Materials Science Forum Vols. 591-593

Abstract: The effect of high energy milling on powders of a FeNi (50/50) alloy and a 316L stainless steel has been evaluated by means of X-Ray Diffraction (XRD). The average microstrain as function of the milling time (1/2h, 1h and 8h) was determined from XRD data. The displacement and broadening of the (XRD) peaks were used for estimate the stacking fault energy (SFE), using the method of Reed and Schramm. It was estimated SFE=79 mJ/m2 for the FeNi (50/50) alloy and SFE=14 mJ/m2 for the 316L stainless steel. The better experimental conditions for determining the SFE by XRD are discussed.

Abstract: It is discussed the difference between the magnetic reversal mechanisms: i) coherent rotation, ii) nucleation, iii) unpinning of domain walls. The main argument to suggest pinning as the coercivity mechanism of Sm(CoFeCuZr)z magnets is the low initial susceptibility of magnetization curves of thermally demagnetized magnets. However, coherent rotation also implies in low initial susceptibility, since the grain size of the magnets is near the single domain particle size. It is unlikely that pinning could be the coercivity mechanism in this case, since the anisotropy field of Sm2Co17 phase is 65 kOe, whereas the coercivity of magnets can be higher than 40 kOe. Such coercive field of 60% of the anisotropy field indicates coherent rotation as mechanism. A model for describing the abnormal coercivity behavior in Sm(CoFeCuZr)z magnets is proposed.

Abstract: ZnO has received great attention in many applications due to its electronic and optical properties. We report on the preparation of ZnO and gallium-containing ZnO (ZnO:Ga) nanoparticles by the precipitation method. The nanoparticles have the wurtzite structure and a high crystallinity. Gallium ions are present as Ga3+, as evidenced by the binding energies through XPS. Porosity and surface area of the powder increased under increasing gallium level, explained by the smaller particle size of ZnO:Ga samples compared with ZnO. The estimated optical band gap of ZnO was 3.2 eV, comparable to ZnO:Ga.

Abstract: The corrosion resistance of 17-4PH stainless steel obtained by powder injection molding (PIM) was investigated in a phosphate buffer solution (PBS) that simulates physiological solution and compared with that of 17-4PH steel obtained by conventional metallurgy. The corrosion resistance was investigated by electrochemical techniques for different immersion times. The cytotoxicity of both types of 17-4PH steel was also investigated using a minimum Eagle’s medium (MEM). The MEM solution is a type of cell culture medium, which simulates physiological fluids. The cytotoxicity assay was carried out by neutral red uptake methodology utilizing NCTC L929 cell line from ATCC bank and none of the steels showed cytotoxic effects. The resulting extracts obtained by immersion of the steel samples in MEM were analyzed by neutron activation analysis and the results indicated liberation of chromium and cobalt as corrosion products but in very low amounts. The electrochemical evaluation of both steels indicated that they are passive in PBS but presented susceptibility to pitting. The 17-4PH PIM steel was slightly more susceptible to pitting than that fabricated by conventional metallurgy due to its inherent porosity.

Abstract: The use of titanium and its alloy as biomaterial is increasing due to their low modulus, superior biocompatibility and enhanced corrosion resistance when compared to more conventional stainless steel and cobalt-based alloys. Ti-13Nb-13Zr is a titanium alloy specifically developed for surgical implants. In this work, highly porous titanium foams, with porosities above from 50%, are reached using an efficient powder metallurgical process, which includes the introduction of a selected spacer into the starting powders. Samples were produced by mixing of initial metallic powders followed by uniaxial and cold isostatic pressing with subsequent densification by sintering. The samples presented a Widmanstättenlike microstructure in an open cellular morphology with pore size between 200-500 μm.

Abstract: Thermomechanical and electrical properties of zirconia-based ceramics have led to a wide range of advanced and engineering ceramic applications like solid electrolyte in oxygen sensors, fuel cells and furnace elements and its low thermal conductivity has allowed its use for thermal barrier coatings for aerospace engine components. A comparison between CoNiCrAlY bond coat and zirconia plasma sprayed coatings on creep tests of the Ti-6Al-4V alloy was studied. The material used was commercial Ti-6Al-4V alloy. 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. Constant load creep tests were conducted on a standard creep machine in air on coated samples, at stress levels of 520 MPa at 500°C to evaluate the oxidation protection on creep of the Ti-6Al-4V alloy. Results indicate that the creep resistance of the ceramic coating was greater than metallic coating.

Abstract: Porosity and pore size are critical features for biomaterial scaffolds as they play an essential role in bone formation and bone ingrowth in vivo. Therefore, techniques for scaffolds evaluation are of great importance for their design and processing. Porous titanium has been used for grafts and implant coatings as it allows the mechanical interlocking of the pores and bone. In this study, porous titanium samples were manufactured by powder metallurgy. The porosity quantification was assessed by optical quantitative metallographic analysis, and non-destructive gamma-ray transmission and X-ray microtomography techniques, in order to compare their efficacy for porosity evaluation. Pore morphology and surface topography were characterized via scanning electron microscopy. These techniques have demonstrated to be suitable for titanium scaffolds evaluation, and micro-CT was the one that allowed the three-dimensional porosity assessment.

Abstract: Fine magnetic powder has been produced using the hydrogenation disproportionation desorption and recombination (HDDR) process. The first goal of this work involved an investigation of a range of disproportionation/desorption temperatures between 800 and 900°C with the purpose of optimizing the HDDR treatment for a Pr14Fe80B6 alloy. The cast alloy was annealed at 1100°C for 20 hours for homogenization. The optimum disproportionation temperature for achieving high anisotropy was 820°C. The influence of the reaction temperature on the microstructure and magnetic properties of Pr14Fe80B6 HDDR powders and magnets has been shown. A second stage of this study involved the characterization, for each temperature, of the HDDR processed powder using X-ray diffraction analysis. Samples of the HDDR material have been studied by synchrotron radiation powder diffraction using the Rietveld method for cell refinement, phase quantification and crystallite sizes determination. Scanning electron microscopy (SEM) has also been employed to reveal the morphology of the HDDR powder.