Mg-Nd Alloys Research Articles

Preparation of Mg-Nd alloys by in situ reduction using Nd2O3

The high cost of producing magnesium alloys containing rare earth with Mg-RE master alloy severely restricts the wide application of rare earth in magnesium alloys. Consequently, we have adopted a novel, cost-effective approach to preparing magnesium alloys containing rare earth elements by calcium thermal reduction of RE₂O₃. This paper presents the successful preparation of neodymium-containing magnesium alloy by the method described, with the process parameters optimized by orthogonal test and single-factor experiment. The optimum process parameters are 780℃ melting point, 1/6 content ratio of Ca to Nd2O3, and 90 min reaction time. The results of SN ratios and ANOVA showed that time had the greatest effect on Nd yield, the content ratio of Ca and Nd2O3 followed, and temperature had the least effect on Nd yield. The actual Nd yield of the optimal alloy is 32.67 %, which is in the range of Taguchi's prediction of 23.36–33.83 %. Based on orthogonal experiments, the relationship between the three factors and Nd yield was studied separately and in detail. The results show that the Nd yield exhibits a trend of initial increase and subsequent decline as a function of reaction time, content ratio of Ca to Nd2O3, and melting temperature. The Nd yield peaked at a reaction time of 90 min, a content ratio of Ca to Nd2O3 of 1/6 and a melting temperature of 780 ℃. A physical phase analysis and microstructure observation of the alloys prepared with the optimal process parameters were conducted. The alloy is comprised primarily of three distinct phases: α-Mg, Mg2Ca, and Mg41Nd5. This proves that Mg-Nd alloy can be prepared successfully by this method, and provides a new idea for the preparation of magnesium alloys containing rare earth.

Emergence of rapid solidification microstructure in additive manufacturing of a Magnesium alloy

Abstract Bioresorbable Mg-based alloys with low density, low elastic modulus, and excellent biocompatibility are outstanding candidates for temporary orthopedic implants. Coincidentally, metal additive manufacturing (AM) is disrupting the biomedical sector by providing fast access to patient-customized implants. Due to the high cooling rates associated with fusion-based AM techniques, they are often described as rapid solidification processes. However, conclusive observations or rapid solidification in metal AM — attested by drastic microstructural changes induced by solute trapping, kinetic undercooling, or morphological transitions of the solid-liquid interface — are scarce. Here we study the formation of banded microstructures during laser powder-bed fusion (LPBF) of a biomedical-grade Magnesium-rare earth alloy, combining advanced characterization and state-of-the-art thermal and phase-field modeling. Our experiments unambiguously identify microstructures as the result of an oscillatory banding instability known from other rapid solidification processes. Our simulations confirm that LPBF-relevant solidification conditions strongly promote the development of banded microstructures in a Mg-Nd alloy. Simulations also allow us to peer into the sub-micrometer nanosecond-scale details of the solid-liquid interface evolution giving rise to the distinctive banded patterns. Since rapidly solidified Mg alloys may exhibit significantly different mechanical and corrosion response compared to their cast counterparts, the ability to predict the emergence of rapid solidification microstructures (and to correlate them with local solidification conditions) may open new pathways for the design of bioresorbable orthopedic implants, not only fitted geometrically to each patient, but also optimized with locally-tuned mechanical and corrosion properties.

Deformation Mechanisms of Magnesium Alloys with Rare-Earth and Zinc Additions under Plane Strain Compression.

The introduction of rare-earth (RE) elements into magnesium (Mg) alloys can significantly improve their ductility, thereby extending the applications of Mg products. However, the impacts of their chemical composition, temperature and processing methods on the mechanical properties of Mg products are highly debatable. In this work, we systematically investigate the deformation behaviors of Mg-Nd and Mg-Zn-Nd alloys using electron backscattered diffraction (EBSD) characterization. The samples were deformed to different stress levels to study the microstructure and texture development during channel die compression. The results reveal that the room temperature formability of the Mg-Nd alloy can be enhanced with the addition of Zn. This is attributed to the higher activities of prismatic slip and tensile twinning in the Mg-Zn-Nd alloy as compared to the binary counterpart, facilitating strain accommodation. When the strain increases, the growing and merging of the same twin variant rapidly consumes the parent grain, which is responsible for the texture modification from the transverse to the basal direction. At elevated temperatures, the twinning is suppressed in both alloys due to the decreased critical resolved shear stress of the non-basal slip systems. Additionally, an obvious sigmoidal yielding phenomenon is observed due to the multiple activation of the different deformation modes. These findings offer valuable insights into the evolution of the microstructure and texture during plane strain compression, elucidating the connections between material chemical composition, processing and mechanical properties, which are important for the advancement of Mg alloy application.

Regulating corrosion resistance of Mg alloys via promoting precipitation with trace Zr alloying

Poor corrosion property has been a bottleneck problem during the application of Mg alloys. This work proposes a new way to improve the corrosion resistance of MgNd alloy by micro-alloying Zr, which promotes the precipitation process to rapidly form a protective corrosion layer. The corrosion rate decreases by three orders of magnitude from 32.69 mL/cm2/day to 0.072 mL/cm2/day. The results show Zr tends to form nano-scale oxide clusters on Mg surface, acting as pre-precipitation sites to accelerate the precipitation and facilitate the formation of dense and compact corrosion layer.

Effect of Superhydrophobic Surface on Corrosion Resistance of Magnesium-Neodymium Alloy in Artificial Hand Sweat

A superhydrophobic surface can endow metals with some intriguing characteristics such as self-cleaning behavior. In this study, a simple solution-immersion method based on the concept of predesigned corrosion is developed to enhance the corrosion resistance of a magnesium-neodymium alloy. The Mg alloy is directly soaked in potassium dihydrogen phosphate solution with the addition of ultrasound, and a layer of rough but dense coating is uniformly formed on the Mg-Nd alloy after the immersion process, which is mainly composed of MgHPO4∙3H2O. A superhydrophobic surface with an average wetting angle of 150.5° and a sliding angle of about 4.5° can be obtained on the Mg alloy by further chemical surface modification with perfluorodecyltriethoxysilane. This superhydrophobic surface has an interesting self-cleaning effect as well as good corrosion resistance in artificial hand sweat. In brief, this study provides a feasible way to prepare a superhydrophobic surface on the Mg-Nd alloy and reveals the effect of a superhydrophobic surface on the corrosion behavior of the Mg-Nd alloy, offering new technical insights into the corrosion protection of magnesium alloys.

Corrosion resistance of a superhydrophobic calcium carbonate coating on magnesium alloy by ultrasonic cavitation-assisted chemical conversion

A superhydrophobic anticorrosive CaCO3 coating was prepared on Mg-Nd alloy by ultrasound-assisted chemical conversion. Results show that the prepared coating consists of an outer durian-peel like CaCO3 layer and a compact inner (Ca, Mg)CO3 layer. Ultrasound treatment enhances the homogeneity and decreases the size of CaCO3 particles in coating, which contributes to its superhydrophobicity. This results in an enhanced anticorrosive property of coating via ultrasonic treatment. Ultrasound possibly accelerates the nucleation of CaCO3 crystals and assists the removal of hydrogen bubbles. A self-healing property of ultrasonically prepared coating was also obtained due to the formation of vaterite and Ca3(OH)2(CO3)2.

Mechanical properties and microstructure of a high-quality Mg-Nd alloy ingot with large size manufactured by direct chill casting

In this work, a high-quality Mg-2.85Nd-0.25Zn-0.5Zr (wt%) alloy ingot with a diameter of 280 mm was successfully fabricated by direct chill casting. There was no obvious composition segregation along the radial direction of the ingot. The large temperature gradient along the radial direction, accurately the higher cooling rate in the surface, gives rise to both grain size and eutectic compounds fraction to gradually decrease from the center to the surface. This results in varying mechanical properties at different positions, such as higher strength and ductility in the surface than in other positions. The peak-aged alloy exhibited a higher strength-ductility synergy, with tensile strength of 282 MPa and elongation of 12.5%, than other Mg-Nd based alloys processed by permanent mold casting and sand casting. Furthermore, it is worth noting that the peak-aged alloy manifested significant strain hardening and high heat resistance, as evidenced by liner strain hardening at temperatures between 100 and 250 °C, which is mostly due to stable solute hardening triggered by the large atomic radius of Nd atoms in the α-Mg matrix. These findings provide references for the application and manufacture of large-scale magnesium alloy ingots. • A high-quality Mg-2.85Nd-0.25Zn-0.5Zr (wt%) alloy ingot with a diameter of 280 mm was successfully fabricated by direct chill casting. • The fraction of eutectic compounds increases and the grain size decreases from the center to the surface. • The alloy exhibited higher strength-ductility synergy than other Mg-Nd based alloys processed by permanent mold casting and sand casting. • The alloy showed abnormal strain hardening at high temperatures, rather than strain softening.

Enhanced corrosion resistance of magnesium-neodymium alloy in simulated concrete pore solution by predesigned corrosion product

The natural surface films on Mg alloys can’t provide enough corrosion protection in aqueous solutions compared to those on Al alloys and Ti alloys. In this study, a strategy is proposed for corrosion protection of Mg alloys by transforming corrosion products into protective coatings. Here, a magnesium-neodymium alloy is immersed in potassium dihydrogen phosphate solution at 50 °C for 4 h. Following this simple corrosion process, a layer of uniform MgHPO4.3 H2O coating with a thickness of about 20 µm is formed on the Mg alloy. Electrochemical test and long-term immersion test reveal that this coating significantly improves the corrosion resistance of the Mg-Nd alloy in Cl--containing simulated concrete pore solution. In summary, it indicates that some corrosion products can be designed to be transformed into protective coatings, providing new technical insights pertaining to the corrosion protection of magnesium alloys.

Effects of Nd content on thermal expansion behavior of Mg-Nd alloys

The effects of annealing temperature, thermal cycle times, precipitates, and Nd content on the thermal expansion behavior of as-extruded Mg-xNd (x=2, 4) binary alloys were investigated. It demonstrated that the coefficient of thermal expansion (CTE) of alloys decreased with the increasing of Nd content. The annealing treatment eliminated partial residual stress inside the alloy, which led to the CTEs of annealed samples higher than that of the unannealed samples at low temperature. However, it was opposite at high temperature after multiple thermal cycles due to the amount of Mg41Nd5 phase increased after annealing treatment. The results of the calculated cumulative residual strain of the Mg-Nd alloys with thermal cycling indicated that the alloys annealed at 325 ℃ has the best thermal stability. The rule of mixtures (ROM) and Kerner model were proposed to predict the CTEs of Mg-Nd alloys. It was found that the prediction of the Kerner model was agrees well with the experimental data of 1st thermal cycle of samples without internal stress.

Enhancing the creep resistance of a dilute Mg-1.5 wt%Nd alloy plate via pre-compression and subsequent peak-aging

Both static precipitation and dynamic precipitation could hardly enhance the creep resistance of dilute Mg-Nd alloys. To solve this problem, the present paper proposed a practical method, which was combining pre-compression and subsequent peak-aging. A binary Mg-1.5 wt%Nd alloy plate, was prepared for researching the tensile creep behaviors at 523 K. Due to the aging hardening was weak, the directly aged sample was not studied. The pre-compressed sample was named PC sample, and the pre-compressed with subsequent peak-aged sample was named PC-AA sample, while the sample without any pre-compression or aging was named NPC sample. The PC-AA sample had the best creep resistance, followed by the NPC sample, and the PC sample had the poorest. For the NPC sample, twinning and dislocation slip (including basal slip, cross-slip and pyramidal<c+a>slip) jointly controlled the creep behavior. Various creep mechanisms led to the easy creep strain increment and the poor creep resistance. In regard to the PC sample, the thermal stability of the preset twins was inferior and twin widening happened, resulting in a faster increment in the creep strain and a poorer creep resistance. With respect to the PC-AA sample, the dominant creep mechanisms were basal slip and dislocation climb. The high-density particles inside the preset twins successfully delayed cross-slip and twin widening, leading to the enhanced creep resistance. Thus, the twins induced by the pre-compression and the particles inside the twins induced by the subsequent peak-aging successfully enhanced the creep resistance of the dilute Mg-Nd alloys.

Influence of the amount of intermetallics on the degradation of Mg-Nd alloys under physiological conditions

The influence of amount of intermetallics on the degradation of as-extruded Mg-Nd alloys with different contents of Nd was investigated via immersion testing in DMEM+10% FBS under cell culture conditions and subsequent microstructural characterizations. It is found that the presence of intermetallic particles Mg41Nd5 affects the corrosion of Mg-Nd alloys in two conflicting ways. One is their negative role that their existence enhances the micro-galvanic corrosion. Another is their positive role. Their existence favours the formation of a continuous and compact corrosion layer. At the early stage of immersion, their negative role predominated. The degradation rate of Mg-Nd alloys monotonously increases with increasing the amount of intermetallics. Mg-5Nd alloy with maximum amount of intermetallics suffered from the most severe corrosion. With the immersion proceeding (≥7 days), then the positive role of these intermetallic particles Mg41Nd5 could not be neglected. Owing to the interaction between their positive and negative roles, at the later stage of immersion the corrosion rate of Mg-Nd alloys first increases with increasing the content of Nd, then reaches to the maximum at 2 wt. % Nd. With a further increase of Nd content, a decrease in corrosion rate occurs. The main corrosion products on the surfaces of Mg-Nd alloys include carbonates, calcium-phosphate, neodymium oxide and/or neodymium hydroxide. They are amorphous at the early stage of immersion. With the immersion proceeding, they are transformed to crystalline. The existence of undegradable Mg41Nd5 particles in the corrosion layer can enhance the crystallization of such amorphous corrosion products.

The effect of precipitation on recrystallization in a Mg-Nd alloy

Neodymium has been shown by Rokhlin (2003) to have an abnormally strong effect on recrystallization in binary Mg-X alloys. Here, we explore the origins of this observation by examining the role of precipitation on recrystallization in a binary Mg-2.8Wt.%Nd alloy. It is shown that precipitation in the deformed state is extremely heterogeneous, leading to strong pinning of recrystallized grains. When combined with fast and effective recovery and the highly heterogeneous nature of new grain formation, this results in little recrystallization at temperatures below 400∘C. At higher temperatures, with reduced driving force for precipitation and increasing recrystallization kinetics, the balance between grain growth and precipitate pinning can be shifted so as to favour increasing recrystallization fractions.

The β1 Triad-Related Configurations in a Mg-RE Alloy

As the building block of a honeycomb precipitate network, the triadic arrangement of β1 strengthening precipitates is technologically important for the potential development of highly tuneable nanostructure in rare-earth (RE) containing Mg alloys. In this work, we provide systematic experimental observations of those impinged β1 variants in a Mg-Nd alloy, which are directly related to the characteristic triadic configuration. It is found that the isolated α-Mg crystal in a β1 triad has a 10.5 deg rotation when it has a perfect equilateral-triangular shape. This rotation angle decreases to ~ 9.4 deg when the isolated α-Mg crystal exhibits a non-equilateral shape. In this case, one or two of its lateral interfaces include a step with a height of 0.56 nm to reduce the lattice mismatch. Furthermore, the pre-stage prior to the formation of β1 triadic configuration is revealed for the first time. It originates from the implement of two β1 variants with the same sense of shear. The presence of such two variants, in the absence of the third variant, leads to a rotation of the α-Mg crystal in the small area close to the approaching ends of these two variants. A low-angle symmetrical tilt boundary around the [0001]α rotation axis is formed between the rotated and unrotated α-Mg crystals. Most of the tilt boundaries observed in this work have a tilt angle of ~ 4.7 deg. The atomic structure of such a tilt boundary is constructed using a crystallographic model and validated by molecular dynamics simulation. The unique distribution of Nd-rich solute clusters along the tilt boundary is qualitatively discussed based on crystallographic analysis.

Recrystallization in an Mg-Nd alloy processed by high-pressure torsion: a calorimetric analysis

Differential scanning calorimetry (DSC) was used to evaluate the recrystallization temperature and activation energy for an Mg-1.43Nd (wt.%) alloy after severe plastic deformation by high-pressure torsion (HPT) at room temperature up to 10 turns. The recrystallization kinetics were determined from DSC analysis. The results show that the recrystallization temperature increases with increasing heating rate and decreases with increasing numbers of HPT turns. Severe plastic deformation by HPT significantly reduces the recrystallization temperature. The estimated activation energy for recrystallization was in the range of ∼ 84−89 kJ mol−1.

An investigation by EXAFS of local atomic structure in an Mg-Nd alloy after processing by high-pressure torsion and ageing

The local atomic structure of an Mg-1.44Nd (wt.%) alloy was investigated after solution annealing, high-pressure torsion (HPT) processing up to 1 and 10 turns and ageing at 250 °C for 5 h using X-ray absorption fine structure (XAFS) measurements at the Nd LIII-edge. The results show that HPT processing has no effect on the atomic structure around Nd atoms compared to the unprocessed state, whereas ageing at 250 °C for 5 h induces a significant modification in the coordination number and interatomic distances around the Nd atoms. These variations are analyzed based on the correlations between precipitation, defects and atomic mobility of the chemical species.

On the Equilibrium Intermetallic Phase in Mg-Nd-Ag Alloys

The equilibrium intermetallic phase in Mg-Nd-Ag alloys has not been well understood. In this work, intermetallic particles in the solution-treated microstructure of commercial magnesium alloy QE22 (Mg-2.5Ag-2.0Nd-0.7Zr, wt pct) have been investigated using scanning electron microscopy, electron diffraction, atomic-resolution imaging and mapping techniques of scanning transmission electron microscopy (STEM) and thermodynamic modelling. The intermetallic particles are distributed in the inter-dendritic regions. They have coarse irregular shape and share the same crystal structure. The intermetallic phase (designated δ) is determined to have an orthorhombic structure (space group Cmcm, a = 1.02 nm, b = 1.18 nm, c = 1.00 nm) and a composition of NdAgMg11, which are different from those reported previously. An atomic model is proposed for the δ phase based on atomic-resolution STEM images and atomic-scale energy-dispersive X-ray spectroscopy maps. The δ lattice is structurally related to that of Mg12Nd phase in binary Mg-Nd alloys. The Gibbs energy of formation of NdAgMg11 is determined from the equilibrium study at 793 K (520 °C), including the entropy of formation using the present experimental phase analysis data obtained at lower temperature. Implications to the formation temperature range and thermal stability of this phase and alloy solidification are discussed based on the calculated Mg-Nd-Ag phase diagram and Scheil solidification paths of alloys.

Effect of Nd Additions on the Mechanical Properties of Mg Binary Alloys

The influence of Nd contents and particles on the microstructure and mechanical properties of extruded Mg alloys are discussed. Pure Mg and binary Mg-xNd (x = 0.2 wt.%, 0.5 wt.%, 1 wt.%, 2 wt.%, 5 wt.%) alloys were cast and extruded. Hardness and tensile/compression tests were measured on those alloys. The results show that the addition of Nd to Mg leads to improvements in hardness, tensile/compression yield strengths and elongation. However, the Nd has little effect on ultimate strengths and compressibility of Mg alloys. The contributions of strengthening mechanisms to the tensile/compressive yield strength of extruded Mg-Nd alloys are investigated. Mg41Nd5 phase was distributed in Mg-2Nd and Mg-5Nd alloys after extrusion, which is the main reason that decreased the grain size of Mg-Nd. The tension-compressing yield asymmetry is decreased from 1.8 to 1 with the increased additions Nd in extruded alloys.

Using synchrotron radiation to improve understanding of deformation of polycrystalline metals by measuring, modelling and publishing 4D information

The evolution of deformation in a Mg-Nd alloy has been investigated using 3D HEDM and crystal plasticity simulation using the PRISMS-Plasticity CPFE Code. Using a simplified representative volume element (RVE), the average stress-strain response is well predicted by the PRISMS-Plasticity simulation. The distribution of the resolved shear stresses corresponding to the basal mode for all grains is obtained using both HEDM and the CPFE. simulation. The results show that the CPFE predicted substantially narrower distributions, thus indicating the need for improved digital representation of the RVE. The results of these HEDM experiments and PRISMS-Plasticity simulations have been stored in the Materials Commons in a straight-forward manner and are being made available as a published dataset. Both PRISMS-Plasticity and the Materials Commons are open-source and available for use by the global materials community.

A new insight into the beta′′′ structure: Three categories of configurations between beta′ precipitates in aged binary Mg-Nd alloy

The morphology and the structure of βF′ metastable phase were studied in aged binary MgNd alloy by atomic-resolution scanning transmission electron microscopy (STEM). The proposed three categories of βF′ − βF′ configurations are respectively aligned, staggered and inserted configurations, which can explain well the β′′′ structure proposed previously based on the experimental images and constructed models. The inserted βF′ − βF′ configuration is the most important one during the structural evolution of aging, which play a crucial role in the in-situ transformation from the D019-like to the quasi-β1 structures.

Role of deformation mechanisms and grain growth in microstructure evolution during recrystallization of Mg-Nd based alloys

The microstructure and texture evolution during static recrystallization of Mg-Nd alloys were investigated. Depending on the third alloying element added into Mg-Nd alloy, either Al or Zn, diverse textures are formed during recrystallization annealing. The rolled Mg-2Nd-1Zn sheet shows a high activity of prismatic <a> slip and the formation of a weak texture with basal pole spread in the sheet transverse direction. In contrast, a strong texture with a basal pole spread in the rolling direction was formed in the Mg-2Nd-1Al sheet. An equivalent growth of the recrystallized grains at the early annealing stage retains their orientations during grain growth.