Materials Science Forum Vols. 660-661

Abstract: This study aimed to evaluate the resistance of a Ti-6Al-4V alloy in creep after heat treatments. It was used a Ti-6Al-4V alloy in cylindrical bars forms, forged condition and annealing at 190oC for 6 hours and cooled in air. The microstructure of Ti-6Al-4V alloy was evaluated after heat treatment and was submitted to creep tests at 600oC and stress conditions from 125 to 319 MPa at constant load. The Widmanstätten structure was obtained by heat treatment. Yttria (8 wt.%) stabilized zirconia (YSZ) with a CoNiCrAlY bond coat was atmospherically plasma sprayed on Ti-6Al-4V substrates by Sulzer Metco Type 9 MB. The alloy with Widmanstätten structure and ceramic coating shows greater resistance to creep and oxidation with a longer life time in creep. At higher stress condition, 600°C and 319 MPa, the Ti-6Al-4V alloy with ceramic coating didn’t show higher creep resistance. This condition presented higher tp value and the value. It occurred because at high stress condition the coating is very fragile, decreasing your creep resistance.

Abstract: It has been studied a steam treatment process in an industrial continuous furnace for sintered iron parts in a temperature range varying from 490 to 570 °C and sample´s sintered densities of 6,52 and 6,77 g / cm3 . The tests have showed oxidation kinetics with two stages, each of them obeying a parabolic law. The first stage was faster than the second. The oxidation time when the oxidation kinetics of the first stage has changed for the second stage showed off very sensitive to the process temperature and sintered densities of the parts which are in agreement with the process rate controlling mechanism that was found to be the iron diffusion through the oxide layer.

Abstract: This study investigate the laser surface hardening in tool steel, avoiding the high reflectivity of the metal to the CO2 radiation by covering the surface sample with black carbon. Hardening process using graphite was effective in many works in the word, although, the particles size of graphite powder was in the range of some micrometers. In order to reduce the particle size, the carbon black was used in the laser thermal treatment of 4340 steel. The microhardness change to 850HV, compared to the substrate of 250HV and the friction coefficient ranged 0.2 and 0.3, because of the covered surface by carbon black. These thin coating of carbon black reduced the wear rate near to a hundred in comparison to the surface without carbon coating.

Abstract: Phosphogypsum or chemical gypsum is a by-product generated in the manufacture of phosphoric acid, generated in the proportion of 4 to 6 tons for every ton of acid produced by the fertilizer industry. Its reuse is important from the economic, social and environmental point of view. The purpose of this paper is to investigate the thermodynamic condition of sulfur dioxide recovering from thermal decomposition of phosphogypsum. The main advantage of this process is to return the sulfur dioxide to sulfuric acid manufacturing. In this case, sulfur demand by fertilizer industry and phosphogypsum generation could be greatly reduced. Experimental results were obtained from a lab scale rotary kiln. Theoretical approach and experimental setup were validated by using the actual data from thermal decomposition of limestone. Theoretical and experimental results showed the thermal decomposition of phosphogypsum is thermodynamically feasible only at high temperatures.

Abstract: The use of biomass for energy generation has aroused great attention and interest because of the global climate changes, environmental pollution and reduction of availability of fossil energy. This study deals with pyrolysis of four agricultural wastes (sawdust, sugarcane straw, chicken litter and cashew nut shell) in a fixed bed pyrolytic reactor. The yields of char, liquid and gas were quantified at 300, 400, 500, 600 and 700oC and the temperature and pressure effects were investigated. Pyrolytic liquids produced were separated into aqueous and oil phases. XRF spectroscopy was used for qualitative and quantitative elemental analysis of the liquids and solids produced at whole temperature range. Calorific value analysis of liquids and solids were also performed for energy content evaluation. Experimental results showed sawdust, sugarcane straw and cashew nut waste have very good potential for using in pyrolysis process for alternative fuel production.

Abstract: Fe-6.5%Si alloy spray formed has the Curie temperature variation measured as a function of the parameters process. In this work the magnetic transition temperature was obtained by Differential Scanning Calorimeter although it can be also gotten by Vibration Samples Magnetometer. The process parameters controlled were gas pressure (P), tube diameter (T) and fly height (H) and the properties analyzed were coercive force, maximum permeability, power loss and Curie temperature. The chemical composition was determined using the Curie temperature. The microstructure of deposits was observed in different conditions of light microscopy, bright field, polarized light and dark field, and the grain size was measured according to ASTM 112-95. The process parameters combination P5T6H360 has produced the minimum power loss of 2.26 W/kg and maximum permeability of 9,000. The Curie temperature measured was 683°C, which indicates a Si content of 6.5wt%.

Abstract: An evaluation of the effect of alloying elements on the microstructure and magnetic properties of Pr15FebalCo8B7Nb0.05Mx (M = Cu, P, Gd and Ga; 0 ≤ x ≤ 0.25) sintered magnets has been carried out. A mixture of alloys and the high-energy milling technique have been used to prepare the magnets. The alloying elements have influenced the remanence, intrinsic coercivity and particularly the squareness factor (SF). Phosphorus addition improved (BH)max (254 kJm-3 ) and SF around 10% (0.89). The same improvement addition on intrinsic coercivity was observed with Gallium (1100mT) compared to the standard composition Pr15FebalCo8B7Nb0.05 (1000mT) magnet. Comparisons between the squareness factors obtained using the J×μ0H curve profile (SF), the estimated (sf) using microstructural parameters and Sf using a (BH)max and Br correlation have also been carried out.

Abstract: In magnets based in phases with high magnetocrystalline anisotropy like Nd2Fe14B or SmCo5 there is a competition between magnetostatic energy and domain wall energies. If the grain size is large, the formation of domain walls is energetically favorable. When the formation of domain walls is an unfavorable process, coercivity is larger. A better comprehension of this phenomenon is possible if the energy necessary for the first domain wall formation is properly evaluated. To address this problem, the magnetostatic energy of a sphere magnetized in two opposite directions, separated by a domain wall, is calculated using Legendre Polynomials. The data allow the determination of the reversible volume for nucleation. It is predicted a “recoil effect”, the magnetization may be reversible until a given volume of reverse magnetization.

Abstract: Sintered NdFeB magnets typically exhibit grain size above 2 micrometers, a value above the single domain particle size (~0.3 micrometers). It is discussed how to obtain relationships between grain size and coercivity with energy balance models, considering formation and annihilation of domain walls as a dissipative process. In the case of nanocrystaline magnets, the Stoner-Wohlfarth model is very suitable. For larger grain sizes, the contribution of the magnetostatic energy of each grain has to be considered. From the concept of meta-stability of domains and domain walls structure, a relation between coercive field and grain size can be estimated.

Abstract: Rare earth transition metals magnets, as for example SmCo5, NdFeB and Sm(CoCuFeZr)z magnets are, very often, submitted to a post-sintering heat treatment where the coercivity increases. In many cases, a slow cooling between the sintering and the annealing temperatures is applied. To optimize the time and cooling rate of heat treatment, it was developed a model using the finite volume method. This model takes into account the diffusivities of the different atom species and also the boundaries of phase diagrams.