Materials Science Forum Vols. 530-531

Abstract: This work is concerned with issues related to using grey cast iron swarf from dry machining in powder metallurgy. In a first stage, a complete study on the characteristics of the raw powder is performed, in which aspects related to the dilution in different proportion of pure iron were studied. Low compacting pressure produces workpieces more resistant and that the ideal sintering temperature is 1160 °C. In a second stage, aiming at reduction of the powder, a heat treatment is performed. New dilutions using the decarburized powder were made and results showed better structural homogeneity and an improvement in the mechanical properties. The steel alloy 50/50% of swarf powder provided the best results. In a third stage, a cylindrical bearing was produced. New dilutions were made. Based on the control of the interconnected porosity, radial and wear resistance, the sample prepared with 100% swarf powder provided the best results.

Abstract: Gamma-TiAl alloys are potential replacements for nickel alloys and conventional titanium alloys in the cooler sections of turbine engines, as well as in orbital platform vehicles. The combination of high specific stiffness and good oxidation resistance at intermediate temperatures can provide significant weight savings. However, they have a limited plasticity at room temperature and the tendency to brittle fracture. Powder metallurgy is a near net shape process that allows the parts production with complex geometry at low costs. An improved plasticity of the Ti-Al alloys is received by adding alloying elements and by microstructure modification. An alloy of two-phase structure Ti–48Al–2Cr–2Nb (at.%) was investigated using the blended elemental technique. Samples were produced by mixing of initial metallic powders followed by uniaxial and cold isostatic pressing with subsequent densification by sintering at 1500 °C, in vacuum. It was shown that the samples presented a two-phase structure consisting of lamellar colonies of alternating layers of gamma and α2 phase.

Abstract: The heat treatment of high speed steel tools consists of austenitizing, quenching and tempering. The size of austenite grains formed during the hardening treatment is an important factor in the final microstructure of the steel, and it also affects properties such as wear resistance and toughness. This paper presents the austenite grain size, matrix composition and hardness of commercial AISI M2, AISI T15, VWM3C and Sinter 23 high speed steels that were austenitized and quenched from five distinct temperatures. This study shows that increase in quenching temperature results in grain growth of steels such as AISI M2 and VWM3C, obtained by the conventional method (cast to ingot and worked). The P/M Sinter 23 high speed steel showed a slight grain growth (about 10%). This effect was not observed in AISI T15 obtained by the powder metallurgy process.

Abstract: Titanium foams have been used for surgical implants and biomedical engineering because they exhibit inert behavior and good corrosion resistance. Substantial progress has been achieved for metallic foam fabrication techniques, however the porosity characterization methods available haven’t been studied sufficiently. A previous research has developed a powder metallurgy route to produce pure titanium foams attaining the porosity requisites for porous surfaced surgical implants. In this study, titanium foams porosity was evaluated employing different techniques: optical quantitative metallographic analysis with automatic image technique, gamma-ray transmission and x-ray microtomography. These techniques can be used for titanium foams analysis, though their results can not be simply compared, because they use quite different methodologies and take different measurement assumptions.

Abstract: Knowledge of particle concentration is of extreme importance for the settling phenomenon. For well-diluted suspensions the solids concentration can be measured by sampling techniques. However, the use of such a technique for more concentrated suspension does not lead to good results. Knowledge of concentration distribution in sediments formed by decantation of aqueous suspensions inside a vertical vessel is fundamental for the evaluation of performance of some pieces of equipment such as continuous thickeners. In this work, the phenomenon of batch settling of aqueous suspensions of calcium carbonate was studied by using a nondestructive technique based upon the measurement of gamma-ray attenuation when the radiation beam goes through the physical medium as a function of the local concentration in several vertical positions of the vessel, and the sampling of suspension by aliquots is not necessary. The main objective of this work is to evaluate the use of gammaray attenuation technique for achieving concentration distributions in the phenomenon of batch settling as well as curves of iso-concentrations.

Abstract: The dynamic analysis of behavior of solid particles in porous media such as settling processes are important for the dimensioning increasingly precise of pieces of equipment that promote the solid-liquid separation. Several factors can influence in dynamics of the fall of solid particles into a fluid medium; among them, the shape, distribution of sizes, and particle density. The main objective of this work is to analyze the behavior of solid-solid system based on the shape, size distribution and density of solid particles. The initial settling velocity in batch settling tests and the accommodation of particles in the sediment formed are evaluated for different materials. The gamma-ray attenuation technique was used in the experimental tests. Comparison of results obtained allowed evaluating in an exploratory research the significance of variables involved.

Abstract: The open photoacoustic cell technique (OPC) was carried out in order to measure the thermal diffusivity of hardmetal. Hardmetal is usually processed by the conventional powder technology techniques: mix of WC + Co powders, compacting, and liquid phase sintering A new method to process hardmetal parts is hereby described. Parts of WC-15%wt Co were processed by using high pressure – high temperature sintering. It was used the pressure of 5GPa, temperature of 1350 oC, and time of 2 minutes of sintering. In addition to the thermal diffusivity, heat capacity was considered and the thermal conductivity achieved. Results matched with the values of the literatures where others photoacoustic techniques have been employed. It was achieved a thermal capacity of 3.34J/cm3K, thermal diffusivity of 0.35cm2/s, and thermal conductivity of 116.9W/mK. It reveals that the HPHT processed WC- 15%wtCo hardmetal is able to work as a cutting tool, in the thermal point of view.

Abstract: The aim of the work was to prepare an overview about the microstructures present in high-speed steel, focused on the crystallography of the carbides. High-speed steels are currently obtained by casting, powder metallurgy and more recently spray forming. High-speed steels have a high hardness resulting from a microstructure, which consists of a steel matrix (martensite and ferrite), in which embedded carbides of different crystal structure, chemical composition, morphology and size, exist. These carbides are commonly named MxC, where M represents one or more metallic atoms. These carbides can be identified by X-ray diffraction considering M as a unique metallic atom. In this work, it is discussed, in basis of the first principles of physics crystallography, the validation of this identification when it is considered that other atoms in the structure are substitutional. Further, it is discussed some requirements for data acquisition that allows the Rietveld refinement to be applied on carbide crystallography and phase amount determination.

Abstract: NiTi alloys with equiatomic composition of NiTi have the highest technological interest for its potencial application in differents areas such as biomedical, naval, aerospace, nuclear, automobilist , robotic,etc. In this work , it was used a 50Ni50Ti at % powder mixture, comercially pure, prepared by mechanical alloying in a Attritor with the following conditions: the milling speed and the ball charge were 1500 rpm and 10:1 respectively. The milling time was 2,4,8 and 16h, under an argon atmosphere at room temperature. After milling it was determined the particle size distribution, the phases by X-ray diffractions (XRD) and the powder morphology by scanning electron microscopy (SEM). The milling promotes dissolution of Titanium in Nickel and continuous amorphization by increasing the milling time. After 16h milling the alloy was almost amorphous. The powders after milling were compacted and heat treated at high temperature and microstructural evolution was characterized. In the heat treated samples were detected different phases showing heterogeneity in the alloy. The detected phases were Ni3Ti, NiTi, Ni2Ti and Ni2Ti4O. Contamination by milling was detected in the powder after milling and in the heat treated samples.

Abstract: This paper presents a new method to separate interfering spectra obtained in wavelength dispersive x-ray fluorescence spectrometry (WDXRF). This method permits improved results to be obtained, compared to conventional analytical results and enables the determination of chemical species of the same element without chemical separation. This is done by separation of the spectra due to electronic transitions of the valence electrons. The Rietveld method [11] overcomes the problem of superimposed peaks of the species present in the specimen and simultaneously enables determination of the species and does not require standard specimens and calibration curves. This signifies a marked improvement in comparison to other techniques. Specimen surface preparation to obtain spectra is a critical stage and its effects can be minimized by using Rietveld refinement, which permits the determination of intensity relationships of superimposed peaks with the aid of mathematical models. This establishes the basic conditions to obtain more accurate results in quantitative analysis. In the determination of chemical species, it is possible to separate, for example, Cr (III) and Cr (IV), with almost 100% superimposition.