Al-Mo Alloys Research Articles

The Corrosion Behavior of Sulfidation-Resistant Fe–Mo–Al Alloys in H2/H2S Atmospheres at 900°C

The corrosion behavior of Fe–22Mo–10Al (a/o, atom %),Fe–20.5Mo–15.7Al, and Fe–10Mo–19Al was examined inflowing H2/H2S gases of 4 Pa sulfur partial pressureat 900°C. Al2O3 was stable on all the alloys inthe atmospheres investigated. Fe–22Mo–10Al andFe–20.5Mo–15.7Al reacted slowly, following the parabolic ratelaw. Multilayered reaction products were formed on these alloys and it isuncertain which layer(s) provided the protection. Fe–10Mo–19Alreacted even more slowly, exhibiting two-stage parabolic kinetics. Duringthe early stage of this alloy's reaction, a preferential reaction zone,consisting of an oxide mixture, possibly Al2O3+FeAl2O4,and nonreacting Fe3Mo2, provided the protection. Duringthe later reaction stage, the formation of a continuous, externalAl2O3 layer further decreased the alloy reaction rate.

Local icosahedral symmetries in Al-Mo alloys prepared by ball milling

Annealing products of mechanically treated Al 75Mo 25 powders have been studied by X-ray powder diffraction with the Rietveld method. The differential scanning calorimetry traces of as-milled powders showed transformation features whose temperature decreased as a function of mechanical treatment time. Different phases were identified across the calorimetric events, which were also reported in the high-temperature phase diagram of the Al-Mo system, such as Al 12Mo, Al 5Mo, Al 4Mo, Al 3Mo and Al 22Mo 5. Nevertheless, the solid state transformation processes activated in the calorimeter at ambient pressure are of different nature with respect to those observed by means of neutron diffraction during in situ annealing under vacuum. In the specimen mechanically alloyed for 32 hours the Al 12Mo phase was observed after annealing with both processes. In the crystal geometry of Al 12Mo, which is regarded as a first order approximation of the quasi-crystalline phase, the molybdenum atoms are arranged in a BCC fashion and are surrounded icosahedrally by twelve aluminum atoms.

A study of Al-Mo alloys synthetized by mechanical treatment and annealed in-situ

Al 75Mo 25 alloys were synthesized by ball milling of pure elements. The decrease of fcc aluminium peaks from the neutron diffraction patterns as a function of mechanical treatment time suggests the formation of an extended Mo(Al) solid solution, even though no significant change of the bcc lattice parameter is observed. Further evidence of the existence of a metastable phase is supplied a posteriori by in situ annealing of the processed metal powders at temperatures below 750 °C. The main product is the intermetallic Al 8Mo 3 compound.

Formation of Microstructures in the Molybdenum-Aluminum-Titanium Ternary System through Eutectoid Decomposition

Formation and evolution processes of microstructure of Mo-Al alloys retarded by addition of 10 at%Ti are studied through various heat treatments. Fine acicular Mo 3 Al precipitates are observed inside a grain for a specimen quenched at 1673 K as an initial stage of the eutectoid decomposition of bcc phase. It suggests that there is a crystallographic orientation relationship between the bcc matrix and A15 precipitates. By changing heat treatment conditions, fraction of fine lamellar structure decreases and spheroidized area increases. Subsequent aging treatment at lower temperature also provides the spheroidized structure. A coarse equiaxed structure is observed for a specimen quenched at 1673 K then aged at 1173 K.

Structure characterization of a laser-processed Al–Mo alloy

The surface structure of a laser-processed Al–Mo alloy has been characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffractory (XRD). The alloy was prepared by first laser alloying a mixture of Al and Mo powders into an Al substrate and then laser remelting the alloyed surface. Following the first laser alloying process, the needle-like equilibrium phases (Al5Mo(h) and Al5Mo(r)) are formed with a broad size ranges and distribute inhomogeneously in the α-Al solid solution matrix. This coarse structure is replaced by a finer, uniform dispersion of dendrites after the subsequent laser remelting. Four basic types of solid states precipitates are observed: (1) irregularly shaped particles constructing the dendrites and having a nearly Al5Mo stoichiometry; (2) needle-like particles which is the Al5Mo (r) phase; (3) Faceted particles having a cubic structure with a stoichiometry close to Al7Mo; (4) tiny, equi-axed particles, with a rather narrow particle size distribution and a cubic structure. © 1998 Chapman & Hall

Passivity of Al–Mo amorphous alloys andtheir electrochemical breakdown

Abstract The passivity of Al–Mo amorphous alloys and stability of the passive state in borate buffer solution containing various concentrations of Cl− (0–1 M NaCl) have been investigated by electrochemical, microscopic and surface analytical methods (AES, XPS). The anodic passivating film had an insulating character supporting voltages over several volts. Surface analytical investigations have revealed that the film consists of oxidized Al and O, whereas Mo signals did not suggest any oxidized molybdenum containing species within the film. Microscopic examinations have shown that at and above the pitting potential, E′np in Cl−-containing solution, an isotropic localized dissolution (pitting) took place. The localized current density within the growing pits was ∼105 times higher than that flowing from the remaining passive surface. A tentative mechanism of passivity of the Al–Mo alloys and of its breakdown is proposed.

Icosahedral crystal in a laser processed Al-Mo alloy

Icosahedral crystal in a laser processed Al-Mo alloy

Analysis of anisotropic SANS patterns obtained from Ni–Al–Mo alloys with bimodal particle size distributions

Analysis of anisotropic SANS patterns obtained from Ni–Al–Mo alloys with bimodal particle size distributions

EXAFS Study of the Local Atomic Order in Ti2AlX (X=Nb, Mo) B2 Intermetallic Compounds

Extended X-ray absorption fine-structure (EXAFS) has been used to study the local atomic order in Ti 2 AlMo and Ti 2 AlNb in the B2 phase. It is found that the Mo and Nb atoms have different behaviors. Whereas the Ti 2 AlMo alloy is ordered as a pseudo-B2 structure in which the Mo and Al atoms are localized on one site and Ti atoms on the other, the Ti 2 AlNb alloy exhibits a more disordered behavior since the Nb and the Ti atoms are distributed on both sites. EXAFS experiments allowed us to quantify the fraction of Nb on each site in Ti 2 AlNb as well as the bond lengths in both alloys.

Temporal Evolution of Bimodal Distribution of γ’ Precipitates in Ni-Al-Mo Alloys under the Influence of Coherency Strains

ABSTRACTThe influence of coherency strains produced by the γ-γ’ lattice mismatch, δ, on the decomposition process of Ni-Al-Mo alloys with a bimodal size distribution is presented. Samples with δ ranging from positive to negative, were investigated in a double-step aging procedure. The evolution of the microstructure and the kinetics of coarsening were studied using transmission electron microscopy (TEM). The lattice mismatch between the matrix and the different classes of precipitates was determined by high-resolution high-temperature x-ray diffraction. It is shown that the strain fields produced by the lattice mismatch can influence dramatically the decomposition of metallic alloys. It is suggested that the reduction of the coarsening rate of the large precipitates, the fast coarsening rate of the small precipitates and the distortions detected in the matrix are all direct consequences of the elastic fields produced by the γ-γ’ lattice mismatch.

The sulphidation behavior of MoAl alloys with low aluminum contents

The beneficial effect of alloying with aluminum to enhance the sulphidation resistance of molybdenum using sputter-deposited amorphous Mo-Al alloys has been studied as a function of temperature (1073–1273 K), aluminum content (1.7, 6.2 and 10.7 at% Al) and sulphur vapor pressure (20–4000 Pa) in He-S 2 gas mixtures. It has been shown that the sulphidation process follows parabolic kinetics, being diffusion controlled. The reaction rate of Mo-Al alloys with low aluminum contents is lower than that of pure molybdenum and decreases with increasing aluminum content in the alloy. Because sulphide scales formed on Mo-Al alloys containing small amounts of aluminum are homogeneous and composed only of MoS 2, the lower sulphidation rate of those alloys is explained in terms of the aluminum doping effect in the MoS 2 scale. It is believed that the better protective properties of the sulphide scale on Mo-Al alloys in comparison with those of the MoS 2 scale on pure molybdenum result from a lower defect concentration in aluminum-doped MoS 2 phase. The experimentally obtained dependence of the sulphidation rate of Mo-1.7Al alloy on the sulphur pressure at 1173 K is in accordance with that predicted from theoretical considerations.

New amorphous alloys resistant to high temperature corrosion

High temperature corrosion of sputter-deposited amorphous Al(34–46)Mo and Al31Mo(6–16)Si alloys has been studied as a function of temperature (973–1273 K) in sulfur vapor (10 3 Pa), as well as in oxygen and air. The sulfidation process follows parabolic kinetics, so is diffusion controlled. Over the whole temperature range studied, the discussed alloys showed excellent resistance to sulfide corrosion, their sulfidation rates being comparable with the oxidation rates of chromia-forming materials. The oxidation resistance of binary AlMo alloys has been found to be satisfactory, but only at temperatures not exceeding 1073 K. Above the melting point of MoO 3 (1069 K), the scale, consisting mainly of Al 2O 3 with about 5% MoO 3, becomes unprotective because of the evaporation of MoO 3. However, ternary AlMoSi alloys show excellent resistance to oxidizing environment up to about 1200 K, their oxidation rates being comparable with those of Al 2O 3 formers. No traces of molybdenum have been found in the Al 2O 3 scale on these alloys.

On the growth mechanism of the sulphide scale on amorphous AlMo alloys

The growth mechanism of the sulphide scale on amorphous Al-Mo alloys has been studied using a marker technique. A very thin (5 nm) gold marker film was deposited onto the surface of the Al-46Mo alloy and after sulphidation for 5 hours at 1073 K the gold marker distribution in the scale has been determined by RBS. It has been found that the main part of the marker remained at the scale surface indicating that the double-layer sulphide scale on the discussed alloy is growing by the inward diffusion of sulphur.

Corrosion-resistant amorphous surface alloys

This is a review of laser and electron beam processing and sputter deposition for the preparation of corrosion-resistant amorphous surface alloys and of the characteristics of thus prepared surface alloys. Amorphous surface alloys with a large surface area were prepared by repetition of instantaneous melting of a very restricted volume of the surface by irradiation with a CO 2 laser or electron beam and subsequent self quenching by the cold bulk substrates. The materials consisting of the amorphous surface alloys and bulk crystalline metals are quite suitable for corrosion-resistant materials with other specific properties. Sputter deposition was used for preparation of various amorphous alloys such as Al-Ti, Al-Zr, Al-Nb, Al-Ta, Al-Cr, Al-Mo, Al-W, Cr-Ti, Cr-Zr, Cr-Nb, Cr-Ta, Cu-Nb and Cu-Ta. The corrosion resistance of amorphous aluminum alloys was very high and could be changed from that comparable to 304 stainless steels to that far exceeding corrosion-resistant nickel-base alloys by changing the alloying element and its concentration. The corrosion rates of amorphous chomium-valve metal alloys are several orders of magnitude lower than those of alloy constituting elements. The amorphous Al-Mo alloys show high resistance to both sulfidation and oxidation at high temperatures. Sputter deposition is a potential method to produce new materials with specific properties.

Phase separation in a ti—al—mo alloy studied by anomalous small-angle x-ray scattering a synchrotron radiation experiment

Abstract The mode of decomposition of the ordered β2 phase (B2) at 450°C of the alloy Ti-21·6Al-3·4Mo has been studied by transmission electron microscopy (TEM) and anomalous small-angle X-ray scattering performed at the K Ti absorption edge. TEM observations have shown that the precipitation occurs according to a two-phase system, and the α2 spherical precipitates are embedded in a B2 matrix. In order to calculate the composition of both phases some parameters such as the Guinier radius R G, the spherical excluded volume (or diameter D), the number of scattering precipitates per unit volume Nv have been determined by fitting the theoretical scattered intensity curves derived from a two phase model of precipitation to the experimental data. The composition of α2 and β2 phases is fairly constant with the ageing times considered. The plot of the ternary diagram shows that the α2 precipitates are slightly depleted in aluminium with ageing time as compared with the B2 matrix.

Metastable phase formation in rapidly solidified Al–Mo alloys

In Al-Mo alloys, rapidly quenched from the melt at rates of approximately 106 KS−1, it has been demonstrated that single phase solid solution alloys can be obtained in materials with up to 1·3 at.-...

Metastable phase formation in rapidly solidified Al–Mo alloys

Metastable phase formation in rapidly solidified Al–Mo alloys

Structure and properties of Al-W, Al-Mo, and Al-Nb alloys obtained from granules

1. The highest solubility of molybdenum, tungsten, and niobium in aluminum with cooling from the liquid phase at rates of 103–104 deg/sec is 1.2%W, 0.6%Mo, and 0.75%Nb. 2. Anomalously supersaturated solid solutions of molybdenum, niobium, and tungsten in aluminum are extremely stable. They begin to decompose at 600–630° (Al with 0.75% Nb), 620–650° (Al with 0.62%Mo), and 650–670° (Al with 1.2%W). 3. Additions of tungsten, molybdenum, and niobium strengthen aluminum notably at room temperature and reduce the rate of softening at 350°.

High-temperature strength and ductility increases of Ti-Mo-Al alloys by step aging

Step-aging programs, based on principles of particle-dislocation interactions, were developed systematically to obtain increases in the high-temperature strength and ductility properties of Ti-7 at. pct Mo-Al alloys. A triple-step aging program applied to Ti-7 Mo-16 Al produced a yield stress σ0.2 = 1,500 MN/m2, elongation to fracture e F = 4 pct at room temperature, and σ0.2 = 900 MN/m2, e F = 12 pct at 600°C. A two-step aging program resulted in σ0.2 = 1,350 MN/m2, e F = 5 pct at room temperature; σ0.2 = 800 MN/m2, e F = 20 pct at 600°C.

Al-Mo系平衡状態圖

Theequilibrium diagram of the Al-Mo (Al side) System is determined as shown in Fig. I. Purity of material employed is given in Table I. As the ordinary thermal analysis was not seemed sensitive enough to determine the liquidus curve and the peritectic temperature, the former was detemined by the chemical analysis of liquid phase which was in coexistence with the primary compounds, and the latter was determined from microstructure of specimens quenched from a supposed range of temperature. Solubility of Mo in solid Al could not be detected by back reflection X-ray analysis nor by the change of hardness after quenching and tempering. Composition of intermetallic compound was estimated by the chemical analysis of insoluble constituents of Al-Mo alloys.