RE Alloys Research Articles

Thermodynamic prediction of thermal diffusivity and thermal conductivity in Mg–Zn–La/Ce system

• Heat dissipating Mg–Zn–La/Ce alloys were designed based on thermodynamic database. • τ 1 phases have more negative influence on thermal diffusivity than REMg 12 . • Solute atom is dominant factor affecting thermal conductivity of Mg–Zn–RE alloys. • The thermal diffusivities of Mg–Zn–La/Ce alloys were predicted by CALPHAD method. Basic thermal properties and mechanical properties are critical parameters for the structural magnesium alloys. Solute atoms and second phases can improve mechanical properties, but are deteriorating the heat dissipation performance. Through experimental determination of alloys, it is found that REMg 12 phases have fewer negative impacts on thermal diffusivities than τ 1 phases. With the same intermetallic compound, the solute atom Zn have more negative influences on thermal diffusivities of Mg–Zn–La/Ce alloys than the content of second phases. In order to quantitatively evaluate thermal conductivities and further design Mg alloys with both high strength and high thermal conductivity, a calculated method is provided to describe the thermal diffusivity of alloys as a function of alloy composition and phase constitution. A set of parameters for expression of thermal diffusivity of Mg–Zn–La/Ce alloys were obtained through assessing the experimental data. The thermal conductivities of Mg–Zn–La/Ce system were predicted and agreed well with experimental values with calculation error of 1.6% and standard error of ±3.0 W/(m K). The calculation method considering thermal diffusivity resistivity improves the calculation accuracy and would be physically significant.

New Development of Permanent Magnets Through Grain Boundary Diffusion and Nanostructuring

High-performance permanent magnets (PMs) have gained high and growing interest due to their excessive demand in energy conversion systems and electric vehicles. PM-based electric machines exhibit great advantages over traditional motors due to their high efficiency of energy conversion. Nd–Fe–B magnet is the best available magnet in terms of energy-product at room temperature. Replacement of Nd by heavy rare earth (HRE) and of Fe by Co results in an enhanced anisotropy field and an improved thermal stability, but also increases the production costs. Developing a strong PM with minimum use of HRE elements is required due to their high cost, low availability and issues associated with international politics. Grain boundary diffusion (GBD) process allows the HRE to diffuse around the grain boundaries, unlike adding expensive HRE to the middle of a grain. Here, we review the recent progress in PMs, especially the novel development of grain boundary-diffused magnets and nanostructured magnets. GBD processes using RE, fluorides or hydrides of RE and eutectic alloys are discussed. Development of nanostructured PMs using physical and chemical methods such as melt spinning, high-energy ball milling, surfactant-assisted ball milling, mechanochemical method, etc. is elucidated. The current and future trends in the area of high performance permanent magnets are outlined.

Microstructure and mechanical behavior of similar butt-joints of ZK60 and ZK60–1.5RE magnesium alloys produced by linear friction stir welding

The microstructure and mechanical properties of butt-joints produced by linear friction stir welding of similar plates of as-cast ZK60 and modified ZK60 with 1.5 wt.% RE (ZK60–1.5RE) are investigated. The thermomechanical affected zone is investigated in both advancing and retrieving sides, and the microstructure is compared to the base metal and the stirred zone. Electron backscattered diffraction measurements provide the average microstructural features of the transformed microstructure. The mechanical properties are assessed using hardness, tensile testing, and surface residual stress measured using X-ray diffraction. Higher torque and heat input are obtained for the ZK60–1.5RE compared to the ZK60. The thermomechanical affected zone is notably larger for the ZK60 compared to the ZK60–1.5RE. A gradient microstructure is formed in thermomechanical affected zones where deformed grains are progressively more recrystallised towards the stirred zone. There is no visible interface between the thermomechanical affected zone and the stirred zone. A fine and partially recrystallised microstructure is formed in the stirred zone. The retrieving side of the ZK60–1.5RE has a slightly more refined microstructure compared to the other investigated zones. Anisotropy measured with increment in the maximum intensity of the (0001) increases towards to stirred zone. The formed microstructure is correlated with the role of twinning, recovery, static, and dynamic recrystallisation that can occur during friction stir welding. Twinning and grains with large misorientation spread are more pronounced in the thermomechanical affected zone and regions towards the base metal. Sharp low angle grain boundaries are observed towards the stirred zone. From refined recrystallised grains decorating the grain in the thermomechanical affected zone, a mosaic-like of low and high angle grain boundaries are observed in the stirred zone. The joints have comparable surface residual stresses. The friction stir welding improved the ductility and strength of the as-cast ZK60–1.5RE alloy since the tensile samples fractured in the BM.

Corrosion Behavior of Al-Mn-RE Alloy for Air Conditioning System

The effects of RE content and cold rolling and annealing on the microstructure and corrosion resistance of Al-Mn-RE alloy were studied by means of metallographic microscope, scanning electron microscope and electrochemical workstation. The results show that the eutectic phases with different sizes and irregular shapes can be seen in Al-Mn-xRE alloys. With the addition of different RE content, the amount of eutectic silicon increases, the long strip eutectic structure fines and gradually turns into spherical or short rod-like structure. When RE content reaches 0.25% or more, the eutectic structure begins to coarsen. With the increase of RE content, the corrosion potential and pore size of Al-Mn-xRE alloys increase. The corrosion potential of Al-Mn-0.18RE alloy shows a trend of positive and negative migration first, and the corrosion current density decreases first and then increases, the maximum corrosion resistance is obtained when the RE content is 0.18%. The order of corrosion potential and pitting corrosion potential of Al-Mn-0.18RE alloy from high to low is: homogenized state> cold rolled + 325 °C annealed state> rolled state> cold rolled + 475 °C annealed state. The corrosion resistance of the Al-Mn-0.18RE alloy after rolling is lower than that of the homogenized state, but the subsequent stabilization annealing treatment at 325°C can improve the corrosion resistance.

Mechanistic Insights into the Synthesis of Platinum–Rare Earth Metal Nanoalloys by a Solid-State Chemical Route

Platinum–rare earth metal (Pt–RE) nanoalloys are among the most active electrocatalysts for the oxygen reduction reaction and are predicted to exhibit long-term stability in proton-exchange membrane fuel cells. We have recently developed a solid-state chemical route for synthesizing this family of alloy materials, which is carried out under seemingly impossible conditions. Here, we report an in-depth understanding of the synthesis mechanism, obtained through systematic investigations of the chemical processes involved at different stages of the synthesis process and the structural evolution of the intermediate products. The formation of Pt–RE nanoalloys is made possible by a series of consecutive chemical and physical processes, including the polymerization processes of the nitrogen-rich precursor, the formation of a porous RE carbodiimide phase, the mobility of the formed metal phases on the carbon support, the reduction of the RE metals driven by the alloying reaction, and so forth. This thorough understanding of the mechanism of the synthesis process lays the foundation for optimizing the synthesis procedures and maneuvering this method to synthesize Pt–RE alloy materials with the desired structures and properties.

Coupled CA-FE simulation for dynamic recrystallization of magnesium alloy during hot extrusion

In the present research, the dynamic recrystallization (DRX) behavior of a newly-developed Mg–Al–Zn–RE alloy with abundant second-phase particles during hot extrusion is investigated by coupling finite element (FE) and cellular automaton (CA) models. A two-dimensional CA model is developed to quantitatively and topologically evaluate the DRX process during deformation with constant forming conditions. Considering the fact that second-phase particles with various sizes extensively exist in the studied Mg–Al–Zn–RE magnesium alloy, models of DRX nucleation and grain growth velocity are modified. The coefficients of the modified CA model are calibrated by isothermal compression experiments of the magnesium alloy. Subsequently, the CA model is coupled with FE analysis to investigate the DRX behavior during the hot extrusions of the Mg–Al–Zn–RE alloy. The DRX behavior of the magnesium alloy at different stages and positions of extruded plates is simulated by the established model. Finer grains near the edge than in the inner of the plates result from higher strain and strain rate. The influence of extrusion conditions on microstructural evolution is explored. Under the employed forming conditions, average grain size decreases 28–62 times from as-cast and solution-treated to as-extruded state due to grain refinement by DRX. With increasing initial billet temperature or extrusion speed, average grain size increases. The finest grains are obtained at the initial billet temperature of 623 K and the extrusion speed of 7.83 mm/s. Low initial billet temperature or high extrusion speed benefits homogeneous grain distribution. The simulated results are in good agreement with experimental ones.

Processing of high strength Mg–9Gd–3Nd–1Sn–1Zn–0.6Zr alloy by hot rolling and subsequent aging

Magnesium alloy plates can be strengthened by rolling, however, it is easy to crack or even break when the reduction of Mg–RE alloys is too large. Herein, the strengthening mechanical of the Mg–9Gd–3Nd–1Sn–1Zn– 0.6Zr alloy under different treatment conditions were investigated after hot-rolling to 80% reduction in thickness (0.8 mm) by multi-step methods. Furthermore, the rolled alloy by aging strengthening are explored. The results show that the hot-rolled alloy with 80% reduction are basically composed of dynamically recrystallized grains with the size of about 60 m, improving the mechanical properties significantly. The precipitates within grains undergo SSSS→ β″ → β′ phase transformation with the aging treatment up to 200 °C. Fine β″ precipitates were found in the grains of the rolled alloy under aged time (2 h), while β″ precipitates changed into β′ phase when the aging time was extended to 32 h. The base phase which is perpendicular to phase was precipitated in the alloy in longer aging time (96 h). In addition, the thickness of precipitates and precipitation-free zone (PFZ) at the grain boundary gradually increased as the time went on. Meanwhile, the discontinuous equilibrium phases at the grain boundary are gradually become continuous. The ultimate tensile strength and hardness were reached to 431.14 MPa, 105.9 HV at peak-aging, in addition, the elongation is reached to 3.11%, respectively. The formation of crack sources is due to the stress concentration between the brittle PFZ and the magnesium matrix, which leads to the decrease of ductility.

Development of biodegradable Zn–1Mg–0.1RE (RE = Er, Dy, and Ho) alloys for biomedical applications

Zinc (Zn) and its alloys are receiving great attention as promising biodegradable materials due to their suitable corrosion resistance, good biocompatibility, and highly desirable biofunctionality. Nevertheless, the low mechanical strength of pure Zn impedes its practical clinical application and there have been calls for further research into the Zn alloys and thermomechanical processes to enhance their mechanical properties and biocompatibility. Here, we report on the alloying efficacy of rare earth elements (REEs) including erbium (Er), dysprosium (Dy), and holmium (Ho) on the microstructure, mechanical properties, corrosion and wear behavior, and in vitro biological properties of Zn–1Mg–0.1RE alloys. Microstructural characterization revealed that the addition of 0.1 wt.% REEs had a significant refining effect on the grain size of the α–Zn matrix and the second phases of the alloys. Alloying of the REEs and hot-rolling effectively improved the mechanical properties due to both precipitation strengthening of the second phases of ErZn5, DyZn5, and Ho2Zn17 and grain-refinement strengthening. The highest ultimate tensile strength of 259.4 MPa and yield strength of 234.8 MPa with elongation of 16.8% were achieved in the hot-rolled Zn–1Mg–0.1Ho. Alloying of REEs also improved the wear and corrosion resistance, and slowed down the degradation rate in Hanks’ solution. Zn–1Mg–0.1Er showed the highest cytocompatibility of MC3T3-E1 cells cultured directly on the alloy surface and of MG-63 cells cultured in the alloy extract. Zn–1Mg–0.1Dy showed the best anticoagulant property among all the alloys. Overall, these Zn–1Mg–0.1RE (Er, Dy, and Ho) alloys can be considered promising biodegradable metallic materials for orthopedic applications.

Corrosion behaviour and cytocompatibility of selected binary magnesium-rare earth alloys

Abstract The corrosion behaviour of as-cast binary Mg–0.3Ce, Mg–1.44Nd, Mg–0.63Gd and Mg–0.41Dy (wt%) alloys was investigated in DMEM+10% FBS solution using electrochemical and weight loss tests. The results revealed that the alloys with heavy RE elements (Gd and Dy) exhibited the lowest corrosion rate compared to the alloys with light RE elements (Ce and Nd). The cytocompatibility of the Mg–RE alloys was assessed via live/dead straining after 3 and 7 days. The results show that Mg–0.63Gd alloy is a suitable candidate for biomedical applications.

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.

Degradation Behavior, Transport Mechanism and Osteogenic Activity of Mg–Zn–RE Alloy Membranes in Critical-Sized Rat Calvarial Defects

In this study, a specific Mg–Zn–RE alloy membrane with 6 wt.% zinc and 2.7 wt.% rare earth elements (Y, Gd, La and Ce) was prepared to investigate implant degradation, transport mechanism and guide bone regeneration in vivo. The Mg-membrane microstructure and precipitates were characterized by the scanning electron microscopy (SEM) and the transmission electron microscopy (TEM). The Mg-membrane degradation process and effect on osteogenesis were investigated in a critical-sized rat calvarial defect model via micro-CT examination and hard tissue slicing after 2-, 5- and 8-week implants. Then, the distribution of elements in organs after 1-, 2- and 4-weeks implantation was examined to explore their transportation routes. Results showed that two types of precipitates had formed in the Mg–membrane after a 10-h heat treatment at 175 °C: γ-phase MgZn precipitation with dissolved La, Ce and Gd, and W-phase Mg3(Y, Gd)2Zn3 precipitation rich in Y and Gd. In the degradation process of the Mg-membrane, the Mg matrix degraded first, and the rare earth-rich precipitation particles were transferred to a more stable phosphate compound. The element release rate was dependent on the precipitate type and composition. Rare earth elements may be transported mainly through the lymph system. The defects were repaired rapidly by the membranes. The Mg-membrane used in the present study showed excellent biocompatibility and enhanced bone formation in the vicinity of the implants.

Investigation of rapidly decomposable AZ91–RE–xCu (x=0, 1, 2, 3, 4) alloys for petroleum fracturing balls

AZ91–RE–xCu (x = 0%, 1%, 2%, 3%, and 4%) alloys have been prepared by a semi-continuous casting followed by hot-extrusion technique, in order to find suitable degradable materials for use in petroleum fracturing balls. Results indicate that the Cu-free AZ91–RE alloy mainly consists of dynamically recrystallized α-Mg grains and the β-Mg17Al12 phase. When Cu is added to AZ91–RE, a new T-AlCuMgZn phase is formed. The grains are also dramatically refined for Cu contents of 0–3%. When the Cu content is increased to 4%, however, the grain size is slightly coarsened to 8.9 μm. Consequently, both the σf and σ0.2 reach maximum values of 244 and 405 MPa, respectively, for AZ91–RE–3Cu. The corrosion rate of AZ91–RE–xCu alloys increases with increasing Cu concentration. The addition of 4% Cu increases the decomposition rate of the AZ91–RE alloy, but slightly reduces its compressive strength.

Cooling curve thermal analysis, phase constitution, and solidification characteristics of cast Mg–5Gd–2Y–xNd–Zr alloys

Mg–RE alloys are high-strength magnesium alloys with great application potential. The solidification pathways and microstructure formation of most Mg–RE alloys are still under discussion. In the present research, phase constitution and solidification characteristics of Mg–5Gd–2Y–xNd–Zr (x = 0, 2, 4, 6) alloys were investigated using a Computer-Aided Cooling Curve Analysis (CA-CCA) system, together with microstructure observation and phase identification methods including SEM, EDS, and XRD. The results show that the solidification microstructure of Mg–5Gd–2Y–Zr alloy was composed of α-Mg and Mg24RE5 phase. And the addition of Nd leads to the formation of β phase through a eutectic reaction, L → Mg24RE5 + α-Mg + β. Combining the CA-CCA and microstructure observation results, the non-equilibrium solidification route and solid fraction–time relationship of Mg–5Gd–2Y–xNd–Zr (x = 0, 2, 4, 6) alloys was summarized. Besides, hot tearing susceptibility of the investigated alloys was evaluated according to their solid fraction–time relationship using Clyne–Davies’ criterion.

The Effect of Precipitation Characteristics on Hardening Behavior in Cu–Cr–Sn Alloy with Sn Variation

The influence of minor solute additions of Sn on hardening behavior and microstructure evolution of the Cu–Cr–Sn alloys has been investigated with help of an optical microscope, scanning electron microscope, transmission electron microscopy, and the Vickers hardness tester. Experimental results show that the Sn solutes, homogeneously distributed within the Cu matrix, provide a weak solid solution strengthening effect in the Cu–Cr–Sn alloy, nevertheless are important in enhancing precipitation hardening. The Cu–0.43Cr–0.36Sn alloy exhibits a significant age-hardening effect with a hardness increase by 62 HV after peak aging at 450°C, which is stronger than that of the Cu–0.43Cr–0.08Sn and the Cu–0.43Cr–0.17Zr–0.05Mg–0.1RE alloys. A trace amount of Sn is highly effective at suppressing the transformation of the Cr-rich phase from the face-centered cubic (FCC) into the body-centered cubic (BCC) structure in the Cu–Cr alloys during the aging process, and therefore the FCC Cr-rich precipitate is the dominant strengthening phase in the Cu–Cr–Sn alloys. Besides, a higher areal number density of Cr-rich precipitates (~4.2 · 1015 m–2) was observed in peak-aging Cu–0.43Cr–0.36Sn alloy as compared to Cu–0.43Cr–0.08Sn alloy, which shows that the adding Sn to Cu–Cr alloy can accelerate the nucleation of the Cr-rich precipitates. The improvement of precipitation hardening and hardening kinetics in the Cu–0.43Cr–0.36Sn alloy is mainly attributed to the acceleration in the nucleation of Cr-rich precipitates through Sn addition.

A review on thermal conductivity of magnesium and its alloys

Abstract This review summarizes recent researching works the thermal conductivity of Mg–Zn, Mg–Al, Mg–Mn and Mg–RE alloys. Solute atoms, heat treatment, deformation and temperature, which have significant influence on the thermal conductivity of magnesium alloys, are highlighted. For an individual solute atom, its effects on thermal conductivity are highly dependent on its chemical valence, radius and extra-nuclear electrons. The thermal conductivity of Mg alloys is decreased by solution treatment, but improved by aging and/or annealing treatments. As for the deformed Mg alloys, the thermal conductivity along transverse or normal direction is superior to that of along extrusion or rolling direction. We expect this review is helpful for those who are working on developing Mg alloys with superior thermal conductivity.

On the role of yttrium in microstructure evolution and texture modification during magnesium alloy extrusion

Three binary alloys, Mg-1wt%Al, Mg-1wt%Y, and Mg-4wt%Y were forward extruded to investigate the effect of the yttrium addition on the evolution of texture and microstructure of magnesium alloys during extrusion. The non-rare earth (RE) alloy Mg-1wt%Al exhibits a 101¯0 or 101¯0−112¯0 fiber texture depending on the recrystallized fraction, whereas the RE alloys Mg-1wt%Y, and Mg-4wt%Y develop weaker texture and a new 112¯1 “RE texture” component. The microstructure was characterized by using the electronic backscattered diffraction (EBSD) technique, and the formation mechanism of “RE texture” component was probed. The enhanced twin activity in the RE alloys leads to the broadened orientation spread of grains, but is little related to the formation of the “RE texture” component. Further examination on the dynamically recrystallized (DRX) grains reveals that both 112¯0 and 112¯1 texture components are closely related to the continuous dynamic recrystallization (CDRX) during which new grains nucleates in the mantle regions of the original grains.

Analysis of Microstructure and Electric Conductivity in Al-RE Alloy by Heat-Treatment Condition.

In this study, we investigate the microstructure and mechanical properties of as-extruded Al-1.0RE alloys. The molten Aluminum alloy was maintained at 800 °C and then poured into a mould at 200 °C. Aluminum alloys were hot-extruded into a rod measuring 12 mm thick with a reduction ratio of 38:1. The microstructure and electric conductivity properties of as-extruded Al-1.0RE alloy under different annealing processes were investigated and compared. After extrusion, the intermetallic compound having a needle shape in the cast state was finely decomposed based on the direction of extrusion. Significant changes in the microstructure were detected after annealing at 500 °C with fragmentation and sphering of eutectic particles. The annealing temperature of Al-1.0RE alloy increased proportionally to the electrical conductivity. The formation of Al-RE intermetallic compounds increases the electrical conductivity and improves the mechanical properties of the alloy through precipitation hardening.

Microstructure and Mechanical Properties of the Al-RE Alloy with Ca Addition.

Aluminum and its alloys are used in a wide range of industrial applications from low density, high strength and a variety of structural materials. In this study, the effects of Ca addition on the microstructure and mechanical properties of Al-1wt.%RE alloys were investigated. The melt was held at 800 °C for 20 minutes and poured into a mold. The cast Al alloy was hot extruded with a rod having a diameter of 12 mm and a reduction ratio of 38:1. Al-1wt.%RE alloy consists of Al, Al11RE₃ phase. The Al₂Ca phase is increased by increasing the Ca content to 0.2 to 0.4 wt.%. As the Ca content increased from 0 to 0.4 wt.%, the average grain size of the extruded Al alloy decreased by 739.8, 400.8 and 155.0 μm. The tensile strengths were increased to 74.25, 76.53, and 79.52 MPa. The electrical conductivity of Al-RE alloy with Ca addition decreased to 60.32, 58.15 and 57.89% IACS.

Effects of Mg Addition on Microstructure, Mechanical Properties, and Thermal Conductivity of As-Extruded Al-RE Alloys

In this study, we investigated the effect of Mg addition (0, 0.5, and 1.0 wt%) on the microstructure, mechanical properties, and thermal conductivity of as-extruded Al-RE alloys. With an increase in the Mg content from 0 to 1.0 wt%, the average grain size of the alloys decreased remarkably from 740 to 130 μm and the high-angle grain boundary fraction increased from 35 to 54%. The addition of Mg resulted in the grain refinement of the Al-1.0RE alloy because of the dynamic recrystallization caused by the solute Mg atoms during the extrusion. With an increase in the Mg content from 0.5 to 1.0 wt%, thermal conductivity of the alloy decreased from 231 to 193 W/mK because of the electric scattering caused by the solute Mg atoms. With an increase in the Mg content from 0 to 1.0 wt%, the ultimate tensile strength of the alloy increased remarkably from 74 to 120 MPa, while the strain reduced from 44 to 34%. This improvement in the strength resulted from the grain refinement and solid solution strengthening due to the solute Mg atoms. The Mg addition amount affected the thermal conductivity and strength of the alloys significantly.

Effect of Zinc Content on the Microstructure, Thermal Behavior, and Static Mechanical Properties of Sn-xZn Alloys Containing Mixed Trace Rare Earth Elements

Abstract The microstructure, thermal behavior, and mechanical properties of Sn-xZn-0.1RE (x = 5, 10, 20, and 30 wt%) alloys containing mixed trace rare earth elements were investigated in this study. The results showed that the alloys had the same solidus temperature of about 199 °C. Zinc content higher than 10% enhanced slightly the eutectic temperatures and enlarged the eutectic temperature range of the alloys. The microstructures of most of the alloys exhibited Zn-rich coarse clusters, but not for Sn-5Zn-0.1RE. The tensile strength of the alloys increased with increasing zinc concentration.