Al 5Mg Research Articles

Electrochemical reactivity, surface composition and corrosion mechanisms of the complex metallic alloy Al 3Mg 2

A corrosion mechanism is proposed for Al 3Mg 2, based on electrochemical tests, XPS, and depth profiling using XPS and ToF-SIMS. After short (∼2 min) solution exposure, the surface consists of a surface film above dealloying. The dealloying is attributed to selective Mg dissolution and the surface rearrangement of Al into islands, although the metallic Al could alternatively be formed by two reduction reactions. The surface film thickness was ∼10 nm. After exposure to ultra-pure water, the composition was AlMg 1.3O 0.2(OH) 5.1 corresponding to Al(OH) 3·1.1 Mg(OH) 2·0.2MgO. After exposure to 0.01 M Na 2SO 4, the composition was AlMg 0.2O 0.4(OH) 2.5 corresponding to Al(OH) 3·0.1Al 2O 3·0.2MgO. Longer exposure produced a thicker surface film, more pronounced metallic Al islands and more MgH 2. Three possibilities are identified for MgH 2 formation. Al(OH) 3 formation is attributed to a precipitation reaction. Bulk nanoporous Al 3Mg 2 formation is predicted to be possible by Mg dealloying of Mg 17Al 12.

Effect of thermomechanical treatment on the corrosion of AA5083

Corrosion resistance degradation induced by the presence of β-phase (Al 3Mg 2) limits the performance of aluminum alloy 5083 for some critical applications. The effect of thermomechanical treatment on the corrosion of AA5083 is presented in this paper. The samples subjected to the thermomechanical treatment showed superior corrosion resistance compared to the as-received samples. The effects of grain boundary character distribution, grain shape, texture, and precipitates on corrosion are discussed based on experimental observations. Special boundaries are expected to have a weak effect on the overall corrosion resistance of the material compared to the other factors.

Influence of subsequent cold work on the superplastic properties of a friction stir welded (FSW) aluminium alloy

Friction stir welding (FSW) can dramatically reduce grain size conferring excellent superplastic properties in certain aluminium alloys. Consequently, FSW of thick plate followed by rolling to sheet could potentially be used as a method of producing wide superplastic sheet from extrusion blanks for automotive applications.An extruded Al–4Mg–1Zr alloy was FSW prior to rolling to sheet. The structure of the FSW region after rolling was complex and in certain cases significant grain coarsening resulted during heating to the superplastic deformation temperature. A detailed investigation of the factors responsible is available here.

On nucleation and propagation of PLC bands in an Al–3Mg alloy

The processes of nucleation and propagation of Portevin–LeChatelier deformation bands during tensile deformation of the alloy AA5754 have been investigated by a special optical technique for local strain measurements, combining both a line-scan and a CMOS camera to record the changes of surface strain, applying digital image correlation. Examples for bands of types A and B are observed and discussed with respect to the micro-mechanisms of their nucleation and propagation, emphasizing local stress concentrations in addition to the well-known effects of dynamic strain aging and dislocation interactions for explaining the PLC behavior. Finally the nucleation and propagation of PLC bands are simulated by computations based on a simple constitutive model.

Assessment of the surface state behaviour of Al 71Cu 10Fe 9Cr 10 and Al 3Mg 2 complex metallic alloys in sliding contacts

Electrochemical measurements and friction measurements during continuous and intermittent unidirectional sliding tests are used to monitor and to evaluate the surface characteristics of two types of metallic materials characterized by a huge unit cell, namely Al 71Cu 10Fe 9Cr 10 and Al 3Mg 2. The modification of the surface characteristics results from the periodic mechanical removal of a surface film during sliding, and the subsequent (electro)chemical re-growth of a surface film in-between successive sliding contacts. Al 71Cu 10Fe 9Cr 10 and Al 3Mg 2 materials were tested in a phosphate buffer solution pH 7 at 25 °C to compare their depassivation and subsequent repassivation behaviour. The Al 3Mg 2 material was also tested in a 0.1 M KOH solution pH 13 and 25 °C to reveal the role of constituting metallic elements on the surface film formation. The effect of film formation and removal on the coefficient of friction recorded during unidirectional sliding is discussed.

Studies on friction and formation of transfer layer when Al–4Mg alloy pins slid at various numbers of cycles on steel plates of different surface texture

In the present investigation, various kinds of textures, namely, unidirectional, 8-ground, and random were attained on the die surfaces. Roughness of the textures was varied using different grits of emery papers or polishing powders. Then pins made of Al–4Mg alloys were slid against steel plates at various numbers of cycles, namely 1, 2, 6, 10 and 20 under both dry and lubricated conditions using an inclined pin-on-plate sliding tester. The morphologies of the worn surfaces of the pins and the formation of transfer layer on the counter surfaces were observed using a scanning electron microscope. Surface roughness parameters of the plate were measured using an optical profilometer. It was observed that the coefficient of friction and formation of transfer layer during the first few cycles depend on the die surface textures under both dry and lubricated conditions. It was also observed that under lubricated condition, the coefficient of friction decreases with number of cycles for all kinds of textures. However, under dry condition, it decreases for unidirectional and 8-ground surfaces while for random surfaces it increases with number of cycles.

The α-Mg solvus and isothermal section of Mg-rich corner in the Mg–Zn–Al ternary system at 320 °C

The phase equilibria and compositions in Mg-rich corner at 320 °C have been investigated in detail in the Mg–Zn–Al ternary system through the equilibrated alloy method by using X-ray diffraction (XRD), scanning electron microscopy (SEM) assisted with energy dispersive spectroscopy of X-ray (EDS). Besides the binary compounds Mg 17Al 12 (γ) and MgZn in equilibrium with α-Mg solid solution, there exist two ternary compounds Al 5Mg 11Zn 4 (φ) and Mg 32(Al, Zn) 49 (τ). A new two-phase region (α-Mg + τ) has been experimentally identified. The whole composition region of the φ phase has been also obtained at 320 °C, i.e., ∼53.7–57.0 at.% Mg, ∼14.7–27.4 at.% Al, ∼17.7–30.5 at.% Zn. The solubility of Al in the α-Mg solid solution in equilibrium with the γ phase decreases due to the dissolution of Zn, while the solubility of Zn increases for the addition of Al when the MgZn/α-Mg phases are in equilibrium, compared to the Mg–Al and Mg–Zn binary systems, respectively. The maximum solubility of Zn in the γ phase and that of Al in the MgZn phase are both about 8.3 at.% in the Mg-rich corner. Moreover, the element Al mainly substitutes the Zn in the MgZn phase while the Mg content hardly changes.

A first quantitative XPS study of the surface films formed, by exposure to water, on Mg and on the Mg–Al intermetallics: Al 3Mg 2 and Mg 17Al 12

An XPS investigation was carried out of the surface films, formed by exposure to ultrapure water, on mechanically ground Mg and the two Mg–Al intermetallic compounds: Al 3Mg 2 and Mg 17Al 12. The mechanically ground Mg surface had a film of MgO at the Mg metal surface covered by a Mg(OH) 2 layer, formed by the reaction of the MgO with water vapour in the air. Upon immersion in ultrapure water, this film converted to a duplex film with an inner MgO layer next to the Mg metal and an external porous hydroxide layer. For both intermetallics, the XPS data is consistent with (i) preferential dissolution of Mg and (ii) a 10 nm thick film on the surface after immersion in ultrapure water; the film composition on Al 3Mg 2 was AlMg 1.4 O 0.2 (OH) 5.4 whilst on Mg 17Al 12 the composition was AlMg 2.5 (OH) 8 .

Effects of Sr and Ca on the microstructure and properties of Mg–12Zn–4Al–0.3Mn alloy

Microstructures and properties of the Mg–12Zn–4Al alloy with calcium and strontium addition have been investigated. The results indicate that the as-cast microstructure of the ZA124 (Mg–12Zn–4Al) alloy consists of the α-Mg matrix and block shaped quasi-crystal at grain boundaries. Calcium addition to the ZA124 alloy results in the formation of a lamellar eutectic phase ϕ (Al 2Mg 5Zn 2) and the transition of the non-equilibrium quasi-crystalline phase Q to the equilibrium τ (Mg 32(Al,Zn) 49) phase. The as-cast microstructure of the alloy with Sr addition consists of a binary dense eutectic ɛ (Mg 51Zn 20) phase and equilibrium τ phase. A small amount of calcium addition to the base alloy results in decrease of ultimate strength as well as ductility, whereas addition of strontium causes increase of both ultimate and yield strengths but decrease of ductility. The creep resistance of the ZA124 alloy is significantly improved by a small amount of calcium addition due to the formation of a grain boundary network consisting of the ϕ phase. However, strontium addition to the ZA124 alloy causes reduction on the creep resistance. Microstructure observations performed on the sample with strontium addition after creep test reveal that the ɛ phase is distorted during creep, reflecting its formation in the as-cast microstructure is not beneficial to creep properties of the ZA124 alloys. The creep mechanism responsible for the alloys studied has also been illustrated based on stress exponent and activation energy obtained from a series of creep tests.

Mechanical alloying and milling of Al–Mg alloys

Solid solubility extension of Mg in Al was achieved for the binary Al–Mg system in the range of 10–40 at.% Mg by mechanical alloying of elemental powder mixtures and by mechanical milling of pre-alloyed ingots. No indication for the formation of the equilibrium phases (β-Al 3Mg 2 and γ-Al 12Mg 17) after milling was observed. The cell parameters of the solid solutions increase linearly with increasing Mg content with a slope that depends on the processing route used. During heating, the Al(Mg) solid solutions prepared from the different starting materials display a complex behavior characterized by several exothermic events. At low temperatures, an increasing amount of Mg is rejected from the solid solution with increasing temperature. At higher temperature, the β′-phase, a hexagonal intermediate phase with approximate composition Al 3Mg 2, is formed. The β′-phase most likely serves as precursor for the formation of the equilibrium β-Al 3Mg 2 phase, which occurs in the following exothermic event.

Hot deformation of AA6082-T4 aluminum alloy

High-temperature tensile deformation of 6082-T4 Al alloy was conducted in the range of 623–773 K at various strain rates in the range of 5 × 10−5 to 2 × 10−2 s−1. Stress dependence of the strain rate revealed a stress exponent, n of 7 throughout the ranges of temperatures and strain rates tested. This stress exponent is higher than what is usually observed in Al–Mg alloys under similar experimental conditions, which implies the presence of threshold stress. This behavior results from dislocation interaction with second phase particles (Mg2Si). The experimental threshold stress values were calculated, based on the finding that creep rate is viscous glide controlled, based on creep tests conducted on binary Al–1Mg at 673 K, that gave n a value of 3. The threshold stress (σo) values were seen to decrease exponentially with temperature. The apparent activation energy for 6082-T4 was calculated to be about 245 kJ mol−1, which is higher than the activation energy for self-diffusion in Al (Qd = 143 kJ mol−1) and for the diffusion of Mg in Al (115–130 kJ mol−1). By incorporating the threshold stress in the analysis, the true activation energy was calculated to be about 107 kJ mol−1. Analysis of strain rate dependence in terms of the effective stress (σ − σo) using normalized parameters, revealed a single type of deformation behavior. A plot of normalized strain rate (\( \dot{\varepsilon }kT/DGb \)) versus normalized effective stress (σ − σo)/G, on a double logarithmic scale, gave an n value of 3.

Phase selection of ternary intermetallic compounds during solidification of high zinc magnesium alloy

The phase selection of ternary intermetallic compound τ phase (Mg 32(Al, Zn) 49) and Φ phase (Al 2Mg 5Zn 2) in high zinc magnesium alloys was studied by using scanning electron microscope, X-ray diffractometer and differential scanning calorimeter, etc. The results indicate that, when adding element Si in Mg-8Zn-4Al-0.3Mn (ZA84) alloy, Φ phase is promoted, whereas τ phase is inhibited. The Chinese script-type Mg 2Si and matrix microstructure are greatly refined, the formation of τ phase is facilitated and Φ phase is restrained when modifier Al-AlP master alloy is added in ZA84 alloy containing Si. The kinetics study of phase selection indicates that there is a critical degree of undercooling of the melt. If the undercooling exceeds the critical value, τ phase preferentially forms while Φ phase is restrained; otherwise, Φ phase preferentially forms while τ phase is restrained.

Vibration fracture resistance of 5052H112 aluminum alloy processed by cold rolling and friction stirring

Abstract Hot-rolled Al–3Mg alloy processed by various cold-rolled reductions exhibited excellent vibration fracture resistance, however subsequent friction stirring did not endow better vibration fracture resistance than cold-rolled samples in spite of possessing microstructural refinement and homogenization. The D–N curve under a constant initial deflection amplitude (6.5 mm) also showed that the vibration fracture resistance of cold-rolled specimens increased as a result of the slower crack propagation rate. All samples used in this investigation showed a work hardening feature from the initial stage of the D–N curve, and the deflection amplitude tended to increase as the number of vibration cycles increased. During this ascending stage, slip band cracking and strain hardening in the vicinity of the main crack could be recognized. The D–N curve feature of the cold-rolled samples depicted a longer plateau region while the deflection amplitude was at its maximum. It is suggested that the dislocation tangles introduced by cold rolling can strengthen the matrix. As a result, the deflection amplitude drops could be associated with increasing the dislocation tangles due to prior cold rolling while the main crack achieved a critical crack length.

Wear properties of copper-coated short steel fiber reinforced stir cast Al–2Mg alloy composites

In the present investigation, wear properties on 2.5, 5 and 10 wt% copper-coated short steel fiber reinforced Al–2Mg alloy composites fabricated by stir casting process were carried out using pin-on-disc wear testing apparatus. The effects of sliding distance, applied load and wt% of fiber on the dry sliding behaviour were evaluated. Dry sliding wear tests at room temperature revealed that the wear rate of MMCs were significantly lower than alloy. The coefficient of friction and the wear rate also decreased with increasing fiber content. The worn surface and debris of specimens were examined under scanning electron microscopy (SEM) to find out the wear mechanism. SEM observations revealed that extensive microcracks occur on the surface of the alloy tested at lower loads. The growth of these microcracks eventually led to the delamination of debris from the alloy surface. The copper-coated steel fiber addition tended to reduce the plastic deformation in the surface layer there by reducing the occurrence of microcracking in the MMCs. The wear mechanisms of MMCs are dominated by oxidative wear at lower load but it changed to severe wear when applied load are increased. The composites containing 2.5 and 5 wt% of fibers exhibited a load dependent transition from mild to severe wear with increasing load. In case of 10-wt% fiber composites showed only mild wear even at higher applied load.

The influence of mischmetal and tin on the microstructure and mechanical properties of Mg–6Zn–5Al-based alloys

The influence of the addition of mischmetal (MM) and tin (Sn) (total content of mischmetal and tin = 4 wt.%) on the microstructure, aging behavior and mechanical properties of Mg–6Zn–5Al-based alloys has been investigated. The microstructure of the as-cast alloys consists of α-Mg, Mg 32(Al, Zn) 49, Al 2Mg 5Zn 2, Mg 2Sn and Al 2MMZn 2 phases, and the morphology of these intermetallic phases varies with different MM and Sn additions. The hardness vs. aging time curves of all the alloys exhibit two peaks, and nanocrystalline Mg 32(Al, Zn) 49 and Mg 2Sn precipitates are formed in the matrix. The alloys exhibit high tensile properties at 200 °C, which indicates that the high thermal stability of the Mg 32(Al, Zn) 49, Mg 2Sn and Al 2MMZn 2 phases can hinder dislocation and grain-boundary sliding at elevated temperatures.

Wetting of pure aluminum and selected alloys on polycrystalline alumina and sapphire

Using a modification of the dispensed drop method to measure true contact angles of readily oxidizing metals and alloys, the wettability of polycrystalline alumina and A-plane sapphire by pure aluminum and selected aluminum alloys was investigated. The experiments were performed under high vacuum in a horizontal tube furnace. The experimental setup produces a sessile drop free of its natural surface oxide layer minimizing flight time of the drop, and maintaining a drop impingement on the substrate. The experiments showed that there is no significant difference in the wettability of alumina and sapphire by aluminum as well as Al–11.5Si, Al–1Mg and Al–7Cu. On both substrates, aluminum shows a strong increase in contact angle well into the non-wetting regime just above the melting point. The wetting behavior of Al–7Cu on both substrates is slightly but significantly reduced in comparison to pure aluminum. The contact angles of Al–1Mg and Al–11.5Si remain rather constant between the respective liquidus temperatures of the alloys and 800 °C with θ (Al–1Mg) < θ (Al–11.5Si). Only Al–7Cu above 730 °C achieves the contact angle interval of 70–86° suggested to be most beneficial in terms of aluminum foam stabilization.

Phase transformations in mechanically milled and annealed single-phase β-Al 3Mg 2

Mechanical deformation drastically affects both microstructure and thermal stability of polycrystalline β-Al 3Mg 2. Upon milling, the β-Al 3Mg 2 phase transforms into a nanoscale supersaturated Al(Mg) solid solution with 40 at.% Mg. Upon heating, the milled powders display a complex thermal behavior characterized by four distinct exothermic events. At low temperatures, an increasing amount of Mg is rejected from the solid solution with increasing temperature. At higher temperatures, the β′-phase, a hexagonal phase with composition Al 3Mg 2, is formed and no traces of the solid solution can be detected, indicating that the solid solution is metastable and transforms into more stable phase(s). Finally, the subsequent exothermic events reveal the formation and growth of the β-Al 3Mg 2 phase.

Predicting the J integral fracture toughness of Al 6061 using the small punch test

ABSTRACTThe 6000 series aluminium alloys (Al–Mg–Si systems) are commonly used as medium‐strength structural materials; in particular, the 6061 (Al–1Mg–0.6Si) alloy is widely utilized as a general‐purpose structural material due to its excellent formability and corrosion‐resisting capabilities. The objective of this study was to obtain a correlation between the small punch (SP) test estimated equivalent fracture strain (ɛqf) and fracture toughness (J1C) property for 6061 aluminium, and determine its viability as a non‐destructive fracture toughness test technique for remaining life assessment of in‐service components. Samples of 6061‐T6 aluminium were cut from bulk plate, in both the longitudinal and transverse directions, for the as‐received condition as well as subjected to three different over‐ageing heat‐treatment schedules. A strong linear correlation between valid J1C and SP estimated biaxial fracture strain ɛqf is presented for aluminium 6061 at room temperature.

Synthesis of an Al/MgAl2O4In situMetal Matrix Composite from Silica Gel

An aluminum/MgAl2O4in situmetal matrix composite has been synthesized using silica gel containing ∼98% SiO2in an Al–5Mg alloy. The thermodynamics and kinetics of MgAl2O4formation have been discussed in detail. A transition phase of composition between MgO and MgAl2O4has been detected in the SEM‐EDS analysis of the particles extracted from the composite by a 25% NaOH solution. This confirms the gradual transformation of MgO to MgAl2O4by the reaction 3SiO2(s)+2MgO(s)+4Al(l)→2MgAl2O4(s)+3Si(l). The stoichiometry,n, of MgAl2O4has been found to sustain close to 1 and the crystallite growth of MgAl2O4has been stopped atD∼30 nm in the composites held at 750°C up to 10 h.

Effect of wt% reinforcement on microstructure and mechanical properties of Al–2Mg base short steel fiber composites

Abstract Al–2Mg alloy composites with varying wt% of copper-coated short steel fiber reinforced were prepared through liquid process vortex method. Steel fibers were coated with copper by electroless deposition method. It is observed that density, hardness and porosity increased with increasing wt% of fibers. The mechanical properties of these composites were measured and the results are correlated with the microstructure observation. It was found that copper-coated short steel fiber reinforced composites showed considerable improvement in strength with good ductility. Tensile strength increased with increasing wt% of copper-coated steel fiber but % of elongation reasonably good for 2.5 and 5 wt% fiber composites. It is observed that 10 wt% fiber composites shows minimum strength among the composites due to high porosity. Fracture surface of tensile specimen was examined under SEM, which revealed a dimple formation and fiber pull out. Fiber pull out from the matrix increased with increasing wt% of fibers as a result composites lose ductility. Copper coating on steel fiber improved the strength properties while retaining a high level of ductility due to better interface bonding.