Al-Zr Alloys Research Articles

Effects of Si and Ti Additions on the Nucleation and Phase Stability of the L1<sub>2</sub>-Type Al<sub>3</sub>Zr Phase in Al-Zr Alloys

Effects of Si and Ti Additions on the Nucleation and Phase Stability of the L1<sub>2</sub>-Type Al<sub>3</sub>Zr Phase in Al-Zr Alloys

Application of Solution Temperature - alculated Resistivity (T-ρ) Diagram Obtained from Binary Solubility Curves to Al-Zr Alloys

Application of Solution Temperature - alculated Resistivity (T-ρ) Diagram Obtained from Binary Solubility Curves to Al-Zr Alloys

Anodic film formation on sputter-deposited amorphous Al-Zr alloys

The anodic oxidation of amorphous Al-32 at% Zr alloy films at high current efficiency results in amorphous anodic films, comprising a mixture of Al2O3 and ZrO2 units, of uniform composition and thickness. The use of an immobile xenon marker reveals that the films develop, under the high field, at the alloy/film and film/electrolyte interfaces by the inward transport of O2- and OH- ions and the outward transport of cations respectively. This mechanism is typical of growth of amorphous anodic films involving the cooperative transport of cations and anions. The total transport number of cations is about 0.35. Zirconium and aluminium are incorporated into the anodic films, in their alloy proportions, without the development of significant enrichments or depletions of zirconium at the alloy/film interface. Zr4+ ions, Which are immobile in the usual crystalline anodic zirconia, migrate outwards at about the same rate as Al3+ ions. Boron species, incorporated into the films from the electrolyte, are immobile and are distributed uniformly in the film material formed at the film/electrolyte interface by outward migration of cations.

Positron annihilation studies of the dynamics of the evolution of free volume in amorphous alloy Ni25Zr55Al20

The dynamics of microstructural changes taking place in amorphous as well as in the undercooled-liquid states (UCL) have been studied for NiZrAl alloys. The samples have been prepared by planar flow casting. The glass transition and crystallization temperatures have been determined from both dilatation and resistivity measurements to be around 700 and 800 K, respectively. An annealing regime has been selected to treat the samples sequentially in the amorphous and UCL states, i.e., at temperatures above (740 K) and below (640 K) the glass transition temperature. The values of the positron lifetimes, τ, and the intensities of the lifetime components, I, indicate that positrons annihilate at different sites; on annealing, the formation of positron capture sites is observed which gradually change their properties, finally relaxing to a state dependent on the annealing sequence. The change of the positron lifetime distribution can be attributed to a free volume relaxation.

Elevated temperature creep behavior of RSP/PM Al-13%Zr

The promise of Rapid Solidification Processing (RSP) has opened new avenues for alloy development. Major goals have been to develop aluminum alloys for elevated temperature applications because of their superior strength to weight ratio. A number of investigations have been carried out on the precipitation behavior in Al-Zr alloys, using transmission electron microscopy. An important strengthening precipitate in the Al-Zr alloy is Al{sub 3}Zr which essentially belongs to tetragonal crystal system with a c/a ratio of 4.06, a lattice mismatch in the range 1.0 to 2.88% and interfacial energy in the range 10 to 2.13 J/m{sup 2}. The purpose of this paper is to evaluate the creep deformation and fracture behavior of an Al-Zr alloy containing about 13 wt% Zr at an elevated temperature of 698 K.

Fabrication and mechanical properties of in situ formed carbide particulate reinforced aluminium composite

Stable carbide particles of TiC, ZrC and TaC were in situ synthesized in liquid aluminium by the reaction between Al-Ti, Al-Zr or Al-Ta systems liquid alloy and SiC or Al4C3 particles. It was possible to generate TiC particles of nearly 1 μm diameter, even utilizing SiC of 14 μm. However, the dispersion behaviour of TiC particles in the matrix depended on the size of the raw carbide. Finer SiC made the dispersion of TiC particles more uniform, resulting in a greater improvement of the mechanical properties. Furthermore, although Al-Ti-Si system intermetallic compound was detected in a TiCp/Al-Si composite fabricated by the melt-stirring method, those compounds considerably decreased in the composite fabricated by the in situ method. The mechanical properties of in situ formed TiCp/Al-5 wt% Mg and TiCp/Al-5 wt% Cu composites were better than those fabricated by the melt-stirring method and by T6 heat treatment, the properties of in situ formed TiCp/Al-5 wt% Cu composite were further improved. The experimental results were analysed by the reaction model based on the assumption that the overall reaction rate was controlled by both the reaction and the diffusion.

Mechanically driven phase transformation from crystal to glass in Ti-Al binary system

Metastable phases such as metallic glass have been widely studied during the past few years. A number of techniques have been employed to synthesize these alloys, among which, mechanical alloying (MA) was considered to be a promising tool for large production and economical operation. However, the mechanism of formation of the metastable phases during mechanical alloying/ball milling have not been well determined. Previous studies of amorphization by mechanical alloying of pure elements or ball milling of intermetallics have proposed the following mechanics: rapid quenching of local melts produced by mechanical impacts, high density of defects generated by mechanical deformation which raises the free energy of compound to above that of the amorphous phase, effective local temperature rise at the collision site which gives rise to a solid-state reaction similar to thin film diffusion couples. Most of the studies have suggested that solid-state-type reactions are most likely to be responsible for vitrification during mechanical alloying, and the kinetics was controlled by the point and lattice defects generated by mechanical deformation. In the Zr-Al alloy system, however, another mechanisms was proposed by Fecht et al. based on a chemically induced catastrophic transition. The amorphization was interpreted with the elastic instability of crystalmore » caused by the atomic mismatch of two components in supersaturated solid solution; when solute concentration reaches a critical value, the supersaturated solid solution becomes metastable and a polymorphous melting'' transition to a glass may occur. This hypothesis has been further examined by Ma and Atzmon with calorimeter measurements. In this paper, it will be shown experimentally that this mechanism may be valid in the Ti-Al binary system.« less

The bamboo boundary peak in an AlZr alloy

An inverted torsion pendulum is used to study the high temperature internal friction of Al-0.073 wt% Zr alloy. Two internal friction peaks well separated from each other, the bamboo boundary peak and the conventional grain boundary peak, are observed in the original internal friction curve when the size of some grains is larger than the diameter of the specimen. The height of the bamboo boundary peak increases while that of the grain boundary peak decreases with increasing number of bamboo-like grains. The bamboo boundary peak in the completely bamboo-structured specimen decreases linearly in height with increasing size of bamboo grains, until the peak is completely absent in the single crystal specimen. The peak temperature of the bamboo boundary peak and the associated activation energy are 690 K (fp = 0.29 Hz) and 3.20 eV when bamboo grains coexist with fine grains, and 635 K (fp = 0.31 Hz) and 2.74 eV when the complete bamboo structure is formed, respectively. Combining the electron microscopic observation of the second phase particles in boundaries, it is suggested that the bamboo boundary peak is originated by the stress relaxation of the particle-bearing boundary.

New metastable phases in rapidly solidified AlZr and AlTi alloys with high solute contents

Al 100- M Zr M (M = 5, 10, 15 at.%) and Al 100- X Ti X ( X = 2–25 at.%) alloys were rapidly solidified into a ribbon form by a single roller melt spinning method. All the rapidly solidified AlZr alloys were composed of α-Al and L1 2-Al 3Zr phases, while the phase of rapidly solidified AlTi alloys is closely related to the Ti content. With increasing Ti content, a metastable phase appears. The decomposition processes of these metastable phases were also investigated. When the rapidly solidified AlZr alloys are heated in the temperature range 573–773 K, the L1 2-Al 3Zr phase changes into a new metastable tetragonal structure. As the temperature rises to about 873 K, the new metastable phase decomposes and transfers into the equilibrium DO23-Al 3Zr phase.

The metastable Al-Zr alloy thin films prepared by alternate sputtering deposition

Al-Zr alloy thin films are prepared via alternate sputtering deposition. The as prepared films exhibit metastable phases including amorphous, cubic ZrAl3(L12), supersaturated Al-in-Zr close-packed hexagonal structures, and Zr-in-Al face-centered cubic solid solutions with uniform composition distribution. The amorphous phase occurs in the compositions ranging from 16.8 to 69.9 at. % Al. The formation of amorphous structure is considered to follow the mechanism of solid state reaction amorphization or surface diffusion induced mixing. Additionally, the formation of a metastable cubic ZrAl3 intermetallic compound is attributed to its low nucleation energy barrier with respect to the stable tetragonal ZrAl3 (DO23). The heat of crystallization of the amorphous phase is also measured to investigate the metastability of the amorphous phase.

Insight into the pitting corrosion behavior of aluminum alloys

On the basis of the literature data and the current results, a mechanism for the pitting corrosion of Al alloys is proposed. An assumption is made that the transport of Cl − ions through defects in the passive film of aluminum and aluminum alloys is not a rate determining step in pitting. The pit development is controlled by the solubility of the oxidized alloying elements in acid solutions. A very good correlation was found between the pitting potential and the oxidized alloying elements for metastable AlCr, AlZr, AlW and AlZn alloys.

The corrosion behavior of sputter-deposited AlZr alloys in 1 M HCl solution

Using electrochemical measurements and XPS analysis, the corrosion behavior of sputter-deposited Al-Zr alloys in 1 M HCl solution at 30°C was investigated. Their open circuit corrosion resistance, pitting corrosion resistance and passivating ability were enhanced with increasing zirconium. Alloying of aluminum with zirconium extends the time to reach the steady passive state by potentiostatic polarization and decreases the steady state current density. XPS analysis revealed that when potentiostatic polarization was continued in the passive region, Zr 4+ cations in the surface film was slightly concentrated simultaneously with the concentration of zirconium in the underlying alloy surface, due to preferential dissolution of aluminum. The cationic fraction of the surface film finally consisted of Al 3+ and Zr 4+ according to the bulk alloy composition, although the Zr 4+ content was apt to be higher than the alloy zirconium content. Therefore, the higher the alloy zirconium content, the higher the corrosion resistance of the sputter-deposited AlZr alloys.

Alloying Effects on Hydrogen Gettering Properties for Zr-Al System

This paper discusses alloying effects on absorption and desorption kinetics of deuterium for Zr-Al alloys, studied with mass analyzed thermal desorption spectroscopy using a conventional high vacuum system. It is found that the absorption rate of deuterium was proportional to the 1/2 power of deuterium gas pressure. On the other hand, the desorption process obeyed the second order kinetics with respect to the amount of absorbed deuterium. The temperature dependence of the rate constants revealed that the activation energies for both the absorption and desorption processes were lowered by the increase in the Al content in the alloys. Through potential diagrams for the absorption and desorption of deuterium, it was also found that the heat of deuterium (hydrogen) solution decreased with increasing Al composition. In addition, the x-ray diffraction spectroscopy showed the formation of a Zr{sub 4}Al{sub 3} phase in the Zr{sub 3}Al{sub 2} sample owing to repeated absorption and desorption cycles. The results suggest that the electronic factors, for example, work function, electron density, d-band character and so on, play an important role for the alloying effects rather than crystallographic structures.

Reinvestigation of the Zr-rich end of the Zr-Al equilibrium phase diagram

The Zr-rich part of the Zr-Al phase diagram has been redetermined in the range 500–1300°C using 3.3 to 31.1 at% Al alloys with 800 atppm O, 300 atppm N, and 230 atppm C. Techniques used were optical metallography, X-ray diffractometry, electron microprobe analysis, and electrical resistance measurements. The experimental results have been analyzed taking into account the effects of oxygen- and nitrogen-impurities. The main results obtained are 1.(i) Peritectoid compositions (at% Al) and temperature: (19.9)β + Zr5Al3 → Zr2Al at 1215 ± 10°C; (12.0)β + Zr2Al → Zr3Al at 1019 ± 2°C; and, (6.8)β + Zr3Al → (8.3)α at 910 ± 3°C.2.(ii) Redetermination of the α/α + Zr3Al boundary, thereby solving a discrepancy existing in the literature. At 700 ± 10°C the solubility is 3.3 at% Al.3.(iii) The dissolution of oxygen-impurities in Zr-Al alloys stabilizes the Zr2Al phase relative to β and Zr3Al.

Composition and lattice parameters of silicide and matrix in cast Ti–Si–Al–Zr alloys

Composition and lattice parameters of silicide and matrix in cast Ti–Si–Al–Zr alloys

Composition and lattice parameters of silicide and matrix in cast Ti–Si–Al–Zr alloys

The silicide chemistry, i.e. the type, composition, and lattice parameters of the silicide in as cast titanium based Ti–Si–Al–Zr alloys, has been studied. It has been shown that the stoichiometry o...

Effects of processing parameters on aluminide morphology in aluminium grain refining master alloys

AbstractAn investigation has been made of the effects of processing parameters on the microstructure of Al–Ti and Al–Zr alloys produced by the reduction of potassium fluorotitanate and potassium fluorozirconate by aluminium. Blocky aluminides (TiAl3 and ZrAl3 respectively) are produced when the reduction is performed in the liquid+solid phase region and flaky aluminides are produced when it is performed in the liquid phase region. The use of Na2TiF6 rather than K2TiF6 has no apparent effect on the alloy structure. The microstructure and the size of aluminide blocks or flakes produced are further influenced by the subsequent cooling rate experienced by the alloy. Post-fabrication heat treatment of Al–5Ti–0·2B (wt-%) alloys (produced by combined reduction of potassium fluqrotitanate and potassium fluoroborate by aluminium) causes boride particles to precipitate on the aluminide surfaces. These borides grow in size and decrease in number with heat treatment time.MST/1355

Hydrogen and oxygen at metal/oxide interfaces

Using mostly electrochemical methods the chemical potential, diffusion coefficient, partial molar volume and the resistivity increment of hydrogen were determined at room temperature in internally oxidized PdAl, PdMg, PdZn and PdZr alloys. In all samples hydrogen is trapped at the formed metal/oxide interfaces, where irreversible trapping (with respect to anodic polarization) can be distinguished from reversible one. In the cases of Pd Al 2 O 3 and Pd MgO phase boundaries the oxide precipitates were characterized by electron microscopy with regard to their size and their orientation and, therefore, the coverage of the phase boundaries with irreversibly bound hydrogen could be calculated as 1.5·10 15 H/cm 2 or 7·10 14 H/cm 2, respectively, which is about one monolayer of the densest packed oxygen planes of the corresponding oxides. This equivalence and the irreversible nature of the bonding indicates the formation of OH bonds at the phase boundaries. During irreversible trapping a larger volume change is observed as compared to dissolution of hydrogen in regular octahedral sites of palladium. The measured partial molar volumes correspond to an increase of the phase boundary “thickness” of 0.3 to 0.9 Å between Pd and oxide. The irreversible component of hydrogen trapping disapears after annealing in the vapor of the alloying addition (Al, Mg, …) or in vacuum and appears again after a second treatment in air. As an explanation of this effect we propose that annealing in metal vapor or in vacuum, respectively, changes the composition at the phase boundary. At high oxygen activities oxygen segregates to the phase boundaries. In comparison to the oxygen in the oxide the segregated one is less strongly bound and, therefore, can be desorbed at low O-activities. Furthermore the missing bonds to the stable oxide allow the formation of OH bonds.

Precipitation and excess solid solubility in MgAlZr and MgZnZr processed by spray atomization and deposition

The microstructures of two magnesium alloys (Mg-5.6wt.%Zn-0.3wt.%Zr and Mg-8.4wt.%Al-0.2wt.%Zr) processed by spray atomization and deposition were examined and subsequently correlated with the processing history. The microstructural characterization was accomplished utilizing semiquantitative energy-dispersive X-ray analysis (EDAX), scanning electron microscopy and X-ray diffraction analysis. The microstructure of the as-spray-deposited MgZnZr alloy consisted of relatively coarse (50–100 μm) equiaxed cells. Lattice parameter measurements using X-ray diffraction revealed a super-saturated solid solutio of zinc in the magnesium matrix. The presence of the Mg 7Zn 3 phase at the cell boundaries of the MgZnZr alloy was confirmed through semiquantitative EDAX analysis. The microstructure of the MgAlZr alloy in the as-deposited condition consisted of relatively coarse (20–50 μm) cells of magnesium surrounded by an intergranular eutectic microsconstituent which was identified as (Mg) + Mg 17Al 12; X-ray diffraction results also revealed a supersaturated solid solution of aluminum in the magnesium matrix. The aforementioned microstructural features were related to the solidification conditions of the droplets at the moment of impact with the substrate; these conditions were computed from previously developed numerical models. Furthermore, the effect of the magnitude of K m (growth coefficient in the kinetic relationship between growth rate R and undercooling ΔT ∗ ) on the computed fraction solidified as a function of flight distance is discussed. The present results indicate a good correlation between the experimental findings and the numerical predictions.

Anodic polarization behaviour of sputter-deposited AlZr alloys in a neutral chloride-containing buffer solution

Sputter-deposited AlZr alloys consist of a single α-Al phase supersaturated with zirconium at the alloy zirconium content of 11 at.% or less, and become amorphous at higher zirconium contents. The sputter-deposited alloys possess better passivating ability and pitting corrosion resistance in a deaerated borate—boric acid solution containing Cl − ions at pH 8.4 and 25°C than do argon arc-melted counterparts. The pitting potentials of the sputter-deposited alloys continuously increase with the alloy zirconium content, and are significantly higher than those of argon arc-melted counterparts. XPS analysis reveals that the surface film consists of a solid solution oxide of Al 3+ and Zr 4+ ions containing a small amount of OH –. The ennoblement of the polarization potential in the passive region results in increases in the film thickness and the cationic fraction of zirconium. The cationic fraction of zirconium is almost the same as or slightly higher than the alloy composition. The surface films forms on these alloys are not composed of a simple mixture of aluminium oxide and zirconium oxide but consist of a solid solution, such as (Al 3+ a Zr 4+ 1- a )O 2– b (OH) − 4- a-2 b .