DTA/TG study of tungsten oxide and ammonium tungstate reduction by (Mg + C) combined reducers at non-isothermal conditions

In this work the reaction mechanism in the WO₃–Mg/C systems and ammonium paratungstate (APT)–Mg/C systems are studied. As reducer magnesium, carbon or combinations of both are explored. It is shown that in the WO₃–Mg system the reduction undergoes by solid–solid mechanism before melting Mg, where metallic tungsten and MgO are formed. Unlike this system, in the WO₃–C system mainly WO_x (< 880 °C) and WO₂ (> 960 °C) and small amount W is formed. In the WO₃–Mg–C ternary system reduction temperature shifts to higher temperature range and depends on amount of carbon. Similar to WO₃–Mg system, APT–Mg reaction starts and completes in the solid state. Thus, firstly the APT decomposes, then reduction of formed WO₃ takes place at ~ 600 °C yielding W and MgO. Likewise to WO₃–Mg system adding carbon into APT–Mg mixture shifts reduction temperature to even higher temperature zone which can exceed melting point of Mg and further reduction undergoes with molten magnesium. It is shown that the reduction products are MgO and W.     

Grain refinement and improved age hardening of Mg–Zn alloy by a trace amount of V

A microalloying addition of V acts as an effective grain refiner of Mg–Zn alloy. V exhibits limited but noticeable solubility in solid magnesium alloyed with Zn. V is exceptionally effective in promoting the nucleation of precipitates during age hardening and it also markedly accelerates the kinetics of precipitation. A near-peak hardness value can be reached after only 4 h of ageing at 160 °C. A considerably higher number density of the precipitates forms in Mg–Zn–V alloy as compared to Mg–Zn, resulting in a doubling of the hardness increment produced by artificial ageing. The highest level of hardening is achieved by ageing at intermediate temperatures. Natural ageing in Mg–Zn–V alloy is notably accelerated compared to that of the binary alloy. The details of the mechanisms involved with grain refinement and enhanced nucleation of precipitates by V are yet to be clarified.     

Effect of the magnesium content on the mechanochemical behavior in ternary system Mg–B2O3–C

The influence of Mg content in Mg–B₂O₃–C mixture on the MgO–B₄C composite formation and mechanism of reactions during ball milling process was investigated. In keeping with this aim, a mixture of boron oxide powder along with different amounts of Mg (4–10 mol) and C (1–3 mol) was activated in a ball mill. Thermodynamic calculations and differential thermal analysis (DTA) results revealed that Mg value played a main role, thereby; overall reaction enthalpy and adiabatic temperature (T_ad) changed by variation of magnesium content. T_ad enhanced with the increase in the Mg content and found maximum value (2852 °C) at stoichiometric ratio (Mg = 6 mol). DTA results showed that increasing the activation time to 5.5 h could decrease the temperature of combustion reaction to 530°C before magnesium melting. According to experimental findings, in the mixture of powder with 4 mol Mg, magnesiothermic reaction occurred in MSR mode and no carbothermal reaction took place. However, when the Mg content reached within a range of 9/2–6 mol, the magnesiothermic reaction occurred in MSR mode and activated the carbothermal reaction. Further enhancement in Mg content (10 mol Mg), as a diluent agent, led to MSR magnesiothermic reaction and no carbothermal reduction occurred.     

Corrosion properties of novel γ′–strengthened Co-base superalloys

Electrochemical properties of γ′-strengthened Co-base superalloys with the composition Co–Al–W–B and Co–Al–W–B–Y are studied in comparison with pure Co at ambient temperature in 0.5 M NaCl aqueous solution (pH 5.8). The different materials exhibit comparable corrosion resistance with a limited initial passivation followed by severe pitting corrosion at higher potentials. Oxide layers, formed under isothermal oxidation at 800 and 900 °C, provide exceptional protection against pitting corrosion. No breakdown events are observed, even for thin layers formed during short-term oxidation (1 h at 900 °C). Si as alloying element further improves the corrosion behaviour of the oxidised alloy in NaCl.     

Precipitation-hardenable Mg–2.4Zn–0.1Ag–0.1Ca–0.16Zr (at.%) wrought magnesium alloy

A new precipitation-hardenable wrought magnesium alloy based on the Mg–Zn system with an excellent combination of high tensile yield strength, good ductility and low tensile-compression anisotropy has been developed. The Mg–2.4Zn–0.1Ag–0.1Ca(–0.16Zr) (at.%) alloys show significantly higher age-hardening responses compared to that of the binary Mg–2.4Zn alloy due to the increased number density and refinement of rod-like MgZn₂ precipitates. The addition of Zr to the Mg–2.4Zn–0.1Ag–0.1Ca alloy resulted in a significant refinement of the grain size. A high number density of precipitates was observed in the Mg–2.4Zn–0.1Ag–0.1Ca–0.16Zr alloy in both the as-extruded condition and following isothermal ageing at 160 °C. The tensile yield strength of the as-extruded and aged alloys was 289 and 325 MPa, with an elongation of 17% and 14%, respectively. These alloys show relatively low compression and tensile anisotropy. The origins of these unique mechanical properties are discussed based on the detailed microstructural investigation.     

Atom probe tomographic study of elemental segregation at grain boundaries for a peak-aged Al–Zn–Mg alloy

Atom probe tomography (APT) has been used to characterize the element segregation at the grain boundary (GB) for a peak-aged Al–Zn–Mg alloy with high stress corrosion cracking (SCC) susceptibility. The results show that Mg segregates along the GB with a peak concentration of 1.38 at.% and width of 3 nm. Zn does not segregate at GB. However, segregation of Zn and H atoms at oxide-containing clusters on GB has been observed. APT atom maps also reveal that Mg₂Si is the H trapping site, but MgZn₂ is not.     

Stress corrosion cracking of a recent rare-earth containing magnesium alloy,EV31A,and a common Al-containing alloy,AZ91E

Stress corrosion cracking of the magnesium alloy Elektron 21 (ASTM–EV31A) and AZ91E was studied using constant load test in 0.1 M NaCl solution (saturated with Mg(OH)₂), and slow strain rate test using glycerol, distilled water and Mg(OH)₂ saturated, 0.01 M and 0.1 M NaCl solutions. Slow strain rate test indicated that EV31A was less susceptible to stress corrosion cracking than AZ91E. Under less intense loading of constant load, EV31A was found to be resistant to stress corrosion cracking. Fractography of EV31A specimens showed little evidence of hydrogen embrittlement. The superior resistance of EV31A is attributed to a more robust oxide/hydroxide layer.     

Characterization of mechanically alloyed nanocomposites in TiO2–B2O3–Mg–C quaternary system

The influence of the simultaneous presence of magnesium and graphite on mechanosynthesis of various nanocomposite powders in TiO₂–B₂O₃–Mg–C quaternary system was investigated. A mixture of boron oxide and titanium dioxide powders along with different amounts of magnesium and graphite was milled using a high-energy planetary ball mill to provide necessary conditions for the occurrence of a mechanically induced self-sustaining reaction (MSR). In the absence of C (100 wt.% Mg), TiB₂ nanopowder was formed as a result of combustion reaction after 34 min of milling. In the presence of both Mg and C, the mechanochemical reaction was completed after different milling times depending on the weight fraction of the reducing agents in the powder mixture. In the presence of x wt.% Mg–y wt.% C (x = 85 and 90; y = 100 − x), the mechanosynthesized composites contained TiB₂ and TiC as major compounds as well as MgO and Mg₃B₂O₆ as unwanted phases. With further increasing the graphite content to 30 wt.%, no mechanical activation was observed after 90 min of milling. The nanocomposite powders showed a bimodal particle size distribution characterized by the presence of several coarse particles (≈ 250 nm) along with finer particles with a mean size of about 75 nm. Formation mechanism of nanocomposites was explained through the analysis of the relevant sub-reactions.     

Effects of grain size on the corrosion resistance of wrought magnesium alloys containing neodymium

A range of grain size from 70 μm to 0.7 μm was studied for corrosion resistance of Mg–Y–RE magnesium alloy using electrochemical and constant immersion testing in 3.5 wt.% NaCl solution. The linear polarization resistance (R_p) showed a clear trend of increasing R_p value with grain refinement. The ultrafine grained sample showed the most positive pitting potential as compared to coarse grained samples. One order of magnitude decrease in corrosion rate was observed between coarsest and ultrafine grained microstructure.     

The improved performance of Mg-rich epoxy primer on AZ91D magnesium alloy by addition of ZnO

ZnO particles were added in Mg-rich epoxy primer to improve the protection for AZ91D magnesium alloy. The well dispersed ZnO particles could play a role in electrical conduction instead of Mg particles, consequently the Mg–ZnO-rich primer exhibited good conductivity while the dissolution rate of Mg particles decreased. ZnO particles also improved physical crosslink density of the epoxy matrix, which could reduce defects and enhance the barrier property and adhesion of the coating. As the results, the epoxy primer with 40 wt.% Mg and 10 wt.% ZnO showed better protection and prolonged lifetime than the primer with 50 wt.% Mg.     

Enhancing room temperature mechanical properties of Mg–9Al–Zn alloy by multi-pass equal channel angular extrusion

Influence of equal channel angular extrusion on room temperature mechanical properties of cast Mg–9Al–Zn alloy was investigated. The results show that room temperature mechanical properties of Mg–9Al–Zn alloy, such as yield strength, ultimate tensile strength and elongation, can be improved heavily by equal channel angular extrusion. Processing routes, processing temperature and extrusion passes have important influence on room temperature mechanical properties of processed Mg–9Al–Zn alloy by equal channel angular extrusion. The optimum room temperature mechanical properties such as yield strength of 209 MPa, ultimate tensile strength of 339 MPa and elongation of 14.1%, can be obtained when Mg–9Al–Zn alloy was processed by equal channel angular extrusion for 6 passes at route B_C at 498 K. Large bulk materials of Mg–9Al–Zn alloy with average grain size of 4 μm and high mechanical properties can be prepared.     

Wetting of porous titanium carbonitride by Al–Mg–Si alloys

Wetting of porous TiC_0.17N_0.83 by six alloys from the Al–Mg–Si system (pure Al, pure Mg, Al–15 at.% Mg, Al–10 at.% Si, Mg–5 at.% Si, and Al–10 at.% Mg–10 at.% Si) in an argon atmosphere was studied using the sessile drop experiment. The contact angle of the liquid drops on TiC_0.17N_0.83 substrates was measured as a function of temperature. Aluminium, Al–10 at.% Si, and Al–10 at.% Mg–10 at.% Si did not wet TiC_0.17N_0.83 in the studied temperature range. Magnesium always wetted TiC_0.17N_0.83 with a minimum contact angle of ≈44° at 900°C, and alloying with Mg significantly lowered the contact angle of Al on TiCN. Alloying with Si deteriorated the wetting of TiCN by Mg. A comparative study between the systems was conducted, based on the results and on data available in the literature. The improvement of the wetting of TiCN by Al due to alloying with Mg can be explained by the segregation of Mg to the interface with TiCN, where it lowers the interface energy. The addition of Si to pure Mg or to Al–Mg results in an increase in the contact angle on TiCN.     

Corrosion stress relaxation and tensile strength effects in an extruded AZ31 magnesium alloy

The corrosion effects on the tensile and stress relaxation behavior of an extruded AZ31 magnesium alloy subjected to immersion and salt-spray environments have been investigated. Specimens were simultaneously corroded and stress relaxed in a 3.5 wt.% NaCl solution and then put under a tensile test to failure to determine the stress–strain response over a 60 h test matrix. The AZ31 magnesium alloy shows an evident relaxation in 3.5 wt.% NaCl at room temperature. According to optical and scanning electron microscopy investigations, the fracture surfaces for the immersion environment show a high sensitivity to stress corrosion cracking.     

Effect of Zn additions on the age-hardening of Mg–2.0Gd–1.2Y–0.2Zr alloys

We have investigated the microstructures of age hardened Mg–2.0Gd–1.2Y–xZn–0.2Zr (x = 0, 0.3, and 1.0) (at.%) alloys to understand the remarkable age-hardening and unusual plastic elongation behavior. The age-hardening of the alloys occurs through the sequential precipitations of β′ and β₁ phases. The β₁ phase heterogeneously nucleates at the interface of the β′ phase, and relaxes strain fields around the β′. Although the addition of Zn degrades the age-hardening response, it causes the discontinuous precipitation of a 14H-type long-period stacking (LPS) phase at grain boundaries as well as within grains in the over-aged condition, which enhances the maximum tensile elongation. The composition of the β₁ phase was determined to be Mg–23.3 at.% RE–9.7 at.% Zn–2.0 at.% Zr (RE: rare-earth, Gd and Y), whereas that of the LPS is Mg–5.6 at.% RE–1.8 at.% Zn–1.0 at.% Zr.     

Corrosion and electrochemical evaluation of an Al–Si–Cu aluminum alloy in ethanol solutions

The corrosion of aluminum alloy AlSi8Cu3Fe(Zn) in ethanol and ethanol solutions containing 10 vol.% water and 10 vol.% acetic acid, respectively, was investigated by means of electrochemical impedance spectroscopy (EIS), polarization curve, immersion, optical microscopy, scanning electron microscopy and element mapping. The Al alloy in the ethanol and its solutions exhibited a capacitive loop in the measured Nyquist EIS spectra at high frequencies, which can be attributed to the ethanol’s dielectric response. Addition of 10 vol.% acetic acid increased the ethanol corrosivity more significantly than the same amount of water addition. The Al–Si–Cu–Mg precipitated zones in the alloy were susceptible to corrosion attack due to the micro-galvanic effect by the Cu-containing precipitates.     

Corrosion of ultra-high-purity Mg in 3.5% NaCl solution saturated with Mg(OH)2

Corrosion was evaluated for ultra-high-purity magnesium (Mg) immersed in 3.5% NaCl solution saturated with Mg(OH)₂. The intrinsic corrosion rate measured with weight loss, P_W = 0.25 ± 0.07 mm y⁻¹, was slightly smaller than that for high-purity Mg. Some specimens had somewhat higher corrosion rates attributed to localised corrosion. The average corrosion rate measured from hydrogen evolution, P_AH, was lower than that measured with weight loss, P_W, attributed to dissolution of some hydrogen in the Mg specimen. The amount of dissolution under electrochemical control was a small amount of the total dissolution. A new hydride dissolution mechanism is suggested.     

Use of pre-oxidation to improve reactive wetting of high manganese alloyed steel during hot-dip galvanizing

The present study discusses hot-dip galvanizing of a Fe–23% Mn–0.6% C–0.3% Si steel using a Zn–0.22%Al bath. The paper concentrates on reactive Zn wetting on top a covering external oxide layer occurring after in-line annealing. Annealing was performed by soaking at 800 °C/60 s in 5% H₂–N₂ at different dewpoints. In-line pre-oxidation at 600 °C/10 s in 1.8% O₂–N₂ was further performed and the impact on selective oxidation as well as reactive Zn wetting was examined. After conventional annealing Zn wetting turns to increase if a roughly globular MnO layer appears on the external steel surface and Si is internally oxidized. Reactive wetting including the formation of Fe₂Al₅ crystals occurs on top of the MnO layer, because metallic-bond Fe exists incorporated within this MnO layer (→MnO·Fe_metall layer). The amount of metallic-bond Fe within the MnO·Fe_metall layer increases considerably if pre-oxidation is conducted. This results in an intensified Fe₂Al₅ formation on top of a MnO·Fe_metall, which improves liquid Zn wetting. Brittle Fe–Zn intermetallics were absent in all trials. These results offer a new way for hot-dip galvanizing (high) Mn alloyed steels. The absence of Fe–Zn intermetallics and (partial) MnO reduction implies that the currently discussed model of aluminothermic MnO reduction during hot-dip galvanizing Mn alloyed steel seems not to be dominating in the present case of reactive Zn wetting on top of a covering MnO·Fe_metall layer.     

An investigation of the corrosion mechanisms of WE43 Mg alloy in a modified simulated body fluid solution: The influence of immersion time

The corrosion mechanisms and kinetics of WE43 Mg alloy in a modified simulated body fluid (m-SBF) are investigated by electrochemical, hydrogen evolution and analytical techniques. The changes in the impedance response over time are related to four corrosion stages involving the formation of a partially protective corrosion layer and adsorption of Mg intermediates, formation of an inner passive MgO layer with increasing coverage over time, rupture of the corrosion layer and lateral growth of stable pits. ATR-FTIR, XRD and XPS results show the presence of an amorphous carbonated apatite/Mg(OH)₂ mixed corrosion layer.     

Characterization of planar features in Mg–Y–Zn alloys

The planar features in a Mg–8Y–2Zn–0.6Zr (wt.%) alloy solution-treated at 500 °C for 1 h have been examined using conventional transmission electron microscopy and atomic-resolution high-angle annular dark-field scanning transmission electron microscopy. Three types of planar features are detected in the microstructure. The first type, which was previously reported to be an intrinsic stacking fault I₁ bounded by a Frank partial dislocation, is shown to be the 14H precipitate phase that is associated with Shockley partial dislocations. The second type is also a precipitate phase that has a single unit cell height and is associated with Shockley partial dislocations. The third type of planar feature comprises small ribbon-like stacking faults. These stacking faults are determined as intrinsic I₂ type bounded by two Shockley partial dislocations, which is further confirmed by computer simulation. The stacking fault energy associated with the faults is much smaller than that of pure magnesium.     

2-Hydroxy-4-methoxy-acetophenone as an environment-friendly corrosion inhibitor for AZ91D magnesium alloy

The inhibition behavior of 2-hydroxy-4-methoxy-acetophenone (paeonol) as an environment-friendly corrosion inhibitor for AZ91D magnesium alloy was investigated in 0.05 wt.% NaCl solution by means of polarization curve, AC impedance, weight loss measurement, scanning electron microscopy, Fourier transformation infrared spectroscopy, ultraviolet analysis, and computer molecular simulation. The results show that paeonol can inhibit the corrosion of AZ91D. The maximum inhibition efficiency is achieved when paeonol concentration is 50 ppm by weight in this study. It is proposed that paeonol chelates with Mg to form a paeonol-Mg complex mixing with the original Mg(OH)₂ film on the surface to inhibit the anodic dissolution of AZ91D.