybdenum 0.4 to 0.5 mm thick interlayer on bonding at 1273 K for 3 h under pressure of 12 MPa.     

The fundamental determining factor of angular emission of multiple charged ions ejected by laser ablation of different metals and their binary alloys

Ions up to ionization state q = 4, emitted from laser produced plasma of Al, Ti, Ti₅₀Al₅₀ and Ti₇₅Al₂₅ targets ablated by Srivastava et al. (2006), are distributed angularly in the form of a cone, and for each ionization state the angular distribution has been shown to follow the cosine power-law: F = F_ocosⁿθ. It is found that the value of exponent n of cosⁿθ distribution function increases with the increase in ionization state. For each target, the value of exponent n of individual ionization states as well as total charge exhibits an excellent linear correlation with the room temperature Debye–Waller thermal parameter B or the mean-square amplitude of the atomic vibrations <u²> of the targets. It is further reported that the FWHM of ion distribution with Gaussian function fitting done by Srivastava et al. (2006) also depends linearly on B rather better than its dependence on the atomic mass of pure metal targets or average atomic mass in the case of their binary alloy targets. The FWHM of ion distribution for Al, Ti, Cu, Mo, W and their alloys Ti₂₅Al₇₅, Ti₃₄Al₆₆, Ti₅₀Al₅₀, Ti₇₅Al₂₅, W₆₀Cu₄₀, W₈₀Cu₂₀, W₉₀Cu₁₀ and Mo₇₀Cu₃₀ laser ablated by Srivastava and Rohr (2005) are also found to have much better correlation with the room temperature Debye–Waller thermal parameter B as compared to the atomic mass of the target.     

Effect of nano-CeO2 on microstructure properties of TiC/TiN+nTi(CN) reinforced composite coating

TiC/TiN+TiCN reinforced composite coatings were fabricated on Ti?C6Al?C4V alloy by laser cladding, which improved surface performance of the substrate. Nano-CeO₂ was able to suppress crystallization and growth of the crystals in the laser-cladded coating to a certain extent. With the addition of proper content of nano-CeO₂, this coating exhibited fine microstructure. In this study, the Al₃Ti+TiC/TiN+nano-CeO₂ laser-cladded coatings were studied by means of X-ray diffraction and scanning electron microscope. The X-ray diffraction results indicated that the Al₃Ti+TiC/TiN+nano-CeO₂ laser-cladded coating consisted of Ti₃Al, TiC, TiN, Ti₂Al₂₀Ce, TiC_0·3N_0·7, Ce(CN)₃ and CeO₂, this phase constituent was beneficial to increase the microhardness and wear resistance of Ti?C6Al?C6V alloy.     

Thermal stability of Al-1%Si-0.5%Cu/TiSi2 contact structure

Stable TiSi₂ was formed by rapid thermal annealing (RTA) on single-crystal Si. Subsequently a 600 nm-thick Al-1%Si-0.5%Cu layer was deposited on the top of the formed TiSi₂ followed by furnace annealing for 30 min at 400–600 C in N₂ ambient atmosphere. The thermal stability of Al-1%Si-0.5%Cu/TiSi₂ bilayer and interfacial reaction were investigated by employing four-point probe, scanning electron microscopy (SEM) and Auger electron spectroscopy (AES). The composition and the phase of precipitates formed by the reaction of Al-1%Si-0.5%Cu with TiSi₂ were studied by energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). It was found that the TiSi₂ layer was consumed by the reaction between TiSi₂ and Al-1%Si-0.5%Cu layer, resulting in precipitates at 550 C. The results from EDS revealed that the precipitates were composed of Ti, Al and Si. The precipitates were identified as Ti₇Al₅Si₁₂ ternary compound from XRD analysis.     

Study of interfacial reactions in ionized metal plasma (IMP) deposited Al-0.5%wtCu/Ti/SiO2/Si structure

Interfacial reactions in Al-0.5%wtCu/Ti/SiO₂/Si structure have been investigated up to the annealing temperature of 600°C for 30 min in Argon ambient. Annealing temperature at above 500°C, Al alloy and Ti start to react and produce Al₃Ti, which was already reported. Annealing at higher temperatures (550°C, and 600°C) made Al₃Ti transformed into Al₅Ti₂, which is thermodynamically more stable than Al₃Ti. The unreacted 52 nm thick Ti which existed underneath of Al₅Ti₂ might lead to retardation of the reaction between Al₅Ti₂ and the underlying SiO₂. Hence, the formation of ternary compound (Al _x Ti _y Si _z ) which is believed to be detrimental to the contact metallization layers was protected.