14H-type Long Period Stacking Ordered Research Articles

TiO2@C catalyzed hydrogen storage performance of Mg-Ni-Y alloy with LPSO and ternary eutectic structure

A designed Mg88.7Ni6.3Y5 hydrogen storage alloy containing 14H type LPSO (long-period stacking ordered) and ternary eutectic structure was prepared by regulating the alloy composition and casting. The hydrogen storage performance of the alloy was improved by adding nano-flower-like TiO2@C catalyst. The decomposition of the LPSO structure during hydrogenation led to the formation of plenty of nanocrystals which provided abundant interphase boundaries and activation sites. The nanoscale TiO2@C catalyst was uniformly dispersed on the surface of alloy particles, and the "hydrogen overflow'' effect of TiO2@C accelerated the dissociation and diffusion of hydrogen on the surface of the alloy particles. As a result, the in-situ endogenous nanocrystals of the LPSO structure decomposition and the externally added flower-like TiO2@C catalyst uniformly dispersed on the surface of the nanoparticles played a synergistic catalytic role in improving the hydrogen storage performance of the Mg-based alloy. With the addition of the TiO2@C catalyst, the beginning hydrogen desorption temperature was reduced to 200 °C. Furthermore, the saturated hydrogen absorption capacity of the sample was 5.32 wt.%, and it reached 4.25 wt.% H2 in 1 min at 200 °C and 30 bar.

Comparative study of solid-solution treatment and hot-extrusion on hydrogen storage performance for Mg96Y2Zn2 alloy: The nonnegligible role of elements distribution

The utilization of Mg alloys for solid-state hydrogen storage has attracted considerable attention, and the Mg-based alloys with long period stacking ordered (LPSO) structure show optimized hydrogen storage properties. In this study, solid-solution treatment and hot-extrusion are utilized to modify the hydrogen storage properties of Mg96Y2Zn2 alloy. The results indicate 14H-type LPSO structure is precipitated after solid-solution treatment. Dynamic recrystallization occurs during deformation process with the refined grain size of 2 μm approximately and the dynamic precipitation of Mg3Y2Zn3 particles leads to the aggregation of Y and Zn atoms. Hot-extrusion can enhance the pulverization resistance of Mg96Y2Zn2 alloy by increasing the lattice parameter ratio of c to a. The reduced hydrogen capacity of hot-extruded Mg96Y2Zn2 alloy is attributed to the increased pulverization resistance and decreased lattice parameters. The apparent activation energies for hydrogenation and dehydrogenation of solid-solution treated alloy are 6.86 and 13.65 kJ·mol−1 less than those of hot-extruded alloy, respectively. The dehydrogenation temperature of solid-solution treated alloy hydride is 18 °C lower than that of hot-extruded alloy. More abundant and uniform distribution of Y atoms can generate accordingly more abundant and uniform YH2 nanoscale particles that embed in the α-Mg matrix, which contributes to faster de/hydrogenation kinetics.

Enhanced hydrogen absorption kinetics by introducing fine eutectic and long-period stacking ordered structure in ternary eutectic Mg–Ni–Y alloy

We employ a ternary eutectic Mg76.87Ni12.78Y10.35 alloy to achieve a fine eutectic structure with long-period stacking ordered (LPSO) structure, and investigate the corresponding hydrogen storage behavior. The as-cast Mg76.87Ni12.78Y10.35 alloy is composed of Mg15NiY, Mg2Ni, MgNi4Y and Mg phases. A high density of stacking faults and 14H-type LPSO structures form in the Mg15NiY phase. This fine ternary eutectic structure has shown significant improvement in promotion of the activation and following hydrogen absorption kinetics compared with as-cast non-eutectic structure. Pressure-Composition-Temperature (PCT) curves demonstrate two major pressure platforms due to the presence of Mg15NiY and Mg2Ni in our Mg–Ni–Y alloys. Nanosized Mg2NiHx, MgH2 and YHx are insitu generated from the decomposition of LPSO structure during hydrogenation. The fine ternary eutectic structure and dispersed nanocatalysts from decomposition of LPSO structure synergistically facilitate the activation and hydrogen absorption kinetics of ternary eutectic Mg–Ni–Y alloy.

Effects of Trace Al Addition on the Microstructure, Hot Deformation Behavior and Mechanical Properties of High-Strength Mg-Gd-Y-Zn Magnesium Alloy

The effects of 0.2 wt pct Al addition on the microstructure, hot deformation behavior and mechanical properties of Mg-8Gd-4Y-1Zn alloy were investigated. The results showed that the Al addition obviously refined the grain structure and increased the 18R long period stacking ordered (LPSO) phase. In addition, a little Al2(Y, Gd) phase formed in the as-cast Al-doped alloy. After solid solution treatment, the Al2(Y, Gd) phase and Mg5(Gd, Y, Zn) eutectic phase were dissolved, and both 18R- and 14H-type LPSO phases could be observed at the grain boundaries of the Al-doped alloy. For the Al-free alloy, the 18R LPSO phase disappeared, and lamellar 14H-LPSO was generated in the grain interior. The calculated results showed that the Al-doped alloy had higher hot deformation activation energy and better hot workability. Under the same extrusion condition, the Al-doped alloy showed faster dynamic recrystallization but similar fully recrystallized grain size as the Al-free alloy. The average tensile strength, yield strength and elongation of the extruded alloy were concurrently improved from 329 MPa, 268 MPa and 6.0 pct to 365 MPa, 281 MPa and 11.1 pct by trace Al addition.

Effect of Er contents on the microstructure of long period stacking ordered phase and the corresponding mechanical properties in Mg-Dy-Er-Zn alloys

The influence of Er contents on the mechanical properties and microstructure of the long period stacking ordered (LPSO) phases in Mg96.6Zn0.91Dy2.17Er0.32 and Mg96.23Zn0.88Dy2.21Er0.68 alloys have been studied. The results showed that the LPSO structures in the two alloys are 18R-type in the as-cast state. And co-existence of 18R-type and 14H-type LPSO phases after heat-treated has been observed in the two alloys. After hot extrusion, the ultimate tensile strength (UTS), yield strength (YS) and elongation (EL) of the Mg96.6Zn0.91Dy2.17Er0.32 alloy have reached the value of 303.42 MPa, 172.76 MPa and 18.31%, respectively. The UTS, YS and EL of the as-extruded Mg96.23Zn0.88Dy2.21Er0.68 alloy are 334.63 MPa, 214.85 MPa and 19.59%, respectively.

Insignificant elastic-modulus mismatch and stress partitioning in two-phase Mg–Zn–Y alloys comprised of [formula omitted]-Mg and long-period stacking ordered phases

Elastic-modulus mismatch and the resultant stress partitioning in two-phase Mg–Zn–Y alloys comprised of α-Mg and long-period stacking ordered (LPSO) phases were studied. Two-phase polycrystals containing anisotropically oriented 18R- or 14H-type LPSO phase and single-phase polycrystals consisting of α-Mg or 18R-type LPSO phase were prepared by extrusion and directional solidification processes and their complete sets of anisotropic elastic properties were measured using resonant ultrasound spectroscopy. Elastic properties of the single and two-phase alloys were analyzed using Eshelby's inclusion theory, effective-medium approximation, and inverse Voigt-Reuss-Hill approximation, in which the crystallographic textures and microstructures formed by the preparation processes were taken into account. The analyses revealed that the elastic properties of 18R-LPSO phase were not unique and they depended on the solute Zn and Y atom concentrations. Additionally, the elastic modulus of 18R-LPSO phase embedded in the two-phase alloy was lower than that of the alloy consisting of single-phase 18R-LPSO phase. The analysis using first-principles calculations based on density functional theory indicated that the low elastic modulus was caused by low density and low stability of short-range ordered solute atom clusters embedded in the LPSO phase of the two-phase alloy. Because of low elastic modulus in the LPSO phase, the elastic mismatch and resultant elastic interaction between the α-Mg and LPSO phases were very small. As a result, the formation of LPSO phase had little effect on the stress partitioning to the LPSO phase, which was independent of the LPSO-phase morphology.

Effects of Ni levels on microstructure and mechanical properties of Mg-Ni-Y alloy reinforced with LPSO structure

The evolution of phase constituent of Mg99.0-xNixY1.0 (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, Mg24Y5 phase or Mg2Ni 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.

Microstructure evolution and mechanical properties of a large-sized ingot of Mg−9Gd−3Y−1.5Zn−0.5Zr (wt%) alloy after a lower-temperature homogenization treatment

In this paper, a large-sized ingot of Mg−9Gd−3Y−1.5Zn−0.5Zr (wt%) alloy with a diameter of 600 mm was successfully prepared by the semi-continuous casting method. The alloy was subsequently annealed at a relatively low temperature of 430°C for 12 h as a homogenization treatment. The microstructure and room-temperature mechanical properties of the alloy were investigated systematically. The results show that the as-cast alloy contained a mass of discontinuous lamellar-shaped 18R long-period stacking ordered (LPSO) phases with a composition of Mg10ZnY and an α-Mg matrix, along with net-shaped Mg5(Y,Gd) eutectic compounds at the grain boundaries. Most of the eutectic compounds dissolved after the homogenization treatment. Moreover, the amount and dimensions of the lamellar-shaped LPSO phase obviously increased after the homogenization treatment. The structure of the phase transformed into 14H-type LPSO with composition Mg12Zn(Y,Gd). The mechanical properties of the heat-treated large-sized alloy ingot are uniform. The ultimate tensile strength (UTS) and tensile yield strength (TYS) of the alloy reached 207.2 MPa and 134.8 MPa, respectively, and the elongation was 3.4%. The high performances of the large-sized alloy ingot after the homogenization treatment is attributed to the strengthening of the α-Mg solid solution and to the plentiful LPSO phase distributed over the α-Mg matrix.

Formation of lamellar phase with 18R-type LPSO structure in an as-cast Mg96Gd3Zn1(at%) alloy

A lamellar Mg10GdZn phase was observed near the secondary eutectic β-phase ((Mg,Zn)3Gd) in an as-cast Mg–Gd–Zn alloy. It shows that the Mg10GdZn phase exhibits a 18R-type long period stacking ordered (LPSO) structure, while the lamellae within the α-Mg matrix exhibits a 14H-type LPSO structure. Moreover, it reveals that the hardness and elastic modulus of Mg10GdZn phase are much higher than those of the α-Mg by nano-mechanical measurements.

Microstructure and tribological behavior of Mg–Gd–Zn–Zr alloy with LPSO structure

A Mg–14.28Gd–2.44Zn–0.54Zr (mass fraction, %) alloy was prepared by conventional ingot metallurgy (I/M). The microstructure differences in as-cast and solution-treated alloys were investigated. Sliding tribological behaviors of the as-cast and solution-treated alloys were investigated under oil lubricant condition by pin-on-disc configuration. The wear loss and friction coefficients were measured at a load of 40 N and sliding speeds of 30–300 mm/s with a sliding distance of 5000 m at room temperature. The results show that the as-cast alloy is mainly composed of α-Mg solid solution, the lamellar 14H-type long period stacking ordered (LPSO) structure within matrix, and β-[(Mg,Zn)3Gd] phase. However, most of the β-phase transforms to X-phase with 14H-type LPSO structure after solution heat treatment at 773 K for 35 h (T4). The solution-treated alloy presents low wear-resistance, because the hard β-phase is converted into thermally-stable, ductile and soft X-Mg12GdZn phase with LPSO structure in the alloy.

The crystal structure of the LPSO phase of the 14H-type in the Mg–Al–Gd alloy system

The crystal structure of a long-period stacking-ordered (LPSO) phase of the 14H-type formed in a Mg–Al–Gd alloy as a local small part in the intergrowth structure together with that of the 18R-type (the majority) has been investigated by scanning transmission and transmission electron microscopy. The LPSO phase of the 14H-type in the Mg–Al–Gd system is found to form by stacking structural blocks, each of which consists of seven close-packed atomic planes. In each of the structural blocks, a long-range ordering occurs for the constituent Mg, Al and Gd atoms with the enrichment of Gd atoms in the four consecutive atomic planes. The in-plane long-range ordering in the four consecutive atomic planes occurs so as to form Al6Gd8 clusters in a periodic manner. This is exactly the same as what is observed in the LPSO phase of the 18R-type. The crystal structure of the 14H-type LPSO phase can thus be described to form simply by adding a Mg layer to the crystal structure of the 18R-type LPSO phase so as to form triple (three consecutive) Mg layers to sandwich the Gd-enriched quadruple layers. The ideal chemical formula of the structural block is Mg35Al3Gd4 (Mg – 7.1 at.%Al – 9.5 at.%Gd). The crystal structure of the LPSO phase can thus be crystallographically described as one of the order–disorder (OD) structures, and the space group of either P6322 or R3¯c, is assigned when the simplest stacking of structural blocks is assumed.

Biocorrosion behavior and cytotoxicity of a Mg–Gd–Zn–Zr alloy with long period stacking ordered structure

Microstructure, biocorrosion behavior and cytotoxicity of an as-extruded Mg–11.3Gd–2.5Zn–0.7Zr (wt%) alloy were studied, in order to develop new biodegradable magnesium alloy. The microstructure of the as-extruded alloy mainly consists of the refinement grains, a lamellar X phase with the 14H-type long period stacking ordered (LPSO) structure in grain boundaries and the 14H-type LPSO structure within initial grains and refinement grains, and a small amount of β phase. The as-extruded alloy has high strength at room temperature. The corrosion rate in Hanks' solution is only 0.17mm/year and the corrosion mode of the alloy is uniform corrosion, which is much better than that of as-extruded Mg–10.2Gd–3.3Y–0.6Zr (wt%) alloy. Moreover, the cell toxicity grade of the Mg–11.3Gd–2.5Zn–0.7Zr alloy is Grade 1, indicating only slightly having cytotoxicity. The desired results indicate the Mg–Gd–Zn–Zr alloy with LPSO structure will become one of novel promising high-strength biodegradable magnesium alloys.

Control of the Nugget Nanostructure of Mg<sub>96</sub>Zn<sub>2</sub>Y<sub>2</sub> Welded by Resistance Spot Welding

Recently, Mg96Zn2Y2 alloy was widely studied due to its excellent mechanical properties. These properties were considered to relate to the nanostructure of the alloy such as α-Mg matrix and bent long period stacking ordered (LPSO) structure. In this study, as the first step to the mechanical property control, we applied various spot welding methods to the Mg alloy to control the structure of the joints. The specimen used was an extruded Mg96Zn2Y2 alloy. The sheets were cut from this alloy rod for resistance spot welding. Three kinds of spot welding methods were applied to join the Mg alloy, such as spot welding with cover plates, long-time spot welding, and water-cooled spot welding. In the base metal, secondary phases extended along extruded direction at approximately 30μm intervals were produced in the α-Mg matrix. The secondary phase contained 14H-type LPSO structure. In the nugget of the joint welded with cover plates, a secondary phase was produced like a net in the α-Mg grain which was approximately 30μm in diameter. Both 14H-type and 18R-type LPSO were observed in the secondary phase. In the nugget produced by the long-time spot welding, a secondary phase tended to segregate to the grain boundary of the α-Mg grains. The structure of LPSO was 18R-type. In the water-cooled welding, strong directionality of the secondary phase texture was observed around the edge of the nugget. These results showed that the nugget nanostructure of Mg96Zn2Y2 dramatically varied depending on the above adopted methods of spot welding.

LPSO STRUCTURE AND AGING PHASES IN Mg--Gd--Zn--Zr ALLOY

At present,long period stacking ordered(LPSO) structures in Mg-RE-Zn(RE=Y, Dy,Ho,Er,Gd,Tb,Tm) alloys have been focused on.According to some reports,Mg-Gd-Zn alloys are classified as typeⅡ,i.e.,there are no LPSO structures in as-cast alloys,but LPSO structures appear after heat treatment.To further study the formation of LPSO structure in Mg-Gd-Zn(Zr) alloys,a Mg_(96.32)Gd_(2.50)Zn_(1.00)Zr_(0.18) alloy was prepared by ingot metallurgy(I/M) in this work.Based on OM,SEM and TEM observations,the as-cast microstructure of Mg-Gd-Zn-Zr alloy consists ofα-Mg solid solution,lamellar 14H-type LPSO structure withinα-Mg grains and dendritic eutectic structure at grain boundaries,in which the eutectic phase is theβ-phase((Mg,Zn)_3Gd)),thus,Mg-Gd-Zn(Zr) alloys should be attributed to typeⅠ,i.e.,there are LPSO structures in as-cast alloys.During solid solution treatment at 500℃for 35 h,a solid transformation,dendriticβ-phase with fcc structure→lamellar X-phase with 14H structure,was observed at grain boundaries.During subsequent peak-aging treatment at 200℃for 128 h,ellipsoidalβ' and rhombusβ_1 phases precipitated withinα-Mg grains. Tensile tests at room temperature and Vickers hardness tests show that the alloy solution-treated and aging-treated has higher mechanical properties,i.e.,σ_b=290.7 MPa,σ_s=162.5 MPa,δ=10.35%and 108.0 HV.The improvement of mechanical properties are attributed to the composite strengthening- and-toughening effect of the 14H-type LPSO structures,and theβ' andβ_1 aging phases.

Formation of a Novel X Phase in Mg–Gd–Zn–Zr Alloy

The coherent fine-lamellae consisting of the 2H-Mg and the 14H-type long period stacking ordered (LPSO) structure within α'-Mg matrix have been observed in an as-cast Mg–Gd–Zn–Zr alloy. During subsequent solid solution heat treatment at 773 K, in addition to the lamellae within matrix, a novel lamellar X phase [Mg–(8.37±1.0)Zn–(11.32±1.0)Gd] with the 14H-type LPSO structure was transformed from the dendritical β phase. The 14H-type LPSO structure existing in Mg–Gd–Zn–Zr alloys derives from two variant ways: formation of the 14H-type LPSO structure comes from two variant means: i.e., the formation within matrix and the phase transformation from the β phase to the X phase in grain boundaries.

Formation of 14H-type long period stacking ordered structure in the as-cast and solid solution treated Mg–Gd–Zn–Zr alloys

The coherent fine lamellae consisting of the 2H-Mg and the 14H-type long period stacking ordered (LPSO) structure within α′-Mg matrix have been first observed in an as-cast Mg96.32Gd2.5Zn1Zr0.18 alloy. During subsequent solid solution heat treatment at 698–813 K, in addition to the lamellae within matrix, a novel lamellar X phase (Mg–8.37±1.0Zn–11.32±1.0Gd, at.%) with the 14H-type LPSO structure was transformed from the dendritical β phase, and a corresponding time–temperature–transformation (TTT) diagram was established. The 14H-type LPSO structure existing in Mg–Gd–Zn–Zr alloys derives from two variant means: the formation of LPSO structure within α′-Mg matrix and the transformation of the dendritical β phase to a lamellar X phase with the LPSO structure. The alloy solid solution treated at 773 K for 35 h exhibits higher tensile strength and better elongation than the nonheated alloy because of the lamellar X phase with the 14H-type LPSO structure and the 14H-type LPSO structure within matrix.

Formation of a lamellar 14H-type long period stacking ordered structure in an as-cast Mg–Gd–Zn–Zr alloy

A novel as-cast Mg96.82Gd2Zn1Zr0.18 alloy was produced by conventional ingot metallurgy. By scanning electron microscope and transmission electron microscope observations, its as-cast microstructure is mainly composed of the α′-Mg solid solution, the coherent fine-lamellae and the eutectic. The β-phase ((Mg,Zn)3Gd) as the second phase in eutectic has a face-center cubic structure. While, the coherent lamellae consist of the 2H–Mg and the 14H-type long period stacking ordered (LPSO) structure. At present, the lamellar 14H-type LPSO structure has first been observed within α′-Mg matrix in the as-cast Mg–Gd–Zn–Zr alloys. It can be concluded that the lamellar 14H-type LPSO structure within α′-Mg matrix is a special structure obviously different from α-Mg matrix (2H-type structure) in structure, composition, and formation condition.

The microstructure evolution with lamellar 14H-type LPSO structure in an Mg 96.5Gd 2.5Zn 1 alloy during solid solution heat treatment at 773 K

In an as-cast Mg 96.5Gd 2.5Zn 1 alloy, by optical microscope (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations, the as-cast microstructure was confirmed to be composed of the α′-Mg solid solution, and the coherent fine-lamellae and the eutectic, in which the second phase is the β-phase ((Mg, Zn) 3Gd) with a fcc structure. While, the coherent fine-lamellae consist of the 2H-Mg and the 14H-type long period stacking ordered (LPSO) structure formed in α′-Mg matrix during solidification. The 14H-type LPSO structure has first been observed in as-cast Mg–Gd–Zn alloys. Moreover, during subsequent solid solution heat treatment at 773 K for 0.5–45 h, the microstructure evolutions, especially the lamellae with 14H-type LPSO structure and the β-phase were studied. It was concluded that a lamellar X-phase with a 14H-type LPSO structure can be transformed from the β-phase with the fcc structure at 773 K in addition to the 14H-type LPSO structure in the matrix. The lamellar X-phase with 14H-type LPSO structure has first been issued in Mg–Gd–Zn alloys.