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A new ternary compound, Mg11-xZnxSr in the Mg-Zn-Sr system was observed and studied using Scanning Electron Microscopy (SEM), Energy-Dispersive Spectroscope (EDS), X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). The XRD patterns were refined by the Rietveld refinement method and the results revealed that the crystallized Mg11-xZnxSr phase belonged to tetragonal I41/amd space group and had the Cd11Ba prototype. The Mg atoms were successfully doped into Zn11Sr crystal lattice by occupying Zn atomic sites. Moreover, the Rietveld refinement and computational results demonstrated a gradual decrease in the a-axis and c-axis lattice parameters with decreasing concentration levels of Mg coordination substitution in the lattice of Mg11-xZnxSr compound. The elastic constants and modulus of the Mg11-xZnxSr compounds calculated by first-principles calculations (FPC) indicated they were increased with the increasing of Zn content. The variation of hardness, d -band widths and the total density of states for Mg11-xZnxSr compounds with Zn content was discussed.

The hydrogen embrittlement (HE) behavior of a selective laser-melted (SLM) 316L austenitic stainless steel has been investigated by hydrogen charging experiments and slow strain rate tensile tests (SSRTs) at room temperature. The results revealed that compared to the samples without H, the ultimate tensile strength (UTS) and elongation (EL) of specimens were decreased from 572 MPa to 552 MPa and from 60% to 36%, respectively, after 4 h of electrochemical hydrogenation with a current density of 100 mA/cm2. The negative effects of hydrogen charging were more pronounced on the samples’ ductility than on their strength. A quasi in situ EBSD observation proved that there was little phase transformation in the samples but an increased density of low angle grain boundaries, after 4 h H charging. After strain was applied, the surface of the H-sample displayed many hydrogen-induced cracks along the melt pool boundaries (MPBs) showing that these MPBs were the preferred areas for the gathering and transferring of hydrogen.

The microstructure and chemical compositions of the solid solution-treated Mg-3Nd-1Li-0.2Zn alloy were characterized using optical microscope, scanning electron microscope (SEM), transmission electron microscope (TEM), electron probe micro-analyzer (EPMA) and X-ray photoelectron spectroscopy (XPS). The corrosion behaviour of the alloy was investigated via electrochemical polarization, electrochemical impedance spectroscopy (EIS), hydrogen evolution test and scanning Kelvin probe (SKP). The results showed that the microstructure of the as-extruded Mg-3Nd-1Li-0.2Zn alloy contained α-Mg matrix and nanometric second phase Mg41Nd5. The grain size of the alloy increased significantly with the increase in the heat-treatment duration, whereas the volume fraction of the second phase decreased after the solid solution treatment. The surface film was composed of oxides (Nd2O3, MgO, Li2O and ZnO) and carbonates (MgCO3 and Li2CO3), in addition to Nd. The as-extruded alloy exhibited the best corrosion resistance after an initial soaking of 10 min, whereas the alloy with 4h-solution-treatment possessed the lowest corrosion rate after a longer immersion (1 h). This can be attributed to the formation of Nd-containing oxide film on the alloys and a dense corrosion product layer. The dealloying corrosion of the second phase was related to the anodic Mg41Nd5 with a more negative Volta potential relative to α-Mg phase. The preferential corrosion of Mg41Nd5 is proven by in-situ observation and SEM. The solid solution treatment of Mg-3Nd-1Li-0.2Zn alloy led to a shift in corrosion type from pitting corrosion to uniform corrosion under long-term exposure.

Calcium phosphate chemical conversion coatings with the additions of sodium lauryl sulfate (SLS) and dodecanesulfonic acid sodium (DSS), respectively, were prepared on the surface of the Mg-8Li alloy. The surface and cross-sectional corrosion morphologies, compositions, and corrosion behavior of the coated surfaces in 3.5 wt.% NaCl solution were respectively investigated by using a scanning electron microscope (SEM), energy dispersive spectrometer (EDS), electrochemical workstation, hydrogen evolution apparatus, and optical microscope (OM). The results demonstrated that Ca-P coatings had a petal-like structure being composed of leaf-like particles. After being respectively performed for 30 min in conversion solutions containing SLS and DSS, the corresponding average film thicknesses of surface coatings were 27 μm and 7 μm. In addition, the corrosion current densities of coated surfaces by using the conversion solutions containing SLS and DSS were 1.438 × 10−5 A·cm−2 and 4.719 × 10−5 A·cm−2, respectively. The effect of surfactants on phosphate chemical conversion coating was discussed in detail.

In this study, the effect of electrochemical hydrogen charging on the corrosion behavior of an as-cast Mg–8%Li alloy was investigated. It was revealed that after being cathodically hydrogen charged in a 0.1 M NaCl solution at a constant current density of 50 mA/cm2 for 3 h, a product film with an average thickness of 20 μm was formed in the α-Mg phase, whilst the average thickness of the product film being formed in the β-Li phase was 6 μm. When the charging time was prolonged to 18 h, the thicknesses of the product films being formed on the α-Mg and β-Li phases were increased to 75 and 20 μm, respectively. The results of the grazing incidence X-ray diffraction (GIXRD) testing showed that the product films of the differently charged samples mainly consisted of Mg(OH)2, LiOH and Li2CO3. The formed product films on the two matrix phases were dense and could hinder the erosion of Cl− in a solution, and hence improved the corrosion resistance of the alloy. After being hydrogen charged for 3 h, the charge-transfer resistance (Rct) value of the alloy was increased from 527 to 1219 Ω·cm2. However, when the hydrogen charging time was prolonged to 18 h, the Rct was slightly reduced to 1039 Ω·cm2 due to the cracking of the surface product films and the interfacial cracking of the film/substrate matrix.

The effect of hydrogen charging on microstructural evolution and corrosion behavior of a Ti-4Al-2V-1Mo-1Fe alloy in a 3.5 wt.% NaCl solution was investigated. The results showed that the hydrogen charging induced the formation and growth of γ-TiH and δ-TiH2 phases, leading to the initiation and propagation of hydrogen-induced cracks. It was also found that hydrogen charging can change the passivity of this alloy and increase its pitting corrosion susceptibility. The main reason for these was attributed to the formation of hydrides in α phase in the Ti-4Al-2V-1Mo-1Fe alloy, leading to the preferential dissolution of the α phase and thus the deterioration in the protective ability of passive film.

By performing the immersion, hydrogen evolution and electrochemical corrosion testing in a 0.1 mol/L NaCl solution, the corrosion performance of differently oriented samples cut from an as-rolled Mg-8 wt.%Li alloy plate has been investigated and compared. It demonstrated that obvious anisotropy in corrosion resistance existed between the sample surfaces with different orientations. Among them, the corrosion rate of “ND” (the normal direction) samples with their surfaces perpendicular to the normal direction of the plate was the highest. The corrosion rate of “TD” (the transverse direction) samples with their surfaces perpendicular to the transverse direction of the plate took the second place. For the “RD” (the rolling direction) samples with their surfaces perpendicular to the rolling direction of the plate, they had the lowest corrosion rate. For all the samples, their corrosion performance and the pitting severity were mainly ascribed to the crystallographic texture of α-Mg matrix phases because the formed surface films on β-Li phases could provide good protection to the substrate. For the “ND” and “TD” samples, the exposed surfaces were composed of {0002}, {10 1 ¯ 0} and {11 2 ¯ 0} planes of α-Mg phases and subsequently resulted in their severe corrosion attack due to the corrosion couples between basal and prism planes. Since the crystallographic planes of exposed α-Mg phases on the surfaces of “RD” samples were mainly {10 1 ¯ 0} and {11 2 ¯ 0} prism planes, the pitting severity was the weakest.

In this study, the corrosion behaviors of an as-extruded Mg–4%Zn–2%Y–1.8%Nd (in wt.%) alloy in different physiological saline solutions were investigated and compared. The results indicated that the alloy in the 9 g/L NaCl had the higher corrosion resistance than that containing with CaCl2 and NaHCO3. Moreover, it demonstrated that the corrosion behaviors of the alloy in two types of solutions were all dependent on the pre-immersion time. In the 9 g/L NaCl solution, the corrosion current densities of the alloy decreased firstly with the pre-immersion time being

By means of an electrochemical workstation, hydrogen evolution device, optical microscope (OM) and scanning electron microscope (SEM), the corrosion behaviors of an as-rolled Mg-8%Li alloy with a dual phase structure in 0.9 wt.% NaCl and 3.5 wt.% NaCl solutions have been investigated and compared. The results show that when the immersion time exceeds 8 h, the hydrogen evolution rate of the alloy in the 0.9 wt.% NaCl is 3 times higher than that in the 3.5 wt.% NaCl solution. Moreover, the corrosion behaviors of the alloy are obviously different in the two differently concentrated NaCl solutions. In the 3.5 wt.% NaCl solution, the localized corrosion is much more severe and can occur simultaneously in the interior of both the α-Mg and β-Li matrix phases. However, the localized corrosion in the 0.9 wt.% NaCl solution is obviously weak and mainly occurs at the α-Mg phase.

Conventional ultrafine-grains can generate high strength in Mg alloys, but significant tradeoff of corrosion resistance due to inclusion of a large number of non-equilibrium grain boundaries. Herein, an ultrafine-grain structure consisting of dense ultrafine twins is prepared, yielding a high strength up to 469 MPa and decreasing the corrosion rate by one order of magnitude. Generally, the formation of dense ultrafine twins in Mg alloys is rather difficult, but a carefully designed multi-directional compression treatment effectively stimulates twinning nucleation within twins and refines grain size down to 300 nm after 12-passes compressions. Grain-refinement by low-energy twins not only circumvents the detrimental effects of non-equilibrium grain boundaries on corrosion resistance, but also alters both the morphology and distribution of precipitates. Consequently, micro-galvanic corrosion tendency decreases, and severe localized corrosion is suppressed completely. This technique has a high commercial viability as it can be readily implemented in industrial production., Conventional ultrafine grains can generate high-strength Mg alloys, but non-equilibrium grain boundaries deteriorates their corrosion resistance. Here, the authors present ultrafine grained Mg alloys with dense twins that display high strength and reduced corrosion rate by one order of magnitude.

Through investigating and comparing the mechanical behavior of an as-rolled Mg-3%Al-1%Zn (wt%) alloy before and after annealing treatments, it was revealed that the formation of annealing-tailored bimodal grain structure ensured the 330 °C/4 h samples having a good combination of tensile strength and plasticity. Failure analysis demonstrated that for the as-rolled and 330 °C/1 h samples with fine grain structure, their plastic deformation was mainly attributed to basal slip, high densities of twins were activated. Meanwhile, basal slips occurred in the twinned and un-twinned areas of coarse grains and could pass through twin boundaries. For differently treated samples, cracking preferentially occurred along slip bands, resulting in their transgranular fractures.

Through investigating and comparing microstructure and crystallographic texture of as-extruded Mg-14Li and Mg-14Li-6Zn-1Y (in wt%) alloys, the differences in their mechanical anisotropy were investigated. It revealed that the formation of I-phase (Mg3Zn6Y, icosahedral structure) can effectively refine grain size. Moreover, compared with Mg-14Li alloy, the texture type of Mg-14Li-6Zn-1Y alloy changed slightly, but its texture intensity decreased remarkably. As a result, the stronger texture contributed to the “normal” mechanical anisotropy of Mg-14Li alloy with higher tensile strength and a lower elongation ratio along transverse direction (TD) than those along extrusion direction (ED). However, for Mg-14Li-6Zn-1Y alloy, the zonal distribution of I-phase particles along ED caused “abnormal” mechanical anisotropy, i.e. higher tensile strength and better plasticity along ED.

In this work, the corrosion fatigue behaviors of an as-forged Mg-6.7%Zn-1.3%Y-0.6%Zr alloy before and after solid solution treatment (T4) in 3.5 wt% NaCl solution have been investigated and compared. S-N curves showed that the fatigue strength of as-forged samples corresponding to 5 × 106 cycles was 30 MPa, whereas the fatigue strength of T4 samples was 50 MPa. Fracture observations showed that for the as-forged samples, fatigue cracks mainly initiated at localized corrosion sites on sample surfaces. However, for the T4 samples, the crack initiation was respectively related to the localized corrosion and TB cracking at low and high stress amplitudes.

A SnO2-doped calcium phosphate (Ca-P-Sn) coating was constructed on Mg-1Li-1Ca alloy by a hydrothermal process. The fabricated functional coatings were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). A triple-layered structure, which is composed of Ca3(PO4)2, (Ca, Mg)3(PO4)2, SnO2, and MgHPO4·3H2O, is evident and leads to the formation of Ca10(PO4)6(OH)2 in Hank’s solution. Electrochemical measurements, hydrogen evolution tests and plating counts reveal that the corrosion resistance and antibacterial activity were improved through the coating treatment. The embedded SnO2 nanoparticles enhanced crystallisation of the coating. The formation and degradation mechanisms of the coating were discussed.

In this review paper, the research progress on corrosion behavior of hexagonal close-packed (HCP) singular phase, body cubic-centered (BCC) singular phase and (HCP + BCC) duplex-structured Mg–Li alloys has been summarized and reviewed, and the future trend about the studies on corrosion behavior of Mg–Li-based alloys and possible solving methods for the improvement in corrosion resistance are discussed also.

Exfoliation on as-extruded Mg-1Li-1Ca magnesium alloy was investigated after an immersion in 3.5 wt% NaCl aqueous solution for 90, 120 and 150 days through optical microscope, digital camera, scanning electron microscope, electrochemical workstation, scanning Kalvin probe, X-ray diffraction and Fourier transform infrared spectroscope. The results demonstrated that exfoliation corrosion occurred on extruded Mg-1Li-1Ca alloy due to elongated microstructure parallel to surface, and delamination of lamellar structure resulted from galvanic effect and wedge effect. Skin layer with fine grains exhibited better corrosion resistance, whereas the interior with coarse grains and the intermetallic compound, Mg2Ca particles existing in a fibrous structure, dispersed along grain boundaries and extrusion direction in a line. Furthermore, galvanic effect between Mg2Ca particles and their neighboring α-Mg matrix facilitated dissolution of Mg2Ca particles and α-Mg matrix; wedge effect was caused by formation of corrosion products. Exfoliation corrosion of extruded Mg-Li-Ca alloys might be a synergic effect of pitting corrosion, filiform corrosion, intergranular corrosion and stress corrosion. Finally, exfoliation corrosion mechanism was proposed.

In the present study, the effect of Zn content on the microstructure and deformation behavior of the as-cast Mg–Zn–Y–Nd alloy has been investigated. The results showed that as Zn content increased, the volume fraction of secondary phases increased. Moreover, the phase transformation from W-phase to W-phase and I-phase occurred. In the as-cast state, W-phase exists as eutectic and large block form. When Zn content increases to 6 and 8% (wt%), small I-phase could precipitate around W-phase particles. Additionally, the effect of Zn content on the tensile properties and deformation behavior varies with the testing temperature. At room temperature, the tensile strength increases with Zn content, whereas the elongation increases initially and then decreases. At 250 °C, as Zn content increases, the tensile strength decreases initially and then increases slightly, whereas the elongation decreases. At 350 °C, the elongation increases with Zn content, whereas the tensile strength decreases initially and then increases slightly.

Mg–Zn–Y–Nd alloy was prepared by casting and hot extrusion. The microstructure and mechanical properties of OM, SEM, XRD, TEM, and tensile tests were investigated with casting and hot extruded alloys. The results demonstrate that in a casting Mg–Y–Nd–Zr alloy, the α-Mg matrix is separated into the cell structure by a discontinuously distributed coarse Mg24Y5/α-Mg eutectic structure and fine Mg12Nd particles. TEM analysis shows that the Mg12Nd and Mg24Y5 phases have the orientation of $$ {\left[001\right]}_{{\mathrm{Mg}}_{12}\mathrm{Nd}}//{\left[0221\right]}_{\upalpha -\mathrm{Mg}} $$ , and $$ {\left[111\right]}_{{\mathrm{Mg}}_{24}{\mathrm{Y}}_5}//{\left[0001\right]}_{\upalpha -\mathrm{Mg}} $$ and $$ {\left(10\overline{1}\right)}_{{\mathrm{Mg}}_{24}{\mathrm{Y}}_5}//{\left(10\overline{1}0\right)}_{\upalpha -\mathrm{Mg}} $$ , respectively. The hot extrusion separates the Mg24Y5/α-Mg eutectic structure into fragments and aligns fragmentary Mg24Y5 particles along the extrusion direction. The interaction of hot extrusion and strengthening particles refines the α-Mg matrix greatly. Moreover, large strains result in the stacking faults of the matrix. As compared to the casting alloy, the hot-extruded one exhibits high yield strength, ultimate tensile strength, and elongation, which should be ascribed to the grain fineness, optimum distribution of strengthening particles and multiple substructures.

High corrosion kinetics and localised corrosion progress are the primary concerns arising from the clinical implementation of magnesium (Mg) based implantable devices. In this study, a binary Mg-lithium (Li) alloy consisting a record high Li content of 14% (in weight) was employed as model material aiming to yield homogenous and slow corrosion behaviour in a simulated body fluid, i.e. minimum essential medium (MEM), in comparison to that of generic Mg alloy AZ31 and biocompatible Mg-0.5Zn-0.5Ca counterparts. Scanning electron microscopy examination reveals single-phase microstructural characteristics of Mg-14Li (β-Li), whilst the presence of insoluble phases, cathodic to α-Mg matrix, in AZ31 and Mg-0.5Zn-0.5Ca. Though slight differences exist in the corrosion kinetics of all the specimens over a short-term time scale (no longer than 60 min), as indicated by potentiodynamic polarisation and electrochemical impedance spectroscopy, profound variations are apparent in terms of immersion tests, i.e. mass loss and hydrogen evolution measurements (up to 7 days). Cross-sectional micrographs unveil severe pitting corrosion in AZ31 and Mg-0.5Zn-0.5Ca, but not the case for Mg-14Li. X-ray diffraction patterns and X-ray photoelectron spectroscopy confirm that a compact film (25 μm in thickness) consisting of lithium carbonate (Li2CO3) and calcium hydroxide was generated on the surface of Mg-14Li in MEM, which contributes greatly to its low corrosion rate. It is proposed therefore that the single-phase structure and formation of protective and defect-free Li2CO3 film give rise to the controlled and homogenous corrosion behaviour of Mg-14Li in MEM, providing new insights for the exploration of biodegradable Mg materials., Graphical abstract Image 1, Highlights • Mg-14Li (wt.%) binary alloy was studied as a potential degradable material. • Single phase of β-Li existed in Mg-14Li. • Homogenous corrosion morphology was observed in Mg-14Li in MEM. • Corrosion rate of Mg-14Li is lower than that of Mg-0.5Za-0.5Ca and AZ31.

The aim of the present work is to elucidate the interaction between dislocation dipoles decorated by trace addition of solute atoms in the model material of as-annealed coarse-grained Ti-3Al (weight percent, wt%) alloys. The morphology, activity, structure, strain field, and interaction of dislocation cores were systematically characterized. Theoretical predictions of equilibrium distribution of edge dislocation dipoles presenting an angle of 45° relative to each other were experimentally demonstrated. More importantly, we observed for the first time the 45°-rotating of strain fields around the dislocation core along a clockwise direction for stress equilibrium which is driven by the dislocation-dislocation interactions. Keywords: Titanium alloys, Homogenization/solutionization, Strengthening, Crystal defects, High-resolution electron microscopy (HREM)

In this work, the corrosion behavior of two differently oriented surfaces of an as-extruded Mg–3%Al–1%Zn (AZ31) bar in a simulated body fluid of phosphate buffer saline (PBS) medium was investigated and compared, and the effect of crystallographic texture on corrosion resistance of the alloy was deeply described. The results showed that at the early stage of immersion, a layer of compact and flat film formed easily on surfaces of both oriented samples. With prolonged immersion time, the degradation of formed corrosive films started and their severity was quite sensitive to the composed crystallographic planes of sample surfaces. For the surface containing highly concentrated orientation of {10–10} and {11–20} prism planes, the degradation of formed corrosive film was quite slight and only occurred at some particular sites even after immersion for 48 h. Thus, the film could keep good corrosive protection to the underneath substrate. However, for the surface containing {0002} basal planes, {10–10} and {11–20} prism planes, the degradation of corrosive production film occurred widely, resulting in further decrease in the corrosion resistance of immersed samples.

Magnesium alloys containing long period stacking ordered (LPSO) phase have been received a great deal of attention in the last decade owing to their excellent comprehensive properties of mechanical strength and corrosion resistance. In this paper, some fundamental aspects of LPSO containing Mg alloys have been reviewed, including: (1) microstructural characterization, formation conditions and the associated phase transformation of LPSO phases in Mg alloys; (2) deformation mechanism of LPSO phases and their influence on the deformation mechanism of the Mg matrix; (3) effect of LPSO structure on the mechanical performance such as tensile strength, creep resistance, fracture toughness and fatigue strength; (4) corrosion behavior of LPSO containing Mg alloys and their possible applications as the biomaterials. Moreover, some remaining unsolved issues of the LPSO containing Mg alloys and the future target about how to further improve their service properties have been also described.

The galvanic corrosion of the coupled Al2Cu and Al as well as Al3Fe and Al in 0.005 M NaCl in and without the presence of cerium cinnamate was studied. Scanning vibrating electrode technique and scanning ion-selective electrode technique were used to measure the distribution of current densities and pH. The results indicate that cerium cinnamate played inhibiting effect on the coupling corrosion of Al2Cu–Al and Al3Fe–Al. For Al2Cu–Al, the inhibition was mainly influenced by the production of cerium oxide/hydroxide on Al2Cu surface. However, for Al3Fe–Al, the inhibition was mainly influenced by the adsorbed cerium cinnamate on the Al3Fe surface.

In the super-long life regime, the fatigue behavior of as-extruded Mg-6wt%Zn-xY-0.8wt%Zr Mg alloys with Y content of 0, 1, 2 and 3 wt% have been investigated, respectively. The result indicated that for all measured S-N curves, a plateau existed in the regime of 5×106-108 cyc, and then the fatigue strength gradually decreased between 108 and 109 cycles. Therefore, only fatigue strength corresponding to 109 cycles can be determined. Compared with other alloys, the alloy with Y content of 2 wt% has the highest fatigue strength and its value is 105 MPa. SEM observations to fracture surfaces revealed that for all alloys, the fatigue crack mostly initiated at the surface or subsurface of samples failed within 106-109 cycles. Further observation indicated that the crack initiation was related with activated slip bands instead of phase particles and activated twins. Based on the measured results and Murakami equation, it demonstrates that the fatigue strength of alloys is more dependent on the hardness values.

By investigating the corrosion behavior of two oriented surfaces for an as-extruded Mg–3% Al–1% Zn alloy bar, we demonstrate that the surface with a high concentration of {1 0 −1 0} and {1 1 −2 0} oriented prism planes is more electrochemically stable and corrosion resistant than the surface with a high concentration of {0 0 0 2}, {1 0 −1 0} and {1 1 −2 0} oriented planes. Further analysis confirms that the corrosion performance is mainly determined by the galvanic corrosion between grains and between twinned and untwinned areas due to their different crystallographic orientations.

The in situ formation of I-phase improves the corrosion resistance of the duplex structured Mg–6% Li alloy. Corrosion attack on the surfaces of Mg–6% Li alloy is very inhomogeneous and severe pits and filiform corrosion clearly occur on the surfaces after immersion in 0.1 M NaCl solution. For the I-phase-containing Mg–6% Li–6% Zn–1.2% Y alloy, the corrosion attack is homogeneous and no severe pits and filiform corrosion can be observed.

Through investigating tensile properties of the duplex Mg–Li alloys with and without I-phase (Mg 3 Zn 6 Y, icosahedral quasicrystal structure) tested both at ambient and elevated temperatures, it demonstrates that I-phase can not only improve the tensile strength at room temperature, but also be beneficial for the mechanical improvement at elevated temperatures. Based on the microstructural observations of the sample surfaces before and after tests, the I-phase particles can effectively suppress the plastic flow of β phase at 220 °C, resulting in the I-phase reinforced duplex Mg–Li alloy having higher mechanical properties at elevated temperatures.

An investigation of the fatigue behaviour of as-extruded pure Mg demonstrates that the fatigue strength corresponding to 10 6 cycles is 35 MPa. An analysis of the microstructural evolution during the fatigue process reveals that the activated twins are mainly { 1 0 1 ¯ 2 } type and their density increases with the number of fatigue cycles. The twin boundary (TB) cracking due to the interactions between TBs and slip bands is the key reason for the crack initiation of fatigue samples.

Both the solid solution and precipitation are mainly strengthening mechanism for the magnesium-based alloys. A great number of alloying elements can be dissolved into the Mg matrix to form the solutes and precipitates. Moreover, the type of precipitates varies with different alloying elements and heat treatments, which makes it quite difficult to understand the formation mechanism of the precipitates in Mg-based alloys in depth. Thus, it is very hard to give a systematical regularity in precipitation process for the Mg-based alloys. This review is mainly focused on the formation and microstructural evolution of the precipitates, as a hot topic for the past few years, including Guinier-Preston Zones, quasicrystals and long-period stacking ordered phases formed in a number of Mg-TM-RE alloy systems, where TM = Al, Zn, Zr and RE = Y, Gd, Hd, Ce and La.

Icosahedral phase (I-phase) is a relatively excellent strengthening phase in Mg alloys. Depending on their volume fraction, the yield strength of Mg-Zn-Y-Zr alloys can vary from 150 to 450 MPa at room temperature. Recently, the formation of I-phase has been considered as one of the most effective methods for developing high strength lightweight Mg alloys for automotive and aerospace applications. In this review article, a series of research work about I-phase containing Mg alloys have been systematically investigated including I-phase formation mechanism and their effects on mechanical properties of Mg alloys. Particular emphases have been given to: (1) Structure of I-phase and its orientation relationship with the alpha-Mg matrix. (2) Influence of alloying elements and solidification conditions on I-phase formation. (3) Effects of I-phase on microstructural evolution and mechanical improvement of Mg-Zn-Y-(Zr) alloys. Moreover, the applications of I-phase for the mechanical improvement of other Mg alloys such as AZ91 and super-lightweight Mg-Li alloys are also reviewed. (c) 2012 Chinese Materials Research Society. Production and hosting by Elsevier Ltd. All rights reserved.

The main coarse constituents in as-rolled aluminium alloy AA7150 were determined by electron backscattered diffraction (EBSD) phase identification to be MgZn2 (eta), Al2CuMg (S), Mg2Si and Al7Cu2Fe. Based on differential scanning calorimetry (DSC) analysis and microstructural observations of single-step solution treated samples, the melting onset temperatures of (alpha + eta) and (alpha + eta + S) are 478(-2)(+1)degrees C and 478(-2)(+1)degrees C, respectively. However, Mg2Si and Al7Cu2Fe particles are very stable and insoluble during typical solution treatments. When the re-melting of (alpha + eta) or (alpha + eta + S) occurs, holes will be left at the original sites of these particles. Since the size of the S-phase particles is larger than that of the eta particles, the holes caused by the re-melting of (alpha + eta + S) are relatively larger. When samples were rapidly heated up to 515 degrees C and solution treated at this temperature, network eutectics composed of MgZn2, Al2CuMg, Mg2Si and alpha-Al were formed at the grain boundaries. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.

In this work, improved solution treatments and their effects on the microstructure and mechanical properties of as-rolled Al–6.22Zn–2.11Mg–2.39Cu alloy have been investigated. By exploring the dissolution process of soluble constituent particles and monitoring changes in the grain structure, a solution treatment sequence with a good balance between a minimum volume fraction of remaining constituents and a partially recrystallised grain structure has been obtained. The results indicate that MgZn2 (η) particles can be completely dissolved after holding at 475 °C for only 5 min, whereas the dissolution of Al2CuMg (S) particles is relatively difficult and their complete dissolution requires a stepped solution treatment. By conducting a final solution treatment step at an increased temperature of 495 °C, all the S-phase particles can be dissolved, whereas the recrystallised fraction can still be controlled to be less than 50%. After retrogression and re-ageing (RRA), samples subjected to a final-stage solution treatment at 495 °C have a relatively higher volume fraction of η′ and η precipitates and can therefore have better mechanical properties.

The effects of processing condition variations in the retrogression and re-ageing (RRA) treatment of AA7150 have been investigated. The results reveal that the corrosion resistance and strength of RRA-treated samples are sensitive to the pre-ageing temperature and retrogression heating rate. With an intermediate pre-ageing temperature of 60 °C and a slow retrogression heating rate of 5 °C/min, RRA-treated samples can be tailored towards a good combination of both corrosion resistance and strength, owing to a smaller potential difference between grain-boundary precipitates and the matrix, and a larger volume fraction of strengthening precipitates in the matrix.

The thermal stability of constituent particles in both as-cast and homogenised alloy AA7150 has been investigated by means of scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The results indicate that the as-cast eutectic starts to melt at 474°C and that the melting onset temperatures of the η- and S-phase particles after homogenisation are 474°C and 484°C, respectively. Two types of homogenisation treatments were used to dissolve eutectic particles: a two-step homogenisation treatment (40 h at 465°C + 4 h at 480°C) can completely dissolve the η- and S-based constituents, whereas a significant amount of S-phase still exists after a single-step homogenisation treatment of 40 h at 465°C. After utilising the two-step homogenisation treatment to dissolve all the η- and S-based constituents without overheating, samples were cooled at different controlled cooling rates and analysed by SEM and DSC to understand the effect of cooling rate on the type, size, distribution and density of precipitates formed during the cooling process. Slow cooling was found to result in the precipitation of coarse η-phase particles, with both the amounts and sizes of these η-phase particles increasing with decreasing cooling rate.

Studies for Al-alloys normally employ specimens subject to a fixed solution heat treatment (SHT), and hence the specific role of solutionising is overlooked. It is revealed that SHT has a major role in corrosion of AA7150 as judged by electrochemical, microscopic and profilometry studies. SHT dictates the constituent particle type and population remaining in the alloy for subsequent processing, and is thus not only a principal factor in corrosion, but a factor that can be engineered in developing more damage tolerant AA7150. Effect of an optimised SHT minimising the residual MgZn 2 and Al 2 MgCu content on corrosion resistance is demonstrated.

In this work, the influence of multi-step solution (MSS) treatments on the constituent particle dissolution, overheating and associated quench cracking behaviour in room temperature water-quenched 7150 Al alloy has been investigated. For comparison, the microstructure and quench cracking behaviour of single step solution treated samples water-quenched from 505°C were also investigated. Based on optical microscopy of differently quenched samples, the quench cracking mode and the influence of overheating of constituents on the quench cracking behaviour have been demonstrated. The results reveal that the constituent particles can be effectively dissolved in the MSS-505°C samples. When the quench temperature of MSS-505°C samples is equal to or higher than 485°C , macro quench cracks can be clearly observed. Moreover, the density and length of the quench cracks increase with increasing quench temperature. Etched microstructures indicate that the quench crack propagation mode is intergranular. However, for samples directly heated to 505°C , typical overheating can be observed at the triple junctions and these regions preferentially act as crack propagation routes.

This work mainly investigated the microstructure and mechanical properties of Mg-Zn-Y-Zr alloys with Zn/Y ratios of 5 and 10. An X-ray diffraction (XRD) analysis indicated that the two alloys were mainly composed of an icosahedral phase (I-phase) and α-Mg matrix. For the alloy with a Zn/Y ratio of 10, however, the diffraction peaks of the I-phase were stronger. Microstructure observation showed that the I-phase preferentially existed in the form of I-phase/α-Mg matrix interdendritic eutectic pockets at grain boundaries. Moreover, when the Zn/Y ratio was increased 2 times, the volume fraction of the I-phase in the α-Mg matrix increased 1.5 times and a tiny Mg7Zn3 phase formed. Energy-dispersive spectroscopy (EDS) mapping and electron probe microanalysis (EPMA) results suggested that the chemical composition of the I-phase was not a constant value. Computer-aided cooling curve analysis (CA-CCA) indicated that, for the alloy with a Zn/Y ratio of 5, formation of the I-phase relied on the W-phase transformation and the eutectic reaction of the residual melt. However, the I-phase formation for the alloy with a Zn/Y ratio of 10 depended on the eutectic reaction of the melt. Tensile tests indicated that the mechanical properties of the two as-cast alloys were poor. After hot extrusion processing, the mechanical properties of the alloy with a Zn/Y ratio of 10 were noticeably increased. The ultimate tensile strength (UTS) and elongation to failure reached 320 MPa and 13 pct, respectively.