Materials Science Forum Vols. 660-661

Abstract: The hydrogen absorption capability of PrxFe77.9-xCo16.0B6.0Nb0.1 (12.0 ≤ x ≤ 14.0) magnetic alloys was evaluated. A practical methodology was developed so that the hydrogen pressure decrease inside a closed system was correlated to the mass gain (%W) of the sample. %W increases linearly with the Pr concentration in the magnetic alloy either in the as-cast state or annealed during 20 hours at 1070°C. Comparisons between %Wtheoretical and %Wexperimental showed a satisfactory agreement.

Abstract: The first goal of this work involved the study of the effect of variables the HDDR processing, such as: the added pressure of H2 in the system, the time of heat treatment and recombination of Pr12Fe65.9Co16B6Nb0.1 alloy with the aim of improving the magnetic properties like the magnetic properties of the Pr14Fe63.9Co16B6Nb0.1 alloy (Br= 865mT and iHc= 790mT). The second aim of the work involved the characterization of HDDR powders that were analyzed by X-ray diffraction for identification and quantification of crystalline phases. These materials were analyzed by scanning electron microscopy (SEM).

Abstract: The first goal of this work involved the study of HDDR powders obtained from annealed alloys with the general formula: PrxFe77.9-xCo16B6Nb0.1 (x = 12; 12.5; 13; 13.5 and 14). The alloys were processed at desorption / recombination temperature of 840°C. The highest magnetic properties were obtained with 13.5 at. % Pr (Br= 1000mT and µ0iHc= 890mT). The alloy with a minimum praseodymium content (12 at. %) exhibited the lowest magnetic properties (Br= 350mT e iHc= 120mT). The second aim of the work involved the characterization of HDDR powders using X-ray diffraction for phase quantification and mean crystallite sizes determination of the hard magnetic phase. The processed powders were characterized by scanning electron microscopy (SEM).

Abstract: Mixtures of AA2124 aluminum alloy powder and SiC particles at volume fractions of 10 vol.% and 20 vol.% were milled in a high energy planetary ball mill under an argon atmosphere, for times of 2.5h to 60 h, aiming to produce Al alloy-SiC nanocomposites. Optical microscopy (MO) and scanning electron microscopy (SEM) were used to evaluate the morphological and microstructural evolution of the powder composite, occurred during mechanical alloying. The crystallite size was determined using the Williamson-Hall method to analyze the X-ray peak broadening. It was observed that increasing the volume fraction of SiC, the mechanical alloying stages were accelerated: a finer composite powder was obtained at a shorter milling time as well as the morphology of the particles became more equiaxed. The XRD analysis showed the reduction of crystallite size of the aluminum alloy matrix with increasing milling time and that this effect is more pronounced with high volume fraction of SiC. The results show that the increase in the volume fraction of reinforcement particles increases the work hardening and fracture occurrence in the aluminum alloy powder during the milling, affecting the structural evolution of the composite.

Abstract: Ultra-high molecular weight polyethylene (UHMWPE) is a polyethylene with a very long chain, which provides excellent features, however it makes the processing difficult due to high melt viscosity. Many studies intend to found out means to make its processing easier. Recently, the high-energy mechanical milling has been used for polymeric materials and it was detected that physical and chemical changes occur during milling. In such case, powder of UHMWPE was milled in three types of mills: SPEX, attritor e planetary, in different times of milling. The polymer was characterized by SEM and XRD. Thus, it was observed that the material processed in attritor mill showed larger phase transformation from orthorhombic to monoclinic. This is most likely due to the smaller milling temperature of attritor mill when compared with the other two mills and the high shear force generated during milling.

Abstract: This work reports the efforts to obtain TiFe intermetallic compound by high-energy ball milling of Ti and Fe powder mixtures. This process route has been used to provide a better hydrogen intake in this compound. Milling was carried out in a SPEX mill at different times. Strong adherence of material at the vial walls was seen to be the main problem at milling times higher than 1 hour. Attempts to solve this problem were accomplished by adding different process control agents, like ethanol, stearic acid, low density polyethylene, benzene and cyclohexane at variable quantities and keeping constant other milling parameters like ball to powder ration and balls size. Better results were attained with benzene and cyclohexane, but with partial formation of TiFe compound even after a heat treatment (annealing) of the milled samples.

Abstract: The effect of mechanical activation procedures on the combustion synthesis of NbAl3 was investigated. The activation was carried out by a two-step high energy ball milling procedure. In the first milling, aluminum and niobium were milled separately (pre-activation). The mixture of pre-activated powders was then activated in the second milling. Reaction synthesis, by simultaneous combustion mode, was conducted on compacted pellets made of powder mixtures with and without pre-activation. The thermal behavior of the compacted pellets upon heating was recorded and the main thermal combustion reaction characteristics were evaluated. The two-step procedure produced aggregates with a globular dispersion of niobium due to increased particle hardness and decreasing mean particle size during pre-activation milling. Analysis of pellet thermal behavior showed the two-step milling procedure could enhance reaction performance by increasing maximum reaction heating rate and temperature gain during reaction.

Abstract: In general, semiconductor materials for thermoelectric generation prepared by vacuum metallurgy shows a relatively high value of figure-of-merit. However, differences in some properties of alloys elemental constituents can cause processing problems. Recently, Mechanical Alloying (MA) has been used to produce polycrystalline thermoelectric materials, such as (Bi,Sb)2 and (Te,Se)3(1). The industry is using this process since early 70’s to produce oxide dispersion strengthened alloys and those with widely different melting temperatures (2) In the present work, Si0.80Ge0.20 alloys were prepared via Mechanical Alloying (MA), using 99.9 % pure silicon and germanium powders, with a sieve size of 100 mesh. The MA has been performed, for several balls - to - powder ratio, in a SPEX 8000 vibratory high energy milling with tungsten carbide balls. Time for alloy formation was in a range from 3 to 9 hours, corresponding to charge ratio of 12:1 and 4:1, respectively. After two hours of processing time, the grinding temperature reached 80 0 C, and remained at this level until the end of the process. It was possible to follow the SiGe alloy formation by x-ray diffractometry, as the peak lines positions of elemental Si and Ge were continuously shifted, and end up to merge into a single broad peak. There was a convergence of the individual lattice parameters of Si and Ge to a single value of 5.470 A, measured within the limit of  0.005 A. For the Si0.80Ge0.20 system evaluated in this work, the alloying progress occurred continuously, and changed inversely with charge ratio.

Abstract: The formation of PbTe intermetallic compound by mechanical alloying (MA) has been investigated. The elemental starting materials were 99.5% pure lead and tellurium, with a sieve size of 80 and 200 mesh, respectively. A SPEX 8000 shaker milling was used to perform the MA, using WC balls as milling media in a cylindrical hardened tool steel vial. X-ray diffraction analysis was performed with a profile-fitting program, to evaluate time evolution of the alloy formation. An exotermical reaction occurs on PbTe formation, with entalphy H= - 16.3 Kcal/mol. The *T value is confirmed by the heat exchange equation *Q = |*Hf | =* i (mici ) *T, where the summation comprises the mass and specific heat of vial, balls and powder material. For the standard milling conditions employed, the PbTe formation occurs at aproximately 90 seconds of milling, when using charge ratios between 3:1 and 7.5:1. However, for lower charge ratios (8:1 to 10:1), isolated reactions at the mixture occurs, but the amount of material is not enough to raise the temperature of adjacent regions, and the propagation of the reaction is avoided. There is therefore a minimum amount of powder (“critical mass”), and below this value the reaction will not be self-sustained.

Abstract: This work reports an investigation about the influence of the environment of milling on the characteristics of the powders and on the structure and density of sintered samples made of these powders. Mixtures of composition W-30wt%Cu were milled for 51 hours in a high energy planetary mill in dry and wet (cyclohexane) conditions. The milled powders have composite particles. The powders were pressed and sintered at 1050º, 1150º and 1200°C under flowing hydrogen. The isothermal times were 0 minutes for the first two temperatures and 60 minutes for the latter. The samples reached around 95% of relative density. The powders were characterized by means of XRD and SEM. The sintered samples were characterized by means of SEM, optical microscopy and density measurement.