Materials Science Forum Vols. 591-593

Abstract: Pr14FebalCoxB6Nb0.1 magnets have been produced using the hidrogenation disproportionation desorption recombination (HDDR) process. The effect of the Co content (x= 0, 4, 8, 10, 12, 16) and the reaction temperature (800-900 °C) on the microstructure and magnetic properties of the HDDR material have been investigated. The processing temperature (desorption/recombination) affected the microstructure and magnetic properties of the bonded magnets. The alloy with low cobalt content (4 at.%) required the highest reaction temperature (880°C) to yield anisotropic bonded magnets. The optimum temperature for alloys with 8 at.% Co and 10 at.% Co were 840°C and 820°C, respectively. Alloys with high cobalt content (12 at.% and 16 at.%) were processed at 840°C. Each alloy required an optimum reaction temperature and exhibited a particular microstructure according to the composition. Pr14Fe80B6Nb0.1 magnets have been processed for comparison.

Abstract: Roller-ball milling (RBM) or planetary ball milling (PBM) have been used together with the hydrogen decrepitation (HD) process to produce sintered permanent magnets based on a mixture of Pr16Fe76B8 and Pr14.00Fe63.90Co16.00B6.00Nb0.10 magnetic alloys. Five distinct compositions have been studied comparing low- and high-energy milling. Magnets with a particular composition and prepared using these two routes exhibited similar magnetic properties. Modifications have been carried out in the procedure of the HD stage for PBM in order to guarantee a high degree of crystallographic alignment. Pr15.00Fe69.95Co8.00B7.00Nb0.05 magnets showed the best maximum energy product for both processing routes (~ 247 kJm-3). A significant reduction in the milling time (93%) has been achieved with high-energy processing, the greatest advantage over the low-energy route.

Abstract: This work involved a morphological, microstructural and magnetic characterization of nanosized powders and sintered samples of Ni-Zn ferrite doped with chromium. The effect of substituting Fe3+ for Cr3+ on the final characteristics of the powders and sintered samples was investigated. The Ni-Zn ferrite powders were prepared by combustion reaction using nitrates and urea as fuel, based on the concepts of propellant chemistry. The samples were uniaxially compacted by dry pressing (385 MPa) and sintered at 1200oC/2h, using a heating rate of 5°C/min. The Ni-Zn powders and compacted samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and the measures of their magnetic properties. The results revealed the formation of the cubic crystalline phase of the inverse spinel of Ni-Zn-Cr ferrite. The average crystallite size was 21 nm and 57 nm while the saturation magnetization was 47.0 and 73.1 emu/g for the powder and the sintered sample, respectively.

Abstract: Ni-Zn ferrites are magnetic ceramics that are widely used in electric and electronic components. Among Ni1-xZnxFe2O4 ferrites, Ni0.3Zn0.7Fe2O4 is known to be the one of highest magnetic permeability in lower frequencies. This work is divided into two parts: the first part deals with the influence of sintering temperature and forming conditions on the microstructure of Ni0.3Zn0.7Fe2O4 ferrite. The second part deals with the influence of sintering temperature and environmental variations of temperature on the complex magnetic permeability of such ferrite. The ferrite studied in this work was fabricated by means of the conventional ceramic method. Complex magnetic permeability between 100 kHz – 100 MHz is discussed in relation to sintering temperature. The influence of environmental variations in temperature (-40 oC to +50 oC) on the complex magnetic permeability of the ferrite sintered at 1300 oC is discussed.

Abstract: High-energy milling has been used for production of nano-structured WC-Co powders. During the High-Energy Milling, the powders suffer severe high-energy impacts in the process of ball-to-ball and ball-to-vial wall collisions of the grinding media. Hard metal produced from nanostructured powders have better mechanical properties after appropriate sintering process. During the milling the particles size of WC and Co can be reduced and plastic deformed. In the present work, a mixture of WC-10%Co was produced by high energy milling. The starting powders of the WC (0.87 μm - Wolfran Bergau) and Co (0.93 μm - H.C.Starck) were used to produce the hard metal. The influence of the milling time on the particle size distributions and in the lattice strain was investigated. Milling time of the 2, 10, 20, 50, 70, 100 and 150 hours were used. The powders after milling were characterized by X-ray diffraction (XRD) and Scanning Electronic Microscopy (SEM). The results show that 10 h milling were enough to reduce the crystallite size of WC and the increase of the milling time reduces the crystallite size.

Abstract: This work discusses on the effect of milling parameters on the TiB and TiB2 formation in Ti-50at%B and Ti-66at%B powders, respectively. Both powder mixtures were processed in a planetary ball Fritsch P-5 ball mill under Ar atmosphere, varying the milling parameters: rotary speed (150 and 200 rpm), size of balls (10 and 19mm diameter) and ball-to-powder weight ratio (2:1 and 10:1). In order to obtain the equilibrium structures the milled powders were heated at 1200oC for 1h. Samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermal analysis (DSC). XRD results indicated that extended Ti(B) solid solutions were formed during ball milling of Ti-50at%B and Ti-66at%B powders. After milling for 170h it was noted the TiB and TiB2 formation in Ti-50B and Ti-66B powders processed under higher-energy condition. DSC analysis revealed that the TiB2 formation was completed during heating of mechanically alloyed Ti-66at%B powders only. After heating at 1200oC for 1h, a large amount of TiB and TiB2 was found in Ti-B powders milled under higher energy condition.

Abstract: This work discusses on the structural evaluation of mechanically alloyed Ti-Nb powders. The Nb amount was varied between 20 and 50 wt-%. The milling process was carried out in a planetary Fritsch P-5 ball mill under Ar atmosphere. The structural evaluation was conducted by scanning electron microscopy, X-ray diffraction, and energy dispersive spectrometry. During ball milling it was noted an excessive agglomeration of ductile Ti-Nb powders on the balls and vial surfaces, and the final amount of remaining powders was then drastically reduced into the vials. This fact was more pronounced with the increased Nb amount in starting powders. Typical lamella structures were formed during ball milling, which were refined for the longest milling times, and fine and homogeneous structures were formed in Ti-Nb (Nb=20-50wt-%) powders. XRD results indicated that the full width at half maximum values of Ti peaks were continuously increased while that the crystallite sizes were reduced for longer milling times due to the severe plastic deformation provided during ball milling of Ti-Nb powders. However, the EDS analysis revealed the presence of Nb-rich regions in Ti-Nb powders after ball milling. The critical ball milling behavior of ductile Ti- Nb powders contributed for reducing the yield powder and increasing the structural heterogeneity.

Abstract: A large amount of the Ti6Si2B compound can be formed by mechanical alloying and subsequent heat treatment from the elemental Ti-22.2at%Si-11.1at%B powder mixture, but the yield powder after ball milling is reduced due to an excessive agglomeration of ductile particles on the balls and vial surfaces. This work reports on the structural evaluation of Ti-22.2at%Si-11.1at%B powders milled with PCA addition, varying its amount between 1 and 2 wt-%. The milling process was carried out in a planetary ball mill under argon atmosphere, and the milled powders were then heated at 1200oC for 1h under Ar atmosphere in order to obtain equilibrium structures. Samples were characterized by X-ray diffraction, scanning electron microscopy, and thermal analysis. Results revealed that the PCA addition reduced the excessive agglomeration during the ball milling of Ti-22.2at-%Si-11.1at-%B powders. After heating at 1200oC for 1h, the Ti5Si3, Ti3O and/or Ti2C phases were preferentially formed in Ti-22.2at%Si-11.1at%B powders milled with PCA addition, and the Ti6Si2B formation was inhibited.

Abstract: W-Cu composite powders were prepared by high energy milling under two milling environments: cyclohexane and air. Composite particles are formed in both cases. The W particles are fragmented and embedded into the Cu particles. Both, W and Cu, are heavily strained, mainly in the first hours of milling. The composite powder has high homogeneity and is much finer than the original Cu powder. The mean particle size of the powders milled in both conditions is very close, but the wet milling was near 25% longer than dry milling and the size distribution is wider. This is consequence of the higher milling intensity of dry milling.

Abstract: The behavior of different process control agents (PCAs) during mechanical activation of Nb75Al powder mixtures was investigated. Mechanical activation by high-energy ball milling was carried out on a shaker mill (SPEX®8000) for 1 hour. Each PCA (Stearic acid, ethanol and methanol) was added to the powder charge in two proportions (1 and 2 wt%). Shape and microstructure of activated powders (aggregates) were analyzed by scanning electron microscopy. Milled powder mixtures were uniaxially pressed in cylindrical compacts that were further vacuum reacted at a constant heating rate (30°C/min) in order to produce NbAl3 intermetallic compound. The temperature of the samples was monitored by an S-type thermocouple. The results show that the shape and the microstructure of the milled powders were strongly affected by the type and quantity of PCAs, therefore changing the reaction behavior and the densification of the produced pellets. Although ethanol was more effective to control aggregate size, best densification results were attained with 2 wt% of stearic acid.