Long Period Stacking Ordered Phase Research Articles

Dual phases strengthening behavior of Mg–10Gd–1Er–1Zn–0.6Zr alloy

Abstract The microstructure evolution and strengthening mechanisms of Mg–10Gd–1Er–1Zn–0.6Zr (wt.%) alloy were focused in the view of the size parameters and volume fraction (fp) of dual phases (long period stacking ordered (LPSO) structures and β′ precipitates). Results show that two types of LPSO phases with different morphologies formed, and the morphology and size of both LPSO phases varied with the solution conditions. However, the volume fraction decreased monotonously with increasing solution temperature, which in turn raised the volume fraction of β′ phase during aging. The alloy exhibited an ultimate tensile strength of 352 MPa, a yield strength of 271 MPa, and an elongation of 3.5% after solution treatment at 500 °C for 12 h and aging at 200 °C for 114 h. In contrast to the LPSO phase, the β′ phase seems to play a more important role in enhancing the yield strength, and consequently, a decreased fLPSO/fβ′ ratio results in an increased yield strength.

Loading Orientation Dependence of the Formation Behavior of Deformation Kink Bands in the Mg-Based Long-Period Stacking Ordered (LPSO) Phase

The variation in the deformation behavior of a directionally solidified (DS) Mg-based long-period stacking ordered (LPSO)-phase crystal depending on the loading orientation was examined. The frequency of formation of the beak-like shape of the deformation band, which is known as one of the important deformation mechanisms in the LPSO phase, monotonically decreased as the inclination angle of the loading orientation with respect to the crystal growth direction in the DS crystal increased, and was accompanied by a decrease in the yield stress due to the activation of basal slip. Deformation bands formed along a direction approximately perpendicular to the grain boundary independent of the loading orientation. The crystal rotation axes selected in the deformation bands were perpendicular to [0001] in almost all grains, independent of the loading orientation. However, the rotation axes in the bands were not fixed but varied between ⟨1010⟩ and ⟨1120⟩; this variation was correlated with the loading axis. These observed features strongly suggest that the deformation bands formed in the LPSO phase are predominantly deformation kink bands and that the formation mechanism itself does not vary with the loading orientation but instead its details. The selectivity of the crystal rotation axis in the kink band is strongly affected by the loading orientation.

Microstructure and mechanical properties of caliber rolled Mg–Y–Zn alloys

The microstructural evolution and mechanical properties were investigated using a caliber rolled Mg–5Y-2.5Zn (in at.%) alloy. Deformation kink bands could be induced by the caliber rolling process, which is the same as that of conventional wrought processed (extruded or rolled) Mg–Y–Zn alloys. The number of rolling passes was an influential factor for the microstructure, i.e., the bending angle of the long periodic stacking ordered (LPSO) phase associated with deformation kink bands. The mechanical properties of strength and hardness also depended on the number of rolling passes. These properties were improved with increased number of rolling passes (up to 6 passes). The yield strength and hardness before the rolling process were 260 MPa and ~90 Hv. On the other hands, after caliber rolling, these properties showed over 500 MPa and 129 Hv, respectively, for the alloy with 6 rolling passes. Microstructural observations revealed that such good properties resulted from the existence of deformation kink bands and dense dislocations induced by caliber rolling. Controlling the morphology of the LPSO phase associated with deformation kink bands was found to be an effective strategy; the bending angle of this phase of more than 20° is outstanding for further improving the mechanical properties.

Influence of Solution Heat Treatment on the Microstructure, Hardness and Stress Corrosion Behavior of Extruded Resoloy\xae

The microstructural changes and corrosion response of Resoloy®, a resorbable Mg-Dy-based alloy, are the focus of this study. Hardness, tensile and compressive, and bending tests are used to monitor the changes in the mechanical properties of this material. The corrosion behavior is investigated by stress corrosion of C-rings. Hot-extruded tubes are solution heat-treated at different temperatures and times. The as-extruded condition shows a homogeneous fine-grained microstructure with matrix long-period stacking-ordered (LPSO) structures. Heat treatment at low temperatures and for short times does not significantly change the microstructure but reduces the hardness. Solution heat treatment at relatively high annealing temperatures and long annealing times causes grain growth, resulting in reduced hardness. The microstructure becomes inhomogeneous, medium-sized grains grow, the matrix LPSO structures dissolve, and small bulk LPSO phases develop. The matrix LPSO structures have a positive effect on the corrosion behavior. In particular, the short-term annealing condition shows the most uniform corrosion morphology. Resoloy® is not free of pitting corrosion, but none of the samples fails by cracking.

Basal shearing of twinned stacking faults and its effect on mechanical properties in an Mg–Zn–Y alloy with LPSO phase

The precipitates inside deformation twins may block the dislocation motion and consequently affect the mechanical property of materials. Herein, at the atomic level, we directly visualize that the basal dislocation slips shear the twinned stacking faults (TSFs) within the deformation twins in an Mg–Zn–Y alloy containing long-period stacking ordered (LPSO) structures. The TSFs, enriched with solute atoms, could be considered as precipitates inside deformation twins. They are sheared by a single step or multiple shearing steps on the basal plane. The microstructural fingerprints, i.e., the width of basal shearing steps, enable a quantitative assessment of the local and total plastic shear strain due to the basal dislocation within the deformation twins. The TSFs can block dislocation slip, while the dislocation shearing induces large lattice distortion and even solute atoms redistribution at local intersection. The TSFs-dislocation interaction is expected to lower the basal dislocation motion and resultantly modulate the mechanical properties of magnesium alloys. These results may offer a novel strategy for strengthening and toughening magnesium alloys via tailoring the shearable precipitates.

Deformation Mechanism of Mg-Gd-Y-Zn-Zr Alloy Containing Long-Period Stacking Ordered Phases During Hot Rolling

The orientation-dependent deformation mechanisms including $$ \{ 10\bar{1}2\} $$ twinning, prismatic 〈a〉 slip and kink in hot rolling of a Mg-5.5Gd-4.4Y-1.1Zn-0.5Zr (wt pct) alloy containing long-period stacking ordered (LPSO) phases were examined in this study. For grains with c-axes close to the rolling direction, $$ \{ 10\bar{1}2\} $$ twinning is the major deformation mode. As for grains with c-axes close to the transverse direction, prismatic 〈a〉 slip and kink dominate the plastic deformation. Kink serves as a supplementary of prismatic 〈a〉 slip and only appears at a larger rolling reduction. The close value of CRSS for prismatic 〈a〉 slip and $$ \{ 10\bar{1}2\} $$ twinning was identified to be the cause of the competitive relationship between the two deformation modes at the temperature of 450 °C. The influence of LPSO phases on twinning was further evaluated by TEM. Twinning generally occurs in the lamellar matrix between LPSO phases with thickness above 100 nm and vanishes in the lamella with a very small thickness, such as 42 nm. Therefore, the occurrence of twinning is dependent on the thickness of the lamellar matrix between adjacent LPSO phases. As the distribution of LPSO phases in the tested material is relatively sparse, the activation of $$ \{ 10\bar{1}2\} $$ twinning deformation cannot be suppressed effectively.

Strain induced transformation, dynamic recrystallization and texture evolution during hot compression of an extruded Mg-Gd-Y-Zn-Zr alloy

The recrystallization mechanism and texture evolution of Mg-8.3Gd-3.6Y-1.6Zn-0.5Zr (wt. %) alloy were investigated in the course of thermomechanical processing at the temperature range of 300–500 °C. The correlation with high temperature flow behavior and the subsequent room temperature mechanical properties have been also assessed. The results indicated that the workability of the alloy was significantly improved owing to the occurrence of continuous dynamic recrystallization, where the recrystallized grain size decreased down to ~3 and ~1 μm at temperatures of 350 and 400 °C, respectively. In addition, the volume fraction of blocky long period stacking ordered (LPSO) phases was increased dynamically at 350 and 400 °C. The presence of blocky LPSO phases in high volumes acted as the particle stimulated nucleation (PSN) mechanism. At temperature of 500 °C, some of the blocky LPSO phases dissolved in the matrix, and this in turn could lead to the precipitation of lamellar ones in the recrystallized grains. Decrease in the basal slip Schmid factor due to the formation of blocky and lamellar LPSOs in the area inside the grains containing these phases demonstrates the important role of these phases in the hardening which is in competition with softening due to dynamic recrystallization. The corresponding texture analysis indicated that recrystallization through PSN could decrease the basal intensity thereby inducing a significant texture weakening effect. Interestingly, an appreciable enhancement in the stretch formability properties was achieved, which were justified considering the effect of grain size, volume fraction of both blocky and lamellar LPSO phases and texture.

Formation of long-period stacking-ordered (LPSO) structures and microhardness of as-cast Mg-4.5Zn–6Y alloy

Long-period stacking-ordered (LPSO) structures formed in both the α-Mg component and the interdendritic component in as-cast Mg-4.5Zn–6Y-0.5Zr (wt.%) alloy were studied from micro to atomic scales. Inside the α-Mg component, stacking-ordered structure took three forms: single unit, scattered cluster, and a single layer of short range 18R structures. Chemical ordering of clusters of Zn and Y formed in the short range scale but not in the long range. Inside the interdendritic component LPSO-layered structure took two forms: 18R and 14H LPSO. Long-range 18R formed at locations with high concentrations of Zn and Y. 14H LPSO phase formed at the edge of any layer of 18R LPSO phase, sandwiched between 18R and α-Mg band. We concluded that the formation of LPSO had occurred in two distinct steps. First, Zn and Y atoms were segregated into two adjacent atomic layers of the original hexagonal close-packed matrix. Then, the nucleation of stacking fault occurred, followed by the growth of a stacking fault, which attracted more Zn and Y atoms into four constitutive close-packed layers, forming a new segment of an LPSO structural unit.

Hot deformation of Mg-Y-Zn alloy with a low content of the LPSO phase studied by in-situ synchrotron radiation diffraction

The compressive deformation behavior of the extruded WZ42 (Mg98.5Y1Zn0.5 in at.%) magnesium alloy containing a low amount of long-period stacking ordered (LPSO) phase was studied by in-situ synchrotron radiation diffraction technique. Tests were conducted at temperatures between room temperature and 350 °C. Detailed microstructure investigation was provided by scanning electron microscopy, particularly the backscattered electron imaging and electron backscatter diffraction technique. The results show that twinning lost its dominance and kinking of the LPSO phase became more pronounced with increasing deformation temperature. No cracks of the LPSO phase and no debonding r at the interface between the LPSO phase and the Mg matrix were observed at temperatures above 200 °C. At 350 °C, the LPSO phase lost its strengthening effect and the deformation of the alloy was mainly realized by the dynamic recrystallization of the Mg matrix.

Ultrahigh strength Mg-Y-Ni alloys obtained by regulating second phases

Mg-Y-Ni alloys with different second phases were designed by changing Y/Ni atomic ratio from 1.5 to 0.5. The microstructure and mechanical properties of as-cast and as-extruded alloys were investigated. The as-cast Mg-Y-Ni alloy with Y/Ni ratio of 1.5 is composed of α-Mg and long period stacking ordered (LPSO) phase. When Y/Ni ratio is equal to 1, nanoscale lamellar γ' phase and eutectic Mg2Ni phase are formed in addition to LPSO phase. As Y/Ni ratio decreases further, the amount of eutectic Mg2Ni phase increases, while the amount of LPSO phase decreases. After extrusion, the LPSO and γ' phases are distributed along the extrusion direction, while eutectic Mg2Ni phase is broken and dispersed in the as-extruded alloys. LPSO phase and Mg2Ni phase in the alloys promote dynamic recrystallization (DRX) during extrusion, while γ' phase inhibits DRX. Consequently, the Mg96Y2Ni2 (at.%) alloy with LPSO phase and γ' phase as the main second phases shows the strongest basal texture after extrusion. The tensile yield strength of the as-extruded Mg-Y-Ni alloys increases first and then decreases with decreasing Y/Ni ratio. The as-extruded Mg96Y2Ni2 (at.%) alloy with Y/Ni = 1 exhibits excellent mechanical properties with tensile yield strength of 465 MPa, ultimate tensile strength of 510 MPa and elongation to failure of 7.2%, which is attributed to the synergistic effect of bulk LPSO phase and nanoscale γ' phase.

A novel severe plastic deformation method and its effect on microstructure, texture and mechanical properties of Mg-Gd-Y-Zn-Zr alloy

In this paper, cyclic expansion extrusion with an asymmetrical extrusion cavity (CEE-AEC) was proposed as a novel severe plastic deformation process for fabricating bulk ultrafine-grained (UFG) metals. Increasing the size of the processed sample and introducing the shear strain by attaching an asymmetrical extrusion cavity are the core advantages of this technology. The CEE-AEC process was applied to Mg-13Gd-4Y–2Zn-0.4Zr alloys for four passes deformation with the decreasing temperature, and then the microstructure evolution, texture analysis, and mechanical properties were investigated. Microstructure results show that a dramatical grain refinement is achieved from initial value of 24 ± 0.3 μm to 1.4 ± 0.3 μm which is mainly led by continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) as well as the particle-stimulated nucleation (PSN) induced by interdendritic long-period stacking ordered (LPSO) phases. With increasing CEE-AEC passes, the intensities of basal texture are gradually weakened, and the basal plane tends to incline to transverse and normal directions with different degrees of different passes, leading to a remarkable increase in Schmid factor for the activation of basal slip system. Tensile tests at room temperature (RT) indicate that the samples after three-passes of CEE-AEC exhibit the best comprehensive mechanical properties with tensile yield strength (TYS) of 302 MPa, ultimate tensile strength (UTS) of 330 MPa, and fracture elongation of 6.1%, which are mainly ascribed to the grain refinement, broken of interdendritic block-shaped LPSO phases, and texture modification.

Thermodynamic origin of solute-enriched stacking-fault in dilute Mg-Zn-Y alloys

We have investigated thermodynamic behaviors of dilute Mg-Zn-Y ternary alloys to form a unique solute-enriched stacking-fault (SESF), which is an intrinsic-II type stacking-fault (I2-SF) enriched by the Zn and Y atoms and represents the structural-unit of the long-period stacking/order (LPSO) phase. SESF in the hexagonal-close-packed (hcp) Mg matrix forms a local face-centered-cubic (fcc) environment, and hence our thermodynamic analysis is based on the Gibbs energy comparison between hcp and fcc phases over the Mg-Zn-Y ternary composition ranges, using the calculation of phase diagrams (CALPHAD) method aided by the first principles calculations. We find that the Zn/Y co-segregations at the SESF provide a remarkable condition that the fcc layers become more stable than the hcp-Mg matrix. Furthermore, within the SESF, the following spinodal-like decomposition into the Mg-rich solid-solution and the Zn/Y-rich L12-type order phase causes a significant reduction of the total Gibbs energy of the system. These spontaneous thermodynamic behaviors explain why the fault layers can be remarkably stabilized in the LPSO-forming ternary Mg alloys, and also support a phenomenological origin of the Zn-Y clustering with the L12-type short-range order, which is known to occur for the LPSO phases and has been confirmed for the present SESF by electron microscopy experiments.

Effect of Zr Content on the Distribution Characteristic of the 14H and 18R LPSO Phases

Abstract Zirconium (Zr) is an essential element in Mg-Zn and Mg-Zn-Y system magnesium alloys. In this study, an interesting phenomenon that the content of Zr element could influence the size and the morphology of the long period stacking ordered (LPSO) phases, which has never been reported by previous works before. The Mg98.5-xZn0.5Y1Zrx (x =0, 0.1, 0.2 and 0.3 at. %) magnesium alloys were fabricated by directional solidification, and the effects of the Zr content on the distribution characteristics of the bulk LPSO phases (18R) and the lamellar LPSO phases (14H) were investigated. The directional solidification technology showed good controllability in LPSO phase’s distribution, and the morphology of LPSO phases in Mg98.5-xZn0.5Y1Zrx (x =0, 0.1, 0.2 and 0.3 at. %) alloys were observed clearly. The results showed that the amount and the morphology of the 14H and 18R LPSO phases within grains continuously decreased with the Zr content increasing. The continuous 14H lamellar structure changed to discontinuous. In addition, Zr element exhibited purification ability on the grain boundaries and refined effect on the 14H and 18R LPSO phases. This can be attributed to the influence of Zr atoms on stacking fault energy (SFE) and the attraction of Zr atoms to Mg atoms.

Dynamic recrystallization and texture evolution of Mg-6.8Y-2.5Zn-0.3Zr alloy during hot rolling

The dynamic recrystallization and changes in the preferred orientation of crystallites during hot rolling at 500 °C have been studied on a cast Mg-6.8Y-2.5Zn-0.3Zr (wt. %) alloy (WZ73) containing long-period stacking ordered (LPSO) phases. The recrystallization began predominantly within twins and also occurred near the LPSO phases. The twinned regions have a much higher stored strain energy compared to the matrix, and are therefore favorable nucleation sites for dynamic recrystallization. Besides, the bits of the LPSO phase located at the grain boundaries of the magnesium matrix grains act as nucleation sites for dynamic recrystallization. With increasing strain from 36 % to 74 %, the average volume fraction of recrystallized grains increased from 3.5 % to 48.8 %. The intensive recrystallization process resulted in basal texture weakening. Thus, the resulting microstructure is a mixture of randomly oriented, dynamically recrystallized and coarse, highly oriented non-recrystallized grains.

Effect of deformation and post-annealing on microstructure and mechanical properties of long-period stacking ordered phase in Mg88Ni5Y7 alloy

Precipitation behavior and its effect on mechanical properties in magnesium-based alloys are necessary to be explored at the atomic scale. Here microstructural properties of as-cast, cold-rolled and post-annealed Mg88Ni5Y7 (at%) alloys containing long-period stacking ordered (LPSO) phase have been investigated by advanced electron microscopy techniques. In comparison, cold-rolling of as-cast alloys leads to form kink boundaries within LPSO phase, and post-annealing of the cold-rolled alloys at different temperatures results in the formation of different types of precipitates at the kink boundaries, which affects the mechanical properties of the LPSO phase. Our findings provide a better understanding of microstructure evolutions of the deformed Mg88Ni5Y7 (at%) alloys subjected to annealing treatment, which is of great significance for the design and optimization of the magnesium-based alloys.

Effect of substitution of Zn with Ni on microstructure evolution and mechanical properties of LPSO dominant Mg–Y–Zn alloys

The microstructures and mechanical properties of Mg96Y2Zn2 (MYZ), Mg96Y2Zn1Ni1 (MYZN), and Mg96Y2Ni2 (MYN) (at.%) alloys have been systematically examined in this study. The as-cast alloys contain α-Mg matrix, long-period stacking order (LPSO) phase and compounds. By substituting Zn with Ni, the volume fractions of the LPSO phase and compounds increase gradually, leading to higher yield strength and poorer ductility of the as-cast alloys. Moreover, the morphology of the LPSO phase transform from block to lamella gradually. After homogenization at 500 °C for 12 h, the fraction of the LPSO phase increase and compounds decrease, moreover, the morphologies of the LPSO phase remain unchanged. Tensile tests indicate that the yield strengths of the extruded MYZ, MYZN, and MYN alloys are 325, 340, and 385 MPa, respectively; the increase can be explained by the higher fraction of the LPSO phase and different stress-withstanding capacity of the LPSO phase with different morphologies. In addition, the enhancement of strength is accompanied by slight drop in ductility.

Mn‐promoting formation of a long‐period stacking‐ordered phase in laser‐melted Mg alloys to enhance degradation resistance

AbstractMagnesium (Mg) alloys are promising candidates for use as biomedical implant materials. However, their very fast degradation rate greatly restricts their application. A rare earth phase possessed with a long‐period stacking‐ordered (LPSO) structure was in favor of enhancing the degradation resistance of Mg alloys. In fact, the formation of the LPSO phase in Mg alloys depends on their stacking fault energy. The lower the stacking fault energy, the more the LPSO phase formed. In this study, manganese (Mn) was alloyed to Mg alloy ZK30–10Gd (containing 3 wt.% Zn and 10 wt.% Gd) via selective laser melting to promote the formation of the LPSO phase. As an alloying element, Mn could be in favor of reducing stacking fault energy due to the fact that the large difference between the atomic radius of Mn and that of Mg induced large lattice distortion to facilitate forming stacking faults. The results showed that as the Mn content increased from 0 to 1.2 wt.%, the area fraction of LPSO phase increased from 12.22% to 22.37%, meanwhile the area fraction of (Mg,Zn)3Gd phase decreased from 9.31% to 2.32%. The ZK30–10Gd–0.6Mn possessed the highest degradation resistance (weight loss rate 0.38 mg · cm−2 · day−1). The enhancement of degradation resistance had two reasons. On the one hand, more LPSO phase provided more sites for nucleation of degradation products, which could promote the formation of a homogeneous and compact degradation product film to protect the Mg matrix. On the other hand, the inhibition of (Mg,Zn)3Gd phase precipitation could reduce galvanic corrosion.

In-situ Investigation of the Microstructure Evolution in Long-Period-Stacking-Ordered (LPSO) Magnesium Alloys as a Function of the Temperature

Deformation behavior of two Mg-Zn-Y magnesium alloys, having a different fraction of the long-period-stacking-ordered (LPSO) phase, has been investigated at room temperature and 200 °C by a combination of in-situ neutron diffraction (ND) and acoustic emission (AE) measurements. The results indicate that the twinning in the magnesium matrix and the kinking in the LPSO phase strongly depend on the composition of the material and the testing temperature. Further, active deformation mechanisms and particularly the load transfer from the magnesium matrix to the LPSO phase define the mechanical properties of the investigated alloys.

The role of long period stacking ordered phase in dynamic recrystallization of a Mg–Zn–Y alloy during hot compression

A Mg–Zn–Y alloy containing long period stacking ordered (LPSO) phases was compressed at temperatures ranging from 623 K to 773 K and strain rates ranging from 0.001 s−1 to 1 s−1. The role of LPSO phase in dynamic recrystallization (DRX) was emphatically studied. The results revealed that: (i) the area filled with kinked LPSO phase is found as potential site favors the initiation of DRX at lower temperatures. (ii) Fragmentation of LPSO phase at initial grain boundaries significantly improves the DRX process via particle stimulated nucleation (PSN) mechanism. (iii) Boundary migration of DRX grains is frequently pinned by LPSO phase at elevated temperature. Understanding the role of LPSO phase in DRX may shed light on manufacturing of Mg–Zn–RE alloys with excellent mechanical property and good corrosion resistance.

Direct Observations of Precursor Short-Range Order Clusters of Solute Atoms in a LPSO-Forming Mg-Zn-Gd Ternary Alloy

We have investigated short-range order (SRO) clusters of solute atoms in a Mg97Zn1Gd2 alloy which forms a long-period stacking/order (LPSO) phase, based on aberration-corrected scanning transmission electron microscopy (STEM) observations and first-principles calculations. High-angle annular dark-field (HAADF) STEM provides directly the individual Gd atom positions through significant atomic-number dependent Z-contrast, and their averaged image-processing have immediately shown up the representative SRO-Gd configurations being in favor of the second-nearest networks in the hcp-Mg structure. Interestingly, Zn atoms seem to be essentially placed at the particular sites within the SRO-Gd configurations to form D019-like ZnGd3 clusters, a validity of which are verified by STEM image simulations and first-principles calculations. These D019-like ZnGd3 clusters in the hcp-Mg matrix are likely to be a precursor sub-structure to the L12-type Zn6Gd8 clusters that are embedded in the local fcc layers in the LPSO structure. In fact, we find that these D019-like ZnGd3 clusters seem to form a layered atmosphere prior to the LPSO formation, providing an important clue on how the LPSO phases nucleate and grow.