Materials Science Forum Vols. 530-531

Abstract: Porous materials are successfully utilized for fabrication of many industrial components such as filters and selflubricating bearings. These products are made by powder metallurgy, where mixtured or prealloyed powders can be used. The aluminum bronze is one of the most wanted due its excellent properties in combination with low cost of the raw materials. In this work, single action compacted (100 MPa) prealloyed aluminum bronze (Cu- 9wt%Al-1wt%Fe) cylinders were sintered using a hollow cathode discharge at temperatures between 400 and 750°C with duration on the isotherm for 12 min. Microstructure changes, homogeneity, porosity and composition were analyzed after the treatment. Sintering below 550° C led to uniform but porous structure. Above 550 °C it was observed a solidified central region and a porous structure that changes slightly through out the cross-section. The diameter of the central region increased with treatment temperature. It is concluded that due to the intense plasma heating and subsequent surface melt formation a mass flow direction to the center of compacts occurred.

Abstract: Tool steels have important participation in the worldwide market of steels. They are used by metal-mechanical industry as cutting tools, molds, punches, etc. Because the use is of great responsibility, it is important to discuss the relationship between microstructure and mechanical properties. The starting material was powder of a commercial tool steel with the composition (wt%) 2.5%C, 5.25%Cr, 0.9%Si, 9.75%V, 0.5Mn, 1.3%Mo and balance Fe. The samples were obtained by hot isostatic pressing (hip) process, and than were austenitizing at 1120°C and submitted to tempering in the temperature range varying from 430° to 550°C. The aim of this work was to evaluate the correlation between heat treatment, microstructure and mechanical properties. Results of toughness and hardness are presented.

Abstract: The coercivity of sintered magnets like barium ferrite (BaFe12O19), samarium-cobalt (SmCo5) or neodymium-iron-boron (Nd2Fe14B) is largely affected by the grain size. A method to evaluate coercivity behavior as function of the crystalline orientation, including also the effects of grain size and lattice defects, is presented. Expressions were deduced to estimate the critical size of nucleus for spontaneous reversion of magnetization. The model indicates that the nucleation in grains of materials with high magnetocrystalline anisotropy only can begin by domain rotation. The model also predicts that the surface condition of grains is very important for the coercivity. A qualitative explanation is offered for the fact that materials with higher coercivity (or with smaller grain size) tend to follow an angular dependence of the coercivity similar to that given by the Stoner-Wohlfarth model, while materials with lower coercivity (or with larger grain size) tend to follow an angular dependence of the coercivity similar to 1 / cos theta.

Abstract: The processing of SmCo5 sintered magnets involves a post-sintering heat treatment, where the intrinsic coercivity of the magnets may increase more than one order of magnitude. Variables of the heat treatment like time, temperature and cooling rate have strong influence on coercivity. We describe a method for modeling the heat treatment, which includes microstructural features as precipitate size and 2nd phase volume fraction. The numerical solution was obtained using the Finite Volume Method to solve Fick´s second law. Experimental data like the diffusion coefficient of Sm into SmCo5 phase and the Sm-Co phase Diagram are used for the modeling.

Abstract: Sm(CoFeCuZr)z commercial magnets are manufactured by powder metallurgy techniques. Microstructural investigations of Sm(CoFeCuZr)z magnets have shown that, increasing the Zr content, some impurity phases may appear. An alloy with composition (at%): 60.5% Co – 15.5% Fe – 11.5% Zr - 8.5% Sm - 4% Cu, homogenized at 1050oC, was investigated. Three main phases were identified: rhombohedral 1:3 (ZrSm)1(CoFeCu)3, hexagonal 1:7 (SmZr)1(CoFeCu)7 and cubic 6:23 (Zr)6(CoFe)23. Knowledge of possible phases present in 2:17-type magnets allows a better understanding of the nanocrystalline microstructure responsible for high coercivity of these magnets.

Abstract: This paper reports the results of investigations carried out to determine the microstructure and magnetic properties of some praseodymium-based magnets represented by the formula Pr14Fe63.9-xCo16B6Nb0.1Mx. Bonded magnets were prepared the annealed alloys using the hydrogenation, disproportionation, desorption and recombination (HDDR) process. The HDDR powders were isostatically pressed and bonded with cyanoacrylate adhesive to form permanent magnets. The effect of addition element content on the magnetic properties of these magnets was investigated. The amount of addition has a significant effect on the magnetic behaviour of these bonded magnets.

Abstract: This paper reports the results of investigations carried out to determine the magnetic properties of some praseodymium-based magnets represented by the formula Pr14Fe63.9-xCo16B6Nb0.1Mx. Bonded magnets of various compositions were prepared from annealed alloys using the hydrogenation, disproportionation, desorption and recombination (HDDR) process. The HDDR powders were isostatically pressed and bonded with cyanoacrylate adhesive to form permanent magnets. The effect of the addition element content on the magnetic properties of these magnets was investigated. The amount of Al, Cu and P addition has a significant effect on the magnetic behavior of these magnets. Under the present processing condition 0.3 at%Al enhanced the intrinsic coercivity to 1.27 and a minimal amount of P improved the remanence to 0.77 T. Copper addition was deleterious to the magnetic properties of the HDDR bonded magnets, independently on the content added.

Abstract: This paper reports the results of investigations carried out to determine the Curie temperature (Tc) of some annealed praseodymium-based alloys represented by the formula Pr14Fe79.9-xCoxB6Nb0.1. The Curie temperature of these permanent magnet alloys increase linearly with the cobalt content at about (10.2±0.3) oC/at%. Pr14Fe80B6 and Pr14Fe79.9B6Nb0.1 magnetic alloys with a Tc of 290oC have been used as a standard reference. Magnets were prepared from the alloys using the hydrogenation, disproportionation, desorption and recombination (HDDR) process.

Abstract: The calciothermic reduction-diffusion (CRD) process is a alternative preparation route for Nd15Fe77B8, NdFe11Ti, NdFe10.5Mo1.5 and NdFe10.75Mo1.25 alloys, which eliminates the need for long homogenizing heat treatment; in addition, the resulting alloy is already in powder form. We have examined the effect of various processing variables in the preparation of mother alloys. Compacts made of NdCl3, Fe, Ti, Mo and Fe-B powders and Ca granules were heated under argon for different times and temperatures. The alloys as-prepared contained mostly the hard magnetic phase. The NdFe11Ti, NdFe10.75Mo1.25 and NdFe10.5Mo1.5 alloys have been successfully nitrogenated by heating a mixture of powdered alloys with sodium azide (NaN3) at temperatures between 330 and 450 oC.

Abstract: This paper compared the influence of the iron powder in the production of Sm2Fe17Nx by calciothermic reduction. Two available iron powders were used: Basf CL carbonyl iron and Ancor MH-100 sponge iron. Both alloys were prepared and analyzed with X-ray diffraction, scanning electronic microscope and vibrating sample magnetometer. The results showed that the alloy produced with Ancor Mh-100 presents a higher anisotropy. The alloy produced with Basf CL has a smaller anisotropy but a higher coercivity. This can be explained by the difference in the microstructure of the two powders.