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To study the effects of dynamic precipitation and processing parameters on the dynamic recrystallization (DRX) behavior of 2195 Al-Cu-Li alloy, the hot deformation process was investigated comprehensively with the temperature range of 300-500°C and strain rate range of 0.001-10 s-1. An Arrhenius-type constitutive equation and DRX kinetics model were established based on the corrected stress-strain data to unveil the relationship between processing parameters, flow stress and DRX behavior. The shape of flow curves was associated with the dynamic softening mechanism and the DRX softening fraction gradually increased with the decrease of Zener-Hollomon parameter (Z). Then, the microstructure evolution was further analyzed to elucidate the DRX mechanisms at various Z values. The primary softening mechanism was dynamic recovery (DRV). However, when temperature set between 300 and 400°C, T1 and cubic phases significantly impeded DRV and promoted the occurrence of discontinuous dynamically recrystallization (DDRX) by a strong pinning effect and considerable driving force. As Z value decreased, the recrystallization mechanism transformed from DDRX into continuous dynamic recrystallization (CDRX) attributed to the higher mobility for dislocations. Due to the reduced stored energy and the hindrance of precipitates, grain boundaries were hard to migrate, making it difficult for DDRX grains to grow continuously.

This work reported the effect of extrusion speeds on the microstructures and mechanical properties of Mg-Ca binary alloy. The results showed that yield strength of the as-extruded Mg-1.2 wt.% Ca alloys decrease from ∼ 360 MPa to ∼ 258 MPa as the ram speed increases from 0.4 mm/s to 2.4 mm/s, and the elongation increases from ∼ 3.9% to ∼ 12.2%. The microstructure changes from bimodal grain feature to the complete dynamical recrystallization (DRX) with increase of the extrusion speed. The ultrafine DRXed grains in size of ∼0.85 µm, the numerous nano-Mg2Ca particles dispersing along the grain boundaries and interiors, as well as the high density of residual dislocations, should account for the high strength. It is believed that the high degree of dynamic recrystallization and the resulting texture randomization play the critical roles in the ductility enhancement of the high-speed extruded Mg alloys.

Local scanning electrochemical microscopy (SECM), along with a scanning vibrating electrode technique (SVET), were used to spatially map anodic and cathodic regions in a complex assembly including a friction stir lap welded joint between two Mg-Al-Mn alloys, namely AM60 and AM30. The assembly investigated herein was comprised of the two base materials (AM60 and AM30) and a stir zone (SZ). The transitory electrochemical characteristics of the three regions were perceived from different electrochemical tests and have been correlated to their respective microstructures. Potentiometric and voltametric SECM performed on the assembly in 0.01 M NaCl revealed that during the first few minutes of exposure, AM60 and SZ regions act as dominant anodic regions in the assembly while after 2 h of exposure these regions transition to become cathodic relative to the AM30 region. Galvanic interaction between the different weld regions immersed in a significantly more conductive NaCl electrolyte was investigated by SVET. Under such conditions, filiform-like corrosion initiated, and propagated exclusively within the AM30 region, resulting in cathodic activation of the corroded surface. However, after approximately 5 h of exposure, the AM60 region preferentially dissolves following sufficient cathodic activation of the AM30 region.

Pure magnesium (Mg) develops a strong basal texture after conventional processing of hot rolling or extrusion. Consequently, it exhibits anisotropic mechanical properties and is difficult to form at room temperature. Adding appropriate alloying elements can weaken the basal texture or even change it, but the improvement in formability and mechanical properties is still far from expectations. Over the past 20 years, considerable efforts have been made and significant progress has been made on wrought Mg alloys at the fundamental and technological levels. At the fundamental level, textures formed in sheets and extrusions of different alloy compositions and produced under different strain paths or thermomechanical processing conditions are relatively well established, with the assistance of the advanced characterization technique of electron backscatter diffraction. At the technological level, room temperature formability of sheet has been significantly improved, and tension–compression yield asymmetry of extrusion is also remarkably reduced or eliminated. This paper starts with an overview of dislocations, stacking faults and twins, and deformation of single crystals of pure Mg along different orientations and under different loading conditions, followed by a review of microstructure (texture and grain size) and deformation of polycrystalline pure Mg with different textures, grain sizes, and loading conditions. With this information as a base, texture, grain size, and deformation of polycrystalline Mg alloy sheets and extrusions produced under different processing conditions are systematically examined and compared. Remaining and emerging scientific and technology issues are then highlighted and discussed in the context of texture and grain size. The need for better-resolution diffraction and spectroscopy techniques is also discussed in the relationship between texture change and grain boundary solute segregation.

High strength–ductility synergy is difficult to achieve in Mg alloys. Although high strength has been achieved through considerable alloying addition and low-temperature extrusion, these techniques result in low ductility (2%–5%). In this work, a novel low-alloy Mg–Ca-based alloy that overcomes this strength–ductility trade-off is designed. The alloy has an excellent tensile yield strength (∼425 MPa) and exhibits a reasonably high elongation capacity (∼11%). A microstructure examination reveals that a high density of submicron grains and nano-precipitates provides the alloy high strength, and the leaner alloy additions and higher extrusion temperatures initially improve ductility. As a result, the density of residual dislocations is reduced, and the formation of low-angle grain boundaries (LAGBs) is enhanced. With fewer residue dislocations, it becomes less probable for the newly activated mobile dislocations to be impeded and transformed into an immobile type during the subsequent tensile test. The LAGBs function as potential sites to emit new dislocations, thus enhancing the dislocation–multiplication capability. More importantly, they can induce evident sub-grain refinement hardening and guarantee that the alloy achieves high strength. The findings lead to a controllable Mg alloy design strategy that can simultaneously afford high strength and ductility.

The unique thermal history of selective laser melting (SLM) can lead to high residual stress and a non-equilibrium state in as-fabricated titanium alloy components and hinders their extensive use. Post heat treatment, as a classical and effective way, could transform non-equilibrium α’ martensite and achieves desirable mechanical performance in SLMed Ti alloys. In this study, we aimed to establish the correlation between the microstructure and mechanical performances of SLMed Ti6Al4V (Ti-64) by using different heat treatment processes. The columnar prior β grain morphology and grain boundary α phase (GB-α) after different heat treatment processes were characterized, with their influences on the tensile property anisotropy fully investigated. Scanning electron microscope (SEM) observation of the fracture surface and its cross-sectional analysis found that the tensile properties, especially the ductility, were affected by the GB-α along the β grain boundary. Furthermore, the discontinuous ratio of GB-α was firstly proposed to quantitatively predict the anisotropic ductility in SLMed Ti-64. This study provides a step forward for achieving the mechanical property manipulation of SLMed Ti-64 parts.

A family of alloys based on the Mg-Al-Zn-Ca-Mn system (Mg-3Al-1Zn-1Ca-0.3Mn, Mg-3Al-1.5Zn-0.5Ca-0.3Mn, and Mg-3Al-1Ca-0.3Mn, wt pct) was developed for extrusion. Their mechanical properties were evaluated by tensile testing at room temperature, and compared to those of the benchmark Mg-alloy Mg-3Al-1Zn-0.3Mn (AZ31). The microstructures of the extruded alloys were characterized in detail in order to reveal the effect of Ca on microstructural evolution, and consequently the alloy strength. The addition of Ca to the AZ31 stifles dynamic recrystallization and grain growth, with only ~30 pct recrystallization and a recrystallized grain size of ~480 nm. In contrast, the benchmark alloy is essentially completely recrystallized with an average grain size of ~2.3 μm. A high density of low-angle grain boundaries (LAGBs) and dislocations were observed in Ca-containing alloys, and were identified as a major factor in the observed strengthening. Such LAGBs form cellular subgrains predominantly along initial grain boundaries, or newly formed boundaries that are closely spaced (~ 600 nm) and nearly parallel to the extrusion direction. The subgrains have an ultrafine size of 100 to 400 nm, and difficult to convert to recrystallized grains. Solute segregation to grain boundaries was also observed. It is hypothesized that it is the Ca segregation to dislocation cores along LAGBs that decreases the dislocation mobility and stabilizes LAGBs, by thermodynamically decreasing the dislocation energy and/or kinetically imposing a solute drag effect.

The 5xxx series aluminium alloys are susceptible to sensitisation during service at elevated temperatures. Sensitisation refers to deleterious grain boundary precipitation resulting in rapid intergranular corrosion in moist environments. A holistic understanding of the variables that can influence the degree of sensitisation in Al-Mg-Mn alloys is presented herein, including the exploration of some custom produced 5xxx series alloys that were prepared to create a significant dataset for which an artificial neural network (ANN) could be applied. An ANN model could reveal complex interactions between various factors that influence sensitisation, with the view to designing sensitisation resistant Al-Mg-Mn alloys.

Extruded polycrystalline pure magnesium (Mg) with fine grain size (~1.2 µm) exhibits ductility of over 100% at room temperature, in spite of the presence of a strong basal texture. In this study, a set of complementary in-situ characterisation techniques over multiple-length scales were utilised to reveal the deformation modes enabling such ductility. Synchrotron X-ray diffraction results show that the elastic lattice strain of fine-grained sample for tensile elongation up to ~55% is 3–10 times lower than that in the coarse-grained sample, indicating the absence of significant strain accumulation inside fine grains and potential inter-granular deformation in bulk. In-situ scanning electron microscopy validates the predominant operation of the inter-granular deformation, and it further reveals that the inter-granular deformation occurs by the relative sliding between groups of grains having similar orientations. The deformation resulting from such sliding is substantial, and it is accommodated by the rotation of grains located between slid grouped grains, from hard to softer orientations to allowing dislocation slip to readily occur. The accommodating mode of dislocation slip is further supported by in-situ transmission electron microscopy observations. Dislocations glide readily to, and annihilate at, grain boundaries. The observations and direct evidences presented herein suggest that the major deformation mode is sliding between grouped grains that is accommodated by grain rotation and dislocation slip, in contrast to dislocation slip in coarse-grained Mg. The coordinated deformation processes postpone the occurrence of localised stress concentration and greatly increases the ductility of pure Mg at room temperature.

An electrochemical flow cell was constructed to permit the application of a mechanical scratch to the exposed material while monitoring the open circuit dissolution rates of the elemental alloy components. Elemental dissolution rates were determined where measured downstream using inductively coupled plasma atomic emission spectrometry (ICP-AES). The technique is demonstrated with Mg-Li(-Al-Y-Zr) alloy exposed to 0.01 M NaCl. A mechanical scratch was applied manually using a non-conductive and non-contaminating sapphire lancet without interrupting the on-line measurement. The ion dissolution rate from the newly exposed surface was significantly higher than the dissolution rate from the remaining surface area, corresponding to a high local current density. Notably, the increase in the dissolution rate from the scratched area occurred gradually after the scratch; then subsided within 200 s after the scratch, indicating that this particular Mg-alloy can repassivate by the formation of a surface film after a scratch was applied. This is the first reported use of an in situ scratch cell coupled with ICP-AES. Keywords: Magnesium, Magnesium‑lithium alloy, In situ, Electrochemistry, Online ICP

The surface film formed on an ultra-lightweight Mg-Li(-Al-Y-Zr) alloy was investigated. Previous research reported that this body-centered cubic (bcc) Mg-Li(-Al-Y-Zr) alloy demonstrated high corros...

© 2020 Elsevier B.V. The production of magnesium alloy WE43 was achieved by selective laser melting (SLM). The alloy was investigated after SLM, hot isostatic pressing (HIP), and solutionising heat treatment. The microstructure and corrosion behaviour of the specimens were carefully characterised, whilst assessed and contrast relative to the conventionally cast alloy counterpart. The SLM prepared specimens possess a unique microstructure comprising fine grains growing with a strong [0001] texture along the building direction with a low fraction of process-induced and metallurgical defects, reaching < 0.1 %, after optimising the SLM parameters and the HIP treatment. Electrochemical measurements demonstrated that the SLM prepared WE43 is cathodically more active as compared with its cast counterpart. It is proposed that this behaviour is due to a high density of zirconium-rich oxide particles uniformly distributed throughout the alloy microstructure as well as the alterations in the chemical composition of the solid-solution matrix originating from the high cooling rates of SLM. It was also noted that the oxide particles are mainly sourced by powder. The present results suggest that the corrosion of SLM prepared Mg alloys could be greatly improved once the influence of powder characteristics is further understood and controlled.

Exceptionally high strength Mg 3Al 1Zn-0.3Mn wt.% (AZ31) is demonstrated herein, revealing a yield strength of 380 MPa in both tension and compression, for extrusions prepared at 175 °C. Extruded AZ31 nominally has a low-to-moderate yield strength amongst typical magnesium (Mg) extrusion alloys, generally less than 250 MPa. The low strength is predominantly due to a large grain size and the absence of effective precipitation strengthening. In this study, AZ31 was extruded at different temperatures to reveal an exceptionally high strength with ultrafine grains of 0.65 μm in diameter. To reveal the origin of high strength in this AZ31 alloy, the microstructure of AZ31 was compared with those of Mg 1Zn alloy and pure Mg with similar grain size and textures. The solute atoms were identified to be the key to alloy strengthening (∼210 MPa). In contrast to grain boundary sliding, grain boundary migration and grain rotation that is observed submicron-grained pure Mg; the solute in submicron-grained AZ31 suppressed such intergranular deformation modes, with the grain boundaries in submicron-grained AZ31 providing significant strengthening (via the Hall-Petch relationship). In the high-strength AZ31 presented, Al reacts with Mn to form uniformly distributed particles, whilst Zn solute atoms segregated to grain boundaries, the latter posited to stabilize the boundaries of submicron grains by reducing grain boundary energy and thus suppressing the intergranular deformation. The results herein reveal high strength Mg-alloys are readily achievable, the related concepts of which have implications for numerous Mg alloy systems.

Satisfactory corrosion resistance remains an issue in the widespread implementation of magnesium (Mg). The use of alloying to improve mechanical properties of Mg generally accelerates corrosion due to microstructural heterogeneity. However, recent works have revealed that additions of elements serving as ‘cathodic poisons’ such as arsenic (As) and germanium (Ge) can reduce cathodic reaction rates and suppress cathodic activation - imparting corrosion resistance. The effect of Ge was translated into a ternary (and mechanically relevant) Mg-alloy system for the first time, revealing an alloy system with a balance of properties, and low rate of corrosion relative to Mg-alloys to date.

Microstructural evolution, corrosion behavior and mechanical properties of cast Mg-Zn-Y alloys as a function of volume fraction of long period stacking ordered (LPSO) phases were investigated. Results revealed that 18R-LPSO phase was formed in Mg-0.9%Zn-1.6%Y (ZW12), Mg-2.1%Zn-5.2%Y (ZW25) and Mg-3.1%Zn-7.6%Y (ZW38) alloys. LPSO phases in these alloys acted as micro-cathode to accelerate corrosion progress due to their nobler nature than that of α-Mg matrix. The galvanic-couple effect between LPSO phases and α-Mg matrix dominated the corrosion rate of the cast Mg-Zn-Y alloys in the long-term corrosion process. As a result, ZW12 alloy displayed the best corrosion resistance due to the least volume fraction of micro-cathodes (i.e. LPSO phases). In addition, compared with ZW25 alloy, the compact and thick LPSO phases in ZW38 alloy hindered the corrosion progress to a small degree since it was not preferentially oriented against corrosion front. The shape of corrosion pits was correlated to the shape of Mg dendrites that was a result of the volume fraction and distribution of LPSO phases. LPSO phases could enhance mechanical strength, but the increment was slight when the volume fraction of LPSO phases reached up to 20.3%. Keywords: Mg alloys, LPSO phases, Corrosion, Electrochemical analysis, Mechanical properties

A unique thermomechanical processing method combining fast rolling with large thickness reduction and short-time annealing (FR-STA) was developed to produce lower-cost magnesium (Mg) sheets with improved tensile properties. Sheets of Mg-3Al-1Zn-0.3Mn (wt pct) and Mg-1Zn-0.2Nd-0.2Zr (wt pct) were produced by FR incorporating large thickness reduction, exhibiting enhanced strength and improved ductility relative to slow rolling with small thickness reduction after STA.

The present study seeks to clarify the mechanism by which dilute additions of zinc (Zn) and calcium (Ca) improve ductility of Mg alloy sheet. Herein, the ductility and microstructure of fully annealed pure Mg and Mg-0.1Ca, Mg-0.4Ca, Mg-0.4Zn, and Mg-0.3Zn-0.1Ca (at.%) alloy sheet were systematically investigated and compared. It is found that the ternary alloy displays better ductility than either pure Mg or binary Mg-alloys, when fully recrystallized and possessing similar grain size. In the deformed grains of the ternary alloy, traces of basal and pyramidal slip and { 10 1 2 } twins are observed, whilst only basal slip traces and { 10 1 2 } twins are observed in the pure Mg and binary alloys. Grain boundary cracks are observed in all the tensile-tested alloys. However, significantly less grain boundary cracks are observed in the ternary alloy, posited to be due to enhanced grain boundary cohesion. These observations suggest that the combination of enhanced pyramidal slip and suppressed grain boundary cracking leads to the appreciably improved ductility of the Mg-0.3Zn-0.1Ca alloy sheet.

Addition of 0.1 at% Zn to the Mg–0.5Ca–0.1Zr alloy is found to significantly improve the stretch formability and ductility. The remarkable improvement in the stretch formability and ductility is attributable to the enhanced grain boundary cohesion and weakened texture. The as-annealed Mg–0.5Ca–0.1Zr–0.1Zn alloy sheet shows a 0.2% proof strength of ≈134 MPa along the rolling direction, which is lower than that of as-annealed AZ31 alloy sheet (≈147 MPa). However, by post-forming ageing, the 0.2% proof strength of the Mg–0.5Ca–0.1Zr–0.1Zn alloy sheet is considerably increased to ≈201 MPa, exceeding that for the annealed AZ31 alloy sheet.

Dynamic recrystallization (DRX), dynamic precipitation and mechanical properties of the hot-extruded Mg89Y4Zn2Li5 (at. %) alloys with different extrusion ratio and speed are systematically investigated in this work. The results reveal the formation details of recrystallized grains in the presence of long-period stacking ordered (LPSO) phases during extrusion at 350 °C. In a low extrusion ratio of 4, the recrystallized grains start to form discontinuously along the fluctuant 18R/α-Mg interface and continuously along the 14H kink boundaries with high misorientation angles. As increasing of extrusion ratio from 4 to 16, besides discontinuous and continuous DRX, particle-stimulated nucleation (PSN) of recrystallized grains is accelerated by small 18R and 14H fragments. Moreover, various thin LPSO lamellae including 14H, 18R and 24H as well as γ′ phase are dynamically precipitated inside the DRX grains. Under extrusion with a high ratio of 25, the DRX is mainly produced by PSN mechanism, leading to an almost completely recrystallized microstructure. Also, most of the 18R blocks are broken down to small fragments while the 14H lamellae in the original α-Mg matrix are dissolved at the large deformation strain. In addition, decreasing extrusion speed from 1 to 0.1 mm/s at this extrusion ratio effectively limits the growth of DRX grains and facilitates the dynamic precipitation of thin 14H lamellae within the fine DRX grains. Consequently, the full and fine DRX microstructure, small 18R fragment and deformation kinking, abundant thin 14H lamellae in fine DRX grains result in the excellent strength-ductility balance through hot extrusion with ratio of 25 and speed of 0.1 mm/s.

The aqueous stability of a corrosion resistant Mg-Li(-Al-Y-Zr)-alloy was investigated by combining in-situ confocal Raman Microscopy, Atomic Emission SpectroElectroChemistry, ex-situ Photoluminiscence Spectroscopy, Auger Electron Spectroscopy and Glow Discharge Optical Emission Spectroscopy. Li and Mg dissolved from visually intact anodic areas, leaving a Li-depleted metallic layer under approximately 100 nm thick Li-doped MgO. The transformation MgO→Mg(OH)2 was inhibited. Li2[Al2(OH)6]2·CO3·nH2O, LiAlO2, Y2O3 and Mg(OH)2 accumulated locally around active cathodic sites. New corrosion mechanism is proposed, which associates the improved corrosion resistance of Mg-Li alloys with an enhanced chemical stability and modified catalytic activity of MgO in presence of Li+.

Texture evolution in cold-rolled Mg-0.3Zn-0.1Ca, Mg-0.4Zn and Mg-0.1Ca (at.%) alloys during static recrystallization is monitored using a quasi-in-situ electron backscatter diffraction (EBSD) method. The quasi-in-situ EBSD results show that most of recrystallized grains formed in the early stage of recrystallization have randomised orientations in the ternary alloy and they grow uniformly during the recrystallization process. The formation and uniform growth of these recrystallized grains with randomised orientations give rise to a weak texture in fully recrystallized samples of the ternary alloy. A weak recrystallization texture also forms in the early stage of recrystallization in the two binary alloys, but it is gradually replaced by a strong basal texture via the preferential growth of recrystallized grains with specific orientations. The grain size in the ternary alloy is smaller than those in the two binary alloys at each stage of recrystallization, and the grain size distribution in the ternary alloy is significantly narrower than those in the two binary alloys after full recrystallization. Solute segregation to grain boundaries is observed in all three alloys in the fully recrystallized state. It is hypothesised that Zn and Ca atoms in the ternary alloy segregate strongly to high-energy boundaries of the recrystallized grains that would otherwise grow preferentially in the counterpart binary alloys, and that this co-segregation would significantly reduce the boundary mobility, by reducing grain boundary energy and enhancing solute dragging effect, and therefore lead to a more uniform growth of recrystallized grains with randomised orientations.

Effects of alloying additions on deformation texture in Mg sheet alloys are still controversial. In this study, a quasi-in-situ electron backscatter diffraction method is used for the first time to monitor the texture evolution during the cold rolling process of Mg–Zn–Ca and Mg–Zn alloys. It is found that the Ca addition does not cause any texture weakening effect — it only delays the development of a strong basal texture by reducing the growth of deformation twins.

Magnesium sheet alloy of dilute composition, Mg–0.3Zn–0.1Ca (at.%), does not exhibit any hardening response during heating. However, an annealing treatment at 80–200 °C, after some plastic strain in tension, leads to a remarkable strengthening, rather than softening, effect. By using electron backscattered diffraction and scanning transmission electron microscopy, it is found that the strengthening effect is caused by the pinning of gliding basal dislocations by GP zones and possibly solute atoms segregated to the dislocations.

The preferential orientation of \( \left\langle {10\bar{1}0} \right\rangle \) axis along tensile direction in magnesium (Mg) sheet alloys is still controversial. By using a quasi-in situ electron backscatter diffraction (EBSD) method, the present study provides an interpretation about the origin of the \( \left\langle {10\bar{1}0} \right\rangle \) texture. It is found that a strong \( \left\langle {10\bar{1}0} \right\rangle \) texture is developed after tensile stretching in the Mg alloys with different compositions. The occurrence of the \( \left\langle {10\bar{1}0} \right\rangle \) texture is predominantly caused by the preferential formation and growth of twins, whose \( \left\langle {10\bar{1}0} \right\rangle \) axes are nearly parallel to the tensile direction. In contrast, the dislocation slip only leads to a small-scale change of grain orientations, and therefore is unlikely to result in the occurrence of \( \left\langle {10\bar{1}0} \right\rangle \) texture.