Materials Science Forum Vols. 587-588

Abstract: On this paper, a study that evaluates the influence of some variables on the mechanical properties of the vertical centrifugal casting is made. It is emphasized the fact that the centrifugal effect from vertical centrifugal casting brings special features on mechanical properties. It has been observed that the centrifugal effect may substantially increase, in some alloys, the rupture strength, rupture strain, and Young modulus, as compared to the gravity casting technique. When compared to gravity casting, the centrifugal casting process, besides the centrifugal force (pressure effect), always has an inherent associated vibration during the casting. In this study, as an attempt to isolate the vibration effect from the overall centrifugal effect, tests on castings obtained by vibrating gravity casting process are made. A comparison between castings obtained by centrifugal casting technique, vibrating casting technique and gravity casting technique is made in order to fully understand the features that allow the improvement on mechanical properties during the vertical centrifugal casting technique. An analysis of the most important effects, on both mechanical properties and on some metallurgical features is made.

Abstract: The Incremental Melting and Solidification Process (IMSP) is a relatively new field for material processing for the production of functionally graded materials. In this process a controlled liquid bath is maintained at the top of the component where new materials are added changing the components composition. Thus, a functionally graded material is obtained with a varying composition along one direction of the component. This paper deals with the influence of one of the process parameters, namely displacement rates between heating coil and mould, in order to evaluate its influence on both metallurgical and mechanical properties of different Al-Si alloys. Hardness and phase distribution, along the main castings axis, were measured. To better assess and characterize the process, two different Al-Si alloys with and without variation of chemical composition along the specimen were analysed. Results demonstrate that a gradual variation of metallurgical and mechanical properties along the component is obtained. It is also shown that Al-Si functionally graded materials can be produced by the incremental melting and solidification process. Results show that the displacement rate is very important on metallurgical and mechanical properties of the obtained alloy.

Abstract: Aluminum alloys have found many applications in different branches of industry. In spite of the valuable properties, there is a significant drawback because of the strong corrosion susceptibility, especially in chloride-containing medium. The present work is focused on study of the 2024 aluminum alloy corrosion mechanism on early stages using Scanning Kelvin Probe Force Microscopy (SKPFM). The corrosion impact was studied measuring the Volta potential (VP) and topography of alloy matrix and S-phase intermetallics after immersion in different electrolytes and pH. It is shown that presence of the chloride anions in the electrolyte leads to increase of aluminum matrix potential for about 100 mV. This can be resulted from the adsorption of chloride ions and their incorporation into the native oxide layer changing semiconductive properties of the oxide. The zones surrounding the S-phase intermetallics are changed more significantly demonstrating higher increase of VP close to the inclusion. These regions are correlated with the increased oxygen content suggesting formation of thicker oxide layer due to local polarization. Addition of an inhibitor to electrolyte also leads to change in Volta potential that is reflected on lower corrosion impact of aggressive environment.

Abstract: A β-FeSi2 sample was ball-milled for different periods in a vibratory ball-mill and studied by X-ray diffraction and Mössbauer spectroscopy. It transforms gradually with milling time into an α-FeSi2 phase.

Abstract: Al 2024-T3 is an important alloy very prone to localized corrosion. In this investigation the effect of chloride concentration on the corrosion resistance of the Al 2024-T3 alloy has been studied, focusing on the dissolution of the intermetallics (IMCs). Sodium chloride solutions of two concentrations, 0.6 M and 0.01mM, were used as test electrolytes. During the investigation selected regions of polished samples had their IMCs analyzed by Scanning Electron Microscopy (SEM) and X-ray Energy Analysis (EDX) prior and after different immersion times in the two test electrolytes. The results showed that even in the lowest chloride concentration the electrolyte was highly corrosive to the Al-Cu-Mg IMCs leading to their partial dissolution and to the attack of the surrounding matrix after only one hour of immersion. On the other hand, the corrosion behavior of the Al-Cu-Mn-Fe IMCs was random, and no correlation could be established between corrosive attack and chloride concentration or time of immersion for this type of particle. Atomic Force Microscopy (AFM) analyses have indicated a stronger dissolution of the matrix in the more concentrated electrolyte. This seems to lead to a milder attack of the IMCs in this solution when compared to the less concentrated one, as indicated in the SEM images.

Abstract: Stacking fault energy (SFE) plays an important role in face centred cubic (f.c.c.) metals and alloys in determining the prevailing mechanisms of plastic deformation. Low SFE metals and alloys have a tendency to develop mechanical twinning, besides dislocation slip, during plastic deformations. Deformation behaviour and microstructure evolution under simple and complex strain paths were studied in 70/30 brass, with small and intermediate grain sizes, which corresponds to a f.c.c. material with low SFE. Simple (rolling and tension) and complex (tension normal to previous rolling) strain paths were performed. The macroscopic deformation behaviour of materials studied is discussed in terms of equivalent true stress vs. equivalent true strain responses and strain hardening rates normalized by shear modulus (dσ/dε)/G as vs. (σ – σ0)/G (σ0 is the initial yield stress of the material and G is the shear modulus). The mechanical behaviour is discussed with respect to dislocation and twin microstructure evolution developed in both, simple and complex strain paths.

Abstract: A detail study focussing the microstructural evolution of the interfacial zone in the course of the processing of Ti-47Al-2Cr-2Nb joints using Tini 67 as filler alloy was carried out in this investigation. Experiments, aiming the understanding of the mechanisms that promote the melting of the braze alloy, were performed below the solidus temperature of the filler, at 750 and 900°C. Diffusion brazed samples were joined at 1000 and 1100°C, with no dwelling stage and subsequently quenched in water in order to frozen the microstructure formed at the bonding temperature. The interfaces were analysed by scanning electron microscopy (SEM) and by energy dispersive X-ray spectroscopy (EDS), respectively. In the course of the heating stage, several single phase layers were formed within the filler alloy due to the solid state interdiffusion of Ti and Ni atoms. At 900°C, the microstructure of the filler evolved form the initial Ti (α)/(Ni)/Ti/ (α) layers to a Ti (β)/Ti2Ni/TiNi/TiNi3/TiNi/Ti2Ni/Ti (β) layered microstructure. The filler alloy begun to melt due to the eutectic reaction between the contiguous layers composed of Ti (β) and Ti2Ni. After joining, the main phases detected at the interfaces were α2-Ti3Al, Ti-Ni-Al and Ti-Ni intermetallics. For joining at 1000°C, a substantial amount of residual filler (Ti2Ni and Ti (α) particles) was also detected at the central zone of the interface. No marked evidences of residual filler zones were noticed for joining at 1100°C; instead, a mixture α2-Ti3Al with Ti-Ni-Al or Ti2Ni intermetallics was detected at the centre of the interface.

Abstract: A study on the mechanical characterization of friction stir welds between aluminium alloys 6061-T6 and 6082-T6 was carried out. For comparison, single alloy joints made from each one of the two alloys were also performed. The work included microstructure examination, microhardness tests, tensile tests and bending tests of all joint types. An approximate finite element model of the joint, taking into account the spatial dependence of the tensile strength properties, was made, modelling a bending test of the weldments.

Abstract: The aim of this study is to determine the thermodynamic influence of the presence of Cu3SbS3, Cu2S and Sb2S3 in the formation of Cu12Sb4S13 - equivalent to mineral tetrahedrite. Thus, the Cu-Sb-S system is studied. Twenty three samples, with compositions that essentially lay along the vertical section Cu2S-Sb2S3, were prepared. The samples were analyzed by Electron Microprobe (EPMA) in order to determine the room temperature phases' composition. Samples were also analyzed by Differential Thermal Analysis (DTA/DSC) in order to establish thermal transitions, and by X-Ray Diffraction (XRD), at room and high temperatures, so as to determine the phases that are present in the equilibria at certain temperatures. The experimental phase diagram was established and the results were compared with those available in the literature.

Abstract: Copper has widespread use as engineering material, because of its structural and functional properties, notably high thermal and electrical conductivity. A major drawback of this base metal and its alloys is a relatively low hardness. This precludes its utilization in applications in which both high conductivity and high strength/hardness are needed, e.g. in injection moulds for plastics. Nanostructured metals and nanocomposites are ways to address the low hardness problem, provided the nanostructured material is thermally stable during processing and service. In the present research, composite powders, with 5 to 30 at % nanodiamond, were consolidated into bulk samples. The copper-nanodiamond composite powders were vacuum encapsulated and extruded at 600°C. A significant proportion of the initial hardness in the powders is retained after extrusion. Transmission electron microscopy (TEM) of the extruded material indicates good bonding between the nanodiamond particles and the copper matrix. Raman spectroscopy on the consolidated samples evidences the presence of graphite, possibly due to partial disintegration of ultradisperse nanodiamond agglomerates.