Strength and ductility with {10͞11} - {10͞12} double twinning in a magnesium alloy

Responsive Protection of Magnesium Alloys From Multicorrosive Media by Constructing Nanofluidic Channels in Self-Repairing Coatings

Low-density magnesium (Mg) alloys are excellent engineering materials, and can significantly reduce energy consumption by replacing existing steel and aluminum materials. However, Mg species are susceptible to corrosion, especially in harsh environments (high-temperature or acidic), severely limiting the range of practical applications. Here, 2D covalent organic framework (COF) is synthesized with pore diameters ranging from 1.5 to 2.9 nm to obtain ultrafast nanofluidic channels. Loaded with silver (Ag+) ions, 2-mercaptobenzimidazole (2-MB) inhibitors are immobilized in the COF channels through the silver bridges. Based on the strong metal-complexing capability, Ag+ ions precipitated with various corrosive media (Cl-, Br-, I-, SO3 2-, S2-, S2O3 2- SO4 2-, CO3 2-, PO4 3-); meanwhile, the 2-MB inhibitors are rapidly released through the nanofluidic channels, forming a passivation film as a corrosion barrier to protect the Mg substrate. After integration with commercial polyethersulfone (PES), the COF-based coating exhibits high repairing capability achieving 100% damage restoration within 7 h, outperforming all existing coatings of Mg alloys. Notably, the coating shows almost complete protection of Mg alloys after being treated in respective 473 K, acidic (pH ≈4.0), and alkaline (pH ≈10.0) environments.

Twin Boundary Superstructures Assembled by Periodic Segregation of Solute Atoms

Twinning is a common deformation mechanism in metals, and twin boundary (TB) segregation of impurities/solutes plays an important role in the performances of alloys such as thermostability, mobility, and even strengthening. The occurrence of such segregation phenomena is generally believed as a one-layer coverage of solutes alternately distributed at extension/compression sites, in an orderly, continuous manner. However, in the Mn-free and Mn-containing Mg-Nd model systems, we reported unexpected three- and five-layered discontinuous segregation patterns of the coherent {101̅1} TBs, and not all the extension sites occupied by solutes larger in size than Mg, and even some larger sized solutes taking the compression sites. Nd/Mn solutes selectively segregate at substitutional sites and thus to generate two new types of ordered two-dimensional TB superstructures or complexions. These findings refresh the understanding of solute segregation in the perfect coherent TBs and provide a meaningful theoretical guidance for designing materials via targeted TB segregation.

Twin boundary migration mechanisms in quasi-statically compressed and plate-impacted Mg single crystals

Twinning is a prominent deformation mode that accommodates plasticity in many materials. This study elucidates the role of deformation rate on the atomic-scale mechanisms that govern twin boundary migration. Examination of Mg single crystals deformed under quasi-static compression was compared with crystals deformed via plate impact. Evidence of two mechanisms was uncovered. Atomic-level observations using high-resolution transmission electron microscopy revealed that twin boundaries in the <a>-axis quasi-statically compressed single crystals are relatively smooth. At these modest stresses and rates, the twin boundaries were found to migrate predominantly via shear (i.e., disconnection nucleation and propagation). By contrast, in the plate-impacted crystals, which are subjected to higher stresses and rates, twin boundary migration was facilitated by local atomic shuffling and rearrangement, resulting in rumpled twin boundaries. This rate dependency also leads to marked variations in twin variant, size, and number density in Mg. Analogous effects are anticipated in other hexagonal closed-packed crystals.

Unusual solute segregation phenomenon in coherent twin boundaries

Interface segregation of solute atoms has a profound effect on properties of engineering alloys. The occurrence of solute segregation in coherent twin boundaries (CTBs) in Mg alloys is commonly considered to be induced by atomic size effect where solute atoms larger than Mg take extension sites and those smaller ones take compression sites in CTBs. Here we report an unusual solute segregation phenomenon in a group of Mg alloys-solute atoms larger than Mg unexpectedly segregate to compression sites of {10[Formula: see text]1} fully coherent twin boundary and do not segregate to the extension or compression site of {10[Formula: see text]2} fully coherent twin boundary. We propose that such segregation is dominated by chemical bonding (coordination and solute electronic configuration) rather than elastic strain minimization. We further demonstrate that the chemical bonding factor can also predict the solute segregation phenomena reported previously. Our findings advance the atomic-level understanding of the role of electronic structure in solute segregation in fully coherent twin boundaries, and more broadly grain boundaries, in Mg alloys. They are likely to provide insights into interface boundaries in other metals and alloys of different structures.

Embracing the Chaos: Alloying Adds Stochasticity to Twin Embryo Growth

High-throughput atomistic simulations reveal the unique effect of solute atoms on twin variant selection in Mg-Al alloys. Twin embryo growth first undergoes a stochastic incubation stage when embryos choose which twin variant to adopt, and then a deterministic growth stage when embryos expand without changing the selected twin variant. An increase in Al composition promotes the stochastic incubation behavior on the atomic level due to nucleation and pinning of interfacial disconnections. At compositions above a critical value, disconnection pinning results in multiple twin variant selection.

Microstructure and Texture Evolutions During Deep Drawing of Mg-Al-Mn Sheets at Elevated Temperatures

Texture and microstructure evolution of ingot and twin-roll casted Mg–Al–Mn magnesium sheets were examined during deep drawing at elevated temperatures. The twin-roll casted sheets possessed smaller grain sizes and weaker basal intensity levels than the ingot-casted sheets. The strength and elongation at room temperature for the twin-roll casted sheets were greater than those of the ingot-casted sheets. At elevated temperatures, the ingot-casted sheets showed better elongation than the twin-roll casted sheets. Different size and density of precipitates were examined using transmission electron microscopy (TEM) for both ingot-casted and twin-roll-casted sheets. The deep drawing process was also carried out at various working temperatures and deformation rates, 225 °C to 350 °C and 30 mm/min to 50 mm/min, respectively. The middle wall part of cups were mainly tensile deformation, and the lower bent regions of drawn cups were most thinned region. Overall, the ingot-casted sheets revealed better deep drawability than the twin-roll casted sheets. Microstructure and texture evolution of the top, middle and lower parts of drawn cups were investigated using electron backscatter diffraction. Increased deformation rate is important to activate tensile twins both near the bent and flange areas. Ingot casted sheets revealed more tensile twins than twin-roll casted sheets. Increased working temperature is important to activate non-basal slips and produce the DRXed grain structure in the flange. Dynamic recrystallization were frequently found in the top flanges of the cups. Both tensile twins and non-basal slips contributed to occurrence of the dynamic recrystallization in the flange.

Microstructural Study of a Mg-Zn-Zr Alloy Hot Compressed at a High Strain Rate

Understanding the correlation of plasticity with deformation and dynamic recrystallization (DRX) behaviors, in magnesium (Mg) alloys deformed under high-strain-rate conditions, is increasingly important for wrought Mg processing. In the present study, a ZK30 (Mg-2.61%Zn-0.66%Zr by weight percent (wt.%)) alloy in the as-forged state was hot compressed to various strain levels at a temperature of 350 °C and a strain rate of 10 s⁻¹. Heterogeneous deformation and dynamic recrystallization (DRX) behaviors of the complicated microstructures in the deformed samples were analyzed via a grain-partitioning approach based on intra-grain misorientation analysis from electron back-scattered diffraction (EBSD). The ZK30 alloy showed excellent formability, remaining intact at a true strain of −1.11. Continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) via grain boundary corrugation/bulging are the dominant mechanisms for the relaxation of strain energy during hot compression. Initial Zr-rich coarse grains undertook a significant portion of the plastic strain as the compression progressed, reflected by the increased misorientations within their interior and marked change in their aspect ratios. The results indicate that the excellent plasticity of the as-forged ZK30 alloy can be attributed to the operative CDRX mechanisms and the reduced deformation anisotropy of Zr-rich coarse grains containing Zn–Zr nano–precipitates.

Capture Deformation Twinning in Mg during Shock Compression with Ultrafast Synchrotron X-Ray Diffraction

Deformation twinning plays a vital role in accommodating plastic deformation of hexagonal-close-packed (hcp) metals, but its mechanisms are still unsettled under high strain rate shock compression. Here we investigate deformation twinning in shock-compressed Mg as a typical hcp metal with in situ, ultrafast synchrotron x-ray diffraction. Extension twinning occurs upon shock compression along ⟨112[over ¯]0⟩ and ⟨101[over ¯]0⟩, but only upon release for loading along ⟨0001⟩. Such deformation mechanisms are a result of the polarity of deformation twinning, which depends on directionality and relative magnitude of resolved shear stress and may be common for Mg and its alloys in a wide range of strain rates.

Direct observation and impact of co-segregated atoms in magnesium having multiple alloying elements

Modern engineering alloys contain multiple alloying elements, but their direct observation when segregated at the atomic scale is challenging because segregation is susceptible to electron beam damage. This is very severe for magnesium alloys, especially when solute atoms segregate to form single atomic columns. Here we show that we can image segregation in magnesium alloys with atomic-resolution X-ray dispersive spectroscopy at a much lower electron voltage. We report a co-segregation pattern at twin boundaries in a magnesium alloy with both larger and smaller solutes forming alternating columns that fully occupy the twin boundary, in contrast to previous observations of half occupancy where mixed-solute columns alternate with magnesium. We further show that the solute co-segregation affects the twin migration mechanism and increases the twin boundary pinning. Our work demonstrates that the atomic-scale analysis of the structure and chemistry of solute segregation in metallic alloys with complex compositions is now possible.

Commercial alloys contain trace solutes that segregate at grain boundaries but have been difficult to directly image due to electron beam damage. Here, the authors use atomic-resolution energy dispersive X-ray spectroscopy at lower electron voltage to image segregation at magnesium alloy twin boundaries.

Deformation twinning mechanism in hexagonal-close-packed crystals

The atomic structure of {10 [Formula: see text] 2} twin boundary (TB) from a deformed Mg-3Al-1Zn (AZ31) magnesium alloy was examined by using high-resolution transmission electron microscopy (HRTEM). By comparing the lattice structure of TB with the previously established model, a kind of special atomic combinations, here named primitive cells (PCs), were discovered at the TB. The PCs reorientation induced mechanism of twinning in hexagonal-close-packed (HCP) crystals was hence verificated. Meanwhile, the relationship between the misorientation of adjacent layers of PCs and the width of TB was discussed. The verification of the mechanism clarifies the twinning mechanism in HCP crystals and opens up opportunities for further researches.

Numerical Investigation of Secondary Deformation Mechanisms on Plastic Deformation of AZ31 Magnesium Alloy Using Viscoplastic Self-Consistent Model

Uniaxial tension and compression of AZ31 magnesium alloy were numerically investigated via the viscoplastic self-consistent (VPSC) model to shed a light on the effect of secondary deformation mechanisms (prismatic <a> slip, pyramidal <c+a> slip, and { 10 1 ¯ 1 } contraction twinning) during plastic deformation. The method adopted in the present study used different combinations of deformation mechanisms in the VPSC modeling. In terms of the pyramidal <c+a> slip, it served as the first candidate for sustaining the extra plastic strain during the plastic deformation. The improvement of activity in the pyramidal <c+a> slip contributed to the increase in the mechanical response and the splitting of pole densities in { 0002 } pole figure during uniaxial tension. As for the prismatic <a> slip, its increasing activity was not only conducive to the improvement of flow stress in mechanical response, but also responsible for the splitting of pole densities in { 0002 } pole figure during uniaxial compression. With respect to the { 10 1 ¯ 1 } contraction twinning, it had a negligible influence on the plastic deformation of AZ31 magnesium alloy in terms of the mechanical response as well as the slip and the twinning activities. However, it is better to include the { 10 1 ¯ 1 } contraction twinning in the VPSC modeling to more accurately predict the texture evolution.

Nanoscale coherent interface strengthening of Mg alloys

Structural materials with higher strength and ductility could reduce weight and improve energy efficiency from the ecological and economical viewpoints. However, most classical strengthening strategies, such as refining grain size and forming secondary particles, can effectively hinder dislocation motion but remarkably decrease ductility. In this research, a nanoscale contraction twins-stacking faults (CTWSFs) hierarchical structure was achieved in a model sample of magnesium-lithium (Mg-Li) alloy using an industrial ultrahigh pressure technique. The specific yield strength and elongation of the treated Mg-Li alloy were ∼179 kN m kg-1 and ∼25% at room temperature, respectively. Both of these are the highest values reported so far, even compared to commercial aluminum alloys and steel. Depending on molecular dynamics simulations, it was demonstrated that the basal-plane stacking faults (SFs) attribute to the nucleation of contraction twins (CTWs), but prevented the broadening of the CTWs. The results were confirmed using ex situ transmission electron microscopy. The models also verified that the unique structure of nanoscale coherent boundaries-basal-plane SFs were effective in preventing dislocation motion in a single Mg crystal, which was analogous to twin boundaries. Finding this new CTWSFs structure might provide an alternative perspective for designing more innovative hexagonal close packed (hcp)-type structural materials with superior mechanical properties.

Characterization of Deformation Mechanisms in Mg Alloys by Advanced Acoustic Emission Methods

Adaptive sequential k-means (ASK) analysis of acoustic emission (AE) data was used to analyze the sources of AE during compression of three AZ31 magnesium samples with different initial texture. The results were compared to the classical hit-based approach. Observation of the deformed microstructure shows that the ASK analysis can distinguish very well between the signal originating in deformation twinning and dislocation slip. Moreover, together with microstructural analysis, the ASK algorithm revealed another source of AE for one of the samples, which was shown to be the double twinning.

Composition Optimization and Mechanical Properties of Mg-Al-Sn-Mn Alloys by Orthogonal Design

Nine kinds of rare-earth free Mg-Al-Sn-Mn magnesium alloys were designed by orthogonal method. Scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD), electron backscatter diffraction (EBSD), and tension tests were carried out to investigate the microstructures and mechanical properties. As-cast Mg-Al-Sn-Mn alloys have an obvious dendritic structure that is composed of α-Mg, Mg₁₇Al₁₂, and Mg₂Sn phases. After hot extrusion, the cast dendrite structure changed into a recrystallized equiaxed grain. Mg₁₇Al₁₂ dissolved completely into a matrix, and only α-Mg, Mg₂Sn, and a few Al-Mn phases could be observed. The influence of three alloy elements (Al, Sn, and Mn) on grain size, texture intensity, ultimate tensile strength (UTS), tensile yield strength (TYS), and elongation (EL) were studied by extreme difference analysis method. The content of Mn had the greatest influence on grain size. The AT61-0.2Mn and AT73-0.2Mn alloys had the smallest grain, reaching 6.8 μm. The content of Al had the greatest influence on the strength; therefore, the AT73-0.2Mn alloy had the highest UTS, 322 MPa, and TYS, 202 MPa. The content of Sn had the greatest influence on elongation. The AT52-0.4Mn alloy had the highest elongation in theory, but it was not included in the nine designed kinds of alloys yet. AT52-0.2Mn alloy had the highest elongation in the nine alloys (28.4%).

Achieving High Strength and Ductility in Magnesium Alloys via Densely Hierarchical Double Contraction Nanotwins

Light-weight magnesium alloys with high strength are especially desirable for the applications in transportation, aerospace, electronic components, and implants owing to their high stiffness, abundant raw materials, and environmental friendliness. Unfortunately, conventional strengthening methods mainly involve the formation of internal defects, in which particles and grain boundaries prohibit dislocation motion as well as compromise ductility invariably. Herein, we report a novel strategy for simultaneously achieving high specific yield strength (∼160 kN m kg^-1) and good elongation (∼23.6%) in a duplex magnesium alloy containing 8 wt % lithium at room temperature, based on the introduction of densely hierarchical {101̅1}-{101̅1} double contraction nanotwins (DCTWs) and full-coherent hexagonal close-packed (hcp) particles in twin boundaries by ultrahigh pressure technique. These hierarchical nanoscaled DCTWs with stable interface characteristics not only bestow a large fraction of twin interface but also form interlaced continuous grids, hindering possible dislocation motions. Meanwhile, orderly aggregated particles offer supplemental pinning effect for overcoming latent softening roles of twin interface movement and detwinning process. The processes lead to a concomitant but unusual situation where double contraction twinning strengthens rather than weakens magnesium alloys. Those cutting-edge results provide underlying insights toward designing alternative and more innovative hcp-type structural materials with superior mechanical properties.

A rare-earth free magnesium alloy with improved intrinsic ductility

Metals are the backbone of manufacturing owing to their strength and formability. Compared to polymers they have high mass density. There is, however, one exception: magnesium. It has a density of only 1.7 g/cm³, making it the lightest structural material, 4.5 times lighter than steels, 1.7 times lighter than aluminum, and even slightly lighter than carbon fibers. Yet, the widespread use of magnesium is hampered by its intrinsic brittleness. While other metallic alloys have multiple dislocation slip systems, enabling their well-known ductility, the hexagonal lattice of magnesium offers insufficient modes of deformation, rendering it intrinsically brittle. We have developed a quantum-mechanically derived treasure map which screens solid solution combinations with electronic bonding, structure and volume descriptors for similarity to the ductile magnesium-rare earth alloys. Using this insight we synthesized a surprisingly simple, compositionally lean, low-cost and industry-compatible new alloy which is over 4 times more ductile and 40% stronger than pure magnesium. The alloy contains 1 wt.% aluminum and 0.1 wt.% calcium, two inexpensive elements which are compatible with downstream recycling constraints.

Strong, Ductile, and Thermally Stable bcc-Mg Nanolaminates

Magnesium has attracted attention worldwide because it is the lightest structural metal. However, a high strength-to-weight ratio remains its only attribute, since an intrinsic lack of strength, ductility and low melting temperature severely restricts practical applications of Mg. Through interface strains, the crystal structure of Mg can be transformed and stabilized from a simple hexagonal (hexagonal close packed hcp) to body center cubic (bcc) crystal structure at ambient pressures. We demonstrate that when introduced into a nanocomposite bcc Mg is far more ductile, 50% stronger, and retains its strength after extended exposure to 200 C, which is 0.5 times its homologous temperature. These findings reveal an alternative solution to obtaining lightweight metals critically needed for future energy efficiency and fuel savings.

Novel Bio-functional Magnesium Coating on Porous Ti6Al4V Orthopaedic Implants: In vitro and In vivo Study

Titanium and its alloys with various porous structures are one of the most important metals used in orthopaedic implants due to favourable properties as replacement for hard tissues. However, surface modification is critical to improve the osteointegration of titanium and its alloys. In this study, a bioactive magnesium coating was successfully fabricated on porous Ti6Al4V by means of arc ion plating, which was proved with fine grain size and high film/substrate adhesion. The surface composition and morphology were characterized by X-ray diffraction and SEM equipped with energy dispersive spectroscopy. Furthermore, the in vitro study of cytotoxicity and proliferation of MC3T3-E1 cells showed that magnesium coated porous Ti6Al4V had suitable degradation and biocompatibility. Moreover, the in vivo studies including fluorescent labelling, micro-computed tomography analysis scan and Van-Gieson staining of histological sections indicated that magnesium coated porous Ti6Al4V could significantly promote bone regeneration in rabbit femoral condylar defects after implantation for 4 and 8 weeks, and has better osteogenesis and osteointegration than the bare porous Ti6Al4V. Therefore, it is expected that this bioactive magnesium coating on porous Ti6Al4V scaffolds with improved osteointegration and osteogenesis functions can be used for orthopedic applications.

Mechanical properties, in vitro corrosion and biocompatibility of newly developed biodegradable Mg-Zr-Sr-Ho alloys for biomedical applications

Our previous studies have demonstrated that Mg-Zr-Sr alloys can be anticipated as excellent biodegradable implant materials for load-bearing applications. In general, rare earth elements (REEs) are widely used in magnesium (Mg) alloys with the aim of enhancing the mechanical properties of Mg-based alloys. In this study, the REE holmium (Ho) was added to an Mg-1Zr-2Sr alloy at different concentrations of Mg1Zr2SrxHo alloys (x = 0, 1, 3, 5 wt. %) and the microstructure, mechanical properties, degradation behaviour and biocompatibility of the alloys were systematically investigated. The results indicate that the addition of Ho to Mg1Zr2Sr led to the formation of the intermetallic phases MgHo3, Mg2Ho and Mg17Sr2 which resulted in enhanced mechanical strength and decreased degradation rates of the Mg-Zr-Sr-Ho alloys. Furthermore, Ho addition (≤5 wt. %) to Mg-Zr-Sr alloys led to enhancement of cell adhesion and proliferation of osteoblast cells on the Mg-Zr-Sr-Ho alloys. The in vitro biodegradation and the biocompatibility of the Mg-Zr-Sr-Ho alloys were both influenced by the Ho concentration in the Mg alloys; Mg1Zr2Sr3Ho exhibited lower degradation rates than Mg1Zr2Sr and displayed the best biocompatibility compared with the other alloys.