RE-rich Particles Research Articles

Wire-arc additive manufacturing of Mg-Gd-Y-Zn-Zr alloy: Microstructure and mechanical properties

Mg-Gd-Y-Zn-Zr alloy is an essential high strength weight ratio material in the aerospace field. It has been fabricated gradually using wire-arc additive manufacturing (WAAM), a key integrated preparation technology for large-scale parts. In this investigation, a Mg-9.4Gd-3.3Y-1.75Zn-0.38Zr (wt.%) alloy was prepared by WAAM based in the cold metal transfer (CMT) process. Investigations were conducted into the microstructural evolution and mechanical properties during the solution and aging heat treatment. With an average grain size of 18.30 ± 9.89 μm, the as-deposited Mg-Gd-Y-Zn-Zr alloy is mainly composed of α-Mg matrix, (Mg,Zn)3(Gd,Y) eutectic phase, RE-rich particles, and rare-earth (RE) segregation zone. Following a 12-h heat treatment at 500 °C, the average grain size of the α-Mg matrix showed no appreciable changes. The α-Mg matrix also dissolved the (Mg, Zn)3(Gd, Y) phases, which led to the production of long period stacking ordered (LPSO) phases and the disappearance of the RE-segregation zone. The appearance of the nanoscale β′ phase following aging heat treatment. The deposited alloys had the following tensile properties: 196.5 ± 24 MPa for ultimate tensile strength (UTS), 157.2 ± 14 MPa for yield strength (YS), and 2.08 ± 0.8% for elongation (EL). The formation of LSPO phase can improve the ductility while the formation of nanoscale β′ phase can improve the strength of Mg-Gd-Y-Zn-Zr alloy. Thus, after heat treatment at 500 °C for 12 h, the alloy's EL rose to 9.27 ± 0.94%. After further aging heat treatment for 225 °C at 24h, the alloy's YS rose to 234±3 MPa and its EL was 4.56 ± 0.25%.

Phase structure evolution and coercivity mechanism of high-Co containing permanent magnets

Abstract The phase structure and magnetic properties of high-Co containing permanent magnets with high thermal stability have been systematically studied in this work. It is abnormal that the coercivity of annealed samples is slightly lower than that of sintered samples, while the coercivity was usually enhanced after annealing in conventional Nd-Fe-B samples. Further analysis showed that in addition to RE2(Fe,Co)14B main phase and RE-rich grain boundary phase, there are also new Co-rich magnetic phase located in the grain boundary. During annealing, the phase structure of high-Co containing magnets were readjusted, especially the increasing Co-rich magnetic phase and emerging RE-rich particles precipitated from the main phase. Eventually, the isolated RE-rich particles would act as the pinning center of the domain wall movement in demagnetization process. It was confirmed that the coercivity of annealed high-Co containing magnets was controlled by both nucleation and pinning. Pinning mechanism can partially compensate for the weakening of magnetic isolation due to increased Co-rich magnetic phase, which explained the moderate decrease in coercivity of annealed high-Co containing magnets. The discovery of new coercivity mechanism contributed to in-depth understanding of high-Co containing magnets.

Enhancement of the magnetic property of Nd-Fe-B sintered magnets through novel powder rounding modification

Two types of sintered Nd-Fe-B magnets were fabricated by the process of powder metallurgy technology through two different powder preparation processes i.e., conventional powder preparation process and novel powder rounding modification. The microstructure, composition and properties of the two type powders have been studied including the particle size distributions and flowabilities. Furthermore, magnetic properties, microstructure and alignment degree of the magnets prepared by the corresponding powder also have been investigated. The novel powder rounding modification rounds the sharp edges and corners of the particles compared to the conventional powder preparation process, thereby reducing friction between the particles and improving flowability of the powder. As a result, the alignment degree of the sintered magnet is enhanced, which contribute to the improvement of the remanence and the energy product density. The powder that experienced rounding process changed the distribution of RE (rare earth)-rich particles, causing tiny RE-rich particles to adhere around the main phase particles, facilitating the compact of bulk during liquid-phase sintering process and achieving uniform distribution of RE-rich phases around the Nd2Fe14B main phase, resulting in increasing the magnet density and coercivity. As a consequence, we have produced the magnet with Br = 1.43 T, Hcj = 1049 kA/m and (BH)max = 393.2 kJ/m3, which is fully superior to conventional magnet with Br = 1.41 T, Hcj = 969 kA/m and (BH)max = 383.7 kJ/m3 by adopting powder rounding process. This indicates that the novel powder rounding modification is an efficient approach for synchronous improvement of magnetic performance of sintered Nd-Fe-B without any heavy rare earth elements.

Novel RE-texture component and bimodal microstructure formation during post-annealing of an accumulative back extruded WE43 alloy

This study offers a new approach to attain recrystallized textures with rare-earth (RE) texture components through post-annealing of accumulative back extruded microstructures. It was found that annealing of deformed microstructures at 400 °C for 30 min can lead to a homogeneous microstructure with RE texture components formed upon precipitation of rE-rich particles and further stimulation of new grains nucleation. Further increasing the annealing time to 240 min (or increasing temperature to 450 °C) resulted in vanishing of RE-texture component and a significant rise in the texture intensity.

The Re/Mo redox proxy reconsidered

Compared to continental crust, marine sediments accumulating beneath O2 depleted waters are enriched in Re and Mo, with Re/Mo ratios that vary by more than two orders of magnitude, a remarkably large range. Deposition of both elements begins simultaneously, and both come to finite aqueous concentration asymptotes together, neither being drawn down to analytical detection limits. To explain these aqueous-phase observations and to reconcile them with laboratory evidence, a previous coprecipitation hypothesis is further developed here. The proposed coprecipitate is a nanoscale sulfide, Fe(Mo,Re)S4 that transforms irreversibly by internal electron transfer to a product represented as Fe(Mo,Re)(S2)S2. Irreversibility preserves information about redox conditions at the time of sedimentation unless sedimentary rocks later become subjected to oxidation or thermal maturation. Compelling support for this mechanism comes from the continuously euxinic Black Sea. A Rayleigh fractionation process characterized by a partition coefficient of 0.38 explains Re/Mo ratios in both its water column and its sediments. On the other hand, in sediments where overlying waters range from oxic to intermittently euxinic, and where Re and Mo deposition occurs only beneath the sediment–water interface, the observed >100-fold range of Re/Mo ratios is too large to be explained solely by Rayleigh fractionation. Instead, it is attributed to modification of pore waters by decomposition of unstable precursory phases. Low solid phase Re/Mo ratios are produced by pore waters modified by dissolution of Mo-rich, Mn,Fe oxyhydroxides; high ratios reflect dissolution of the so-far uncharacterized Re-rich particles that are known to create dissolved Re anomalies in shallow pore waters. Rhenium often follows iodine in biology, suggesting that the Re-rich particles could be biogenic and might participate in the marine iodine cycle. A significant correlation between particulate Re/Mo ratios and biogenic CaCO3 in the Cariaco Basin water column suggests that some of these Re-rich particles could be unstable biocalcification precursors like vaterite and amorphous CaCO3. Compared to δ98Mo, another redox proxy, Re/Mo ratios provide more nuanced redox information, especially under conditions that fall between oxic and sulfidic extremes. Thus Re/Mo ratios are potentially advantageous redox proxies that so far have been underutilized.

Microstructure, texture and mechanical properties of Mg–8Gd–4Y–1Nd–0.5Zr alloy prepared by pre-deformation annealing, hot compression and ageing

Abstract An extruded Mg–8Gd–4Y–1Nd–0.5Zr alloy was pre-heated at 470 °C for 1 h and subsequently compressed at 470 °C and two strain rates of 0.2 and 0.0003 s–1. Microstructure, texture and mechanical properties of the alloy were examined by optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), hardness test and tensile test. The results show that the post-deformed microstructures of alloy are non-uniform at both strain rates due to the dissolution of RE-rich particles and the occurrence of DRX. The textures of post-deformed alloy are affected by strain rate. The alloy exhibits a strong basal texture of ?0001〈//ND (normal direction) after compression at 0.2 s–1, while a weak texture component of ?0001〈//ED (extrusion direction) is formed in the compression obtained at 0.0003 s–1. Compared with the alloy compressed at 0.0003 s–1, the compressed alloy obtained at 0.2 s–1 presents better comprehensive mechanical properties with the ultimate tensile strength of 426 MPa, yield strength of 345 MPa and ductility of 2.1% when being aged at 225 °C for 8 h.

Dynamic dissolution and transformation of LPSO phase during thermomechanical processing of a GWZ magnesium alloy

An extruded Mg-8.2Gd-3.6Y–1.6Zn–0.5Zr alloy containing blocky long-period stacking-ordered (LPSO) phase was subjected to a set of single and multi-pass friction stir processing trials. The evolutions of the microstructure and the secondary phases were investigated in details, and the correlations between the microstructure, the macro-texture and the room-temperature mechanical properties were explained. An outstanding grain refinement was achieved after applying just one friction stir processing pass, while the coarse blocky long-period stacking ordered phase was significantly broken into the fine particles. The occurrence of particle-stimulated nucleation process was identified as the main recrystallization mechanism, and this in turn could lead to appreciable texture weakening. In addition, some of the secondary phases were dissolved in the matrix and successively transformed into cubic RE-rich particles dynamically. The capability of phase transformation and dynamic precipitation were intensified through higher passes and the volume fraction of dispersed RE-rich particles located at the grain boundaries was increased. An ultrafine and relatively homogenous microstructure was also developed at the highest level of imposed equivalent strain. In the current work, the room-temperature mechanical properties of the thermomechanically processed materials were also explored. In spite of previous reports on the severely deformed magnesium alloys, the strength and ductility were significantly improved after friction stir processing. These were explained considering the grain refinement, fragmentation, transformation, suitable distribution of the second phases, and texture weakening which was resulted from the activation of non-basal slip systems.

Formation of Secondary Phases in the Boundary between Surface Defect Grains and Matrix in Third Generation Nickel-Based Single Crystal Superalloy Turbine Blades

Ni-based single crystal superalloy turbine blades have excellent mechanical strength and resistance to corrosion and oxidation due to a uniformly distributed gamma prime phase in a gamma matrix. However, defect grains have been often found on the surface of turbine blades after manufacturing, which can be potential sites of crack initiation. In this study, several different types of surface defect grains formed in third generation Ni-based single crystal turbine blades, such as stray grains, freckle chain grains, equiax grains, and a new grain formed in surface scale, had been investigated. The grain boundary regions were observed by high resolution electron microscopy. Although the formation mechanism of each grain defect is different, secondary phases, such as rhenium-rich particles, have been always found in each grain boundary. In addition, depending on the existence of the secondary phases as well as the size of defect grains, different microstructures were observed even in the same defect grain boundary. Finally, the observed results suggest that if there is any boundary region in a turbine blade, secondary phases, such as Re-rich particles, can be found.

Effects of Gd, Y Content on the Microstructure and Mechanical Properties of Mg-Gd-Y-Nd-Zr Alloy

The effects of Gd, Y content on the microstructure and mechanical properties of Mg-Gd-Y-Nd-Zr alloy were investigated using hardness measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and uniaxial tensile testing. The results indicate that the alloys in as-cast condition mainly consist of α-Mg matrix and non-equilibrium eutectic Mg5.05RE (RE = Gd, Y, Nd). After solution treatment, the non-equilibrium eutectics dissolved into the matrix but some block shaped RE-rich particles were left at the grain boundaries and within grains. These particles are especially Y-rich and deteriorate the mechanical properties of the alloys. Both the compositions of the eutectic and the block shaped particle were independent of the total Gd, Y content of the alloys, but the number of the particles increases as the total Gd, Y content increases. The ultimate tensile strength increases as the total Gd, Y content decreases. A Mg-5.56Gd-3.38Y-1.11Nd-0.48Zr alloy with the highest ultimate tensile strength of 280 MPa and an elongation of 1.3% was fabricated. The high strength is attributed to the age hardening behavior and the decrease in block shaped particles.

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

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

Effect of rhenium content on microstructures and mechanical properties of electron beam welded TZM alloy joints

In order to strengthen weld zones of molybdenum-titanium-zirconium (TZM) alloy joints, electron beam welding experiments using rhenium (Re) addition into weld metals were conducted. The effect of Re content on the phase constitution, microstructural evolution, and mechanical properties of weld zone was investigated by optical microscope (OM), scanning electron microscope (SEM), electron backscattering diffraction (EBSD), transmission electron microscope (TEM), microhardness and tensile tests. Weld narrowing and grain refinement were produced in the welded TZM alloy joints by adding Re interlayers. Re-rich particles segregated in grain boundaries and led to the promotions of stress relaxation and cohesive forces of GBs. Besides, low-angle grain boundaries emerged and increased with Re contents in weld zones, which can hinder the initiations and expandings of microcracks. Consequently, the tensile strengths of the joints were enhanced with the increasing Re contents in weld zones. When the Re content in the weld zone reached 48.7 wt. %, the tensile strength of the joint can be up to 524 MPa and the fracture location was also transferred from weld zone to heat affected zone.

Thermal treatment for the detachment of RE-rich particles precipitated by the high-temperature oxidation of (U,RE)3O8 powder

In this study, the thermal detachment of rare earth (RE)-rich particles adhered to RE-poor particles separated by the high-temperature oxidation of (U,RE)3O8 powder was investigated by repeated oxidation after reduction at low temperature using air and ozone as oxidizing agents. The RE-rich particles detached from RE-poor particles were collected using a magnet of 0.8T. The relative degree for the detachment and collection of RE-rich particles from RE-poor particles was evaluated by the observation of particle morphologies by SEM. As most of the RE-poor U3O8 particles during oxidation at low temperature were broken into plates, the detachment of RE-rich particles from RE-poor particles increased with increasing RE-rich particle sizes dependent on the oxidation temperature of (U,RE)3O8 powder. The detachment of the RE-rich particles increased with decreasing oxidation temperature from 500 to 360°C under air. Under ozone, the (U,RE)O2 pellets, RE-rich and poor UO2 particles, and RE-rich U4O9 particles were not oxidized, while the RE-poor U3O8 particles were converted to the RE-poor UO3 particles with very low magnetic susceptibility. The collection of RE-rich particles increased with increasing magnetic susceptibility difference between the RE-rich and RE-poor particles.

Preparation and characterization of MgB2 with Pd, Pt and Re doping

Samples with Mg1−xDxB2.04 (D = Pt, Pd or Re) nominal compositions have been synthesised by a solid-state route. None of these doping elements can be substituted for Mg in a detectable amount and their presence in the samples has no influence on the critical temperature and on the lattice parameters of the MgB2 superconductor. Impurity phases are formed by reaction mostly with Mg. The microstructure of the Pt, Pd and Re-based phases depends on the elements. Re-rich particles with large sizes up to 8 μm form, whereas Pt- and Pd-containing impurities are finely dispersed with a particle size that does not exceed 1 μm. The field dependence of the normalised critical current density is improved when Pt, Pd or Re are present in the samples.

Effect of compression conditions on the microstructure and texture of a Mg–RE alloy

The effect of compression conditions on the microstructure and texture of a Mg–8Gd–4Y–1Nd–0.5Zr alloy has been investigated by hardness test, optical microscopy, scanning electron microscopy and electron backscatter diffraction. The post-deformed grain size is directly influenced by the area fraction and the distribution of RE-rich particles. A uniform microstructure of refined grains is accompanied by randomly distributed particles due to particle pinning, while a bimodal grain structure is attributed to the dissolution of particles during compression. The textures of all compressions are weak and the dominant component changes with the compression condition. Compression at temperature 450°C and a low strain rate of 0.0003s−1, the texture is a result of dynamic recrystallization (DRX), such as discontinuous DRX (DDRX) and particle stimulated nucleation (PSN), and grain boundary sliding. The alloy exhibits a typical basal texture of 〈0001〉//ND (the normal direction) after compression at temperature 500°C and a strain rate of 0.2s−1, which suggests that RE-rich particles rather than the high concentration of RE solute atoms in the matrix are necessary to generate the weak and randomized texture during hot compression of the studied alloy.

Effect of strain ratio on cyclic deformation behavior of a rare-earth containing extruded magnesium alloy

Cyclic deformation characteristics of an extruded Mg–10Gd–3Y–0.5Zr (GW103K) magnesium alloy were determined via the strain-controlled low cycle fatigue tests with varying strain ratios at a constant strain amplitude. Unlike the rare-earth (RE)-free extruded magnesium alloys, the present alloy exhibited symmetrical hysteresis loops in tension and compression in the fully reversed strain-control tests at a strain ratio of Rε=−1. This was due to the presence of relatively weak crystallographic textures and the suppression of twinning–detwinning activities arising from the fine grain sizes and RE-rich particles. At a strain ratio of Rε=0 and 0.5, a large amount of plastic deformation occurred in the tensile phase of the first cycle of hysteresis loops due to the high positive mean strain values. With decreasing strain ratio, the hysteresis loops became wider. Fatigue life of this alloy was observed to be the longest in the fully reversed strain control at Rε=−1, and it decreased as the strain ratio was deviated from Rε=−1. A certain degree of mean stress relaxation was also observed in the non-fully reversed strain control (i.e., Rε≠−1 tests).

Effect of cooling rate on the microstructure evolution and mechanical properties of homogenized Mg–Gd–Y–Zn–Zr alloy

Different cooling processes, such as quenching in warm water and cooling in furnace, were introduced to homogenize Mg–8.2Gd–3.8Y–1.0Zn–0.4Zr (wt%) alloy. Microstructure evolution and mechanical properties of the homogenized alloy were investigated. The as-quenched sample was comprised of α-Mg matrix, Mg5RE phase and 18R LPSO phase distributed at the grain boundaries and a few of RE-rich particles distributed randomly. During the process of cooling in furnace, Mg5RE and 18R LPSO phases were transformed into block-shaped 14H LPSO phase and lamellar-shaped 14H LPSO phase, respectively, due to the diffusion of solute atoms into the α-Mg matrix. Furthermore, the lamellar-shaped 14H LPSO phase grew and ran through the whole grains. The as-quenched sample exhibits tensile yield strength of 130MPa, ultimate tensile strength of 206MPa and elongation to failure of 5.5%, while the sample cooled in the furnace exhibits higher tensile yield strength but lower ultimate tensile strength and ductility due to the coarse grains and formation of block-shaped 14H LPSO phase.

Microstructural Investigation of the Initial Oxidation of the FeCrAlRE Alloy Kanthal AF in Dry and Wet O2 at 600 and 800 ° C

The FeCrAlRE (where RE is reactive element) alloy Kanthal AF was exposed isothermally at 600 and for 72 h in dry and in with 10 vol % . The mass gains were 3–5 times higher at the higher temperature. The presence of water vapor increased the oxidation rate at , while no significant effect was observed at . A thin two-layered oxide formed at : an outer corundum-type oxide, containing some Al, and an inner, probably amorphous, Al-rich oxide. At a two-layered oxide formed in both environments. The inner layer consisted of inward grown . In dry the originally formed outward grown had transformed to after 72 h. Water vapor stabilized the outward grown and hence no transformation occurred after 72 h in humid environment. RE-rich oxide particles with varying composition (Y, Zr, and Ti) were distributed in the base oxide at both temperatures and in both environments. The RE-rich particles were separated from the alloy substrate by a layer of Al-rich oxide. At the Y-rich RE particles were surrounded by thick oxide patches in both dry and humid .

Oxidation of FeCrAl alloys at 500–900°C in dry O2

AbstractThis paper reports on the oxidation of a commercial FeCrAl alloy, Kanthal AF, in the temperature range 500–900°C. The samples are exposed isothermally in dry oxygen for up to 72 h using a thermo‐balance. In addition, 168 h exposures are carried out in a tube furnace. The exposed samples are investigated by grazing angle X‐ray diffraction (XRD), scanning electron microscopy/energy dispersive X‐ray analysis (SEM/EDX), and auger electron spectroscopy (AES).The rate of oxidation increases with temperature, the kinetics being parabolic in the range 700–900°C. At all exposure temperatures, most of the sample surface is covered by a thin smooth base oxide. In addition, RE‐rich particles, with a typical size of 1–3 μm form. At 800 and 900°C patches of thick oxide appear, featuring needle‐formed crystallites situated on top of the base oxide. The thick oxide usually forms around Y‐rich oxide particles. The concentration of iron and chromium in the oxide decreases with increasing temperature. XRD proves the formation of α‐Al2O3 already at 700°C. The low temperature of formation of α‐Al2O3 is attributed to the presence of chromium in the initial oxide. It is proposed that corundum nucleation is facilitated on a surface consisting of the isostructural escolaite, (Cr2O3). After exposure at 900°C AES shows large amounts of Mg in the outer part of the oxide, MgAl2O4 being detected by XRD together with γ‐ and γ‐Al2O3.

The effect of reactive elements on the segregation behavior and microstructure of α-Al2O3 scales on β-NiAl

In order to improve oxidation resistance, small quantities of certain oxygen-active or “reactive” elements (RE) such as Y, Zr, Hf, or Ce are added to chromia- and alumina-forming high temperature alloys. Many theories have been proposed to explain the effects of RE additions, with a number of these theories including some aspect of the observed segregation of RE ions to oxide scale grain boundaries and the metalscale interface. The RE-ion segregation to the scale grain boundaries has been proposed to affect the oxidation behavior by suppressing cation transport along grain boundaries. It has been demonstrated that the quantity of RE segregation at α-Al2O3 scale grain boundaries varies with oxidation time and temperature and is greatly affected by the formation of RE-rich oxide particles in the scale (particularly at the gas interface). A previous study of scale formation on Y-implanted β-NiAl found that the Y implant had only a short-term (<1 h) effect at high temperatures.

The effect of time and temperature on the segregation of foreign ions to grain boundaries in growing α-Al2O3 scales

The addition of small quantities of certain oxygen-active or “reactive” elements (REs) such as Y, Zr, Hf and Ce produces a tremendous improvement in the oxidation behavior of both chromia and alumina-forming alloys. However, few of the hypotheses that have been put forth to explain this phenomenon have been confirmed by careful characterization of the oxidation process.The segregation theory emphasizes the role of RE segregation to oxide grain boundaries and has been successful in correlating the presence of Y and Zr segregants to changes in the oxidation mechanism (thus reducing the oxidation rate) of chromia and alumina scales. Improvements in oxide adhesion have been attributed to the segregation of REs to the metal-alumina interface on doped β-NiAl and FeCrAl alloys. All of these studies employed FEG-STEM equipped with XEDS in order to detect the interfacial segregation.The RE is also found in the scale as RE-rich particles which have been observed after high temperature oxidation using XEDS on SEM and FEG-STEM and detected using glancing-angle (0.5° incident angle) x ray diffraction.