Hf Ti Alloys Research Articles

Кристаллографические особенности структуры alpha-фазы гафния и сплавов гафний--титан

AbstractThe structure of a hafnium crystal undergoing β → α (bcc → hcp) polymorphic transformation upon gradual cooling and the structure of Hf_55Ti_45 and Hf_30Ti_70 alloys formed under various kinetic conditions of polymorphic transformation are studied. The structure of the α phase in cast hafnium is shown to consist of lath crystals grouped into packets. The misorientations between separate laths in a packet are less than 1°. The Hf–Ti alloys in the cast state exhibit a mixed structure consisting of α-phase crystals of several morphological types. A structure of packet martensite is observed in the Hf–Ti alloys after quenching. Each packet includes laths of several crystallographic orientations. There is no regular alternation of differently orientated laths in the packet. The same set of α-phase orientations within an initial β-phase grain is observed independently of the cooling rate of the Hf–Ti alloys upon β → α polymorphic transformation. The misorientation of substructural elements within an α-phase crystal in the Hf–Ti alloys is ~5° for the cast state and ~2.2° after quenching.

Crystallographic Features of the α-Phase Structure in Hafnium and Hafnium–Titanium Alloys

The structure of a hafnium crystal undergoing β → α (bcc → hcp) polymorphic transformation upon gradual cooling and the structure of Hf55Ti45 and Hf30Ti70 alloys formed under various kinetic conditions of polymorphic transformation are studied. The structure of the α phase in cast hafnium is shown to consist of lath crystals grouped into packets. The misorientations between separate laths in a packet are less than 1°. The Hf–Ti alloys in the cast state exhibit a mixed structure consisting of α-phase crystals of several morphological types. A structure of packet martensite is observed in the Hf–Ti alloys after quenching. Each packet includes laths of several crystallographic orientations. There is no regular alternation of differently orientated laths in the packet. The same set of α-phase orientations within an initial β-phase grain is observed independently of the cooling rate of the Hf–Ti alloys upon β → α polymorphic transformation. The misorientation of substructural elements within an α-phase crystal in the Hf–Ti alloys is ~5° for the cast state and ~2.2° after quenching.

ESBD Analysis of the Structure of Cast and Quenched Hafnium–Titanium Alloys

This paper presents the results of investigation of alloys Hf–45% Ti and Hf–70% Ti structures which were formed under different kinetic conditions of the β → α (bcc → hcp) polymorphic transformation occurence. We used methods of metallography and EBSD analysis during our investigation. We have shown that the Hf–Ti alloys in the cast state exhibit a mixed structure consisting of α-phase crystals of several morphological types: large plates, groups of small parallel plates, and fine crystallites of the shape close to polyhedral. The α-phase structure of the Hf–Ti alloys after quenching consists of martensite crystals grouped into packets. In each packet, there are several crystallographic orientations of 12 possible orientations corresponding to Burgers orientation relationships, and no regular alternation of different orientations of the α phase were revealed in the packet. Irrespective of the cooling rate of the Hf–Ti alloys upon the β → α polymorphic transformation, within the initial β-phase grain, the same set of orientations of the α phase occurs. In the studied Hf–Ti alloys, the misorientation of substructure elements within the α-phase crystal reaches ~5.0° for the as-cast state and ~2.2° after quenching.

Mixing Enthalpies of Liquid Cu–Hf–Ti Alloys Studied by High-Temperature Calorimetry

The partial enthalpies of mixing of titanium and hafnium in liquid Cu–Hf–Ti alloys were investigated at 1873 K along the xCu/xHf = 3 (at xTi = 0–0.25) and xCu/xTi = 3 (at xHf = 0–0.5) sections by a hightemperature isoperibolic calorimeter. The limiting partial enthalpy of mixing of supercooled liquid titanium in liquid Cu–Hf alloy is 2.0 ± 1.5 kJ/mol. The limiting partial enthalpy of mixing of supercooled liquid hafnium in liquid Cu–Ti alloy is –43.0 ± 4.5 kJ/mol. The integral mixing enthalpies of components along the investigated sections are calculated by integrating the Gibbs–Duhem equation. The integral mixing enthalpy is negative in the investigated composition range. The comparison of our experimental results with the calculations according to Miedema’s model shows that this model’s prediction for the ΔmixH function should be treated with caution.

Microstructure and Corrosion Behavior of Hf-40 Wt Pct Ti Alloy in Nitric Acid Medium for Reprocessing Applications

The Hf-40 wt pct Ti (Hf-Ti) alloy was developed for neutron poison application in the spent nuclear fuel reprocessing plant. The furnace-cooled Hf-Ti sample exhibited the microstructure comprising equiaxed-α, lamellar-α, and feathery-α. The water-quenched Hf-Ti sample confirmed the presence of lath and internally twinned martensite. In comparison to the furnace-cooled sample, low corrosion current density and passivation current density values obtained for the water-quenched Hf-Ti in 6 M HNO3 at 298 K (25 °C) indicated better passivation ability. The martensitic structure exhibited high hardness (660 HV) and negligible corrosion rate in 6 M nitric acid at 298 K (25 °C). X-ray photoelectron spectroscopic (XPS) analysis confirmed that passivation behavior of this alloy was due to the protective passive film composed of TiO2 and HfO2.

Sputtered Hf–Ti nanostructures: A segregation and high-temperature stability study

High-temperature stability and segregation tendency of a Hf–Ti alloy is investigated using specimens sputtered in monolithic and multilayer configurations. Upon annealing at 800 °C for 96 h, the alloy shows segregation of Hf and Ti at the nanoscale even though the bulk Hf–Ti phase diagram predicts a homogeneous solid solution. The length scale of segregation is found to depend upon the initial as-deposited structure, with the monolithic film showing Ti segregation at grain boundaries, and the multilayered specimens showing a nanostructure of Hf-rich grains and Ti-rich intergranular amorphous regions. The multilayer specimens exhibit similar post-annealing grain diameters even when the initial layer thickness is varied. Thermodynamic Monte Carlo simulations show a solid solution when the bulk constraint is imposed, in line with the phase diagram expectation, and a polycrystalline nanostructure with solute segregated intergranular regions with full equilibration, in general agreement with the experimental observations.

Elasticity behavior, phonon spectra, and the pressure–temperature phase diagram of HfTi alloy: A density-functional theory study

The pressure-induced phase transition, elasticity behavior, thermodynamic properties, and P–T phase diagram of α, ω, and β equiatomic HfTi alloy are investigated using first-principles density-functional theory (DFT). The simulated pressure-induced phase transition of the alloy follows the sequence of α→ω→β, in agreement with the experimental results of Hf and Ti metals. Our calculated elastic constants show that the α and ω phases are mechanically stable at ambient pressure, while the β phase is unstable, where a critical pressure of 18.5GPa is predicted for its mechanical stability. All the elastic constants, bulk modulus, and shear modulus increase upon compression for the three phases of HfTi. The ductility of the alloy is shown to be well improved with respect to pure Hf and Ti metals. The Mulliken charge population analysis illustrates that the increase of the d-band occupancy will stabilize the β phase under pressure. The phonon spectra and phonon density of states are studied using the supercell approach for the three phases, and the stable nature of α and ω phases at ambient pressure are observed, while the β phase is only stable along the [110] direction. With the Gibbs free energy calculated from DFT-parametrized Debye model as a function of temperature and pressure, the phase transformation boundaries of the α, ω, and β phases of HfTi are identified.

Interfacial, optical properties and band offsets of HfTiON thin films with different nitrogen concentrations

The effect of nitrogen concentration on the interfacial and optical properties, as well as band offsets of HfTiO thin films by rf sputtering HfTi alloy target has been systematically investigated. The results indicate that an interfacial layer is unavoidably formed between HfTiON thin films and Si substrate, and the main content of the interfacial layer is silicate. No silicide is formed in the interfacial layer which is partly responsible for the poor electrical properties of high-k gate dielectrics. The optical properties of HfTiON films change, such as the refractive index decreases, while the extinction coefficient increases with the increase of N content, due to the defects increase in the films. The results also indicate that the bandgap and VB offset reduce with the introduction of N into HfTiO thin films. The CB offset of the HfTiON thin films is almost unchanged indicating that the N concentration has little effect on CB offset. However, the bandgap and band offsets are all higher than 1 eV, the sufficient band offsets still makes sputtering-derived HfTiON films by HfTi alloy target a promising high-k gate dielectric for future complementary metal oxide semiconductor technology.

High glass formability for Cu–Hf–Ti alloys with small additions of Y and Si

The effects of small substitutions of Si and Y on the glass-forming ability of a Cu55Hf25Ti20 glassy alloy are reported and discussed. Fully glassy rods with diameters up to 7 and 6.5 mm were produced for Cu54.5Hf25Ti20Si0.5 and Cu55− x Hf25Ti20Y0.3 alloys, respectively. The addition of Si enlarged ΔTx (= Tx − T g, where T g and Tx are crystallisation and glass transition temperatures, respectively) considerably, from 25 to 53 K for the Cu54Hf25Ti20Si1 alloy. However, the results showed that the parameters obtained from thermal analysis, such as T rg , ΔTx and γ[= Tx /(T g + T l)] are not reliably correlated with the glass-forming ability (GFA), at least for these bulk glass-forming alloys. The scavenging effects of the Y and Si, in particular the possibility of Y reducing the oxides, could be responsible for enhancing the GFA. It is proposed that the effectiveness of small additions of Si in enhancing the GFA may be the result of the possible formation of HfSiO4 having a very large negative enthalpy of formation and, as a strong network former, it would form glassy particles which would be ineffective as nucleating agents.

High-throughput study of the anodic oxidation of Hf–Ti thin films

A comprehensive study of the anodic oxide formation on Hf–Ti alloys over the entire range of composition was conducted. Combinatorial thin film libraries were prepared by co-sputtering. HRSEM and XRD were used to characterize the thin films and to confirm that a continuous change of the composition was obtained for this fully miscible system. Electrochemical investigations were performed by means of an automatic scanning droplet cell with a droplet radius of 100 μm. Subsequent potentiodynamic scans with intermittent impedance measurements allowed a quantitative determination of film formation factor and dielectric constant for each composition with a resolution of 0.5 at.%. Mott–Schottky analysis of the oxides was used to evaluate the relationship between parent metal composition and oxide properties, namely flat band potential and donor density. The structure–property and composition–property relationships are discussed in detail.

Investigation of Cracks Generated During Flow Forming of Nb–Hf–Ti Alloy Sheet

Niobium alloy Nb–Hf–Ti sheets are flow formed to make a conical divergent. During cold forming of the sheet, cracking was noticed on the surface. Visual observations of the cracks and microstructural and fractographic analysis revealed the presence of built-up layer and delaminations at the cracked site. Serration marks on the formed surface indicated a relatively higher feed rate. Work hardening of the material was also observed. This paper discusses the reasons for crack generation and provides the remedial measures to avoid such cracks during flow forming.

Amorphous alloys in the Cu–Hf–Ti system

Good bulk glass forming alloys can be found over a wide range of compositions in the neighborhood of Cu 60Hf 25Ti 15. The glass forming ability, thermal and mechanical properties of the Cu–Hf–Ti system is examined for alloys in the composition range of 57.5 < Cu < 62.5, 22.5 < Hf < 28.75, 11.25 < Ti < 17.5 (at.%). These alloys typically have strength ∼2 GPa, Hv ≥ 600, T g ∼ 450 °C, and Δ T ∼ 50 °C, and these properties are uniform independent of particular composition. Increasing Hf content while decreasing Ti increases the density and liquidus temperature of Cu–Hf–Ti alloys, while increasing Cu content decreases glass forming ability. The effect on GFA of small additions of Si, Ni, Y and Ta was evaluated. A 2 at.% addition of Y improved the GFA of the most robust bulk glass forming alloy in this series, Cu 57.5Hf 27.5Ti 15.

A novel Cu-based BMG composite with high corrosion resistance and excellent mechanical properties

A newly designed (Cu 0.6Hf 0.25Ti 0.15) 90Nb 10 bulk metallic glass (BMG) composite with an excellent combination of high corrosion resistance and superior mechanical properties was successfully synthesized. The micrometer-sized ductile Nb-rich dendrite phase disperses homogeneously in the glassy matrix. Addition of Nb to the Cu–Hf–Ti alloy results in a significant decrease in the corrosion rates in the solutions examined. The high corrosion resistance of the (Cu 0.6Hf 0.25Ti 0.15) 90Nb 10 BMG composite is attributed to the formation of Hf-, Ti-, and Nb-enriched highly protective surface films during immersion in acid- and chloride-ion-containing solutions. The composite alloy exhibits a high compressive true yield strength of 2073 MPa and true fracture strength of 2232 MPa together with a large true plastic strain of 14.1%. The embedded micrometer-sized dendrites in the glassy matrix retard inhomogeneous shear deformation and contribute to the large plastic strain.

Evaluation of nanoscale inhomogeneity in as-quenched state of Cu–Hf–Ti alloys

The microstructure of the melt-spun Cu60Hf30Ti10 and Cu50Hf40Ti10 alloys was investigated. The high-resolution transmission electron microscopy (HRTEM) image in the Cu60Hf30Ti10 alloy reveals the existence of nanoscale inhomogeneity, which appears to originate from nanocrystalline particles. The compositional segregation with a diameter of approximately 5 nm was detected, and its size agrees with the scale of nanocrystalline particle. Small-angle x-ray scattering measurement of the melt-spun Cu60Hf30Ti10 alloy also indicates the development of nanoscale inhomogeneity with the same size as that of nanocrystalline particles. It is found that the Cu element is enriched in and Hf element is rejected from the nanocrystalline particles. Since the melt-spun Cu50Hf40Ti10 alloy, which has a similar alloy composition to that of the glassy region of melt-spun Cu60Hf30Ti10 alloy, has a homogeneous glassy structure without fringe contrast in the HRTEM image, it is suggested that the excess of Cu causes the nanoscale inhomogeneity with crystalline structure in the rapidly quenched Cu–Hf–Ti alloys.

Nano structure of rapidly quenched Cu–(Zr or Hf) – Ti alloys and their devitrification process

The structure and primary devitrification process of the melt-spun Cu60(Zr or Hf)30Ti10 alloys were investigated. It was confirmed that the compositional segregation in the diameter range of 5–10 nm exists in the as-quenched state. The nanocrystalline particles with cubic structure are observed in the glassy matrix in thehigh-resolution transmission electron microscopy images, of which size is corresponding to the scale of compositional segregation. Small-angle X-ray scattering measurement also indicates the development of nanoscale inhomogeneity with the same size as that of nanocrystalline particles. The nanocrystalline region has high Cu content. In contrast, Zr or Hf and Ti elements are enriched in the glassy region. These results are recognized as the formation of novel structure consisting of the glassy and nanocrystalline phases. It is suggested that the precipitation of bcc CuZr phase as a primary crystallization phase proceeds in the glassy phase remaining the nanocrystalline phase in the Cu–Zr–Ti alloy. Meanwhile, the glassy and nanocrystalline phases are transformed to an orthorhombic Cu8Hf3 phase at the initial crystallization stage in the Cu–Hf–Ti alloy. These differences of crystallization process are consistent with the results of thermodynamic and kinetic analyses of the transformation mode.

Structure and crystallization of rapidly quenched Cu–(Zr or Hf)–Ti alloyscontaining nanocrystalline particles

The structure and primary crystallization process of the melt-spun Cu60 (Zr orHf)30Ti10 alloys were investigated. Compositional segregation in the diameterrange of 5–10 nm was observed in the as-quenched state. In the high-resolutiontransmission electron microscopy images, nanocrystalline particles areobserved in the glassy matrix, the size of which corresponds to the scale ofcompositional segregation. The glassy region has comparatively high Zror Hf and Ti contents. In contrast, the Cu element is enriched in thenanocrystalline phases. The nanocrystalline phases are identified as thecubic structure with a lattice constant of approximately 0.5 nm. Theseresults are recognized as the formation of novel structure consisting of theglassy and nanocrystalline phases. It is suggested that the precipitation ofbody-centred-cubic CuZr phase as a primary crystallization phase proceeds in theglassy phase, the nanocrystalline phase remaining in the Cu–Zr–Ti alloy.Meanwhile, the glassy and nanocrystalline phases are transformed to anorthorhombic Cu8Hf3 phase in the initial crystallization stage in the Cu–Hf–Tialloy.