Magnesium-rare Earth Alloys Research Articles

Microstructures and hydrogen storage properties of Mg-Y-Zn rare earth magnesium alloys with different Zn content: Experimental and first-principles studies

In this work, as-cast Mg-9.1Y-1.0Zn, Mg-9.1Y-1.8Zn and Mg-9.2Y-3.1Zn (wt%) alloys with different Zn content are prepared by the semi-continuous casting method. The microstructures and hydrogen storage properties are systematically investigated by experimental and first-principles calculations approaches. The results show that more LPSO phases form with increasing Zn content and they mainly distribute at the grain boundaries of alloys. The LPSO phases decompose and in-situ form YH2/YH3 hydrides upon hydrogenation. These YH2/YH3 hydrides are more likely to be concentrated at the grain boundaries with increasing Zn content, which leads to the non-uniform distribution of YH2/YH3 hydrides in these alloys. The more non-uniform distribution of these YH2/YH3 hydrides on the Mg matrix will lead to the worse hydrogen storage performance of alloys. Comparatively, the three alloys exhibit similar hydrogen absorption kinetics, while the as-cast Mg-9.1Y-1.0Zn (wt%) shows the best hydrogen desorption kinetics. First-principles calculations show that the H removal energies Erom and H2 recombination energies Erecom from MgH2(110) surface both decrease upon the role of YH2, which further validates the catalytic effect of YH2 on hydrogen sorption kinetics of Mg-Y-Zn alloys.

Defect, microstructure and mechanical properties of Mg–Gd binary alloy additively manufactured by selective laser melting

The high-performance rare earth magnesium alloys such as Mg–Gd based alloys are becoming the research focus of Mg industry. In this paper, a series of Mg-xGd (x = 3, 6, 10, 15, wt. %) binary alloys were additively manufactured by using the selective laser melting (SLM) technology. The influence of Gd content on the defects, microstructures, and mechanical properties of the SLM alloys were studied. The result shows that the Mg-3Gd, Mg-6Gd, and Mg-10Gd SLM samples are well-built with high density. In contrast, macro-cracks growing perpendicular to build direction occasionally occur on the Mg-15Gd samples. The phase composition of all the SLM samples includes α-Mg matrix and Gd2O3 oxidation inclusion, and the Mg-6Gd, Mg-10Gd, and Mg-15Gd samples also contains eutectic β-Mg5Gd. With the increase of Gd content, the morphology of the eutectic β-Mg5Gd turns from lamellar to network-like, whilst the grain morphology of the α-Mg matrix changes from coarse columnar to fine equiaxed and the texture intensity decreases. The increase of Gd content enhances the effects of fine grain strengthening and secondary phase strengthening, so that the microhardness and tensile strength of the SLM samples improve accordingly. The average microhardness and ultimate tensile strength of the Mg-15Gd SLM sample are as high as 106.9 HV and 386 MPa, respectively. On the other hand, the elongation of the SLM alloys shows a “Λ-shaped” variation trend with the increase of Gd content, reaching the maximum value when the Gd content is 10 wt %, due to the combined influence of the changes in grain size, texture intensity, and the morphology of eutectic β-Mg5Gd. Despite the SLM Mg-15Gd alloy has the highest microhardness and tensile strength, its practicality is restricted by the occasional formation of macro-cracks.

Microstructure and properties of vacuum electron beam welded WE43 magnesium alloy joint

This study presents the microstructure and properties of vacuum electron beam welded WE43 magnesium alloy joint. The process parameters of acceleration voltage 150 kV, electron beam current 120 mA and welding speed 35 mm/s are used for vacuum electron beam welding of WE43 rare earth magnesium alloy plate. In this study, the main compositions of the weld are α-Mg and a small amount of eutectic rare earth phase β-Mg24Y5. The mass fraction of the rare earth phase β-Mg24Y5 in the weld area is more than that of the base metal. Segregation of Zr-rich particles can occur in weld zone. The average hardness of the weld is about 27% higher than that of the base metal, and the hardness near the center line of the weld is the highest. The yield strength, tensile strength and elongation of the joint are higher than that of the base metal by approximately 17%, 14% and 41%. Tensile fracture morphology of the welded joint is characterized by ductile and brittle mixed fracture. So, electron beam welding can achieve the connection of WE43 magnesium alloy plate with excellent microstructure and performance. It may be of great significance for this study to expand the application of rare earth magnesium alloy.

Research on formability, microstructure and mechanical properties of selective laser melted Mg-Y-Sm-Zn-Zr magnesium alloy

Selective laser melting (SLM) refers to a laser additive manufacturing technology. It shows its advantages of high efficiency and is capable of processing arbitrary complex structural parts. However, the SLM of magnesium alloy is highly challenging and should be studied in depth due to the low melting and boiling point of magnesium alloy. In this study, selective laser melting (SLM) technology was used to manufacture the Mg-Y-Sm-Zn-Zr alloy. Microstructure characteristics and performance mechanism of the SLMed samples were investigated. As revealed by the results, samples characterized by relatively high densities and low surface roughness could be produced when the energy density was 83.3–166.7 J/mm 3 . The highest density of 97.8% could be obtained when the energy density was 125.7 J/mm 3 . The molten pool was found to consist of slender columnar grains at the edge and a small amount of equiaxed grains at the top, while the grains below the molten pool were coarsened under the action of the thermal influence. The role played by Y 2 O 3 in the solidification of SLM was characterized using the degree of lattice mismatch. Impacted by the high lattice mismatch between Y 2 O 3 and Mg, Y 2 O 3 could not serve as an effective heterogeneous nucleation particle. The highest comperssive performance was obtained at energy density of 125.7 J/mm 3 (YS = 304 ± 5 Mpa, UTS = 394 ± 5 Mpa). The main strengthening mechanism was fine-grain strengthening, followed by precipitation strengthening and the effect of solid solution strengthening was not obvious. This work provides a certain guiding significance for the follow-up research of SLMed rare earth magnesium alloy. • SLM process parameters of Mg-Y-Sm-Zn-Zr which can obtain ideal relative density and surface roughness were determined. • The influence of Y and Zr elements on the solidification behavior of SLMed Mg-Y-Sm-Zn-Zr was determined. • The orientation relationship between precipitated particles and the magnesium matrix was studied. • The improvement of the compressive properties of the SLMed sample was analyzed.

Anisotropic cyclic deformation behavior of an extruded Mg-3Y alloy sheet with rare earth texture

Mg-RE (magnesium-rare earth) alloys exhibit pronounced in-plane anisotropy of mechanical response under quasi-static monotonic loading resulting from the RE texture, as extensively reported. In this work, an obvious in-plane anisotropy of cyclic deformation behavior was observed in an extruded Mg-3Y alloy sheet during strain-controlled tension-compression low-cycle fatigue (LCF) at room temperature. The extrusion direction (ED) samples displayed better fatigue resistance with almost symmetrical hysteresis loops and longer fatigue life compared with the transverse direction (TD) samples. The influences of texture on the deformation modes, cracking modes, and mechanical behavior of Mg-Y alloy sheets under cyclic loading were studied quantitatively and statistically. The activation of various slip/twinning-detwinning systems was measured at desired fatigue stages via EBSD observations together with in-grain misorientation axes (IGMA) analysis. The results indicate that the activation of deformation modes in the TD sample was featured by the cyclic transition, i.e., prismatic slip (at the tensile interval) →{10–12}tension twinning (at the compressive reversal) → detwinning + prismatic slip (at the re-tensile reversal). In the case of the ED sample, the cyclic deformation was dominated by the basal slip throughout the fatigue life. For cracking modes, intergranular cracking and persistent slip bands (PSB) cracking were the primary cracking modes in the ED sample while the TD sample showed a high tendency of {10–12} tension twinning cracking (TTW cracking). The underlying mechanisms influencing the activation of various slip/twinning-detwinning systems, as well as cracking modes and cyclic mechanical behavior, were discussed.

Selective Laser Melted Rare Earth Magnesium Alloy with High Corrosion Resistance.

Magnesium (Mg) degrades too fast in human body, which limits its orthopedic application. Single-phase Mg-based supersaturated solid solution is expected to possess high corrosion resistance. In this work, rare earth scandium (Sc) was used as alloying element to prepare Mg(Sc) solid solution powder by mechanical alloying (MA) and then shaped into implant using selective laser melting (SLM). MA utilizes powerful mechanical force to introduce numerous lattice defects, which promotes the dissolution of Sc in Mg matrix and forms supersaturated solid solution particles. Subsequently, SLM with fast heating and cooling rate maintains the original supersaturated solid solution structure. Immersion tests revealed that high Sc content significantly enhanced the corrosion resistance of Mg matrix because of the formation of protective corrosion product film, which was also proved by the electrochemical impedance spectroscopy measurements. Thereby, Mg(Sc) alloy showed a relatively low degradation rate of 0.61 mm/year. In addition, cell tests showed that the Mg(Sc) exhibited favorable biocompatibility and was suitable for medical application.

Plastic deformation mechanism and hardening mechanism of rolled Rare-Earth magnesium alloy thin sheet

The as-cast rare earth magnesium alloy is extremely difficult to be rolled due to poor plasticity. In this paper, the rolled rare earth magnesium alloy thin sheet was successfully prepared by repeated rolling with small thickness reductions in multiple passes. The microstructure evolution of rare earth magnesium alloys during rolling and forming and its plastic deformation mechanism was explored. The results show that the shape of the precipitates affects the plastic deformation of rare earth magnesium alloys. The slender precipitates can activate more types of non-basal slip to coordinate the deformation, while the agglomerated precipitates cause cracks easily. Intragranular multi-slip and inter-grain slip transfer are the plastic deformation mechanism and hardening mechanism of rolled rare-earth magnesium alloys. The competitive relationship between local strain and macroscopic strain affects the selection and activation order of slip variants, and geometric compatibility is not a necessary condition for coordinated deformation between adjacent grains. The rolled rare earth magnesium alloy sheet not only improves the strength and plasticity significantly but also breaks through the limitation of the application shape of rare earth magnesium alloy, which is of great significance for expanding the practical application of rare earth magnesium alloy in industrial production.

Effect of Heat Treatments on the Corrosion Resistance of a High Strength Mg-Gd-Y-Zn-Zr Alloy.

Magnesium-rare earth (Mg-Re) alloys are very promising structural materials in lightweight industries, while the poor corrosion resistance limits their widespread application. In this work, to provide insights into the functions of precipitate characteristics on the corrosion behaviors of Mg-Re alloys, the influence of heat treatments on the corrosion behavior of Mg-11.46Gd-4.08Y-2.09Zn-0.56Zr alloy was investigated via an immersion test, electrochemical experiment and scanning electron microscope (SEM). The results showed that the corrosion rate of Mg-11.46Gd-4.08Y-2.09Zn-0.56Zr alloy specimens decreased by 17.58% and 20.44% after T5 and T6 heat treatment, respectively. In the heat-treated specimens, the corrosion did not extend further into the matrix but formed fine corrosion grooves along the extrusion direction. The improved homogeneity reduced the residual stress and the β’ precipitate introduced as a corrosion barrier after T5 and T6 heat treatment reduced the corrosion rate of the studied Mg alloy. In addition, the volume fraction of the long-period stacking-ordered (LPSO) phase decreased after heat treatment, which effectively reduced galvanic corrosion and enhanced the protective effect on the Mg matrix.

Fluidized Bed Heat Treating of a Magnesium-Rare Earth Alloy

Fluidized Bed Heat Treating of a Magnesium-Rare Earth Alloy

Multi-scale simulation of the dendrite growth during selective laser melting of rare earth magnesium alloy

The solidification microstructure of the alloy fabricated by the selective-laser-melting (SLM) process can significantly impact its mechanical properties. In this study, a multi-scale model which couples the macroscale model for thermal-fluid and microscale cellular automata (CA) was proposed to simulate the complex solidification evolution and the dendrite growth (from planar to cellular to dendritic growth) during the SLM process. The solid–liquid interface of CA was dispersed with the bilinear interpolation method. On that basis, the curvature was accurately determined, and the calculation result was well verified by employing the Kurz–Giovanola–Trivedi analytical solution. The dendrite morphology, solute distribution, and primary dendrite arm spacing during the solidification of the SLM molten pool were quantitatively analyzed with the proposed model, well consistent with the experiment. The distribution of the undercooling field and the concentration field at the tip of dendrites different orientations were analyzed, and the two competing growth mechanisms of converging and diverging growth were revealed. Moreover, the research also indicates that during the growth of dendrites, the result of dendrite competition is determined by the height of the dendrite tip position in the direction of the thermal gradient, while the distribution of the concentration field (symmetrical or asymmetric) at the tip of the dendrite critically impacted the competing growth form of dendrites.

Influence of plasma electrolytic oxidation coating on corrosion characteristics of friction stir welded ZE41 rare earth magnesium alloy

Magnesium and its alloys are extensively used in the defence, aerospace and automotive industries, owing to their excellent mechanical properties. But there is a requirement of corrosion prevention treatment before using it for applications, since the corrosion resistance of Mg alloy is poor. In this work, a study on the mechanical properties of friction stir welded ZE41 rare earth magnesium alloy thick plates of 15 mm and 25 mm, was carried out. A metallographic examination and mechanical tests were conducted on both the welds. Furthermore, Plasma Electrolytic Oxidation (PEO), a surface conversion treatment was carried out on both the base material as well as on the friction stir butt joints. X-Ray diffraction and scanning electron microscopy analysis were carried out on all the samples, with and without coating. The coated and uncoated samples were assessed by potentiodynamic polarisation (PDP) tests and electrochemical impedance spectroscopy (EIS) to study the electrochemical corrosion behaviour. An outer porous layer and an inner dense layer, have been identified from the SEM images. From the corrosion tests results it was revealed that there was an improvement in the corrosion resistance of both the base metal as well as the weldments by the PEO coating process.

Study on the Strengthening Mechanism of Rare Earth Ce in Magnesium Alloys, Based on First-Principle Calculations and Electronegativity Theory.

Since the commercial applications of rare earth magnesium alloys are increasing gradually, there are considerable advantages to developing lower cost and higher performance magnesium alloys with high abundance rare earth (RE) elements. However, the alloying order of a matrix magnesium alloy is completely changed with the addition of RE elements. Therefore, further study of the strengthening mechanism of Ce element in magnesium alloys is required. In this work, the thermodynamic stability of the possible second phases in a Mg-Al-Mn-Ce multicomponent magnesium alloy were analyzed, based on first-principle calculations, and the precipitation sequence of the key RE phases was deduced as a consequence. Combined with Scanning Electron Microscope (SEM), X-ray Diffractometer (XRD), Energy Dispersive Spectrometer (EDS), and other experimental methods, it was investigated whether the preferentially precipitated second phases were the nucleation core of primary α-Mg. The complex alloying problem and strengthening mechanism in a multi-elemental magnesium alloy system were simplified with the aid of electronegativity theory. The results showed that the preferentially precipitated Al11Ce3 and Al10Ce2Mn7 phases could not be the nucleation core of primary α-Mg, and the grain refinement mechanism was such that the second phases at the grain boundary prevented the growth of magnesium grains. Moreover, the tensile test results showed that the reinforced structure, in which the Al-Ce phase was mixed with Mg-Al phase, was beneficial for improving the mechanical properties of magnesium alloys, at both ambient temperature and high temperature.

Strengthening of low-cost rare earth magnesium alloy Mg-7Gd-2Y–1Zn-0.5Zr through multi-directional forging

Conventional rare-earth magnesium alloys have high strength but are not accepted by some fields because of their relatively high price. Magnesium alloys with the addition of small amounts of rare earth elements can improve the mechanical properties by severe plastic deformation while saving cost. In this paper, the effects of multidirectional forging (MDF) process on the microstructure and mechanical properties of low-cost rare earth magnesium alloy Mg-7Gd-2Y–1Zn-0.5Zr were investigated, and the plastic deformation mechanism, dynamic recrystallization mechanism and property strengthening mechanism during the MDF process were analyzed. The results showed that the yield strength, ultimate tensile strength and elongation of Mg-7Gd-2Y–1Zn-0.5Zr alloy were 395.2 MPa, 435.3 MPa and 17.6% after extrusion, MDF at reduced temperature conditions and T5 treatment. The increase in alloy strength is related to solid solution strengthening and is more attributed to the combined effect of grain refinement and aging precipitation. The dynamic recrystallization mechanism leading to grain refinement converts from discontinuous dynamic recrystallization to continuous dynamic recrystallization during MDF at reduced temperatures. And the decrease in deformation temperature leads to difficulties in prismatic and pyramidal slipping, where grain boundary sliding from fine grains and twinning in a few relatively coarse grains play a role in coordinating deformation.

Enhanced Grain Refinement and Precipitation of the IEECAPed Mg-Sm-Zn-Zr Alloy by Nd Addition.

Achieving magnesium-rare earth alloys with excellent mechanical properties remains a challenging goal in the aerospace industry. The integrated extrusion and equal channel angular pressing were employed to refine grain and improve the mechanical properties of Mg-xNd-2.0Sm-0.4Zn-0.4Zr alloys. The effect of Nd element on microstructure and mechanical properties of the extruded and subsequently aged alloys were carried out by varying the amount of the Nd element from 0 wt.% to 2.5 wt.%. The optical microscopy results indicated that the grain size was remarkably refined by the addition of Nd element. The grain size decreased from 29.7 μm to 10.9 μm with increasing of the Nd element from 0 wt.% to 2.5 wt.%. The transmission electron microscopy results showed that the nano-scaled basal lamellar precipitates, prismatic lamellar precipitates and granular precipitates were formed in α-Mg matrix. The amount of the precipitates increased significantly by the addition of Nd. Moreover, the strength of the alloys significantly improved with Nd. Superior strength and considerable plasticity were obtained as the content of Nd element reached 2.0 wt.%, while the tensile strength of the Mg-2.0Nd-Sm-Zn-Zr alloy (315 ± 5 MPa) increased by 35.8% with respect to the Nd-free alloy (232 ± 3 MPa).

Unraveling the effect of deformation-induced phase transformation on microstructure and micro-texture evolution of a multi-axially forged Mg-Gd-Y-Zn-Zr alloy containing the LPSO phase

The effect of blocky to lamellar phase transformation on the microstructure and micro-texture of a Magnesium-Rare earth alloy containing long-period stacking order (LPSO) phases were examined comprehensively at the isothermal temperature of 400 °C via multiaxial forging. A very fine-grained microstructure with an average grain size of 1 μm was achieved after applying three multi-axial forging (MAF) passes. Particle stimulated nucleation (PSN) along with continuous dynamic recrystallizations (CDRX) due to the blocky LPSO phases were realized to be the main reasons for the achievement of such fine microstructure. The deformation-induced blocky to lamellar phase transformation began at 1/3 pass and expanded through the whole microstructure after the third pass. Such phase transformation was found to be initiated by the fragmentation of blocky phases. As a result of PSN and CDRX mechanisms, two new rare earth (RE) texture components of <10-11> and <2-1-10> || Transverse Direction (TD) were formed at the second MAF pass that led to a highly randomized deformation texture. Nevertheless, the continuous breakdown of blocky phases and their subsequent phase transformation to lamellar LPSO suppressed PSN at the third deformation pass. Hence, the formed RE texture components were disappeared which in turn increased the texture intensity at this deformation pass.

Multi-scale study on microstructure of ME21 rare-earth magnesium alloy sheet

Multi-scale study on microstructure of ME21 rare-earth magnesium alloy sheet

Achieving ultra-high hardness of Mg-Sm-Ca alloy with the unique nanostructure

Most magnesium rare-earth (Mg-RE) alloys have limited hardness. Here a new strategy is proposed, that is, the solution treated Mg-4.97Sm-0.84Ca (wt.%) alloy was subjected to hot rolling processing and high-pressure torsion (HPT) processing, followed by aging for various periods. The ultra-high hardness value of ~175 H V for Mg-Sm-Ca alloys processed by hot rolling and HPT after aging at 150 °C for 3.5 h is achieved which is ~109 H V higher than as-solution alloy (~66 H V). Such an unprecedentedly high hardness value is mainly attributed to the unique nanostructure in the alloy which contains high-density nano-precipitates which have low lattice misfit with the matrix. We found that the morphological of the Mg41Sm5 phase was changed from rod-like to kink band-like by HPT processing and then convert into dense, near-spherical shape particles during the subsequent aging process to form isotropic structure which provides exceptional strengthening for the alloy.

Segregation behavior and its regulating process in as-cast magnesium alloy containing heavy rare earth

Specific gravity segregation that occurs during the smelting process always leads to the low composition homogeneity and poor performance stability of the magnesium-rare earth (Mg-RE) alloys. In this study, the segregation behavior of Mg–Gd alloy was investigated by sampling from different locations in the ingot fabricated in a resistance furnace without a pouring process. The combined application of induction-heating and mechanical stirring with various speeds (0–130 r/min) was applied to promote the distribution homogeneity of Gd atoms. In the resistance-heating fabricated ingot, Gd content at the bottom section reaches 407% of that at the top. The coarse dendrites surrounded by the network-like eutectic structures are responsible for the brittle fracture with a poor elongation of 3.7%. By the combined employment of the induction heating and mechanical stirring with the speed of 87 r/min at 740 °C for 40 s, the variation of the Gd content within the whole ingot can be reduced to be the minimum of 0.23 wt%. Corresponding formation and regulating models of segregation were also proposed. However, the cooling rate of the melt is reduced by the continuous increase of the stirring speed to 130 r/min, which results in the grain coarsening and lower homogeneity of the ingot.

Automated ex-situ detection of pitting corrosion and its effect on the mechanical integrity of rare earth magnesium alloy - WE43

This study develops a three-dimensional automated detection framework (PitScan) that systematically evaluates the severity and phenomenology of pitting corrosion. This framework uses a python-based algorithm to analyse microcomputer-tomography scans (μCT) of cylindrical specimens undergoing corrosion. The approach systematically identifies several surface-based corrosion features, enabling full spatial characterisation of pitting parameters, including pit density, pit size, pit depth as well as pitting factor according to ASTM G46-94. Furthermore, it is used to evaluate pitting formation in tensile specimens of a Rare Earth Magnesium alloy undergoing corrosion, and relationships between key pitting parameters and mechanical performance are established. Results demonstrated that several of the parameters described in ASTM G46-94, including pit number, pit density and pitting factor, showed little correlation to mechanical performance. However, this study did identify that other parameters showed strong correlations with the ultimate tensile strength and these tended to be directly linked to the reduction of the cross-sectional area of the specimen. Specifically, our results indicate, that parameters directly linked to the loss of the cross-sectional area (e.g. minimum material width), are parameters that are most suited to provide an indication of a specimen's mechanical performance. The automated detection framework developed in this study has the potential to provide a basis to standardise measurements of pitting corrosion across a range of metals and future prediction of mechanical strength over degradation time.

Isothermal compression bonding mechanism and mechanical properties of WE43 magnesium-rare earth alloy

Isothermal compression bonding (ICB), or high temperature plastic deformation bonding, has become a prospective solid phase bonding technique without risk of casting defects in the joints compared to the conventional welding. In this study, a rare-earth magnesium alloy is subjected to the ICB process under a range of thermo-mechanical processing parameters. The results show that large grains and low grain orientation spread (GOS) values are observed in the bonding zone at high temperature and low strain rate, which leads to a lower bonding strength but a higher elongation (El). In the other cases (high strain rate or relatively low temperature & low strain rate), the grain size in the bonding zone is refined but the average GOS value increases, resulting in a stronger bonding strength but a weaker El. The former case belongs to a strong grain boundary sliding and thus the metallurgical bonding of the interfacial grain boundaries is enhanced while the latter is a DRX-dominated mechanism by the evaluation of stress exponent and apparent activation energy. In addition, the flow instability criterion is utilized to evaluate whether the ICB process is in a steady condition.