14H Phase Research Articles

Elucidating the evolution of long-period stacking ordered phase and its effect on deformation behavior in the as-cast Mg-6Gd-1Zn-0.6Zr alloy

Herein, the evolution of long-period stacking ordered (LPSO) phases in the as-cast Mg-6Gd-1Zn-0.6Zr (wt.%) alloy are investigated via transmission electron microscopy (TEM) and atom probe tomography (APT). The TEM results reveal that two types of LPSO phase (a bulky interdendritic phase and a plate-like matrix LPSO phase) are formed in the as-cast sample. Most of the LPSO phases are confirmed to be of the 14H type, with a smaller proportion being of the 18R LPSO. Further, the APT results reveal that the composition of the interdendritic LPSO phase is closer to that of the ideal 14H phase compared to the matrix LPSO phase, and both the interdendritic and matrix LPSO phases exhibit a Gd/Zn ratio of 2.5, thereby indicating a deficient Zn content compared to the ideal 14H phase (i.e., 1.3). In addition, the influence of the LPSO phases on the deformation behavior is investigated at different compressive plastic strains using electron backscatter diffraction (EBSD) analysis to reveal twinning and slip behavior during deformation. The results indicate that the LPSO phase induces additional work hardening in the late stage of deformation via the suppression of {101¯1} compressive twinning and the activation of non-basal slip systems.

Tailoring strength and ductility of Mg97Y2Zn1 alloy through the architecture of heterogeneous lamellar structure by large-ratio hot extrusion

In this study, a heterogeneous lamellar structure is achieved in Mg97Y2Zn1 alloy via large-ratio hot extrusion of the alloys in as-cast (CE) and as-annealed (AE) stages. The ultimate tensile strength (UTS) of CE and AE alloys is 426.5 and 471.4 MPa, and their fracture elongation (FE) is 8.1% and 1.0%, respectively. Mechanical properties of the CE and AE alloys exceed that of the majority of wrought Mg97Y2Zn1 alloys due to the heterostructure induced strengthening. Both in the two extruded alloys, α-Mg layers and LPSO layers acting as soft and hard domains, respectively, constitute the heterogeneous lamellar structure. As a result of the narrower spacing of α-Mg and LPSO layers, more refined DRX grains and LPSO particles, and the increased volume fraction of LPSO phase, the strength of the AE alloy is higher than that of CE alloy. However, the AE alloy exhibits poor ductility owing to the easy aggregation of dislocations and stress at the interfaces of narrower layers. Based on discussion of the strengthening mechanisms, the preparation of Mg97Y2Zn1 alloys with tailored mechanical properties of high UTS ranging from 420 to 480 MPa, and FE 1~ 8%, could be realized via the combination of annealing (controlling the content of 14H phase) and large-ratio hot extrusion (promoting the formation of heterogeneous lamellar structure).

Deformation-induced dynamic precipitation of 14H-LPSO structure and its effect on dynamic recrystallization in hot-extruded Mg-Y-Zn alloys

Understanding dynamic precipitation in thermo-mechanical processing can shed light on the manufacturing of high-performance Mg-based alloys. This study investigates the dynamic precipitation of long-period stacking order (LPSO) phase with 14H structure and its effect on dynamic recrystallization (DRX) in the hot-extruded Mg98.5Y1Zn0.5, Mg97Y2Zn1 and Mg94Y4Zn2 (at. %) alloys. First, the simultaneous increase of Y and Zn atoms not only promotes the formation of interdendritic block 18R-LPSO phases during solidification but also facilitates the dynamic precipitation of intragranular lamellar and thin 14H-LPSO phases in coarse non-DRX and fine DRX grains, respectively, during hot extrusion. Second, the increased block 18R volume fraction and lamellar 14H number density significantly delays DRX. However, as the deformation strain increases, broken 18R fragments accelerate the DRX particle-stimulated nucleation (PSN) mechanism and fluctuant α-Mg/18R interfaces initiate the discontinuous DRX mechanism. In particular, the deformation kinking and dissolving of lamellar 14H activate the continuous DRX mechanism. Finally, the mechanical strength increases but ductility decreases with increasing the content of Y and Zn atoms for the hot-extruded Mg-Y-Zn alloys. The high strength of the hot-extruded Mg94Y4Zn2 alloy is attributed to fine DRX grain size, strong un-DRX grains, major block 18R and lamellar/thin 14H phases, and deformation kinking, while the high ductility of the hot-extruded Mg98.5Y1Zn0.5 alloy is responsible for large DRX volume fraction, high activity of deformation twins, and easy mobility of basal slip.

Investigation on Thermodynamic Stability of 10H and 14H Long Period Stacking Ordered Phase in Mg-Zn-Y Alloy

In this paper, enthalpies of formation and electronic structures of long period stacking ordered phases in ternary Mg-Zn-Y alloys are calculated by first-principles. Through calculations of the enthalpies of formation, it was found that enthalpies of formation of 10H phase and 14H phase are negative, and the enthalpy of formation of 10H phase less than that of 14H phase. It shows that both phases can form stable structure. The energy of transformation calculations show that 14H phase is more stable than 10H phase. The analysis of electronic structure shows that the difference between the structural stability of 10H phase and 14H phase is due to the different number of bonding electrons at the lower energy level of Fermi level. More bonding electrons determine that 14H phase has better stability than 10H phase.

Improving microstructure and corrosion resistance of LPSO-containing Mg–Y–Zn–Mn alloy through ECAP integrated with prior solution treatment

Unique Mg-RE (rare earth) alloys with long-period stacking ordered (LPSO) structures have attracted extensive attention for their excellent mechanical properties and bright prospects. Thus it is urgent to improve their corrosion resistance simultaneously. The present work focused on the improvement of microstructure and corrosion behaviours in new Mg-2.2Y-1.1Zn-0.4Mn (wt. %) alloys with mixed LPSO phases through warm equal-channel angular pressing (ECAP) integrated with prior solution treatment (ST). The results show that the multi-pass ECAP process optimizes the grain size and distribution of mixed LPSO phases, which improves the corrosion resistance of the alloy. During the integrated ECAP processing, the massive 18 R phase was broken from 100 μm (as-cast) to 5–8 μm (12 passes), and the precipitated 14H phase was more uniformly distributed in the fined α-Mg matrix. Accordingly, the corrosion resistance of the LPSO-containing samples from 0 to 8 ECAP passes increased for a more uniform and more refined structure. In contrast, the corrosion resistance improvement of the 12-pass ECAP one was reduced slightly for weakened 18 R barriers.

Phase equilibria and microstructure investigation of Mg-Gd-Y-Zn alloy system

• Confirmation of W-phase (Mg 3 Y 2 Zn 3 ) instead of Mg 3 RE in Mg-Gd-Y-Zn alloy system. • Accurate prediction (within 5 mol%) of LPSO 14H phase due to improved thermodynamic model. • Critical phase equilibria understanding of Mg-Gd-Y-Zn alloy system for alloy design. In order to develop high strength Mg-Gd-Y-Zn alloys, key experiments coupled with CALPHAD (CALculation of PHAse Diagrams) calculations were carried out in the current work to provide critical understanding of this important alloy system. Three Mg-10Gd-xY-yZn ( x = 4 or 5, y = 3 or 5, wt.%) were mapped on Mg-Gd-Y-Zn phase diagrams for phase equilibria and microstructure investigation. Electron microscopy was performed for phase identification and phase fraction determination in as-cast and solution treated conditions. In all three alloys, the major phases were Mg-matrix and long period stacking order (LPSO) 14H phase. With ST at 400 and 500 °C, the phase fraction of LPSO 14H increased, particularly the fine lamellar morphology in the Mg matrix. The as-cast and 400 °C Mg10Gd5Y3Zn samples had Mg 5 (Gd,Y) present. At 500 °C, Mg 5 (Gd,Y) is not stable and transforms into LPSO 14H. The Mg10Gd5Y5Zn alloy included the W-Phase, which showed a reduction in phase fraction with solution treatment. These experimental results were used to validate and improve the thermodynamic database of the Mg-Gd-Y-Zn system. Thermodynamic calculations using the improved database can well describe the available experimental results and make accurate predictions to guide the development of promising high-strength Mg-Gd-Y-Zn alloys.

Preparation, Microstructure Evolutions, and Mechanical Property of an Ultra-Fine Grained Mg-10Gd-4Y-1.5Zn-0.5Zr Alloy

In this work, the microstructural evolutions and mechanical properties of an as-cast Mg-10Gd-4Y-1.5Zn-0.5Zr (wt %) alloy during successive multi-pass equal channel angular pressing (ECAP) were systematically investigated by X-ray diffractometer, scanning electron microscopy, transmission electron microscopy, and compression test. The obtained results show that the microstructure of as-cast alloy consists of α-Mg grains, Mg3Gd island phase, few Y-rich particles, and lamellar 14H LPSO (long period stacking ordered) phase located at the grain boundaries. During ECAP, the Mg3Gd-type phase is crushed and refined gradually. However, the refined Mg3Gd particles are not distributed uniformly in the matrix, but still aggregated at the interdendritic area. The 14H phase becomes kinked during the early passes of ECAP and then broken at the kinking bands with more severe deformation. Dynamic recrystallization of α-Mg is activated during ECAP, and their average diameter decreases to around 1 μm, which is stabilized in spite of increasing ECAP passes. Moreover, nano-scale γ′ phases were dynamically precipitated in 16p ECAP alloy. Compression tests indicate that 16p ECAP alloy exhibits excellent mechanical property with compressive strength of 548 MPa and fracture strain of 19.1%. The significant improvement for both strength and ductility of deformed alloy could be ascribed to dynamic recrystallization (DRX) grains, refined Mg3Gd-type and 14H particles, and dynamically precipitated γ′ plates.

First-principles study on thermodynamic stability and electronic characteristics of long-period stacking ordered phases in Mg–Zn–Y alloys

Abstract A first-principles pseudopotential plane-wave method based on density functional theory has been used to investigate the thermodynamic stability and electronic characteristics of long-period stacking ordered (LPSO) phases in Mg–Zn–Y alloys. The obtained lattice constants of each LPSO phase considered are in good agreement with the experimentally determined values, with relative errors ranging from 0.21% to 2.0%. The calculated enthalpies of formation show that 14H and 18R phases can co-exist in the Mg–Zn–Y system, and the 18R phase is easier to form than the 14H phase while solidifying. But the 14H phase is more stable than the 18R phase by the calculation results of reaction energy. And the calculated results are in accord with the experiments for the thermodynamic stability of the two phases. The density of states of these phases reveals that the discrepancy in the stability of LPSO phases could be attributed to the variation of bonding electron numbers at low-energy region of Fermi level. In addition, the calculation of the charge density shows that the Zn Y bond exhibits covalent feature in both 14H and 18R phases, and the covalent bonding of the 14H phase is stronger than that of the 18R phase, which means the 14H phase is more stable than the 18R phase.

Precipitation behavior of 14H LPSO structure in single 18R phase Mg–Y–Zn alloy during annealing at 773 K

The microstructural evolution of a 18R single phase (S18) alloy during annealing at 773 K for 100 h was investigated in order to reveal the formation mechanism of 14H phase. The results showed that the as-cast S18 alloy was composed of 18R phase (its volume fraction exceeds 93%), W particles and α–Mg phase. The 18R phase in S18 alloy was thermally stable and was not transformed into 14H long period stacking ordered (LPSO) phase during annealing. However, 14H lamellas formed within tiny α–Mg slices, and their average size and volume fraction increased with prolonging annealing time. Moreover, the 14H phase is nucleated within α–Mg independently on the basis of basal stacking faults (SFs). The broadening growth of 14H lamellas is an interface-controlled process which involves ledges on basal planes, while the lengthening growth is a diffusion-controlled process and is associated with diffusion of solute atoms. The formation mechanism of 14H phase in this alloy could be explained as α–Mg'→ α–Mg+14H.

Formation Behavior of 14H Long Period Stacking Ordered Structure in Mg–Y–Zn Cast Alloys with Different α-Mg Fractions

Phase compositions and microstructure evolutions of three Mg–Y–Zn cast alloys during isothermal annealing at 773 K have been systematically investigated to clarify the formation behavior of 14H long period stacking ordered (LPSO) structure from α-Mg grains. The annealed microstructure characteristics indicate that the 18R phase is thermal stable in Mg86Y8Zn6 alloy where 18R serves as matrix, and 14H lamellar phase only forms within tiny α-Mg slices (less than 1% for volume fraction). The α-Mg grains in Mg88Y8Zn4 and Mg89Y8Zn3 alloys exhibit cellular shape, and 14H phase forms and develops into lamellar shape in these cellular grains after annealing. The results suggest that the presence of α-Mg grains is a requirement for the generation of 14H phase. The nucleation and growth rates of 14H lamellas are accelerated in α-Mg grains with higher concentrations of stacking faults and solute atoms. Moreover, the 14H lamellas are parallel to adjacent 18R plates in Mg86Y8Zn6 alloy, but the 14H phase precipitated in cellular α-Mg grains of Mg88Y8Zn4 and Mg89Y8Zn3 alloys exhibits random orientation relationship with surrounding 18R phase, indicating that the orientation relationship between 14H and 18R phases depends on the relationship between α-Mg grains and 18R phase.

First-principles study of the thermodynamic stabilities and electronic structures of long-period stacking ordered phases in Mg-Y-Cu alloys

The long-period stacking ordered (LPSO) phases in magnesium alloys possess excellent mechanical performances, and have received considerable attention. The strengthening LPSO phases, such as 14H and 18R structures, are found experimentally in some Mg-Y-Cu alloys, which can significantly enhance the mechanical performances of the alloys.However, it is unknown which phase is more stable thermodynamically, and easier to form during the solidification. In this paper, thermodynamic stabilities and electronic characteristics of LPSO phases 14H and 18R (18R(m), 18R(t)) in Mg-Y-Cu alloys are investigated by the first-principles pseudopotential method based on the density functional theory. The present calculations are performed by using Vienna ab-initio simulation package (VASP) with projector-augmented plane wave pseudopotential, and the generalized gradient approximation is used to deal with and describe the exchange-correlation interaction. The plane wave cutoff energy is set to be 360 eV, the forces on all the atoms are less than 0.02 eV/. The k-point meshes of Brillouin zone sampling in a primitive cell are based on the Monkhorst-Pack scheme. The calculated enthalpies of formation indicate that the 14H and 18R phases coexist in Mg-Y-Cu alloys. The 18R phase has a larger absolute value of formation enthalpy, which means that it is easier to form than the 14H phase. The reaction energy is also computed for the transformation from the 18R phase to 14H phase, which shows that the 14H phase is more stable than the 18R phase. The results for density of states (DOS) reveal that the bondings of the 14H and 18R phases occur mainly among the valence electrons of Cu 3d, Y 4d, Mg 3s and Mg 2p orbits while those of Cu 4s, Y 4s and Y 4p orbits are very weak in the whole region. The bonding peaks of the 14H, 18R(m), and 18R(t) phases are localized, and the corresponding hybridization orbits, which are all or part of Mg 3s, Mg 2p, Cu 3d and Y 4d orbits, are determined. At the same time, there are sharp peaks on both sides of the Fermi level of the 14H, 18R(m) and 18R(t) phases, which shows that there exist pseudogaps in those phases. The presence of pseudogap indicates that the bonds in the 14H and 18R phases are noticeable covalent. In addition, the charge densities both on (0 0 0 1) plane of the 14H and 18R phases are analyzed in detail. The results show that the Cu-Y bond exhibits the covalent feature in the 14H and 18R phases, the covalent bonding of the 14H phase is stronger than that of the 18R phase, and it is the key reason that the 14H is more stable than the 18R. The calculated results for thermodynamic stabilities and electronic structures of LPSO phases will provide useful data for analyzing and designing Mg-Y-Cu alloys.

Comparative studies on evolution behaviors of 14H LPSO precipitates in as-cast and as-extruded Mg–Y–Zn alloys during annealing at 773 K

The microstructural evolutions of as-cast and as-extruded Mg97Y2Zn1 (at.%) alloys during annealing at 773K were systematically investigated in this work. The results show that the evolutions of 14H long period stacking ordered (LPSO) phases are obviously different for the two alloys. In cast alloy, 14H LPSO precipitates firstly and then dissolves into α-Mg, while 18R shows no obvious change. Only a few 14H phases remain near 18R/α-Mg interfaces of cast alloy after annealed for 240h. As for extruded alloy, 14H precipitates and grows continuously with increasing annealing time, while 18R gradually dissolves. The extruded alloy exhibits an α-Mg/14H dual microstructure after annealed for 240h. The formation of 14H in cast alloy follows the reaction of 18R+α-Mg→14H, while it is generated in extruded alloy through a precipitation process. Moreover, the 14H preferentially forms near α-Mg/18R interfaces where SFs and solute atoms are enriched in cast alloy, but they are uniformly generated within α-Mg grains of extruded alloy. The differences between microstructural evolutions of cast and extruded alloys during annealing could be mainly attributed to the segregations of solute atoms and SFs within α-Mg grains.

Phase equilibria of the long-period stacking ordered phase in the Mg–Ni–Y system

Phase equilibria in the Mg-rich Mg–Ni–Y system at 300, 400 and 500 °C have been experimentally investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), electron probe micro-analyzer (EPMA) and transmission electron microscopy (TEM). The results show that a long-period stacking ordered (LPSO) phase with 14H structure is thermodynamically stable in the Mg–Ni–Y system in a wide temperature range, but it dissolves varying from 492 to 559 °C depending on the alloy composition. The equilibrium 14H phase has a very limited solid solution range, and can be nearly regarded as a ternary stoichiometric compound with a formulae as Mg91Ni4Y5. The isothermal sections of the Mg-rich Mg–Ni–Y system at 300, 400 and 500 °C have been finally established, and a eutectic reaction, Liquid ↔ α-Mg + 14H + Mg2Ni, has been determined occurring at 492 °C with a liquid composition about Mg84.8Ni12.0Y3.2.

The phase equilibria and thermal stability of the long-period stacking ordered phase in the Mg–Cu–Y system

Phase equilibria in the Mg-rich Mg–Cu–Y system at 300, 400 and 450°C have been experimentally investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), electron probe micro-analyzer (EPMA) and transmission electron microscope (TEM). The results show that a long-period stacking ordered (LPSO) phase 14H is stable in the Mg–Cu–Y system, which is the only one ternary intermetallic compound that gets a thermodynamic equilibrium with the a-Mg phase. The equilibrium 14H phase has a very limited solid solution range, and can be nearly regarded as a ternary stoichiometric compound with a formulae as Mg91Cu4Y5. The thermal stability of the 14H phase in the Mg–Cu–Y system has been well studied, which shows that the 14H phase disappears varying from 442°C to 490°C depending on the alloy composition. The isothermal sections of the Mg-rich Mg–Cu–Y system at 300, 400 and 450°C have been finally established, and moreover, a quasi-peritectic reaction L+α-Mg↔14H+Mg2Cu has been determined occurring at 442°C with an estimated liquid composition of Mg77Cu18Y5.

Growth and transformation mechanisms of 18R and 14H in Mg–Y–Zn alloys

The growth of and transformation between 18R and 14H phases in a Mg–8Y–2Zn–0.6Zr (wt.%) alloy have been examined using conventional transmission electron microscopy and atomic resolution high-angle annular dark-field scanning transmission electron microscopy. The growth of both 18R and 14H within the α-Mg matrix occurs via a ledge mechanism, with the thickness of the particle increasing by the height of the ledge as it propagates. The unit height of the growth ledges or disconnections is 1.563nm for 18R and 1.824nm for 14H, and the displacement vector is a/3〈1¯010〉α. 18R transforms in-situ to 14H during prolonged heat treatment at 500°C. The 18R to 14H transformation is shown to occur most readily in regions where the 18R structure has irregularities in the building block stacking, in particular where a pair of adjacent building blocks is separated by four rather than two α-Mg layers. It is proposed that the diffusional-displacive 18R to 14H transformation rate is controlled by the diffusion rate of Y and Zn atoms into the segregation layers.