Single Α-phase Research Articles

Phase transformation kinetics of ω-phase in pure Ti formed by high-pressure torsion

High-pressure torsion (HPT) process is the only method which can obtain a 100 vol% of high-pressure ω-phase sample at ambient condition in pure Ti. In this paper, the mechanism of ω-phase stabilization by the HPT process is discussed on the basis of the reverse phase transformation kinetics of ω-phase in pure titanium formed by the HPT process and then measured using electrical resistivity and calorimetric experiments. The single ω-phase sample showed much higher electrical resistivity of 0.95 μΩ m at 350 K compared with that of the single α-phase sample (0.62 μΩ m). The ω-to-α reverse transformation behavior was clearly observed through both electrical resistivity and calorimetric measurements. The activation energy for ω-to-α reverse transformation, derived from the kinetics, showed a value close to that for the self-diffusion of Ti. The ω-phase obtained after the HPT process has an equiaxed submicron microstructure. The microstructure of reverse transformed α-phase showed no evidence of the occurrence of martensitic transformation. These results suggest that the mechanism governing ω-to-α phase transformation changed from diffusionless martensitic transformation to diffusion-controlled transformation after severe plastic deformation using the HPT process.

Influence of phase composition on fretting wear behavior of thermally sprayed aluminum bronze coatings

Single α-phase and duplex ( α+ β′) phase aluminum bronze (CuAl) coatings were deposited on 1Cr18Ni9Ti stainless steels by atmosphere plasma spray. The fretting wear behaviors of the two types of coatings were investigated in relation to their phase composition. The damage mechanisms of the coatings were identified by analysing their worn surfaces using scanning electron microscopy and three-dimensional non-contact surface profiler. A discussion about the identified fretting damage mechanisms was carried out. Results show that duplex CuAl coating has higher hardness than the single α-phase CuAl coating but the former has poorer resistance against fretting wear than the latter in mixed stick-slip regime and gross slip regime. The toughness reduction due to the dendritic distribution of the β′ phase in duplex CuAl coating could explain the low fretting wear resistance observed in the duplex coating. In stick regime, single α-phase CuAl coating has a lower friction coefficient but it has a higher wear rate than the duplex coating, which is attributed to the lower hardness and hence more severe plastic deformation.

LiFe5O8 prepared by sol–gel synthesis: microstructural and morphological effects by organic and inorganic templates

Lithium ferrite was prepared using a sol–gel growth process using different fuels and inorganic templates. The aim of this study is to evaluate the influence of inorganic template agent like KCl, KBr and KI as well as the effect of different fuels like urea, glycine and citric acid on the morphology change of lithium ferrite. The structure, morphology and magnetic properties of the ferrite were investigated using powder X-ray diffraction (XRD), Infrared red spectroscopy and vibrating sample magnetometer. Thermal decomposition studies reveal the formation of lithium ferrite at a low temperature ~440 °C. Powder XRD pattern has shown the formation of a single α-phase lithium ferrite except the sample with inorganic template KBr, in which the presence of hematite as a secondary phase was observed. Scanning electron microscopy studies show that the structural morphology is highly sensitive to the inorganic template as well as on the fuel. The rod shaped nanoparticles are observed with the inorganic template KCl and KBr. The decrease in grain size is observed for LiFe5O8/glycine as compared to LiFe5O8/citric acid and flake shaped dense particles are observed for LiFe5O8/urea. The magnetic properties of the ferrite have also been investigated.

Structure dependent luminescence characterization of green–yellow emitting Sr 2SiO 4:Eu 2+ phosphors for near UV LEDs

This paper reports on the luminescence properties of mixtures of α- and β-(Sr 0.97Eu 0.03) 2SiO 4 phosphors. These phosphors were prepared by 3 different synthesis techniques: a modified sol–gel/Pechini method, a co-precipitation method and a combustion method. The structural and optical properties of these phosphors were compared to those of solid state synthesized powders. The emission spectra consist of a weak broad blue band centered near 460 nm and a strong broad green–yellow band centered between 543 and 573 nm depending on the crystal structure. The green–yellow emission peak blue-shifts as the amount of β phase increases and the photoluminescence emission intensity and quantum efficiency of the mixed phase powders is greater than those of predominant α-phase powders when excited between 370 and 410 nm. Thus, (Sr 1− x Eu x ) 2SiO 4 with larger proportion of the β phase are more promising candidates than single α-phase powders for use as a green–yellow emitting phosphor for near UV LED applications. Finally the phosphors prepared by the sol–gel/Pechini method, which have larger amount of β phase, have a higher emission intensity and quantum efficiency than those prepared by co-precipitation or combustion synthesis.

Wear property of low-temperature bainite in the surface layer of a carburized low carbon steel

► An approach is proposed for the development of low-temperature bainitic microstructure in the surface layer of a low-carbon steel. ► Wear properties of the low-temperature bainitic microstructure were investigated comparisons were made with carburized commercial gear steel of 20CrMnTi. The low-temperature bainitic steel exhibits an excellent wear resistance as a consequence of three main factors: high strength plus high toughness, carbon enriched austenite, and extremely fine α-phase microstructure. ► This material which has low-temperature bainite in the surface layer is potentially of use in the manufacture of heavy-duty gears if the process can be further optimized. A process primarily based on carburization and successive low-temperature austempering, was proposed for the generation of low-temperature bainitic microstructure in the surface layer (∼2.5 mm in thickness) of low-carbon steel. For a potential use in the manufacture of heavy-duty gears, wear properties of this type of microstructure were studied. Comparisons were made with carburized commercial steel of 20CrMnTi subjected to quenching followed by tempering, which is widely used in the area of gear. The result showed that the experimental steel and the control steel present different wear mechanisms under the same sliding wear process. The low-temperature bainitic steel exhibits an excellent wear resistance as a consequence of the following three main factors: (i) high strength plus high toughness, (ii) carbon enriched austenite, and (iii) extremely fine α-phase microstructure. The low-temperature bainitic microstructure exhibits high strength and high toughness due to the refinement of the microstructure. Secondly, the film-like carbon enriched austenite (∼30 nm in thickness) uniformly distributed between the carbide-free bainitic ferrite plates can retard the crack propagation during sliding friction and play an advantageous role in wear resistance. Lastly, an extremely fine single α-phase microstructure was promptly induced in the dry sliding friction of the surface layer of the low-temperature bainitic steel; such fine refined microstructure lead to an increase in hardness, which can improve wear resistance.

Can local hot spots induce α2/ γ lamellae during incomplete massive transformation of γ-TiAl alloys?

Oil quenching is applied to a Ti-45Al-7.5b alloy (in at%) in order to investigate the occurrence of local “hot spots” during rapid cooling from the single α-phase region. After quenching the microstructure consists of large α 2-Ti 3Al grains and a small volume fraction of massively transformed γ m-TiAl particles. The majority of the γ m-particles show a featureless α 2/ γ m interface, whereas in the immediate vicinity of some γ m-particles lamellar α 2/ γ-structures exist, exhibiting a Blackburn orientation relationship. The extension into the α 2-parent grain is in the range of 1–10 μm. In this study it is investigated, if the diffusion-controlled formation of the γ-laths is triggered by the release of latent heat during the α → γ m transformation, which leads to a local increase in temperature by a hot spot effect. A theoretical model is presented, which describes the temperature conditions around the hot spot. The model predicts that the life time of the hot spot is much too short to generate γ-laths observed with a length of 1–10 μm.

Study of nanometer-scaled lamellar microstructure in a Ti–45Al–7.5Nb alloy – Experiments and modeling

Quenching of Ti–45 at%Al–7.5 at%Nb from the single α-phase region to room temperature followed by aging below the eutectoid temperature leads to the precipitation of ultra-fine γ-TiAl lamellae. In addition to an extensive experimental program, reported by Cha et al. in Intermetallics 16 (2008) 868–875, in this work a micromechanical and thermodynamical model is presented for the formation of γ-TiAl lamellae within the α 2-Ti 3Al parent phase. A global transformation condition allows to predict a thickness to length ratio in accordance with experimental observations. Furthermore, a local transformation condition offers the basis for a kinetic law. The modeling concept can be applied to similar problems of combined diffusive and displacive phase transformations.

Abnormal Austenite-Ferrite Transformation Kinetics of Ultra-Low-Nitrogen Fe-N Alloy

The kinetics of the isochronal austenite (γ) → ferrite (α) transformation of ultra-low-nitrogen Fe-N alloy were investigated for cooling rates in the range of 5 to 15 Kmin−1 by high-resolution dilatometry. The explored γ → α transformation takes place in the single α-phase region, and the onset temperature of the transformation decreases with increasing applied cooling rate. According to the variation of the ferrite formation rate data, an abnormal transformation mechanism, i.e., multiply peaked nature with rate maxima appearing at fixed transformed fractions, was recognized. For the later (main) part of the transformation, a phase-transformation model, incorporating site saturation, interface-controlled growth, and an appropriate impingement correction, has been employed to extract the migration velocity of the γ/α interface from the measured data. The thus determined interface migration velocity is approximately constant for all applied cooling rates. The first abnormal part of the transformation was discussed in comparison to the recently observed abnormal phenomena for the γ → α transformation in pure iron and ultra-low-carbon Fe-C alloy. On the basis of the thermodynamic and kinetic results, it is shown that autocatalytic nucleation occurs in the first part of the transformation and that interface-controlled growth prevails for the entire transformation.

Initial Stages of Lamellae Formation in High Nb Containing γ-TiAl Based Alloys

AbstractIn this study a two-step heat treatment was applied to high Nb containing γ-TiAl based alloys in order to investigate the initial stage of the lamellae formation in ordered α2-grains as well as in massively transformed γm-grains. The first heat treatment step, conducted in the single α-phase field followed by oil quenching, leads to a microstructure consisting of supersaturated α2-grains and a small volume fraction of γm-grains. The second step of the heat treatment was performed below the eutectoid temperature, i.e. within the (α2+γ)-phase field region and was again followed by oil quenching. There, the formation of ultra-fine γ-lamellae takes place in the α2-grains and (some) fine α2-lamellae are formed in the γm-grains. In both cases the lamellae show a Blackburn orientation relationship with the matrix grain. It was found that the precipitation of γ-laths in the supersaturated α2-grains is faster than the formation of α2-laths in γm-grains. The characteristics of the initial stage of formation were investigated by transmission electron microscopy.

69Ga NMR in Pu 1− xGa x ( x < 0.01) alloy

69Ga nuclear magnetic resonance (NMR) experiments have been carried out on the Pu 0.995Ga 0.005 alloy at temperatures between 20 and 420 K at magnetic field of 9.4 T to study the local charge symmetry and magnetic behavior of the Pu atoms surrounding solute Ga. In accordance with optical metallography and X-ray diffraction studies the investigated plates of alloy (20 mm × 3 mm × 0.2 mm) represent at room temperature the single α-phase material without any detectable macroscopic segregation of other phases (the corresponding volume fraction <0.03). It was revealed that the magnetic shift of the 69Ga NMR central line and the electric quadrupole broadening of the Ga NMR spectrum trace temperature dependence of the NMR parameters is observed for the δ-phase plutonium alloy Pu 0.95Ga 0.05. On the basis of 69Ga NMR and X-ray data obtained, it is suggested that specific microscopic areas with a size less than 100 nm exist in the α-phase plutonium alloys Pu 0.995Ga 0.005. The local charge and magnetic environment of the solute Ga in these microscopic areas are similar to the observed in the stabilized δ-Pu alloy.

Investigation of Stresses Generated during Lithium Transport through the RF Sputter-Deposited Li1 − δCoO2 Film by a DQCR Technique

In this work, stresses generated during lithium transport through radio-frequency (rf) sputter-deposited films with different thicknesses were investigated by a double quartz crystal resonator (DQCR) technique. For this purpose, in situ resonant frequency changes of the -coated AT- and BT-cut quartz crystals were first recorded along with the galvanostatic charge (lithium deintercalation) and discharge (lithium intercalation) curves obtained in a 1 M -propylene carbonate solution. From the measured resonant frequency changes, the lateral stresses of the films were then estimated as a function of lithium stoichiometry, (1−δ). Compressive and tensile stresses were developed in the films during the lithium deintercalation and intercalation, respectively. The remarkable variation of compressive and tensile stresses with lithium stoichiometry appeared in a two-phase (a Li-poor α-phase and a Li-rich β-phase) region. Compressive and tensile stresses decreased in absolute magnitude with increasing film thickness. The contribution of the electrostrictive stress to the total stress was theoretically calculated to be about From the extremely small contribution of the electrostrictive stress, it is strongly suggested that the stresses result mainly from the volume contraction and expansion of the films due to the lithium intercalation and deintercalation, respectively. Furthermore, the relaxation of the compressive stress was developed during the lithium deintercalation in a single α-phase region, causing the cracking of the films. © 2003 The Electrochemical Society. All rights reserved.

Observations of room-temperature creep recovery in titanium alloys

Conventional α(hcp) and α(hcp)/β(bcc) titanium alloys exhibit significant primary creep strains at room temperature and at stresses well below their macroscopic yield strength. It has been previously reported in various materials systems that repeated unloading during primary creep testing may either accelerate or retard the accumulation of creep strains. These effects have been demonstrated to depend on both microstructure and the applied stress. This article demonstrates that significant room-temperature recovery occurs in technologically relevant titanium alloys. These recovery mechanisms are manifested as a dramatic increase in creep rates (by several orders of magnitude) upon the introduction of individual unloading events, ranging from 1 minute to 365 days, during primary creep tests. Significant increases in both creep rate and the total accumulated creep strain were observed in polycrystalline single α-phase Ti-6Al, polycrystalline α/β Ti-6Al-2Sn-4Zr-2Mo-0.1Si, and individual α/β colonies of Ti-6242. Based on transmission electron microscopy (TEM) studies of the active deformation mechanisms, it is proposed that the presence of significant stress concentrations within the α phase of these materials, in the form of dislocation pileups, is a prerequisite for significant room-temperature recovery.

Observations of room-temperature creep recovery in titanium alloys

Conventional α(hcp) and α(hcp)/β(bcc) titanium alloys exhibit significant primary creep strains at room temperature and at stresses well below their macroscopic yield strength. It has been previously reported in various materials systems that repeated unloading during primary creep testing may either accelerate or retard the accumulation of creep strains. These effects have been demonstrated to depend on both microstructure and the applied stress. This article demonstrates that significant room-temperature recovery occurs in technologically relevant titanium alloys. These recovery mechanisms are manifested as a dramatic increase in creep rates (by several orders of magnitude) upon the introduction of individual unloading events, ranging from 1 minute to 365 days, during primary creep tests. Significant increases in both creep rate and the total accumulated creep strain were observed in polycrystalline single α-phase Ti-6Al, polycrystalline α/β Ti-6Al-2Sn-4Zr-2Mo-0.1Si, and individual α/β colonies of Ti-6242. Based on transmission electron microscopy (TEM) studies of the active deformation mechanisms, it is proposed that the presence of significant stress concentrations within the α phase of these materials, in the form of dislocation pileups, is a prerequisite for significant room-temperature recovery.

Formation of highly saturated Al–Ag precursor by rapid solidification for skeletal silver synthesis

The solubility of silver in the Al phase is the most important factor for the synthesis of skeletal silver. Rapid solidification process was used to obtain Al precursor particles with much dissolved silver from various Al100−xAgx (x=15–39at%) alloys and their effect was compared with that obtained by the conventional solidification process. The conventionally solidified specimens showed the primary dendritic α-Al phase and the eutectic was composed of the α and Ag2Al phases in the inter-dendritic region as expected from the equilibrium phase diagram. On the other hand, the X-ray diffraction patterns of the rapidly solidified specimens, except Al61Ag39 alloy, showed almost single α-phase though their micro structure and had a clear network showing different solubilities of silver. The solubility of silver in these alloys has a maximum for Al70Ag30 alloy. The precipitation rate of the Ag2Al phase from the α-Al phase was quite high as derived from a Kissinger plot using DSC measurements. The powder specimens solution treated at 823 K for 1.8×103 s followed by water quenching showed significant precipitation of the Ag2Al phase though most of the silver can be dissolved in the α phase by holding at that temperature. The cooling rate of the powder was not high enough to suppress the precipitation. The rapid solidification had two advantages over the conventional solidification process. The first is that the rapid solidification formed the highly saturated α phase beyond 23.5at%. The second is that particles directly formed from the melt and could circumvent pulverizing of the conventionally solidified ingot.

A discussion on the formation of bainite and other precipitates in Cu-Zn and Ag-Zn alloys

Various transformation products appearing in Cu-Zn and Ag-Zn alloys upon aging at temperatures in the α + β and single α-phase range were examined by optical and electron microscopy. Upon aging at relatively low temperature, bainite plates with the 9R structure are first formed and then retransformed to face-centered cubic (fcc). At higher temperature, other transformation products with the fcc structure, such as rod-shaped α and massive α, are concurrently observed with the bainite. Measurements of solute concentration in bainite were performed by using analytical electron microscopy. Bainite plates were found to have lower solute concentration than that of stable α, not only in Cu-Zn but also in Ag-Zn alloys. These results were discussed in terms of the free-energy change in the transformation. The first appearance of the 9R bainite in place of the stable fcc phase in low-temperature aging should be attributed to the “shear” process which lowers the nonchemical energy and diminishes the necessary driving force for the transformation. The measured solute concentration of the bainite is well reproduced theoretically on the view that the bainite is a product of shear transformation combined with the concentration change.

Effect of aluminium content on the anodic behaviour of copper-aluminium alloys in 3.5 wt% NaCl solution

Anodic behaviour of heterogeneous Cu-Al alloys in 3.5 wt% NaCl solution has been investigated as a function of aluminium content by potentiodynamic polarization, cyclic voltammetry and potentiostatic current transient experiments and compared to pure Cu and homogeneous Cu-Al alloys. The specimens were subjected to various anodic potentials and subsequent analyses by scanning electron microscopy (SEM) supplemented with energy dispersion spectroscopy (EDS). The pure Cu and single α-phase Cu-Al alloys showed that a relatively thick protective CuCl film forms at active-passive transition potentials and transforms into CuO or Cu(OH) 2 at the passivation potential, resulting in high limiting current densities. However, the Cu-10 wt% Al alloy is passivated to Al(OH) 3, impeding the transformation of the thick CuCl layer which contains AlCl or Al(OH) 2Cl salt into CuO or Cu(OH) 2. At high applied anodic potential, the Cu 2(OH) 3Cl phase forms on the deteriorated passive film Al(OH) 3.

Local heating effect in point-contact spectroscopy for magnetic metals of Ni and ?-Mn

The current-voltage characteristics at a homojunction point-contact of ferromagnetic Ni single crystal and antiferromagnetic α-phase Mn polycrystalline metal have been measured at 4.2 K. Several singularities in the first and second derivative of I–V curves are observed at specific bias voltages for both metals. Taking into account the observed temperature dependence of the bulk electrical resistivity and thermal conductivity (and thus mean free path of electrons), we have performed computer simulations of these nonlinear I–V characteristics based on a modified “local heating model” and compared with the results by a traditional “spherical spreading-out” (SSO) model. When a constant additive correction to the bulk resistivity near a contact interface is incorporated, our calculated curves are in good agreement with the experimental results.

Formation of helium bubbles and dislocation loops in tritium-charged vanadium

Abstract Dislocation loops and small He bubbles were observed by transmission electron microscopy in V samples which were tritium-charged to produce single α-phase and (α+ β) two-phase regions and subsequently aged up to 14 months at room temperature and at — 196°C. In the β-phase areas, large densities of He bubbles, 1–2 nm in diameter and dislocation loops were observed. Preliminary observations after 14 months of ageing showed that the loop pattern had coarsened into a dislocation network. In the α-phase areas, bubbles and tube-shaped cavities were found along dislocations and in dislocation tangles. Also, isolated bubbles which had grown by prismatic loop punching were observed. In low-angle grain boundaries bubbles were formed at dislocations. These results show that microstructures similar to those observed after energetic He implantation can form in the absence of irradiation-induced defects. It is concluded that He must be mobile at room temperature in both phases and precipitates by a mechanism which does not require He trapping in vacancies as a first step.

Mechanical properties changes of Fe-Cr alloys by fast neutron irradiation

The changes in the mechanical properties of Fe-Cr ferritic, single α-phase alloys, containing 0–15 wt% Cr, by fast neutron irradiation were studied by means of tensile tests in the temperature range between 77 and 283 K, mainly with regard to the effects of the Cr content. The temperature dependence of the yield stress decreased with increasing Cr content. The irradiation raised the thermal activated component of the yield stress in low Cr alloys severely, but not in high Cr alloys. The fracture stress in the brittle fracture mode increased with Cr content up to 10 wt%, beyond which it decreased. The frequency of twinning deformation showed an opposite tendency to the fracture stress with or without irradiation. The transition temperature of the unstable plastic flow, which occurred intensively after irradiation, decreased with increasing Cr content. The last condition mainly governed the irradiation embrittlement of Fe-Cr alloys determined from absorbed energy-temperature curves in tensile testing. Thus, high Cr alloying can make the steel more resistant to irradiation embrittlement.