Defect and Diffusion Forum Vols. 289-292

Abstract: Many studies have emphasized the beneficial effect of niobium on the physical metallurgy of Ni-Cr-Fe alloy 718. Among the different strengthening actions of niobium, such as solid solution hardening and carbide precipitation, the precipitation of niobium with nickel in a strengthening phase γ” (Ni3Nb) during the aging heat treatment has the largest influence on the mechanical properties of alloy 718. The improvement of the niobium distribution and diffusion in the Ni-matrix may allow a more homogenized repartition of γ” precipitates and seems then to be an effective way to upgrade the mechanical properties. As γ” precipitates decompose to the stable δ phase at very long aging times, the study of the effect of carbon, nitrogen and oxygen concentrations on precipitation and dissolution of the δ phase may give information on γ” precipitation and on niobium distribution. It is the purpose of the present work to examine the role that the alloy content of interstitial species plays with the niobium-rich δ phase evolution in alloy 718. Alloy 718 samples were heat treated under hydrogenated argon at 980°C for 0 to 96 hours in order to gradually curb the content of interstitial species by reaction with the reducing atmosphere. Chemical analyses realized by glow discharge mass spectrometry (GDMS) confirmed the reduction of the concentration of these species. Specimens were solution-treated for 1h at 1050°C in an inert atmosphere and furnace cooled. Some of the samples were then aged at 920°C for times ranging from 10 min to 1 hour. The precipitation was measured quantitatively in terms of volume fraction and the morphology of the precipitates was appreciated using scanning electron microscopy (SEM). The differences in the precipitation kinetics and in the microstructure evolution for each interstitial concentration are then discussed.

Abstract: Mg-rich alloys of the binary system Mg-Ni are prospective hydrogen-storage materials. In the present study, desorption characteristics of hydrided Mg2Ni intermetalic and hydrided Mg/Mg2Ni eutectic mixture were investigated. Structure of experimental materials during the hydrogenation was observed by SEM. Three modifications of (Mg2Ni)Hx (x ~ 4) were prepared differing in the ratio of two low-temperature phases f = LT2/LT1: with (i) f >1, (ii) f ~ 1 and with (iii) f <1. Evolution of the ratio f during hydrogen desorption was checked by XRD. It was found that the micro-twinned phase LT2 is not desirable in hydrogen-storage materials containing Mg2Ni intermetallic. Diffusion coefficient of hydrogen in LT2 is about 20 times lower than in LT1.

Abstract: Two austenitic stainless steels have been implanted at 150 keV with Mo at a fluence of 3.5x1015 ions/cm2. The effects of ion implantation in the chemical composition of the passive films was evaluated by x-ray photoelectron spectroscopy (XPS) and glancing angle x-ray diffraction (GAXRD) was used to determine the induced structural modifications. The results of the pitting corrosion studies carried out in neutral chloride medium as well as the morphology of the localized attack are discussed.

Abstract: Removal of fine particles from some gas-product effluents from motors and industries, using filters, is an important subject in the field of public health and environment. In the present work, a porous silicon filter was produced, which is able to capture most of the particles undesirable for the environment (transported by gases), larger than the pore diameter (micrometer) of the filter and even smaller size particles. The development of whiskers inside of the pores of the silicon filter, improve its ability to catch smaller particles than the filter’s size pores. Those whiskers are made of Silicon Nitride, produced by a Nitridation process. A different time-temperature schedule for the formation of -silicon nitride (-Si3N4) whiskers by direct Nitridation of the porous silicon filter was studied, in order to optimize the amount of whiskers and improve the filter quality. Four different temperatures (1000, 1100, 1200 and 1300 °C) were selected, each with two different holding times (15 min and 1 hour) for complete Nitridation with N2 and N2+H2 gases. The as-formed whiskers were characterized by SEM, XRD techniques and the process conditions were studied. The filter with the Si3N4 whiskers was characterized evaluating mechanical properties of the porous silicon filter (Micro Hardness and Young Modulus). The permeability measurements were made before and after the Nitridation process. Analysis indicates that the higher Si3N4 whiskers formation temperature was 1300 °C for the gas (N2+H2) phase reaction results from the lower PSiO2/Psio ratio in the Si-N system. Titanium (99% pure) was used with the purpose of reduction of the oxygen partial pressure and the increase of the amount of -silicon nitride whiskers. The porous silicon filter improved its conditions with the silicon nitride whiskers, even though decreases also the fluid permeability measurement. However, it has a smaller flow decrement than filters with smaller porosity. The mechanical properties did not have variation at all, the porosity size increased because of the diffusion of Si to form whiskers in the Nitridation process.

Abstract: Non-commercial, 434L ferritic stainless steel powders prealloyed with 2% of Si were uniaxially compacted at 700 MPa and sintered in 100% H2 atmosphere for 30 min at three different temperatures (1225, 1250 and 1275 °C). For comparison, standard, commercial 434L powders were sintered in the same conditions, and 434L+2%Si powders were also sintered in vacuum. The porosity and grain size of the 9 sintered stainless steels were measured by image analysis methods. Tensile strength and Vickers hardness tests show that the steels with 2% of Si exhibit higher mechanical properties. Oxidation tests were carried out in air at 800 and 900 °C. H2-sintered 434L steels prealloyed with Si exhibit higher weight gains per unit of apparent surface than 434L steels sintered in the same conditions, due to the higher porosity of the former. The sintering atmosphere affects the nature and morphology of the scales observed by XRD and SEM. Vacuum sintering of 434L+2%Si stainless steels promotes the formation of more protective scales and lower mass gains (for identical porosity volume) than H2 sintering.

Abstract: Hydrogen embrittlement is believed to be one of the main reasons for cracking of structures under stress. High strength steels in these structures often include a ferritic core made of alpha-iron (body centered cubic lattice). We compute the interaction of atomic hydrogen with iron using first principles. The interstitial hydrogen can be placed in two high symmetry positions: octahedral and tetrahedral sites. Our calculations provide diffusion barriers between these sites. These barriers have been analyzed to understand the propagation of hydrogen through the iron lattice. We analyze how these barriers can be modified by the hydrogen concentration. The results show the main site for high and low hydrogen density and they show the diffusion coefficient variation by the hydrogen density.

Abstract: Grain boundary (GB) diffusion of 59Fe in high purity polycrystalline copper was measured using the radiotracer technique and precision parallel sectioning in an extended temperature interval. The results of diffusion measurements below 900 K are consistent with Harrison's C kinetics and yield the GB diffusion coefficient of Fe in Cu with the pre-exponential factor of 5.610-6 m2/s and the activation enthalpy of 121 kJ/mol. Unexpectedly strong GB segregation of Fe in Cu hindered reliable determination of the Fe diffusivity in the B kinetics. Additionally, unconventional penetration profiles were measured for GB diffusion of 59Fe in Cu at high temperatures. Molecular dynamics simulation with the literature Finnis-Sinclair type interatomic potentials was performed to shed light into the observed features. A strong effect of the Fe coverage on GB structure and kinetics in pure Cu is predicted at increased temperatures above 900 K.

Abstract: In rolling Industries, cold rolls are chosen of low alloy steel heat treated to about 60 RC in 50 millimeter depth on surface. Troostite phase which happens in some cases in tempered martensite has lower fracture toughness, although its ductility is a little more. In rolling process, some technological problems seldom take place, and the roll might be locally heated accidentally. In cold rolling technology, this effect may cause a phase change in metallurgical structure on the surface of the cold work forged steel rolls, named Troostite. However, the rolls which subjected to this problem have no mechanical defect to be detected by NDT, but these kinds of affected rolls may be failed in working (rolling) by fracture, spalling or cracks. So Troostite is a harmful phase for rolls and causes lots of losses for rolling companies. The following paper is a practical industrial approach to show and follow thermal instability of roll surface and metallurgical change of the surface from sorbite to troostite by diffusion mechanism, due to any improper mill usage.

Abstract: In this work, β-NiAl aluminide coatings (cubic B2 structure) deposited on a DS substrate have been isothermally as well as cyclically oxidised at 1100°C for up to 240 h to study the diffusion mechanisms associated with the growth of the oxide scales. A 24 h cycle has been shown to promote enhanced Al depletion, thus requiring a sufficient Al flux to maintain a protective oxide scale. Glancing incidence X-ray diffraction (GI-XRD) combined to electron microscopy (FEG-ESEM / EDS) has been carried out to characterize the evolution of the phases induced by the progressive Al depletion into the coating. The results show that upon cycling, specimens undergo significant oxide scale spallation and increased roughness that can be ascribed to both the growth stresses and the phase transformation contribution whereas the coating has barely evolved after 240 h of isothermal exposure. In particular, the martensitic transformation (tetragonal L10 structure) that accompanies thermal cycling was found to be much more significant than the evolution of the γ’-Ni3Al (cubic L12 structure) phase over the same thermal cycle and therefore the B2 to martensite transformation could originate the progressive roughening of the surface. Conversely, upon isothermal exposure, the coating exhibited a typical alumina scale with almost no spallation and the appearance of rumples.

Abstract: Among the new environmentally friendly alternatives to improve the corrosion resistance of metallic substrates, cerium oxide based electrodeposited layers have raised particular attention. In this work, various electrodeposited layers on zinc substrate have been elaborated at different conditions to produce tailored compositions, microstructures and different degrees of porosity. The use of electrochemical techniques (polarisation resistance measurements, electrochemical impedance spectroscopy) has allowed to ascertain the porosity, which coupled to Scanning Electron Microscopy (FEG-SEM) and Energy Dispersive Spectrometry (EDS) studies along the cross sections have revealed the inward and/or outward diffusion of the different elements through the film till the metallic surface. The resulting corrosion mechanisms of these coatings immersed in 3% NaCl are finally discussed.