Al6Mn Phase Research Articles

Influence of second phase on corrosion performance and formation mechanism of PEO coating on AZ91 Mg alloy

Plasma electrolytic oxidation (PEO) treatment has been performed on AZ91 Mg alloy in a silicate-containing alkaline electrolyte to disclose the relationship between second phase and resultant coating. The morphology and corrosion behavior were characterized for PEO coatings on three different AZ91 substrates with microstructures altered by heat treatment. In the presence of β phase, both the substrate and PEO coating exhibit enhanced corrosion resistance with finer and more evenly distributed β phase precipitates. In the as-cast AZ91, the anodized coating displays chemically and morphologically discontinuity on α and β phase with very coarse and defective regions on two-phase boundaries. In aged AZ91 with nano-sized β phase precipitates, the PEO coating shows no obvious difference on two-phase boundaries and is more homogenous with fewer pores and defects. The residual stress values given by X-ray diffraction (XRD) analysis indicate that PEO coating on aged AZ91 with finest β phase has the smallest stress which might be the reason for its improved corrosion resistance under synergetic effect of corrosive ions and residual stress. Conversely, in the absence of β phase, i.e. after solid solution treatment, the coating is characterized by the most inhomogeneous microstructure and the worst corrosion resistance, presumably owing to the absence of protective aluminum-containing oxides in the coating and more severe localized oxidation behavior adjacent to AlMn phase. Understanding the effect of second phase on PEO coating will enable us to optimize porous oxide structures for improved corrosion resistance by properly altering the substrate microstructure.

Crystal structure, homogeneity range and electronic structure of rhombohedral γ-Mn5Al8

Abstract The γ-region of the Mn–Al phase diagram between 45 and 70 at.% Al was re-investigated by a combination of powder and single crystal X-ray diffraction as well as EDS analysis to establish the distribution of Mn and Al atoms. Single crystals of γ-Mn5–x Al8+x were grown using Sn-flux at 650 °C. The crystal structure, atomic coordinates and site occupancy parameters of γ-Mn5−x Al8+x phases were refined from single crystal X-ray data. The γ-Mn5-x Al8+x phase adopts the rhombohedral Cr5Al8-type structure rather than a cubic γ-brass structure. The refined compositions from two crystals extracted from the Al-rich and Mn-rich sides are, respectively, Mn4.76Al8.24(2) (I) and Mn6.32Al6.68(2) (II). The structure was refined in the acentric R3m space group (No.160, Z=6), in order to compare with other reported rhombohedral γ-brasses. In addition, according to X-ray powder diffraction analysis, at the Al-rich side the γ-phase coexists with LT–Mn4Al11 and, at the Mn-rich side, with a hitherto unknown phase. The refined lattice parameters from powder patterns fall in the range a=12.6814(7)−12.6012(5) Å and c=7.9444(2)−7.9311(2) Å from Al-rich to Mn-rich loadings, and the corresponding rhombohedral angles distorted from a pseudo-cubic cell were found to be 89.1(1)°−88.9(1)°. Magnetic susceptibility and magnetization studies of Mn4.92Al8.08(2) are consistent with moment bearing Mn and suggest a spin glass state below 27 K. Tight-binding electronic structure calculations (LMTO-ASA with LSDA) showed that the calculated Fermi level for γ-“Mn5Al8” falls within a pseudogap of the density of states, a result which is in accordance with a Hume-Rothery stabilization mechanism γ-brass type phases.

Microstructure and electrochemical corrosion behavior of extruded Mg–Al–Pb–La alloy as anode for seawater-activated battery

A seawater-activated battery is a disposable reserve battery that is specifically designed to pollute less due to the absence or relatively less use of heavy metals. Mg–Al–Pb–La alloy is a typical anode material that is used in high-power seawater-activated batteries. In this study, we use microstructure characterization to demonstrate that hot extrusion refines the grains of the alloy and leads to the formation of banded structures composed of the lamellar β-Al11La3 and lump AlMn phases. Our electrochemical measurements reveal that the extruded Mg–Al–Pb–La alloy is a promising anode material that exhibits excellent discharge activity and high anodic efficiency at a large current density. Moreover, we also examine the correlation between the grain orientation induced by extrusion and the electrochemical corrosion behavior of Mg–Al–Pb–La alloy. We believe that our results can significantly contribute to the development of seawater-activated batteries.

Grain Refinement of AZ61 Alloy after ECAP Processing

Electron microscopy techniques were used to characterize the microstructure and deformation behavior of AZ61 alloy proceeded by ECAP. The commercial AZ61 alloy subjected to severe plastic deformation possesses a two-phase microstructure consisting of solid solution matrix and massive γ-phase Mg17Al12, or Mg17(Al,Zn)12 distributed mostly at grain boundaries. Based on selected area diffraction and electron back scattered diffraction applied to a sample after the third pass, it can be concluded that plastic deformation induced by ECAP occurs mainly by slip mode forming a high density network of dislocation inside the grains. The grains size was significantly refined to 1.4 μm after the third pass of ECAP. The refinement of grain size is probably due to polygonization process associated with formation of high angle grain boundaries due to dislocations rearrangement. (Al, Zn)12Mg17 precipitates of size scattered from 100 to 200 nm and also the primary precipitates of Al6Mn phase were observed in this alloy.

Phase control and formation mechanism of Al–Mn(–Fe) intermetallic particles in Mg–Al-based alloys with FeCl3 addition or melt superheating

In this study, we identify the crystal structures and elucidate the formation mechanism of Al–Mn(–Fe) intermetallic particles formed in Mg–Al-based alloys with varied Al contents, FeCl3 addition or melt superheating. The formed Al–Mn(–Fe) particles were collected through a newly developed particle extraction method for structural, chemical and morphological characterizations. It is found that increasing the Al content from 3 wt% to 9 wt% changes the major Al–Mn phase from τ-Al0.89Mn1.11 in AZ31 to γ2-Al8Mn5 in AZ91, and both FeCl3 addition and melt superheating promote the formation of γ2-Al8(Mn,Fe)5 in AZ31 and AZ63. In addition, through tuning FeCl3 addition level or melt superheating temperature, different morphologies of the γ2-Al8(Mn,Fe)5 particles are formed in AZ31 and AZ63, including quasi-cubes, octa-pod microstructures and polyhedral particles. This study provides several important approaches to control the crystal structure and morphology of Al–Mn(–Fe) intermetallic particles formed in Mg–Al-based alloys.

Precipitation Processes in Hot-Rolled Al-Mn-Sc-Zr Alloy

The effect of hot rolling on mechanical and electrical properties, microstructure and recrystallization behaviour of the AlMnScZr alloy was studied. The mould-cast alloy and the alloy after hot rolling at 300 °C was studied during step-by-step quasilinear annealing from 200 °C up to 600 °C with heating rate 100 K/h followed by subsequent isothermal annealing at 600 °C/5 h. Precipitation reactions were studied by electrical resistometry, differential scanning calorimetry and hardness measurements. Transmission electron microscopy and electron backscatter diffraction examination of specimens quenched from temperatures of significant resistivity changes were used to identify microstructural processes responsible for these changes. Only occasional irregular sharp-edged polygonal particles of the AlMnFeSi system were found in the as-prepared state of the mould-cast alloy. The as-prepared state of the hot-rolled alloy was characterized by a dispersion of fine coherent Al3Sc and/or Al3(Sc,Zr) particles and furthermore a fine (sub) grain structure was observed. The hardening effect in the alloys is due to presence and/or precipitation of the Sc,Zr-containing particles with L12 structure. The distinct resistivity changes of the alloys are mainly caused by precipitation of Mn-containing particles. Two-stage development of the Al6Mn phase (in (sub) grain interiors and at (sub) grain boundaries) in the hot-rolled alloy was observed. The presence of Sc,Zr-and Mn-containing particles has an anti-recrystallization effect that prevents recrystallization minimally up to 600 °C and annealing of 1 hour in the hot-rolled alloy. The apparent activation energy for the Al3(Sc,Zr)-phase and Al6Mn-phase precipitation was also determined. The activation energy values obtained in the hot-rolled AlMnScZr alloy are comparable to those observed in the hot deformed AlMnScZr alloys prepared by powder metallurgy.

Microstructural study of cold forged and annealed Mg alloys

Cold forging deformations (3 and 8%) were carried out on extruded AZ31 and AZ61 Mg alloys, and subsequent annealing is performed at 573 K for different durations. The microstructure evolution is investigated and further analysed. The results show that the thick lenticular twinning is mainly formed at initial deformation stage and subsequently transforms into narrow band twinning. Compared to AZ31, AZ61 produces broader twins with lower volume fraction in response to 3% cold forging due to Al–Mn phase hindering the twinning activity and plastic deformation. Twin boundary plays an important role in refining grains as it provides the nucleation sites of static recrystallisation. Moreover, as for AZ61, the starting and ending times of static recrystallisation are much later than those of AZ31.

Effect of T6 heat treatment on corrosion resistance and mechanical properties of AM50 magnesium alloy

To improve the corrosion resistance and mechanical properties of AM50 magnesium alloy, T6 heat treatment was applied. The microstructure, mechanical properties and corrosion properties of as-cast and T6 heat-treated alloy were studied. The results indicated that the β phase presented at the grain boundaries along with Al–Mn phase after T6 heat-treatment. Because of solid solution and aging treatments, the tensile strength of the AM50 magnesium alloy was increased. The decreasing of the β phase owing to the solidsolution during T6 treatment, which causes Al solid solution in the α-Mg phase, and it is favourable for increasing the corrosion resistance of AM50 magnesium alloy. The fracture mechanisms of the as-cast and T6 heat-treated alloy were quasi-cleavage fracture. The corrosion residual strength of the T6 heat-treated alloy was higher than that of the as-cast alloy. Compared with the as-cast alloy, the corrosion rate of the T6 heat-treated alloy decreased gradually.

The Al-rich region of the Al–Fe–Mn alloy system

Phase equilibria in the Al-rich region of the Al–Fe–Mn alloy system were studied at 1070, 1020, 950, 875, 800, 740, 695 and 650°C. The continuous region of the bcc solid solution was estimated between the Al–Mn and Al–Fe terminals. Also the isostructural high-temperature Al–Mn and Al–Fe γ1-phases (γ-brass type structure) form continuous regions. The Al6Mn, high-temperature T-Al11Mn4 and low-temperature γ2 phases dissolve up to 9.0, 14.5 and 31.0at.% Fe, respectively, while the M-Al13Fe4, Al5Fe2 and Al2Fe phases dissolve up to 15.5, 11.5 and 10.0at.% Mn, respectively. The thermodynamically stable decagonal D3-phase with periodicity of 1.25nm in the specific direction and two periodic intermetallics designated φ (P63/mmc; a=0.7554, c=0.7872nm) and κ (P63/m; a=1.7630, c=1.2506nm) were identified. An additional ternary phase of unknown structure was also revealed.

Mechanical properties, intergranular corrosion behavior and microstructure of Zn modified Al–Mg alloys

The effects of the alloying elements Zn on the mechanical properties, intergranular corrosion behavior and microstructure of Al–Mg alloys have been examined. The type, morphology and distribution of the precipitates both at the grain boundary and in the matrix were systematically observed by optical microscopy and transmission electron microscopy. Integrated intergranular corrosion test and transmission electron microscopy results indicated the stable ternary Al–Mg–Zn phase precipitated at grain boundary rather than Al–Mg binary phase with Zn addition to Al–Mg alloy, and discontinuously precipitated at the grain boundary. Also, the area fraction of Al–Mg–Zn phase at the grain boundary decreased with an increase in the Zn content. The intergranular corrosion resistance and strength of the alloy enhanced by Zn addition. In grains, the amount of Al–Mg–Zn phase and Al–Mg increasing with Zn content increases. The Al–Mg phase was found to be nucleation at preexisting Al–Mn phase. Finally, detailed explanation for controlling and optimizing the microstructure in the Zn modified alloy was discussed.

Phase stability in a powder-processed Al–Mn–Ce alloy

There has been considerable interest in Al-rich Al–Mn–Ce alloys due to the variety of crystalline and quasi-crystalline metastable phases that can be formed. Here we report a study of the effects of heat treatment on an Al–5Mn–2Ce (at.%) alloy processed by gas atomization and consolidated by warm extrusion. Characterization using X-ray diffraction and electron microscopy showed that the powder microstructure consists mainly of an amorphous phase, FCC Al, and a previously unreported phase, Al20Mn2Ce. The extrudate is fully devitrified and contains a mixture of FCC Al, Al20Mn2Ce, and Al6Mn, with a small amount of Al12Mn and Al11Ce3. Upon heat-treatment at up to 450 °C, the Al20Mn2Ce and Al6Mn phases decompose to give a hard stable phase mixture with 72–73 % Al12Mn plus 13–14 % each of Al11Ce3 and FCC Al. Heat treatments at 500 °C give a much softer phase mixture consisting of 60 % FCC Al, 22 % of an unknown Al3(Mn,Ce) phase, 9 % Al12Mn, 8 % Al6Mn, and 1 % Al11Ce3. The formation of large volume fractions of Al12Mn for heat-treatments at up to 450 °C suggests that the presence of Ce may stabilize this phase, and that more dilute Al–Mn–Ce compositions could form the basis for new high-strength, low-density Al-based alloys with enhanced elevated temperature properties.

Intermetallic phase evolution of 5059 aluminum alloy during homogenization

Intermetallic phase evolution of 5059 aluminum alloy during homogenization was investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectrometry (EDS), differential scanning calorimetry (DSC) and X-ray diffraction analysis (XRD). The results show that severe dendritic segregation exists in as-cast alloy. The dissolvable intermetallic phases in as-cast alloy consist of Zn- and Cu-rich non-equilibrium β (Al3Mg2) phase, Fe-rich eutectic Al6Mn phase and equilibrium Mg2Si phase. During the homogenization, Zn- and Cu-rich non-equilibrium β (Al3Mg2) phase, Fe-rich eutectic Al6Mn phase and equilibrium Mg2Si gradually dissolve into matrix. Fine dispersed β(Al3Mg2) particles and rod-shaped Al6Mn particles form in the Al matrix after homogenization. The proper homogenization processing is at 450 °C for 24 h, which is consistent with the results of homogenizing kinetic analysis.

Impression creep behavior of Al–1.9%Ni–1.6%Mn–1%Mg alloy

In the present study, microstructure and creep behavior of an Al–1.9%Ni–1.6%Mn–1%Mg alloy were studied at temperature ranging from 493 to 513K and under stresses between 420 and 530MPa. The creep test was carried out by impression creep technique in which a flat ended cylindrical indenter was impressed on the specimens. The results showed that microstructure of the alloy is composed of primary α(Al) phase covered by a mantle of α(Al)+Ni3Al intermetallic compound. Mn segregated into AlxMnyNiz or Al6Mn phases distributed inside the matrix phase. It was found that the stress exponent, n, decreases from 5.2 to 3.6 with increasing temperature. Creep activation energies between 115kJ/mol and 151kJ/mol were estimated for the alloy and it decreases with rising stress. According to the stress exponent and creep activation energies, the lattice and pipe diffusion- climb controlled dislocation creep were the dominant creep mechanism.

Corrosion Mechanism of AM50 Magnesium Alloy in Simulated Acid Rain

The poor corrosion resistance of magnesium alloys is limiting their widespread applications. Some patents and literatures have reported corrosion-resistant magnesium alloys and surface treatment techniques to improve the corrosion resistance. However, these acknowledges are not enough for well understanding the corrosion behavior of magnesium alloys in actual application environments. In this paper, the influence of Cl- on the corrosion rate of AM50 alloy in simulated acid rain (SAR) was studied using electrochemical tests and scanning electronic microscopy (SEM) observation, and then the corrosion mechanism of magnesium alloys in the acid solution was established. The results show that Cl- can accelerate the corrosion of Mg matrix around the AlMn phases in the initial stage of corrosion. The predominant corrosion type of the AM50 alloy is the uniform type of corrosion in SAR. The coverage fraction of Mg(OH)2, MgH2, and the concentration of Mg+ are three key factors influencing the corrosion processes of magnesium alloys in SAR. Keywords: Acid solutions, Mg alloys, corrosion mechanism, negative difference effect, simulated acid rain (SAR).

Duplex-layered manganese phosphate conversion coating on AZ31 Mg alloy and its initial formation mechanism

Abstract A duplex-layered phosphate conversion coating was obtained on AZ31 Mg alloy by substituting NaF bath with a citric bath. The morphology, composition and corrosion resistance of the coating were investigated using SEM, EDS, SPM and electrochemical methods. A three-stage mechanism for initial formation of the coating was proposed: Dissolution of the loose oxide film and deposition of Mg3(PO4)2 and AlPO4, formation of a composite intermediate layer of Mg3(PO4)2, AlPO4 and Mg(OH)2, and deposition of manganese phosphate nuclei followed by growth and lamination of the nuclei. The nuclei preferentially deposit at the Al–Mn phase surface and near the grain boundary.

Structural modifications in a Mn54Al43C3 melt-spun alloy induced by mechanical milling and subsequent annealing investigated by atom probe tomography

The structural changes upon milling and subsequent annealing of a Mn54Al43C3 alloy containing the intermetallic tetragonal L10 MnAl phase (τ phase) as the major phase were investigated by X-ray diffraction and atom probe tomography. The analyses show that milling the starting powder for 10h leads to the nanostructuration of the sample. The milled sample is partly oxidised and contains both non oxidised Mn60±5Al40±5 regions and oxidised regions. Annealing the powder for 1h at 500°C leads to enrichment in Al of the oxidised regions, and to the phase transformation of the non-oxidised regions into a nanostructured β-Mn-like phase with a composition close to Mn3Al2.

Effect of a high magnetic field on the morphological and crystallographic features of primary Al6Mn phase formed during solidification process

Abstract

Laser-tungsten inert gas hybrid welding of dissimilar metals AZ31B Mg alloys to Zn coated steel

Laser-tungsten inert gas (TIG) hybrid welding has been developed for joining Mg alloys to Zn coated steel in a lap joint configuration. The joint could not be produced in laser or arc welding only, while acceptable joints without obvious defects were obtained with a relatively wide processing window in the hybrid process. Two reaction layers were observed to form at the interface and were identified as Mg–Zn eutectic structure (α-Mg+MgZn) and Fe3Al phase by TEM analysis. In some cases, Al6Mn phase also formed adjacent to the Fe–Al reaction layer. The tensile-shear strength attained the maximum value of 68MPa, representing 52.3% joint efficiency relative to Mg base metal. The element Al from AZ31B Mg alloys diffused to the liquid/solid interface and then reacted with the elements from steel, such as Fe and Mn, contributing to the metallurgical bonding at the interface. The weak bonding between Mg–Zn reaction layer and newly formed Fe–Al layer resulted in the interfacial failure.

STRUCTURE AND MAGNETIC PROPERTIES OF MECHANICAL ALLOYED Mn-15at.%Al

Mechanical alloying (MA) method has been used to produce nanocrystallite Mn -15at.% Al alloy. X-ray diffraction (XRD) patterns for the as-milled elemental α- Mn and aluminum powder samples show a mixture of α + β- MnAl phases after 20 h of milling and changes to a dominant β- MnAl phase structure after 50 h. An average crystallite size of 40 nm was determined from Hall–Williamson method analysis after 5 h of milling. Moreover, the thermal analysis results using differential thermal analysis (DTA), suggested a possible phase transformation after 20 h of milling. Isothermal treatments are carried in the temperature range of 450°C to 1000°C. Room-temperature vibrating sample magnetometer (VSM) measurements of the hysteretic response revealed that the saturation magnetization Bs and coercivity Hc for 10 h ball milled sample are ~ 2.1 emu/g and ~ 92 Oe, respectively.

A high strength and ductility Mg–Zn–Al–Cu–Mn magnesium alloy

A high strength Mg–8.0Zn–1.0Al–0.5Cu–0.5Mn(wt.%) magnesium alloy with outstanding ductility was developed using a common casting technique and heat treatment. The microstructure of the as-cast alloy is composed of α-Mg, MgZn, MgZnCu and Al–Mn phases. After the solution treatment and subsequent two-step aging treatment, the yield strength (YS), ultimate tensile strength (UTS) and elongation of the alloy at peak hardness reach 228MPa, 328MPa and 16.0% at room temperature, respectively. The comprehensive mechanical properties of the alloy are superior to almost all other high performance casting Mg alloys.