Mg5Gd Phase Research Articles

Mechanism of corrosion behavior induced by precipitates under plastic compressive stress in Mg-Gd-Y alloys

Through independently developed stress-loading equipment, stress corrosion tests on Mg-Gd-Y alloy were conducted in a 3.5 wt% NaCl solution. The effects of plastic compressive stress on the corrosion behavior of the alloy were thoroughly investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) among other microscopic analysis techniques. The results indicate that the alloy mainly consists of α-Mg grains, Mg24Y5 phase, Mg5Gd phase, and LPSO phase. The corrosion behavior of the Mg-Gd-Y alloy is significantly influenced by the microstructure of the interface between the precipitates and the matrix, the potential difference, and the stress state. In the unstressed state, the Mg24Y5 phase first induces corrosion at the edges of the α-Mg grain boundaries, which then spreads internally. Upon the application of plastic stress, the corrosion-inducing capability of the LPSO phase on α-Mg grains notably increases. This discovery provides new insights into the mechanisms by which plastic compressive stress affects the corrosion behavior of Mg-Gd-Y alloys and offers an important basis for the theoretical research and anti-corrosion design in the engineering applications of this alloy.

Roles of micro-alloyed Y and Gd in improving the corrosion resistance and mechanical properties of pure Mg

The microstructure, corrosion resistance and mechanical properties of extruded pure Mg, Mg–2Y and Mg-2Gd were studied by means of scanning electron microscope (SEM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD), scanning probe microscope (SPM), immersion test, electrochemical test and tensile test. The results demonstrated that Mg–2Y and Mg-2Gd were composed of Mg, Mg24Y5, and Mg5Gd phases with the addition of Y and Gd. The addition of Y and Gd to pure Mg noticeably reduced the grain size and textural strength of the alloy. Mg-2Gd alloy had the smallest grain size and the lowest textural strength. The corrosion rate of Mg-2Gd was the slowest due to the influence of grain size. Y slowed the corrosion of pure Mg in the early stages due to the grain refinement, but speeded up the corrosion because of the galvanic corrosion produced by the precipitation of the second phase in the latter stages. The elongation of pure Mg, Mg–2Y, and Mg-2Gd were 16.5 %, 38.67 %, and 48.67 %, respectively. The inclusion of Y and Gd refined the grain, softened the texture strength, and activated basal slip, which improved the elasticity of alloys. Gd was more significant than Y in improving the corrosion resistance and mechanical properties of pure Mg.

Effects of temperature-induced variation of the second phase on the microstructure, texture and properties of Mg-RE-Zn alloys

In this paper, the Mg-9.32Gd-3.72Y-1.68Zn-0.72Zr (wt%) alloy has undergone three times of repetitive upsetting extrusion deformation. Alloys with different morphologies and distribution patterns of the second phase have been prepared by varying the deformation temperature in each pass. The effects of the second phase on microstructure, texture and mechanical properties are investigated. The results show that (i) the second phase, including Mg5Gd and LPSO phase, has an important effect on the dynamic recrystallization (DRX) behavior of the alloy. (ii) Appropriate lamella distance and block phase size can promote the activation of the slip system and effectively weaken the texture strength. (iii) The DT sample has an effective combination of fine grain strengthening and second phase strengthening due to the reasonable second phase distribution and size, which greatly improves the UTS and YS of the alloy.

Dynamic precipitation and dynamic recrystallization behaviors of Mg-Gd-Nd-Zr magnesium alloy during thermal compression deformation

In this paper, the dynamic precipitation and dynamic recrystallization (DRX) behaviors of Mg-7Gd-2Nd-0.5Zr (wt.%) alloy during thermal compression were investigated by optical microscopy (OM), scanning electron microscope (SEM) and transmission electron microscopy (TEM). The hot compression tests were performed at 350–500 °C with strain rates of 0.003–1 s−1 and a strain of 0.7. The results indicate that dynamic precipitation and DRX occur in the deformed region under appropriate conditions, and that dynamic precipitation precedes DRX. Elevated deformation temperature and strain rate facilitate DRX but weaken dynamic precipitation. The dynamic precipitates were β (Mg5Gd) phases and were mainly located at the grain boundaries in the deformed regions. DRX can be promoted when the size of the granular precipitates is larger than 200 nm. The second phase particles located at the grain boundaries of the recrystallized grains can pin the grain boundaries to refine the recrystallized grains. There are three mechanisms of recrystallization nucleation: grain boundary bowing nucleation, dislocation pile-up induced nucleation, and particle-stimulated nucleation (PSN). Moreover, a critical strain model and a kinetic model for the DRX of the alloy were developed.

Microstructure and mechanical properties of Mg–Ni–Gd alloy synthesised by powder metallurgy

ABSTRACT The hexagonal close-packed structure renders magnesium weak at room and high temperatures for structural applications despite its low density. Inducing thermally stable and coherent second phases would enhance/retain the strength of Mg-based alloys even at high temperatures. This paper aims to develop a high-strength Mg-based nanocomposite. A master alloy composed of Ni and Gd was cast and the composition of Mg97.56Ni1.22Gd1.22 (at.-%) was prepared using ball milling for 150 h. XRD plots of the as-milled powder having nano-size crystallites confirm the partial dissolution of the master alloy. Consolidation through sintering with 5, 7 and 9 h of exposure at 550°C and extrusion at 500°C resulted in the formation of Mg5Gd, Mg2Ni, Gd2O3 and MgO phases. The extruded samples possessed a high strength of 804 MPa, which can be attributed to ultra-fine grains and dispersoid strengthening by homogeneously distributed second-phase particles in the 100–200 nm range.

Effect of Nd on microstructure and mechanical properties of Mg-7Gd-0.5Zr alloy

Alloying studies of Mg-Gd system alloys are an important way to improve alloy properties, reduce Gd content, and thus cost and density. Nd is excellent in promoting aging precipitation and enhancing heat resistance properties. In this work, low Gd content Mg-7Gd-xNd-0.5Zr (x = 0, 0.5, 2, 3.5 wt%) alloys were prepared by melting casting and processed by solution aging. The effects of Nd on the microstructure and mechanical properties of the aged alloys were investigated by SEM, XRD, EDS, TEM, and electron tensile testing machine. The findings show that the Nd-free alloy consists mainly of α-Mg and Mg5Gd, and after Nd addition, a new phase Mg41Nd5 is generated and the precipitation of the Mg5Gd phase is promoted, and due to the heterogeneous nucleation ability of Mg41Nd5 particles leads to grain refinement. The principal precipitate phases in the matrix and grain boundaries of Nd-containing alloys are the β′ phase, which is the alloy's primary strengthening phase. Due to the influence of fine grain strengthening and precipitation strengthening, the ultimate tensile strength of the Mg-7Gd-2Nd-0.5Zr alloy achieves a maximum of 328 MPa and 294 MPa at ambient temperature and 250 °C, respectively.

Effects of Static Magnetic Field on Compression Properties of Mg-Al-Gd Alloys Containing Gd-Rich Ferromagnetic Phase.

The Mg–0.6Al–20.8Gd (wt.%) alloys were homogenized at 620 °C for 20 min under 0 T and 1 T, followed by furnace cooling, quenching, and air cooling, respectively. The effects of the magnetic field on the phase constituent, microstructure, secondary phase precipitation, and mechanical properties of the Mg–Al–Gd alloys were investigated. The Mg–Al–Gd alloys contained α-Mg, Mg5Gd, Al2Gd, and GdH2 phases, and the phase constituents were hardly influenced by the applied magnetic field. However, the precipitation of the paramagnetic Mg5Gd upon cooling was accelerated by the magnetic field, and that of the ferromagnetic Al2Gd phases was inhibited. In addition, the Al2Gd phase was significantly refined and driven to segregate at the grain boundaries by the magnetic field, and the resultant pinning effect led to the microstructure change from dendritic α-Mg grains to rosette-like ones. When the magnetic field was only applied to the homogenization stage, the content of the Mg5Gd phase remained unchanged in the quenched alloy, whereas the Mg5Gd laths were significantly refined. By contrast, the contents of the Al2Gd and GdH2 phases were increased, while the precipitation sites were still within the α-Mg grains. The Mg5Gd laths were incapable of providing precipitation strengthening, while the Al2Gd and GdH2 particles brought positive effects on the enhancement of the mechanical properties. In the quenching condition, the hardness, compression strength, and ductility can be improved by the magnetic treatment, whereas these mechanical properties can be suppressed in the furnace cooled condition by the magnetic treatment.

Microstructure evolution and quench sensitivity characterizations of Mg-9.5Gd-0.9Zn-0.5Zr alloy

Mg-9.5Gd-0.9Zn-0.5Zr alloys were water quenched (WQ), air cooled (AC) and furnace cooled + water quenched (FC + WQ) after solution treatment. Effects of quenching conditions on the microstructure, mechanical properties and age hardening behavior were investigated, then the quench sensitivity and strengthening mechanism were analyzed. Results showed that only rod-like Zn2Zr3 phases were found in the WQ alloy, while Mg5Gd and 14H LPSO phases were also detected in the AC and FC + WQ alloys. The WQ alloy exhibited the most significant age hardening response, and the peak-aged alloy was consisted of densely distributed γ' + γ'' precipitates, Zn2Zr3 and Mg5Gd particles. In contrast, decreased amount of γ' + γ'' precipitates were observed in the AC and FC + WQ peak aged alloys. The increment of yield strength and ultimate tensile strength after aging treatment decreased with the decreasing quenching rate, and the elongation decreased sharply in the WQ and AC peak-aged alloys, while it only decreased slightly in the FC + WQ peak-aged alloy. The calculated strengthening contribution exhibited that the variation of precipitation strengthening in peak-aged alloys was more dramatic than that of solid solution strengthening in solution treated alloys, then lead to a higher quench sensitivity.

Effect of Zn on the Mechanical Properties and Microstructure of as-Cast and Solution-Treated Mg–6Y–2Nd–1Gd–0.5Zr Alloys

The effect of varying Zn contents (0, 0.4, and 0.8 wt.%) on the microstructure and mechanical properties of an Mg–6Y–2Nd–1Gd–0.5Zr alloy was systematically evaluated to improve the above characteristics. An as-cast alloy is shown to mainly consist of α-Mg, Mg24Y5, Mg12Nd, and Mg5Gd phases. An addition of Zn to the alloy results in the Mg12YZn phase formation and grain refinement. An increase in a Zn volume from 0.4 to 0.8 wt.% brings about a narrower solidification temperature range. After solution treatment, Mg12Nd and Mg24Y5 mainly disappear, but a Mg12YZn content grows. The tensile strength, yield strength, and elongation of a solution-treated Mg–6Y–2Nd–1Gd–0.5Zr–0.4Zn alloy are 284 MPa, 218 MPa, and 6.5%, respectively.

Microstructure and mechanical properties of Mg-Gd-Y-Sm-Al alloy and analysis of grain refinement and strengthening mechanism

The effects of Sm on the microstructure and mechanical properties of Mg-11Gd-2Y-0.6Al alloy were investigated by X-ray diffraction, optical microscopy, scanning electron microscopy, energy dispersive spectrometry and high resolution transmission electron microscopy. Based on the theory of edge–edge matching and electronegativity theory, the mechanism of grain refinement is discussed. The strengthening mechanism is expounded conveniently from fine grain strengthening, coherent strengthening, precipitation strengthening and grain boundary strengthening. The results show that the microstructure of Mg-11Gd-2Y-0.6Al alloy is mainly composed of α-Mg matrix, Mg5Gd and Mg24Y5 phases. The addition of Sm forms Mg41Sm5 phase in the alloy and refines the alloy. The addition of Sm significantly improves the mechanical properties of the alloy at room and high temperatures. When the addition of Sm is 3 wt%, the tensile strengths of the alloy at room temperature and high temperature (200 °C) reach the maximum value 292 and 321 MPa, respectively. The fracture mode of the alloy at different temperatures is mainly brittle fracture and intercrystalline fracture.

Optimisation of Heat Treatment Process for Damping Properties of Mg-13Gd-4Y-2Zn-0.5Zr Magnesium Alloy Using Box–Behnken Design Method

High damping magnesium alloys have poor mechanical properties, so it is necessary to investigate the damping properties of high-strength wrought magnesium alloys to effectively reduce vibration and noise in mechanical engineering. The aim of this work is to improve the mechanical damping performance of a novel high-strength Mg-13Gd-4Y-2Zn-0.5Zr magnesium alloy by optimising the heat treatment process. The mechanical damping coefficient, considering not only damping capacity but also the yield strength, is selected as one of the evaluation indexes. The other evaluation index is the tensile strength. The solid solution and ageing treatment were optimised by Box-Behnken method, an efficient experimental design technique. Heat treatment experiments based on the optimal parameters verified that the best process is a solution at 520 °C for 10 h followed by ageing at 239 °C for 22 h. The damping coefficient reaches 0.296, which is 73.1% higher than that before heat treatment. There was a good agreement between the experimental and Box-Behnken predicted results. The microstructure, morphology and composition of the second phases after heat treatment were analysed by SEM, XRD and EDS. Due to the high content of alloying elements in Mg-13Gd-4Y-2Zn-0.5Zr alloy, there are a large number of second phases after heat treated. They mainly include layer, short rod-shaped, bulk long period stacking order (LPSO) Mg12YZn and granular Mg5Gd phases. It was found that the area fraction of the second phases has an extreme effect on the damping capacity and short rod-shaped LPSO can effectively improve the damping capacity of heat-treated Mg-13Gd-4Y-2Zn-0.5Zr alloy. The volume fraction of the second phases was analysed by ImageJ software. It was concluded that the smaller the area occupied by the second phases, the better the mobility of the dislocation, and the better the damping performance of the alloy. The statistical analysis results obtained using ImageJ software are consistent with the experimental results damping capacity.

Enhanced mechanical properties of Mg-Gd-Y-Zn-Mn alloy by tailoring the morphology of long period stacking ordered phase

High-performance as-extruded and peak-aged Mg-9.2Gd-3.3Y-1.2Zn-0.9Mn (wt%) alloys have been fabricated by semi-continuous casting, heat-treating, extruding and ageing processes. The effects of different heat treatments on the microstructure and mechanical properties have been investigated. Lamellar and bulk-shaped 14H-type long-period stacking ordered (LPSO) phase and few RE-rich (RE: rare earth element) particles form by different heat treatments. After hot extrusion, the heat-treated alloy with bulk-shaped LPSO phase exhibits an equal-axial microstructure, while the heat-treated alloy with lamellar LPSO phase across the grains shows bimodal grain microstructure consisting of fine equal-axial grains and coarse deformed grains. The more profuse lamellar LPSO phase in heat-treated alloy generates higher volume fraction of deformed grains and finer equal-axial grains, hence leads to higher strength but lower ductility. After ageing, the alloy with profuse LPSO and β′ phases achieves the superior mechanical properties with ultimate tensile strength (UTS) of 525 MPa, tensile yield strength (TYS) of 420 MPa and elongation to failure (EL) of 6.3%. The alloy strengthening is mainly attributed to the bimodal grain microstructure, LPSO phase, Mg5Gd (β) phase and β′ phase.

Mechanical properties and microstructure of magnesium alloy Mg22Gd processed by severe plastic deformation

Mg22Gd alloy was processed by high pressure torsion (HPT) at room temperature and the pressure of 2 GPa. A series of specimens with different number of rotations N (N = 0-15) was prepared from the initial coarse grained as cast material. Mechanical properties were investigated by microhardness mapping. The microhardness was found to increase with increasing strain imposed by HPT and tend to saturate at about HV = 145. The microstructure (phase morphology and composition, etc.) evolution with strain was investigated by scanning electron microscopy and EDS. High Gd content in the alloy resulted in the precipitation of stable Mg5Gd phase. This phase exhibited apparently higher hardness than the magnesium matrix. During straining the phase was continuously fragmented and only tiny particles were found in heavily strained material. Electron back scatter diffraction (EBSD) and automated crystallographic orientation mapping in transmission electron (ACOM-TEM) were employed to characterize the fragmentation of the grain structure. HPT was found to result in strong grain refinement by the factor of approximately 1000. The dislocation density was determined by positron annihilation spectroscopy. Significant twinning was found in the initial stages of HPT straining. At high strains twin formation was suppressed and only dislocation storage in the material occurs.

Synergistic effect of Al and Gd on enhancement of mechanical properties of magnesium alloys

The effect of Gd/Al ratio on the properties of as-cast Mg-Gd-Al-Zn alloys was investigated by changing the chemical composition from that of AZ61 to GZ61. At the ratio of 1, the Al2Gd phase becomes predominant and Mg17Al12 is hardly seen in the microstructure. As a potent inoculant, the Al2Gd phase resulted in intense grain refinement and enhancement of strength, ductility and toughness. For instance, the tensile strength and elongation to failure of Mg-3Gd-3Al-1Zn alloy were enhanced by ~4% and 180% compared with those of AZ61 alloy, respectively. However, at high Gd/Al ratios, the Al2Gd phase was replaced by (Mg,Al)3Gd and Mg5Gd phases and very large grain sizes were achieved, which led to poor tensile properties and the appearance of cleavage facets on the fracture surfaces. Therefore, it can be deduced that the presence of Gd and Al, in appropriate amounts to reach Gd/Al ratio of ~ 1, is required for the achievement of grain refinement, good ductility, high strength, and the appearance of ductile fracture surfaces in the Mg-Gd-Al-Zn system. Conclusively, the Mg-Gd-Al-Zn alloys can be considered as a new class of structural magnesium alloy and it is superior to both AZ (Mg-Al-Zn) and GZ (Mg-Gd-Zn) series of alloys.

Equilibrium and metastable phases in a designed precipitation hardenable Mg–3Gd–3Nd–0.6Zr alloy

A precipitation hardenable Mg–3Gd–3Nd–0.6Zr alloy was designed by thermodynamic calculations firstly and then investigated by experiment. In the as–cast alloy, the solution of Nd atoms into equilibrium Mg5Gd phase facilitated its formation and increased its lattice constant. After a two–step solution and a two–step aging treatment, the alloy showed remarkable age–hardening response, which was caused by the formation of massive fine β1 precipitates. It is quite possible that the β1 phase is a continuous solid solution with metastable Mg3Nd and Mg3Gd phases.

Microstructures and Microsegregation of Directionally Solidified Mg-1.5Gd Magnesium Alloy with Different Growth Rates

Directional solidification of Mg-1.5Gd (wt%) magnesium alloy was carried out to investigate the effects of the growth rate on the microstructures under controlled solidification conditions. A Bridgman-type directional solidification furnace with a liquid metal cooling (LMC) technique was used to solidify the specimens, which could provide steady state conditions with a constant temperature gradient (40 K/mm) at a wide range of growth rate (10∼200 μm/s). Results show that the microstructures are cellular, and the relationship between cellular spacing (λ) and growth rate (V) is established in the form: λ = 130.2827V−0.2228 by a linear regression analysis, which is in good agreement with the calculated values by Trivedi model. The thermodynamics solidification path calculations by Scheil model and experimental observations confirm that the solidification microstructure in the alloy consists of primary α(Mg) phase and binary eutectic α(Mg)+Mg5Gd phase. Meanwhile, the microsegregation of the alloying element predicted by the Scheil model agrees reasonably with the electron probe microanalysis (EPMA) measurements.

Creep properties and creep microstructure evolution of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy

The creep properties and creep microstructure evolution of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy were investigated as a function of creep stresses and creep temperatures. It was found that the creep strain and the creep rate increases but the creep life decreases with increasing creep temperatures and creep stresses. At a defined creep stress, the creep strain increases with increasing creep temperatures. At a defined creep temperature, the creep strain increases with increasing creep stresses. α(Mg), Mg12Nd phase and Mg5Gd phase are present in the microstructure after creep. Furthermore, the Mg12Nd phase appears to increase with increasing the creep stress, creep temperature and creep time, indicating that the creep stress, creep temperature and creep time may have a significant effect on the formation of the equilibrium Mg12Nd phase during the creep. At the temperature range less than 225℃, the calculated Qc value is about 74–98kJ/mol, which is very close to the grain boundary diffusion activation energy (80kJ/mol−1), indicating that the grain boundary sliding may be a dominant factor affecting creep properties. While, at the temperature range higher than 225℃, the calculated Qc value is about 227–289kJ/mol, which is much higher than the self - diffusion activation energy (135kJ/mol−1), indicating that the dislocation climbing may be a dominant factor affecting creep properties. The fracture was observed to occur along the grain boundaries. The initiation of the crack is from a trigeminal grain boundary, indicating that the Mg12Nd phase at the grain boundary has a significant effect on the initiation of the crack.

Mg-Gd-Y-Nd-Zr合金的微观组织和力学性能

利用光学金相显微镜、扫描电镜、X射线衍射和透射电镜等研究了名义成分为Mg-8.0Gd-2.5Y-0.5Nd-2.0Zr的合金的显微组织，测试了合金的力学性能。研究表明，Mg-8.0%Gd-2.5%Y-0.5%Nd-2.0%Zr合金的铸态组织主要由α-Mg基体和主要沿晶界分布的第二相Mg5Gd和Mg24Y5组成，固溶处理后，Mg5Gd和Mg24Y5相基本消失，再经时效处理后，合金达到较好的力学性能，其室温、250℃和300℃的抗拉强度分别为312.04 MPa，271.99 MPa和199.81 MPa，其对应的延伸率分别为：3.4%，7.0%和12.7%。 The microstructure of magnesium alloy whose nominal composition is Mg-8.0Gd-2.5Y-0.5Nd-2.0Zr was investigated by means of optical microscopy, scanning electron microscopy, X-ray texture analysis and transmission electron microscopy, and the mechanical properties were obtained. The results show that the as-cast microstructures of Mg-8.0%Gd-2.5%Y-0.5%Nd-2.0%Zr alloy consists of α-Mg matrix and lamellar second-phase (Mg5Gd and Mg24Y5 phase) which distributes around α-Mg matrix. The Mg5Gd and Mg24Y5 phase basically disappeared after solution treatment. Then by aging treatment, alloys reach a good mechanical property. The mechanical properties of studied alloy at room temperature, 250˚C and 300˚C reach 312.04 MPa, 271.99 MPa and 199.81 MPa; the corresponding elongations are 3.4%, 7.0% and 12.7%.

Characterization of the reaction products and precipitates at the interface of carbon fiber reinforced magnesium–gadolinium composite

In the present work, carbon fiber reinforced magnesium-gadolinium composite was fabricated by pressure infiltration method. The phase composition, micro-morphology, and crystal structure of reaction products and precipitates at the interface of the composite were investigated. Scanning electron microscopy and energy dispersive spectroscopy analysis revealed the segregation of gadolinium element at the interface between carbon fiber and matrix alloy. It was shown that block-shaped Gd4C5, GdC2 and nano-sized Gd2O3 were formed at the interface during the fabrication process due to the interfacial reaction. Furthermore, magnesium-gadolinium precipitates including needle-like Mg5Gd (or Mg24Gd5) and thin plate-shaped long period stacking-ordered phase, were also observed at the interface and in the matrix near the interface. The interfacial microstructure and bonding mode were influenced by these interfacial products, which were beneficial for the improvement of the interfacial bonding strength.

Effect of Ag addition on microstructure and mechanical properties of Mg–14Gd–0.5Zr alloy

Microstructure and mechanical properties of the Mg–14Gd–0.5Zr (wt.%) alloy and Mg–14Gd–2Ag–0.5Zr (wt.%) alloy in homogenized, extruded and aged conditions have been investigated. Addition of 2% Ag enhances the tensile ultimate strength of the homogenized Mg–14Gd–0.5Zr alloy due to producing a larger strain field in magnesium crystal lattice. For the hot extruded alloys, the Ag-containing alloy shows a different type of texture, finer grains, higher hardness and a weak age-hardening response compared with the extruded Mg–14Gd–0.5Zr alloy, which are influenced by more Mg5Gd phases precipitating during extrusion in the Ag-containing alloy. The extruded Ag-containing alloy in peak-aged condition exhibit a lower yield strength due to a lower density of the prismatic β′ precipitate, while its better elongation is ascribed to the finer grain size and the basal precipitate.