LPSO Phase Research Articles

Effect of heat treatment on mechanical properties and microstructure evolution of Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy

Regarding the as-cast Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy, the effect of heat treatment on its properties at room temperature (RT), as well as the mechanical properties and microstructure evolution of various peak-aging samples at different tensile temperatures were discussed in this article. The results indicated that the optimal heat treatment process of the alloy was: 520 °C × 24 h + 200 °C × 112 h. Under this condition, the yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) at RT were: 238 MPa, 327 MPa and 2.5 %, respectively. As the tensile temperature increases, the strength increases firstly and then decreases, but the ductility increases monotonously. The microstructures evolution of 200 °C peak-aging (200PA) and 250 °C peak-aging (250PA) samples were different with the increasing tensile tenperature. When tensile test processed at 150°C, the dense β' phase and rod-shaped basal γ' phase will be formed in the 200PA sample. However, at 300 °C, the β' phases disappeared. The β' and LPSO phases in the 250PA sample coarsened gradually as the tensile temperature increased, and 14H-LPSO phases were formed during tensile at 300 °C. The 200PA sample reached the highest strength when tensile at 150 °C, which was attributed to the hindrance of the basal dislocation and non-basal dislocation slip by the prismatic β' phases and the newly formed basal γ' precipitates.

α-Mg/LPSO (Long-Period Stacking Ordered) phase interfaces as obstacles against dislocation slip in as-cast Mg-Zn-Y alloys

The influence of α-Mg/LPSO (Long-Period Stacking Ordered) phase interfaces on the flow stress of as-cast Mg-Zn-Y alloys was experimentally and numerically evaluated by compressive loading samples with different volume fractions of LPSO phase. The experimental results showed that the flow stresses of Mg-Zn-Y alloys with LPSO volume fractions ranging from 40 to 85% are clearly higher than those of single phase α-Mg or LPSO alloys. Comparison of deformation behavior between Mg-Zn-Y alloys and Mg-Zn-Y-Zr alloys, where Zr effectively decreases grain size of α-Mg phase, showed that the flow stresses of Mg-Zn-Y alloys with 25 and 40% of LPSO phase were insensitive to grain size whereas the flow stress of α-Mg single-phase alloy showed clear grain size dependence. By including the dependence of critical resolved shear stress (CRSS) on the average interface spacing, crystal plasticity analysis successfully reproduces the observed experimental stress-strain behavior, which suggests that α-Mg/LPSO phase interfaces are effective obstacles against dislocation slip in two-phase Mg alloys containing LPSO phase.

Influence of crystallographic orientation and Al alloying on the corrosion behaviour of extruded α-Mg/LPSO two-phase Mg-Zn-Y alloys with multimodal microstructure

A highly corrosion resistant Mg-Zn-Y-Al alloy featuring an α-Mg/LPSO two-phase multimodal microstructure was developed. The extruded Mg-Zn-Y alloy showed orientation dependent corrosion behaviour, the longitudinal section exhibited a higher corrosion rate than the transverse section. The orientation dependence was attributed to the varying in-plane atomic densities of the crystallographic planes on the longitudinal and transverse sections. The LPSO phase was the cathode and galvanic corrosion occurred in the adjacent α-Mg. The Al alloying into the Mg-Zn-Y alloy facilitated the formation of a protective surface film, which decreased the corrosion rate in spite of geometrical and electrochemical heterogeneities.

Effect of Al addition on microstructure and mechanical properties of Mg−Gd−Zn alloys

The microstructure evolution and mechanical properties of Mg−15.3Gd−1Zn alloys with different Al contents (0, 0.4, 0.7 and 1.0 wt.%) were investigated. Microstructural analysis indicates that the addition of 0.4 wt.% Al facilitates the formation of 18R-LPSO phase (Mg12Gd(Al, Zn)) in the Mg−Gd−Zn alloy. The contents of Al11Gd3 and Al2Gd increase with the increase of Al content, while the content of (Mg, Zn)3Gd decreases. After homogenization treatment, (Mg, Zn)3Gd, 18R-LPSO and some Al11Gd3 phases are transformed into the high-temperature stable 14H-LPSO phases. The particulate Al−Gd phases can stimulate the nucleation of dynamic recrystallization by the particle simulated nucleation (PSN) mechanism. The tensile strength of the as-rolled alloys is improved remarkably due to the grain refinement and the fiber-like reinforcement of LPSO phase. The precipitation of the β′ phase in the peak-aged alloys can significantly improve the strength. The peak-aged alloy containing 0.4 wt.% Al achieves excellent mechanical properties and the UTS, YS and elongation are 458 MPa, 375 MPa and 6.2%, respectively.

Effects of hot compression on the fracture toughness and tensile creep behaviors of a Mg-Gd-Y-Zn-Zr alloy

The effects of hot compression on the fracture toughness and tensile creep behaviors of a Mg-Gd-Y-Zn-Zr alloy were researched in detail. The hot compression was performed to a true strain of 0.5 along the extrusion direction at 703 K with a strain rate of 3 × 10-1 s-1. Two types of samples were used in this work: one was the annealed sample without hot compression (NHC sample), the other was the annealed sample with hot compression (HC sample). Uniaxial tensile tests were performed with a strain rate of 3 × 10-3 s-1 at room temperature and 523 K. Fracture toughness tests were performed with a strain rate of 3 × 10-3 s-1 at room temperature. Tensile creep tests were carried out at 523 K under various applied stresses ranging from 90 to 130 MPa. All tests were conducted along the extrusion direction. The results showed the improved mechanical properties were obtained in the HC sample at room temperature. At elevated temperature, the yield strength of the HC sample was improved, while the ultimate tensile strength and elongation were decreased. Compared with the NHC sample, the room temperature fracture toughness of the HC sample was apparently improved due to 14H LPSO phases and kink bands had excellent blocking effects to cracks growth and turned the crack propagation direction, which helped to release stress and absorb energy. However, the creep resistance of the HC sample was poorer than that of the NHC sample at 523 K. The creep mechanism of the NHC sample was basal slip, while the HC sample were dominated by pyramidal <c+a> slip and grain boundary migration. Besides, the dynamic precipitation of β′ phases occurs during hot compression. Although pyramidal <c+a> slip and β′ phases had positive effects on the creep resistance, they could not offset the detrimental impacts derived from grain boundary migration and cracks nucleation at kink bands. As a result, a decreased creep resistance was obtained in the HC sample.

Effects of Gd on Hot Tearing Susceptibility of As-Cast Mg96.94-Zn1-Y(2-X)-Gdx-Zr0.06 Alloys Reinforced with Lpso Phase

Effects of Gd on Hot Tearing Susceptibility of As-Cast Mg96.94-Zn1-Y(2-X)-Gdx-Zr0.06 Alloys Reinforced with Lpso Phase

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

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

Tailoring the Microstructural Characteristic and Improving the Corrosion Rate of Mg-Gd-Ni Alloy by Heat Treatment with Different Volume Fraction of Lpso Phase

Tailoring the Microstructural Characteristic and Improving the Corrosion Rate of Mg-Gd-Ni Alloy by Heat Treatment with Different Volume Fraction of Lpso Phase

Roles of Lpso Phases on Dynamic Recrystallization of High Strain Rate Multi-Directional Forged Mg-Gd-Er-Zn-Zr Alloy and its Strengthening Mechanisms

Roles of Lpso Phases on Dynamic Recrystallization of High Strain Rate Multi-Directional Forged Mg-Gd-Er-Zn-Zr Alloy and its Strengthening Mechanisms

Compressive deformation of as-extruded LPSO-containing Mg alloys at different temperatures

This work investigates the compressive deformation behavior of as-extruded Mg-6Gd-1.6Y–1Zn-0.4Zr (VZ61) and Mg-6Gd-4.8Y–3Zn-0.4Zr (VZ63) alloys via uniaxial compressive tests at various deformation temperatures. At room temperature, compared with the VZ61 alloy, the compressive yield strength of the VZ63 alloy were obviously enhanced due to its increased amount of LPSO phase. Both alloys exhibited a three-stage strain hardening feature. The strain hardening rate first sharply decreases (stage I), then continues to increase at a slower rate (stage II) and finally decreases (stage III). At stage III, a larger number of dislocation accumulation around the kinked LPSO phase resulted in a higher strain hardening rate for VZ63 alloy than that for VZ61 alloy. During high temperature compression, the true stress–strain curves showed that the flow stress gradually reduced with increasing temperature and also the reducing of strain rate, and the VZ63 alloy displayed a higher peak stress than VZ61 alloy. Constitutive equations were constructed based on the true stress–strain to better understand the relation among flow tress (σ), strain rate (ε⋅) and deformation temperature (T) in VZ alloys during hot deformation. The results showed that the VZ63 alloy had a lower deformation activation energy (Q = 255.6 kJ/mol) than VZ61 alloy (Q = 395.5 kJ/mol). The DRX kinetic models of the VZ61 and VZ63 alloys were also established, indicating that the VZ63 alloy was more prone to DRX with a higher volume fraction of dynamically recrystallized grains (XDRX) at the same deformation conditions.

Effects of heat treatment before extrusion on dynamic recrystallization behavior, texture and mechanical properties of as-extruded Mg-Gd-Y-Zn-Zr alloy

The effects of heat treatment before extrusion on the dynamic recrystallization (DRX) behavior, texture, and mechanical properties of the extruded Mg-9.8Gd-3.5Y-2.0Zn-0.4Zr (wt. %) alloy were comparatively investigated in this work. A large number of dendritic microstructures in as-homogenized alloy and intergranular block-shaped LPSO phases in as-aged alloy were found to promote the operation of particle-stimulated nucleation (PSN) and discontinuous dynamic recrystallization (DDRX) mechanisms, which further accelerate growth of dynamic recrystallized grains due to the lack of solute drag and dynamic precipitation. Also, we determined the nucleation of dynamic recrystallization affects little to the formation of abnormal texture, whereas the shear stress results from the flow velocity gradient and the pyramidal-2 slip during the growth of the dynamic recrystallized grains leading the formation of abnormal texture. However, the solution treatment before extrusion could effectively eliminate the dendritic microstructures and increase the solid solubility of the matrix, which would facilitate the occurrence of continuous dynamic recrystallization (CDRX) and dynamic precipitation during the hot extrusion process. Meanwhile, CDRX and dynamic precipitation co-contribute to the formation of a bimodal microstructure that composed of coarse deformed grains with basal orientation and fine dynamic recrystallized grains with random orientation. The observed bimodal microstructure, fine dynamic precipitations, strong fiber texture, and substructure thus well explained the improved strength and elongation of samples extruded with the solution treated materials.

Effect of Al Content on Microstructure Evolution and Mechanical Properties of As-Cast Mg-11Gd-2Y-1Zn Alloy.

In the present paper, the Mg-11Gd-2Y-1Zn alloys with different Al addition were fabricated by the gravity permanent mold method. The effect of Al content on microstructure evolution and mechanical properties of as-cast Mg-11Gd-2Y-1Zn alloy was studied by metallographic microscope, scanning electron microscope, XRD and tensile testing. The experimental results showed that the microstructure of as-cast Mg-11Gd-2Y-1Zn alloy consisted of α-Mg phase and island-shaped Mg3 (RE, Zn) phase. When Al element was added, Al2RE phase and lamellar Mg12REZn (LPSO) phase were formed in the Mg-11Gd-2Y-1Zn alloy. With increasing Al content, LPSO phase and Mg3 (RE, Zn) phase gradually decreased, while Al2RE phase gradually increased. There were only α-Mg and Al2RE phases in the Mg-11Gd-2Y-1Zn-5Al alloy. With the increase of Al content, the grain size decreased firstly and then increased. When the Al content was 1 wt.%, the grain size of the alloy was the minimum value (28.9 μm). The ultimate tensile strength and elongation increased firstly and then decreased with increasing Al addition. And the fracture mode changed from intergranular fracture to transgranular fracture with increasing addition. When Al addition was 1 wt.%, the maximum ultimate tensile strength reached 225.6 MPa, and the elongation was 7.8%. When the content of Al element was 3 wt.%, the maximum elongation reached 10.2% and the ultimate tensile strength was 207.7 MPa.

Effect of hierarchical multimodal microstructure evolution on tensile properties and fracture toughness of rapidly solidified Mg–Zn–Y–Al alloys with LPSO phase

High fracture toughness Mg96.75Zn0.85Y2.05Al0.35 alloys featuring a multimodal microstructure were developed by rapidly solidified (RS) ribbon-consolidation processing. A non-heat-treated alloy that was prepared by consolidating the as-quenched ribbons exhibited a high tensile yield strength of ∼500 MPa, an elongation of ∼5%, and a plane-strain fracture toughness (KIc) of ∼9.5 MPa m1/2. Microstructure of the non-heat-treated alloy consists of two matrices: coarse-worked grains with a high kernel average misorientation (KAM) (group 1) and ultra-fine dynamically recrystallized (DRXed) grains with an intermediate KAM (group 2). The formation of ultra-fine DRXed grains with the precipitation of fine LPSO phase strengthens the alloy, but causes work-softening. The heat-treated alloy that was prepared by consolidating the RS ribbons heat-treated at 738 K exhibited a good balance of a high KIc (∼15 MPa m1/2), a reasonable yield strength (∼400 MPa), and a large elongation (∼12%) with a positive work-hardening rate. The optimized pre-consolidation heat-treatment produces multimodal microstructure having three matrices; a third group of the fine DRXed grains with a low KAM was formed in addition to groups 1 and 2. The formation of hierarchical multimodal structure in the grain size and KAM evokes strain-hardening, which has a positive effect on the fracture toughness.

Effect of extrusion plus rolling on damping capacity and mechanical properties of Mg–Y–Er–Zn-Zr alloy

Many common strengthening means are not favorable for damping capacity of alloys. While twin and long-period stacking ordered phase (LPSO) in magnesium alloys are beneficial to improve both damping and mechanical properties. To obtain a kind of Mg alloy with both high mechanical properties and damping capacity, it is proposed to introduce twin and LPSO structure into Mg–4Y–2Er–2Zn-0.6Zr (wt.%) alloy through extrusion and rolling. The influence of LPSO structure and twin on the mechanical properties and damping capacity was studied. The results show that, under the combined actions of extrusion and rolling, the twin form in both the matrix and LPSO phase, which is beneficial to synergistically enhance the damping and mechanical properties of the alloy.

Enhanced degradation properties of Mg-Y-Ni alloys by tailoring the LPSO morphology for fracturing tools applications

In this study, rod-shaped LPSO was obtained by heat treatment, and the effect of LPSO morphology on the degradation properties of Mg-7.2Y-2.8Ni alloys was investigated. Mg-7.2Y-2.8Ni alloys mainly consisted of α-Mg and LPSO, and the distribution of LPSO was changed from continuous network to disordered rod-shaped after heat treatment. The results showed that the formation of rod-shaped LPSO not only increased the average degradation rate by 27% but also increased the elongation. The tensile strength and elongation of the annealed alloys reached 280 MPa and 21.8% respectively. • Different morphology of LPSO phase with Mg-7.2Y-2.8Ni alloys was obtained. • The formation of rod-shaped LPSO simultaneously increased the degradation rate and elongation of Mg-7.2Y-2.8Ni alloys. • The combined effects of disordered rod-shaped LPSO and the corrosion cracks significantly increased the degradation rate.

Influence of Al2Y particles on mechanical properties of Mg-11Y-1Al alloy with different grain sizes

Manipulating the grain size and the distribution of precipitates has been widely used for tailoring mechanical properties of magnesium alloys. In this work, we systematically studied the influence of LPSO lamellas, Al2Y particles as well as grain size on the mechanical properties of the Mg-11Y-1Al alloys. The LPSO phase has little effect on the strength and ductility of the Mg-11Y-1Al alloys. Grain refinement gives rise to the simultaneous improvement of the strength and ductility. The yield strength is improved from 160 MPa to above 300 MPa and the elongation is increased from 4% to more than 13% with the grain refinement from 40 μm to 5 μm. Based on the CPFEM simulation results, it is found that stress concentration generally increases at the adjacent of the Al2Y particle at the scenario of the 40 μm grain size. Cracks thus nucleate at the adjacent area of the Al2Y particles, which results in poor ductility. With the reduction of grain size to 5 μm, the ductility is enhanced with alleviating the stress concentration around micrometer-size Al2Y particles and subsequent activating <c+a> dislocations. Such a finding provides a guideline in tailoring mechanical properties of other Mg-RE-Al alloys for demanding applications.

Improved strength in wrought Mg–Y–Ni alloys by adjusting the block-shaped LPSO phase and plate-shaped γ′ phase

The microstructure and mechanical properties of two Mg–Y–Ni alloys with different volume fraction of block-shaped LPSO phase and different number densities of plate-shaped γ′ phase were investigated. The volume fraction of block-shaped LPSO phase increases with the content of Y and Ni in the Mg–Y–Ni alloys with Y/Ni atomic ratio of 1. The plate-shaped γ′ phase is formed in the matrix of the as-cast alloys and grows across the matrix during the preheating before extrusion, and its number density increases with increasing content of Y and Ni. After extrusion, the block-shaped LPSO phase is distributed along extrusion direction, and the plate-shaped γ′ phase can be observed in both non-dynamic recrystallized (non-DRXed) grains with strong basal texture and dynamic recrystallized (DRXed) grains with random texture. The increased volume fraction of LPSO phase improves the tensile yield strength of the as-extruded Mg–Y–Ni alloys, accompanied by the decrease in ductility. In addition to the obvious strengthening from the LPSO phase, the plate-shaped γ′ phase is also an effective strengthening phase which can hinder the movement of the slip systems in non-DRXed and DRXed grains of the Mg–Y–Ni alloys. The Mg94Y3Ni3 (at%) alloy containing the highest volume fraction of block-shaped LPSO phase and the highest number density of plate-shaped γ′ phase exhibits the superior tensile yield strength of 502 MPa after extrusion.

The Formation of 14H-LPSO in Mg-9Gd-2Y-2Zn-0.5Zr Alloy during Heat Treatment.

There is a new long-period stacking ordered structure in Mg–RE–Zn magnesium alloys, namely the LPSO phase, which can effectively improve the yield strength, elongation, and corrosion resistance of Mg alloys. According to different types of Mg–RE–Zn alloy systems, two transformation modes are involved in the heat treatment transformation process. The first is the alloy without LPSO phase in the as-cast alloy, and the MgxRE phase changes to 14H-LPSO phase. The second is the alloy containing LPSO phase in the as-cast state, and the 14H-LPSO phase is obtained by the transformations of 6H, 18R, and 24R. The effects of different solution parameters on the second phase of Mg–9Gd–2Y–2Zn–0.5Zr alloy were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The precipitation mechanism of 14H-LPSO phase during solution treatment was further clarified. At a solution time of 13 h, the grain size increased rapidly initially and then decreased slightly with increasing solution temperature. The analysis of the volume fraction of the second phase and lattice constant showed that Gd and Y elements in the alloy precipitated from the matrix and formed 14H-LPSO phase after solution treatment at 490 °C for 13 h. At this time, the hardness of the alloy reached the maximum of 74.6 HV. After solution treatment at 500 °C for 13 h, the solid solution degree of the alloy increases, and the grain size and hardness of the alloy remain basically unchanged.

Intrinsic kink bands strengthening induced by several wrought-processes in Mg-Y-Zn alloys containing LPSO phase

The kink bands strengthening effect was investigated using Mg-9at.%Y-6at.%Zn alloys produced by caliber rolling or extrusion at intermediate and elevated temperatures (373 K and more than 673 K). The results of microstructural observations showed that deformation kink bands were induced in all the alloys, regardless of the wrought-processing method and the processing temperatures. Micro-Vickers hardness tests showed that the regions including kink bands exhibited a higher hardness than those without kink bands, which reveal intrinsic kink bands strengthening. Interestingly, the trend of this kink bands strengthening effect could be expressed by an empirical relationship, which was divided into two lines associated with the wrought-processing temperature. This tendency did not appear to depend on the wrought-processing method. The alloys produced at the intermediate temperature showed markedly superior hardness, since, in addition to the kink bands strengthening, the residual strains/dislocations around kink boundaries contributed to obstruct dislocation slips. • Kink bands were induced regardless of wrought processing-method and -temperatures. • Intrinsic kink bands strengthening effect could be expressed by number of kink bands. • The alloys produced at the intermediate temperature showed markedly superior hardness. • Residual strains around kink bands contributed to obstruct for dislocation slips.

Hydrogen storage properties of Mg98.5Gd1Zn0.5 and Mg98.5Gd0.5Y0.5Zn0.5 alloys containing LPSO phases

In this work, the as-cast Mg-rich Mg98.5Gd1Zn0.5 and Mg98.5Gd0.5Y0.5Zn0.5 alloys are prepared by the semi-continuous casting method, and their hydrogen storage performance and catalytic mechanisms are investigated by experimental and first-principles calculations approaches. The results show that the LPSO phases decompose and in-situ form the RE(Gd/Y)Hx(x = 2,3) nano-hydrides upon hydrogenation. These nano-hydrides not only serve as the in-situ catalysts to promote the hydrogen ab/desorption of Mg matrix, but also present the pinning effect to inhibit the growth of Mg/MgH2 grains during hydrogenation and dehydrogenation. Comparatively, the two alloys exhibit the similar hydrogen absorption kinetics, while the hydrogen desorption kinetics of Mg98.5Gd1Zn0.5 is superior to that of Mg98.5Gd0.5Y0.5Zn0.5. The first-principles calculations reveal that the GdH2 and YH2 hydrides exhibit different catalytic effects on weakening the bond strength of H–H within H2 and Mg–H within MgH2, which interprets well the differences in the hydrogen ab/desorption kinetics between Mg98.5Gd1Zn0.5 and Mg98.5Gd0.5Y0.5Zn0.5 alloys.