Lamellar Long Period Stacking Ordered Research Articles

Regulating the microstructures and creep behaviors of an LPSO-containing Mg alloy via heat treatment

The microstructures and creep behaviors of a hot-extruded Mg-Gd-Y-Zn-Zr alloy containing long period stacking order (LPSO) phases prepared by different heat treatment procedures were investigated in this work. Three samples were made, including Sample #1, which was in the as-extruded state; Sample #2, which was preserved at 500 °C for 12 h; and Sample #3, which was preserved at 500 °C for 12 h and 460 °C for 10 h. The average grain sizes of the heat treated samples were much larger than those of the as-extruded samples. The second phases included mainly blocky 18R LPSO phases, lamellar 14H LPSO phases and needlelike ZnZr compounds in the three samples, but Sample #2 had much fewer 14H LPSO phases than the other two samples. The grain boundary migration resulting from the small grain size induced the lowest creep resistance in Sample #1. In contrast, the hardening effects generated from the pyramidal <c+a> dislocations and the dynamic precipitation of dense β’ phases dramatically retarded the cross-slip, resulting in the highest creep resistance in Sample #2. Although the aging treatment introduced numerous 14H LPSO phases in Sample #3, they delayed the dislocation movements to a limited degree, and the creep resistance was inferior to that of Sample #2. Therefore, the creep resistance ultimately decreased in the order of Sample #2 > Sample #3 > Sample #1. It is suggested that the microstructures and creep behaviors of LPSO-containing Mg alloys can be well regulated through appropriate heat treatment, and this work can hopefully offer important guidance for the design of highly creep resistant Mg-Gd-Y-Zn-Zr alloys.

Inhomogeneous deformation behavior and recrystallization mechanism of Mg-Gd-Y-Zn-Zr alloy containing only intragranular lamellar LPSO phase

Inhomogeneous plastic deformation and recrystallization behavior of Mg-Gd-Y-Zn-Zr alloy containing only intragranular lamellar long-period stacking ordered (LPSO) phase were investigated by isothermal compression experiments at the temperatures of 350–450 °C, and the strain rates of 0.001 ∼ 10 s−1. Results showed that Mg-Gd-Y-Zn-Zr alloys with varying degrees of bimodal structures were obtained after compression. The lamellar LPSO phase directions in residual coarse grains were aligned in the radial direction (RD). Dynamic recrystallization (DRX) grains induced at grain boundaries, kinked bands, and lamellar shearing regions during compression synergistically promoted coarse grain refinement. Moreover, the condition of high temperature and medium strain rate (450 °C-0.1 s−1) contributed to the uniform extension of "mantle layers" of fine DRX grains. In contrast, the condition of medium temperature and high strain rate (400 °C-10 s−1) could accelerate the deflection of CD-directed (compression direction) LPSO lamellae and the formation of local high-energy regions, facilitating the generation of recrystallization band. In addition, a high strain rate (400 °C-10 s−1) is more conducive to the formation of a heavily bimodal structure than a high temperature (450 °C-0.1 s−1). Only a larger spacing of LPSO lamellae (∼1.4 μm) could meet the spatial requirements of recrystallization. Further analysis revealed that the continuous dynamic recrystallization (CDRX) mechanism characterized by the expansion of "mantle layers" and nucleation along kinked boundaries and lamellar shearing regions was the primary grain refinement mechanism. The discontinuous dynamic recrystallization (DDRX) mechanism only exhibited a limited role due to the inhibition of the highly dense lamellar LPSO phase on grain boundaries bulging. After compression, the orientation of residual coarse grains gradually deviated toward the CD. While the recrystallization with limited proportion and random orientation cannot significantly weaken the basal texture.

Improving strength-ductility of Mg-8.5Gd-4.5Y-0.8Zn-0.4Zr magnesium alloy due to bimodal LPSO and dislocations

Rare-earth (RE) magnesium alloys have attracted lots of attention due to their excellent mechanical properties. In this work, the microstructure and mechanical properties of as-extruded 8.5Gd-4.5Y-0.8Zn-0.4Zr magnesium alloy under different solution treatment were examined with the optical microscope (OM), scanning electron microscope (SEM), high resolution transmission electron microscope (HRTEM), electron back-scattered diffraction (EBSD) and Instron testing machine. The results show that the ES12 alloy (solution treatment for 12 h at 520 °C) has the highest ultimate tensile strength (UTS) of 390 MPa with a fracture elongation of 24.5% at the cost of a minor drop in yielding strength (YS) compared to the as-extruded alloy. During solution treatment, the block-shaped long period stacking ordered (LPSO) in as-extruded alloy evolves into plate-shaped LPSO, which disperses at grain boundaries (GBs), and lamellar LPSO, which distributes in grains. The coexistence of plate-shaped and lamellar LPSO, which impedes the dislocations movement, and the activated <c + a> dislocations are regarded as the primary reasons for mechanical properties improvement. Furthermore, the (11–21) <1–100> texture in as-extruded alloy transforms into the (11–20) <0001> texture in ES12 alloy. The average grain size increases from 3.45 μm in as-extruded alloy to 18.70 μm in ES12 alloy. The Schmid factors of {0001} <11–20>, {10-10} <11–20>, {10–11} <11–20>, and {11–22} <11–23> increase, which indicate that slip systems are more easily activated in plastic deformation. The dynamic recrystallization (DRX) grains fraction increase to 92.8% for ES12 alloy due to the particle-stimulated nucleation (PSN) mechanism triggered by block-shaped and plate-shaped LPSO. The freshly DRXed grains further weaken the texture, and reduce the dislocation density. All of these factors increase elongation of RE magnesium alloy.

Deformation behavior of LPSO phases, grain refinement mechanism, and texture evolution of a Mg-Gd-Y-Zn-Zr alloy processed by forward extrusion combined with dual-directional angular deformation

In this study, a Mg-Gd-Y-Zn-Zr alloy was processed by forward extrusion combined with dual-directional angular extrusion (FE-DDAE), and the corresponding microstructure evolution, grain refinement mechanism, and texture were investigated systematically. The deformed materials were subjected to a combination of three-way compressive stress and shear stress, and five different areas were selected to study the typical deformation behaviors. The results show that microstructures are heterogeneous in different areas, and the grain size is refined from 138.70 μm to 37.18 μm through continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX). The block-shaped and lamellar long period stacking ordered (LPSO) phases can coordinate severe deformation through dissolution, tearing, breaking for the former, and bending, severe kinking, refining into small pieces for the latter. Moreover, the broken block-shaped LPSO can promote DRX process through particle-stimulated nucleation (PSN) mechanism, and the lamellar LPSO phases can hinder the dislocation motion to restrict DRX. A strong basal texture is observed during the initial stage of FE-DDAE process, and the basal texture intensity gradually weakens to 4.9 MRD due to the random orientations of high volume fraction of DRX grains. A mixed texture component is formed with basal planes inclined 50° from ED to TD, showing a trend of spreading.

Developing a novel high-strength Mg-Gd-Y-Zn-Mn alloy for laser powder bed fusion additive manufacturing process

Laser powder bed fusion (LPBF) of Mg alloys mainly focuses on the traditional commercial casting Mg alloys such as AZ91D, ZK60 and WE43, which usually display relatively low tensile strengths. Herein we developed a novel high-strength Mg-12Gd-2Y-1Zn-0.5Mn (wt.%, GWZ1221M) alloy for the LPBF additive manufacturing process, and the evolution of microstructure and mechanical properties from the as-built state to LPBF-T4 and LPBF-T6 states was systematically investigated. The as-built GWZ1221M alloy exhibited fine equiaxed grains with an average grain size of only 4.3 ± 2.2 μm, while the as-cast alloy displayed typical coarse dendrite grains (178.2 ± 73.6 μm). Thus, the as-built alloy showed significantly higher tensile strengths than the as-cast counterpart, and its yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) were 315 ± 8 MPa, 340 ± 7 MPa and 2.7 ± 0.5 % respectively. Solution treatment transformed hard and brittle β-(Mg,Zn)3(Gd,Y) phase into basal X phase and lamellar long period stacking ordered (LPSO) with better plastic deformability, leading to the improvement of EL. Then peak-aging heat treatment introduced numerous nano-sized prismatic β′ precipitates inside grains, resulting in the enhancement of YS. Finally, the LPBF-T6 alloy achieved appreciably high strength with YS, UTS and EL of 320 ± 3 MPa, 395 ± 4 MPa and 2.1 ± 0.4 % respectively. Both as-built and LPBF-T6 GWZ1221M alloys showed remarkably higher tensile strengths than the as-cast counterparts and as-built commercial Mg alloys, highlighting the great potential of high-strength as-built Mg-Gd based alloys for structural applications.

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.

The effect of hydrostatic pressure on grain refinement and deformation behaviors of Mg–Gd–Y–Zn–Zr alloy under complex shear stress

The effects of hydrostatic pressure on grain refinement and deformation behaviors of Mg–13Gd–4Y–2Zn–0.5Zr alloy were investigated. With the increase of hydrostatic pressure, the equivalent stress and forming load increase, and stress triaxiality also increases. The hydrostatic pressure has little effect on dynamic recrystallization (DRX) fraction and grain size, and the DRX fraction difference is not greater than 10%. This is because DRX critical strain and grain nucleation are almost unaffected by hydrostatic pressure. The slight increase in DRX fraction under higher hydrostatic pressure is attributed to the enhanced <a> dislocation activity near the lamellar long-period stacking ordered (LPSO) phase and significant local temperature elevation in the deformation zone. The high hydrostatic pressure is mainly borne by the LPSO phase. The bending deformation energy of the lamellar LPSO phase increases with the increase of hydrostatic pressure. The area fraction of samples is decreased from 18.5% of one direction compression & simple shear (1-CS) sample to 13.6% of three directions compression & simple shear (3-CS) sample. The solute diffusion and deconstruction of the bulk LPSO phase occur at higher hydrostatic pressure. The hydrostatic pressure has little effect on the basal texture strength of α-Mg phase under strong shear stress. The activation of <c+a> dislocation, as the primary source of non-basal slip, shows the same distribution characteristics.

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.

Deformation behaviors and the related high-temperature mechanical properties of Mg–11Gd–5Y–2Zn–0.7Zr via regulating extrusion temperatures

The effects of extrusion temperatures (430, 450, 470 and 490 °C) on the microstructures and mechanical properties of Mg–11Gd–5Y–2Zn–0.7Zr (GW) alloys at room temperature (RT), 250, 300, 350 °C were systematically investigated. The dynamic precipitation of blocky and lamellar long period stacking ordered (LPSO) phases was taken place during hot extrusion. The dominant texture changed from <10 1¯0 >//ED to <0001>//ED with increasing extrusion temperature. Tensile results indicated that the GW alloy extruded at 430 °C (GW430) exhibited the highest mechanical properties at both RT and elevated temperatures, which was ascribed to the fine dynamic recrystallized (DRXed) grains, un-DRXed grains with an orientation of <10 1¯0 >//ED, and the LPSO phase strengthening. When tensile tested below 300 °C, slips on the blocky LPSO phase and kinks of lamellar LPSO phase were the primary deformation modes, followed by grain boundary crack. At 350 °C, grain boundaries became the vulnerable sites to be activated. For GW alloys extruded at 450, 470 and 490 °C, an abnormal increase of mechanical properties was observed at 300 °C. This was due to the occurrence of multiple-slip, which showed the intersection and entanglement of dislocations and thus improving the strength of Mg matrix.

Achieving outstanding heat-resistant Mg-Gd-Y-Zn-Mn alloy via introducing RE/Zn segregation on α-Mn nanoparticles

A novel Mg-8Gd-2Y-1.5Zn-1Mn (GYZ-1Mn, wt.%) alloy with extraordinary high-temperature mechanical properties was developed by introducing RE/Zn segregation on α-Mn nanoparticles. Its ultimate tensile stress (UTS) and yield stress (YS) are 312 MPa and 248 MPa at 300 ℃, which are 87 MPa and 52 MPa higher than the highest strengths of other Mg-Gd-Y-Zn alloys reported in the previous literature. The outstanding performance of GYZ-1Mn alloy was associated with the segregation of RE/Zn atoms on the α-Mn nanoparticles, which facilitated the formation of fine lamellar long period stacking ordered (LPSO) phase and impeded the coarsening of α-Mn. Consequently, retarded dynamic recrystallization behavior occurred in the GYZ-1Mn, resulting in fine grain size, high density of geometrically necessary dislocations and strong fiber texture intensity to strengthen the high-temperature mechanical properties. Hence, this work paved a novel path for the development of heat-resistant Mg alloys by introducing elemental segregation on nanoparticles.

Optimizing LPSO phase to achieve superior heat resistance of Mg–Gd–Y–Zn–Zr alloys by regulating the Gd/Y ratios

The microstructural features of long period stacking ordered (LPSO) phase in Mg–(11-x)Gd–xY–1Zn–0.3Zr (wt.%) alloys were optimized by adjusting Gd/Y ratios to10:1, 8:3, 6:5, 4:7 and 2:9, respectively. Superior high-temperature strength was successfully obtained in the extruded 6Gd–5Y alloy with ultimate tensile strength (UTS) values of 338 MPa and 286 MPa at 250 °C and 300 °C, respectively. In the as-cast alloys, with the decrease of Gd/Y ratio, the type of intermetallic compound gradually changed from (Mg, Zn)3(Gd, Y) (W) to LPSO phase. After homogenization, the W phase was completely converted to LPSO phase except for the 10Gd–1Y alloy. Importantly, the block-shaped LPSO phases with the smallest size and the abundant lamellar LPSO phases were formed in the 6Gd–5Y alloy, which significantly influenced the microstructures of the extruded alloy. Firstly, the small sized block-shaped LPSO phase before extrusion provided favorable conditions for the formation of thin plate-shaped LPSO phase at DRXed regions, which effectively hindered the grain boundary migration at high temperature due to superior thermal stability. Secondly, the profuse lamellar LPSO phases prior to extrusion contributed to the highest density of low angle grain boundaries (LAGBs) and residual dislocations in the extruded 6Gd–5Y alloy, which obstructed dislocation slip at elevated temperatures. In addition, the abundant lamellar LPSO phase also caused the fine DRXed grain size and strong basal texture to enhance the high-temperature strength of the extruded 6Gd–5Y alloy.

Revealing the deformation behavior and microstructure evolution in Mg-12Y–1Al alloy during hot compression

In this study, the hot uniaxial compression of the heat-treated Mg-12Y–1Al alloy containing long-period stacking ordered (LPSO) phase was conducted by a Gleeble-3500 simulator. Compressive mechanical properties at elevated temperatures and the effects of deformation temperature (300–450 ℃) and strain rate (0.001–1 s−1) on the hot deformation behavior of the alloy were systematically investigated. Combining with an optical microscope (OM), electron backscatter diffraction (EBSD), and in-grain misorientation axes (IGMA) analysis, the microstructure and texture evolution, dislocation slip, and dynamic recrystallization (DRX) mechanism of the alloy was studied in detail. The results showed that the flow stress decreased with the increase in temperature and the decrease in strain rate. At a strain rate of 1 s−1, the compressive strength at 300 ℃ and 350 ℃ was basically the same. After deformation, the morphology of Al2Y had almost no change, while the LPSO underwent apparent kinking deformation. The increase in temperature was conducive to DRX. The DRX mechanism of the alloy mainly included discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX), and DDRX was dominant. The dense lamellar LPSO at the grain boundary was detrimental to the occurrence of DDRX. In addition, the deformed microstructure presented a typical [0001]//CD (compression direction) texture. With the increase in temperature, the texture intensity decreased progressively due to the activation of multiple slip systems and the increased DRX fraction. At a strain rate of 0.01 s−1, when the temperature increased to 450 ℃, the number of deformed grains dominated by pyramidal Ⅱ<c+a> slip increased substantially.

Introducing lamellar LPSO phase to regulate room and high-temperature mechanical properties of Mg-Gd-Y-Zn-Zr alloys by altering cooling rate

The mechanical properties of the extruded and peak-aged Mg-12.8Gd-4.7Y-2.6Zn-0.9Zr (wt.%) (GWZ1352) alloys at room and high temperatures were systematically investigated with respect to their microstructures. The microstructural features were tailored through altering the cooling rates after homogenization by water-quenching, air-cooling and furnace-cooling, respectively. As the cooling rate reduced, the number of lamellar long period stacking ordered (LPSO) phase increased substantially and its spacing decreased gradually. Additionally, solute segregation at grain boundaries occurred after furnace-cooling. After extrusion, the quenching (QE) alloy exhibited nearly full dynamic recrystallized (DRXed) grains, whereas the air-cooling (AE) and furnace-cooling (FE) alloys displayed a typical bimodal structure consisting of fine DRXed and coarse un-DRXed grains. The nucleation and development of the DRXed grains were hampered by the nano-spaced lamellar LPSO phase and the solute segregation of grain boundary. The excellent high temperature mechanical properties of the FE alloy were mostly due to the abundant lamellar LPSO phase. After aging treatment, plentiful fine β′ phase was precipitated in the quenching and aged (QEA) alloy. In contrast, the β′ phase with a larger size and lower density, accompanied with a wide precipitation-free zone (PFZ) at grain boundaries was formed in the air-cooling and aged (AEA) and furnace-cooling and aged (FEA) alloys, which severely deteriorated their mechanical properties both at room and high temperatures.

Microstructure, texture evolution and mechanical properties of a large-scale multidirectionally forged Mg-Gd-Y-Zn-Zr-Ag alloy

A large-scale Mg-6.2Gd-3.7Y-0.9Zn-0.3Zr-0.3Ag (wt.%) magnesium component with the dimension of 480 × 250 × 160 mm3 was fabricated via direct-chill (DC) casting, homogenization and multidirectional forging (MDF). The evolution of the microstructure, texture and uniaxial tensile properties during MDF process were comprehensively investigated. 2.5%, 5.6% and 10.9% anisotropy were obtained in the MDF alloy subjected to 9, 18 and 27 passes, respectively. The MDF alloy subjected to 27 passes tensile along FD exhibits superior comprehensive mechanical properties, with a yield strength (TYS) of 292 MPa, ultimate tensile strength (UTS) of 384 MPa and elongation of 9.0%. Interdendritic Mg5(Gd, Y, Zn) phases dissolved after homogenization, with the precipitation of intragranular lamellar 14H long period stacking ordered (LPSO) phases from α-Mg matrix. During the MDF process, intragranular lamellar LPSO phases were initially kinked, suppressing dynamic recrystallization (DRX) behavior in the first 9 passes, and subsequently partially dissolved. Dynamic precipitation of Mg5(Gd, Y) and ultrafine LPSO phase were also induced by MDF. With the cumulative deformation of MDF, increased volume fraction of Mg5(Gd, Y) phases, enhanced texture and refined α-Mg grains are likely responsible for the improved mechanical properties via MDF. We also found that prismatic <a> slip is activated in the deformed grains with the c-axis around FD and multiple slip is activated in the other deformed grains during the MDF process. This paper provides a superior MDF processing route to manufacture high-performance large-scale Mg-Gd-Y-Zn-Zr-Ag components for industrial production.

Investigations on microstructure and mechanical properties of cast Mg-Gd-Y-Zn-Zr-Nd alloy

Different solution treatments were carried out on the semi-continuously cast Mg-9.55Gd-2.27Y-1.28Zn-0.55Zr-0.11Nd (wt.%) alloy at temperatures of 480 °C, 500 °C, and 520 °C. It was found that the different solution treatment temperatures led to different phase constitutions of the alloy. The Mg3RE-type eutectic phases transform into long period stacking order (LPSO) phases with various morphologies after solution treatment. Increasing the solution temperature resulted in larger grain size, greater number of lamellar LPSO phases, and segregation of Zr element. Block LPSO phases also shifted to rod LPSO phase. After ageing treatment, the special spatial structure consisting of prismatic β′ precipitates, basal γ′ precipitates, and lamellar LPSO phases hindered dislocation movement effectively, leading to high strength.However, the accumulation of massive dislocations promoted the premature formation of cracks at the grain boundary and LPSO/Mg matrix interface, resulting in a rapid decrease in ductility of the alloy. After solution treatment at 480 °C for 12 h and aged at 200 °C for 48 h, the Mg-9.55Gd-2.27Y-1.28Zn-0.55Zr-0.11Nd alloy exhibited favorable comprehensive mechanical properties at room temperature: ultimate tensile strength (UTS) of 383 MPa, tensile yield strength (TYS) of 269 MPa, and fracture elongation (EL) of 5.1%.

Dissolution and reprecipitation of 14H-LPSO structure accompanied by dynamic recrystallization in hot-extruded Mg89Y4Zn2Li5 alloy

We investigate the variation induced in long-period stacking ordered (LPSO) structures, dynamic recrystallization (DRX), and mechanical performance of hot-extruded Mg89Y4Zn2Li5 alloys fabricated at different extrusion speeds (Ve = 0.4, 0.8, 1.0, 1.2 mm/s) and die angles (α = 30°, 60°, 90°) under 400 °C, the dissolution and reprecipitation of 14H LPSO structure accompanied by DRX process are then clarified in detail. Upon all extrusion conditions, the block 18R LPSO structures elongate in the extrusion direction, while the lamellar 14H LPSO structures dissolve under the deformation strain. In addition, due to discontinuous and continuous DRX mechanisms, all hot-extruded alloys have a full DRX microstructure consisting of equiaxed recrystallized grains, but the DRX grain size reduces when both extrusion speed and die angle decrease. Note that, in the interior of DRX grains, thin LPSO lamellae mixing 14H, 18R and 24R structures nucleate and dynamically precipitate due to the dissolution of the original lamellar 14H LPSO structures. Furthermore, the hot-extruded Mg89Y4Zn2Li5 alloy becomes stronger as decreasing of the extrusion speed and die angle, whereas the ductility remains nearly constant. Finally, the hot-extruded Mg89Y4Zn2Li5 alloy achieves an excellent strength-ductility balance at a relatively low extrusion speed (0.4 mm/s) and small die angle (30°) mainly due to the elongated 18R LPSO structure, fine and full DRX microstructure, thin mixed LPSO precipitates in the DRX grains, twins and dislocations.

Effect of long-period ordered stacking on twinning behavior and mechanical properties of Mg-Al-Y alloy during uniaxial hot compression

Two alloys with (T4-4h) and without (T4-8h) long-period stacking ordered (LPSO) phase were obtained by solution treatment of as-cast Mg-1Al-12Y alloy at 540 ℃ for 4 h and 8 h, respectively. The compressive tests (300 – 450 ℃, ε=0.01s−1) showed that the strengthening effect of LPSO in T4-4h alloy gradually disappeared with temperature increase. In addition, the corresponding compression deformation behavior of the two alloys was also investigated by electron backscatter diffraction (EBSD). It was found that the twinning of the two alloys was dominated by {101¯2} extension twins, and the large amount of lamellar LPSO in T4-4 h alloy inhibited the activation and growth of twins. The [0001] // compression direction (CD) texture formed after deformation is mostly attributed to the activation and growth of extension twins. By analyzing the activated twin variants, it was found that the activation of twin variants mainly depended on Schmid factor (SF). For twin variants with smaller SF, they may be activated due to local stress concentration. Based on the analysis results of in-grain misorientation axis (IGMA) and SF, the deformation mechanism of the alloys at elevated temperature is dominated by basal slip and non-basal slip. The stronger mechanical properties of T4-4h alloy than those of T4-8h are attributed to the obstruction of non-basal slip movement by LPSO.

Microstructure evolution and mechanical properties of high-strength Mg-Gd-Y-Zn-Mn alloy processed by asymmetric hot rolling

Asymmetric hot rolling was used to fabricate the high-strength Mg-9Gd-4Y–1Zn-0.8Mn (wt.%) alloys, which is strengthened by the lamellar long-period stacking ordered (LPSO) phases inside grains and the block-shaped LPSO phases surrounding grain boundaries. The reduction in total thickness shows a significant impact on the microstructure, texture, and mechanical properties of the alloy sheets. By keeping the velocity ratio between the top and lower rollers at 1.3, the shear strain is introduced all across the sheet thickness. The as-rolled sample exhibits a bimodal-grained structure made up of fine recrystallized (DRXed) grains with relatively random orientation and coarse elongated non-recrystallized (unDRXed) grains with intense basal texture and profuse substructure. As rolling strain increases, the volume fraction of the DRXed region and the basal texture intensity of the unDRXed region gradually increase. The dominating prismatic <a> slip as well as the moderate basal <a> slip and pyramidal <c+a> slip are activated during rolling. The triggering of prismatic <a> slip, among the activated multiple slip systems, results in the development of [101¯0]//RD texture component at high strains. The alloy sheet that underwent severer rolling strain (56.3%R sample) shows higher ultimate tensile strength (411 MPa) and tensile yield strength (348 MPa) but lower elongation to failure (3.5%). The dominant strengthening mechanisms for the preferable mechanical properties should be grain boundary strengthening of fine DRXed grains, texture strengthening of coarse elongated unDRXed grains, short-fiber strengthening of block-shaped LPSO phases, and dispersion strengthening of profuse lamellar LPSO phases and dense tiny Mg5(Gd, Y) particles.

The influence of the LPSO on the deformation mechanisms and tensile properties at elevated temperatures of the Mg-Gd-Zn-Mn alloys

The introduction of long period stacking ordered (LPSO) phase is an effective way to improve the performance of the magnesium alloys at elevated temperatures. In this paper, the hot tensile behaviors of two Mg-4Gd-1Zn-0.5Mn (wt.%) alloys with different lamellar LPSO volume fractions under the strain rate of 2.2 × 10–4 s –1 and at varied temperatures (150 °C–250 °C) were investigated. With the increasing of the deformation temperatures, the peak stresses of the alloy with lower volume fraction of the LPSO decreased (denoted as LL alloy). The hot tensile mechanical properties of the alloy with higher volume fraction of the LPSO (denoted as HL alloy) are significantly higher than those of LL alloy. When the deformation temperature was 250 °C, the LL alloys significantly underwent dynamic recrystallization (DRX) and the DRX dominated by discontinuous dynamic recrystallization (DDRX). However, the HL alloys did not have obvious DRX due to the lamellar LPSO hindering the grain boundary migration. The basal fiber texture of the alloy was produced after hot tensile. The deformation of the LL alloys was dominated by prismatic <a> slip during the hot deformation while the deformation of the HL alloys was dominated by the prismatic <a> slip and pyramidal <c+a> slip. The pyramidal <c+a> slip was activated by the Young's moduli mismatch between the α-Mg matrix and LPSO. Lamellar LPSO has an positive effect on strengthening the mechanical properties of the alloy which is attributed to the hinder of LPSO to non-basal slip, the prick function of kink to basal slip and the inhibition of LPSO to DDRX.