Mg-rich Matrix Research Articles

Synthesis and Characterization of Mg-Based Coatings for Sacrificial Cathodic Protection of Mg Alloy AZ31B

Magnesium (Mg) alloys are becoming increasingly popular for weight reduction in the automotive industry due to their low density and nominal specific strength [1]. However, the deleterious intrinsic corrosion rates of Mg alloys have hindered their widespread application. To date, protection strategies for Mg alloys has consisted primarily of barrier coatings such as Rockhard™ and Tagnite™ which have not demonstrated the ability to protect bare Mg at scratches, cracks, and other defects [2] while conversion coatings have not been shown to survive harsh working conditions and do not provide sacrificial cathodic protection or inhibitor release [3]. The drawbacks of these coating strategies provide motivation for additional research. In addition, sacrificial cathodic protection of Mg alloys has not been widely studied and presents opportunities of high interest. Recent studies have shown that sacrificial cathodic protection of Mg and its alloys is possible and shows some promise albeit complicated by cathodic corrosion of Al rich phases and large local cathodic reaction rates [4]. However, an optimal sacrificial coating has not been produced to date. As such, the overall goal of this research is to design a tunable coating which acts as (1) a corrosion barrier, (2) provides sacrificial cathodic protection, and (3) functions as a reservoir for ionic inhibitor release. Such a coating has been designed recently for aluminum alloy AA2024 using an amorphous Al-Co-Ce alloy [5]. The present work aims to study sacrificial Mg-based coatings produced by pulsed laser deposition (PLD) tuned for the Mg alloy AZ31B-H24 (Mg-3Al-1Zn). Binary and ternary coatings using rare earth elements known to decrease the OCP of Mg [6] and exhibit some ability for inhibitor release (RE = La, Gd, Y) as well as elements such as Ge and Sn [7] which poison cathodic kinetics, will be explored. Preliminary results of a Mg88Gd9Y3 based coating on AZ31B revealed a two-phase microstructure consisting of an extended solid solution Mg-rich matrix with a grain size on the order of 100 nm as well as a small fraction of Gd-Y rich particles < 1 μm in diameter. Immersion testing of PLD coated AZ31B in 0.6 M NaCl revealed an open circuit potential of -1.58 VSCE which is mildly cathodic to uncoated AZ31B (-1.56 VSCE) after a 24-hour exposure. In addition, the coating was observed to dissolve preferentially on an AZ31B surface demonstrating ability as a sacrificial anodic coating. Future testing will systematically explore the effects of alloying element concentration on the corrosion behavior of PLD coated AZ31B utilizing both AC and DC electrochemical techniques. Such an understanding will provide a framework to optimize a sacrificial cathodic coating for AZ31 based on critical factors such as open circuit potential, corrosion rate, galvanic couple potential, and galvanic couple current. Acknowledgments: This project is funded by the US Air Force Academy under agreement number FA7000-13-2-0020 as part of Corrosion University Pilot Program under the direction of Mr. Daniel Dunmire and by the National Science Foundation under NSF DMR#1309999 as well as the Army Research Office under agreement number W911NF-14-2-0005 with project manager Joe Labukas. The U.S. Government is authorized to reproduce and distribute reprints for government purposes notwithstanding any copyright notation thereon. The views and conclusions contained are those of the authors and should not be interpreted as necessarily representing the official policies or endorsement of the US Air Force Academy or the US Government.

Microstructure and mechanical properties of a newly developed high strength Mg54.7Cu11.5Ag3.3Gd5.5Sc25 alloy

Starting from the bulk glass-forming composition Mg59.5Cu22.9Ag6.6Gd11, the development of a bulk glassy matrix composite alloy with high strength and ductility was attempted by adding a large amount of Sc to induce the formation of the MgSc intermetallic compound. However, copper-mould casting of the modified composition, Mg54.7Cu11.5Ag3.3Gd5.5Sc25, as rods with 3 and 5 mm diameter, yields besides the expected MgSc, a second (CuSc) and a third (Mg3Gd-type) intermetallic. These compounds are surrounded by a Mg-rich matrix, possibly composed of glassy regions and a Mg-based terminal solid solution. Although the obtained microstructure deviates largely from the expected one, the new alloy exhibits very high strength, i.e. 790 MPa, and visible plastic deformation, i.e. 0.9%. Shear dimples were found in MgSc.

Growth kinetics of cellular precipitation in a Mg–8.5Al–0.5Zn–0.2Mn (wt.%) alloy

Microstructural evolution and growth kinetics were studied in an isothermally aged Mg–8.5Al–0.5Zn–0.2Mn (wt.%) alloy by means of X-ray diffraction, scanning electron microscopy, Vickers hardness measurements and transmission electron microscopy. Specimens were solution-treated and then aged at 373, 473 and 573 K for different time period. The characterization results indicated the presence of both continuous and discontinuous precipitations of the Mg 17Al 12-γ phase in a Mg-rich matrix. The discontinuous or cellular precipitation was present with a lamellar structure, and the growth kinetics was evaluated using the Johnson–Mehl–Avrami–Kolmogorov equation analysis, which gives a time exponent close to 1. This value confirms that cellular precipitation takes place on the saturation sites corresponding to grain boundaries. In addition, the activation energy for cellular precipitation was determined to be about 64.6 kJ mol −1. This also indicates a grain boundary diffusion process. The variation of cellular spacing with temperature follows the behavior expected by Turnbull theory. The highest hardness peak corresponded to the lowest aging temperature and it is associated with a fine continuous precipitation; while the lowest hardness peak was detected at the highest aging temperature and it is attributed to the rapid coarsening process of both precipitations.

Corrosion of magnesium alloy ZE41 – The role of microstructural features

Magnesium alloy ZE41, used extensively in the aerospace industry, possesses excellent mechanical properties albeit poor corrosion resistance. This paper investigates the mechanism of corrosion and the interaction between the grain boundary intermetallic phases, the Zr-rich regions within the grains and the bulk Mg-rich matrix. The results of optical and scanning electron microscopy (SEM) together with energy-dispersive X-ray (EDX) and atomic force microscopy (AFM) potential map measurements have shown the importance of the microstructure in the initiation and propagation of corrosion in an aqueous environment, indicating that the Zr-rich regions play a distinct role in the early stages of corrosion in this alloy.

Effects of Al/Zn Ratio on the Microstructure and Strengthening of Mg-Al-Zn Alloys

The morphology and crystallography of precipitates formed in three Mg-Al-Zn alloys containing various Al/Zn ratios, conventionally designated as AZ91, AZ64 and AZ46, were characterized, and the mechanical properties determined. Three morphologically and crystallographically different types of lath- or rod-shaped γ-(Mg17All2) precipitates were identified in AZ91, and needle-like η-(MgZn2) in addition to γ-(Mg17All2) occurred in the alloys containing higher Zn. With increasing Zn content, i.e. with decreasing Al/Zn ratio, the fraction of γ-(Mg17All2) decreased, while that of η-(MgZn2) increased. The peak aged hardness and tensile properties of the alloys were slightly decreased, rather than increased, with increasing the Zn content from AZ91 through AZ64 to AZ46. The mechanism by which the γ-(Mg17All2) and η-(MgZn2) precipitates affected the age strengthening of the alloys was discussed in terms of the morphology, crystallography, interfacial structure, and relative quantity of the precipitates, and of their ability to block dislocation basal slip in the Mg-rich matrix.

Influence of ionic strength and cation charge on transport of colloidal particles in fractured shale saprolite.

The role of solution chemistry (cation charge and concentration) and particle size on colloid transport was examined in an intact monolith of fractured shale saprolite (highly weathered rock). Recovery of the microsphere tracers consistently decreased with increasing ionic strength of either mono- (Na+) or divalent- (Ca2+) dominated solutions, but a much greater concentration of Na+ in the influent solution was required to result in a similar reduction in colloid recovery as compared to Ca2+. However, composition of the solution along the flow path, and hence the degree of microsphere retention, was also strongly influenced by cation exchange and diffusive exchange between pore water in the fractures and in the fine-grained, Ca- and Mg-rich matrix. The influence of "matrix diffusion" on solute transport is also evident in the 5-fold difference between the arrival of the center-of-mass of microspheres as compared to the much later arrival of a bromide tracer. Particle size affected the extent of microsphere transport, but the solution chemistry appears to be a more dominant control. While confirming the importance of ionic strength, counterion charge, and particle size on colloid migration, this study emphasizes the profound effect that pore structure and geochemical processes such as cation exchange have on solution chemistry and thus on colloid transport.

The effects of the distribution aspect of precipitate on the corrosion behavior of as-cast magnesium alloys

In the present study, the corrosion behavior of AZ91D as-cast alloy was investigated form the viewpoint of the distribution aspect of precipitate (Mg17Al12) and the variation of Al concentration in the Mg-rich matrix. The dendrite arm spacing (DAS) of an as-cast specimen was measured as a function of degree which describes the distribution aspect of the precipitate, and the salt spray test was conducted for various grain-sized specimens for 20 days. The dendrite arm spacing increased as the grain size increased to about 150 μm, but a constant value is indicated when the grain size exceeds that range. Although the relationship between the corrosion rate and grain size is of a nonlinear type, the linear trend between the corrosion rate and the dendrite arm spacing is maintained for the overall range of dendrite arm spacing. Since the precipitate in the as-cast alloy is discontinuously distributed, this linear relationship means that the variation of Al-solute concentration in the Mg-rich matrix has a more potent effect than the protective action of the precipitate on the corrosion behavior of an as-cast alloy.

Composition and temperature dependence of cation ordering in Ni-Mg olivine solid solutions: a time-of-flight neutron powder diffraction and EXAFS study

The non-convergent ordering of Mg and Ni over the M1 and M2 sites of synthetic olivines has been studied using “time of flight” neutron powder diffraction and X-ray absorption spectroscopy (EXAFS). The compositional dependence of order/disorder at room temperature was established for solid solutions of general formula (Mg1− X Ni X )2SiO4, where X = 0.15, 0.2, 0.25, 0.3, 0.5, and 0.8 atoms Ni ( X Ni; i.e., mole fraction of Ni-olivine end-member). Ni orders into M1 with K D = (Ni/Mg in M1)/(Ni/Mg in M2) reaching a maximum of 9.5 at a composition of Mg1.6Ni0.4SiO4. The temperature dependence of order/disorder at up to 1100 °C was determined for two samples ( X Ni = 0.2 and 0.5). Between about 600 and 750 °C the samples show an increase in order due to kinetic effects, while above 750 °C the samples show a progressive decrease in order and describe an equilibrium disordering path. Equilibrium data define a Ni-Mg, M1-M2 intersite exchange energy of 21.5 ± 1.9 kJ/mol. On cooling, the blocking temperature for cation exchange is about 800 °C. The kinetics of disordering behavior were analyzed using a Ginzburg-Landau model giving activation energies for Mg-Ni exchange between M1 and M2 for samples of composition Mg1.6Ni0.4SiO4 and Mg1.0Ni1.0SiO4 of 145 ± 5 and 160 ± 5 kJ/mol, respectively. The model also shows that the characteristic time scale for re-equilibration of M1-M2 order decreases from around 2.5 s at 1000° to 0.03 s at 1300 °C. This points to the inapplicability of intracrystalline Ni-Mg partitioning for obtaining geothermometry and geospeedometry information for magmatic conditions. Ni K -edge EXAFS data show that samples with X Ni = 0.15, 0.2, 0.25 and 0.3 all show Ni clustering on adjacent M1 sites, indicating the presence of domains of Ni-rich and Mg-rich regions on a nanolength scale of < 10 A. These “precipitates” are at least an order of magnitude too small to be detectable by neutron powder diffraction. We suggest that the elastic strain at the interfaces between the Ni-rich precipitates and the Mg-rich matrix is responsible for the plateau or possible maximum in the b unit-cell parameter as a function of composition across the solid solution, which is observed at a composition of Mg1.6Ni0.4SiO4 at room temperature. Comparison of our data with earlier studies at high P and T on Mg1.0Ni1.0SiO4 olivine suggests that the effect of P is to increase the degree of order of Ni into M1 and to slow down the kinetics of intersite exchange with a Δ V disorder of 0.039 J/bar.

Effects of chunk breakage and surface protective film on negative difference effect of magnesium alloys

In the present study, the factors having an affect on the difference effect of Mg-alloy, such as the role of monovalent ion, chunk breakage and surface protective film are discussed in detail. The degree of difference effect of as-cast magnesium alloy increases with Al content and applied external potential. The chunk breakage of precipitate was observed under open circuit potential, as well as applied external potential. Also, the precipitate which is disrupted as chunk type maintained its original morphology as that of as-cast alloy among the corrosion products after a corrosion test. It is thought that chunk breakage is a secondary factor bearing on the difference effect, which varies only the degree of the difference effect, Δ. The main cause for the difference effect of Mg-alloy which reveals a negative sign is due to the corrosion behavior of the surface protective film which is influenced on the Al content of the Mg-rich matrix, e.g., the repassivation tendency.

Effect of an Mg-rich matrix on the corrosion behavior of As-cast magnesium-aluminum alloys

In the present study, the corrosion behavior of as-cast Mg-Al and Mg-Al-Zn alloys was studied as a function of the Al content in the matrix. Corrosion properties such as the corrosion rate, corrosion potential, and repassivation tendency were estimated through immersion and electrochemical tests. The corrosion potential and corrosion rate of a solutionized alloy depend mainly on the Al content of the as-cast alloy. The variation of Al content in the Mg-rich matrix influences the stability of the passive film and the repassivation tendency, i.e., as the Al content of the matrix increases, the repassivation tendency of the surface protective film after its breakage deteriorates. Also, it was proven that the enhancement of corrosion resistance by heat treatment, as in T6, is due to the decrease of solute concentration in the matrix, in addition to the effect of the precipitate, which plays the role of a barrier against corrosion.

Effect of galvanic corrosion between precipitate and matrix on corrosion behavior of As-cast magnesium-aluminum alloys

In the present study, the corrosion behavior of an as-cast magnesium alloy was studies focusing on the galvanic corrosion between a precipitate and Mg-rich matrix. Through immersion and electrochemical tests, the variation of the corrosion behavior with the alloy composition and alloy system was discussed in detail. The corrosion rate of an as-cast alloy increased abruptly to 9 wt.% Al in both alloys, but in the composition range of over 12 wt.% Al, the corrosion rate reveals a different tendency than the alloy system. The β-phase that is a typical precipitate in an Mg-xAl alloy is a more potent cathodic phase than is the ternary precipitate in a Mg-xAl-LZn alloy. In the case of the Mg-xAl alloy, the formation of a galvanic cell between the precipitate and matrix promotes the preferred dissolution of the matrix, but the precipitate in the Mg-xAl-lZn alloy has a minor effect on the corrosion behavior of the Mg-rich matrix. However, the corrosion rate of as-cast Mg-xAl and Mg-xAl-lZn alloys which contain precipitate, depends mainly upon the corrosion behavior of the Mg-rich matrix, which is influenced by the Al content. It depends additionally upon the variation of the Anode-Cathode Area Ratio (ACAR) and the chunk breakage of precipitate during corrosion.

The primitive matrix components of the unique carbonaceous chondrite Acfer 094: A TEM study

The mineralogical and chemical characteristics of the fine-grained matrix (≤53μm) of the unique primitive carbonaceous chondrite Acfer 094 have been investigated in detail by scanning electron microscopy (SEM) and analytical transmission electron microscopy (ATEM). Generally, the fine-grained matrix represents a highly unequilibrated assemblage of an amorphous material, small forsteritic olivines (200–300 nm), low Ca-pyroxenes (300–400 nm), and Fe,Ni-sulfides (100–300 nm). The matrix is basically unaffected by secondary processes. Only minor amounts of serpentine and ferrihydrite, as products of hydrous alteration, are present. Texturally, the amorphous material acts as a groundmass to olivines, pyroxenes, and sulfides, mostly exhibiting rounded or elongated morphologies. Only very few clastic mineral grains have been found. The texture and chemical composition of the amorphous material are consistent with an origin by disequilibrium condensation in either the cooling solar nebula or a circumstellar environment. As such, the amorphous material may be considered as a possible precursor of matrix materials in other types of chondrites. The non-clastic matrix olivines (Fo 98−99) and pyroxenes (En 97−100) are suggested to have formed either by condensation in the solar nebula under highly oxidizing conditions or by recrystallization from the amorphous material. The formation of these grains by fragmentation of chondrule components is unlikely due to chemical and microstructural reasons. Rapid cooling caused the observed intergrowths of clino/orthoenstatite in the Mg-rich matrix pyroxenes. Although some similarities exist comparing the fine-grained matrix of Acfer 094 with the matrices of the unequilibrated C03 chondrite ALHA77307 and the unique type 3 chondrite Kakangari, Acfer 094 remains unique. Since it contains the highest measured concentrations of circumstellar SiC and the second highest of diamond (highest is Orgueil), it seems reasonable to suggested that at least parts of the amorphous material in the fine-grained matrix may be of circumstellar origin.

Mechanism of Exfoliation (Layer Corrosion) of AI-5%Zn-1%Mg

AbstractThe mechanism of exfoliation in AlZn5Mg1 alloy has been studied by making a survey of the phasesb occurring in the alloy in the naturally aged and artificially aged conditions, preparing the phases in pure state and investigating their electrochemical properties. It has beenfound that in the naturally aged alloy, the attack is confined to lamellar zones in the structure giving rise to the exfoliation. Finely dispersed α-Al(Fe, Me)Si phase particles arranged in streaks along the extrusion or rolling direction act as cathodes and the anodic areas consist of narrow zones of the Zn- and Mg-rich matrix, next to the particles. Between the streaks of α-Al(Fe,Me)Si particles, layers of matrix are left unattacked. Since the main factor determining the amount of α-Al(Fe,Me)Si phase is the Fe content, an increase in Fe content will decrease resistance to exfoliation. In the artificially aged condition, the alloy is not prone to exfoliation, but shows a type of general attack. α-Al(Fe,Me)Si particles again co...