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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 Y3+/Ho3+ from SESFs to form a more protective film with an outer Ca10(PO4)6(OH)2/Y2O3/Ho2O3 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.

Customized heat treatment is essential for enhancing the mechanical properties of additively manufactured metallic materials, especially for alloys with complex phase constituents and heterogenous microstructure. However, the interrelated evolutions of different microstructure features make it difficult to establish optimal heat treatment processes. Herein, we proposed a method for customized heat treatment process exploration and establishment to overcome this challenge for such kind of alloys, and a wire arc additively manufactured (WAAM) Mg-Gd-Y-Zn-Zr alloy with layered heterostructure was used for feasibility verification. Through this method, the optimal microstructures (fine grain, controllable amount of long period stacking ordered (LPSO) structure and nano-scale β ′ precipitates) and the corresponding customized heat treatment processes (520 °C/30 min + 200 °C/48 h) were obtained to achieve a good combination of a high strength of 364 MPa and a considerable elongation of 6.2%, which surpassed those of other state-of-the-art WAAM-processed Mg alloys. Furthermore, we evidenced that the favorable effect of the undeformed LPSO structures on the mechanical properties was emphasized only when the nano-scale β ′ precipitates were present. It is believed that the findings promote the application of magnesium alloy workpieces and help to establish customized heat treatment processes for additively manufactured materials.

The non-uniform mixture microstructure is always revealed in laser welds of Al–Si coated hot stamping steel. It is very necessary to understand aluminum-rich segregation on phase morphology evolution. At same time the uniform and high aluminum welds were tried to obtain by replacing keyhole mode with conduction mode. The uniform fusion zones contained 3–6 wt.% Al, higher than normal keyhole welds. The weld microstructure was mixed of delta ferrite, lath martensite and few discussed needle phases with different morphology. Lower Al region revealed high fraction of island martensite, when aluminum content w(Al) > 5 wt.%, the needle phase was precipitated from the matrix of delta ferrite. High resolution transmission electron microscopy confirmed it long periodical stacking ordered structure in body-cubic crystal matrix.

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 β ′ F 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. [ABSTRACT FROM AUTHOR]

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 Y3+/Ho3+ 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. [ABSTRACT FROM AUTHOR]

This paper optimizes experimental parameters on controlling corrosion performance of the Mg-rare-earth alloys with long period stacking ordered structure using response surface methodology. Different NaCl concentrations, temperatures, and varying pH values are selected as input parameters. The corrosion current density (icorr) and open circuit potential (OCP) are used as the multiple responses to evaluate the corrosion performances. The corrosion surface morphology and corrosion mechanism of the Mg alloys are also investigated by scanning electron microscopy for the optimization. Analysis of variance (ANOVA) has given the impact of individual factors and interactions on the corrosion rate. The results indicated that the three test parameters had significant impacts in controlling the corrosion behavior of Mg alloy. Moreover, the increased NaCl concentration decreased the pitting potential (Epit) of the target materials. Filiform corrosion can be detected in high pH solutions, whereas the matrix suffered from severe dissolution phenomenon in low pH solutions. High temperature aggravated the local destruction and dissolution of the protective film. The interaction of the experimental parameters showed a sizable influence on corrosion performance. [ABSTRACT FROM AUTHOR]

In this study, microstructures, phase compositions, phase morphologies of the as-cast and as-extruded Mg–8Y–2Ho–2Zn (wt%) alloys were investigated by optical microscope, scanning electron microscope, energy-dispersive spectrometer, transmission electron microscope and X-ray diffraction. The mechanical properties at room temperature and elevated temperatures were evaluated by tensile test. The primary secondary phases in the as-cast alloy are 18R-type long period stacking ordered (LPSO) phase. After extrusion, the microstructure mainly consists of considerable fine dynamically recrystallized (DRX) grains, the long strip 18R-type LPSO-phase at grain boundary along the extrusion direction and the fine lamellar 14H-type LPSO-phase in the interior of DRX grains. When the temperature is increased, the strength of the alloy is reduced, but the elongation progressively rises. The as-extruded alloy shows higher strength and larger elongation than the as-cast alloy at each test temperature. The superior mechanical properties of the as-extruded alloy are ascribed to various strengthening mechanisms induced by co-addition of Y and Ho element, e.g., solid solution strengthening, fine-grain strengthening and strengthening of fine LPSO-phase. [ABSTRACT FROM AUTHOR]

The transitional state of ordering in the long-period stacking ordered (LPSO) structure in Mg–Al–Gd alloy has been investigated by scanning transmission electron microscopy. It is found that the domains are the origin of observed transitional state. Three new patterns of clusters at this state can be fully explained by the superimposition of L1 2 type clusters located in different domains. The domains are further verified by different tilted views. Growth of preferred domains during ageing process will result in final ordered LPSO structure. Based on the idea of domains, less ordered LPSO structure in other dilute alloys can be also understood. Different cluster patterns, viewed along [10 1 ¯ 0], due to the superimposition of two domain structures specified by a relative translational vector t . Image, graphical abstract [ABSTRACT FROM AUTHOR]

Ultrahigh pressure (UP) plays a crucial role in modifying structures and properties of functional materials. The effects of UP treatment (4 GPa) on phase transition and hydrogen storage properties of Mg 12 NiY alloys has been investigated at the temperature range of 800–1300 °C. The results show that the dimension of 18R-type long period stacking ordered (LPSO) structure in the Mg 12 NiY sample after UP treatment at 1300 °C is two orders of magnitude smaller than that in the as-cast sample. The hydrogen storage capacity, kinetics and cycle properties of Mg 12 NiY alloys are concurrently improved after UP treatment. The two-step reaction process is confirmed during hydrogenation process by combining cycle testing and in-situ transmission electron microscopy (TEM) observations. The reasons for high hydrogen storage properties are mainly related to three aspects: the increased volume fraction of high angle interfaces between LPSO phase and matrix, the reduction of hydrogen diffusion distance, and the low energy barrier of hydrogen diffusion in the interior of superfine LPSO structures. • The high interface fraction is firstly achieved by ultrahigh pressure technique. • A superfine LPSO structure improves hydrogen-storage capacity by increasing sites. • The formation of fine LPSO structure reduces the onset dehydriding temperature. • The hydrogenation of LPSO phase has been clarified by in-situ TEM observations. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to study the imaging of cyanide in alloys in an electron microscope, and exploring new LPSO structures in magnesium alloys, studying defects in the LPSO structure and interface characteristics, understanding the mechanism of its formation and growth. This paper characterizes LPSO and its related structures in Mg97Zn1Y2 alloy by (scanning) transmission electron microscopy. At the same time, a new method for determining the Bravais cell in any three-dimensional lattice is proposed, That is, by finding the symmetry of the lattice to determine its Bravais lattice type and lattice parameters, Summarized the methods and specific steps for determining the position of the unknown crystal structure Bravais lattice, space group and three-dimensional atom by transmission electron microscopy systematically. Determined the Bravais lattice of the MgZnY phase, and the space group of the Mg3 (Co, Y) phase and the position of the three-dimensional atom. The experimental results show that the shape of the particles has not changed before and after adsorption. Which shows that cyanide has good mechanical stability; however, the particles become larger after adsorption, which may be due to cyanide entering the film during the adsorption process, causing the cyanide particles to aggregate, resulting in the particles becoming larger? The dislocation column of the b1 part of the periodic distribution and the b2, b3 double nuclear dislocation column form two pure tilt transition interface of 54 R / Mg and 54 R / 18 R, its tilt angle is 2.23°.Through the coordinated movement of b1 and b2, b3 dislocations can achieve the growth of 18R, and since the sum of the dislocations in one cycle is zero, all macroscopic strains are not generated. [ABSTRACT FROM AUTHOR]

The deformation mechanisms and microstructure evolution of Mg 88 Co 5 Y 7 (at.%) alloy with long period stacking ordered (LPSO) phase during compression at 373–673 K have been studied using transmission electron microscopy. The alloy consists of α-Mg matrix, interdendritic LPSO phase, and other Mg-Co-Y intermetallic compounds. Increasing temperature leads to a general decrease in strength of the Mg 88 Co 5 Y 7 alloy. A large number of {10 1 ¯ 2 } tension twins and deformation bands are activated in the matrix, whereas basal slip and deformation kink dominate in the LPSO structures during compression at 373 and 473 K. Non-basal slip in matrix and almost no dynamic recrystallization are responsible for high strength and the good ductility of sample deformed at 573 K. Recrystallization of Mg matrix occurs upon deformation at 673 K, dramatically lowering the corresponding strength. Further, the grain boundary pinning effect from LPSO phase, MgYCo 4 phase, and broken Mg 3 (Co, Y) segments is supposed to account for the relatively high strength of Mg 88 Co 5 Y 7 alloy at high temperatures. • The Mg 88 Co 5 Y 7 (at. %) alloy show high compression stress at hot compression. • The microstructural evolution and deformation mechanisms are characterized by transmission electron microscopy. • The grain boundary pinning effect is important for the relatively high strength. [ABSTRACT FROM AUTHOR]

Abstracts The role of long period stacking ordered (LPSO) structure in the corrosion behavior of Mg-Gd-Zn-Zr alloys was investigated by means of potentiodynamic polarization experiments, hydrogen evolution tests, and microstructure characterization. The corrosion rate of the Mg-Gd-Zn-Zr alloys is mainly determined by the acceleration effect of the second phases in the micro-galvanic corrosion. After T4 heat-treatment, the β-(Mg,Zn) 3 Gd eutectic phase in the as-cast Mg-Gd-Zn-Zr alloys transforms into an X phase with a 14H-LPSO structure. According to the results of scanning Kelvin probe force microscopy (SKPFM), the potential difference between LPSO phase and the α-Mg matrix is 243 mV, whereas, it is 290 mV between the eutectic phase and matrix in the as-cast alloys. The low relative potential and volume fraction of the LPSO phase reduce the acceleration degree of micro-galvanic corrosion. As a result, the corrosion resistance of Mg-Gd-Zn-Zr alloys improves significantly by the LPSO structure. Highlights • Corrosion rate of T4 treated alloys with LPSO structure is extremely lower than as-cast alloys without LPSO structure. • Micro-galvanic corrosion dominates the corrosion behavior of Mg-Gd-Zn-Zr alloys. • LPSO phase can improve the corrosion resistance of Mg-Gd-Zn-Zr alloys significantly due to its lower potential. [ABSTRACT FROM AUTHOR]

Abstract The effects of 0.5 wt% Zn addition on microstructural evolution and mechanical properties in Mg-7Y-5Sm-0.3Zr alloy were investigated. The results showed that after solution treatment, LPSO phase precipitated only at grain boundaries in Zn-containing alloy, while RE-containing cuboid compound appeared in both alloys (Zn-free and Zn-containing). A large number of Mg 5 (Y,Sm) precipitated dynamically during hot extrusion in Zn-free alloy, while there was only a few Mg 5 (Y,Sm) in Zn-containing alloy after hot extrusion. Furthermore, many basal γ′ precipitates and lamellar LPSO were observed in grain interior in extruded 0.5Zn alloy. In subsequent aging process, the two alloys exhibited the same peak-aged Vicker hardness and identical precipitates (β'). The extruded 0.5Zn alloy after peak-aging treatment exhibited a yield strength (YS) of 413 MPa and an ultimate tensile strength (UTS) of 465 MPa with an elongation to failure (EL) of 6.5%, which were 370 MPa, 413 MPa and 5.8% respectively, for the 0Zn alloy. The strength improvement after Zn addition is mainly attributed to the synergistic contribution of LPSO structure, γ′ precipitates and β' phase. Graphical abstract fx1 Highlights • Addition of Zn changed the microstructures of Mg-Y-Sm-Zr alloy at all the stages. • LPSO, γ′ and Mg 5 (Y, Sm) phases precipitated in 0.5Zn alloy while only Mg 5 (Y, Sm) in 0Zn alloy during hot extrusion. • After aging treatment, 0.5Zn alloy exhibited better strength and elongation than those of 0Zn alloy. • The synergistic strengthening effect of the LPSO, γ′ and β′ in 0.5Zn alloy is higher than that of β′ phase in 0Zn alloy. [ABSTRACT FROM AUTHOR]

Abstract This paper demonstrates that spheroidizing treatment greatly affects the microstructure and mechanical properties of the extruded Mg 94 Zn 2.5 Y 2.5 Mn 1 (at%) alloy. The extruded alloy with spheroidizing treatment exhibits higher strength but lower yield symmetry than that without spheroidizing treatment. The detailed reasons are given and analyzed. [ABSTRACT FROM AUTHOR]

It has been reported that functional and mechanical properties of Ti-Ni shape memory alloys could be degraded by hydrogen absorption. Many studies have indicated the formation of a hardened layer on the surface in hydrogen-absorbed Ti-Ni alloys. However, the underlying microstructural features and their impact on the functional and mechanical properties remain unclear. In the present study, ex situ and in situ electron microscopy characterizations of the high-hardness layer in a hydrogen-charged Ti-Ni alloy were performed for the first time. Electron diffraction and atomic resolution imaging revealed that the hardened layers have an orthorhombic structure with a four-layer stacking sequence and a martensitic-like crystallographic nature such as lattice correspondence including 12 variants of the orthorhombic phase against each orientation of the parent cubic (B2) phase. In situ and ex situ observations in an electron microscope confirmed that the orthorhombic phase undergoes athermal transformation upon cooling and stress- and/or strain-induced transformation upon deformation, which further characterizes the martensitic-like nature of this phase. The effect of hydrogen on the original phase transformation from the cubic (B2) to the monoclinic (B19′) structure is also discussed. [Display omitted] • Electron microscopy study was performed for a hardened layer induced by hydrogen charging in Ti-Ni shape memory alloys. • A novel hydrogen-induced phase transformation in Ti-Ni shape memory alloys was observed. • Hydrogen-induced phase has orthorhombic structure with a four-layer stacking sequence. • Hydrogen-induced phase has martensitic-like characteristics such as lattice correspondence and athermal transformation nature. • Hydrogen-induced phase can be induced by deformation in hydrogen-charged Ti-Ni alloys. [ABSTRACT FROM AUTHOR]

We calculate long-range interactions between Al-Y clusters along the stacking direction in Mg-Al-Y alloys with long-period stacking ordered (LPSO) structure by using first-principles calculation based on density functional theory (DFT). We find that the highly ordered LPSO structure, recently observed in Mg 75 Al 10 Y 15 alloy with 10H-type stacking sequence, is the most stable in thirteen polytypic structures. Relative energy differences in the thirteen polytypic structures suggest that significant long-range interactions between the separated stacking faults bring the highly ordered LPSO structure although short-range interactions are dominant for the stability of the alloy. In addition, we calculate and compare the long-range interactions for sixteen Mg-Metal (M)-Rare Earth (RE) alloys which have been experimentally observed to form the LPSO phase, and reveal that the long-range interactions in the alloys with M = Al are one digit larger than those with M = Zn. These results suggest that the long-range interactions between Al-RE clusters play an important role to form the highly ordered LPSO structure in Mg-Al-RE alloys. [ABSTRACT FROM AUTHOR]

Effects of semisolid squeeze casting (SSC) process on microstructures and mechanical properties of the LPSO structure reinforced Mg 97 Zn 1 Y 2 alloy were studied by optical microscopy, SEM equipped with EDS, TEM, XRD and Nanoscale indentation for the first time. The LPSO structure and α-Mg grains in the Mg 97 Zn 1 Y 2 alloy can be refined significantly by SSC. The gran size of α-Mg was decreased from 170 µm to 109 µm, and the LPSO structure could be distributed uniformly in the alloy under the combined action of ultrasonic vibration (UV, preparation for semisolid slurry) and squeeze casting (SC). The type of LPSO structure was remained as the initial 18R-type structure after SSC, but a strip YZn 3 phase precipitated out inner the block LPSO structure, which has never been found in the LPSO-containing Mg 97 Zn 1 Y 2 alloy. With the increase of squeeze pressure, the amount of YZn 3 phase was increased and the tensile strength and elongation of the alloy were increased at first and then maintained nearly constant. When the pressure was 100 MPa, the optimum mechanical properties were obtained. The yield strength, ultimate tensile strength and elongation were 141 MPa, 234 MPa and 11.6%, which were enhanced by 28.2%, 11.1% and 45.1%, respectively, compared with those of the alloy without SSC. [ABSTRACT FROM AUTHOR]

Effect of ultrasonic treatment (UT) on the microstructures and mechanical properties of Mg 99.0−x Ni x Y 1.0 (x = 0.5, 1.0, 1.5, at.%) alloys was investigated. With the increase of Ni content, the amount of eutectic structure, consisting of Mg, Mg 2 Ni phase and LPSO structure, increased while the mechanical properties decreased in as-cast alloys. After introducing UT to the melt, the secondary phases in these alloys were refined significantly and distributed uniformly, especially for long period stacking ordered (LPSO) structure. In Mg 98.5 Ni 0.5 Y 1.0 alloy, the formation of MgNi 4 Y phase, which was distributed adjacent to the LPSO structure at the grain boundaries, was stimulated by UT. In Mg 97.5 Ni 1.5 Y 1.0 alloy with UT, not only the width of grain boundaries were reduced, but also both the width of LPSO structure and that of eutectic structure were reduced. The optimal mechanical properties were obtained in Mg 98.5 Ni 0.5 Y 1.0 alloy, which exhibited 206 MPa in ultimate tensile stress and 7.96% in elongation, respectively. After UT, these properties were enhanced to 231 MPa and 14.56%, respectively. The other alloys’ mechanical properties were also enhanced significantly by UT. [ABSTRACT FROM AUTHOR]

The evolution of phase constituent of Mg 99.0-x Ni x Y 1.0 (x = 0.2, 0.5, 0.8, 1.0, 1.5, at.%) alloys was investigated by OM, SEM/EDS, TEM and XRD. Apart from the α-Mg phase, the secondary phases in these ternary alloys mainly consist of LPSO (long period stacking ordered) structure, Mg 24 Y 5 phase or Mg 2 Ni phase, which depend on the atomic ratio of Ni and Y. The amount of LPSO structure in the alloys increases firstly and then decreases with the increase of Ni level, but the change in the amount of LPSO structure is not proportional to that of Ni level. In the as-cast alloys containing excessive Ni, there exist two forms of LPSO structures with different characteristics of bulk and lamellar shapes, respectively. During solidification process, part of the bulk 18R-type LPSO structure formed firstly is transformed to the lamellar 14H-type LPSO structure. The tensile properties of different alloys are tested and the changing trend of strength is similar to that of the amount of LPSO structure. When the Ni content is 0.5 at.%, the ultimate tensile strength, yield stress and elongation are 208 MPa, 93 MPa and 8.0%, respectively, which are the optimal properties among these alloys. [ABSTRACT FROM AUTHOR]

Ternary Mg 94 Ni 2 Y 4 (at%) alloy was prepared by conventional gravity casting or direct squeezing casting and then subjected to T6 heat treatment. The influence of squeeze casting pressure and heat treatment on microstructure evolution and mechanical properties of Mg 94 Ni 2 Y 4 alloy containing long period stacking ordered (LPSO) structure was studied. The results show that gravity cast Mg 94 Ni 2 Y 4 alloy consisted of primary α-Mg, 18 R LPSO and eutectic structure containing α-Mg, LPSO and Mg 2 Ni phases. The application of pressure cannot change phase constituents and the type of LPSO phase. With the increase of squeeze casting pressure, microstructure was refined, the volume fraction of LPSO was decreased while that of eutectic structure was increased. Under the highest pressure of 320 MPa, kinking of LPSO phase was induced and the best mechanical properties were obtained, with the ultimate tensile strength of 250 MPa, the yield strength of 140 MPa and the elongation of 8%. They were improved by 20.2%, 22.8% and 45.5%, respectively, than those of gravity cast Mg 94 Ni 2 Y 4 . After heat treatment, bulk-shaped LPSO phase transformed into rod-shaped one, but no lamellar 14 H LPSO was precipitated in the matrix. Besides, the application of squeeze casting pressure prevented LPSO from delaminating during heat treatment. [ABSTRACT FROM AUTHOR]

Dynamic precipitation in an Mg-Gd-Zn-Zr alloy during hot compression at 350 ° C and 450 ° C and at strain rates of 0.001-0.1 s −1 is investigated. The deformation conditions were designed such as to promote twinning or slip of dislocations. The results indicated that both twinned and non-twinned grains were present at 350 ° C , with the extent being a function of the applied strain rate. The formation and propagation of { 10 1 ̅ 2 } 〈 1 ̅ 011 〉 primary tensile twins within the twinned grains was identified as being at root of the twin-assisted precipitation phenomenon. The precipitates had spherical-morphology in the twinned grains whereas parallel arrays of rod-shaped precipitates formed in the non-twinned grains due to dynamic precipitation on the slip bands and the stacking faults. Twin-assisted precipitates were suppressed by increases in the strain rate at 350 ° C because of the shorter time required for solute element diffusion towards the nucleated precipitates at that temperature. Increasing the deformation temperature to 450 ° C changed the dominant deformation mechanism from twinning to slip of dislocations, resulting in the formation of rod-shaped precipitates similar to those observed in non-twinned grains at 350 ° C . An EBSD analysis was used to study the dominant deformation mechanism as a function of the Schmid factor, and the twinning deformation mechanism was found to be predominant in the grains having a lower Schmid factor. [ABSTRACT FROM AUTHOR]

The effects of different manganese (Mn) content of 0–0.8 wt% on the microstructures and room-temperature tensile properties of Mg-15Gd-1Zn (wt%) alloys have been investigated. The results reveal that the solution-treated microstructures of the alloys are mainly composed of α-Mg matrix, the secondary eutectic phase [(Mg, Zn) 3 Gd], X phase with 14H long period stacking ordered (LPSO) structure. However, small α-Mn particles precipitate within the α-Mg matrix with the increase of Mn content from 0 to 0.8 wt% during the solution treatment at 773 K. It shows that the α-Mn particles can inhibit grain growth, which can result in grain refinement. Moreover, Mn addition can promote the formation of 14H-LPSO structure in the solution-treated alloy by increasing stacking fault probability of α-Mg matrix. In this study, the influence mechanisms of Mn addition were suggested according to observation of microstructure and calculation of stacking fault probability. Besides, the room-temperature tensile properties can be improved as Mn addition content increases. In conclusion, the solution-treated Mg-15Gd-1Zn-0.8Mn (wt%) alloy exhibits an ultimate tensile strength of 275 MPa, a yield strength of 154 MPa and an elongation of 7.4%. [ABSTRACT FROM AUTHOR]

The microstructural and chemical features of deformation-induced interfaces are one of key issues in engineering materials because they determine plastic deformation behavior and thus affect mechanical properties of the materials. Using atomic-resolution high-angle annular dark-field scanning transmission electron microscopy, we characterized deformation-induced low angle kink boundaries (LAKBs) in long period stacking ordered (LPSO) structures in an extruded Mg–2.3Zn–6.6Y–0.56Zr (wt%) alloy. We clarified that the LAKB in LPSO phase consists of an array of dislocations, while the LAKB in Mg interlayers sandwiched between LPSO phases is composed of an array of dissociated and/or dislocations. Correspondingly, the former and the latter LAKBs are depleted and segregated with Zn/Y/Zr elements, respectively. I 2 stacking fault (SF) is meanwhile generated in Mg layers, and its energy is evaluated approximately 0.1–1.6 mJ m −2 . Deformation-induced LAKBs, the resultant redistribution of solute elements, and precipitated I 2 SFs, are proposed to be responsible for the high strength of extruded Mg alloys containing LPSO structures. [ABSTRACT FROM AUTHOR]

The effects of Zr and Mn additions to Mg-15Gd-1Zn alloy on the formation of long period stacking ordered (LPSO) structure under solution-treated (T4-treated) condition and the dynamic recrystallization (DRX) behavior alloy during extrusion have been investigated. The underneath mechanisms have also been proposed. The results indicate that Zr hinders the formation of LPSO structure by consuming Zn, while the Mn promotes it. On the other hand, Zr suppresses the grain growth by introducing Mg(Zn, Zr) precipitates during the DRX, while Mn effectively hinders the recrystallization process due to increasing LPSO structure. As the LPSO structure has dissolved in recrystallized grains, Mn addition affects little on suppressing grain growth in the grain growth step. Zr addition improves the mechanical properties of both the casting and extruded Mg-15Gd-1Zn alloys by refining grains and introducing Mg(Zn, Zr) precipitates. By contrast, the Mn addition is beneficial for the tensile properties of casting Mg-15Gd-1Zn alloy because of the LPSO structure with high ductility but harmful for the properties of the extruded alloys due to the coarse un-recrystallized grains. [ABSTRACT FROM AUTHOR]

An effect of alloying element content on mechanical properties and precipitate formation in Mg–RE alloys was studied for Mg–8Gd–4Y–1Zn–0.4Zr (wt%) and Mg–10Gd–5Y–1.8Zn–0.4Zr (wt%). It is shown that small variations in the alloying element concentration can be used to manipulate the alloy microstructure and precipitate formation towards eliminating the asymmetry (tension/compression) and anisotropy of yield stress. [ABSTRACT FROM AUTHOR]

Microstructures and mechanical properties of the Mg–5Gd–2Y–2Zn–0·5Zr alloy (wt-%) in the as cast and solution treated conditions have been investigated using optical microscopy, scanning electron microscope, X-ray diffraction, electron probe X-ray microanalyser and transmission electron microscope. The results show that after adding the element Y, the as cast Mg–5Gd–2Y–2Zn–0·5Zr alloy forms the Mg12Zn(Y, Gd) phase with 18R-long period stacking ordered (LPSO) structure, and the Gd atoms in the phase can slow down the transition process of 18R to 14H in high temperature treatment. LPSO phase in the grain boundary can ensure the strength of the alloy at room and elevated temperature through effectively preventing grain boundary sliding. At 250°C, the alloy has shown the similar ultimate tensile strength and tensile yield strength under room temperature, 241 and 130 MPa respectively, and both with excellent elongation. [ABSTRACT FROM AUTHOR]

Abstract: Microstructures and mechanical properties of the Mg–8Gd–5Y–2Zn–0.5Zr–xNd (x=0, 0.5, 1 and 2wt%) alloys in the as-cast, solid solution and as-aged conditions have been investigated. The experimental results reveal that Nd can impede the formation of LPSO phases during the solidification process and suppress the transformation of Mg5(Gd, Y, Zn) to LPSO phase at high temperature heat-treatment. Addition of 2% Nd leads to the disappearance of the LPSO phases in the as-cast alloy. After heat-treatment at 520°C for 16h, the uniform LPSO phases, which are transformed from Mg5(Gd, Y, Zn) phases and refined by the Nd, result in a higher solid solubility and greater aging effect. With increasing Nd content, the residual Nd-rich Mg5(Gd, Y, Zn) phases in the solid solution alloys gradually increase and produce an adverse effect on the mechanical properties. The alloy with the addition of 0.5% Nd shows the optimal mechanical properties in the as-aged alloys, and its ultimate tensile strength, yield strength and elongation are 308MPa, 252MPa and 5.3%, respectively. [Copyright &y& Elsevier]

Formation of profuse I 2 -type solute-enriched stacking faults (SESFs) in fine grains and transformation of SESFs to 18R-long period stacking ordered (LPSO) phase are attained in Mg-Er-Zn-Zr alloys. It is first confirmed by scanning Kelvin probe force microscopy (SKPFM) that SESFs with nanostructure act as weak anodes and the 18R-LPSO phase serves as cathode, which are verified by the theoretical calculated electronic work function and observation of scanning transmission electron microscope. Furthermore, the preferential corrosion of uniformly dispersed SESFs is demonstrated to be conducive to forming effective passivation film. It provides a new insight for developing high-strength and anti-corrosion Mg alloys. [Display omitted] • The PD of weak anode SFs is measured at the nanoscale by SKPFM for the first time. • The reliability of SKPFM results is verified by the theoretical calculated WF. • Preferential corrosion of SFs is demonstrated using STEM-EDS for the first time. • Preferential corrosion of solute-enriched SFs is conducive to formation of passivation film. • A new insight for designing high-strength and anti-corrosion Mg alloys is proposed. [ABSTRACT FROM AUTHOR]

Abstract: Microstructures and tensile mechanical properties of Mg–10Gd–6Y–2Zn–0.6Zr alloy were systematically studied. Four phases were found in the as-cast specimen: α-Mg, Mg3(GdYZn), Mg12(GdY)Zn and Mg24(GdYZn)5. The long-period stacking order (LPSO) structure is found, which is the phase of Mg12(GdY)Zn. The LPSO structure has two existing forms: lamellar structure in the inner grains and block-like structure at grain boundaries. 6H-type LPSO structure with a stacking sequence of ABCBCB′ is defined in homogenized specimen, where A and B′ layers are significantly enriched by Gd, Y and Zn. The ageing hardening behavior of as-extruded specimens at 200°C has been investigated. The ultimate tensile strengths of the as-extruded and peak-aged alloys are 360MPa and 432MPa, and the elongations are 18% and 5% respectively. The effective strengthening models have been considered to predict the strength. The results suggested that the sub-micron metastable β′ phase was the main strengthening factor of the peak-aged alloy. [Copyright &y& Elsevier]

This work investigates the influence of heat treatment on the discharge and electrochemical properties of Mg 95.28 Gd 3.72 Zn 1.00 alloy as an anode for the Mg-air battery. The heat treatment removes the (Mg, Zn) 3 Gd eutectic phase and produces the long period stacking ordered (LPSO) structure. The T6 heat treated Mg 95.28 Gd 3.72 Zn 1.00 alloy shows good discharge performance, including the high and stable cell voltage, high discharge capacity and anodic efficiency, and good specific energy output. The T6 treated Mg 95.28 Gd 3.72 Zn 1.00 anode has (i) the best discharge capacity and anodic efficiency of 1329 mAh g−1 and 59.9%, respectively, at 40 mA cm−2, which is 21.4% and 23.3% higher than the as-cast anode, and (ii) the highest specific energy of 1504 mW h g−1, which is 46.4% higher than the as-cast anode. These improved discharge properties are attributed to the LPSO structure, which provides moderate and even stimulation for the dissolution of the α-Mg and helps to exfoliate the discharge products. In contrast, the (Mg, Zn) 3 Gd phase produces a passive film, leading to the voltage drop. [ABSTRACT FROM AUTHOR]

In this work, a high-strength Mg-12Gd-2Y-1Zn-Mn alloy with an ultimate tensile strength of 509 MPa and a fracture elongation of 5% at room temperature tensile was developed by using hot extrusion and subsequent ageing. Progressive observations of crack initiation and early propagation of the as-extruded and peak-aged alloys were conducted during the tensile process. The results show that the activation of fine-grain boundary sliding and multiple non-basal slip systems in coarse unDRXed grains is the dominant deformation mechanisms of the as-extruded alloy, which eventually lead to crack initiation and propagation. After peak-ageing treatment, dense β′ precipitates are formed within the bimodal grains, resulting in changes in deformation mechanisms and crack initiation and propagation behaviours during the tensile process. The grain boundary sliding is suppressed and the grain boundary strength is enhanced due to the precipitation of β ′ phases on the hindering effect of dislocation mobility. Therefore, no-basal slip and cross-slip in coarse unDRXed grains become the major roles in crack propagation. The intra-grain LPSO (long-period stacking ordered) phase can suppress crack initiation and propagation in both as-extruded and peak-aged alloys. For the thick LPSO phase at the grain boundaries of the as-extruded alloy, it inhibits crack propagation to some extent. While the thick-deformed LPSO phase at the grain boundaries of the extruded-peak-aged alloy more easily causes crack initiation with increasing tensile strains. Unlabelled Image • High-strength Mg-RE alloy with UTS of 509 MPa and Elongation of 5% was developed. • Crack initiation and propagation of as-extruded and aged alloys were investigated. • β ′ nanoprecipitates suppressed grain-boundary sliding during plastic deformation. • Thick-deformed LPSO phases caused crack initiation in the peak-aged Mg-RE alloy. [ABSTRACT FROM AUTHOR]

• A part of the formation mechanism of LPSO structures was studied by DFT. • Energy profile for the HCP-LPSO transition was obtained. • Y-concentrated layer in the HCP structure can easily form a stacking fault. • We considered that chemical pressure to Y layer is important for LPSO formation. To study the formation mechanism of the long-period stacking ordered (LPSO) structures, the reaction pathways of solid–solid transformations from a hexagonal close-packed (HCP) structure to LPSO structures in Mg-Y-Zn alloys were calculated using the generalized solid-state nudged elastic band method. The energy increases along the transition from HCP to 18R, and the peak positions represent the activation energy for the transition. Y substitution hardly changes the activation energy but makes the 18R-type LPSO structure more stable than HCP. In contrast, Zn or Y + Zn substitution results in higher activation energy and makes the 18R-type LPSO structure less stable than HCP. The calculated results for 14H and 24R LPSO structures also show similar activation energy and LPSO stability to the HCP-18R transition. Therefore, Y substitution plays an important role in stabilizing the stacking faults in LPSO systems. For the microscopic mechanism, the volume dependence of the total energy in pure FCC and HCP Y were examined, and the result suggests that FCC-Y is stable than HCP-Y under pressure. Therefore, the effect of substitution of Y in HCP Mg can be explained by the characteristics of Y under the chemical pressure exerted by the small size of Mg lattice. [ABSTRACT FROM AUTHOR]