Area Fraction Of Phase Research Articles

Influence of forging pretreatments on microstructure evolution and surface roughness of Al 6061 alloy

Achieving ultra-smooth surfaces is the goal of aluminum optical manufacturing. Under certain processing conditions, improving the microstructure of aluminum and understanding its relationship with surface roughness requires systematic study. The grain structure and various types of second-phase particles are of paramount importance. This study analyzed the microstructure of 6061 alloy after undergoing severe plastic deformation under various processing conditions followed by T6 homogenization heat treatment. Utilizing a white light interferometer, a comparative analysis of the surface roughness was conducted on specimens that underwent single-point diamond turning to achieve a mirror finish. The assessment of surface roughness on machined surfaces is solely based on white light interferometry. The analysis and discussion focus on the effects of phases (causing scratches and voids), the grains and grain boundaries. Experimental findings signify: the grain size, grain boundary and residual second phase can both influence the surface quality, the increase in deformation temperature and accumulated strain both facilitate the dissolution and fragmentation of the secondary phases. However, they also contribute to some extent to grain growth, resulting in a minimum secondary phase area fraction of 0.87% and grain sizes reaching 147.8 μm. Subsequent heat treatments, while effective in reducing the negative impact of the phases, reveal noticeable step-like structures affecting the quality of surface roughness, with the lowest obtained Ra value being 0.8 nm. A proposed pretreatment method in cleaner ingot processing with lower alloy element content addresses the trade-off between reducing phases and controlling grain growth, aiming to achieve improved surface roughness, promoting the application of polycrystalline aluminum alloys in the field of optics manufacturing.

Understanding wetting behaviors on rough PTFE surfaces towards n-octane/water mixture separation by molecular dynamics simulation

The wetting behaviors of pure n-octane and water nanodroplets on rectangular pitted, grooved, and rectangular pillared polytetrafluoroethylene (PTFE) surfaces were examined by molecular dynamics simulation. The results showed that the three rough surfaces could achieve superlipophilicity and near superhydrophobicity by changing the rough morphology of surface. The n-octane droplets always presented the Wenzel state on the rough surfaces, and the water droplets were the Wenzel state, Transition state and Cassie state under different phase area fractions and/or roughness factors. Based on completely opposite wetting properties of the two droplets on the same surface, separation performances of mixture nanodroplets were further studied on the three rough PTFE surfaces. The results showed that the n-octane/water separation efficiency of pillared surface was above 98.4 %, significantly higher than those of pitted and grooved surfaces. The rough PTFE surface with special wettability enabled efficient separation of oil/water mixture. This work is expected to pave the way for the rational design and industrial application of oil/water separation materials.

The effect of stress gradient on the formation of topologically close packed phases in a Ni-based single crystal superalloy at 1050 °C

The formation of topologically close packed (TCP) phases is detrimental to the mechanical properties of Ni-based single crystal (SX) superalloys, which are used in harsh environments with high temperature and gradient stress caused by centrifugal force during service. In order to study the precipitation behavior of TCP phases in coupling with temperature and stress gradient, a third generation Ni-based SX superalloy is selected to conduct force holding experiments at 1050 °C for 8 hours. Stress gradient can be realized under constant load by designing tensile samples with cross-sectional area continuously changed. The results indicate that the area fraction of TCP phases varies at different positions of the sample, where the effective tensile stress is different. The area fraction of TCP phases decreases at the thinner region of the sample, which means that the amount of TCP phases decreases as applied stress increases. At the thinnest position, where the stress is estimated to be 400 MPa, almost no TCP was observed. The inhibition of stress on the formation of TCP phase may be due to the reduction of Re concentration in the γ matrix, because of the segregation of Re at γ/γ′ phase interfacial dislocation cores and the dissolving of γ′ phase.

Cryo-EM images of phase-separated lipid bilayer vesicles analyzed with a machine-learning approach

Lateral lipid heterogeneity (i.e., raft formation) in biomembranes plays a functional role in living cells. Three-component mixtures of low- and high-melting lipids plus cholesterol offer a simplified experimental model for raft domains in which a liquid-disordered (Ld) phase coexists with a liquid-ordered (Lo) phase. Using such models, we recently showed that cryogenic electron microscopy (cryo-EM) can detect phase separation in lipid vesicles based on differences in bilayer thickness. However, the considerable noise within cryo-EM data poses a significant challenge for accurately determining the membrane phase state at high spatial resolution. To this end, we have developed an image-processing pipeline that utilizes machine learning (ML) to predict the bilayer phase in projection images of lipid vesicles. Importantly, the ML method exploits differences in both the thickness and molecular density of Lo compared to Ld, which leads to improved phase identification. To assess accuracy, we used artificial images of phase-separated lipid vesicles generated from all-atom molecular dynamics simulations of Lo and Ld phases. Synthetic ground-truth data sets mimicking a series of compositions along a tieline of Ld + Lo coexistence were created and then analyzed with various ML models. For all tieline compositions, we find that the ML approach can correctly identify the bilayer phase with >90% accuracy, thus providing a means to isolate the intensity profiles of coexisting Ld and Lo phases, as well as accurately determine domain-size distributions, number of domains, and phase-area fractions. The method described here provides a framework for characterizing nanoscopic lateral heterogeneities in membranes and paves the way for a more detailed understanding of raft properties in biological contexts.

Dissolution of η phase and evolution of dynamically recrystallized grains in ATI 718Plus superalloy during hot compressive deformation

The dissolution mechanism of the η-Ni3(Al0.5Nb0.5) phase during hot compression in Ni-based ATI718Plus superalloy has been investigated. The results indicate that with increasing deformation, the amount of dynamic recrystallization grains in the γ matrix increases, and the dissolution of the η phase tends to be accelerated. At the precipitation temperature of the η phase of 950 °C, the η phase area fraction decreased from 9.8% to 3.0% after deformation. The deformation promotes dislocation accumulation at the boundaries of the η phase, developing into subgrains and then evolving into DRX grains due to subgrain rotation. With increased strain, the subgrain rotations and DRX grain growths result in extrusion and subsequent fracture of the η phase. Due to strain incompatibility between the η phase and γ matrix, dislocation sustains are accumulated at their interfaces, which disrupts the η/γ semi-coherent interface, leading to localized amorphization and the decomposition of the η phase. Furthermore, dislocation surrounding the η phase can act as diffusion channels for solute elements, promoting the dissolution of the η phase and resulting in the element concentration gradient distribution from the η phase to the γ matrix.

Failure analysis of an AISI 310 stainless steel beam reinforcement fracture during service in a rolling beam furnace

This work investigated the premature failure of a beam reinforcement made of austenitic stainless steel AISI 310 used in the entrance of a rolling beam furnace working continuously at cycles from 650 to 850 °C. Fracture surface and microstructural investigations allowed to identify the fracture mechanism as fatigue failure. The fracture was probably initiated at the oxidized grain boundaries at an early stage of service life, when no microstructure degradation was present, while most of the fatigue crack propagation occurred after the precipitation of secondary phases. The microstructure investigation revealed severe microstructure degradation with the presence of intermetallic sigma (σ) phase (area fraction = 19 %) and M23C6 carbides (area fraction = 1 %), which were formed during the high-temperature exposure. Cracking of σ phases and decohesion at σ /austenite matrix interfaces were observed near the fracture and in the small punch test samples. Such discontinuities provide an easy path for crack propagation.

Role of high-speed nanoindentation mapping to assess the structure-performance correlation of HVAF-sprayed Cr3C2-25NiCr coating

Cr3C2-NiCr coatings deposited by high-velocity air-fuel (HVAF) spray can potentially exhibit superior wear performance. However, the microstructural variations caused by altering HVAF process parameters and secondary heat treatment considerably impact coating characteristics and wear performance. The effective characterization of such microstructural variations at the micrometer length scale using conventional techniques is limited. In this study, we employ a high-speed nanoindentation mapping technique to spatially map the properties of HVAF sprayed Cr3C2-NiCr coating deposited by varying particle size-combustion mode combination, which was in the as-deposited as well as heat-treated state. Despite the significant microstructural heterogeneity at a small scale, the hardness maps show excellent correlation with the microstructure. The data extracted from the nanoindentation maps estimate the area fraction and average hardness of individual phases (carbide, binder, interacted region) in the coatings. Insights on the role of individual constituents on the coating microstructure and mechanical property evolution are presented. The hardness evolution of the individual constituents with processing conditions and secondary heat treatment is correlated with the solid particle erosion performance of the coatings. Overall, Cr3C2-NiCr coatings deposited through the correct choice of particle size-combustion mode combination and heat treatment improved the desired phase distribution, resulting in superior erosion resistance.

Effects of Cu content and heat treatment process on microstructures and mechanical properties of Al−Si−Mg−Mn−xCu cast aluminum alloys

The effects of Cu content and heat treatment process on the microstructures and mechanical properties of a series of Al−Si−Mg−Mn−xCu cast aluminum alloys prepared by the vacuum die casting process were investigated by three-dimensional X-ray microscopy, optical microscopy, scanning electron microscopy, transmission electron microscopy and microhardness testing. It was found that the number density and size of gas porosities increase with increasing Cu content. However, the Cu addition will promote the formation of Cu-containing primary phases (Q-Al5Cu2Mg8Si6 and θ-Al2Cu) during the solidification, which will improve the properties of the alloys. Five different primary phases were observed, namely eutectic Si, α-Al(Fe, Mn)Si, β-Mg2Si, Q-Al5Cu2Mg8Si6, and θ-Al2Cu phases. With increasing the Cu content, the θ phase area fraction increases significantly, while the α-Al(Fe, Mn)Si phase area fraction decreases initially, followed by a slight increase, with the Q phase area fraction displaying the opposite trend relative to α-Al(Fe, Mn)Si phase. These primary phases present different evolution rules during heat treatment process. During subsequent aging, the synergic effect of precipitating Q' and θ' phases can significantly increase the alloy hardening response.

Microstructure and Tensile Properties of HPDC Mg–RE Alloys with Varying Y Additions

AbstractHigh-pressure die-casting Mg–2.6RE–xY (EW) alloys with Y contents between 0 and 3% (in wt%) were investigated for their microstructure and tensile properties. In the Y-containing alloy, the intermetallic phases at the grain boundaries consisted of skeletal Mg12RE phase, bulk Mg24Y5 phase and irregular Mg3Y phase, while {011} twins were observed in the Mg12RE phase. The yield strength was improved by Y addition at both room temperature and high temperatures. Compared with Y-free alloy, the yield strength of 3% Y alloy increased from 143.1 to 174.8 MPa and improved by 22.2% at room temperature, while it was increased from 72.2 to 104.6 MPa and enhanced by 44.9% at 300 °C. The area fraction of intermetallic phase increased dramatically from 14.5 to 18.4% with 3% Y addition. Second phase strengthening was the major contributor to the yield strength increase at ambient temperature. The increment of the area fraction of the high-thermally stable Mg–RE intermetallic phases with Y addition contributed to the consequent improvement in yield strength at high temperatures. At ambient temperature, the mechanism for the fracture of EW alloys was a ductile and quasi-cleavage fracture blend.

Phospholipase-catalyzed degradation drives domain morphology and rheology transitions in model lung surfactant monolayers.

Lung surfactant is inactivated in acute respiratory distress syndrome (ARDS) by a mechanism that remains unclear. Phospholipase (PLA2) plays an essential role in the normal lipid recycling processes, but is present in elevated levels in ARDS, suggesting it plays a role in ARDS pathophysiology. PLA2 hydrolyzes lipids such as DPPC-the primary component of lung surfactant-into palmitic acid (PA) and lyso-PC (LPC). Because PA co-crystallizes with DPPC to form rigid, elastic domains, we hypothesize that PLA2-catalyzed degradation establishes a stiff, heterogeneous rheology in the monolayer, and suggests a potential mechanical role in disrupting lung surfactant function during ARDS. Here we study the morphological and rheological changes of DPPC monolayers as they are degraded by PLA2 using interfacial microbutton microrheometry coupled with fluorescence microscopy. While degrading, domain morphology passes through qualitatively distinct transitions: compactification, coarsening, solidification, aggregation, network percolation, network erosion, and nucleation of PLA2-rich domains. Initially, condensed domains relax to more compact shapes, and coarsen via Ostwald ripening and coalescence up until the domains solidify, marked by a distinct roughening of domain boundaries that does not relax. Domains aggregate and eventually form a percolated network, whose elements then erode and whose connections are broken as degradation continues. The relative enzymatic activity of PLA2, set by the age of the sample, impacts the order and the duration of morphology transitions. The fresher the PLA2, the faster the overall degradation, and the earlier the onset of domain solidification: domains solidify before aggregating with fresh PLA2 samples, but aggregate and percolate before solidification with aged PLA2. Irrespective of the activity of the PLA2, all measured linear viscoelastic surface shear moduli obey the same dependence on condensed phase area fraction (log|G*| ∝ ϕ) throughout monolayer degradation. Moreover, the onset of domain solidification coincides with the time when the relative surface elasticity begins to increase.

The role of strain rate in microstructure evolution, deformation heterogeneity and cracking mode of high-pressure die-casting Al7Si0.2Mg alloy

The quasi-static and dynamic deformation behaviours of high-pressure die-casting (HPDC) Al7Si0.2 Mg alloy were studied in this work. Competing effects among porosities, α-Al(Fe/Mn)Si phase, eutectic phase and α-Al dendrites on strain concentration and crack propagation at different strain rates were investigated using X-ray computed tomography (CT), scanning electron microscopy (SEM), in-situ electron backscattering diffraction (EBSD), and high-resolution digital image correlation (HR-DIC). The results indicate that yield strength and ductility of Al7Si0.2 Mg alloy were improved at high strain rates compare to quasi-static loading. Weak zones, including porosities and particle interfaces, primarily contributed to deformation heterogeneity under quasi-static tension, whereas both weak zones and Al matrix played a role under dynamic loading. Moreover, crack propagation was hardly observed on porosities during dynamic deformation, thereby reducing the sensitivity of ductility to porosity. The slip and rotation of α-Al dendrites became more pronounced during dynamic deformation. As a result, the area fraction of fractured eutectic phase increased dramatically and surpassed the area fraction of α-Al(Fe/Mn)Si phase, which became the dominant factor of crack propagation during dynamic deformation.

Exceptional abrasive wear resistance of immiscible Mg-Fe80Cr20 composites with 3D interconnected structure developed by liquid metal dealloying

Three-dimensional (3D) interconnected heterogeneous composites prepared through liquid metal dealloying (LMD) have been studied owing to their unique structural features and interesting deformation behavior beyond the strength-ductility trade-off. In this work, we systematically investigated the abrasive wear mechanism of 3D interconnected composites comprising immiscible Mg and Fe80Cr20 as a function of the fraction of hard Fe80Cr20 phase. (Fe80Cr20)xNi100−x precursor alloys were used in the LMD process, and the miscible Ni element dissolved in the Mg melt. Subsequently, the self-organization of the solid Fe80Cr20 led to a 3D interconnected composite containing penetrated Mg structures. The volume fraction of Mg phase and microstructural features of the composites were highly influenced by the initial Ni concentration. Although the composites had a brittle interface because of the immiscibility of Mg and Fe80Cr20, the 3D interconnected structures exhibited exceptional abrasive wear resistance beyond expectations. Highly deformed structures were observed on worn surfaces, and wear mechanisms differed from those of bulk Mg and Fe80Cr20 materials. During the wear test, the area fraction of Fe80Cr20 phase on the worn surface increased significantly, and the presence of nanograined Fe80Cr20 layers with a high dislocation density effectively protected the underlying soft Mg phase. In particular, the Mg-based tribofilm acted as a lubricant in the 3D interconnected structure. These findings related to the improvement of wear resistance through structural design could serve as guidelines for various applications of heterogeneous 3D architecture materials.

Semi-Continuous Functionally Graded Material Austenitic to Super Duplex Stainless Steel Obtained by Laser-Based Directed Energy Deposition

In this work, a semi-continuous functionally graded material (FGM) between an austenitic and a super duplex stainless steel was obtained. These materials are of great interest for the chemical, offshore, and oil and gas sectors since the austenitic stainless steel type 316L is common (and not so expensive) and super duplex stainless steels have better mechanical and corrosion resistance but are more expensive and complex in their microstructural phases formation and the obtention of the balance between their main phases. Using directed energy deposition, it was possible to efficiently combine two powders of different chemical compositions by automated mixing prior to their delivery into the nozzle, coaxially to the laser beam for melting. A dense material via additive manufacturing was obtained, with minimum defectology and with a semi-continuous and controlled chemical compositional gradient in the manufactured part. The evolution of ferrite formation has been verified and the phase fraction measured. The resulting microstructure, austenite/ferrite ratio, and hardness variations were evaluated, starting from 100% austenitic stainless-steel composition and with variants of 5% in wt.% until achieving 100% of super duplex steel at the end of the part. Finally, the correlation between the increase in hardness of the FGM with the increase in the ferrite phase area fraction was verified.

Effect of Homogenization Process on Microstructure of Al–Zn–Mg–Cu Aluminum Alloys

Herein, the best homogenization process of 466.5 °C × 36 h + 490 °C × (14–26.4 h) that can completely eliminate the coarse phases σ[Mg(Zn, Al, Cu)2] and S(Al2CuMg) in the Al–Zn–Mg–Cu aluminum alloy is developed. The homogenization process is determined by the method of calculation phase diagram, and the experimental verification. It is shown in the results that, first, in the microstructure of the as‐cast alloys, the crystal structure of the σ[Mg(Zn, Al, Cu)2], Al7Cu2Fe, and Mg2Si phases is determined. Second, during the homogenization process, the σ[Mg(Zn, Al, Cu)2] phase dissolves and also transforms into the S(Al2CuMg) phase. Most importantly, the dissolution temperature range of the σ[Mg(Zn, Al, Cu)2], S(Al2CuMg), and Al7Cu2Fe phases is determined from 472.56 to 476.36 °C, from 484.09 to 485.39 °C, and from 540.18 to 547.23 °C, respectively. At best homogenization process, the residual Al7Cu2Fe phase area fraction ranges from 1.28 ± 0.16% to 1.60 ± 0.18%. In addition, dispersed η(MgZn2) phase precipitates in supersaturated Al‐matrix during differential scanning calorimeter heating. And, the concentration differences between the grain center and the eutectic of structure of Zn, Mg and Cu regression equations are established, which can provide some reference for the design of experimental parameters, thus reducing the experimental workload.

Effects of cooling manner on Laves phase precipitation and mechanical properties of Al modified super ferritic stainless steels

The solution treating and cooling experiments were conducted to study the microstructure and mechanical properties of Al modified super ferritic stainless steels with various cooling methods. The results demonstrated that the grain was characterized by the stratified distribution. The grain coarsened gradually with the decrease in cooling rate. And the surface grain coarsened obviously, Nb (C, N) particles were precipitated in the samples in different cooling manners. However, the Laves phase was distributed both at the grain boundaries and in the grains in the furnace-cooled specimen, and the area fraction of Laves phase increased significantly with the increase of Al content. With the cooling rate decreased, the yield strength, ultimate tensile strength and Vickers hardness of the Al-free samples increased, while the elongation decreased, and the furnace-cooled specimens showed brittle fracture during the tensile test. The addition of Al also increased yield strength, ultimate tensile strength and Vickers hardness and deteriorated plasticity of samples with different cooling manners. Meanwhile, the plasticity of furnace-cooled samples was more sensitive to addition of Al than water-quenched and air-cooled specimens. The analysis results showed that the plastic deterioration of furnace-cooled samples was mainly related to the coarsening of Laves phase at grain boundaries. Hence, the precipitation of large size Laves phase at grain boundaries should be inhibited in the process of industrial production.

The microstructure and mechanical properties of 2219 alloy during ultrasonic treatment

High-intensity ultrasonic melt treatment technique plays a key role in improving the performance of large-scale 2219 alloy ingot (Φ630 mm × 4500 mm). In this current paper, ultrasonic treatment-assisted casting techniques were developed and applied to manufacture large-scale 2219 aluminium (Al) alloy ingot in direct chill casting (DC). The industry-level experiments revealed that the α-Al grains of the ingot with ultrasonic (USDC) were refined, the area fraction of the coarse Al2Cu eutectic phase showed a decreasing trend, and the mechanical properties were lightly improved than without ultrasonic ingot (DC). Based on the present results and previous proposed hypotheses, the synergistic effects of cavitation and acoustic streaming were considered as the dominant mechanisms of USDC.

Dynamic Precipitation During Forging of a γ–γ′ Nickel-Based Superalloy

Hot compression and creep tests under different conditions were performed on a typical γ–γ′ polycrystalline nickel base superalloy below the γ′-phase solvus temperature, to shed light on the mechanisms of γ′ precipitation during deformation and the driving forces behind these mechanisms. Scanning Electron Microscopy (SEM) and image analysis were employed to characterize the different populations of γ′ precipitates and their evolutions. The γ′-phase area fractions observed after hot compression are systematically higher than the fraction prior to deformation within the investigated range of thermomechanical conditions. Two new populations of precipitates formed during deformation are identified and characterized. These are interpreted to result from a dynamic precipitation process, the hypothesis of their formation during the quenching step being very unlikely with regards to their size and amount. The fraction of γ′-phase produced during deformation increases with increasing deformation magnitude, decreasing deformation temperature and for slower strain rates. Finally, a scenario for dynamic γ′ precipitate nucleation and growth is proposed to explain the experimental observations and the influence of dislocation density.

Microstructure and mechanical properties of the new cladding austenitic stainless steel of 15–15Ti–Y after aging at 500–700 °C for up to 1000 h

As a promising material for fuel cladding, 15Cr–15Ni–Ti (15-15Ti) austenitic stainless steel is widely used in core materials of nuclear reactors due to its excellent creep and oxidation resistance. To further enhance the stability of 15–15Ti steel in ultra-long service time at high temperature, the yttrium-modified 15Cr–15Ni–Ti (15-15Ti–Y) was aged at 500 °C, 600 °C and 700 °C for up to 1000 h. The grain size and precipitation behavior including precipitates species, morphology, quantity, formation sequence and location were systematically investigated. Through the detection and analysis of electron backscatter diffraction, scanning electron microscope, X-ray diffraction and transmission electron microscope, the results indicates that Y is a strong solute segregation element, which hinders the formation and coarsening of M23C6 phase along the grain boundary, thus facilitating the formation of fine intragranular precipitation. At the same time, the addition of Y inhibits recrystallization, grain growth and intergranular precipitation, and promotes intragranular precipitation. Therefore, after aging at 500 °C for 1000 h, the Chib particles only precipitated in 15–15Ti–Y steel, while the size of precipitates in grains interiors and area fraction of M23C6 phase along grain boundaries in 15–15Ti–Y steel were always smaller than that of 15–15Ti steel. Besides, the effect mechanism of Y addition on precipitation behavior and mechanical properties of 15–15Ti–Y after aging is also discussed. The comparison shows that the hardness of the two steels has little change after aging, but the Charpy impact toughness and tensile properties have decreased to varying degrees. As the main reason affecting the mechanical properties, the intergranular precipitation in the chain after low temperature aging weakens the impact toughness of 15–15Ti, while the fine intragranular precipitation after high temperature aging can improve the strength of 15–15Ti–Y. Therefore, compared with 15–15Ti steel, 15-15Ti–Y steel shows higher hardness, impact toughness and uniform elongation after aging at 500–700 °C for 1000 h.

In Situ Observation on the Heterogeneous Nucleation of MnS in Heavy Rail Steels with Varied Aluminum Content

In situ observation, scanning electron microscope/energy‐dispersive spectrometer detection, and lattice disregistry calculation are combined to investigate the evolution mechanism of inclusions in heavy rail steels with varied aluminum contents from SiO2–Al2O3–MnO to SiO2–Al2O3–MnO–MnS at 1323 K and 1393 K. The precipitation of MnS inclusions on SiO2–Al2O3–MnO inclusions in the steel occurs when the Al2O3 content in identical oxide inclusions is 30% at 1393 K and 30%, 40%, and 59% at 1323 K. The MnS outer layer shows visible precipitation from 3 min after heating at experimental temperature. The area fraction of MnS phase in SiO2–Al2O3–MnO–MnS inclusions with varied Al2O3 content at 1323 K decreases in the following order: SiO2–Al2O3 (30%)–MnO > SiO2–Al2O3 (40%) –MnO > SiO2–Al2O3 (59%)–MnO. The precipitation of MnS is prone to occur on the Al2O3‐less region of the SiO2–Al2O3–MnO inclusion. The solubility of Mn and S decreases with decreasing temperature. Lower heating temperature and lower Al2O3 content in SiO2–Al2O3–MnO oxide cores are profitable to the heterogeneous nucleation of MnS due to lower disregistry between MnS with the SiO2–Al2O3–MnO inclusion.

Influence of intermetallic Al-Mn particles on in-situ steam Mg-Al-LDH coating on AZ31 magnesium alloy

The influence of intermetallic Al-Mn particles on the corrosion behavior of in-situ formed Mg-Al layered double hydroxide (Mg-Al-CO32--LDH) steam coating on AZ31 Mg alloy was investigated. The alloy was pretreated with H3PO4, HCl, HNO3 or citric acid (CA), followed by hydrothermal treatment, for the fabrication of Mg-Al-LDH coating. The microstructure, composition and corrosion resistance of the coated samples were investigated. The results showed that the surface area fraction of Al-Mn phase exposed on the surface of the alloy was significantly increased after CA pretreatment, which promotes the growth of the Mg-Al-LDH steam coating. Further, the LDH-coated alloy pretreated with CA possessed the most compact surface and the maximum coating thickness among all the coatings. The corrosion current density of the coated alloy was decreased by three orders of magnitude as compared to that of the bare alloy.