Long Period Stacking Ordered Research Articles

Tailoring the microstructure and mechanical properties of LPSO-containing Mg–Ni cast alloy by yttrium addition

The effect of yttrium addition on the evolution of phases, grain refinement, and mechanical behavior of Mg-0.5Ni cast alloy was investigated. It was observed that while the base alloy contained a eutectic structure containing α-Mg and Mg2Ni, in the Y-containing alloys, the long period stacking ordered (LPSO) structures and Mg24Y5 phase appeared at the expense of the eutectic structure. The dendrite arm spacing (DAS) was significantly refined, so that adding 0.5 and 2 at% Y led to the refinement of DAS from 141 to 56 and 22 μm, respectively. Accordingly, hardness, tensile and shear strength were improved, being successfully represented by the Hall-Petch-type relationships in terms of DAS. The highest elongation to failure in tensile tests was obtained at 0.5 % Y addition, which was ascribed to the refinement of microstructure and partial replacement of the deleterious α-Mg + Mg2Ni eutectic structure by the LPSO phase. However, further addition of Y led to the formation of intergranular interconnected network of LPSO structure with deteriorated ductility. Therefore, the best strength-ductility synergy was obtained for the Mg99Ni0.5Y0.5 alloy, as evaluated by the tensile toughness. Finally, the tensile and shear properties were correlated according to the von-Mises criterion.

Study on the Microstructure and Properties of Mg-Gd-Ni-Y Alloy Containing LPSO Phase

The long-period stacking ordered (LPSO) structure, functioning as a strengthening phase in magnesium alloys, plays a pivotal role in compensating for inherent performance limitations. In this study, an as-cast Mg-Gd-Ni-Y alloy, including the LPSO phase, was initially obtained through an ingot metallurgy process. Subsequently, the alloy underwent distinct thermal treatments: annealing at 500 °C for 10 h, and extrusion using an extrusion ratio of 10 at a speed of 5 mm/s. Comparative analysis of the microstructure and corrosion characteristics was performed across these three alloy states. Comprising primarily of α-Mg, LPSO phase, and eutectic structures (ES), the alloy exhibited distinctive microstructural features. Immersion experiments conducted in a 3.5% NaCl solution revealed that the as-cast alloy displayed the highest dissolution rate at various temperatures, from room temperature, to 50 °C, and 70 °C. Following annealing, a reduction in the second phase content within the alloy significantly contributed to the observed decrease in its dissolution rate. Extrusion processes resulted in a denser network structure within the microarchitecture, to some extent impeding the spread of corrosion to some extent. By emloying scanning Kelvin probe force microscopy (SKPFM) and micro-electrochemical testing, it was discerned that predominantly the electrochemical system involving α-Mg and the second phases predominantly dictated the heightened dissolution rate of the alloy. This study presents valuable insights into understanding the dissolution mechanisms and potential strategies for controlling the dissolution performance of magnesium alloys containing the LPSO phase.

Effects of Bulk LPSO Phases on Mechanical Properties and Fracture Behavior of As-Extruded Mg-Gd-Y-Zn-Zr Alloys

Despite the consensus on the constructive effect of LPSO (long-period stacking-ordered) phases, the true effect of bulk LPSO phases on strengthening and toughening in deformed magnesium alloys is still controversial. This article, which introduces the alloys Mg-8Gd-4Y-0.6Zn-0.5Zr, without bulk LPSO phases, and Mg-8Gd-4Y-1.6Zn-0.5Zr, containing bulk LPSO phases, details a systematically comparative analysis conducted to clarify the true contribution of bulk LPSO phases to the properties of as-extruded alloys. The results indicate that bulk LPSO phases significantly improve strength by refining grain sizes remarkably. But contrary to expectations, bulk LPSO phases themselves only provide a small strengthening effect and deteriorate plasticity, ascribed to the poor compatible plastic deformation of bulk LPSO phases. More importantly, this work may offer new insights into the strengthening and toughening of LPSO phases for further research and engineering applications of this series of alloys. Additionally, an example of a design strategy for Mg-Gd-Y-Zn alloys with superior strength and excellent plasticity is proposed at the end of this article.

Probing the varying ranges of damping capacity of magnesium alloys containing long-period stacking ordered phases

Traditional high damping Mg alloys, based on the concept of purified matrix, cannot withstand large vibrational strain, inhibiting their practical use. The current study employed Mg alloys containing long-period stacking ordered (LPSO) phase and designed representative microstructures, to probe the varying ranges of damping capacity. It is found that intragranular LPSO lamellae effectively purify the matrix without hindering dislocation vibrations, hence contribute to improving damping capacity at small strain. They also provide additional boost to damping capacity at intermediate strain. Precipitates, as matrix purifiers, are detrimental to damping capacity. Quantitative analyses of damping curves and the correlation with stress-strain data reveal that the transition to the rapidly rising part on damping curves physically corresponds to plastic yielding. This further inspired two perspectives of viewing damping capacity at the characteristic strain values of different materials, representing a paradigm shift of material selection and providing a rational criterion for future alloy development.

Microstructure and mechanical properties of extruded and rolled Mg–Y–Zn–Ni–Co alloy

The as-cast Mg–2Y-0.5Zn-0.5Ni-0.5Co alloy (MYZNC, at%) underwent composite hot working, extrusion and rolling to further improve the mechanical properties, and the corresponding microstructure and mechanical properties were also investigated. The extruded and rolled MYZNC alloy (MYZNC-R) consisted of α-Mg, a long period stacking order (LPSO) phase comprising lamellar and fine block 18R LPSO phase as well as a thin strip 14H LPSO phase, along small amounts of Mg2Y, MgY(Co,Zn,Ni)4 and Mg3(Co,Zn,Ni,Y) particle phases. After composite processing, the dispersion and amounts of LPSO phase in the alloy was significantly enhanced, accompanied by a substantial introduction of low angle grain boundaries (LAGBs) during rolling process. The MYZNC-R alloy exhibited the high mechanical properties, with an ultimate tensile strength (σb) of 350 MPa, a yield strength (σ0.2) of 305 MPa and an elongation to failure (εf) of 10.6 %. In comparison to the extruded MYZNC alloy, both σb and σ0.2 demonstrated significant improvements by 25 % and 35.6 %, respectively. The elongation to failure only decreased by mere 1 %, indicating that the alloy had a good strength-plasticity. The high tensile strengths of MYZNC-R alloy were mainly attributed to grain refinement, LPSO phase strengthening and texture strengthening. The good ductility of MYZNC-R alloy was associated with the coordinated deformed LPSO phase and the formation of a high-volume fraction (∼76.5 %) of LAGBs.

The effect of silica NPs incorporation on protective properties of oxide layers formed by PEO on Mg97Y2Zn1 alloy with LPSO-phase

The inherited chemical inhomogeneity in oxide layers obtained by plasma electrolytic oxidation (PEO) on the magnesium alloy Mg97Y2Zn1 is associated to long-period stacking-ordered (LPSO) phase present in the treated alloy. This heterogeneity results in decrease of corrosion resistance and adhesion strength. The problem was solved by adding silica nanoparticles (NPs) into the electrolyte under PEO. According to the model developed, NPs which are harder than the oxide layer and being electrically charged, can be accelerated by an electric field and penetrate deep into the layer. The near-surface incorporation of NPs results in branching of the local breakdowns of vapor-gas bubbles that leads to an increase of the volumes of oxide layer and improvement of its properties.

Development of low rare-earth containing magnesium alloy with high strength-ductility synergy by engineering nano-spaced lamellae

Conventional Mg-RE-based alloys containing a high content of RE solutes can generate high strength, but there is often low ductility. Herein, we reported a low RE alloying Mg–5Er-1Nd-1Zn-0.2Zr extrusion alloy with only 6 wt% RE addition, showing a high strength-ductility synergy, such as a yield strength of ∼380 MPa and an elongation of ∼12.5 %), which is superior to the majority of traditional Mg-RE-based wrought alloys with at least 12 wt% RE addition. Nano-spaced lamellae was formed in the α-Mg matrix during extrusion, such as stacking faults (SFs) evolved in dynamically recrystallized (DRXed) grains and the 14H long-period stacking-ordered (LPSO) phase developed in unDRXed grains as a result of dynamical solute partitioning during extrusion. And this alloy characterizes a high proportion (∼70 %) of elongated unDRXed grains with a strong basal texture. Thus, high strength is mostly related to SFs/LPSO lamellae strengthening and texture hardening, instead of the commonly accepted refinement strengthening and precipitation hardening in those of high RE containing Mg alloys. While high ductility is heavily dependent on SFs/LPSO lamellae to trigger strain hardening and <c + a> dislocations glide, independent of fine grains as in traditional Mg alloys with high elongation. The findings give an insight into the development of high-performance Mg wrought alloys with a high strength-ductility synergy.

Effects of B4C on the formation of LPSO phase and mechanical properties of Mg–Zn–Y–Mn alloy

Effects of B4C on the microstructure and properties of Mg94Zn2.5Y2.5Mn1 alloy were investigated by means of optical microscope, scanning electron microscope, X-ray diffraction, transmission electron microscope, nano indentation tester and electronic universal testing machine. The results indicated that the as-cast Mg94Zn2.5Y2.5Mn1 alloy was mainly composed of α-Mg, fishbone-like W phase, and block 18 R long period stacking ordered (LPSO) phase. The addition of an appropriate amount of B4C could effectively refine grains, promote the formation of the LPSO phase and inhibit the generation of the W phase. The microstructure and mechanical properties of the alloy were both optimal at the addition of 0.5 at.% B4C. The microstructure exhibited a refined grain size from 41.2 μm to 20.7 μm, and the volume fraction of the LPSO phase increased from 8.1 % to 22.5 %. The yield strength increases from 112 MPa to 145 MPa, the tensile strength of the alloy was enhanced from 192 MPa to 256 MPa, and the elongation increased from 4.8 % to 8.6 %.

Thermodynamic modelling for the long-period stacking ordered phase in Mg–Y–Al system: Construction and application

An adequate thermodynamic model is required for describing the long-period stacking ordered (LPSO) phase because previously developed models cannot account for the accurate atomic occupancy of the LPSO structure. In this study, atomic structures of the equilibrated 18R and 10H phases in Mg–Y–Al alloys were systematically investigated via atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-TEM) and first-principles calculations. Three structures (α-type, β-type, and γ-type) with different numbers of Mg layers (one, two, and three, respectively) sandwiched between L12 structures were present in the 18R phase. Meanwhile, δ-type (without Mg layers), ε-type (with one Mg layer), and ζ-type (with two Mg layers) structures were detected in the 10H phase. The presence of the interstitial atoms in the L12 structure was confirmed by TEM and theoretical calculations. Based on the obtained results, a new thermodynamic model for the LPSO phase was constructed and successfully applied to establish the thermodynamic description of the Mg–Y–Al system. The CALPHAD-type calculation data obtained using the proposed thermodynamic description were in good agreement with the experimental results. The findings of this study can help elucidate the formation mechanism of LPSO phases and guide Mg–Y–Al alloy design.

Electronic and optical characteristics of long-period stacking ordered phases in Mg-Y-Zn alloys: A first-principles study

Electronic and optical properties of long-period stacking order (LPSO) phase in Mg-Y-Zn alloy are calculated and analyzed by first principles in this paper. The electronic properties and thermodynamic stability of 14 H and 18 R LPSO phases were studied by means of electron density of state (DOS), COHP analysis, electron localization function (ELF) and charge density distribution, and their optical properties, including absorption coefficient, reflectivity, refractive index, energy loss function and extinction coefficient, were discussed, and their variation trends with photon energy and related factors were analyzed. The findings indicate that both phases possess metallic characteristics and exhibit no energy gap. Furthermore, the thermodynamic stability of the 14 H phase surpasses that of the 18 R phase, owing to its stronger covalent bonds metal mixed bonds. In terms of absorption coefficients, both phases display high values within the energy range of 0–19 eV, with their most potent absorption peaks occurring at photon energies of 4.48 eV and 4.8 eV for the 14 H and 18 R phases, respectively. Moreover, the photon absorption rate is inversely proportional to the density of L12 clusters in both phases.

Formation process, microstructure, and mechanical properties of an ultrafine dual-phase alloy formed through phase transition of 18R-type long-period stacking ordered in Mg85Zn6Y9 under high pressure

The 18R-type long-period stacking ordered (LPSO) structure in Mg85Zn6Y9 exhibits phase transition into D03 and α-Mg (D03/α-Mg) before it transforms into a liquid phase with the increase in temperature at 7.8 GPa. First-principles calculations suggest that the energy barrier between 18R-type LPSO and D03/α-Mg decreases with pressure. A further increase in the temperature decreases the energy difference between LPSO and D03/α-Mg. Therefore, the free energy variations due to the effects of both pressure and temperature cause phase transition. The D03/α-Mg phases formed at high pressure can be recovered at 0.1 MPa and room temperature. The phase transition from 18R-type LPSO structure to D03/α-Mg causes inner-grain diffusion and generates a lamellar structure. The thickness of the lamellar structure decreases with the increasing pressure until it reaches 65 nm at 15 GPa. Moreover, the alloy solidified above 5 GPa also comprises D03/α-Mg phases. The alloy solidified at 5 GPa exhibits a typical eutectic lamellar structure. However, spherulite-like polycrystalline microstructures containing ultrafine grains are obtained when the alloys are solidified at 10 and 15 GPa. The microstructure variations are caused by an increase in the degree of supercooling due to atomic diffusion suppression with the increasing pressure. The alloys containing D03/α-Mg exhibit both low modulus and ultrahigh strength. The Young’s modulus is related to the presence of the D03 and α-Mg phases in the alloy structure. In contrast, the increase in σy can be attributed to the grain refinement strengthening.

Dual influences of deformation-induced W precipitates on dynamic recrystallization and fracture mechanism of the hot-extruded Mg-Y-Zn alloys: An experimental and phase field study

Understanding the relationship between nano-sized precipitates and dynamic recrystallization (DRX) during thermo-mechanical processing is vital for developing high-performance Mg-based alloys. This work uses experiment and phase field model to investigate the dynamic precipitation of W particles and DRX grains in the hot-extruded Mg98.5Y1Zn0.5 alloys at 360, 380 and 400 °C and clarifies the fracture mechanisms in detail. The results reveal that under all temperatures, the hot-extruded Mg98.5Y1Zn0.5 alloys are composed of block 18R long-period stacking order (LPSO) phase, lamellar 14H-LPSO phase, W particles and a typical bimodal structure containing fine DRX grains and coarse non-DRX grains. Noted that the W particles not only promote the DRX nucleation by hindering dislocation motion to increase elastic energy, but also inhibit the growth of DRX grains by pinning grain boundaries during deformation. In addition, the strength decreases while the plasticity increases with increasing temperature for the hot-extruded Mg98.5Y1Zn0.5 alloys, but an excellent strength-plasticity balance is achieved at 400 °C. More importantly, analyzing the crack extension in non-DRX, DRX and non-DRX/DRX coexistence regions shows that a ductile-brittle fracture mechanism dominates in the non-DRX regions due to the difficulty of basal slip and low fracture toughness. But a hole-joining fracture mechanism dominates in the DRX regions. The fracture mechanism in the non-DRX/DRX coexistence region is mainly determined by the DRX volume fraction. These results can provide insights and valuable references for developing high-performance Mg-based alloys.

Design of LPSO Phases in Mg-Y-Ni Alloys to Impact Hydrogenation Kinetics

A series of Mg-Y-Ni alloys with different volume fractions of long-period stacking-ordered (LPSO) phase were prepared, by controlling the alloy composition, heat treatment, and single-pass extrusion, to assess the influence of increasing LPSO phase volume fraction on the hydrogen absorption and desorption properties of the extruded alloys. The LPSO phase volume fraction in the alloys increased with increasing solute concentration, from ~24% LPSO in Mg97Y2Ni1 (at.%) to ~60% LPSO in Mg93Y4Ni3 (at.%) up to ~92% LPSO in Mg91Y5Ni4 (at.%). The most refined microstructure was obtained in the alloy with highest volume fraction of LPSO phase. After 100 s at 300 °C, the Mg91Y5Ni4 alloy absorbed 4.6 ± 0.2 wt.% H while the Mg97Y2Ni1 and Mg93Y4Ni3 alloys each absorbed 3.8 ± 0.2 wt.% H. After 10,000 s at 300 °C, all three alloys had absorbed a maximum of 5.3 ± 0.2 wt.% H with no further significant difference in hydrogen absorption kinetics. The Mg91Y5Ni4 alloy desorbed 1.8 ± 0.2 wt.% H after 100 s at 300 °C against a vacuum while the Mg97Y2Ni1 and Mg93Y4Ni3 alloys desorbed 0.8 ± 0.2 wt. H and 0.6 ± 0.2 wt.% H, respectively. After 10,000 s at 300 °C, the Mg91Y5Ni4 and Mg97Y2Ni1 alloys completely desorbed 5.2 ± 0.2 wt.% H and 5.4 ± 0.2 wt.% H, respectively, but the Mg93Y4Ni3 alloy desorbed only 3.7 ± 0.2 wt.% H. Hydrogen absorption and desorption kinetics were fastest in the Mg91Y5Ni4 alloy with the highest LPSO volume fraction, but no consistent trend with LPSO phase volume fraction was observed with the Mg93Y4Ni3 alloy, which showed the slowest absorption and desorption kinetics. The hydrogen pressures corresponding to metal–hydride equilibrium did not vary with LPSO phase volume fraction or alloy composition, indicating that the (de)hydrogenation thermodynamics were not significantly changed in any of the alloys. Hydrogen absorption experiments with thin foils, made of extruded Mg91Y5Ni4 alloy with the highest LPSO phase fraction, demonstrated that the LPSO structures decompose into Mg phase, Mg2Ni phase, lamellar Mg/Mg-Y structures, and YHx particles. This study shows that hydrogen kinetics can be impacted in Mg-Y-Ni alloys by controlling the LPSO phases using common metallurgical techniques.

The Development of a High-Strength Mg-10.3Gd-4.4Y-0.9Zn-0.7Mn Alloy Subjected to Large Differential-Thermal Extrusion and Isothermal Aging.

The large differential-thermal extrusion (LDTE) process, a novel approach for efficiently fabricating a high-strength Mg-10.3Gd-4.4Y-0.9Zn-0.7Mn (wt.%) alloy, is introduced in this work. Unlike typical isothermal extrusion processes, where the ingot and die temperatures are kept the same, LDTE involves significantly higher ingot temperatures (~120 °C) compared to the die temperature. For high-strength Mg-RE alloys, the maximum isothermal extrusion ram speed is normally limited to 1 mm/s. This research uses the LDTE process to significantly increase the ram speed to 2.0 mm/s. The LPTE-processed alloy possesses a phase composition that is similar to that of isothermal extruded alloys, including α-Mg, 14H-type long-period stacking ordered (LPSO) and β-Mg5(Gd, Y) phases. The weakly preferentially oriented α-Mg grains in the LDTE-processed alloy have <101¯0>Mg//ED fibrous and <0001>Mg//ED anomalous textures as their two main constituents. After isothermal aging, high quantitative densities of prismatic β' and basal γ' precipitates are produced, which have the beneficial effect of precipitation hardening. With a yield tensile strength of 344 MPa, an ultimate tensile strength of 488 MPa, and an elongation of 9.7%, the alloy produced by the LDTE process exhibits an exceptional strength-ductility balance, further demonstrating the potential of this method for efficiently producing high-strength Mg alloys.

Refining 18R-LPSO phase into sub-micron range by pre-kinking design and its prominent strengthening effect on Mg97Y2Zn1 alloy

In this study, a composite deformation strategy of pre-kinking (equal channel angular pressing (ECAP)) followed by large-ratio hot extrusion (HE) was designed to refine the 18R long period stacking ordered (LPSO) phase into sub-micron range in a Mg97Y2Zn1 (at.%) alloy. After the composite processing, the mechanical properties of the alloy are significantly enhanced, superior to the majority of reported Mg97Y2Zn1 and other LPSO-containing Mg alloys. Among the composite deformed alloys, the 16P-HE alloy exhibits the best mechanical properties with tensile yield strength (TYS) of 475 MPa, ultimate tensile strength (UTS) of 526 MPa, and fracture elongation (FE) of 14.5%. Quantitative analysis of 18R phase indicates that increasing ECAP pass from 1 to 16 gradually decreases the average size of 18R phase from 5.1 µm to 2.3 µm. After HE, the 18R phase is further refined with a corresponding decrease in the average size in the descending order of 1P-HE (4.3 µm), 4P-HE (3.2 µm), and 16P-HE (1.4 µm) alloys. Calculation of the strengthening contributions confirms that the superior mechanical properties of 16P-HE alloy are mainly due to its strongest interface strengthening (145 MPa) and grain boundary strengthening (189 MPa) from the sub-micron 18R phase and α-Mg grains. Moreover, the strengthening effect of 18R phase decreases gradually with their morphology changing from particles to fibers, and to blocks. The obtained results further deepen and broaden the strengthening-toughening theory of 18R phase.

Deformation behavior of LPSO phases with regulated morphology and distribution and their role on dynamic recrystallization in hot-rolled Mg–Gd–Y–Zn–Zr alloy

Three relatively novel initial microstructures of Mg–Gd–Y–Zn–Zr alloy were regulated by heat treatment, the deformation behavior of long period stacking ordered (LPSO) phases with different morphology and distribution and their role on recrystallization during thermal deformation were systematically investigated. The interdendritic block-shaped LPSO phases exhibit higher deformation resistance, and shearing, bending, kinking, tearing, dissolution, together with the breaking of plate-shaped lamellae produced by dissolution, are the main deformation mechanisms for them to coordinate deformation. As for the lamellar LPSO phases, deformation behaviors such as shearing, bending, kinking and breaking mainly occur. The interdendritic block-shaped LPSO promote recrystallization through the particle-stimulated nucleation (PSN) mechanism, and the PSN effect is enhanced with the increasing phase fraction, also, the broken block-shaped LPSO have better PSN effect. Recrystallized grains tend to nucleate at the kink of lamellar LPSO and form recrystallized grains bands along the kink bands; the fragmented fine lamellae have better PSN effect, promoting numerous recrystallization nucleation in the severely deformation zone, meanwhile, the joint pinning effect of the fragmented lamellae and precipitates β-phase results in the smaller recrystallized grains size. However, the intragranular continuous large-sized lamellar LPSO strongly pin the dislocation movement and hinder migration of recrystallized grain boundaries along the basal plane and c-axis of lamellar LPSO, making it difficult to achieve extensive lattice rotation, which is not conducive to the recrystallization nucleation. The most remarkable work in this paper is that we clarified the mechanism of different LPSO phases, especially lamellar LPSO phase, on recrystallization behavior, and these findings will provide microstructure regulation guidance for thermal-deformed Mg–Gd–Y–Zn–Zr alloys containing LPSO.

In-situ investigate on the microstructure evolution, deformation and fracture mechanism of Mg-Gd-Zn-Mn alloy with varies of LPSO volume fraction

The induction of long-period stacking ordered (LPSO) phase through heat treatment was an effective method to strengthen magnesium alloys. In this study, the volume fraction of lamellar LPSO in the Mg-4Gd-1Zn-0.5Mn alloy was controlled by optimizing the heat treatment process. The research findings showed that with increasing heat treatment time, the volume fraction of LPSO initially increased then decreased until disappearing. The alloy treated at 500 °C for 2 h exhibited the highest volume fraction of lamellar LPSO, increasing from 1.64% to 13.68%. Further in-situ tensile tests were conducted on two types of alloys contain higher LPSO fraction (HL) and lower LPSO fraction (LL) at ambient temperature. The increased critical shear stress (CRSS) and obstacle effects on dislocations caused by LPSO effectively enhanced the strength of the alloy. The dominant deformation mechanisms of both HL and LL alloys are basal slip and twinning during tensile deformation. However, the HL alloy activated more non-basal slip significantly, which was ascribed to the activation of non-basal slip by LPSO kinking during deformation. Additionally, the bending and kinking of LPSO effectively restricted crack initiation and propagation, thereby enhancing the plasticity. The HL alloy exhibited fewer twin boundaries after deformation, which was attributed to the hindrance of twinning extension by LPSO.

Small-angle X-ray scattering analysis on the in-plane cluster distribution during heating amorphous Mg85Y9Cu6 alloys

In-plane cluster arrangements of a long-period stacking ordered (LPSO) structure in MgYCu alloys were examined by reverse Monte Carlo fitting of small-angle X-ray scattering (SAXS) intensities. In situ SAXS measurements were performed at a heating rate of 10 K min−1. from the glass state to the mp. The clusters were characterized as so-called L12 clusters commonly observed in the LPSO structures. The results suggest that the average in-plane distance of the clusters decreased during heating. The distance showed two preferred average distances, in contrast to the MgYZn case where the average distance decrease monotonically during heating.

Comparison of Thermal Deformation Behavior and Characteristics of Mg-Gd-Y-Zn Alloys with and without Bulk LPSO Phase.

Alloys Mg-8Gd-4Y-0.6Zn-0.5Zr (referred to as 0.6Zn) without the bulk long-period stacking ordered (LPSO) phase and Mg-8Gd-4Y-1.1Zn-0.5Zr (referred to as 1.1Zn) containing the bulk LPSO phase were prepared and a series of hot compression tests were conducted to examine and evaluate the influence of the bulk LPSO phase on the thermal deformation behavior and characteristics of the Mg-Gd-Y-Zn-Zr alloy. The bulk LPSO phase affects the dynamic recrystallization behavior, resulting in differences in flow stress between two alloys under different conditions. Specifically, in the temperature range of 380~460 °C, compression at lower strain rates is beneficial for the LPSO phase to promote dynamic recrystallization, while compression at a high strain rate inhibits the dynamic recrystallization due to the severe deformation of the bulk LPSO phase to release the stress concentration instead. The increase in temperature helps the LPSO promote dynamic recrystallization. As a result, the LPSO phase promotes dynamic recrystallization at all experimental strain rates at 500 °C. Furthermore, the thermal processing maps of the 0.6Zn and 1.1Zn alloys are established, and their optimal processing windows are located at 500 °C/0.001~0.01 s-1 and 500 °C/0.01 s-1, respectively. In addition, the instability zones for the 1.1Zn alloy are much larger than that for the 0.6Zn alloy, which corresponds to the microcracks generated at the interfaces between α-Mg and bulk LPSO phases.

Effect of dynamic recrystallization, LPSO phase, and grain boundary segregation on heterogeneous bimodal microstructure in Mg-9.8Gd-3.5Y-2.0Zn-0.4Zr alloy

The formation conditions and influence mechanisms of the heterogeneous bimodal microstructure of Mg-9.8Gd-3.5Y-2.0Zn-0.4Zr alloy were systematically studied by thermal compression experiments in the range of 0.001–1.0 s−1 and 350–500 °C. Combined with the microstructural characterization, the inherent relationships between the grain structure and the deformation parameters are deeply analyzed. The results indicate that heterogeneous bimodal microstructures composed of fine dynamically recrystallized (DRXed) grains with random orientation and coarsely deformed grains with basal orientation can only be formed under deformation conditions of 40.51 < lnZ < 46.33. This mainly stems from the dominant continuous dynamic recrystallization (CDRX) and particle stimulated nucleation (PSN) mechanisms under this deformation condition. However, dynamic recovery (DRV) and discontinuous dynamic recrystallization (DDRX) dominate isothermal compression process with lnZ > 47.41 and lnZ < 39.74, which eventually lead to unimodal distributions of deformed grains and DRXed grains, respectively. Furthermore, results reveal that deformation parameters have a strong influence on the long-period stacking order (LPSO) phase and grain boundary (GB) segregation. The lamellar LPSO phases and grain boundary co-segregation of Zn, Y, and Gd atoms jointly promote CDRX and inhibit DDRX by preventing GB migration under the condition of 40.51 < lnZ < 46.33, which is ultimately beneficial to the development of heterogeneous bimodal Mg-Gd-Y-Zn-Zr alloy.