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In-situ pre-decorated different content of TiB2 particle reinforced 2024 Al matrix composites (xTiB2/2024Al composite, x = 0, 1, 2, 4, 6, 8 wt.%) were printed by laser powder bed fusion (LPBF). A quantitative equation was established to quantify heterogeneous nucleating potency by TiB2 nanoparticles on the grain size of the LPBFed samples. Optimized mechanical properties were obtained in the as-printed 4TiB2/2024Al alloy which proved the feasibility of replacing the wrought part with the LPBFed one. This manuscript provides a method to determine the optimized content of TiB2 nanoparticles to change the noncastable material to the printable one. This manuscript provides a method to determine the optimized TiB2 content to change the noncastable material to the printable one, and can potentially guide the introduction of many other nanoparticles.

Many additives are preferred to extend the usage of concrete constructions in order to increase durability. These compounds have to be non-poisonous and have no undesirable consequences on the environs. Out of all these concerns, amino acids are mainly used in food and drug industries as well as safe corrosion inhibitors in a number of acid solutions. Here, the corrosion inhibition efficacy of optimum concentration (50 ppm) of caffeine and L-arginine has been investigated for reinforcement steel in mixture water (MW) environment. This was performed for a duration of 2 h, 7, 28, 56 and 90 d using electrochemical impedance spectroscopy (EIS), dynamic EIS (DEIS), and potentiodynamic polarization (PDP) techniques. The surface of specimens was also investigated by atomic force microscopy (AFM), optical profilometer (OP) and scanning electron microscopy (SEM). DEIS results indicate that selected inhibitors protected reinforcement steel approximately up to 62% against corrosion till 7 d, then the effectiveness of the protection decreased to 30% on 90th day. It was also observed that L-arginine showed better inhibition efficiency than caffeine. Duzce University Research FundDuzce University [2019.06.05.1036] The authors acknowledge the financial support provided by the Duzce University Research Fund (Project No: 2019.06.05.1036). The authors also thank Dr. Marziya Rizvi (ORCID ID: 0000-0001-8928-529X) for his assistance in the interpretation of the EIS measurements. Africa Centre of Excellence in Future Energies and Electrochemical System, Federal University of Technology, Owerri, Nigeria is acknowledged for their contribution to this study. WOS:000650506200001 2-s2.0-85106258076

Orientation relationships (ORs) can form during solidification by a variety of mechanisms that are often difficult to distinguish after solidification. Here we study three ORs formed by the nucleation of Al3Ti on TiB2, and by the pushing and engulfment of TiB2 by growing Al3Ti facets in hyperperitectic Al-rich melts. The nucleation OR is identified by growing a relatively large TiB2 crystal, solidifying multiple small Al3Ti crystals on one (0001) facet of TiB2, and measuring the resulting OR by electron backscatter diffraction (EBSD). Pushing and engulfment ORs are investigated by statistical analysis of EBSD measurements, density functional theory (DFT) calculations of interface energies, and imaging of cross-sections of TiB2 particles being pushed and engulfed by Al3Ti facets. It is shown that the lowest energy OR is formed by nucleation as well as by pushing/engulfment. The higher energy ORs, formed by pushing and engulfment, correspond to local interfacial energy minima and can be explained by rotation of TiB2 particles on Al3Ti facets during pushing.

This paper provides insights into the effect of high thermal gradients and cooling rates on equiaxed grain nucleation and growth in conditions similar to those experienced during Additive Manufacturing (AM) processes. Bridgman type solidification is numerically simulated with columnar grains growing at a fixed pull rate under a user-imposed thermal gradient. Controlled inoculants of known nucleation undercooling were placed ahead of the growing columnar grains to allow quantitative analysis of nucleation events. At low thermal gradient and cooling rate only the inoculants with low nucleation undercooling were activated due to low melt undercooling driven by constitutional supercooling (CS). As the cooling rate is increased, for a given thermal gradient, a larger number of inoculants with higher nucleation undercoolings were activated. At higher cooling rates, thermal undercooling was generated by a lag in the growth rate of the solid-liquid (S-L) interface compared to the theoretical pull rate. Thus, thermal undercooling becomes dominant leading to the facilitation of nucleation on less potent substrates requiring higher undercooling. The results show a transition from solute-driven undercooling to cooling rate driven thermal undercooling which contributes to the undercooling that activates the nucleation events. Invoking the Interdependence model, it is also shown that the high cooling rate induced thermal undercooling reduces the size of the nucleation free zone substantially.

A novel set of unified constitutive equations has been developed and validated to describe stress relaxation ageing (SRA) behaviour of 7xxx series aluminium alloys. The model, based on dynamic ageing and power-law creep relations, can predict the stress relaxation, age hardening response and their interactions at different temperatures, through considering the microstructure evolutions (precipitate radius, volume fraction and dislocation density) during SRA. In addition, the model newly incorporates the effects of prior plastic strain. This model was verified through T74 multi-step SRA experiments for different pre-strain conditions. Excellent agreement was achieved between the predicted and experimental results for stress relaxation and yield strength variation. The evolution of micro-internal variables (e.g. normalised precipitate radius) within the model were calibrated by observing transmission electron microscopy (TEM) images both performed in this work and available in literature. The advanced constitutive model developed predicts the mechanical properties and residual stresses in components after ageing. Therefore, the model provides a valuable tool to optimise manufacturing processes leading to many benefits including reduced scrap rates and financial losses.

Sn-Ag-Cu electronic solder joints typically solidify from a single nucleation event producing either a single βSn grain or twinned grains. With so few and variable βSn orientations, each joint is mechanically unique due to the anisotropy of tetragonal βSn. Here we explore the potential of increasing the number of βSn orientations in solder joints by promoting heterogeneous nucleation during solidification. It is shown that large (60 g) samples can be grain refined effectively by combining growth restricting solute with potent heterogeneous nucleant particles introduced by alloying additions of Zn, Ti, Co, Ir, Pd, or Pt. These mechanisms are discussed with reference to the grain refinement of structural casting alloys. In 500 μm solder balls, these grain refinement approaches were less effective and a relatively high cooling rate of 17 K/s combined with solute and nucleant particles were required to trigger multiple nucleation events. With this approach, up to 12 independent grains formed in 500 μm balls of Sn-3Ag-0.5Cu alloyed with Pt, Co, or Ti, compared with one independent grain in balls without nucleant particle additions.

The solution heat treatment (SHT) process resolving hardening precipitates in high strength aluminium alloys is a critical step for high-efficient forming processes, such as Hot Form Quench (HFQ®). SHT largely determines the overall cycle time of a forming process. However, effects of heating process parameters, such as the heating rate and soaking time, on the microstructure and the associated mechanical properties of aluminium alloy 6082, one of the most commonly used aluminium alloys, for HFQ applications, have not been systematically investigated. The aim of this study is to explore and understand the relationships among heat treatment conditions, grain microstructure and associated mechanical properties for AA6082. A series of uniaxial tensile tests conducted under various SHT conditions revealed significant variations on mechanical behaviour characterised by stress-strain curves. To correlate these stress-strain relationship with underlying microstructure, the grain and orientation distribution of each heat-treated sample were characterised by the electron backscatter diffraction (EBSD) technique. Due to the presence of a large number of microscopic variables, such as grain size, morphology, texture, grain boundary and etc., the crystal plasticity finite element (CPFE) modelling was employed to identify the key microscopic factors which determine the differences in the observed strength and ductility for all samples. A new CPFE model integrated with local strain criterion was proposed and validated to correlate the ductility and the strength with the material microstructure. This rigorous investigation provides more insights on how microstructure (grain size and texture) affects the mechanical behaviour for AA6082, which enables to enlarge the capability of HFQ for industrial applications.

Annealing behavior of Al-Mg-Sc-Zr alloy processed by equal-channel angular pressing (ECAP) at 325 degrees C to various strains and undergone subsequent cold rolling to similar to 80% reduction was investigated. ECAP to strain, e similar to 3 formed partially recrystallized structure with ultrafine-grain (UFG) mantle zone along the boundaries of initial grains of similar to 25 mm size in as-cast and homogenized condition, whilst ECAP to e similar to 8 and 10 resulted in nearly 1 mm grain size, with their volume fractions being 0.7 and 0.9, respectively. These structures after ECAP exhibited sufficient stability, within normal grain growth up to similar to 5 mm grain size only during annealing for 1 h in the temperature range of 350-520 degrees C. However, subsequent to ECAP cold rolling and further annealing at 425 degrees C and above revealed a distinct effect of prior ECAP. While the materials undergone ECAP to e similar to 3 and e similar to 8 and having lower UFG fractions revealed quite similar behavior, the material undergone ECAP to e similar to 10 (and nearly complete recrystallization at ECAP) exhibited abnormal grain growth. The sources of microstructure evolution at different stages of processing are analyzed along with their effects on hardness at ambient temperature. (C) 2016 Elsevier B.V. All rights reserved.

Two types of A356 alloy castings in initial and modified with nanoparticles condition produced by gravitational casting were studied. Samples, as-cut from the castings, were subjected to light optical microscopy (LM), thermal analyses, Electron Backscattered Diffraction (EBSD) and Scanning Transmission Electron Microscopy (STEM) analyses. Results, obtained by EBSD, confirmed that there is grain refinement in samples from castings with added nanoparticles compared to the initial ones. STEM analysis shows agglomerates of nanoparticles in examined foils. Nanoparticles&rsquo, position in the microstructure confirms the hypothesis that they act as nucleating sites during the alloy solidification, which is the reason for observed fine-grained microstructure.

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2019; Aachen 1 Online-Ressource (152 Seiten) : Illustrationen, Diagramme (2019). = Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2019, In-situ X-radiography and X-ray tomoscopy is used to investigate the evolution of dendrites in Al-Ge alloys towards the final grain morphology. The focus was drawn on the investigation of the globulitic to dendritic transition (GDT) with respect to destabilization of the solid-liquid interface, cooling rate and grain density. Furthermore, the dendritic to globulitic transition (DGT) was investigated during advanced stages of solidification. As in most solidification cases the solid-liquid interface of a growing grain destabilizes at an early stage of the solidification process. A nucleus first grows with a spherical shape into the undercooled melt. Dendritic arms are formed due to perturbations of the solid-liquid interface. Synchrotron high speed tomoscopy was employed to get a deeper insight into this phenomenon. For the first time, the GDT was directly observed and the critical radius at which the GDT takes place was measured. Comparison between the measured radii from the experiment and the model predictions by Dantzig and Rappaz showed that the model underestimates the critical stable radius for globulitic growth in the Al-Ge system. To avoid destabilization of the solid-liquid interface the cooling rate and the grain density play an important role. The cooling rate has a direct impact on the nucleation undercooling which is needed for the nucleation process. A higher cooling rate leads to higher nucleation undercooling, which triggers more inoculants to nucleate. The thermal gradient between nuclei and melt causes perturbations of the solid-liquid interface. A smaller and more homogeneous nucleation undercooling was thought to prevent destabilization of the solid-liquid interface. A higher grain density leads to an early interaction of the growing dendrites. Solutal fields due to enrichment of the melt by germanium, rejected by the growing Al-dendrite or impingement of the grains can also promote globulitic grain morphology. More grains impinge earlier and suppress the formation of dendritic branches. The impact of cooling rates from 0.0016 Ks-1 to 1.5 Ks-1 was in-situ investigated with X-radiography. Grain refiner was used to vary the grain density. The Al-Ge alloy compositions of Al-10 wt.%Ge, Al-20 wt.%Ge, Al-33 wt.%Ge and Al-46 wt.%Ge were analyzed. The near-isothermal, X-ray transparent furnace developed at DLR (ITF1) enables the measurement of the solutal fields around a nucleating and growing grain in an isothermal field. The X-ray transparent Bridgman type gradient furnace (XRMON-GF), at NTNU, was operated in isothermal mode and used to achieve larger cooling rates. One challenge in increasing the grain density of Al-Ge alloys was to find a suitable grain refiner. For the standard grain refiner for Al-alloys, Al-5Ti-1B, a poisoning effect was observed, getting stronger with increasing germanium or titanium concentration. Germanium and titanium form stable interlayers around the grain refiner particle, which prevents the epitactical growth of the Al-matrix on the particle and reduces the efficiency of the grain refiner. SEM and TEM analysis techniques were employed to characterize those stable Ti-Ge layers. A new, niobium-based grain refiner showed promising results for application in the Al-Ge system. In this work, Al-4.6Nb-0.5B was tested the first time as a grain refiner for Al-Ge alloys. Stable and reproducible results in Al-Ge alloys were found and an overall better performance compared to Al-5Ti-1B was observed. Comparing the industrial grain refiner AlB3 with Al-4.6Nb-0.5B the performance was found to be similar. Only for small amounts of germanium, the niobium-based grain refiner showed better results. Both grain refiners Al-5Ti-1B and Al-4.6Nb-0.5B were used for nucleation studies in the system Al-Ge and were compared with the “solute segregation nucleation stifling model”) by Li and Mathiesen et al. (model 1) and the “interdependence theory” of St.John et al. (model 2). Model 1 supposes recalescence-free solidification and a nucleation stifling caused by solute segregation around the nucleated and growing grain. Modeling data were compared with the experimental results obtained at the XRMON-GF furnace at NTNU. Due to rapid heat extraction, good heat transfer between the sample, furnace and environment and constantly applied cooling rates, a recalescence-free solidification process could be assumed. For sample inoculated with Al-5Ti-1B grain refiner a deviation of the prediction of the grain density and the final average grain size predicted by model 1 and experimental results was found. Only a few and large grains were growing. The results can be attributed to poisoning of the grain refiner particles in the Al-Ge system, whereby less grain refiner particles were active in the melt. Inoculation with Al-4.6Nb-0.5B grain refiner showed conformity with the model results for high cooling rates and well grain refined samples. For low cooling rates and a poorly grain refined melt, the model parameters need further adaption. The experimental result from the near-isothermal furnace (ITF1) developed at the DLR were compared with the “interdependence theory” of St.John et al. (model 2). Assuming a recalescence event after the nucleation takes place, the model can be used to predict the final grain size in the cast and a resulting nucleation free zone (NFZ) due to recalescence. Measurements of the enrichment of the solute around the growing grain from the in-situ experiments with Al-33 wt.%Ge inoculated with Al-5Ti-1B grain refiner were compared with model 2. The comparison showed an underestimation of the size of the NFZ and the final grain size by model 2. Key determinants for the deviation between the prediction by the model and the experimental length of the NFZ were: i) the poisoned performance of the Al-5Ti-1B grain refiner particles. Fewer dendrites nucleate and have more time to grow. ii) the treatment of the dendritic growth velocity by the “interdependence theory”. Model 2 assumes a steady-state growth velocity of the dendritic tip, while the experimental results show that the dendritic tip velocity decrease over time, before a steady-state growth velocity is reached. iii) in addition, the influence of the calculation of the solute supersaturation in front of the growing dendrite were found to have a major impact. Different approaches to calculate the germanium concentration in front of the dendritic tip were made and compared with experimental values. A more precise examination of the germanium concentration in front of the dendritic tip is proposed. Besides the investigation of the GDT the transition from dendritic to globulitic morphology (DGT) was investigated. Here, two aspects were found to be the determining factor: the coarsening of the dendrites and globulitic growth from a semi-solid state. For the investigation of the DGT during advanced stages of solidification, a new, X-ray transparent, isothermal, furnace setup (ITF2) for long term solidification experiments was designed and successfully tested. A new heater concept enabled a constant heat input parallel to the sample surface. Temperature gradient zone melting (TGZM), caused by a thermal gradient could be prevented with this new furnace setup. Improvement of the temperature measurement inside the furnace additionally enables a more precise determination of the temperature in the melt., Published by Aachen

The homogenous redistribution of nanosized TiB2 particles in a fine-grained in-situ AA6063/TiB2 composite has been achieved by friction stir processing (FSP). The effects of TiB2 particles on as-deformed structure in the nugget zone and its thermal stability after additional T6 heat treatment are quantitatively evaluated by using neutron diffraction, scanning electron microscopy associated with electron backscatter diffraction and synchrotron X-ray line profile analysis. The results are compared to those obtained from a FSPed AA6063 alloy and, show first that, average grain size of the FSPed AA6063/TiB2 composite is smaller than that of the FSPed AA6063 alloy, while grain size distribution is more uniform. Second, dislocation density estimated in the FSPed AA6063/TiB2 composite is higher than that in the FSPed AA6063 alloy. Third, a small number of grains are dynamically recrystallized in the FSPed AA6063/TiB2 composite compared to its alloy counterpart, which indicates that recrystallization and grain growth during FSP and T6 heat treatment are effectively inhibited by the presence of nanosized TiB2 particles. Keywords: Metal-matrix composites (MMCs), X-ray diffraction (XRD), Transmission electron microscopy (TEM), Neutron diffraction, Friction stir processing

This paper investigates the effect of 0.05 wt.% Ni on the formation and growth of primary Cu6Sn5 in Sn-0.7 wt.%Cu solder paste soldered on a Cu substrate, using a real-time synchrotron imaging technique. It was found that small additions of Ni significantly alter the formation and growth of the primary Cu6Sn5 intermetallics, making them small. In contrast, without Ni, primary Cu6Sn5 intermetallics tend to continue growth throughout solidification and end up much larger and coarser. The primary effect of the Ni addition appears to be in promoting the nucleation of a larger amount of small Cu6Sn5. The results provide direct evidence of the sequence of events in the reaction of Ni-containing Sn-0.7 wt.%Cu solder paste with a Cu substrate, and in particular the formation and growth of the primary Cu6Sn5 intermetallic.

WOS: 000435426100004, The present study focuses on the applicability of friction stir welding to pure Pb plates. The joint performances are evaluated by optical microscopy, microhardness measurements, notch impact tests and three point bending tests. The effects of the tool rotational speed (1500, 2000 and 2500 rev/min) and welding speed (72, 100 and 125 mm/min) on the joint performance are determined. The results indicate that the welded joints exhibit similar or higher mechanical properties than those of the base material. Optimal joint performance was achieved with a rotational speed of 1500 rev/min and welding speed of 100 mm/min.

International audience; A three-stage mechanistic model for solidification cracking during TIG welding of steel is proposed from in situ synchrotron X-ray imaging of solidification cracking and subsequent analysis of fracture surfaces. Stage 1-Nucleation of inter-granular hot cracks: cracks nucleate inter-granularly in sub-surface where maximum volumetric strain is localized and volume fraction of liquid is less than 0.1; the crack nuclei occur at solute-enriched liquid pockets which remain trapped in increasingly impermeable semi-solid skeleton. Stage 2-Coalescence of cracks via inter-granular fracture: as the applied strain increases, cracks coalesce through inter-granular fracture; the coalescence path is preferential to the direction of the heat source and propagates through the grain boundaries to solidifying dendrites. Stage 3-Propagation through inter-dendritic hot tearing: inter-dendritic hot tearing occurs along the boundaries between solidifying columnar dendrites with higher liquid fraction. It is recommended that future solidification cracking criterion shall be based on the application of multiphase mechanics and fracture mechanics to the failure of semi-solid materials. (C) The Minerals, Metals & Materials Society and ASM International 2018

In the present investigation, steel plates were joined at different tool traversing speed by friction stir welding keeping other parameters same. Microstructural characterization was carried out with optical and scanning electron microscopes. At weld nugget pearlite and bainite were present within ferrite matrix. Thermo-mechanically and heat affected zones microstructure consisted of pearlite and ferrite. Second phase area fraction and matrix grain size at different regions were varied depending on welding parameters. Weld nugget exhibited substantial improvement in microhardness with respect to base metal. In this respect heat affected zone revealed minimum microhardness and was below base metal value. Tensile tests were carried out on standard and miniature specimens in scanning electron microscope. Highest joint efficiency to the tune of approximate to 82% and approximate to 120% of that of base metal obtained for standard and miniature specimens, respectively machined from weld fabricated at lowest welding speed. With increment in welding speed assembly strength was reduced for both types of specimens. Standard specimens failed from heat affected zone and miniature specimens failed through centre of weld nugget. Apart from matrix grain size and second phase area fraction, precipitation of microalloyed carbide / carbonitride was responsible for altering the joint strength.

The purpose of the present work is to study on the effect of material properties tool on friction stir butt welding of AA2124-T4 alloy matrix MMC. Uncoated tool, coated tool with a CrN, and coated tool with AlTiN were used to weld aluminum MMC plates. Macrostructure and microstructure observations, ultimate tensile strength, wear resistance, and chemical analysis were carried out to determine the appropriate tool for joining these composite plates. Results showed that the good welded joints could be obtained when a tool is coated with AlTiN. (C) 2017 Brazilian Metallurgical, Materials and Mining Association. Published by Elsevier Editora Ltda. This is an open access article under the CC BY-NC-ND license.

The work described in this paper concerns a novel method for directly forming curved profiles/sections from billets in one extrusion operation using two opposing punches. Its mechanics are based on internal differential material flow, and it has been given the acronym, differential velocity sideways extrusion (DVSE). A tool set enabling sideways extrusion to be performed using opposing punches moving with different velocities was used for a series of experiments in which punch velocity ratio and extrusion ratio were process parameters. Plasticine was used as a model work-piece material and a series of compression tests were undertaken, to determine its constitutive properties and gain an estimate of work-piece die friction for use in process simulation. Curvature of extrudate can be controlled and varied using a difference between the velocities of the two punches, defined by velocity ratio. Greater curvature is achieved with lower velocity ratio. Curvature is also dependent on extrusion ratio, an increase in which increases curvature, although curvature is less sensitive to it than to velocity ratio. The extent of work-piece flow velocity gradient across the die exit orifice, which causes curvature, has been identified. Severe plastic deformation of the extrudate occurs in a way similar to channel angular extrusion (CAE), thus a greatly promoted effective strain level is achieved, though it is not always uniform across a section. The inner bending region of an extrudate experiences maximum localised effective strain, which decreases with decrease in curvature. To the authors' knowledge this is the first publication in which extrudate curvature is deliberately induced using opposing punches with differential velocities. Although only fixed velocity ratio values have been used in the work described in this paper the ability to change during operation exists and the process has the potential for the production of a profile with different curvature along its length.

While many aspects of electronics manufacturing are controlled with great precision, the nucleation of tin in solder joints is currently left to chance. This leads to a widely varying melt undercooling and different crystal orientations in each joint, which results in a different resistance to electromigration, thermomechanical fatigue, and other failure modes in each joint. Here we identify a family of nucleants for tin, prove their effectiveness using a novel droplet solidification technique, and demonstrate an approach to incorporate the nucleants into solder joints to control the orientation of the tin nucleation event. With this approach, it is possible to change tin nucleation from a stochastic to a deterministic process, and to generate single-crystal joints with their c-axis orientation tailored to best combat a selected failure mode., Control over the crystallographic orientation of solder joints based on βSn will improve the reliability of electronic interconnects. Using a technique based on droplet solidification and lattice matching, Ma et al. are able to control the βSn nucleation events, hence control the grain orientation.

Rapidly solidified nano-quasicrystalline Al93Fe3Cr2Ti2 at% alloy has previously shown outstanding tensile and compressive strength and microstructural stability up to elevated temperatures. Despite this, no study had previously assessed the effect of plastic deformation at elevated temperature to simulate thermal-mechanical forging processes for the production of engineering components. The present work analysed bars consisting of a nano-quasicrystalline Al93Fe3Cr2Ti2 at% alloy matrix, with the addition of 10 and 20 vol% pure Al ductilising fibres, produced through gas atomisation and warm extrusion. The microstructure was made primarily of nanometre-sized icosahedral particles in an α-Al matrix. Compression tests were performed across a range of temperatures and strain rates. The measured yield strength at 350 °C was over 3x that of “high strength” 7075 T6 Al alloy, showing outstanding thermal stability and mechanical performance. However, the microstructure was shown by XRD to undergo a phase transformation which resulted in the decomposition of the icosahedral phase around ~500 °C into more stable intermetallic phases. Serrated flow associated with dynamic strain ageing was observed and a semi-quantitative analysis matching elemental diffusion speeds with dislocation speed at specific strain rates was performed, which tentatively identified Ti as the solute species responsible within the selected range of temperatures and strain rates. Fil: Pedrazzini, S.. University of Oxford; Reino Unido. University of Cambridge; Reino Unido Fil: Galano, M.. University of Oxford; Reino Unido Fil: Audebert, Fernando Enrique. University of Oxford; Reino Unido. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long". Universidad de Buenos Aires. Facultad de Ingeniería. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long"; Argentina Fil: Smith, G. D. W.. University of Oxford; Reino Unido

The microstructural patterns formed during liquid to solid phase transformations control the properties of a wide range of materials. We developed a novel methodology that enables in situ quantification of the microstructures formed during solidification of high temperature advanced alloys via synchrotron tomography. The patterns formed are captured in 4D (3D plus time) using a methodology which exploits three separate advances: a bespoke high temperature environment cell; the development of high X-ray contrast alloys; and a novel environmental encapsulation system. This methodology is demonstrated on Ni, Fe, and Co advanced alloy systems, revealing dendritic pattern formation. We present detailed quantification of microstructural pattern evolution in a novel high attenuation contrast Co-Hf alloy, including microstructural patterning and dendrite tip velocity. The images are quantified to provide 4D experimental data of growth and coarsening mechanisms in Co alloys, which are used for a range of applications from energy to aerospace. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The work was partly supported by the Russian Foundation for Basic Research and the Government of Sverdlovsk region (grant nos 13-03-96055-ural and 13-02-96036-ural). Using a combination of the synchrotron X-ray absorption spectroscopy, transmission electron microscopy, vibrating magnetometry, Faraday's method and thermal analysis, we studied Al92Ce8 alloy in the glassy, crystalline and liquid states. The cerium in the alloy demonstrates mixed valence behavior with the noticeable portion of tetravalent states. The local order and physical properties of the investigated composition are discussed in the assumption of specific chemical bonding between Al and Ce with partial involvement of 4f-electrons. (C) 2014 Elsevier B.V. All rights reserved.

In previous work, we have established a scheme that exploits a three-dimensional heat and mass flow model to assess tool durability and define the domains of satisfactory tool life in the context of welding difficult aluminium alloys. We now apply this scheme to the friction stir welding of steel and extend the calculations to cover consequences on the microstructure of the steel while optimising tool life. This is the first published model that covers both the processing parameters and the consequences on the physical metallurgy of the steel.

Forming limit diagram of aluminum AA7075 and titanium Ti-6Al-4V alloys was examined. For this aim, various strain paths from uniaxial to equi-biaxial tensions were considered. It was established that titanium alloy stretching potential is more than of aluminum alloy due to higher average strain hardening exponent. Also, the average anisotropy factor of AA7075 is about five times lower than that of Ti-6Al-4V resulting in the reduction of resistance to thinning during sheet metal forming processes. Furthermore, employment of BBC2000 yield criterion for AA7075 and Hill’s 1993 for Ti-6Al-4V in determining numerical forming limit diagrams resulted in a closer coincidence with the experimentations. Finally, imperfection factors of 0.970 and 0.955 for AA7075, and 0.935 and 0.960 for Ti-6Al-4V are respectively attained by yield surfaces of BBC2000 and Hill’s 1993. Keywords: Bulge test, Strain paths, Imperfection factor, Strain hardening exponent, Strain hardening coefficient

Studies were conducted to investigate the effect of polymer matrix composite stiffener (patch) on environmentally assisted cracking (EAC) susceptibility of pre-cracked alclad Al-Zn-Mg-Cu-Cr alloy in 3.5 wt.% NaCl environment. The EAC threshold stress intensity of the peak and two-step aged alloy was reduced from 26 to 7 MPa m(1/2) and 31to 8 MPa m(1/2) respectively under the influence of patch. The plateau crack velocity was increased from 2.5 x 10(-9) to 5.1 x 10(-9) m/s for peak aged and 1.2 x 10(-9) to 2.0 x 10(-9) m/s for two-step aged tempers in patched condition. The significant increase in EAC susceptibility was attributed to crevice effect generated by the stiffener. (C) 2013 Elsevier Ltd. All rights reserved.

Al-Cu-Li alloy (AA2195) was produced using vacuum induction melting (VIM) furnace under dynamic argon atmosphere. The as-cast billets were homogenized using a two-step homogenization cycle. The hot deformation behavior of homogenized and forged AA2195 alloy was studied by hot isothermal compression in a thermo-mechanical simulator. The contour maps of efficiency of power dissipation and instability maps have been generated within the temperature range of 250-450 degrees C and strain rate range of 10(-3)-10(2) s(-1). Various deformation mechanisms, which operate in different temperature-strain rate regimes, were identified with the aid of these maps and complementary microstructural analysis of the deformed specimens was carried out. Results indicate four distinct deformation domains within the range of experimental conditions examined. Out of these four domains, the optimum temperature and strain rate range for obtaining a completely reconstituted microstructure is T: 400 degrees C-450 degrees C and epsilon: 10(-2) 10(-1.5) s(-1) and epsilon: 10(-0.5)-10(1) s(-1). Instability mechanisms in the material are attributed to localized plastic flow and cracking. A constitutive equation that describes the flow stress of AA2195 alloy as a function of strain rate and deformation temperature was also established. (C) 2017 Elsevier B.V. All rights reserved.

A Cu6Sn5 layer is an integral part of many electronic interconnections. Here we show that, although primary Cu6Sn5 is not a potent nucleant for Sn, the Cu6Sn5 layer plays a key role in Sn nucleation and microstructure formation in solder joints. Combining thermal analysis, FIB-tomography and EBSD, we show that conical cavities are present between the scallops of the Cu6Sn5 layer that act as geometric nucleation sites for Sn, that Sn grows from the Cu6Sn5 layer, and that reproducible nucleation orientation relationships (ORs) exist between Cu6Sn5 and Sn. With these ORs, a near-random distribution of Sn orientations is predicted from joint to joint even for Cu6Sn5 layers with a strong [0001] fibre texture. It is shown that the nucleation undercooling is strongly affected by manipulation of the Cu6Sn5 layer shape, and that it is possible to prevent nucleation on the Cu6Sn5 layer by adding more potent nucleants.

Conventional experimental approaches used to generate forming limit diagrams (FLDs) for sheet metals at different linear strain paths are not applicable to hot stamping and cold die quenching processes because cooling occurs prior to deformation and consistent values of heating rate, cooling rate, deformation temperature and strain rate are not easy to obtain. A novel biaxial testing system for use in a Gleeble testing machine has been adopted to generate forming limits of sheet metals, including aluminium alloys, magnesium alloys and boron steel, under practical hot stamping conditions in which heating and cooling occur. For example, the soaking temperature is about 900 °C and the deformation temperature range is 550–850 °C for boron steel [1] and the soaking temperature is about 535 °C and the deformation temperature range is 370–510 °C for AA6082 [2]. Resistance heating and air cooling were introduced in this pioneering system and the thermal analysis of different heating and cooling strategies was investigated based on a type of cruciform specimen. FE models with a UAMP subroutine were used to predict temperature fields on a specimen in ABAQUS 6.12. Digital image correlation (DIC) system was used to record strain fields of a specimen by capturing images throughout the deformation history and its post-processing software ARAMIS was used to determine forming limits according to ISO standards embedded in the software. Heating and cooling strategies were determined after the analysis. Preliminary results of forming limit curves at the designated temperatures are presented in order to verify the feasibility of this new method.

A phase-field model has been developed to describe the morphology of pores constrained by a dendritic solid network, and are forced to adopt complex non-spherical shapes. The distribution of the solid, liquid and gas phases was calculated with a multiphase-field approach which accounts for the pressure difference between the liquid and the gas. The model considers the partitioning of the dissolved gas at interfaces, gas diffusion and capillary forces at the solid/liquid, liquid/gas and gas/solid interfaces. The model was used to study the influence of the dendrite arm spacing (DAS) and the solid fraction on the state of a pore. The calculations show that a pore constrained to grow in a narrow liquid channel exhibits a substantially higher mean curvature, a larger pressure and a smaller volume than an unconstrained pore. Comparisons with simple geometrical models indicate that analytical approaches show a good trend but tend to underestimate the pore curvature, in particular at high solid fractions, where pores have to penetrate the thin liquid channels. For pores spanning over distances larger than the average DAS, the simulations showed that the radius of curvature can vary between two limits, which are given by the size of the narrowest section that the pore needs to pass in order to expand and by the largest sphere that can be fitted in the interdendritic liquid. The pore curvature is therefore a complex non-monotonic function of the DAS, the solid fraction, the hydrogen content and statistical variations of the liquid channel width. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Spray formed billets of Al-4Li-0 center dot 14Zr (wt-%) were processed by a combination of hot isostatic pressing (HIPing), homogenisation, and solution treatment and aging. Extensive oxidation damage occurred after homogenisation in air, but could be alleviated by using controlled atmosphere or by prior HIPing. The microstructure and tensile properties were studied at various stages of processing. The volume fraction of AlLi (delta) decreased successively after HIPing, homogenisation and T6 temper, redistributing itself towards grain boundaries. The Al(3)Zr (beta') phase was absent in the as formed condition, but precipitated after HIPing at 838 K/4 h and/or stepped homogenisation finishing at 838 K/4 h. Room temperature tensile properties were poor in the as spray formed condition, but improved after HIPing, homogenisation and T6 treatment successively. The tensile properties of the alloy were also evaluated between 648 and 848 K, for preliminary identification of optimum hot working parameters. HIPing did not affect high temperature yield strength significantly, but increased the ductility.

The aim of the work was to inhibit recrystallization in Al-Zn-Mg-Cu-Zr alloy and to evaluate the stress corrosion cracking (SCC) behavior of the alloy in the peak aged condition. For this purpose, scandium addition was made to an Al-Zn-Mg-Cu-Zr alloy as the former inhibits recrystallization The scandium-containing alloy was heat-treated to peak aged condition and compared with the base peak aged alloy which contained recrystallized grains. The SCC susceptibilities of the alloys were evaluated using slow strain rate testing (SSRT) and U-bend techniques. While. the base alloy having recrystallized grains showed drastic loss in ductility in the corrosive environment (3.5 wt.% NaCl solution), the scandium-containing alloy having un-recrystallized and fine grains showed no significant loss in ductility in the similar environment. The fracture surface analysis revealed typical inter-granular cracking of recrystallized grains in the base alloy. whereas in the scandium-containing alloy predominant ductile failure was observed. Thus, the study clearly indicated that inhibiting recrystallization in Al-Zn-Mg-Cu-Zr alloy through scandium addition, the SCC resistance of the alloy can be substantially improved even in the peak aged condition (C) 2009

Micromechanical models for ductile failure have been developed in the 1970s and 1980s essentially to address cracking in structural applications and complement the fracture mechanics approach. Later, this approach has become attractive for physical metallurgists interested by the prediction of failure during forming operations and as a guide for the design of more ductile and/or high-toughness microstructures. Nowadays, a realistic treatment of damage evolution in complex metallic microstructures is becoming feasible when sufficiently sophisticated constitutive laws are used within the context of a multilevel modelling strategy. The current understanding and the state of the art models for the nucleation, growth and coalescence of voids are reviewed with a focus on the underlying physics. Considerations are made about the introduction of the different length scales associated with the microstructure and damage process. Two applications of the methodology are then described to illustrate the potential of the current models. The first application concerns the competition between intergranular and transgranular ductile fracture in aluminum alloys involving soft precipitate free zones along the grain boundaries. The second application concerns the modeling of ductile failure in friction stir welded joints, a problem which also involves soft and hard zones, albeit at a larger scale. (C) 2010 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

The influence of powder particle injection velocity on the microstructure of coatings consisting of an Al-Si matrix reinforced with SiC particles prepared by laser cladding from mixtures of powders of Al-12 wt.% Si alloy and SiC was investigated both experimentally and by modeling. At low injection velocities SiC particles react with the molten aluminum alloy. Only a small fraction of SiC remains in the microstructure, which contains large amounts of particles of the reaction products Al4SiC4 and Si dispersed in the alpha-Al + Si eutectic matrix. By contrast, at high injection velocities chemical reactions between SiC and molten aluminum are almost entirely suppressed and the resulting microstructure consists only of SiC particles dispersed in the matrix. To investigate whether this behavior could be explained by the different temperatures reached by the injected particles as they fly through the laser beam, a physical-mathematical model describing the interaction between the laser beam and the powder stream in the off-axis blown powder laser cladding process was developed and applied to calculate the temperature attained by the powder particles as a result of their interaction with an Nd:YAG laser beam (lambda = 1.06 mu m). At an injection velocity of 1 m/s the maximum temperature attained by SiC and Al-12Si particles is 3150 and 180 degrees C, respectively. This result demonstrates that particle injection velocity is a major parameter affecting the microstructure of coatings produced by laser cladding, and must be carefully controlled. (C) 2009 Elsevier B.V. All rights reserved.

Under certain thermal conditions (G approximate to 1 x 10(4) K/m, v(s) approximate to 1 x 10(-3) m/s), < 1 1 0 > twinned dendrites appear in aluminum alloys and can overgrow regular columnar dendrites, provided that some convection is also present in the melt. In order to check the stability of such morphologies, directionally solidified twinned samples of Al-Zn were partially remelted in a Bridgman furnace and then resolidified under controlled conditions, with minimal convection. It was found that, although twin planes remain stable during partial remelting, non-twinned dendrites regrow during solidification. They have a crystallographic orientation given by those of the twinned and untwinned "seed" regions, and grow along preferred directions that tend to be those of normal specimens. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A 3-5 vol.% TiC particulate Al-Si-Cu composite was prepared by diluting Al/20 vol.% TiC composite in an Al-7Si-4Cu alloy matrix. TiC particle distribution consists of isolated and clustered particles which are both located at the primary-alpha grain boundaries and at the areas of the last solidified liquid. Particle pushing by the solidification front is responsible for the final particle location. The solidified microstructure consists of primary and intermetallic phases formed by a sequence of possible eutectic reactions. No evidence of TiC particle degradation was observed. Journal of Materials Engineering and Performance

International audience; Medium strength Al-Zn-Mg age hardening alloys are widely used when a low quench sensitivity is required. such as in welding applications. In this work we present a detailed characterization of the precipitate microstructures resulting from different quench rates from the solution treatment, and from the subsequent artificial ageing to the T6 state, in an Al-4.5Zn-1 Mg (wt%) alloy. This work is carried out using differential scanning calorimetry, transmission electron microscopy and in situ small-angle X-ray scattering. It is shown that for quench rate between 5 and 200 degrees C/min substantial heterogeneous precipitation is observed, nucleated on dispersoids and on grain boundaries, the former being of much larger size than the latter. During subsequent ageing, it is shown that the precipitation kinetics in the material unaffected by the quench-induced precipitates is independent on the quench rate used. (c) 2008 Elsevier B.V. All rights reserved.

Indentation is a well-established technique for measuring mechanical properties, such as hardness and creep, in solid materials at a continuum level. In this study, we performed indentation of a semi-solid granular alloy with an equiaxed dendritic microstructure. The resulting microstructural effects were quantified using a novel thermo-mechanical setup combined with 4D (three spatial dimensions plus time) synchrotron tomography and digital volume correlation. The experiments not only revealed the multitude of deformation mechanisms occurring at a microstructural level, (e.g. dilatancy, liquid flow, macrosegregation, shrinkage voids, and intra-granular deformation), but also allowed quantification of the evolution of the strain fields within the material. The resulting methodology is a powerful tool for assessing the evolution of localized deformation and hence material properties. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd.

We report a study of the structure-processing-property relationships in a high strength AlFeCrTi nano-quasicrystalline alloy and composites containing 10 and 20 vol% ductilising pure Al fibres. The superimposed contributions of several different strengthening mechanisms have been modelled analytically using data obtained from systematic characterisation of the monolithic alloy bar. An observed yield strength of 544 MPa has been substantiated from a combination of solid solution strengthening, work hardening, precipitation hardening and Hall-Petch grain size dependent effects. These materials have been shown by other authors in previous published work to be highly sensitive to the size distribution of particles in the powder from which they are made, and the subsequent thermomechanical processing conditions. The processing condition employed in this study provided micron-sized grains with a strong [111] preferential orientation along the extrusion direction and a bimodal size distribution of the icosahedral nano-quasicrystalline precipitates. Both were deemed to be a significant contributor to the high yield strength observed. The addition of pure Al fibres was found to decrease the yield strength linearly with increasing Al content, and to augment the ductility of the composites., Industrial collaborator ALPOCO Ltd. (and more specifically Steve McArthur) provided the powders. Dr. Karen Kruska and Dr. Alan Xu assisted with sample preparation of FIB lift-out specimens of the atomised powder for TEM analysis. EPSRC Project EP/E040608/1 provided financial support. M. Galano thanks the RAEng for their support by means of a Research Fellowship. F. Audebert and M. Galano thank PICT-Oxford2010/2831. F. Audebert also thanks UBACyT20020130100663 and FONARSEC FS Nano 2010/11 for financial support.

Sc-containing Al alloys are potentially applied in saline environment. However, the corrosion behavior and its mechanisms of Sc-containing Al alloys have not been paid much attention. In this research, the corrosion behavior of Al-4Mg, Al-4Mg-0.3Sc, and Al-4Mg-0.24Sc-0.06Yb alloys was investigated in 3.5wt% NaCl solution by immersion and potentiodynamic polarization analysis. Scanning electron microscope (SEM), transmission electron microscopy (TEM) and high-resolution TEM were used to characterize intermetallic phases and localized corrosion. The results showed that the Fe-containing intermetallic particles are cathodic with respect to the matrix and can lead to localized corrosion, whereas the high content of Mg makes the Fe-containing intermetallic particle less noble to the matrix. Yb-containing intermetallic particles did not show any dissolution of the matrix at the periphery of these particles. The results indicated that active dissolution and localized corrosion were the dominant corrosion mechanisms for Al-4Mg alloy. However the polarization curves of Al-4Mg-0.3Sc and Al-4Mg-0.24Sc-0.06Yb alloys showed spontaneous passivation followed by pitting corrosion. Results suggested that age-hardening heat treatment improved the stability of the passive film on Al-4Mg-0.3Sc and Al-4Mg-0.24Sc-0.06Yb alloys., This research was supported by The Project Bridging The Gap, funded by the Erasmus Mundus External Cooperation Window Program. Acknowledgements also to the University of Minho, for the provision of research facilities., info:eu-repo/semantics/publishedVersion

C ORROSION is one of the degradation mechanisms in aerospace structures [1]. Among various types of corrosion, pitting corrosion associatedwith the dissolution ofmetal is caused by the breakdown of the passive film on the metal surface and is known to be one of the major damage mechanisms affecting the integrity of many aerospace structures. Pitting corrosion is a complex process, and a fundamental aspect of pitting corrosion failure mechanisms is that they tend to initiate at the micro/nanostructure level [2]. The details of the mechanisms vary with material composition, electrolyte, and other environmental conditions [3]. It is well known that pitting corrosion has a strong effect on the fatigue life of aluminum alloys used in aircraft structures [4–6]. Prediction of pitting corrosion damage is therefore very important for the structural integrity of aerospace materials and structures. Although pitting corrosion has been studied extensively over the past two decades, computational modeling of pit initiation and growth is still open to investigation. Several pitting corrosionmodels exist in the literature [7–13] and are mostly empirical, mechanistic, deterministic, or phenomenological in nature. Pitting corrosion involves repassivation, mass transport, electrochemistry, and other mechanisms that operate at different length scales, thus making modeling a difficult problem. Recently, Burstein et al. [14] demonstrated that pit nucleation occurs at the microscopic level and is random in nature and that somemetals showpreferential sites of pit nucleation. Even though there are experimental studies aimed at understanding the basic mechanisms, the observed complexity is very difficult to interpret, with many controlled variables affecting the pitting process. In general, the corrosion damage modeling should involve not only physicochemical and environmental factors, but also various parameters, random in nature. Therefore, a more realistic computational corrosion damage model should integrate various parameters that are random in nature from electrochemistry, materials science, and probability and statistics. In contrast to mechanistic approaches, computational modeling of pitting corrosion based on local rules with evolving patterns may open up the possibility of getting insight into pitting corrosion from a different point of view. Recently, Pidaparti et al. [15] developed a two-dimensional model for studying pitting corrosion growth based on the cellular automata (CA) approach. However, they did not consider the initiation phase of pitting corrosion in their model. The main objective of this work is to develop an approach to modeling three-dimensional surface-corrosion-damage initiation and growth using specific rules governing the electrochemical reactions in a cellular automata environment. The results of corrosion damage growth obtained from the three-dimensional model are compared with the experimental data obtained from the Center for Materials Diagnostics at theUniversity ofDaytonResearch Institute.

Laser beam welding experiments have been carried out on 3-mm thick aluminium-magnesium alloy 5083 specimens in butt-joint configuration. The mechanical properties of the joints have been evaluated by performing tensile tests, hardness profiles across the weld sections, porosity measurements and EDX analyses. A design of experiment technique has been used to study the effects of the welding speed (v) and the incident laser power (P) on the aforementioned response variables that are considered to be representative for the weld quality. By comparing the welds obtained by operating at constant linear energy input released onto the material (P/v ratio), the best results have been found for higher laser powers and welding speeds. A clear correlation was found between the incidence of porosity, the tensile strength and the hardness of the fused zone. Welding reliability was enhanced for selected sets of process parameters capable of producing butt-joints showing mechanical properties very competitive if compared with the performances obtained, on similar aluminium alloys, using alternative joining technologies like friction stir welding or gas tungsten arc welding. (C) 2007 Elsevier B. V. All rights reserved.

We present an innovative real-time monitoring technique based on the correlation analysis of the plasma plume optical spectra generated during laser welding of Al alloys. In particular we show that it is possible to determine the transition between the two main welding modes: keyhole and conduction. (C) 2007 Elsevier B.V. All rights reserved.

Grain refinement via semi-solid deformation is desired to obtain superior mechanical properties of cast components. Using quantitative in situ synchrotron X-ray tomographic microscopy, we show an additional mechanism for the reduction of grain size, via liquation assisted transgranular cracking of semi-solid globular microstructures. Here we perform localized indentation of Al-15wt.%Cu globular microstructures, with an average grain size of ∼480 μm, at 555 °C (74% solid fraction). Although transgranular fracture has been observed in brittle materials, our results show transgranular fracture can also occur in metallic alloys in semi-solid state. This transgranular liquation cracking (TLC) occurs at very low contact stresses (between 1.1 and 38 MPa). With increasing strain, TLC continues to refine the size of the microstructure until the grain distribution reaches log-normal packing. The results demonstrate that this refinement, previously attributed to fragmentation of secondary arms by melt-shearing, is also controlled by an additional TLC mechanism., Nature Communications, 6, ISSN:2041-1723

One strategy for minimizing corrosive attack on carbon steel involves the application of hybrid coatings rich in rare earths, on the basis of their low toxicity and environmental sustainability. Tetraethoxysilane (TEOS) and 3-glycidoxypropyl-trimethoxysilane (GPTMS) were each used here as precursors, together with various quantities of colloidal silica suspension, to improve barrier properties of the coatings. Ce(NO3)3·6H2O was added as a source of Ce(III) for further inhibition of corrosion. The corrosion resistance of each kind of sample was evaluated using potentiodynamic polarization measurements and electrochemical impedance spectroscopy (EIS) in 0.35 wt.% NaCl solution. The electrochemical behaviour at various immersion times was modelled by equivalent circuits to demonstrate the effect of silica nanoparticle loading on corrosion. Additional testing was performed under harsh conditions of salt spray chamber. The microstructure and composition of sol-gel coatings were characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM/EDS), focused-ion beam SEM/EDS analysis, X-ray photoelectron spectroscopy and confocal microscopy. Fil: Santana, Ianina Cecilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Mar del Plata. Instituto de Investigación en Ciencia y Tecnología de Materiales (i); Argentina. Universidad Nacional de Mar del Plata. Facultad de Ingenieria; Argentina Fil: Pepe, Andrés. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Mar del Plata. Instituto de Investigación en Ciencia y Tecnología de Materiales (i); Argentina. Universidad Nacional de Mar del Plata. Facultad de Ingenieria; Argentina Fil: Jimenez Pique, Emilio. Universidad Politecnica de Catalunya; España Fil: Pellice, Sergio Antonio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Mar del Plata. Instituto de Investigación En Ciencia y Tecnología de Materiales (i); Argentina. Universidad Nacional de Mar del Plata. Facultad de Ingenieria; Argentina Fil: Milosev, Ingrid. Jožef Stefan Institute. Department of Physical and Organic Chemistry; Eslovenia Fil: Cere, Silvia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Mar del Plata. Instituto de Investigación en Ciencia y Tecnología de Materiales (i); Argentina

This thesis focuses on the use of x-ray diffraction to measure residual stresses around welds in 5XXX series aluminum-alloys used in naval ship structures both in the laboratory and the field. Tensile residual stresses are commonly generated during welding and, in sensitized alloys, can cause stress corrosion cracking. Peening techniques, such as ultrasonic impact treatment (UIT), can mitigate and possibly reverse these tensile residual stresses. This research uses x-ray diffraction to measure residual stresses around welds in AA5456 after UIT, around welds in AA5083 installed on-board a U.S. naval combatant and in AA5083 after in situ surface preparation. In the AA5456, we examined the importance of UIT parameters such as peening amplitude and pin size. It was found that all combinations of UIT parameters produced significant compressive stress but that some combinations resulted in extensive subsurface intergranular cracking in the sensitized AA5456. Optimal UIT parameters for mitigating the production of subsurface cracking were determined. In the AA5083, we examined the effect of field-based in situ surface preparation on residual stress measurements. The use of a portable x-ray diffractometry system to experimentally measure the distribution of residual stresses in aluminum-alloy ship structures on U.S. Navy vessels has been successfully demonstrated. http://archive.org/details/systematicreview1094545184 Lieutenant, United States Navy Approved for public release; distribution is unlimited.

Slow strain rate testing (SSRT) was carried out on over aged 7010 Al-alloy in laboratory air, glycerin and 3.5 wt.% NaCl solution with and without cathodic charging to study the hydrogen embrittlement susceptibility of the alloy in over aged condition. It was found that the over aged alloy exhibited high resistance to stress corrosion cracking (SCC) than hydrogen embrittlement (HE). The high SCC resistance is due to the modification in the grain boundary precipitate morphology and chemistry due to over aging, however it is suggested that the dislocations in the alloy are not completed annealed during over aging to arrest HE.

In this work, nanocontainers based on layered double hydroxide (LDH) loaded with two different corrosion inhibitors, namely 2-mercaptobenzothiazole (MBT) and cerium(III) nitrate, were prepared. MBT was intercalated into LDH galleries in anionic form by anion-exchange, while Ce3+ was fixed between polyelectrolyte layers on the surface of LDH-MBT nanoplatelets by the Layer-by-Layer (LbL) method. Both inhibitors were found in the modified LDHs (LDH-Mod) by X-ray diffraction and scanning electron microscopy coupled with energy dispersive spectroscopy. The release studies performed by UV-vis spectrophotometry indicated that the main triggering conditions for release of MBT from LDHs changed when LDH nanoplatelets were covered with polyelectrolytes. Furthermore, electrochemical impedance spectroscopy and DC polarization were used to investigate the effect in combining MBT− and Ce3+ within the same nanocontainer, for the corrosion protection of 2024-T3 aluminium alloy directly in solution as well as in a hybrid sol–gel coating. The obtained results open prospects for application of these systems as additives in multifunctional smart coatings.

The localized corrosion and electrochemical behaviour of Al-0.28 wt.% Sc and Al-0.24 wt.% Sc-0.07 wt.% Yb alloys have been studied in 3.5 wt.% NaCl solution. The results showed that the impurity intermetallic phases containing Fe can act as cathodes and promote localized corrosion in the surrounding matrix. Yb had strong effect on the polarization behaviour in the anodic domain of Al-Sc alloy by improving the pitting corrosion resistance of this alloy. It has been deduced that a high number density of fully coherent nano-sized Al3Sc or Al-3(Sc, Yb) precipitates in heat treated alloys can enhance the passivation tendency of these alloys., This research was supported by The Project Bridging The Gap, funded by the Erasmus Mundus External Cooperation Window Program. Acknowledgements also to the University of Minho, for the provision of research facilities, info:eu-repo/semantics/publishedVersion

A steady axial magnetic field is applied to a liquid metal zone heated by induction currents. The resulting alternating Lorentz force causes pressure oscillations that being strong enough lead to cavitation in the molten metal. Amplitude of the pressure oscillations is proportional to the product of the induced currents and the steady axial magnetic field induction. We follow an approach where the acoustic pressure is maximized by the induction currents. The onset of cavitation is identified by the occurrence of sub-harmonics of the drive frequency in sound recorded at the surface of the experimental cell. It is demonstrated that cavitation in a liquid metal may be excited by a superimposed axial magnetic field of a moderate 0.5 T induction.