Mg-rare Earth Alloys Research Articles

Optimizing microstructure and strength of CMT-wire arc additive manufactured WE43 Mg alloy through a novel active cooling technique

Cold metal transfer (CMT)-Wire arc additive manufacturing (WAAM) has been widely utilized in the production of large Magnesium (Mg) alloy components. However, the thermal cycling and heat input inherent in the CMT-WAAM process can adversely affect the microstructure and properties of Mg-Rare earth (RE) alloys. Currently, there is a lack of effective, low-cost, and easy-to-implement methods to mitigate these issues. In this study, an active cooling technique (ACT) comprising base cooling and bypass cooling was proposed. The ACT facilitates in-situ cooling of CMT-WAAMed WE43 components through uninterrupted water flow in the substrate and bypass plate, offering advantages in cost-effectiveness, safety, and ease of implementation. It was found that the CMT-WAAMed WE43 alloy thin wall component manufactured with the ACT assistance had fewer hard-brittle eutectic phases and finer grains due to the accelerated cooling rate. ACT effectively reduced the heat accumulation and peak temperatures in the component. Furthermore, it inhibited the transformation of the β1 phase to the β phase and suppressed the coarsening of the β1 phase. ACT also improved the ultimate tensile strength (UTS), yield strength (YS), and elongation (EL) of the CMT-WAAMed WE43 component by 11.7 %, 5.8 %, and 72.3 %, respectively. This study provides a novel approach for the microstructure optimization of CMT-WAAMed Mg-RE alloy, which is crucial for the production of high-performance and large-size components in the aerospace industry.

The effect of medical biodegradable magnesium alloy in vivo degradation and bone response in a rat femur model with long-term fixation.

It is of great significance to understand the effect of the different corrosion behaviors of magnesium (Mg) alloys manufactured using different casting methods and implanted with different methods on the long-term implantation to expand the application of Mg-based biomedical implants. The effects of four different casting and rolling speeds on the microstructure of an Mg-rare earth (Mg-Re) alloy were analyzed using electron backscatter diffraction (EBSD). Four Mg alloys were obtained using vertical two-roll casting (TRC) at 10m/min, 16m/min, 24m/min, and 30m/min, and their microstructure, corrosion behavior and bone reaction in vivo were studied. The corrosion resistance of the alloy increases with an increase in casting speed and finer grain size of the cast-rolled parts. The Mg-Re alloys with TRC-10m/min and TRC-30m/min were selected for animal experiments. The two Mg alloys were made into metal rods and inserted into the rat femur to simulate the effect of Mg-Re on femoral healing under an injury condition. The rods were implanted for a long time to judge the effects of the Mg-Re alloy on the body. The TRC-30m/min implants obtained highly mature new bone tissue in the case of bone injury. The in vivo experiments showed that the corrosion resistance of the TRC-30m/min implant was better than that of the TRC-10m/min implant. After 32 weeks of implantation, there were no pathological changes in the liver, heart, or kidney of rats in the TRC-30m/min group, and the cell structure was normal.

Fatigue-induced oxidation assisting microcrack nucleation in Mg-RE alloy under ultrasonic fatigue

Characterizing the transition from a crack-free to a cracked state remains a challenging topic in fatigue. Mg-rare earth alloys, containing the long-period stacking ordered (LPSO) phase, exhibit superior mechanical properties. Here, microcracks are found to nucleate at the soft α-Mg nano-layers, away from the LPSO lamellae. Notably, severe oxidation is observed along the damage bands. Based on the detailed characterizations, it is suggested that dislocation motions continuously bring the new α-Mg matrix to oxidation transformation, resulting in the thickening MgO layer. However, once the fatigue-induced oxide reaches a certain thickness, it starts acting as a barrier for the further dislocation motions. As a result, dislocation accumulation and cumulative damage occur in the region ahead of the thick oxide, causing microcrack nucleation. This fatigue-induced oxidation, assisting microcrack nucleation, is distinct from the existing fatigue mechanisms.

Multiscale thermo-kinetic characterization for β′ and β1 precipitation in Mg-Sm alloys

Two types of metastable β-series phases have been reported to exist in aged Mg-Sm alloys. The first type refers to preferentially precipitated β′-series phases (including β′s, β′l and β′h), which are intrinsically different Sm orderings on the hexagonal close-packed (HCP) lattice of α-Mg, while the second type refers to subsequently precipitated β1 phase having a face-centered cubic (FCC) lattice. Up to now, whether β1 forms from the α-Mg matrix or the pre-existing βʹ is still on debate, due to unavailable convincing experimental evidence. Employing the first-principles calculations and a statistical mechanics approach consisting of the cluster expansion (CLEX) model and the Monte Carlo simulation, a multiscale phase-field model is herein constructed to thermo-kinetically characterize the precipitations of β′s, β′l, β′h and β1 in Mg-Sm alloys. Equipped with a newly revised explicit nucleation algorithm developed from so-called thermo-kinetic correlation, the present phase-field model can reproduce multi-particle morphologies upon independent precipitations of β′s, β′l, β′h and β1. And then, correlative precipitations of β′ (referring to β′s, β′l or β′h) and β1 are focused on, where, it has been proved that, any pre-existing βʹ variant can transform to a specific single variant or multi-variant configurations of β1. Notably, the precipitations above should follow a rule determined by so-called strain energy dominated thermo-kinetic connectivity. The newly proposed nucleation algorithm is widely applicable for precipitation modeling, and the revealed thermo-kinetic mechanism for precipitations of β-series phases should be enlightening for the rational design of Mg-rare earth (RE) alloys.

Comparative study on the microgalvanic corrosion phenomena of WE43 alloy in Cl- / HCO3- / CO32- environments

With rare-earth elements gradually coming into view, Mg-rare earth alloys like WE43, which inherit the properties of Mg alloys as lightweight structural materials, are gradually being explored. Anomalous preferential dissolution of the second phase of Mg-rare earth alloy (WE43) in the CO32- environment was identified. Further research revealed that this anomaly was due to the anodic role of the second phase in the microgalvanic corrosion with Mg-matrix protection. The interesting thing is that the second phase as cathode role was found in the HCO3- environment, like the phenomenon in Cl- environment, and the value of pH was not the decisive fact. The different microgalvanic corrosion behavior could be attributed to the reaction between metal atoms and ions in solution. In addition, the surface film of WE43 was systematically studied in three environments and specific corrosion models were developed for each environment.

Microstructure and strengthening mechanism of TIG welded joints of a Mg-Nd-Gd alloy: Effects of heat input and pulse current

As a newly-developed Mg-rare earth (Mg-RE) alloy, Mg-3Nd-3Gd-0.2Zn-0.5Zr (EV33) alloy has been reported to show desirable mechanical properties. Further research on the welding behavior of EV33 alloy can facilitate its application in practical production. In this work, the effects of heat input and pulse current on the microstructural evolution and mechanical properties of tungsten inert gas (TIG) welded EV33 alloy were systematically investigated. The results show that the increment of welding current or pulse current frequency led to increased grain size in the fusion zone (FZ), on account of the combined effect of reduced cooling rate and enhanced constitutional supercooling. Notably, the TIG welding with pulse current (PC-TIG) efficiently broke the dendrites and promoted the dissolution of Zr particle (Zrp), producing a better grain refinement effect and more homogeneous microstructure in the FZ as compared to the ordinary TIG (O-TIG) welding. A physical model has been proposed for depicting the microstructure evolution of the joints fabricated with different welding parameters. The highest joint efficiency 88.4% with a good combination of strength-ductility (UTS = 220 ± 14 MPa, YS = 129 ± 9 MPa, EL = 11.9 ± 1.2%) of the joint was obtained by optimizing the welding process, and the related strengthening mechanisms were quantified. It is believed that this work can provide guidance for controlling the microstructure of the Mg-RE alloy joints.

Mechanistic origin of the enhanced strength and ductility in Mg-rare earth alloys

Magnesium (Mg) alloys with low concentrations of rare earth additions are known to exhibit strengths and ductility that are significantly higher than those obtained in traditional Mg alloys. However, the mechanisms that underlie these improvements are still open to debate. We assessed these mechanism(s) by carrying out in-depth analysis of the deformation behavior in single crystals of pure Mg and a homogenized Mg-0.75 at.% Gd alloy oriented for twinning, pyramidal- and basal-slip. We observed a fivefold increase in basal CRSS, an eightfold increase in twinning CRSS and a fourfold decrease of the pyramidal/basal CRSS (P/B) ratio due to Gd addition. We also observed that while twinning and pyramidal slip activities were similar in the two material systems, basal slip was radically different. Specifically, basal slip was planar in the alloy but wavy in pure Mg. Our work reveals that these observations are a consequence of Gd-rich short-range ordered (SRO) clusters in the alloy. We show that interactions between dislocations and the SRO clusters would lead to significant increases in strength and 〈c+a〉 slip activity, and consequently, ductility improvements in homogenized polycrystalline Mg-Gd alloys.

Prediction of mechanical properties of Mg-rare earth alloys by machine learning

In this work, the quantitative relationship among the composition, processing history and mechanical properties of Magnesium-rare earth alloys was established by machine learning (ML). Based on support vector regression (SVR) algorithm, ML models were established with inputs of 310 sets of data, which can predict ultimate tensile strength (UTS), yield strength (YS) and elongation (EL) with well accuracy. In order to verify the general applicability of our model, new data were collected from the literature, and the ML models was used to predict their mechanical properties respectively. The MAPE of UTS, YS and EL predicted by SVR model are 9%, 12% and 36%, respectively. The reasons for the deviation of the predicted results were also analyzed. The effects of rare earth elements on UTS, YS and EL were analyzed by the SVR models. The established ML model was used to recommend the composition and processing history of new Magnesium-rare earth alloys with high mechanical properties.

Texture tailoring of a cold-rolled Mg-Zn-Gd alloy by electropulse treatment: The effect of electropulse and Gd element

Electropulse treatment (EPT) was utilized to tailor the texture of a cold-rolled Mg-rare earth (RE) alloy (Mg-1Zn-1Gd, ZG11). The respective effect and combined effect of electropulse and Gd element on the texture modification were evaluated by using conventional heat treatment (HT) and AZ31 alloy for comparison. It was found that in addition to significantly accelerating static recrystallization (SRX), electropulse was effective in suppressing the growth advantage of basal oriented grains and facilitating uniform growth, according to the evolution of 30° 〈0001〉 boundaries. As widely reported, addition of Gd element was observed to promote shear band-related nucleation and preferential growth of off-basal oriented grains via grain boundary segregation; unfortunately, the growth advantage of basal oriented grains seemed not to be well-suppressed. When electropulse was exerted on cold-rolled ZG11, the SRXed grains showed obvious off-basal orientations at the early stage benefited from various nucleation sites, e.g., shear bands, extension twin boundaries, and the off-basal orientations were retained to a great extent during grain growth due to the synergetic effect of electropulse and grain boundary segregation on grain boundary migration. As a consequence, the ZG11 alloy evolved into a quite distinctive SRX texture after EPT.

Opposite effects of Zn addition on the creep resistance at low and high temperatures for as-cast Mg-15Gd alloy

The effect of Zn addition on the mechanical properties in Mg-rare earth (RE) alloys has been controversial. In this study, we selected the as-cast Mg-15Gd (wt.%) alloy as a model system to illustrate the effect of Zn addition on the microstructures and creep properties. The results revealed that the main strengthening phase transformed from β' in the Mg-15Gd alloy to long-period stacking ordered (LPSO) structure in the Mg-15Gd-1Zn (wt.%) alloy. At low temperatures (≤285 °C) when the creep mechanism was dislocation slip, the prismatic β' phase hindered the movement of dislocations, leading to better creep resistance of the Mg-15Gd alloy. On the contrary, at high temperatures (>285 °C) when the creep was controlled by dislocation climbing, the Mg-15Gd-1Zn alloy with lower stacking fault energy and basal LPSO phase exhibited better creep resistance. These findings inspire us to design the composition of Mg-RE alloys in different service environments for better performance.

Partitioning of Ca to Metastable Precipitates in a Mg-Rare Earth Alloy

Partitioning of Ca to Metastable Precipitates in a Mg-Rare Earth Alloy

Microstructure evolution and mechanical properties of a high-strength Mg-10Gd-3Y–1Zn-0.4Zr alloy fabricated by laser powder bed fusion

A high-strength Mg-10Gd-3Y–1Zn-0.4Zr (GWZ1031K, wt%) alloy was prepared by laser powder bed fusion (LPBF), and the microstructure and mechanical properties of the as built, LPBF-T5, LPBF-T4, and LPBF-T6 states were systematically studied. The as built alloy is composed of fine equiaxed ɑ-Mg grains with an average grain size of 4.1 ± 0.5 µm, reticular β-(Mg,Zn)3(Gd,Y) eutectic phase and flaky Y2O3 oxide phase, and exhibits yield strength (YS) of 310 ± 8 MPa, ultimate tensile strength (UTS) of 347 ± 6 MPa and elongation (EL) of 4.1 ± 0.8%. A simple direct aging heat treatment at 175 ℃ for 64 h after LPBF leads to an ultra-high YS of 365 ± 12 MPa but a low EL of 0.8 ± 0.3% in the LPBF-T5 alloy. The solution heat treatment improves ductility by transforming the hard and brittle eutectic phase into the relatively soft and deformable lamellar long period stacking ordered (LPSO) structure inside grains and X phase at grain boundaries without obvious grain growth. Moreover, the LPBF-T4 alloy solution-treated at 450 ℃ for 12 h exhibits YS of 255 ± 8 MPa, UTS of 328 ± 9 MPa, and EL of 10.3 ± 0.5%. Aging heat treatment after solution introduces numerous prismatic β′ and β1 precipitates, which help to increase tensile strength. The YS, UTS, and EL of the LPBF-T6 alloy are 316 ± 5 MPa, 400 ± 7 MPa, and 2.2 ± 0.3%, respectively. It can be concluded that the LPBF process when combined with specially designed post-processing holds great promise for the manufacturing of high-performance components of the Mg-rare earth alloys with significantly higher YS for various applications.

Intercalation of Y in Mg-Al layered double hydroxide films on anodized AZ31 and Mg-Y alloys to influence corrosion protective performance

Yttrium-doped Mg-Al layered double hydroxide films (MgAlY-LDHs) were firstly fabricated by a hydrothermal method on anodized AZ31 and Mg-Y alloys. The structure, morphology, and composition of MgAlY-LDHs were characterized by X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), X-ray fluorescence spectrometry (XRF), backscattered electron spectrometer (BSE), field-emission scanning electronic microscope (FE-SEM), energy dispersive spectrometry (EDS) and glow discharge optical emission spectroscopy (GDOES), respectively. The electrochemical behavior was investigated using polarization curves, electrochemical impedance spectroscopy (EIS), and immersion tests. The results indicated that the Y ions can be successfully intercalated in the MgAl-LDH lamella by isomorphous replacement, which can enhance corrosion protective performance and possess the self-healing ability of coatings. Furthermore, the in-situ growth of ternary LDHs film on high-strength Mg-rare earth (RE) alloys demonstrated the considerable potential of anti-corrosion protection for industrial application. The schematic representation of the growth mechanism and electrochemical behavior of MgAlY-LDHs nanosheet was put forward finally.

Role of slip and {10-12} twin on the crystal plasticity in Mg-RE alloy during deformation process at room temperature

The deformation mechanism of slips and twins has a considerable influence on the plasticity of magnesium alloys. However, the roles of slips and twins in the room-temperature deformation of Mg-rare earth (Mg-RE) alloys with high contents of rare earth elements is rarely investigated. Here, the microstructural evolution and deformation mechanism of an aged Mg-5Y-2Nd-3Sm-0.5Zr alloy during uniaxial compression at room temperature were systematically investigated using in-situ electron-backscattered diffraction and transmission electron microscopy. The results indicated that in the early stage of deformation, the Mg-RE alloy was mainly controlled by the slip of <a> dislocations in the basal plane and the coordinated c-axis strain of the {10-12} twin. With an increase in the strain, the grain orientation became more suitable for the initiation of pyramidal II <c + a> dislocations in the later stage of deformation; these dominated the deformation mechanism. In the twin evolution of the Mg-RE alloy, there were three types of twin-twin interaction behaviors: (i) single twin variant ‘parallel’ structure, (ii) single twin variant ‘cross’ structure, and (iii) multi twin variant ‘cross’ structure. In addition, three types of twin-grain boundary interaction behaviors were summarized: (i) twin ‘refracting through’ grain boundary, (ii) twin ‘parallel through’ grain boundary, and (iii) twin ‘fusing through’ grain boundary, which are expected to act as new means and solutions for the twin strengthening of magnesium alloys.

Effect of casting speed on microstructure, corrosion behaviour and in vivo bone reaction of Mg-rare earth alloys

In this study, two casting speeds of 10 and 30 r/min were used in vertical twin-roll casting (TRC) to obtain Mg-rare earth (Mg-RE) alloys, and their microstructures, corrosion behaviours and in vivo bone reactions were investigated in detail. The results indicated that the roll-castings of TRC-30 r/min exhibited a finer grain size and higher volume fraction of non-crystallization than those in castings of TRC-10 r/min. Moreover, the results of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization indicated that the castings of TRC-30 r/min displayed a higher corrosion resistance compared with those in the castings of TRC-10 r/min. Animal tests showed that a higher degree of newly formed bone tissues was achieved by implants of TRC-30 r/min. Additionally, in vivo tests displayed that degradation properties of the TRC-30-r/min implants were better than those of the TRC-10-r/min implants; furthermore, the degradation layer was a two-layer structure, and P and Ca were enriched in the outer degradation layer. In summary, these findings elucidated that casting speed has a substantial effect on the microstructure and degradation property of Mg-based implants, and the degradation property performs better with increased casting speed.

In vivo degradation behaviour and bone response of a new Mg-rare earth alloy immobilized in a rat femoral model

A new type of Mg-rare earth (Ce, La) and the AZ31 alloy sheets were prepared by vertical twin-roll casting (TRC) technology under identical casting conditions, and their microstructural features, degradation behaviours and bone responses were investigated. The microstructural characterization showed that the Mg-RE (rare earth) exhibited a higher amorphous forming ability than the AZ31. Moreover, the results of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization indicated that the Mg-RE sheets displayed a higher corrosion resistance compared with the AZ31 sheets. Additionally, the Ti, Mg-RE and AZ31 sheet implants were immobilized and implanted in a rat femur model to observe degradation behavior during 16 weeks. in vivo tests showed that no significant change in the femur surrounding the Ti group, which excluded the external factor that the new bone formation resulting from bone remodeling. Furthermore, the Mg-RE group induced more newly formed bones, which met the necessary conditions for the prevention of pathological fractures. Therefore, the novel Mg-RE alloy appear to hold a healing candidate as the biodegradable implant material.

Laser cladding of Mg–Zn–Y–Zr alloy using Al with different ratios of micro- and nano-SiC powder

In this study, a Mg–Zn–Y–Zr alloy was laser cladded with Al–SiC powder using different weight percentages and different grain sizes of SiC to improve its surface properties. We observed that the laser-cladded layer primarily comprised α-Mg, Mg2Si, Mg17Al12, Al2Y and Al3Y phases. Moreover, the cladded layer exhibited a progressive microstructure and composition, and the volume fraction and size of the Mg17Al12, Al2Y and Al3Y phases decreased with the increase in depth. Because of the formation of these hardened phases, the average hardness of the sample with 10% of nSiCp was 4.06 times that of the substrate. In comparison with four laser-cladded samples, the sample cladded with 5% of nSiCp had the lowest friction coefficient and the highest wear resistance. The potentio-dynamic polarisation curves, measured in an aqueous NaCl (3.5 wt%) solution, demonstrated that for samples with a cladding of 10% nm, the corrosion potential increased from − 1.63 V for the untreated Mg–Zn–Y–Zr alloy to − 1.32 V for the laser-cladded alloy. The corrosion current density significantly decreased from 3.60 × 10−5 to 4.03 × 10−6 A cm−2. These results demonstrate that laser cladding with nano-SiC is an effective method to significantly improve the surface properties of Mg-rare earth alloys.

Environmentally assisted cracking behavior of U-bend specimens of Mg–RE alloys in chloride containing basic solution

Abstract Environmentally assisted cracking (EAC) behavior of two Mg-rare earth (RE) alloys such as Mg–Zn–Gd–Nd–Zr (EV31A) and Mg–Y–Nd (WE43C) alloys was investigated by using U-bend specimens. Open circuit potentials (OCP) of the U-bend specimens were monitored during the EAC tests in 0.1 M NaOH solution with different chloride concentrations at room temperature. EV31A (as-received, and peak aged) and WE43C (peak aged) specimens failed by SCC in 80 ppm chloride containing 0.1 M NaOH solution at OCP. When the EAC initiation occurred, the OCP decreased continuously. Irregular fluctuations of the OCP were observed in the absence of EAC. The OCP versus time profile could be used for monitoring EAC failure of the Mg–RE alloy components in real life service. Applied potentials did not cause cracking of the EV31A alloy in 80 ppm Cl− containing 0.1 M NaOH. Accelerated cracking was observed on the WE43C alloy in peak-aged condition under the applied potentials in the transpassive region when compared to that of OCP condition. Overaging decreased the susceptibility to cracking.

Mg-Based Materials with Quasiamorphous Phase Produced by Vertical Twin-Roll Casting Process

Metallic materials with micron grains, submicron grains, or amorphous structures have attracted great interest in recent decades owing to their excellent mechanical properties and corrosion resistance. Compared with traditional forming processes, rapid solidification technology has shown great superiority and potential in the preparation of materials in such structures. In this study, fine-grained quasiamorphous Mg-based alloy strips fabricated by a twin-roll strip casting process were explored using simulation and experimental methods. The concept of critical casting speed was proposed to reflect the optimum casting conditions. The product of critical casting speed and strip thickness was used to evaluate the cooling capacity of the casting system. Based on simulation results, a twin-roll strip-casting experiment was performed on a Mg-rare earth alloy. A novel puddle-like microstructure of the as-cast alloy strip was obtained. Tensile testing results showed that the novel strip exhibited improved ductility.

Role of grain boundaries on ductility in Mg-Y alloys

The impact of the grain size and the chemical composition of an alloying element on mechanical properties in ductility was investigated using wrought processed various Mg-Xat%Y binary alloys (where, X = 0.1, 0.3, 1.0 and 2.0). The average grain sizes of these Mg-Y alloys after a subsequent annealing process varied from around 5 µm to over 100 µm. The elongation-to-failure in tension was influenced by the grain size, irrespective of the yttrium content. The Mg-Y alloys consisting of meso‑grained structures (average grain size of ∼20–30 µm) exhibited good elongation-to-failure of ∼25–30%; however, “further” grain refinement led to a decrease in the properties of ductility. This tendency was confirmed to have no relation to the yttrium content. Deformed microstructural observations revealed that grain boundaries were the source of non-basal dislocation slips, but became crack-propagation sites. Although grain refinement from coarse- to meso‑sizes (up to around ∼20–10 µm) was effective, fine-grained alloys were unlikely to show a large elongation-to-failure due to grain boundary embrittlement. Not only the critical resolved shear stress but also the segregation energy of grain boundary is a considerable characteristic for alloying elements to improve ductility in magnesium alloys. Furthermore, in the case of fine-grained Mg-rare earth alloys, it is necessary to take into consideration that grain boundaries become not only the source of non-basal dislocation slips but also the route for crack-propagation.