Lamellar Long Period Stacking Ordered Phase Research Articles

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.

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.

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.

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.

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.

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.

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.

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.

Mechanical and Corrosion Properties of Mg–Gd–Cu–Zr Alloy for Degradable Fracturing Ball Applications

Generally, excellent mechanical properties of Mg alloys are desired, but their rapid degradation properties are seldom utilized. Petroleum fracturing techniques are required to take full advantage of this rapid degradation. Therefore, we have prepared an as-extruded Mg–6.0Gd–1.2Cu–1.2Zr (wt.%) alloy and treated it with peak aging to analyze its potential as a degradable fracture ball. The results show that the as-extruded alloy mainly consists of an α-Mg matrix, second phase, and large elongated α-Mg grains (LEGs). After aging, the LEGs undergo static recrystallization, which improves the mechanical properties of the alloy, and a lamellar long period stacking ordered (LPSO) phase is observed. Under simulated underground temperature conditions (93 °C), the ultimate tensile strength and elongation of both as-extruded and as-aged alloys are over Ȧ MPa and 11.1%, respectively, and the ultimate compressive strength and elongation of both alloys are over 336 MPa and 16.9%, respectively. The corrosion rate of the as-extruded alloy in 3 wt.% KCl solution at 93 °C reaches 1660.8 mm/y by mass loss test, and that of the as-aged alloy increases to 1955.1 mm/y. The atomic force microscope analysis result confirms that the second phase shows the highest corrosion potential, followed by the lamellar LPSO phase and α-Mg matrix. The as-extruded and as-aged Mg–6.0Gd–1.2Cu–1.2Zr alloy with good mechanical properties and a high corrosion rate in this work shows promising potential for degradable fracturing ball applications.

A novel Mg-Gd-Y-Zn-Cu-Ni alloy with excellent combination of strength and dissolution via peak-aging treatment

Inferior absolute strength and dissolution properties are the main bottlenecks for the widespread application of dissolvable magnesium alloys in complex working environments for unconventional oil and gas resources. Here, a novel functional peak-aged Mg-9.5Gd-2.7Y-0.9Zn-0.8Cu-0.4Ni (wt.%) alloy for fracturing tools is reported, and it possesses an ultimate tensile strength of 457.6 MPa, ultimate compressive strength of 620.7 MPa and dissolution rate of ∼43.7 mg·cm−2·h−1 in 3 wt.% KCl solutions at 93 °C. The excellent strength of the aged-alloy is primarily attributed to the combination of grain refinement, long-period stacking ordered (LPSO) strengthening, and precipitation strengthening induced by stacking fault and β’ phase, among which the precipitation strengthening is dominant. Further investigations confirm that the corrosion is triggered from the micro-galvanic coupling between the Mg matrix and the cathodic lamellar and block LPSO phases. Strip-shaped corrosion pits along with LPSO phases are subsequently formed, significantly accelerating corrosion. The β’ precipitates can effectively improve the strength without compromising the dissolution rate because of their nanoscale size. This study provides an excellent material selection for dissolvable fracturing tools and presents a strategy by which a synergistic combination of strength and dissolution rate is achieved via peak-aging treatment.

Nanoindentation on the Transformation of LPSO Phases during Different Solution Heat Treatments in an Mg-Dy-Nd-Zn-Zr Alloy

The objective of this study is the investigation of nanomechanical properties using nanoindentation of extruded and heat-treated Mg-Dy-Nd-Zn-Zr, with an emphasis on the transformation of long-period stacking-ordered (LPSO) phases. Solution heat treatment was performed with different heat treatment for durations on hot extruded Mg-Dy-Nd-Zn-Zr to monitor the transformation of LPSO phases, as well as to keep track of microstructural changes. The initial fine-grained microstructure, with blocky and lamellar LPSO structures within the matrix, first transformed into coarser grains with fewer LPSO lamellae, which then increased in amount again at higher annealing duration. The blocky LPSO phases, which have the highest hardness compared to the matrix grains with and without LPSO lamellae, consistently decrease in quantity, as so does the trend in their hardness value. The Mg matrix grains with LPSO lamellae show a lower hardness compared to the Mg matrix grains without or with a just few lamellar LPSO phases, and increase in quantity at long annealing durations. The overall hardness of the microstructure is essentially determined by the LPSO lamellae-containing grains and reaches a peak at 24 h. There is another peak found for the grain size values; however, this is at later annealing duration, at 72 h. The reduction in grain size towards longer annealing durations goes along with a reactivated formation of LPSO lamellae.

An investigation of the deformation and cracking mechanism of Mg–1Al–12Y alloys based on in-situ SEM/EBSD slip trace analysis

In this work, the deformation mechanism and cracking behavior of Mg–1Al–12Y alloys with different contents of long-period stacking ordered (LPSO) phase during in-situ tensile testing at ambient temperature were investigated in detail using a scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). The slip trace analysis results showed that the deformation of the three alloys was all dominated by dislocation slip and supplemented by extension twinning. The predominant slip deformation mode was basal <a> slip. The Mg–1Al–12Y alloy after solution treatment at 540 °C for 4 h (T4-4h) exhibited higher elongation due to greater non-basal slip activation (31.8%) and cross slip occurrence (10.4%) and lower twinning activity. This was primarily attributed to the fact that profuse lamellar LPSO phases in the matrix of T4-4h alloy promoted the activation of non-basal slip and inhibited the initiation of extension twins. In addition, crack nucleation of the three alloys was mainly concentrated at the grain boundaries and triple junction of the grain boundaries. Nucleation and crack propagation were mostly attributed to the surrounding grains with a hard orientation and the second phase at the grain boundaries, which was not conducive to coordinated deformation.

Crack Propagation in As-Extruded and Heat-Treated Mg-Dy-Nd-Zn-Zr Alloy Explained by the Effect of LPSO Structures and Their Micro- and Nanohardness.

The investigation of the crack propagation in as-extruded and heat-treated Mg-Dy-Nd-Zn-Zr alloy with a focus on the interaction of long-period stacking-ordered (LPSO) structures is the aim of this study. Solution heat treatment on a hot extruded Mg-Dy-Nd-Zn-Zr (RESOLOY®) was done to change the initial fine-grained microstructure, consisting of grain boundary blocky LPSO and lamellar LPSO structures within the matrix, into coarser grains of less lamellar and blocky LPSO phases. C-ring compression tests in Ringer solution were used to cause a fracture. Crack initiation and propagation is influenced by twin boundaries and LPSO lamellae. The blocky LPSO phases also clearly hinder crack growth, by increasing the energy to pass either through the phase or along its interface. The microstructural features were characterized by micro- and nanohardness as well as the amount and location of LPSO phases in dependence on the heat treatment condition. By applying nanoindentation, blocky LPSO phases show a higher hardness than the grains with or without lamellar LPSO phases and their hardness decreases with heat treatment time. On the other hand, the matrix increases in hardness by solid solution strengthening. The microstructure consisting of a good balance of grain size, matrix and blocky LPSO phases and twins shows the highest fracture energy.

Microstructure Evolution and Mechanical Properties of Mg-Gd-Y-Zn-Zr Alloy during Compression

The compression behavior and mechanical properties of the Mg-13Gd-4Y-2Zn-0.5Zr (wt.%) alloy filled with intragranular long-period stacking ordered (LPSO) phases at different temperatures were investigated. The results showed that the higher the compression temperature, the smaller the plastic strain that the grains withstand. The grains changed from equiaxed to flat strips when compressed at 350°C, and the morphology of the grains did not change at 450°C. Due to the existence of DRX grains, compression at 450 °C didn’t cause large-angle kink, but the kink angle at 350°C was very large. DRX grains only appeared at the grain boundaries and around the intergranular LPSO phase at the beginning of compression, and only appear at the kink bands (KBs) after the lamellar LPSO phases begin to kink. DRX grains gradually increased with the KBs increasing.

Influence of heat treatment on the tensile properties and fatigue properties of Mg-8.8Gd-3.5Y-1.5Zn-0.5Zr alloy

The microstructure, tensile properties and fatigue properties of Mg- 8.8Gd- 3.5Y- 1.5Zn- 0.5Zr alloy under T4 and T6 heat treatment have been studied. The as-cast alloy was homogenized to eliminate the inhomogeneous structure and the strength of the alloy was increased by aging treatment. The microstructure evolution and mechanical properties of the alloy were investigated by optical microscope, scanning electron microscope, energy dispersive spectrometer and universal mechanical testing machine. The as-cast alloy is composed of dendritic α-Mg matrix, metastable lamellar stacking fault, eutectic Mg24(Gd, Y, Zn)5, massive long-period stacking ordered (LPSO) phase Mg12(Gd, Y)Zn and a few rare earth rich phases. In the process of homogenization, the lamellar phase and Mg24(Gd, Y, Zn)5 eutectic phase dissolved in the α-Mg matrix gradually, and the massive LPSO phase disappeared gradually, while the lamellar LPSO phase steadily grew into grains and formed some precipitate particles near the grain boundary. As the tensile temperature increases, the ductility of solid solution alloy increases, but decreases at 250 °C, which is related to the melting reaction of γ phase at high temperature and the cracking caused by grain boundary sliding. Intergranular fracture occurs at 250 °C. When the tensile temperature rises to 150 °C, the ultimate tensile strength of the alloy increases first before 150 °C and decreases after 150 °C, which is related to two strengthening mechanisms, namely, dissolution of Gd and Y elements into α-Mg matrix results in solid solution strengthening and the co-lattice strengthening between petal β′ and α-Mg matrix. Compared with the fatigue properties of the alloy after solution treatment and aging treatment, the fatigue life of the alloy after aging treatment is longer.

Corrosion behaviour of Mg-Gd-Y-Zn-Ag alloy components with different sizes after cooling

The corrosion behaviour of Mg-6Gd-3Y-1Zn-0.3Ag (wt.%) alloy components with different sizes after cooling was investigated. The alloys in the small components (SC) cooled fast, which were composed of α -Mg matrix and coarse long-period stacking ordered (LPSO) phases. The alloys in the large components (LC) cooled slowly, and there were thin lamellar LPSO phases precipitating inside the grains, except for α -Mg matrix and coarse LPSO phases. The hydrogen evolution test revealed that the corrosion rate of LC sample was higher than that of SC sample. Electrochemical impedance spectroscopy (EIS) test showed that the surface film on LC alloys provided worse protection. The corrosion morphologies indicated that the precipitation of the thin lamellar LPSO phases in LC sample caused severe micro-galvanic corrosion, which accelerated the rupture of the surface film.

Hot Deformation and Dynamic Recrystallisation Behaviour of Twin-Roll Cast Mg-6.8Y-2.5Zn-0.4Zr Magnesium Alloy.

In this work, the deformation behaviour of a twin-roll cast (TRC) Mg-6.8Y-2.5Zn-0.4Zr alloy during plane strain compression was characterised by high-temperature testing. Based on the experimental data, the values of strain-rate sensitivity, the efficiency of power dissipation and the instability parameter were investigated under the conditions of various hot deformation parameters. In contrast to conventionally cast material, no lamellae of the LPSO (long period stacking ordered) phase were precipitated in the magnesium matrix after TRC. The precipitation of fine lamellar LPSO phases only occurred during cooling to forming temperature after the heat treatment. Dynamic recrystallization (DRX) hardly occurred during deformation at temperatures between 350 °C and 400 °C. This can be attributed to the precipitation of the lamellar LSPO phases, which contribute to retardation of the DRX process. At higher deformation temperatures and strain rates DRX is pronounced and the twin-induced (TRDX) as well as continuous dynamic recrystallization could be identified as the dominant softening mechanisms. The processing maps were established by superimposing the instability map over the power dissipation map, this being associated with microstructural evolution analysis in the hot deformation processes. Two instability zones could be recognised for the twin-roll cast and heat-treated Mg-6.8Y-2.5Zn-0.4Zr alloy: (1) 350 °C to 460 °C and 0.01 s−1 to 0.3 s−1 and (2) 485 °C to 525 °C and 2.5 s−1 to 10 s−1, where deformation is not favourable.

A Study on the Damping Capacities of Mg–Zn–Y-Based Alloys with Lamellar Long Period Stacking Ordered Phases by Preparation Process

Mg alloys with fine mechanical properties and high damping capacities are essential in engineering applications. In this work, Mg–Zn–Y based alloys with lamellar long period stacking ordered (LPSO) phases were obtained by different processes. The results show that a more lamellar second phase can be obtained in the samples with more solid solution atoms. The density of the lamellar LPSO phase has an obvious effect on the damping of the magnesium alloy. The compact LPSO phase is not conducive to dislocation damping, but sparse lamellar phases can improve the damping capacity without significantly reducing the mechanical properties. The Mg95.3Zn2Y2.7 alloy with lamellar LPSO phases and ~100 μm grain size exhibited a fine damping property of 0.110 at ε = 10–3.

Microstructure Evolution of Mg96.9Gd2.7Zn0.4 Alloy Containing Multiple Phases Prepared by Spark Plasma Sintering Method

The synergic strengthening of multiple phases is an essential way to achieve high-performance Mg alloys. Herein, Mg-Gd-Zn alloy containing four phases was prepared by rapid solidification (RS) ribbons and spark plasma sintering (SPS). The microstructure of the alloy consisted of α-Mg, nanosized β1 phase particles, lamellar long period stacking ordered (LPSO) phase, and β′ phase precipitates. The microstructural evolution was also investigated. The results show that the metastable β1 phase was formed in the as-cast solidification through rapid solidification, because both Zn atoms and the short holding-time at molten liquid facilitated the formation of the β1 phase. The β1 phase grew from 35.6 to 154 nm during the sintering process. Meanwhile, the fine lamellar LPSO phase was simultaneously formed after the Zn-Gd clusters were generated from the supersaturated solid solution, and the width of the LPSO phase was only in the range of 2–30 nm. The third strengthening phase, the metastable β′ phase, was obtained by aging treatment. The results of hardness testing implied that the hardness of the alloy containing the aforementioned three nanosized strengthening phases significantly improved about 47% to 126 HV compared with that of the as-cast ingot.