Amorphous TiAl Research Articles

Structure Evolution of Nanocrystalline–Amorphous TiAl Biphase Films during Helium Ion Implantation

Building nanocrystalline–amorphous biphase nanostructure has recently emerged as an advanced route to improve radiation tolerance, as the nanocrystalline–amorphous interface is expected to enhance the sink efficiencies of helium atoms. However, the structure evolution and degradation mechanisms during helium ion implantation in nanocrystalline–amorphous biphase films are still unclear. This study aimed to further understand these mechanisms through in situ observation of nanocrystalline–amorphous TiAl biphase films deposited via magnetron sputtering in a helium ion microscope. Results demonstrate that during the helium implantation process (the final fluence was 4 × 1017 ions cm−2), a partial swelling occurred in the implantation region without blisters, cracks, or exfoliation on the surface. The AFM and TEM results revealed that the partial bulge originated from the differential in the swelling rate between the amorphous and grain areas during helium ion implantation. These findings offer promising insights into designing radiation-tolerant materials for advanced nuclear reactors.

Structural evolution of amorphous and nanocrystalline TiAl films under helium implantation

The structure evolution and mechanism of the material during helium ion implantation are directly related to their performance and service life, towards the nuclear application. Despite the growing interest in amorphous and nanocrystalline films for a range of nuclear applications, many specifics about how their structure evolves during helium implantation are unclear. Here, the amorphous and nanocrystalline TiAl films produced by magnetron sputtering were in situ observed in a helium ion microscope to further understand the mechanism of microstructural evolution under severe ion implantation of two distinct structured nanomaterials. The result shows that a blister suddenly appeared on the surface of the nanocrystalline TiAl film, caused by a laterally ordered distribution of bubbles along the 〈111〉 crystal direction when the implantation dose reached 3.5 × 1017 ions cm−2. Instead, significant swelling of the whole implantation region of amorphous TiAl film was observed. At the final dose (9 × 1017 ions cm−2), the swelling thickness of the amorphous film was 200 nm and no exfoliation and cracking occurred. Due to long-range disorder, the degradation processes of amorphous TiAl film significantly differ from those of nanocrystalline TiAl film. This research has the potential to classify applications of materials in the nuclear field while providing a new viewpoint on the material structure design.

Electrochemical corrosion performances of laser thermal sprayed amorphous Al–Ti–Ni coatings in marine environment

PurposeThe purpose of this paper is to investigate the effects of laser power on the electrochemical corrosion performance in 3.5% NaCl, 0.1 M H2SO4 and 0.1 M NaOH solutions, which provided an experimental basis for the application of Al–Ti–Ni amorphous coating in marine environment.Design/methodology/approachAmorphous Al–Ti–Ni coatings were fabricated on S355 structural steel by laser thermal spraying (LTS) at different laser powers. The surface and cross-section morphologies, chemical element distribution, phases and crystallization behaviors of obtained coatings were analyzed using a scanning electron microscope, energy-dispersive X-ray spectroscope, X-ray diffraction and differential scanning calorimetry, respectively. The effects of laser power on the electrochemical corrosion performances of Al–Ti–Ni coatings in 3.5% NaCl, 0.1 M H2SO4 and 0.1 M NaOH solutions were investigated using an electrochemical workstation.FindingsThe crystallization temperature of Al–Ti–Ni coatings fabricated at the laser power of 1,300 and 1,700 W is ∼520°C, whereas that fabricated at the laser power of 1,500 W is ∼310°C. The coatings display excellent corrosion resistance in 3.5% NaCl and 0.1 M NaOH solutions, while a faster dissolution rate in 0.1 M H2SO4 solution. The coatings fabricated at the laser power of 1,300 and 1,700 W present the better electrochemical corrosion resistance in 3.5% NaCl and 0.1 M NaOH solutions, whereas that fabricated at the laser power of 1,500 W exhibits the better electrochemical corrosion resistance in 0.1 M H2SO4 solution.Originality/valueIn this work, Al-wire-cored Ti–Ni powder was first on S355 steel with the laser power of 1,300, 1,500 and 1,700 W, and the effects of laser power on the electrochemical corrosion performance in 3.5% NaCl, 0.1 M H2SO4 and 0.1 M NaOH solutions were investigated using an electrochemical workstation.

Amorphous Al-Ti Powders Prepared by Mechanical Alloying and Consolidated by Electrical Resistance Sintering

A novel processing method for amorphous Al50Ti50 alloy, obtained by mechanical alloying and subsequently consolidated by electrical resistance sintering, has been investigated. The characterisation of the powders and the confirmation of the presence of amorphous phase have been carried out by laser diffraction, scanning electron microscopy, X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. The amorphous Al50Ti50 powders, milled for 75 h, have a high hardness and small plastic deformation capacity, not being possible to achieve green compacts for conventional sintering. Moreover, conventional sintering takes a long time, being not possible to avoid crystallisation. Amorphous powders have been consolidated by electrical resistance sintering. Electrically sintered compacts with different current intensities (7–8 kA) and processing times (0.8–1.6 s) show a porosity between 16.5 and 20%. The highest Vickers hardness of 662 HV is reached in the centre of an electrically sintered compact with 8 kA and 1.2 s from amorphous Al50Ti50 powder. The hardness results are compared with the values found in the literature.

Electron beam induced rejuvenation in a metallic glass film during in-situ TEM tensile straining

Rejuvenation of an amorphous TiAl thin film under external tensile stress by high energy electron irradiation is observed via in-situ transmission electron microscopy (TEM). Electron beam (e-beam) irradiation results in a characteristic change of the elastic properties over time, as revealed by the atomic-level elastic strain obtained from the TEM diffraction pattern. Specifically, a time dependent increase/decrease of elastic strain is observed along the tensile direction, the saturation value of which correlates linearly with the preceding stress increment/decrement but shows little dependence on the e-beam condition. The low sensitivity of the saturation value to the e-beam condition indicates that the elastic strain change is induced by the structural transitions of a population (dependent on stress increment/decrement) of unstable atomic configurations to local, elastically soft areas. Classical molecular dynamics (MD) simulations including high energy electron scattering events are performed under tensile load to obtain insights into the structural modification that leads to time dependent changes in elastic strain under irradiation. The simulations reveal a change in quantities that are characteristic of structural rejuvenation, with a reduction of the local shear modulus manifesting as time dependent increase in the atomic-level elastic strain at fixed external stress. This link to the experimental data is confirmed by tracking elliptic distortions of simulated diffraction patterns calculated from MD configurations. The presented findings are highly relevant for experimental characterization of amorphous materials using TEM and give a new perspective on local structural modifications by electron irradiation.

Effects of laser power on corrosion behaviors of laser thermal sprayed Al–Ti–Ni amorphous coatings in H2SO4 and NaOH solutions

Amorphous Al–Ti–Ni coatings were fabricated on S355 steel using a laser thermal spraying (LTS). The surface and cross–section morphologies, chemical compositions and phases of obtained coatings were analyzed using a field emission scanning electron microscope (FESEM), energy dispersive spectrometer (EDS), and x–ray diffraction (XRD), respectively. The effects of laser power on the immersion corrosion and polarization curves in 0.1 M H2SO4 and 0.1 M NaOH solutions were investigated using a salt spray corrosion (SSC) chamber and electrochemical workstation. The results show that the amorphous Al–Ti–Ni coatings are primarily composed of AlFe3, AlFe, Ni2MnAl, AlCrFe2, and Al2O3 with the amorphous structure, which form the metallurgical bonding at the coating interface. The Al2O3 film on Al–Ti–Ni coating is destroyed severely by the aggressive H+ in 0.1 M H2SO4 solution, leading to the higher corrosion rate, the lowest corrosion current density and highest polarization resistance of Al–Ti–Ni coating fabricated at the laser power of 1500 W are 1.722 × 10–4 A·cm–2 and 195 Ω, respectively, whose corrosion rate is the lowest. While those fabricated at the laser power of 1700 W in 0.1 M NaOH solution are 2.484 × 10–6 A·cm–2 and 15 243 Ω, respectively, its corrosion resistance is the best among the three kinds of coatings.

Electrical and structural properties of direct current magnetron sputtered amorphous TiAl thin films

Structural and electrical properties of amorphous TiAl films with 50 at.% Ti and 50 at.% Al deposited with a compound target by dc magnetron sputtering were characterized by X-ray diffraction (XRD), atomic force microscopy, scanning electron microscopy, and electrical measurements. We found that as-deposited Ti0.5Al0.5 films with a thickness below 1000 nm were amorphous, whereas small hexagonal crystallites could be detected in thicker films. Such layers exhibited a compressive stress which increased with decreasing the thickness of TiAl. The behaviour of stress with the temperature was qualitatively similar in films independently on their thickness. Thermal annealing at 300 °C reduced the initial compressive stress and no changes of the phase of TiAl were observed. We correlated these observations with annihilation of point-like defects created by the deposition in thin Ti0.5Al0.5 films or with appearance of nanocrystallites in the amorphous matrix of TiAl. These effects were also responsible for the changes of the resistivity observed at elevated temperatures. We showed that a heat treatment induced by an electronic current with a density of about 2 MA/cm2 led to the crystallization of TiAl films independent on their thickness. A similar effect could be achieved by annealing the samples at 520–550 °C in air or N atmosphere. A significant drop of the resistivity down to 90 μΩcm was detected in the annealed films at room temperature. XRD confirmed that two different phases of TiAl (γ and α2) with randomly oriented crystallites appeared in the annealed films.

Effects of laser remelting speeds on microstructure, immersion corrosion, and electrochemical corrosion of arc–sprayed amorphous Al–Ti–Ni coatings

An arc–sprayed amorphous Al–Ti–Ni coating on S355 structural steel was processed using a laser remelting (LR). The surface and cross–section morphologies, chemical compositions, phases and residual stresses of obtained Al–Ti–Ni coatings at the LR speeds of 5, 10, and 15 mm/s were analyzed using a field emission scanning electron microscope (FESEM), energy dispersive spectrometer (EDS), X–ray diffractometer (XRD), and X–ray diffraction stress tester, respectively, and the effects of LR speeds on their immersion corrosion and potentiodynamic polarization curves of Al–Ti–Ni coatings in 3.5% NaCl solution were also investigated to analyze the mechanism of corrosion resistance. The results show that the metallurgical bonding is formed at the Al–Ti–Ni coating interface due to the interdiffusions and recombinations of Al, Ti, Ni, Fe, Cr and Mn. The Al–Ti–Ni coatings at the different LR speeds obtain a certain amount of amorphous phases, which are detected as AlFe, AlFe3, AlCrFe2, Ni2MnAl, and AlNi phases. The residual stresses of as–obtained Al–Ti–Ni coatings at the LR speeds of 5, 10, and 15 mm/s are −12 ± 8, 43.9 ± 3, and −34.4 ± 8 MPa, respectively, of which the tensile residual stress exacerbates the immersion corrosion and the compressive stress restrains the crack expansion. The corrosion potentials of Al–Ti–Ni coatings at the LR speeds of 5, 10, and 15 mm/s are −1.046, −1.106, and −0.986 V, respectively, which shift positively than S355 steel and effectively increase the corrosion resistance of substrate, the electrochemical corrosion resistance of Al–Ti–Ni coating at the LR speed of 15 mm/s is the best among the three kinds of coating.

Amorphous Phase Formation and Heat Treating Evolution in Mechanically Alloyed Al-Ti Powders

This paper focuses on the microstructural characterization of Al25Ti75, Al37Ti63, Al50Ti50, Al63Ti37and Al75Ti25powders mixtures prepared by mechanical alloying (MA). The high-energy ball-milling, up to 75 h, of aluminium and titanium powders leads to a nanocrystalline or an amorphous structure. It is showed that a stable amorphous Al–Ti phase with uniform elemental distribution forms after 50 h of milling in Al50Ti50alloy. Heat treatment of the different alloys leads to the crystallization of AlTi3, AlTi, Al2Ti and Al3Ti intermetallic compounds. A comprehensive study by laser granulometry, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) was carried out on the structure, surface morphology and thermal behaviour of the MA Al-Ti mixtures, both of milled and heat treated powders.

Effects of Environment on the Wear Behavior of P/M Ti-47Al-2Cr-0.2Mo

The wear behavior of powder metallurgical Ti-47Al-2Cr-0.2Mo alloy prepared by pre-alloyed powders was investigated using pin-on-disk wear tests in different environments, viz, argon, 4% hydrogen in nitrogen, air and oxygen. The disk material was sinter-hot isostatically pressed, yttria-stabilized zirconia. Lower wear rates were found for the TiAl pins in oxygen-free environments, indicating that oxygen play a key role in the wear rate. In contrast, the presence of molecular hydrogen and moisture have nearly no effect. A combination of X-ray diffraction and energy dispersive X-ray spectroscopy indicated that the abrasive particles present mainly consisted of the zirconia. In addition, zirconia particles were embedded in the worn tips of the TiAl pins and mixed into the tribolayers. The high stress and high contact temperature at the wear surface made the zirconia disk undergo a phase transformation during the dry sliding wear, accompanied by grain pullout, surface uplifting and microcracking. The hard tribolayer with embedded zirconia particles provides some protection against further wear of the TiAl pin. The highly localized, repetitive shear stress during the wear tests may bring about amorphous TiAl in the wear debris. The main wear mechanisms were abrasive wear of two-body and three-body, some delamination and plastic deformation.

Effect of Laser Remelting Power on Immersion Corrosion of Amorphous Al–Ti–Ni Coatings

An arc-sprayed amorphous Al–Ti–Ni coating on S355 structural steel was processed by laser remelting (LR) at powers of 600, 800, and 1000 W. The surface-cross-sectional morphologies, chemical element distributions, and phase compositions of the as-obtained Al–Ti–Ni coatings were analyzed using a scanning electron microscope (SEM), energy-dispersive spectrometer (EDS), and X-ray diffractometer (XRD), respectively. The immersion corrosion tests of Al–Ti–Ni coatings in 3.5% NaCl solution for 720 h were performed to investigate the effects of LR power on their immersion corrosion behaviors. The test results show that the amorphous Al–Ti–Ni coatings form good metallurgical bonding with the substrate after LR. The AlNi, Al3Ti, Al3Ni2, Ti3O5, and Al2O3 amorphous phases are detected in the Al–Ti–Ni coatings after LR. The corrosion potentials of Al–Ti–Ni coatings after LR show a positive shift relative to that of S355 steel, implying that the corrosion resistance of Al–Ti–Ni coatings was superior to that of S355 steel. A dense protective Al2O3 film is formed on the Al–Ti–Ni coating surface at an LR power of 1000 W, at which power the highest corrosion potential of −0.233 V is observed. The corrosion mechanisms of Al–Ti–Ni coating at the LR power of 1000 W are uniform corrosion and pitting corrosion, while those of Al–Ti–Ni coatings at the LR powers of 600 and 800 W are localized corrosion and pitting corrosion. The corrosion resistance of Al–Ti–Ni coating with the LR power of 1000 W is better than those at the LR powers of 600 and 800 W, effectively improving the corrosion resistance of S355 steel.

Optical and electrical properties of polycrystalline and amorphous Al-Ti thin films

The structural, optical, and transport properties of sputter-deposited Al-Ti thin films have been investigated as a function of Ti alloying with a concentration ranging from 2% to 46%. The optical reflectivity of Al-Ti films at visible and near-infrared wavelengths decreases with increasing Ti content. X-ray absorption fine structure measurements reveal that the atomic ordering around Ti atoms increases with increasing Ti content up to 20% and then decreases as a result of a transition from a polycrystalline to amorphous structure. The transport properties of the Al-Ti films are influenced by electron scattering at the grain boundaries in the case of polycrystalline films and static defects, such as anti-site effects and vacancies in the case of the amorphous alloys. The combination of Ti having a real refractive index (n) comparable with the extinction coefficient (k) and Al with n much smaller than k allows us to explore the parameter space for the free-electron behavior in transition metal-Al alloys. The free electron model, applied for the polycrystalline Al-Ti films with Ti content up to 20%, leads to an optical reflectance at near infrared wavelengths that scales linearly with the square root of the electrical resistivity.

A Study on the Characteristics of Amorphous TiAl by P/M Processing

The P/M processing of titanium aluminide using amorphous TiAl is developed by which it is possible to overcome inherent fabricability problems and to obtain a fine microstructure. A high quality amorphous TiAl pow- der produced by reaction ball milling shows clear glass transition far below a temperature at the onset of crystalli- zation in differential scanning calorimetry above a heating rate of 0.05 K/s. We obtained a fully dense compact of amorphous TiAl powders, encapsulated in a vacuumed can, via viscous flow by hot isostatic pressing (HIP). Iso- thermally annealing of HIP'ed amorphous compact under a pressure of 196 MPa shows a progressive growth of γ- TiAl phase with α2 (Ti3Al), which is characterized by increasing sharpness of X-ray peaks with temperature. Fully dense HIP'ed compact of titanium aluminide TiAl shows a high hardness of 505 Hv, suggesting strengthening mechanisms by sub-micron sized grain of γ-TiAl and particle-dispersion by second phase constituent, α2. (Received January 13, 2016; Revised January 18, 2016; Accepted January 22, 2016)

Growth of icosahedral medium-range order in liquid TiAl alloy during rapid solidification

The formation and evolution of icosahedral medium-range order structures of TiAl alloy during the rapid solidification are investigated based on molecular dynamics simulations. The icosahedral medium-range order structural evolutions are described in detail by coordination number and icosahedral connectivity parameters. Simultaneously, the evolution of the medium-range order icosahedral clusters is analyzed by nano-clusters tracing method. The results reveal that the full growths of icosahedral medium-range order structures improve the glass forming ability and stability of metallic glass. Meanwhile, the hexagonal connected icosahedral clusters which link via volume-sharing have excellent structural stability, and the dendritic icosahedral clusters based on the hexagonal formation keep continuous growth below the Tg. The junctions which consist of the hexagonal formations enhance the compactness of icosahedral medium-range order structure. Moreover, the icosahedral medium-range order is an appropriate structural characterization which accurately defines the characteristics of amorphous TiAl alloy during the rapid solidification.

Crystallization Kinetics Study on Magnetron-Sputtered Amorphous TiAl Alloy Thin Films

Crystallization kinetics of magnetron-sputtered amorphous TiAl alloy thin films is investigated by differential scanning calorimetry through isothermal analysis and non-isothermal analysis. In non-isothermal analysis, the Kissinger method and the Ozawa method are used to calculate the apparent activation energy and local activation energy, respectively, in the crystallization processes of amorphous TiAl thin films. Furthermore, the crystallization mechanism is discussed from the investigation of the Avrami exponent by isothermal analysis. In addition, x-ray diffraction is utilized to reveal the grain orientation and evolution during the crystallization of TiAl thin films.

Evolution of icosahedral clusters during the rapid solidification of liquid TiAl alloy

The evolution characteristics of icosahedral clusters during the rapid solidification of TiAl alloy under different cooling rates are investigated based on molecular dynamics simulations. The short-range order structural properties of liquid and amorphous TiAl alloy are analyzed by several structural characterization methods. It is found that the cooling rate plays a key role during the evolution of icosahedral clusters and has significant effect on the glass transition temperature. Simultaneously, the medium-range order structural evolutions are described in detail by quantitative method and visualization technology during the rapid solidification. The results reveal that the medium-range order icosahedral clusters have good structural stability and configural continuity during the rapid cooling process. Furthermore, the icosahedral structures have significant improvements with decreasing cooling rate. The structures block the crystal nucleation and improve the glass forming ability of supercooled liquid.

Compositional Control of Co-deposited TiAl Film using Dual Magnetron System

Titanium Aluminum (TiAl) films have been deposited by physical vapor deposition method using two unbalanced planar magnetrons. Pure Titanium (Ti) and Aluminum (Al) sputtering targets were used for the co-deposition of TiAl films of varying compositions. Precise control of the sputtering yields of target materials were achieved through optimization of applied power. This in turn controlled the composition of the Ti and Al in the grown films. The films were obtained at applied power levels of ‘50W to 900W’ and their compositions were established and affirmed by EDAX measurements. Amorphous TiAl films were formed for a higher Al composition (i.e. from 20 % to 50 Atomic %), while crystalline film was formed for low Al composition (i.e.10%). Crystallinity of the TiAl film was improved at post annealing temperature of 800 C.

Amorphous <inline-formula><math display="inline" overflow="scroll"><mrow><mi>TiAl</mi></mrow></math></inline-formula> films for micromirror arrays with stable analog deflection integrated on complementary metal oxide semiconductors

Large micromechanical mirror arrays (MMA) with analog pixel deflection integrated onto active CMOS address circuitry require both high-quality planar reflective optical surfaces and a stable deflection versus voltage characteristic. However, for implementing a CMOS-compatible surface-micromachining process, certain obstacles such as a restricted thermal budget and a limited selection of suitable materials must be overcome. Amorphous TiAl is presented as a new actuator material for monolithical MEMS integration onto CMOS circuitry. TiAl films may be sputter deposited at room temperature, have an x-ray amorphous structure, and a low stress gradient. The glassy structure and high melting point make TiAl less vulnerable to stress relaxation, which makes TiAl an ideal spring material. One-level actuators with TiAl or Al-TiAl-Al structural layers and two-level actuators with separate TiAl spring and Al-alloy mirror layers were fabricated and tested with respect to their drift stability. The stability of TiAl-based actuators was found to be superior in comparison to one-level Al-alloy actuators. Two-level actuators with TiAl hinges emerge as the most promising design.

Effect of capping layer on the crystallization of amorphous CoFeB

AbstractWe investigated the effects of capping layer on the crystallization of amorphous top‐CoFeB electrode in the glass/CoFeB/MgO(001)/CoFeB/cap stack. Evolution of texture and crystallization of the top‐CoFeB electrode can be affected by the texture and the crystallinity of the capping layer. When the (002)‐textured Ru capping layer was used, a (110) texture of CoFeB was developed during annealing at 350 °C for 90 min. On the other hand, when the amorphous TiAl capping layer was used, a (002) texture of the CoFeB was developed during crystallization due to MgO (001) texture seeding effect. Consequently, the texture evolution of the amorphous top‐CoFeB electrode during annealing could be controlled by the texture and crystallinity of the adjacent capping layer. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Capping층 재료에 따른 CoFeB/MgO/CoFeB 자기터널접합의 미세구조와 자기저항 특성

본 연구에서는 CoFeB/MgO/CoFeB 구조를 가지는 자기터널접합에서 capping층 재료의 종류와 열처리 시간에 따른 비정질 top CoFeB 자성층의 결정화 상태 및 자기터널접합의 자기적 특성 변화에 대한 연구결과를 비교 분석 하였다. Hcp(Hexagonal close-packed)의 결정구조를 가지는 Ru(002)를 capping층 재료로 사용한 자기터널접합 박막의 경우에는 열처리 이후 Ru과 인접한 부분의 top CoFeB이 bcc-CoFe(110)로 성장하는 반면, TiAl과 ZrAl을 capping층 재료로 사용한 자기터널접합의 경우는 열처리 이후 top CoFeB이 MgO와 epitaxial하게 bcc-CoFe(002)로 결정성장 하였다. 이로 인해 Ru을 사용한 자기터널접합의 터널자기 저항비(46.7%)보다 약 1.5배 높은 터널자기저항비(TiAl: 71.8%, ZrAl: 72.7%)를 나타내었다. We investigated the effects of the capping layer materials on the crystallization of the amorphous top-CoFeB (t-CoFeB) electrode and the magnetoresistance properties of the magnetic tunnel junctions (MTJs). When the hcp(002)-textured Ru capping layer was used, the amorphous t-CoFeB was crystallized to bcc-CoFe(110). The CoFe(110)/Ru(002) texture relation can be minimized the lattice mismatch down to 5.6%. However, when the fine polycrystalline but almost amorphous TiAl or amorphous ZrAl were used, the amorphous t-CoFeB was crystallized to bcc-CoFe(002). When the amorphous capping materials were used, the evolution of the t-CoFeB texture was affected mainly by the MgO(001) texture. Consequently, the M ratios of the annealed MTJ capped with the ZrAl and TiAl (72.7 and 71.8%) are relatively higher than that of the MTJ with Ru capping layer (46.7%). In conclusions, the texture evolution of the amorphous t-CoFeB during the post deposition annealing could be controlled by the crystallinity of the adjacent capping layer and in turn, it affects the TMR ratio of MTJs.