Complex Carbides Research Articles

Reactive Hot Pressing of HfB2–SiC–Ta4HfC5 Ultra-High Temperature Ceramics

Highly disperse and reactive (HfB2–SiC)@(Ta2O5–HfO2–C) composite powders were manufactured by sol–gel technology where Ta2O5–HfO2–C amorphous components were nanostructured and distributed in each other as uniformly as possible. The reactive sintering of the prepared composite powders at a relatively low temperature (1800°С) with an exposure time of 30 min and the pressure 30 MPa yielded (HfB2–30 vol % SiC)–xTa4HfC5 ultra-high temperature ceramics (UHTCs), where x = 5, 10, and 15 vol %, with a relative density of 75–78%. X-ray powder diffraction proved the complete conversion of tantalum and hafnium oxides to complex carbide Ta4HfC5. The average grain size as determined by scanning electron microscopy did not exceed 2–3 µm in HfB2 and 30–60 nm for the Ta4HfC5 phase. Thermal analysis in flowing air showed that, in the temperature range 20–1400°С, the increasing percentage of tantalum–hafnium carbide (the least oxidation resistant phase) leads to a systematic increase in oxidation-induced weight gain; however, a tendency to saturation is observed. The microstructural specifics of the oxidized surface are noticed depending on the composition of HfB2–SiC–Ta4HfC5 ceramics.

Structure of Fe-Mn-Al-C Steels after Gleeble Simulations and Hot-Rolling.

In this paper, analytical results are compared for the newly developed steels, Fe-Mn-Al-C (X105) and Fe-Mn-Al-Nb-Ti-C (X98), after being hot-rolled and also after undergoing thermomechanical treatment in a Gleeble simulator. These steels have a relatively low density (~6.68 g/cm3) and a content of approx. 11% aluminum. The multistage compression of axisymmetric samples constituting a simulation of the real technological process and hot-rolling performed on a semi-industrial line were carried out using three cooling variants: in water, in air, and after isothermal heating and cooling in water. The temperature at the end of the thermomechanical treatment for all variants was 850 °C. On the basis of detailed structural studies, it was found that the main mechanism for removing the effects of the strain hardening that occurred during the four-stage compression involved the dynamic recrystallization occurring in the first and second stages, the hot formability and dynamic recovery in successive stages of deformation, and the static and/or metadynamic recrystallization that occurred at intervals between individual deformations, as well as after the last deformation during isothermal heating. Analysis of the phase composition and structure allowed us to conclude that the tested steels have an austenitic-ferritic structure with carbide precipitates. Research using scanning and transmission electron microscopy identified κ-(Fe, Mn)3AlC and M7C3 carbides in both the analyzed steels. In addition, complex carbides based on Nb and Ti were identified in X98 steel; (Ti, Nb)C carbides occurred in the entire volume of the material. Slow cooling after thermomechanical treatment influenced the formation of larger κ-carbides at the border of the austenite and ferrite grains than in the case of rapid cooling. The size and morphology of the carbides found in the examined steels was varied. Back-scattered electron diffraction studies showed that wide-angle boundaries dominated in these steels.

Hybrid Laser Deposition of Fe-Based Metallic Powder under Cryogenic Conditions

The purpose of this study was to demonstrate the novel technique of laser deposition of Fe-based powder under cryogenic conditions provided by a liquid nitrogen bath. Comparative clad layers were produced by conventional laser cladding at free cooling conditions in ambient air and by the developed process combining laser cladding and laser gas nitriding (hybrid) under cryogenic conditions. The influence of process parameters and cooling conditions on the geometry, microstructure, and hardness profiles of the clad layers was determined. The optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive spectrometer (EDS), and XRD test methods were used to determine the microstructure and phase composition. The results indicate that the proposed technique of forced cooling the substrate in a nitrogen bath during the laser deposition of Fe-based powder is advantageous because it provides favorable geometry of the clad, low dilution, a narrow heat-affected zone, a high hardness and uniform profile on the cross-sections, homogeneity, and refinement of the microstructure. The influence of the forced cooling on microstructure refinement was quantitatively determined by measuring the secondary dendrite arm spacing (SDAS). Additionally, highly dispersed nanometric-sized (200–360 nm) precipitations of complex carbides were identified in interdendritic regions.

Influence of boron modification and cooling conditions during solidification on structural and phase state of heat- and wear-resistant white cast iron

The paper is devoted to examination of the effect of boron modification and temperature conditions for metal cooling in a mold on phase composition, morphology and chemical composition of structural components of heatand wear-resistant white cast iron of Fe – C– Cr– Mn – Ni – Ti–Al – Nb system. The phase composition of the metallic base changed from the dualphase (α- and γ-phases) to the completely single-phase (γ-phase). Boron modification influenced on the type of secondary carbides, while secondary hardening in a mold occurs through extraction of dispersed niobium carbides (without boron, but with chromium carbides). The structure of modified cast iron is presented by the primary complex carbides (Ti, Nb, Cr, Fe)C, as well as by solid solution dendrites, eutectics and secondary carbides MeC. Boron addition changes the chemical composition of primary carbides with decrease of niobium content from 44 to 2 % and increase of titanium content from 24 to 65 %; content of eutectic carbides rises as well. As for hypereutectic carbides, they are characterized by increase of ferrum content and lowering of chrome content. Parameters of the primary phases (MeC carbides and solid solution dendrites) were investigated using the methods of quantitative metallography. The special technique of Thixomet PRO image analyzer was used for evaluation of the F form factor which is the criterion of compatibility of the primary phases. The following parameters were used in this work as the parameters of dendrite structure: dispersity of the dendrite structure (δ), volumetric part of dendrites (V), distance between the axes of second order dendrites (λ2 ), form factor (F), average dimensions of dendrites – square (S), length (l) and width (β). All the suggested characteristics (parameters) allowed not only to provide quantitative evaluation of the dendrite structure, but also to determine modification degree as relative variation (in %) of each criterion in modified cast iron in comparison with non-modified iron. Quantitative relation between modification degree and crystallization conditions were established as well.

Dispersion of complex carbides at the oxidation front during the oxidation of 9Cr-1Mo steel

To better understand oxidation and carburisation processes taking place during exposure of 9Cr-1Mo steel experimental samples have been exposed at 600 °C and 620 °C to a CO2 rich atmosphere for various exposure times. Complex multi-elemental carbides form showing a range of compositions, with some showing coring/elemental segregation. Certain carbides show both M7C3 and M23C6 within a single multi-component carbide. At the oxidation front some carbides resist oxidation through growth of a Cr rich oxide shell.Carbides within both substrate and oxide have been characterized with mechanisms being proposed for the complex interactions taking place within both the substrate and oxide.

Microstructure, Microhardness, and Wear Properties of Cobalt Alloy Electrodes Coated with TiO2 Nanoparticles

In this paper, the influence of TiO2 nanoparticle coating on cobalt-based electrodes was studied. Different coating treatment times were applied, and the results were compared to the hard-faced layer obtained with unmodified electrodes. The hard facing was done in three layers, the first being a Ni-based interlayer, followed by two layers of corrosion and wear-resistant Co-based Stellite 6 alloy. Pin-on-disc wear testing was applied, along with the metallographic study and hardness measurements of the hard-faced layers. Furthermore, energy-dispersive X-ray spectroscopy (EDS) analysis was conducted. It was found that the microstructural properties, as well as microhardness profiles, are modified in hard-faced layers obtained with modified electrodes. Interdendritic distances are altered, as are the dendrite growth directions. Titanium oxides are formed, which, along with the present complex carbides, increase the wear resistance of the hard-faced layers compared to layers obtained with untreated electrodes.

Influence of Boron Modifications and Cooling Conditions during the Structural and Phase State Solidification of Heat- and Wear-Resistant White Cast Iron

The paper examines the effect of boron modification and temperature conditions for metal cooling in a mold with the phase composition, morphology and chemical composition of structural components found in a heat- and wear-resistant white cast iron Fe–C–Cr–Mn–Ni–Ti–Al–Nb system. The phase composition of the metallic base changed from the dualphase (α- and γ-phases) to the completely single-phase (γ-phase). Boron modification influenced on the secondary carbide type, while secondary hardens in a mold, occurs through extraction of dispersed niobium carbides (without boron but with chromium carbides). The modified cast iron structure is presented by the primary complex carbides (Ti, Nb, Cr, Fe)C, as well as by solid solution dendrites, eutectics and secondary carbides MeC. Boron addition changes the chemical composition of primary carbides with a decrease of niobium content from 44 to 2% and an increase of titanium content from 24 to 65%; the eutectic carbide content rises as well. Regarding hypereutectic carbides, they are characterized by a ferrum content increase and chrome content decrease. Parameters 893 of the primary phases (MeC carbides and solid solution dendrites) were investigated using quantitative metallography methods. The special technique of Thixomet PRO image analyzer was used for the evaluation of the F form factor which is the criterion of primary phase compatibility. The following parameters were used in this work as dendrite-structure parameters: dispersity of the dendrite structure (δ), volumetric dendrite parts (V), distance between the axes of second-order dendrites (λ2), form factor (F), average dimensions of dendrites—square (S), length (l) and width (β). All the suggested characteristics provided a quantitative evaluation of the dendrite structure, as well as determined the modification degree as the relative variation (in %) of each modified cast iron criterion in comparison with the non-modified iron. The quantitative relation between the modification degree and crystallization conditions were established as well.

Abrasive wear characteristics of complex carbides overlay (CCO) in comparison to low-alloy boron steels

The aim of present study was to evaluate the abrasive wear resistance of complex carbide overlay plates in comparison to low alloy boron steels of different hardness grades. These materials are typical in mining, trasportation and crushing aplications worldwide. The abrasive wear resistance was evaluated by weight loss using a dry sand-rubber wheel laboratory abrasion test that exposed the materials to slidding abrasion. All tests were performed under dry conditions at room temperature. In order to evaluate the tribological response of the different steels and their wear mechanisms, post-characterization of the worn surfaces was performed by SEM/EDX. To better understand the wear process, microstructural SEM/EDX and microhardness analyses were performed as well. The results showed that abrasive wear performance was determined by a combination of high hardness and toughness (fracture strain). The dry sand-rubber wheel using quartz sand with 1100 HV causes severe damage in low alloy boron steels plates due to transition between cutting and plastic deformation. By applying the described methodology it was possible to establish performance comparisons and wear behavior due to individual material features according field applications.

Materials for Gas-Thermal Spraying, Obtained by Diffusion Alloying from Powders Based on Austenitic Steels

The article presents experimental studies of composite powder materials for plasma spraying, obtained by the method of diffusion doping of powder materials based on austenitic steels. It is indicated that the main factors forming the diffusion layer on a powder material are a composition of the required saturating medium, treatment temperature and duration of chemical and thermal exposure. Creation of single-phase diffusion layers is possible only in the case of a minimum level of temperature-time characteristics during heat treatment. This is also facilitated by the use of media with a low concentration of boron and introduction of additives inhibiting saturation process (such as carbon, aluminum, silicon) into a saturating mixture of powder. Structure and composition of powders have been thoroughly investigated with the help of X-ray microanalysis that has made it possible to study location of elements contributing to powder alloying and micro-durametric characteristics. A component of high-boride phase is increasing due to higher degree of powder material alloying. Significant changes in phase composition, as well as the chemical one, are noticeable in diffusion processing of the following alloyed powder materials: РR-Х18N9, РR-Х18N10, РR-Х18N15. Free carbon being displaced by boride into a transition zone creates dispersed complex carbide compounds with chromium. This is confirmed by distribution nature of carbide-forming components in a powder particle. All carbide-forming elements have characteristic concentration peaks-bursts in contrast to non-carbideforming silicon. Silicon is practically not present in the considered FeB phase and it is found only in a very small amount in the studied Fe2B phase; it is pushed aside by high-boride phases to a sublayer. The change in microhardness of the FeB and Fe2B phases under study is associated with dissolution of corresponding alloying elements in them and distortions of a crystal lattice in borides. A similar phenomenon is also characteristic for saturation while using boron or while making chemical and thermal treatment of alloyed steels, it has been noted in a number of studies. The increase in microhardness of a particle nucleus during its boriding is caused by displacement of carbon and alloying elements by growing front of boride phases. A core zone moves with an increased microhardness to a particle core while increasing temperature mode and time of boronization and up to realization of the effect with counter diffusion.

Effect of Molybdenum on Metallurgical and Wear Properties of Fe–Cr–C Hard facing alloy

In this research, Surface hardening was performed by two types of Flux Cored Arc Welding (FCAW) filler metals based on Fe–Cr–C and Fe–Cr–C–Mo at the single, two and three- pass modes on the plain carbon steel. To evaluate the metallurgical and mechanical properties of hard facing layers, the optical microscope metallography, Scanning Electronic microscope, X-ray Diffraction test, hardness testing, Microhardness test and abrasion test was applied. Adding (1.1– 1.3) molybdenum to the Fe–Cr–C filler rod led to the formation of complex carbides of M7C3, in the microstructure. By increasing molybdenum, the wear resistance and hardness of the weld layers increased so that three-layers weld hardness reached to 62 HRC. Also, the hardness of complex carbides is 1299 HV which increased to 1596 HV by adding molybdenum.

Fe based (W,Ti)C EAS and WC-12Co HVOF sprayed coatings: microstructure, mechanical properties and micro-scale abrasion performance

The application of cermet material coatings deposited by thermal spray techniques such as high velocity oxy-fuel (HVOF) has not ceased expanding to many industrial fields, especially for protection of mechanical components and industrial parts against wear and corrosion. Recently, electric arc spray process (EAS), using cored wire as feed-stock, has been considered a step over HVOF technology thanks to its low effective cost and high deposition rate. In this work, the microstructure, hardness and abrasive wear performance of FeCrNiSiB-(W,Ti)C coatings deposited by EAS were compared with WC-12Co coating deposited by HVOF process. The microstructure of the feed stock material and deposited coatings as well as the mechanical properties of the deposited coatings were characterized by scanning electron microscopy/energy dispersive x-ray analysis (SEM/EDAX), x-ray diffraction (XRD) and micro-hardness measurements. The tribological properties were evaluated at room temperature in a micro-scale abrasion device. As expected, coatings exhibited different morphologies, i.e. WC-12Co HVOF coating displayed a more homogeneous and dense microstructure, with low degree of carbide decomposition; whilst, FeCrNiSiB-(W,Ti)C EAS coatings showed an heterogeneous microstructure with more pronounced phase transformation. W-C phase on the EAS coating was dissolved in the metallic matrix which leads to the formation of new and complex carbides. The wear rate of Fe based EAS coatings was just 1.9 times higher than the one of HVOF coating. Despite of this, Fe based-(W,Ti)C EAS coating offers a potential solution for surface protection of parts as compared to HVOF coating, due to the lower production costs of the former.

Fabrication and mechanical properties of WC-10Co cemented carbides with plate-like WC grains

WC-10Co cemented carbides with plate-like WC grains were fabricated using ultrafine WC as seeds via the reaction among tungsten, cobalt and carbon. The effects of ultrafine WC contents on the microstructural characteristics and mechanical properties were investigated. Because the density of W is higher than that of complex carbide particles (CoxWyCz), some cracks could be existed in the thin flake W particles owing to the volume expansion, bringing about the formation of flake WC nuclei. Some flake WC nuclei aggregates could be seen in the alloys without the addition of ultrafine WC. The addition of ultrafine WC could bring about the gradually disappearance of flake WC nuclei aggregates and accelerate the formation of plate-like WC grains. The high content of plate-like WC could bring about high hardness. Furthermore, the palte-like WC grains could efficiently hinder the propagation of the cracks as the crack path deflection and crack bridging, and inevitably improve the transverse rupture strength.

Hot cracking susceptibility of commercial filler metals for Alloy 617 by Varestraint test. Study of hot cracking of Alloy 617 in multipass welds

ABSTRACT Solidification cracking and liquation cracking susceptibility of four kinds of alloy 617 commercial filler metals were evaluated by Varestraint test, in order to clarify a fundamental effect of each solute element in the alloy. Fracture surface of cracking in alloy 617 multipass welds by GTA welding exhibited a possibility of occurrence of both solidification cracking and liquation cracking. Brittle temperature ranges (BTR) of the liquation cracking at each alloy were almost same, however the BTRs of solidification cracking indicated significant difference. Complex carbide composed carbon, titanium and niobium was formed in the weld metal in the case of the smallest BTR for the solidification cracking, in other words alloy 617 including a lot of titanium and niobium indicated a good susceptibility for the solidification cracking. Numerical simulation of solidification segregation of alloy 617 was carried out to reveal the solidification behaviour at terminal stage of the alloy. The solidification segregation analysis suggested that solidification cracking susceptibility was attributed to the carbide formation depending on the solidification segregation of carbon, titanium and niobium. In the alloy containing titanium and niobium, the carbide formation occurred at the terminal stage of the solidification. Therefore, solidification completed temperature increased because the amount of carbon in residual liquid decreased after the carbide formation.

The Evolution of Complex Carbide Precipitates in a Low Alloy Cr-Mo-V Steel After Long-Term Aging Treatment.

Complex carbide precipitates in a quenched and tempered low alloy Cr–Mo–V steel after long-term aging at 650 °C for 13,000 h and 30,000 h were investigated in this study. The mass fraction and sizes of precipitates were quantified by electrolytical extraction technique. The types of precipitate were further studied by combined X-ray diffraction and transmission electron microscopy with selected area electron diffraction and energy dispersive spectrometry. A series of carbide precipitates, namely MC, M7C3, M6C, and M2C, were found existing in the near-equilibrium state. The precipitate sequence of these carbides was identified as MC + M7C3 + M2C → MC + M2C + M7C3 + M6C → MC + M7C3 + M6C. It was clarified that the stable phases for the investigated steel aged at 650 °C were composed of MC, M7C3, and M6C. For the first time, the in-situ transformations of M2C to M6C and M7C3 to M6C were directly observed. It was also observed that the nucleation site of the M6C was located at the interface of M7C3 carbides and the matrix. The orientation relationships between the secondary phases of the in-situ transforming carbides aged for 13,000 h and 30,000 h at 650 °C were established. The coherent interfaces between these secondary phases became incoherent with prolonged aging treatment due to the exerted strain field of the growing carbides.

A study on the effect of stress relief heat treatment on the microstructure and mechanical properties of dissimilar GTAW weld joints of Inconel 625 and A106 carbon steel

In this study, the effect of stress relief heat treatment at different temperatures (900 °C, 1040 °C, and 1200 °C) on the mechanical properties and microstructure of Inconel 625 and A106 GTAW weld joint (using ER-625 as filler rod) has been investigated. The results showed that bainite phase was formed at the HAZ of A106 alloy which resulted in the brittleness of the alloy and hardness increase in the non-heat treated sample (187 and 210 HV at HAZ of A106 and Inconel, respectively). By applying the stress relief process, bainite phase was omitted from the microstructure and ferrite-pearlite structure was formed. By increasing the process temperature to 1200 °C, the austenitic grain boundaries appeared in the microstructure. EDS analysis approved the presence of molybdenum and niobium rich complex carbides in the welding metal microstructure. The results of tensile test also showed that without stress relief process, the sample ductility decreased by ∼27%. By applying the stress relief process, the ductility of samples obviously improved and the sample which was stress relieved at 1040 °C showed the best tensile properties (UTS of ∼493 MPa and ductility of ∼29%). By increasing the stress-relief temperature to 1200 °C, the strength (UTS of ∼443 MPa) and hardness of the sample decreased due to the growth of the grain size.

Preventive Repetitive Maintenance Applications in the Field of High Wear Subjected Surfaces

The paper presents three applications for maintenance of soil processing tools, achieved by loading through welding of rough layers, of the active surfaces, highly subjected to wear. To achieve this objective we used the preventive repetitive maintenance principles and the concept of fair definition of the wear additional material, in such a way that the rendering activity of active surfaces with the additions will take place in the „dead time” for the manufacturing operation. In concrete situations, of preparing and processing soil, to assure the good functioning of cutting tools in contact with the soil to be processed, in quarries that extract and process basaltic rock aggregates we used a common process of depositing wear protection layers or intelligent self-protection systems, by melting with electric arc of new welding materials that assure in the deposited metal, type Fe-25%Cr-4%W-1%V-Ti-La, hardness of approx. 55HRC and a good welding compatibility with a large palette of low alloyed steels or micro-alloyed with boron, with hardness up to 400 HB. The structure and morphology of the deposits are type austenitic with a high volume of complex carbides of chromium, wolfram and vanadium and reduced, clean inter-crystalline areas, determined by the presence of lanthanides, fact that assures the intelligent protection systems a high tenacity and a good resistance to abrasion wear, specific clay soil with a high content of quartz sand, present in the vest area. The recent change in soil processing systems, by introducing new equipment, determined a rise in wear conditions, especially for scarify and plow coulters teeth and hoeing digger used for growing crops, respectively of useful technologies used in exploitation in conditions of high productivity with low costs.

A Hot Cracking on Dissimilar Metal Weld between A106Gr.B and A312 TP316L with Buttering ERNiCr-3

When designing piping systems for various industrial facilities, carbon steel and stainless steel are widely being used. In order to satisfy design requirements in the piping systems, the two different materials are often welded in various cases. Therefore, for quality assurance, it is necessary to understand mechanical and metallurgical properties of dissimilar metal welds thoroughly. In this study, dissimilar metal welds of stainless and carbon steels were produced through the gas tungsten arc welding (GTAW) process. In the middle of the dissimilar weld, buttering welding and butt welding were manufactured using filler wires of ERNiCr-3 and ER316L. The chemical composition of the dissimilar metal weld was analyzed. Tensile test, bending test, and hardness test were additionally performed. The microstructures of the dissimilar metal weld were investigated to analyze the cracks found during the tensile test and the bending test. The metallographic behavior was analyzed in the vicinity of the cracks. The mechanism and cause of the cracks in the dissimilar metal weld were identified. As a result, the precipitates of complex carbide types were observed in segregation bands.

Evaluation of Microstructure at Interfaces of Welded Joint Between Low Alloy Steel and Stainless Steel

In the present investigation, welded joints between low alloy steel and 304LN stainless steel were considered. The joints were fabricated using Inconel 152 and 182 as buttering materials. A thin layer of martensite, Type-I boundary, and Type-II boundary were revealed near the fusion boundary between low alloy steel and the buttering material. The polygonal grain structure and unmixed zone were found near the fusion boundary between the weld metal and austenitic stainless steel. During a uniaxial in situ deformation test in an SEM, it was revealed that crack initiation and propagation were through the buttering material. In this respect, the weld fabricated with the Inconel 152 buttering material exhibited better joint efficiency and a higher strain-hardening exponent than the joint produced with the Inconel 182 buttering material. The improved strength of the former joint was attributed to a fine grain structure, fine-scale distribution of complex carbide phases, and qualitatively high dislocation density within the Inconel 152 buttering material.

Analysis of Fractured Weldment of Hadfield Steel

This contribution is divided into two sections. The first one deals with identifying the cause of fracture in a ring, which fractured after weld surfacing. Optical and scanning electron microscopy and hardness testing were employed. Chemical composition was determined using the GDOES and EDX methods. Although weld surfacing has led to fracture in the base material, the actual problem was in the inadequate microstructure of the base material. Microscopic analysis revealed that it was heterogeneous. It contained distinctive segregation bands with bands of complex carbides and pearlite colonies, which are undesirable. The microstructure of this steel should be fully austenitic. In order to remove the heterogeneities, solution annealing was proposed and carried out successfully. The second section of this paper explores the options for revealing the microstructure of Hadfield steel. Etching reagents from the literature and authors' experience were put to test.

Crystal Chemistry of Ternary Rare Earth Transition Metal Carbides: Studies of the Tb-Fe-C System at 800°C

The isothermal section of the phase diagram of Tb–Fe–C system at 800 °C was studied in the full concentration range using powder X-ray phase and structure analyses, and energy-dispersive X-ray spectroscopy. Six ternary compounds Tb1.88Fe14C, Tb13Fe10C13, TbFeC2, Tb15Fe8C25, Tb5.64Fe2C9, Tb2FeC4 and a limited solid solubility of carbon in the crystal structure of the binary parent compound Tb2Fe17Cх (0≤ х ≤0.8) have been found to exist at 800 °C. The crystal structures of two new ternary carbides have been determined by means of powder X-ray diffraction: Tb15Fe8C25 with structure type Er15Fe8C25, space group P321, a = 11.9706(3) Å, c = 5.1733(2) Å, RB(I) = 0.07, RP = 0.06, RPw = 0.08, and Tb13Fe10C13 with structure type Gd13Fe10C13, space group P3121, a = 9.1800(9) Å, c = 23.703(5) Å, RB(I) = 0.04, RP = 0.16. Both compounds are representatives of the carbometalate class of complex carbides. Tb15Fe8C25 displays an itinerant ferro-or ferrimagnetic ordering of the Fe 3d-moments below TM ≈ 50 K while Tb 4f-moments remain essentially paramagnetic at least down to about 10 K.