Claudiney de Sales Pereira Mendonça

Comparative analysis of niobium and vanadium carbide efficiency in the high energy mechanical milling of aluminum bronze alloy

This study aims to analyze the efficiency of niobium and vanadium carbides in the high energy mechanical milling of aluminum bronze alloy. Two series of experiments were made following the same steps for both niobium carbide (NbC) and vanadium carbide (VC) additions: 30 g of chips were weighed and placed in a stainless steel jar with 3 % of carbide and 1 % of stearic acid for a mass/sphere relationship of 1:10. The milling was realized using a planetary ball mill for 10, 30 and 50 hours in an inert argon atmosphere at 300 rpm. Results shown in laser diffraction indicate a great reduction in the particle sizes of powders when VC is used. For 30 hours milling, D50 values ranged from 1580 μm with NbC to 182.3 μm with VC addition. The D50 values ranged from 251.5 μm with NbC to 52.26 μm with VC addition, for 50 hours milling. The scanning electron microscopy showed that in 10 hours of milling, the energy was not sufficient to achieve the shear of chips in both cases. For 30 hours, it’s possible to observe particles with sizes between 100 μm and 800 μm with NbC addition while for the same milling time, with VC it’s possible to see particles with different sizes, but with many shapes of fine particulates. For 50 hours milling, particles achieved the smaller sizes between 50 and 200 μm with NbC and ranging from 5 until 50 μm with VC addition.

Investigação das propriedades magnéticas e microestrutura da ferrita de chumbo e cobre

Os objetivos deste trabalho foram a obtencao de uma ferrita de chumbo e cobre com a estrutura do espinelio, a caracterizacao da sua microestrutura e a determinacao das suas propriedades magneticas. A ferrita de cobre e chumbo e um material com baixa coercividade magnetica, alta resistividade eletrica e alta permeabilidade magnetica. Amostras com estequiometria PbxCu1-xFe2O4, com (x= 0, 0,1 e 0,2), foram produzidas pelo processo de metalurgia do po e sinterizadas a 800 °C por 6 h em atmosfera ambiente. As caracterizacoes estrutural, microestrutural e magnetica foram realizadas utilizando as tecnicas de difracao de raios X, microscopia eletronica de varredura, magnetometro de amostra vibrante e analise termogravimetrica magnetica. Os resultados mostram que as amostras sao formadas exclusivamente pela estrutura do espinelio tetragonal. As curvas de histerese mostram caracteristicas de materiais magneticos moles e sua magnetizacao de saturacao aumentou com a concentracao de chumbo. O valor da temperatura de Curie diminuiu com o aumento da concentracao de chumbo.

Optimization of the High Energy Milling Process of Chips of a Stainless Steel Using the Response Surface Modeling

The present paper discusses response surface methodology as an efficient approach for predictive model building and optimization of the high energy milling process from the chip duplex stainless steel with a carbide vanadium addition willing to obtain the smaller particle size. The process parameters studied was milling time of 10 h and 50 h. Rotations were performed at 250 and 350 rpm. Vanadium carbide was added from 0% to 3% in weight, and a mass/ball ratio of 1/10 and 1/20. An analysis of particle size and a scanning electronic microscopy were used to measure and characterize particle size. With addition of carbide in milling process resulted on a reduction of particle size compared to the material without carbide added around 66%. The results predicted using factorial regression model showed high values of regression coefficients (R2 = 0.952) indicating good agreement with experimental data. The minimum value of particle size was obtained for following optimal conditions: rotation of 325 rpm, time of 42 h, ball/mass 18:1 and carbide 2, 67%wt. The particle size of fabricated powders after 50 h of milling with 3% vanadium carbide addition was about 186 times lower than that the initial chips.

Avaliação da microestrutura e das propriedades magnéticas de ferrita de cobre dopada com chumbo sinterizada com fase líquida

This paper presents to obtain a lead and copper ferrite with spinel structure, characterizing the microstructure and the determination of their magnetic properties. The copper and lead ferrite is a material with low magnetic coercivity, high electrical resistivity. Samples with stoichiometry PbxCu1-xFe2O4 with (x = 0.00; 0.05; 0.10; 0.15 e 0.20) were prepared by powder metallurgy processes and sintered at 1000 °C for 6 hours. The results show that sample with x = 0.00 is formed exclusively by the cubic spinel structure and samples with x between 0.05 to 0.20 are formed exclusively by the tetragonal spinel structure hysteresis loops show the characteristics of a soft magnetic material and the saturation magnetization and Curie temperature decreased with the lead concentration, however, noted that the increase in the coercive field.

Magnetic properties and potential barrier between crystallites model of MgGa2-xFexO4 ceramics

The aim of this work was to investigate the magnetic properties and the electrical conductivity temperature dependence associated to the potential barrier between the crystallites model. Gallium and magnesium containing spinel ceramic has low magnetic coercivity and high electrical resistivity. MgGa2-xFexO4 samples (x= 0.01, 0.05, 0.15, 0.25, 0.35) were prepared by solid-state method and sintered at 800 °C for 8 h. X-ray diffraction analysis confirmed the formation of a single phase with compact cubic spinel structure. The magnetic measurements show that the saturation magnetization and remanence of all samples increased with increasing iron concentration. The coercive field decreased up to the concentration x= 0.15, and above x= 0.25 it was observed an increase in the coercive field. Through electrical characterization it was found that the samples presented highly insulating behavior for x= 0.01, and further increase in x above 0.15 gives a semiconductor behavior compatible with the potential barrier between the crystallites model, i.e. fulfills the condition L/2 > LD (crystallite size L in comparison with the Debye length LD), and the conduction is limited by potential barriers between the crystallites.

Influência do tratamento térmico de envelhecimento a 850ºC na microestrutura e nas propriedades mecânicas e magnéticas do aço Duplex UNS S31803

Duplex stainless steels are those that have a microstructure constituted by two phases, ferromagnetic ferrite (α) and paramagnetic austenite (γ), which are present in approximately equal volumetric fraction. In this work, the microstructure of duplex steel UNS S31803 was modified by a thermal solubility treatment at temperature of 1050 °C for 30 minutes, followed by water cooling. Next, specimens were aged for 5, 15, 30, 60 and 180 minutes at temperature of 850°C. This treatment promoted the formation of a deleterious phase, the sigma phase, which is formed by the decomposition of ferrite, rich in the elements Cr and Mo. The formation of this phase changes the mechanical and chemical properties of the duplex and super duplex steels. The magnetic and mechanical properties were evaluated in each of the conditions mentioned and a correlation with the microstructure was made. The magnetic properties of the specimens were obtained in a Vibrating Sample Magnetometer (VSM), where the material hysteresis curves were obtained. The mechanical properties of the samples were characterized by Vickers micro-hardness measurements performed with a load of 0.2 kgf. Microstructural characterization was carried out by using an optical microscope. VSM tests results showed that as the aging time increased, the volume fraction of sigma phase also increased, causing a diminution of the steel duplex magnetic properties. The diminution of the magnetic properties of the duplex steel with the aging time is attributed to the lower fraction of ferrite as more sigma phase is formed by the decomposition of ferrite. The hardness results showed an increasing of the hardness of the material as the aging temperature increased, which confirms the higher brittleness of the steel due to the higher amount of sigma phase.