Effect of Magnesium Content on the Corrosion Behaviors of Grf/Al Composite

Graphite-fiber-reinforced aluminum composites (Gr_f/Al) with different magnesium content were fabricated with the pressure infiltration method. Their corrosion behaviors were investigated by potentiodynamic polarization measurements, immersion tests, electrochemical impedance spectroscopy (EIS) analysis, and scanning electron microscopy (SEM). Both the corrosion potential (E _corr) and the pitting potential (E _pit) decreased with the increase of magnesium content, whereas the corrosion current density (i _corr) decreased sharply at first and then increased slightly. The i _corr of Gr_f/Al-3.2Mg was the lowest among the four composites with different magnesium contents, which indicated that Gr_f/Al-3.2Mg had the best corrosion resistance. EIS showed that the capacitive reactance of Gr_f/Al-8.5Mg was 1682 Ω × cm⁻², which was the worst, whereas that of Gr_f/Al-3.2Mg was 3498 Ω × cm⁻², which was the best. SEM results revealed that magnesium and silicon formed the Mg₂Si phase in Gr_f/Al-3.2Mg, which hindered the extension of corrosion crack and improved the corrosion resistance.     

Thermodynamic properties of dilute solutions of magnesium in zinc

Concentration cells of the type: Mg(s) |MgCl₂ in (LiCl-KCl)_eut(l) |Mg-Zn(l) have been employed for a study of the thermodynamic properties of dilute Mg-Zn alloys in the range from 0.01 < X_Mg < 0.17 and at temperatures below the melting point of Mg, 922 K. The data were discussed in terms of the Krupkowski formulae, Wagner's linear relations of In γ_Mg vs X_Mg and Darken's quadratic formalism. This research was done at the Institute of Inorganic Chemistry, The Technical University of Norway, Trondheim, Norway.     

Effect of strontium on the microstructure, mechanical properties, and fracture behavior of AZ31 magnesium alloy

The effect of strontium (Sr) on the microstructure, mechanical properties, and fracture behavior of AZ31 magnesium alloy and its sensitivity to cooling rate are investigated. Three phases—blocky-shaped Mg₁₇Al₁₂, acicular Mg₂₀Al₂₀Mn₅Sr, and insular Mg₁₆(Al,Zn)₂Sr—are identified in the Sr-containing AZ31 alloys. With increasing cooling rate, the blocky-shaped Mg₁₇Al₁₂ phase increases, the acicular Mg₂₀Al₂₀Mn₅Sr phase diminishes, and the insular Mg₁₆(Al,Zn)₂Sr phase is refined and granulated. The study suggests that the grain size decreases with increasing cooling rate for a given composition. However, the grain size decreases first, then increases, and finally decreases again with increasing Sr for a given cooling rate. The yield strength (σ _y ) of AZ31 magnesium alloy can be improved by grain refinement and expressed as σ _y =35.88+279.13d ^−1/2 according to the Hall-Petch relationship. The elongation increases when Sr is added up to 0.01 pct and then decreases with increasing Sr addition. Grain refinement changes the fracture behavior from quasicleavage failure for the original AZ31 alloy to mixed features of quasicleavage and microvoid coalescence fracture.     

Effect of magnesium on the aging behavior of Al-Zn-Mg-Cu/Al2O3 metal matrix composites

The effect of magnesium content on the aging behavior of Al-Zn-Mg-Cu alloy reinforced with alumina (A1₂O₃) was studied by using the differential scanning calorimetry (DSC) technique and hardness measurement. The magnesium contents were studied in the range from 1.23 to 2.97 wt pct. The addition of magnesium was found to increase the coherent Guinier-Preston (GP) zones in com-posites. The apparent formation enthalpy of GP zones of composites (0.1V _f) was 0.932 cal/g for 1.23 wt pct magnesium content and 1.375 cal/g for 2.97 wt pct magnesium content. The precipitation time to achieve the maximum hardness in the composites depends on the magnesium content. The time changed from 12 to 48 hours as the magnesium content increased from 1.23 to 2.97 wt pct. Both Vickers microhardness and Rockwell hardness increased with increasing magnesium content. The maximum hardness occurred in the composites that contained maximum amounts of GP zones and η′ precipitates. However, the microhardness of the composites was always lower than that of monolithic alloys due to the alumina fibers which caused the suppression of GP zones and η′ for-mation in the composites.     

Microstructure Refinement After the Addition of Titanium Particles in AZ31 Magnesium Alloy Resistance Spot Welds

Microstructural evolution of AZ31 magnesium alloy welds without and with the addition of titanium powders during resistance spot welding was studied using optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). The fusion zone of AZ31 magnesium alloy welds could be divided into columnar dendritic zone (CDZ) and equiaxed dendritic zone (EDZ). The well-developed CDZ in the vicinity of the fusion boundary was clearly restricted and the coarse EDZ in the central region was efficiently refined by adding titanium powders into the molten pool, compared with the as-received alloy welds. A microstructural analysis showed that these titanium particles of approximately 8 μm diameter acted as inoculants and promoted the nucleation of α-Mg grains and the formation of equiaxed dendritic grains during resistance spot welding. Tensile-shear testing was applied to evaluate the effect of titanium addition on the mechanical properties of welds. It was found that both strength and ductility of magnesium alloy welds were increased after the titanium addition. A TEM examination showed the existence of an orientation matching relationship between the added Ti particles and Mg matrix, i.e., [ 0 1[`1]0 ]<sub>\textMg</sub> //  [ 1[`2] 1[`3] ]<sub>\textTi</sub>  \textand ( 000 2 )<sub>\textMg</sub> //  ( 10[`1]0)_\textTi \left[ {0 1\bar{1}0} \right]_{\text{Mg}} // \, \left[ { 1\bar{2} 1\bar{3}} \right]_{\text{Ti}} \,{\text{and}}\,\left( {000 2} \right)_{\text{Mg}} // \, ( 10\bar{1}0)_{\text{Ti}} in some grains of Ti polycrystal particles. This local crystallographic matching could promote heterogeneous nucleation of the Mg matrix during welding. The diameter of the added Ti inoculant should be larger than 1.8 μm to make it a potent inoculant.     

Standard gibbs energy of formation of Mg<Subscript>48</Subscript>Zn<Subscript>52</Subscript> determined by solution calorimetry and measurement of heat capacity from near absolute zero kelvin

The thermodynamic properties of Mg₄₈Zn₅₂ were investigated by calorimetry. The standard entropy of formation at 298 K, Δ_f S ₂₉₈ ^o , was determined from measuring the heat capacity, C _p , from near absolute zero (2 K) to 300 K by the relaxation method. The standard enthalpy of formation at 298 K, Δ_f H ₂₉₈ ^o , was determined by solution calorimetry in hydrochloric acid solution. The standard Gibbs energy of formation at 298 K, Δ_f G ₂₉₈ ^o , was determined from these data. The obtained results were as follows: Δ_f H ₂₉₈ ^o (Mg₄₈Zn₅₂)=(−1214±(300) kJ · mol⁻¹;Δ_fS ₂₉₈ ^o (Mg₄₈Zn₅₂)=(−123±0.36) J · K⁻¹ · mol⁻¹; and Δ_f G ₂₉₈ ^o (Mg₄₈Zn₅₂)=(−1177±(300) kJ · mol⁻¹. The electronic contribution to the heat capacity of Mg₄₈Zn₅₂ was found to be approximately equal to pure magnesium, indicating that the density of states in the vicinity of the Fermi level follows the free electron parabolic law.     

The kinetics of molten iron desulfurization using magnesium vapor

Magnesium vapor was injected into 60 kg heats of carbon-saturated iron at 1523 K. The magnesium dissolution and desulfurization rates as well as magnesium bubble size were monitored over sulfur contents ranging from 0.0002 to 0.2 pct S. The magnesium dissolution mass transfer coefficient was found to be 0.046 ± 0.034 mm · s⁻¹ (for 30 mm diam bubbles). Further analysis indicated that most of the desulfurization took place at magnesium sulfide inclusions present in the bath; there was good agreement between the experimental precipitation rate constant and that predicted for diffusion to the number of inclusions observed by chemical analysis and inclusion counts. The seed magnesium sulfide inclusions were probably stripped from the ascending magnesium bubbles. These particles were quickly eliminated from the melt by bubble and induction stirring, resulting in a steady-state number of inclusions. This allowed a pseudosteady-state model to be developed.     

Joint solubility of samarium and dysprosium in solid magnesium

The phase compositions of solid Mg–Sm–Dy alloys corresponding to the magnesium-corner region of the phase diagram are studied by optical and scanning electron microscopy, electrical resistivity measurements, and electron microprobe analysis. The obtained results allowed us to determine the joint solubility of samarium and dysprosium in solid magnesium at 500, 400, and 300°C; it decreases with decreasing temperature. The magnesium solid solution is found to be in equilibrium only with the Mg₄₁Sm₅ and Mg₂₄Dy₅ compounds, which are in equilibrium with the magnesium solid solution in the binary Mg–Sm and Mg–Dy systems.     

Phase equilibria and intermetallic phases in the Ni-Si-Mg ternary system

The Ni-Si-Mg ternary phase diagram has been established after homogenization and slow cooling to room temperature. The chemical compositions of the alloys and their phases were obtained using fully quantitative energy dispersive X-ray spectroscopy (EDS) with standard spectrum files created from intermetallic compounds Mg₂Ni and Ni₂Si. The following intermetallic phases have been observed: (a) four new ternary intermetallic phases, designated as ν, ω, μ, and τ, (b) a ternary intermediate phase Mg(Ni,Si)₂ based on the binary MgNi₂ phase containing Si; (c) three ternary intermetallic phases, η, κ, and ζ, previously reported by the present authors;[10] and (d) Mg₂SiNi₃ (Fe₂Tb type),^[9] previously reported by Noreus et al. ^[8] The MgNi₆Si₆ phase, which was also previously reported,^[7] was not observed at the corresponding composition in the present work. However, the MgNi₆Si₆ phase reported as being of hexagonal symmetry (Cu₇Tb type),^[9] with the lattice parameters a=0.4948 nm and c=0.3738 nm, possibly corresponds to the μ phase (Mg(Si_0.48Ni_0.52)₇) discovered in the present work. The lattice structure of the newly discovered ω phase was determined with the help of the X-ray indexing program TREOR (developed by Werner et al. ^[13]) to be a hexagonal structure of the Ag₇Te₄ type ((Mg_0.52Ni_0.48)₇Si₄) with the lattice parameters a=1.3511 nm and c=0.8267 nm.     

Thermodynamic properties of liquid Mg-ln-Cd ternary solutions

By means of concentration cells of the following type: Mg(s)∣MgCl₂ in (LiCl-KCl)_eut(l)∣Mg-In or Mg-ln-Cd(l), the partial thermodynamic data of Mg in Mg-ln and Mg-ln-Cd liquid solutions have been obtained in the composition range 0.1 ≤ X_Mg ≤ 0.7 for binary while for ternary alloys fort = 0.4, 0.6, and 0.8 (wheret = X_In/(X_In + X_Cd)) and at various mangesium concentrations 0.1≤ X_Mg ≤ 0.6. Both ternary and binary alloys were investigated at a temperature range 750 to 900 K. Experimental partial excess Gibbs energies of Mg were interpreted by the Pelton and Flengas method. Results for Mg-ln system show a slight difference in comparison with previously published data for the same system also from emf studies. Results of this study for Mg-ln system exhibit negative and positive excess entropies of magnesium and the same is observed in ternary system Mg-ln-Cd at the range of concentration close to Mg-ln.     

Experimental Determination of Mg Activities in Fe-Mg Solutions

The thermodynamics of magnesium in liquid iron was determined at 1823 K (1550 °C). For this purpose, liquid iron was equilibrated with Ag-Mg alloys in a semienclosed molybdenum vessel. From the partition of magnesium between iron and silver, the activity coefficient of Mg and the self-interaction parameter e_\textMg^\textMg \varepsilon_{\text{Mg}}^{\text{Mg}} were determined.     

Crystallization kinetics of amorphous magnesium-rich magnesium-copper and magnesium-nickel alloys

Amorphous magnesium-rich alloys Mg _y X_1-y (X=Ni or Cu and 0.82<y<0.89) have been produced by melt spinning. The crystallization kinetics of these alloys have been determined by in situ X-ray diffraction (XRD) and isothermal and isochronal differential scanning calorimetry (DSC) combined with ex situ XRD. Microstructure analysis has been performed by means of transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Crystallization of the Mg-Cu alloys at high temperature takes place in two steps: primary crystallization of Mg, followed by simultaneous crystallization of the remaining amorphous phase to Mg and Mg₂Cu. Crystallization of the Mg-Cu alloys at low temperatures takes place in one step: eutectic crystallization of Mg and Mg₂Cu. Crystallization of the Mg-Ni alloys for a Mg content, y>0.85, takes place in two steps: primary crystallization of Mg and of a metastable phase (Mg_∼5.5Ni, with Mg content y=0.85), followed by the decomposition of Mg_∼5.5Ni. Crystallization of the Mg-Ni alloys for a Mg content y<0.85 predominantly takes place in one step: eutectic crystallization of Mg and Mg₂Ni. Within the experimental window applied (i.e., 356 K<T<520 K and 0.82<y<0.89), composition dependence of the crystallization sequence in the Mg-Cu alloys and temperature dependence of the crystallization sequence in the Mg-Ni alloys has not been observed.     

Effect of Mg and Sr additions on the formation of intermetallics in Al-6 Wt pct Si-3.5 Wt pct Cu-(0.45) to (0.8) Wt pct Fe 319-type alloys

Al-Si alloys are materials that have been developed over the years to meet the increasing demands of the automotive industry for smaller, lighter-weight, high-performance components. An important alloy in this respect is the 319 alloy, wherein silicon and copper are the main alloying elements, and magnesium is often added in automotive versions of the alloy for strengthening purposes. The mechanical properties are also ameliorated by modifying the eutectic silicon structure (strontium being commonly employed) and by reducing the harmful effect of the β-Al₅FeSi iron intermetallic present in the cast structure. Magnesium is also found to refine the silicon structure. The present study was undertaken to investigate the individual and combined roles of Mg and Sr on the morphologies of Si, Mg₂Si, and the iron and copper intermetallics likely to form during the solidification of 319-type alloys at very slow (close to equilibrium) cooling rates. The results show that magnesium leads to the precipitation of Al₈Mg₃FeSi₆, Mg₂Si, and Al₅Mg₈Cu₂Si₆ intermetallics. With a strontium addition, dissolution of a large proportion of the needle-like β-Al₅FeSi intermetallic in the aluminum matrix takes place; no transformation of this phase into any other intermetallics (including the Al₁₅(Fe,Mn)₃Si₂ phase) is observed. When both Mg and Sr are added, the diminution of the β-Al₅FeSi phase is enhanced, through both its dissolution in the aluminum matrix as well as its transformation into Al₈Mg₃FeSi₆. The reactions and phases obtained have been analyzed using thermal analysis, optical microscopy, image analysis, and electron microprobe analysis (EMPA) coupled with energydispersive X-ray (EDX) analysis.     

Kinetics of the growth of spinel,MgAl2O4, on alumina particulate in aluminum alloys containing magnesium

The growth of spinel in melts of alumina-reinforced composites not only consumes the magnesium required for age hardening, but also acts to increase the viscosity and thus adversely affects the castability of the material. The kinetics of this growth have been studied in molten Al-1 pct Mg alloys in the temperature range of 948 to 1073 K. The results are shown to fit an equation of the form ln(1−α = k(t₀ −t) describing deceleratory growth on fine particles, where α is a dimensionless reaction parameter. The rate constant,k, fits an Arrhenius equation giving an activation energy of 103 ± 7 kJ and a time constant of 50 s⁻¹. The incubation time,t ₀, which is about 2000 seconds forT < 1000 K, drops to below 500 seconds forT > 1000 K.     

Effect of retrogression and reaging treatments on the microstructure of Ai-7075-T651

The effect of the retrogression and reaging treatments (RRA) on the microstructure of Al-7075 in the T651 temper, both in the matrix and on grain boundaries, was studied using transmission electron microscopy. The processes occurring in the matrix during the retrogression treatment are principally the dissolution of small particles of the η’ transition phase, transformation to η of the larger particles of η’, coarsening of the three commonly observed variants of the η phase precipitates (η₁, η₂, and η₄), and precipitation of new η phase particles, particularly the η₁ variant. The main process occurring during the reaging treatment is either growth of partially dissolved η’ particles or precipitation of the η’ phase. These lead to a microstructure containing many fine η’ precipitates and some larger η₁ and η₂ particles with a smaller amount of coarse η₄ particles, resulting in a broad particle size distribution. The high strength of the 7075 alloy in the RRA temper is believed to arise from the relatively high overall concentration of particles in this dispersion. The retrogression treatment produces rapid initial coarsening of the grain boundary particles, which are primarily η phase precipitates, resulting in an increase in their volume per unit grain boundary area,V _A . The beneficial effect of the RRA treatment on the susceptibility of 7075-T651 to SCC is believed to be due, at least partially, to the increased value ofV _A produced by the RRA treatment. Formerly Visiting Assistant Research Engineer in the Department of Materials Science and Engineering, University of California, Los Angeles, CA     

The role of microstructure in localized corrosion of magnesium alloys

The article presents new findings on the influence of microstructural changes on corrosion behavior of magnesium alloy AZ91 in chloride solution, with a particular attention to the role of the β phase (Mg₁₇Al₁₂) and the surrounding Al-rich-α area. The as-cast alloy was subjected to solutionizing and aging heat treatments, in order to incorporate variation in morphology and distribution of the intermetallic β phase and the surrounding Al-rich-α (also known as eutectic α). Although previous workers have ascribed the higher corrosion resistance of a fine-grained alloy to the formation of the finely distributed β phase, the present work suggests that it is the ratio of the β phase to the Al-rich-α that governs the localized corrosion of the aged alloy. Corrosion characteristics were investigated by immersion and electrochemical tests. Surface microtopography, optical microscopy, and scanning electron microscopy (SEM) were employed to characterize the localized corrosion.     

Microstructure and second-phase particles in low- and high-pressure die-cast magnesium alloy AM50

The microstructure and phase composition of low-pressure die-cast (LPDC) and high-pressure diecast (HPDC) magnesium alloy AM50 were examined by transmission electron microscopy (TEM) techniques in combination with optical microscopy, scanning electron microscopy (SEM), and electron-probe microanalysis (EPMA). It has been established that the dimensions and morphology of the constituent phases (α-Mg solid solution, Mg₁₇Al₁₂, and Al₈Mn₅) depend on the processing parameters. The results obtained suggest that there is a ternary eutectic with the aforementioned three phases in the Mg-Al-Mn system. Phase transformations leading to the observed microstructures are discussed.     

Thermodynamics and phase relationships of transition metal-sulfur systems: IV. thermodynamic properties of the Ni-S liquid phase and the calculation of the Ni-S phase diagram

An associated solution model is applied to describe the thermodyanmic properties of the liquid Ni-S phase. This model assumes the existence of ‘NiS’ (l) species in the liquid in addition to Ni(l) and S(l). With two solution parameters for the binaries Ni-‘NiS’ and ‘NiS’-S, this model is able to describe the thermodynamic behavior of the liquid phase over a wide range of temperature and composition. Using this model for the liquid phase, a statistical thermodynamic model for the monosulfide phase and empirical thermodynamic equations for β₁-Ni₃S₂ and β₂-Ni₄S₃, the Ni-S phase diagram is calculated. The calculated diagram is in excellent agreement with the available experimental data with the exception that the eutectic composition for the equilibrium L₁ + δ + η and those of the two liquids for the equilibriumL ₁ + L ₂ + η differ from the experimental data by more than 2 at. pct S. Formerly Post-Doctorl Research Associate, University of Wisconsin-Milwaukee is now Lecturer, Department of Metallurgical Engineering, Indian Institute of Technology, Kanpur, India     

A process for debismuthizing lead with magnesium

Significant amounts of bismuth can be removed from magnesium-lead alloys by crystallization of the intermetallic compounds Mg_{0. 6573}Pb_{0. 3427} and Mg_0.662Pb_0.338in the Mg₂Pb, magnesium plumbide phase field of the Pb-Mg-Bi system. The results of the previous studies have been used to develop a process for debismuthizing lead using controlled conditions for the crystallization of magnesium plumbide from alloys containing 0.03 to 0.06 wt Pct bismuth and magnesium whose concentration is determined by the relationship wt Pct Mg = 2.9 + 20 x wt Pct Bi. A flow diagram showing the sequence of operations is presented together with a material balance, which was established from data obtained from individual experiments simulating the previously mentioned unit operations, with a final product containing less than 0.0010 wt Pct Bi. The process also includes the recycling of magnesium recovered by vacuum distillation. Additional procedures are included to extend the process to treat alloys containing up to 1.5 wt pct bismuth.     

The isothermal section at 500 °C of the Y-La-Mg ternary system

Ternary Y-La-Mg alloys have been studied by X-ray diffraction, optical microscopy, scanning electron microscopy, and electron probe microanalysis. Phase equilibria have been established in the isothermal section at 500 °C in the 50 to 100 at. Pct Mg composition range. The sections (Y_xLa_1-x) Mg (continuous solid solution), (Y_xLa_1-xMg₂ (cF24-Cu₂Mg type for 0 ≤x ≤ 0.72 and hP12-MgZn₂ type for 0.82 ≤x ≤ 1 solid solutions), and Y_xLa_1-xMg₃ (0 ≤x ≤ 0.47) have been especially studied. The ≈Y_xLa_1-xMg₅ (0.35 ≤x ≤ 0.77) ternary compound (cF440-GdMg₅ type) has been found. A reinvestigation of selected regions of the binary La-Mg and Y-Mg systems proved necessary. The La₅Mg₄₁ phase, not previously reported, has been found by the annealing of samples at 600 °C. The YMg₂ phase shows a wide homogeneity range up to 73 at. Pct Mg. The ternary section has been compared with a simulated one that was created by placing sections of binary systems of Mg with different rare earth elements side by side. The results are related to the pseudolanthanide concept.