Digital image correlation techniques (DICT), a non-contact deformation measuring technique based on gray value digital images, have become increasingly used over the last years. By using the DIC technique during a tensile test, the deformation behavior of different engineering material under an applied load can be determined and analyzed. Digital images, acquired from a tensile test, can be correlated by using DICT software and from that the local or global mechanical properties can be calculated. The local or global mechanical properties determination of a flat test specimens are based on the displacements or changes in a previous stochastic sprayed or natural pattern. The used material for this purpose is cast silicon (Si) based aluminium (Al) component, designated as AlSi7Mg0.3 (Anticorodal-78 dv). The hypoeutectic Al- Si alloy is widely applicable for engine constructions, vehicle and aerospace constructions, shipbuilding, electrical engineering and constructions for food industry. There are many microstructural parameters in a binary system Al- Si alloys, which the mechanical properties can be depended on, for instance phase distribution, Secondary Dendrite Arm Spacing (SDAS), morphology of Si particles (Roundness) and microscopic defects or pores. All these parameters can contribute to enhance the proper mechanical performance (e.g. Strength and ductility) in the Al-Si cast components.

No presente trabalho quatro ligas Al-Si hipereutéticas, três das quais foram produzidas por conformação por \"spray\" e a outra por fundição, foram caracterizadas quanto a textura, dureza, microestrutura e resistência à corrosão em meio de condensado sintético automotivo (CSA). Duas das ligas conformadas por \"spray\" foram retiradas de camisas de cilindro e a outra de um pré-formado obtido em laboratório. A conformação por \"spray\" envolve a atomização de uma liga e a deposição de gotículas em um substrato, antes mesmo que todas se encontrem no estado sólido. Este processo permite a obtenção de materiais que se caracterizam por uma microestrutura livre de macrossegregações e bastante refinada, implicando melhor trabalhabilidade a quente. A caracterização da microestrutura das quatro ligas hipereutéticas revelou a presença de porosidades na liga obtida em laboratório, e microestrutura com distribuição homogênea de precipitados primários nas três ligas conformadas por \"spray\". A microestrutura de uma das ligas apresentou-se muito diferenciada, com a presença de eutético, sugerindo que esta foi fabricada por fundição. Nas camisas de cilindro foram feitas medidas de rugosidade, e em todas as ligas foram realizados ensaios de microdureza. A liga conformada por \"spray\" e obtida em laboratório foi laminada a quente e a frio. Foram realizados também estudos de textura, para tentar estabelecer uma correlação entre todas as ligas quanto ao processo de fabricação. A avaliação da textura indicou que a presença de fases de silício primário, finamente distribuídas impedem o aparecimento de texturas típicas de deformação de ligas de alumínio, mesmo após severas deformações, como as necessárias para a transformação de pré-formados em tubos que originam as camisas de cilindro. As medidas de rugosidade indicaram características próprias do acabamento superficial usado para a produção das camisas, por brunimento ou por ataque químico. Os ensaios de microdureza apresentaram variações de acordo com as rotas de fabricação sendo que a liga eutética apresentou os maiores valores de microdureza em comparação às ligas conformadas por \"spray\". Todas as ligas foram avaliadas quanto a resistência à corrosão por ensaios de espectroscopia de impedância eletroquímica em dois meios, CSA com pH 3,3 e CSA com pH 11. As quatro ligas hipereutéticas estudadas apresentaram mecanismo de corrosão similar em condensado sintético automotivo (CSA) pH 3,3. Em todas ocorreu o ataque intenso da matriz de alumínio e as partículas de silício primário atuaram como regiões catódicas. A liga 2 apresentou maior resistência à corrosão entre todas as ligas ensaiadas, tanto em CSA pH 3,3 como em pH 11. Neste último meio, uma camada de produtos de corrosão formou-se sobre todas as ligas, e os resultados indicaram mecanismos diferentes para o ataque da matriz de Al e para o crescimento da camada depositada na superfície das ligas estudadas. O ataque da matriz da liga 2 neste meio foi aparentemente mais lento do que sobre as demais ligas, com a formação de uma camada mais compacta de produtos de corrosão, estabelecendo um controle por difusão dos processos interfaciais para maiores períodos de ensaio. A camada formada sobre as demais ligas neste meio apresentou-se mais defeituosa, e nestas ocorreu também menor controle da velocidade das reações interfaciais por processos difusionais. / In the present study four hypereutectic Al-Si alloys, three produced by spray forming and one by casting, were characterized for microhardness, roughness, microstructure, texture and corrosion resistance in a synthetic automotive condensed solution (SACS). Two of the spray formed alloys tested were obtained from cylinder liners and the other was laboratory made. Spray forming involves alloy atomization and droplets deposition on a substrate, previous to the solidification of all of the droplets. This process favours the production of materials with a fine microstructure free of macrosegregation that is related to improved hot workability. The microstructure characterization of the four alloys revealed the presence of porosities in the laboratory made alloy. All the three alloys produced by spray forming showed a homogeneous distribution of primary precipitates. The microstructure of one of the alloys showed eutectic microstructure, indicating that this alloy was fabricated by casting. In the cylinder liners, the surface roughness was measured and the microhardness of all the alloys was also evaluated. Furthermore, the laboratory made alloy was hot and cold rolled. Texture determinations were carried out to investigate the correlation between the alloy type and their fabrication process. The texture investigation indicated that the fine distribution of primary silicon phase in the alloy hindered the development of texture typical of aluminium alloys deformation, even after severe mechanical work, such as those used in the conversion of pre-formed in cylinder liners. The surface roughness results indicated typical characteristics of the surface finishing used, honing or chemical etching. The microhardness results were dependent on the fabrication process used, with higher microhardness associated to the eutectic alloy comparatively to the spray formed ones. All hypereutectic alloys were tested for corrosion resistance using electrochemical impedance spectroscopy in two electrolytes, SACS with pH 3.3 and SACS with pH 11. The four tested alloys showed similar corrosion mechanisms in the acid electrolyte (SACS pH 3.3). The intense attack of the aluminium matrix occurred in ali tested alloys and the primary silicon phase was unattacked and appeared as emerging from the surface after corrosion immersion test. The primary silicon particles acted as cathodic sites. The alloy 2 showed the highest corrosion resistance among the tested alloys in both electrolytes, SACS pH 3.3 and SACS pH 11. In this last medium, a layer of corrosion products formed on all the alloys, and the results indicated different mechanisms for the aluminium matrix corrosive attack and growth of the deposited layer on the alloys surface. The kinetics of aluminium matrix attack was apparently slower in the alloy 2 than for the other alloys, resulting in the formation of a more compact layer of corrosion products, leading to diffusion controlled interfacial processes for longer test periods. The layer of corrosion products on the other three alloys (1, 3, and 4) had more defects and for these alloys diffusional controlled interfacial processes were not as significant as for alloy 2.

Magnesium alloys have emerged as a promising material for light-weighting due to their potential for higher weight saving compared with advanced high-strength steel, aluminium alloys and glass fibre reinforced polymer composites based on equal stiffness or strength. Even though magnesium alloys offer low density, high strength to weight ratio and excellent machinability their poor creep resistance and low yield strength at elevated temperatures (~150oC) restricts their use in automotive powertrain applications. Possible ways of improving the creep resistance include development of creep resistant alloys and/or reinforcing the alloy with ceramic particulates, fibres/whiskers. Several magnesium alloys such as – Mg-Zr based alloys, Mg-Al-RE alloys (RE: rare earth), Mg-Al-Ca alloys, Mg-Al-Sr alloys, Mg-Al-Sr-Ca alloys, Mg-Al-Si alloys offer improvement in creep resistance to different extent. However, these alloys are relatively expensive than the widely used Mg-Al-Zn alloys. Some of these alloys are unsuitable for die casting application due to cracking or die filling problems for example, in Ca and Sr containing alloys their content must be controlled to avoid castability problems. Discontinuous reinforcement of magnesium alloys opens up wide variety of casting techniques for production. Most of the components do not require high performance capability all throughout the component. Squeeze casting technique can be used to economically produce selectively reinforced composites having complex near-net-shape components. By selectively reinforcing only the regions of stress concentration, material property can be optimised at the same time lowering both the cost of manufacturing and machining. In the present study, wear behaviour of Saffil short fibre reinforced AZ91D Mg alloy composite prepared by squeeze casting is explored. The Mg-Saffil composite had a two dimensional planar random fibre orientation which was inherent to the fabrication process of porous fibre preform. In the as-polished specimen fibres protrude out of the Mg matrix due to differential polishing. The Mg-Saffil composites were subject to unidirectional sliding wear against steel counterface under nominally dry condition in ambient atmosphere at low velocities, to simulate the piston reversal at the top dead centre of an engine wherein lubrication starvation results in wear of engine cylinder. The fibres protruding out of the magnesium matrix prevent the softer Mg matrix coming in contact with the counterface material. The worn surface was examined by means of scanning electron microscopy and the physical and chemical changes caused by the wear processes were characterised using different spectroscopic techniques. The effect of fibre distribution and fibre orientation on wear of the composite was studied. Depending on the radius of curvature of the counterface, inhomogeneities in the fibre distribution up to a certain length scale were permissible without deteriorating the wear resistance of the composite. The normal fibre orientation proved deleterious to the wear of the counterface. But wear of the composite was independent of the fibre orientation of the wear surface. In contrast to unreinforced AZ91 Mg alloy which undergoes extensive wear, the steel counterface was machined by the hard alumina fibres protruding out of the matrix. With progressive sliding a discontinuous patch of transferred material formed on the worn surface. Eventually, an oxidised iron-rich transfer layer formed on the worn surface due to compaction of the transferred material and wear debris under the combined action of applied normal load and frictional force. The coefficient of friction plateaus following an initial rapid increase with the increase in the areal coverage of the transfer layer. The abrupt increase in friction coincides with the change in contact at the sliding interface from protruding alumina fibres/steel counterface to mostly between the transferred layer and steel counterface. The increase in friction due to the iron-rich transfer layer formed on the worn surface induced bending stresses in the fibre. A correlation between the incipient fibre fracture and build-up of the transfer layer was observed. Wear of the composite was governed by the dawn of the fibre fracture event, which in turn leads to three-body wear. Diamond-like carbon coatings are well-known for their low friction, high hardness and elastic modulus, chemical inertness and optical transparency. They have found widespread use due to their superior tribological characteristics as protective coatings for magnetic storage media (hard-disk drives), in micro-electromechanical devices (MEMS), biomedical applications (joint implants, artificial heart valves), optical windows (anti-reflection coating) and razor blades. The physical and mechanical properties of these coatings can be tailored by controlling the sp3 /sp2 ratio and modification via alloying with metals such as W, Ti, Cr, Al; or non-metallic elements such as B, N, F, Si. A tungsten doped hydrogenated diamond-like carbon coated steel counterface was used to rule out or minimise any chemical and physical interaction between the composite and the counterface. The composite exhibited a higher wear resistance when slid against a DLC coated steel counterface due to ease of interfacial sliding between the carbon-rich transfer layer and the DLC coated counterface. The transition from ultra-mild to mild wear was not altogether suppressed but delayed to higher loads, prolonging the ultra-mild wear regime. As a result of reduced friction the point of maximum shear stress recedes from the surface, which manifests as subsurface cracks. The dominant wear mechanism of Mg-Saffil composite sliding against the DLC coated counterface was delamination wear.

Due to the increasing use of cyclically loaded cast aluminum components in automotive and aerospace applications, fatigue and fatigue crack growth characteristics of aluminum castings are of great interest. Despite the extensive research efforts dedicated to this topic, a fundamental, mechanistic understanding of these alloys' behavior when subjected to dynamic loading is still lacking. This fundamental research investigated the mechanisms active at the microstructure level during dynamic loading and failure of conventionally cast and SSM Al-Si-Mg alloys. Five model alloys were cast to isolate the individual contribution of constituent phases on fatigue resistance. The major constituent phases, alpha-Al dendrites, Al/Si eutectic phase, and Mg-Si strengthening precipitates were mechanistically investigated to relate microstructure to near-threshold crack growth (Delta Kth) and crack propagation regimes (Regions II and III) for alloys of different Si composition/morphology, grain size, secondary dendrite arm spacing, heat treatment. A procedure to evaluate the actual fracture toughness from fatigue crack growth data was successfully developed based on a complex Elastic-Plastic-Fracture-Mechanics (EPFM/J-integral) approach. Residual stress-microstructure interactions, commonly overlooked by researches in the field, were also comprehensively defined and accounted for both experimentally and mathematically, and future revisions of ASTM E647 are expected.

No description available.

Within the collaborative research center SFB 920 “Multifunctional Filters for Metal Melt Filtration”, the thermally induced formation of metastable aluminum oxides and related microstructural changes were investigated. It was confirmed that the γ-Al₂O₃ phase possesses a defective spinel structure containing Al vacancies that preserve the stoichiometry of this phase. The presence of vacancies fragments apparently the γ-Al₂O₃ crystallites into nanocrystalline domains, which are separated by non-conservative antiphase boundaries (APBs) of the type {100} ¼<110>. These APBs form a 3D network that is randomly distributed over all crystallographically equivalent lattice planes. This phenomenon causes a starlike (and hkl-dependent) broadening of the reciprocal lattice points that correspond to the aluminum sublattice. It was shown that the extent of the broadening of the reciprocal lattice points can be predicted by employing the phase shift factors. With increasing degree of the APBs ordering, the initial streaks representing the broadened reflections start to split, forming superstructure reflections. This superstructure of γ-Al₂O₃ is commonly known as δ-Al₂O₃. Between the ordered APBs, the crystal structure of δ-Al₂O₃ is closely related to the crystal structure of monoclinic θ-Al₂O₃. The phase transition of γ-Al₂O₃/δ-Al₂O₃ to θ-Al₂O₃ proceeds via migration of just three Al³⁺ cations to the neighboring tetrahedral and octahedral sites in the cubic close packed (ccp) oxygen sublattice. The general migration vector is ⅛<111> (γ-Al₂O₃). Diffraction effects associated with different intermediate states can be explained by an improper long-range ordering of equivalent APBs or certain Al³⁺ cations and the local formation of θ-Al₂O₃ within the δ-Al₂O₃ superstructure. The formation of θ-Al₂O₃ is accompanied by an increase of the occupancy of the tetrahedral sites in the oxygen sublattice by the Al³⁺ cations. In surrounding local γ-Al₂O₃ domains, however, some cations migrate from the tetrahedral to the octahedral sites. Thus, the local formation of θ-Al₂O₃ is nearly invisible for the ²⁷Al 1D magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Still, it was recognized by the 2D multiple quantum (MQ) MAS NMR spectroscopy. A continuous formation of the θ-Al₂O₃ domains was confirmed by the Raman spectroscopy, X-ray diffraction (XRD) and selected area electron diffraction (SAED). The proposed microstructure and transformation models helped to explain the thermal stabilization of the metastable alumina phases by Si-doping. For investigation of the thermally induced phase transitions in metastable alumina phases, boehmite (γ-AlO(OH)) was chosen as the starting compound. However, the metastable alumina phases were also observed in endogenous inclusions present in solidified steel melts. For identification of these phases, a procedure for reconstruction of spherical Kikuchi maps from recorded EBSD patterns was developed.

<p>Proučavane su koroziona postojanost i tendencija lepljenja različitih<br />materijala u kontaktu sa tečnom Al&ndash;Si&ndash;Cu legurom. Ispitivanjem su<br />obuhvaćeni čelik za rad na toplo, plazma nitrirani čelik i dupleks<br />slojevi sa CrN, TiAlN, TiAlSiN i CrAlN prevlakama, različitog nivoa<br />površinske hrapavosti. Za ispitivanja pomenutih fenomena<br />primenjena je metoda izvlačenja, koja je unapređena kako bi se povećale<br />njena tačnost i verodostojnost simulacije procesa livenja. Korozioni<br />efekti su pojačani tako što su uzorci osim kratkog kontakta sa odlivkom<br />zadržavani i u dužim periodima u kontaktu sa tečnom legurom (5 i 20<br />min). Uprkos opštim stavovima, za ispitivane materijale je<br />ustanovljeno da su sile izvlačenja uzoraka iz Al&ndash;Si&ndash;Cu odlivaka<br />nezavisne od njihovog hemijskog sastava. Uticaj hrapavosti je izražen<br />kod uzoraka sa prevlakama kod kojih pri smanjenju hrapavosti dolazi do<br />povećanja sile izvlačenja. Sve ispitane prevlake su sklone mehaničkom<br />lepljenju Al&ndash;Si&ndash;Cu legure za svoje površine, ali sa aspekta korozije u<br />tečnom metalu značajno prevazilaze performanse čelika i plazma<br />nitriranog sloja. Duži kontakt livene legure sa površinama prevlaka<br />uzrokovao je niže vrednosti sila izvlačenja, što je posledica<br />oksidacije površina prevlaka. Ustanovljeno je da su ispitivane<br />prevlake inertne ka tečnoj leguri aluminijuma. Međutim, dolazi do<br />oksidacije i korozije materijala podloge kroz greške rasta koje su<br />prisutne u prevlakama. Stečena znanja o identifikovanim<br />mehanizmima habanja i propadanja zaštitnih slojeva prevlaka<br />poslužiće daljem razvoju dupleks slojeva namenjenih za zaštitu alata<br />za livenje pod pritiskom.</p> / <p>Corrosion resistance and soldering tendency of different materials in molten<br />Al&ndash;Si&ndash;Cu alloy were studied. Hot-working tool steel, plasma nitrided steel and<br />duplex layers with CrN, TiAlN, TiAlSiN and CrAlN top coatings, which were<br />produced to various degree of surface roughness, were covered by the study.<br />An ejection test was employed for investigation of the concerned phenomena.<br />The ejection test was improved in order to increase its accuracy and the<br />reliability of process simulation. Samples were examined in both short and<br />extended periods of contact (5 and 20 min) with liquid casting. Casting<br />solidification was extended in order to intensify the corrosion effects. Contrary<br />to common findings, it was found that the ejection force of the investigated<br />materials does not depend on their chemical composition. For the coated<br />samples, a pronounced dependence of the ejection force on the surface<br />roughness was found. The ejection force increases with decrease in surface<br />roughness. All investigated coatings are prone to mechanical soldering by Al&ndash;<br />Si&ndash;Cu alloy. Still, their corrosion resistance substantially exceeds the corrosion<br />resistance of steel and plasma nitrided layer. Longer exposure of coated<br />samples to cast alloy induced lower ejection forces, which is a consequence<br />of coatings oxidation. It was found that the investigated coatings are inert to<br />liquid aluminium. However, the underlying material undergoes oxidation and<br />corrosion through coating growth defects. The findings concerning the wear<br />mechanisms of protective layers support further development of duplex layers<br />intended for die casting tools protection.</p>