Hélio Goldenstein

Porosity dependence of elastic constants in aluminum nitride ceramics

Aluminum nitride is a relatively new engineering ceramic and its mechanical properties have not been extensively studied. In this work, the effects of porosity (~0 to 38%) on sonic velocities, Poissons ratio, and elastic moduli of partially sintered A1N, both pure and with additives, were investigated. The elastic constants, determined by the ultrasonic pulse-echo method, were significantly influenced by the porosity, but not by the different types of additives used (2 wt. (%) of CaCO3, CaO, and Y2O3). All elastic constants evaluated decreased with increasing porosity, but the rates of decrease were higher for elastic moduli compared do sonic velocities and Poissons ratio. The results were analyzed in the light of stress concentration and loading bearing area models proposed in the literature.

Three-body abrasion of Al–SiC composites

Abstract While many works have been focused on the two-body abrasive wear of aluminum matrix composites, there are relatively few results on the three-body abrasion performance of these composites. In the present study, the three-body abrasion of aluminum matrix composites reinforced with silicon carbide particles (SiC p ) has been investigated. The metal matrix composites (MMCs) were fabricated by a powder metallurgy route involving a final hot extrusion step, with Al 1100 matrix and α-SiC p reinforcement with mean sizes of 10, 27 and 43 μm, in the proportions of 5, 10 and 20 vol.%. Using a wet monolayer tester, three-body abrasive wear tests were conducted under a constant load against silicon carbide and alumina abrasives with four different grits of 320, 400, 600 and 1000. The microstructural characterizations were performed using light microscopy. The dominant wear mechanisms were identified using scanning electron microscopy. The influence of type of the abrasive particles on wear rate and dominating wear mechanism is reported. Relationships between size and volume fraction of the SiC p reinforcement and wear rate is discussed. It is shown that SiC p reinforcement increases the abrasion resistance against all the abrasives used. This increase is generally higher against alumina than silicon carbide abrasives.

EMBEDDED ATOM COMPUTER SIMULATION OF LATTICE DISTORTION AND DISLOCATION CORE STRUCTURE AND MOBILITY IN Fe-Cr ALLOYS

Abstract The atomistic structure of dislocation cores of 〈111〉 screw dislocations in disordered FeCr b.c.c. alloys was simulated using embedded atom method potentials and molecular statics computer simulation. The mixed FeCr interatomic potentials used were derived by fitting to the thermodynamic data of the disordered system and the measured lattice parameter changes of Fe upon Cr additions. The potentials predict phase separation as the most stable configuration for the central region of the phase diagram. The next most stable situation is the disordered b.c.c. phase. The structure of the screw 1 2 〈111〉 dislocation core was studied using atomistic computer simulation and an improved visualization method for the representation of the resulting structures. The structure of the dislocation core is different from that typical of 1 2 〈111〉 dislocations in pure b.c.c. materials. The core structure in the alloy tends to lose the threefold symmetry seen in pure b.c.c. materials and the stress necessary to initiate dislocation motion increases with Cr content. The mobility of kinks in these screw dislocations was also simulated and it was found that while the critical stress for kink motion in pure Fe is extremely low, it increases significantly with the addition of Cr. The implications of these differences for mechanical behavior are discussed.

Abrasion and corrosion resistance of new Ni-based coating deposited by HVOF thermal spray process

Abstract Coatings based on NiCrAlC intermetallic based alloy were applied on AISI 316L stainless steel substrates using a high velocity oxygen fuel torch. The influence of the spray parameters on friction and abrasive wear resistance were investigated using an instrumented rubber wheel abrasion test, able to measure the friction forces. The corrosion behaviour of the coatings were studied with electrochemical techniques and compared with the corrosion resistance of the substrate material. Specimens prepared using lower O2/C3H8 ratios showed smaller porosity values. The abrasion wear rate of the NiCrAlC coatings was much smaller than that described in the literature for bulk as cast materials with similar composition and one order of magnitude higher than bulk cast and heat treated (aged) NiCrAlC alloy. All coatings showed higher corrosion resistance than the AISI 316L substrate in HCl (5%) aqueous solution at 40°C.

Atomistic structure of the coherent NiNi3Al interface

Most Ni-based superalloys are strengthened by the presence of coherent precipitates of an ordered fcc bases phase, known as [gamma][prime]. This phase is basically Ni[sub 3]Al. The precipitates are coherent up to a certain size and they present a cubic shape with faces oriented in the (100) planes of both matrix and precipitate. The detailed atomistic structure of this interface has not been studied. Interest in the use of ordered intermetallic compounds as possible structural materials has resulted in a large amount of work in Ni[sub 3]Al and in particular, the development of interatomic potentials for the Ni-Al system using the embedded atom technique. These potentials have been employed in the simulation of a variety of defects in Ni[sub 3]Al, including dislocation cores, grain boundaries and free surfaces. However, there is no simulation of the Ni/Ni[sub 3]Al interface structure using the embedded atom method. The objective of the present work is to carry out such a simulation. Besides the practical importance of the interface in superalloys, it is the simplest type of interface that can be modeled and it is a good starting point for interface work using the embedded atom technique.

Aluminium Matrix Composites Reinforced with Co-continuous Interlaced Phases Aluminium-alumina Needles

An Al-5SiO2 (5 wt% of SiO2) aluminium matrix fiber composite was produced where the reinforcement consists of fossil silica fibers needles. After being heat-treated at 600 °C, the original fiber morphology was retained but its microstructure changed from solid silica to an interconnected (Al-Si)/Al2O3 interlaced structure named co-continuous composite. A technique of powder metallurgy, using commercial aluminium powder and the silica fibers as starting materials, followed by hot extrusion, was used to produce the composite. The co-continuous microstructure was obtained partially or totally on the fibers as a result of the reaction, which occurs during the heat treatment, first by solid diffusion and finally by the liquid Al-Si in local equilibrium, formed with the silicon released by reaction. The internal structure of the fibers was characterized using field emission electron microscope (FEG-SEM) and optical microscopy on polished and fractured samples.

Quenching and partitioning heat treatment in ductile cast irons

A commercial ductile iron alloy was submitted to a quenching and partitioning heat treatment. Samples were austenitized at 900 oC for two hours, quenched at 160 oC and kept at this temperature for 2 minutes and finally were re-heated at temperatures between 300 and 450 oC during time intervals between 2 and 180 minutes. The microstructural evaluation was performed with SEM and X-ray diffraction and the mechanical properties were measured using tensile strength and Charpy tests. In general, the quenching and partitioning treatment is viable to achieving expressive fractions of retained austenite in ductile cast irons. Generally, higher partitioning temperatures produce a higher fraction of retained austenite after shorter times. This behavior can be explained by the increase on diffusion rate of carbon at higher temperatures. For all tested conditions it was possible to see a well-defined process window and the combination of mechanical properties is very similar to the austempered ductile irons.

Magnetic and spontaneous Barkhausen noise techniques used in investigation of a martensitic transformation

Magnetic Barkhausen noise (MBN) was used to characterize the progress of austenite to martensite phase transformation while cooling steel specimens, using a conventional Barkhausen noise emission setup stimulated by an alternating magnetic field. The phase transformation was also followed by electrical resistivity measurements and by optical and scanning electron microscopy. MBN measurements on a AISI D2 tool steel austenitized at 1473 K and cooled to liquid nitrogen temperature presented a clear change near 225 K during cooling, corresponding to the MS (martensite start) temperature, as confirmed by resistivity measurements. Analysis of the resulting signals suggested a novel experimental technique that measures spontaneous magnetic emission during transformation, in the absence of any external field. Spontaneous magnetic noise emission measurements were registered in situ while cooling an initially austenitic sample in liquid nitrogen, showing that local microstructural changes, corresponding to an aval...

Magnetic Barkhausen Noise in Quenched Carburized Nickel-Steels

Two steel sheets, one with 5% Ni and another with 10% Ni, were submitted to carburization and quenching, obtaining a microstructure with martensite and retained austenite. These steels were characterized with magnetic Barkhausen noise (MBN). The Barkhausen signal is distinctively different for the carburized and quenched samples. The carburized and quenched samples present higher coercive field than the annealed samples. X-ray diffraction data indicated that the carburized and quenched samples have high density of dislocations, a consequence of the martensitic transformation.

Comparison of the Magnetic Barkhausen Noise for Low Carbon Steel in Deformed and Annealed Conditions

The magnetic Barkhausen noise (MBN) in low carbon steel (0.034%C) was examined for two situations: after deformation and after recrystalization. The steel sheets previously received 50% of cold rolling and were heat treated for time periods between 15 to 540 min, originating different grain sizes. It was found that the maximum amplitude of MBN decreases with annealing time. Electron back scattered diffraction (EBSD) shows that significant grain fragmentation occurred due to plastic deformation. The results are discussed with help of the Two Gaussian model for the MBN envelope. The major contribution for MBN in the deformed sample was elimination of 90° closure domains. Displacement of 180° domain walls was limited in the deformed sample, due to the grain fragmentation.