Long Period Stacking Ordered Research Articles

Corrosion mechanism of Mg alloys involving elongated long-period stacking ordered phase and intragranular lamellar structure

It is a long-term challenge to further improve the corrosion resistance while ensuring the strength of magnesium (Mg) alloys. Revealing the effect of potential fluctuation on the micro-galvanic corrosion and the subsequent film formation is important for understanding the corrosion mechanism of Mg alloys with multiple strengthening phases/structures. Here, we prepared the high-strength Mg-14.4Er-1.44Zn-0.3Zr (wt.%) alloys containing hybrid structures, i.e., elongated long-period stacking ordered (LPSO) blocks + intragranular stacking faults (SFs)/LPSO lamellae. The Mg alloy with elongated LPSO blocks and intragranular LPSO lamellae (EZ-500 alloy) obtains good corrosion resistance (2.2 mm y–1), while the Mg alloy containing elongated LPSO blocks and intragranular SFs (EZ-400 alloy) shows a significantly higher corrosion rate (6.9 mm y–1). The results of scanning Kelvin probe force microscopy (SKPFM) show the elongated LPSO blocks act as cathode phase (87 mV in EZ-400 alloy), and the SFs serve as the weak anode (30 mV in EZ-400 alloy), resulting in high potential fluctuation in EZ-400 alloy. On the contrary, both elongated blocks and intragranular lamellae are cathodic LPSO phase (67–69 mV) in EZ-500 alloy, leading to a lower potential fluctuation. Quasi in-situ atomic force microscope (AFM) observation indicates that high potential fluctuation would cause strong micro-galvanic corrosion, and subsequently leads to the failure in rapid formation of corrosion film, finally forming a loose and porous film, while relatively low potential fluctuation could result in more uniform corrosion mode and facilitate the rapid formation of protective film. Therefore, we propose that it is an effective way to develop high-strength corrosion-resistant Mg alloys by controlling the potential fluctuation to form a “uniform potential” strengthening microstructure.

Hydrogen storage performance and phase transformations in as-cast and extruded Mg-Ni-Gd-Y-Zn-Cu alloys

Thermal-mechanical processing of magnesium-based materials is an effective method to tailor the hydrogen storage performance. In this study, Mg-Ni-Gd-Y-Zn-Cu alloys were prepared by Direct Chill (DC) casting, with and without extrusion process. The influences of microstructure evolution, introduced by DC casting and thermal-mechanical processing, on the hydrogen storage performance of Mg-Ni-Gd-Y-Zn-Cu alloys were comprehensively explored, using analytical electron microscopy and in-situ synchrotron powder X-ray diffraction. The result shows that the extruded alloy yields higher hydrogen absorption capacity and faster hydrogen ab/desorption kinetics. As subjected to extrusion processing, the α-Mg grains in the microstructure were significantly refined and a large number of 14H type long-period stacking ordered (LPSO) phases appeared on the α-Mg matrix. After activation, there were more nanosized Gd hydride/Mg2Ni intermetallics and finer chips. These modifications synergistically enhance the hydrogen storage properties. The findings have implications for the alloy design and manufacturing of magnesium-based hydrogen storage materials with the advantages of rapid mass production and anti-oxidation.

Deformation behavior of Mg-Y-Ni alloys containing different volume fraction of LPSO phase during tension and compression through in-situ synchrotron diffraction

The deformation behavior of the as-extruded Mg-Y-Ni alloys with different volume fraction of long period stacking ordered (LPSO) phase during tension and compression was investigated by in-situ synchrotron diffraction. The micro-yielding, macro-yielding, tension-compression asymmetry and strain hardening behavior of the alloys were explored by combining with deformation mechanisms. The micro-yielding is dominated by basal slip of dynamic recrystallized (DRXed) grains in tension, while it is dominated by extension twinning of non-dynamic recrystallized (non-DRXed) grains in compression. At macro-yielding, the non-DRXed grains are still elastic deformed in tension and the basal slip of DRXed grains in compression are activated. Meanwhile, the LPSO phase still retains elastic deformation, but can bear more load, so the higher the volume fraction of hard LPSO phase, the higher the tensile/compressive macro-yield strength of the alloys. Benefiting from the low volume fraction of the non-DRXed grains and the delay effect of LPSO and γ′ phases on extension twinning, the as-extruded alloys exhibit excellent tension-compression symmetry. When the volume fraction of LPSO phase reaches ∼50%, tension-compression asymmetry is reversed, which is due to the fact that the LPSO phase is stronger in compression than in tension. The tensile strain hardening behavior is dominated by dislocation slip, while the dominate mechanism for compressive strain hardening changes from twinning in the α-Mg grains to kinking of the LPSO phase with increasing volume fraction of LPSO phase. The activation of kinking leads to the constant compressive strain hardening rate of ∼2500 MPa, which is significantly higher than the tensile strain hardening rate.

A novel Mg-Gd-Y-Zn-Cu-Ni alloy with excellent combination of strength and dissolution via peak-aging treatment

Inferior absolute strength and dissolution properties are the main bottlenecks for the widespread application of dissolvable magnesium alloys in complex working environments for unconventional oil and gas resources. Here, a novel functional peak-aged Mg-9.5Gd-2.7Y-0.9Zn-0.8Cu-0.4Ni (wt.%) alloy for fracturing tools is reported, and it possesses an ultimate tensile strength of 457.6 MPa, ultimate compressive strength of 620.7 MPa and dissolution rate of ∼43.7 mg·cm−2·h−1 in 3 wt.% KCl solutions at 93 °C. The excellent strength of the aged-alloy is primarily attributed to the combination of grain refinement, long-period stacking ordered (LPSO) strengthening, and precipitation strengthening induced by stacking fault and β’ phase, among which the precipitation strengthening is dominant. Further investigations confirm that the corrosion is triggered from the micro-galvanic coupling between the Mg matrix and the cathodic lamellar and block LPSO phases. Strip-shaped corrosion pits along with LPSO phases are subsequently formed, significantly accelerating corrosion. The β’ precipitates can effectively improve the strength without compromising the dissolution rate because of their nanoscale size. This study provides an excellent material selection for dissolvable fracturing tools and presents a strategy by which a synergistic combination of strength and dissolution rate is achieved via peak-aging treatment.

Precursor H-induced pyrolysis towards hydrogen storage properties enhancement of Mg–Y–Zn alloys with diverse Y level

For advancing the hydrogen economy, developing Mg-based alloys for hydrogen storage is an important choice, in which the introduction of long period stacking ordered (LPSO) phase cracked it wide open. Presently, various contents of LPSO phase are obtained in Mg96-xY2+xZn2 (x = 0, 1 and 2) alloys by adjusting Y/Zn ratio. The formation of eutectic (Mg + Mg3Y2Zn3) in Mg96Y2Zn2 alloy and Y(Mg) solid solution in Mg94Y4Zn2 alloy both lead to a greater degree of element segregation, while Mg95Y3Zn2 alloy with only α-Mg and LPSO phase achieves relatively the most uniform element distribution. Hydrogen storage tests show that Mg96-xY2+xZn2 alloys can be activated only after one de/hydrogenation cycle and LPSO phase undergoes irreversible hydrogen-introduced decomposition, in-situ generating YH2/YH3 phases. Mg95Y3Zn2 alloy achieves the fastest kinetics and the highest hydrogenation capacity, ascribing to the most uniformly dispersed YH2/YH3 nanoparticles, the finest α-Mg grains and the most significant lattice constants. Mg95Y3Zn2 hydride exhibits the fastest dehydrogenation rate and lowest decomposition temperature owing to the “hydrogen pump” effect of the most dispersed YH2/YH3 phase. Further tests reveal that Mg95Y3Zn2 alloy has a good cycling stability for hydrogen absorption.

Microstructure and High-Temperature Mechanical Properties of Mg-1Al-12Y Alloy Containing LPSO Phase

This paper describes our study on the effect of the long-period stacking order (LPSO) phase on the comprehensive mechanical properties of the as-cast Mg-1Al-12Y (wt. %) alloy. The microstructure and tensile mechanical properties of as-cast alloy and solution treatment alloy were evaluated. The results showed that α-Mg matrix, Al2Y phase and β-Mg24Y5 phase were present in the as-cast alloy. After solution treatment, the β-Mg24Y5 phase in the alloy was dissolved and the LPSO phase was precipitated. The solution treatment did not cause grain growth or Al2Y phase change, but the comprehensive mechanical properties of the alloy were significantly improved. This was mainly due to the precipitation of the LPSO phase in the solid solution alloy. In addition, the dissolution of the β-Mg24Y5 phase after solution treatment could also improve the mechanical properties. The improvement of the comprehensive mechanical properties of solution treatment alloys at room temperature was due to the dispersion strengthening caused by the intragranular LPSO phase dispersed within the alloy, which had a good blocking effect on the movement of dislocations. The ductility of the solid-solution alloy was greatly improved at a high temperature while maintaining high strength because the LPSO phase is kinked, the dislocation density was reduced, and more non-basal slip was activated during the high-temperature tension.

Effect of Cu micro-alloying on the microstructure, mechanical and corrosion properties of Mg-Gd-Y-Zn based alloy applied as plugging tools

Dissolvable functional magnesium alloys are attracting strong attention due to the prominent advantages of high strength and suitable dissolution rate. Here, the effect of Cu micro-alloying (0, 0.4, 0.8 wt%) on the microstructure, mechanical and corrosion properties of Mg-9.5Gd-2.7Y-0.9Zn alloys are investigated. With the increase of Cu contents, the block 14 H long-period stacking ordered (LPSO) phase gradually increases, which promotes the nucleation of dynamic recrystallized (DRXed) grains. However, the migration of the sub-structure is inhibited caused by the plate LPSO phase and the growth of DRXed grains is suppressed. Eventually, the grain size of DRXed grains decreased. The alloy with 0.8 wt% Cu obtains higher yield strength compared with Cu-free alloys, which is mainly ascribed to multiple strengthening contributions containing second phase, dislocation, grain boundary and solid solution. Due to the potential difference between the LPSO phase and the Mg matrix in the investigated alloys, the LPSO phase acts as cathodic to accelerate corrosion. The 0.8Cu alloy exhibits the highest corrosion rate, which is the synergistic effect of the second phase (its larger volta potential compared with 0Cu and its higher fraction of LPSO phase), dislocation density, grain size and crystal orientation. Ultimately, the Mg-9.5Gd-2.7Y-0.9Zn-0.8Cu alloy exhibits an ultimate tensile strength of 380 MPa, tensile yield strength of 331 MPa, and decent corrosion rate (16.8 mg·cm−2·h−1 at 93 °C in 3 wt% KCl solution). This work strengthens understanding the Cu role in designing high-strength dissolvable Mg-RE alloys.

Developing new Mg alloy as potential bone repair material via constructing weak anode nano-lamellar structure

The mechanics-corrosion and strength-ductility tradeoffs of magnesium (Mg) alloys have limited their applications in fields such as orthopedic implants. Herein, a fine-grain structure consisting of weak anodic nano-lamellar solute-enriched stacking faults (SESFs) with the average thickness of 8 nm and spacing of 16 nm is constructed in an as-extruded Mg96.9Y1.2Ho1.2Zn0.6Zr0.1 (at.%) alloy, obtaining a high yield strength (YS) of 370 MPa, an excellent elongation (EL) of 17%, and a low corrosion rate of 0.30 mm y −1 (close to that of high-pure Mg) in a uniform corrosion mode. Through scanning Kelvin probe force microscopy (SKPFM), one-dimensional nanostructured SESFs are identified as the weak anode (∼24 mV) for the first time. The excellent corrosion resistance is mainly related to the weak anodic nature of SESFs and their nano-lamellar structure, leading to the more uniform potential distribution to weaken galvanic corrosion and the release of abundant Y 3+ /Ho 3+ from SESFs to form a more protective film with an outer Ca 10 (PO 4 ) 6 (OH) 2 /Y 2 O 3 /Ho 2 O 3 layer (thickness percentage of this layer: 72.45%). For comparison, the as-cast alloy containing block 18R long period stacking ordered (LPSO) phase and the heat-treated alloy with fine lamellar 18R-LPSO phase (thickness: 80 nm, spacing: 120 nm) are also studied, and the characteristics of SESFs and 18R-LPSO phase, such as the weak anode nature of the former and the cathode nature of the latter (37-90 mV), are distinguished under the same alloy composition. Ultimately, we put forward the idea of designing Mg alloys with high mechanical and anti-corrosion properties by constructing "homogeneous potential strengthening microstructure", such as the weak anode nano-lamellar SESFs structure.

Effect of electrolyte systems on plasma electrolytic oxidation coatings characteristics on LPSO Mg-Gd-Y-Zn alloy

In order to solve the problem of poor corrosion resistance of Mg-Gd-Y-Zn alloy with high strength and high toughness. In this paper, two-step plasma electrolytic oxidation (PEO) was implemented on Mg-Gd-Y-Zn alloy with long period stacking ordered (LPSO) phase in seven different electrolyte systems. Optical microscope (OM), Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) were used to characterize the structure distribution and composition of the LPSO phase. X-ray diffraction (XRD), SEM, EDS, Mott-Schottky test (M-S), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and immersion hydrogen evolution were used to study the morphology, composition, structure and corrosion resistance of the PEO coatings prepared using the various electrolytes. The PEO coating prepared by the mixed electrolyte of sodium aluminate, sodium phosphate and sodium silicate, referred to as PEO-APS coating, had the most excellent structure and corrosion resistance. The coating was dense and uniform, the coating thickness was 3.5 μm, the open porosity was 4.4 %, the average pore size was 0.12 μm, and the corrosion current density icorr was 2.09 × 10−8 A cm−2. In addition, the formation mechanism and corrosion resistance mechanism of the PEO-APS coating were discussed, and the effect of LPSO phase on the PEO-APS coating was proposed.

Phase evolution and strengthening mechanisms in a cast magnesium-zinc-yttrium-zirconium alloy following different heat treatments

The phase transformation sequences of a quasicrystal-reinforced magnesium-zinc-yttrium-zirconium (Mg–Zn–Y–Zr, labeled as ZWK3) alloy after annealing, T4, and T6 heat treatments, respectively, were determined by using electron microscopes. Nanoscale Mg7Zn3 phase and I–Mg3YZn6 strengthening particles are the two primary precipitates in the annealed ZWK3 alloy. Substantial nanoscale I-phase particles formed during solidification remained stable during the T4 treatment indicating their high stability at elevated temperatures, while a large number of nanoscale MgZn2 precipitates appeared during the following T6 treatment. There is a clear orientation relationship between the precipitates and the matrix; consequently, these precipitates are supposed to improve the strength of the heat-treated ZWK3 alloys significantly. T6 heat-treated ZWK3 alloy appears to contain thin and slender-like twins and long period stacking ordered (LPSO) structure. T4 and T6 heat treatment enhance the tensile strength of ZWK3 alloy, though with a minor reduction in elongation compared to the as-cast one.

In-situ and ex-situ investigation of deformation behaviors of a dual-phase Mg−Ni−Y alloy

The relationship between the deformation behaviors of the bulk α-Mg and Long Period Stacking Ordered (LPSO) phases is important for the design of high-strength alloys. In this work, the deformation behaviors of dual-phase Mg alloy which contains α-Mg and 18R-LPSO phases are investigated by in-situ tensile scanning electron microscopy and ex-situ transmission electron microscope. The results show that the 18R phase accounting for 11.3 vol.% provides 23.0% deformation of the overall alloy. At the early stage of deformation, the stacking faults are bent to coordinate twinning deformation through the slip of basal dislocations in the α-Mg matrix, while the kinking deformation 18R induces the kinking deformation of Mg slices. At the end of deformation, large numbers of T2-type stacking faults are generated. These deformation behaviors improve the ductility of the alloy. This work provides a novel strategy to design the high strength and good ductility dual-phase Mg alloys.

Structural features of 18R-type long-period stacking ordered phase in Mg85Zn6Y9 alloy and the Ni doping effect on its hydrogen storage properties

Magnesium-based alloys with 18R-type long-period stacking ordered (LPSO) structures have attracted wide attention for structural and functional applications. To understand hydrogen storage properties of 18R phase, the Mg85Zn6Y9 alloy with 94 wt.% of 18R-type LPSO phase is prepared in this work. The 18R phase has a layered structure where Y-Zn-Mg and Mg layers alternately stack along the c-axis. In the Y-Zn-Mg layers, Y, Zn and partial Mg sites are co-occupied by Y and Mg, Zn and Mg, and Mg and Zn/Y atoms, respectively. Thus the 18R phase is easily decomposed into α-MgH2, γ-MgH2, YH2, YH3, C14-type Laves phase MgZn2 and minor CsCl-type Y(Mg,Zn) during ball milling under hydrogen atmosphere. After further hydrogen absorption-desorption cycling, Y(Mg,Zn) disappears gradually and C14 phase transforms into C15-type Laves phase. By contrast, the Mg85Zn6Y9 alloy has better hydrogen storage kinetics and cycle durability than pure Mg because of the catalytic effect of YH2/YH3 on hydrogen absorption-desorption and inhibition role of Laves phase in Mg crystallite growth. Moreover, the introduction of Ni into Mg85Zn6Y9 sample leads to a further decrease in activation energy of hydrogen desorption from 106.39 to 96.78 kJ mol−1 due to the formation of Mg2Ni. This work not only provides new insights into structural features and hydrogen storage characteristics of 18R phase but offers an effective method for improving hydrogen storage properties.

Dynamical, mechanical, anisotropic, and thermodynamic properties of Mg-XL-Y (XL = Zn, Al) alloys with long-period stacking ordered structures: A first-principles calculation

The dynamical, mechanical, anisotropic, and thermodynamic properties of Mg-XL-Y (XL = Zn, Al) alloys based on three long-period stacking ordered (LPSO) phases (ie, 10H, 14H, and 18R) have been investigated systematically by first-principles calculations. The LPSO phases of Mg-XL-Y (XL = Zn, Al) alloys are thermodynamically, dynamically, and mechanically stable. The each LPSO phase of Mg–Al–Y alloys has superior mechanical properties with larger resistances to deformation under applied stress and shear pressure, stiffness, as well as hardness than those of corresponding to LPSO phases of Mg–Zn–Y alloys. According to universal elastic anisotropic index, percent shear anisotropy, 3D surface structures of Young's moulus, and sound velocities, the LPSO phases of Mg-XL-Y (XL = Zn, Al) alloys exhibit obvious anisotropy. Besides, all LPSO phases of Mg-XL-Y (XL = Zn, Al) alloys possess low lattice thermal conductivity due to the strong anharmonicity. The calculated adsorption energies indicate that the 14H(001) surface of Mg–Al–Y system at the hollow site is more sensitive to absorb O atom.

Achieving ultra-high strength using densely ultra-fine LPSO phase

• An optimized FSP processing generates densely ultrafine LPSO blocks with an average width of ∼200 nm and an average length of ∼1 μm. • There is an ultrahigh strength up to approximately 800 MPa under the uniaxial compression of micropillars. • The strengthening effect by ultrafine LPSO blocks reaches approximately 630 MPa. Long period stacking ordered (LPSO) structures are an effective strengthening phase in Mg alloys. However, the coarse LPSO phases in as-cast alloys are very difficult to refine, and they dramatically reduce the strengthening effect. In the present study, friction stir processing (FSP) was employed to refine the structure of an Mg-12.8Y-4.7 Zn (wt%) alloy with a very high content of coarse LPSO phases. An optimized FSP regime refined the coarse LPSO phases into densely ultrafine blocks with an average width of ∼200 nm and an average length of ∼1 μm. An ultrahigh yield strength of ∼800 MPa was achieved under compression of the FSP region. Theoretical calculations indicated that the strengthening by the densely ultrafine LPSO phases was up to approximately 630 MPa.

Solid state recycling of Mg–Gd–Y–Zn–Zr alloy chips by isothermal sintering and equal channel angular pressing

The Mg-7Gd-4Y-2Zn-0.5Zr alloy chips were successfully recycled through isothermal sintering and equal channel angular pressing (ECAP). The mechanical properties and microstructure evolution of samples during the recycling process were studied in detail. The eutectic phases in the as-cast alloy transform into long period-stacking ordered (LPSO) phases after homogenization, which can improve the plasticity of the material. After isothermal sintering, the density of the sample is lower than that of the homogenized sample, and oxide films are formed adjacent to the bonding interface of the metal chips. Hence, the plasticity of the sintered sample is poor. Dense samples are fabricated after ECAP. Although the grains are not refined compared to the sintered sample, the microstructure becomes more uniform due to recrystallization. Fiber interdendritic LPSO phase and kinked 14H-LPSO phase are formed in the alloy due to the shear deformation during the ECAP process, which improves the strength and plasticity of the sample significantly. Furthermore, the basal texture is weakened due to the Bc route of the ECAP process, which can increase the Schmid factor of the basal slip system and improve the elongation of the sample. After 2 ECAP passes, the fully densified recycled billet shows superior mechanical properties with an ultimate tensile strength of 307.1 MPa and elongation of 11.1%.

The aging behaviour of a Mg-10Gd-3Y-1.0Zn-0.5Zr (wt.%) alloy with long-period stacking ordered phase at 275°C

ABSTRACT We investigated the aging behaviour of a Mg-10Gd-3Y-1.0Zn-0.5Zr (wt.%) alloy with long-period stacking ordered (LPSO) phase at 275°C. Its peak hardness is about 116.4 HV after aging for 25 h. We unravelled the evolution of prismatic precipitates through transmission electron microscopy (TEM). The formation process is β′ (10h)→ β′ + β1 (15 h)→ β′ + β1 + β (≥20 h), where nano-scale phase benefits the transformation from β′ phase to β1 phase. In addition to the stable basal LPSO structures, the coexistence and competition balance between the prismatic β′, β1 and β phases synergistically contribute to the high hardness of the aged alloy. The number density of the prismatic precipitates is at the magnitude of 1020/m3. These prismatic precipitates during aging largely determine the hardness of the Mg alloys. The interaction between precipitates and stacking faults brings good compression property.

The Role of LPSO Structures in Corrosion Resistance of Mg-Y-Zn Alloys

The growing interest in improving Mg-based alloys’ corrosion properties stimulates the development of Mg-Y-Zn alloys with long-period stacking-ordered (LPSO) structures. In this work, to describe the corrosion performance of Mg-LPSO alloys, a set of experiments, including microstructure observations and corrosion testing in media containing various concentrations of chloride ions, were carried out. It was shown that the main corrosion mechanism occurring on the alloys was not only related to the volume of LPSO structures in the Mg matrix but was also dependent on their distribution. In the chloride-containing solutions, pitting was the predominant corrosion mechanism, and with the increasing chloride concentration, microgalvanic corrosion was accelerated.

Effect of extrusion parameters on microstructure, mechanical properties and damping capacities of Mg-Y-Zn-Zr alloy

In this work, different extrusion processes were carried out on a Mg-3.16Y-1.85Zn-0.37Zr (wt%) alloy containing long period stacking ordered (LPSO) phases. The effect of extrusion parameters on the microstructure, mechanical properties and damping capacities of the alloy were investigated. The results show that the alloy extruded at 360 °C and a ratio of 9:1 shows good comprehensive properties with a tensile yield strength (TYS) of 280 MPa, ultimate tensile strength (UTS) of 330 MPa, elongation (EL) to failure of 21% and damping value Q−1 of 0.023 (strain amplitude ε = 10−3) at room temperature (RT). As the extrusion temperature or ratio increases, the strength decreases and EL increases, while the damping value of ε = 10−3 at RT is slightly down due to the decrease in the LPSO phase and dislocation density. All as-extruded alloys show a similar variation tendency for the relationship between Q−1 and test temperature T, with elevated temperature (ET) damping capacities Q−1 > 0.01 at a frequency of 1 Hz and T > 300 °C. Strengthening mechanisms for mechanical properties of the as-extruded alloys have also been analyzed, and the damping capacities are also discussed in terms of the Granato-Lücke (G-L) dislocation theory and grain boundary sliding. The results of the present work provide a new perspective for further developing high-performance Mg-Y-based alloys with both high damping and excellent comprehensive mechanical properties by careful tailoring of the extrusion process parameters.

Nanoindentation on the Transformation of LPSO Phases during Different Solution Heat Treatments in an Mg-Dy-Nd-Zn-Zr Alloy

The objective of this study is the investigation of nanomechanical properties using nanoindentation of extruded and heat-treated Mg-Dy-Nd-Zn-Zr, with an emphasis on the transformation of long-period stacking-ordered (LPSO) phases. Solution heat treatment was performed with different heat treatment for durations on hot extruded Mg-Dy-Nd-Zn-Zr to monitor the transformation of LPSO phases, as well as to keep track of microstructural changes. The initial fine-grained microstructure, with blocky and lamellar LPSO structures within the matrix, first transformed into coarser grains with fewer LPSO lamellae, which then increased in amount again at higher annealing duration. The blocky LPSO phases, which have the highest hardness compared to the matrix grains with and without LPSO lamellae, consistently decrease in quantity, as so does the trend in their hardness value. The Mg matrix grains with LPSO lamellae show a lower hardness compared to the Mg matrix grains without or with a just few lamellar LPSO phases, and increase in quantity at long annealing durations. The overall hardness of the microstructure is essentially determined by the LPSO lamellae-containing grains and reaches a peak at 24 h. There is another peak found for the grain size values; however, this is at later annealing duration, at 72 h. The reduction in grain size towards longer annealing durations goes along with a reactivated formation of LPSO lamellae.

Atomic-scale characterization of the precipitates in a Mg-Gd-Y-Zn-Mn alloy using scanning transmission electron microscopy

Precipitates are critical in strengthening Mg-Gd-Y-Zn-Mn alloys. However, lack of sufficient atomic-scale characterization restricts further understanding of the precipitates. Therefore, in the present study, precipitates in the as-cast, solution-annealed, and isothermally aged (200 °C) Mg-Gd-Y-Zn-Mn alloy have been comprehensively examined on the atomic scale by aberration-corrected (Cs) high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) combined with energy dispersive X-ray spectrometry (EDS). When observed in the [0001]α direction in the early-aged alloy for 6 min, uneven local rare earth solute clusters in the form of βH and βZ phases suggest a transition from βH to βZ. In the peak-aged alloy for 16 h, Mn atoms are found to segregate into the γ′/long-period stacking ordered (LPSO) phase and β′ phase, participating in their formation. In the over-aged alloys for 200 h, γ″ phase is detected with enrichment of Mn, coexisting with the γ′ and β′ phases. Furthermore, β″-like structures are observed, which may be defective βM or β′T as a result of their overlapping, thus providing a possible explanation for the controversy regarding the existence of the β″ phase. Our findings serve as evidence that deepens the understanding of the precipitates in Mg-Gd-Y-Zn-Mn alloys at the atomic level.