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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.

The December 2009 special issue of Journal of Alloys and Compounds focuses on XV International Symposium on Metastable, Amorphous and Nanostructured Materials (ISMANAM) held on July 6-10, 2009 in Argentina. ISMANAM 2008 had 221 participants from four continents that have contributed to the symposium with 267 works. The symposium discussed metals, ceramics, polymers, composites, powders to bulk materials, including composites and nanocomposites. The ISMANAM Steering Committee acknowledged the outstanding achievements of a senior and a junior scientist by granting the awards.

This thesis looks at the viability of using oxide based ceramic matrix composites for high temperature gas turbine sealing applications. Two material systems namely Nextel312/SiOC, Nextel 312 fibre with SiOC matrix, and Nextel720/Alumina, Nextel 720 fibre with alumina matrix, were studied in detail and modified in order to produce CMC variants that could perform adequately as seals materials. Nextel 312/SiCO composites were optimised for sealing applications at service temperatures of up to 900°C, using repeated matrix impregnation and pyrolysis (polymer infiltration processing, PIP) at different temperatures. The microstructures were analysed using SEM, TEM, XRD and X-ray computed tomography in order to quantify the differences arising from the manufacturing process. The materials were tested mechanically to determine their strength, ductility, interfacial shear strength and fracture behaviour. The thermal stability of the composites was investigated using thermo-gravimetric analysis (TGA). The objective was to identify the PIP temperature that could provide the optimum combination of properties of high elastic flexural deflection, thermal stability, low interfacial shear stress for toughening and low mass loss up to the intended service temperature. Processing at 950°C or above gave a combination of high effective elastic modulus, good resistance to mechanical damage from tensile strain and good thermal stability. The temperature capability of this material system, as studied in literature and by carrying out experiments in this piece of work, suggests that this material system would not be able to perform adequately as a seal at temperatures ≥ 900°C for extended periods, hence it was not investigated further in this study. After identifying the key gaps in literature, an investigation was carried out that looked at the deformation and fracture of a 0/90° Nextel 720 alumina/Mullite fibre with alumina matrix ceramic-matrix composite material by in situ synchrotron X-ray computed micro-tomography at ambient temperature and at 1100°C. A three-point loading configuration was used and samples were loaded monotonically until final fracture. The flexural strength was found to be unaffected by temperature. 3D visualisation of the microstructure showed that fracture occurred by propagation of internal cracks at stresses close to the flexural strength. Shear fractures propagated across fibre bundles that were oriented perpendicular to the flexural stress at both room temperature and 1100°C. At 1100°C however, an additional failure mode of delamination was observed, in the form of needle shaped cracks in between fibre bundles parallel to x axis. This may arise from relaxation of thermal residual stresses or could be a function of the creep strains which could develop within the sample. Once it was established that Nextel 720 with alumina matrix could perform adequately at the given temperature, 8 different variants, each with different fibre layup, weave pattern and processing temperature, were produced. Three-point bend tests were done at 1100°C, to quantify the effective flexural modulus along with the proportional limit stress, PLS, for each of the samples. The results suggested that out of all 8 samples, EF 20, which had a unidirectional fibre weave architecture, sintered at 1250°C, with an involute layup, had the most optimum mechanical properties. Creep tests were carried out on beam specimens in three-point bend test configuration at 25MPa stress, at 1100°C. The results indicated that EF 20, due to its unidirectional weave configuration, developed lower primary and secondary creep strains, in comparison to EF 19 which had 50% fibre tows in the warp and the weft directions. The involute layup outperformed the 0/90° layup, due to higher shear strength between the layers. The samples sintered at 1250°C had similar creep performance to the samples sintered at 1200°C. Using the effective modulus values calculated for EF 19-1 and EF 20-1, piston rings with a gauge diameter of 140 mm were designed and subjected to a constant deflection condition with a nominal stress of 25 MPa, at 1100°C. The EF 19 ring performed adequately, only losing 13% of their spring at 1100°C. When the fitted stress was beyond the PLS limit permanent microstructural deformation was observed in the sample, EF 20, which resulted in higher stress relaxation than normal. Hence future ring design must be done keeping in mind the PLS. The manufacturing process should be reviewed, such that the machining of the piston ring does not result in cutting through the fibre layers, lowering the number of defects and consequently resulting in a higher PLS. This work was successful in identifying and modifying an oxide based ceramic matrix composite system for use in gas turbine sealing applications at 1100°C.

This project aimed to study the atmospheric stress corrosion cracking (SCC) behavior of 6061 T6 aluminum alloy and a composite based on the same alloy, reinforced with SiC nano particles. The materials were machined into taper-shape specimens, and on each specimen, three salt deposits were applied at the center and two ends. The SCC experiment was conducted at a temperature of 40 ºC and a relative humidity of 30%. The 6061 T6 aluminum alloy specimens and the composite specimens were tested at an applied constant load of 948±20 N and 1050±20 N respectively. Cracks were only observed at the center salt deposit location of 6061 T6 aluminum alloy specimens. It is observed that pre-pitting accelerates the development of strain, but does not affect the final strain that is developed in the SCC test. There was no measurable strain change at the end location of all the 6061 T6 aluminum specimens, indicating that there is a threshold of applied stress or strain for stress corrosion cracks to develop. For the aluminum based composite, no crack was found on the specimens. The applied strain of the center location of the composite specimen is equal to that of the end location of the aluminum specimen, within the error range, and it is concluded that the addition of SiC nano particles may make the composite material more susceptible to stress corrosion. DIC technique was adopted to measure the displacement and strain change of specimens, and an effective method to monitor the initiation and propagation of small cracks has been developed. SEM and X-ray tomography were used to observe specimens after SCC testing, verifying the DIC result.

Novel Al matrix nanocomposites have been produced and studied based on a high strength nanoquasicsytalline (NQX) Al93Fe3Cr2Nb2 (at%) alloy. Two types of metallic materials including pure metals (such as Nb or Ti) and amorphous Ti50Cu25Ni25 (at%) alloy ribbons have been used as reinforcements. The constituent materials, extruded alloy/composites bars have been characterised using various techniques including electron microscopy, X-ray diffraction, differential scanning calorimetry, and X-ray computed tomography. The mechanical properties of the extruded alloy/composites bars have been studied by Vicker’s hardness tests, compression tests at room temperature and elevated temperature, and split Hopkinson pressure bar tests. The microstructure of the alloy/composites contains i-phase particles embedded in α-Al, mostly with diameters under 1µm, along with some possible intermetallic precipitates. NQX/Ribbons composite also contains amorphous phase and a crystalline Ti2(Cu, Ni) phase. Compression strength of the alloy/composites at room temperature and elevated temperature are superior to conventional high strength Al alloys and corresponding Al matrix composites. The elastic (non-plastic) reinforcements (the melt-spun amorphous Ti-based alloy ribbons) contributes to an increase of the strength but a reduced ductility, which leads to a lower toughness material respect to the alloy matrix. However, the elastic-plastic reinforcements (pure Nb or pure Ti) contributes to an increased strength and maintain the ductility, which leads to a higher toughness material respect to the alloy matrix. From the tomography results of the NQX/Ribbons composite and Al-QC/Nb composite, their plastic deformation has been visualised and localised strain at the shear stress plane has been identified. The strength of NQX alloy have been attributed to grain boundary and Orowan’s strengthening mechanisms, while solid solution and dislocation strengthening have also contributed to the strength in a minor fraction. Strengthening from the reinforcements in the composites have been explained by composite models such as Rule of Mixture and Shear Lag model which fit well with the experimental results.

The strength of conventional aluminium alloys decreases rapidly above 150°C, which limits their applications at elevated temperature. Rapid solidified (RS) nanoquasicrystalline Al93Fe3Cr2Ti2 (at.%) alloy has previously shown outstanding mechanical performance and microstructural stability at elevated temperatures (>300°C). In order to obtain an outstanding Al-based material with very high strength in a broad range of temperature, it is proposed to produce a nanocomposite consisting of nano-size ceramic particles homogenously distributed in a nanoquasicrystalline alloy matrix. The main challenges of processing nanoquasicrystalline nanocomposites in ball milling are to prevent the metastable quasicrystalline phase from decomposition and to avoid the agglomerations of nano-size γ-Al2O3. [abstract continues in thesis]

Spray forming with co-injection of a solid particulate phase to form a metal-metal composite has been studied as a new route for manufacture. Two Al-based matrices were investigated: Al-12Si for testing the feasibility of the new manufacturing route and Al-4Cu for providing better mechanical performance. For both composite types, Ti was chosen as the particulate phase and the processing-microstructure-property relationships then studied. At Peak Werkstoff GmbH, Germany 12 wt%Ti particles were co-injected into an atomised Al alloy droplet spray and co-deposited to form a ∼300 kg billet. The microstructure comprised refined equiaxed α-Al grains (∼5μm), spherical Si particles (∼5μm) and uniformly distributed Ti particles (∼80μm). Sections of the billet were extruded under a range of conditions into long strips 20mm wide and 6mm, 2.5mm and 1mm thickness. At high strains, the Ti particles were deformed into continuous fibres of a few microns in thickness. Accumulative roll bonding was then performed to higher total strains, while maintaining a constant cross-section, reducing the Ti fibres to sub-micron thickness. The fibres were studied by extraction after selective dissolution of the α-Al matrix. There was no interfacial reaction between α-Al and Ti or any measurable oxide formation, thus providing encouragement for the manufacture of metal-metal composites by co-spray forming. A powder injection pump was successfully integrated and commissioned on the spray forming facility at Oxford University. The pump was calibrated to optimise powder flow rates. Three Al-4Cu+Ti composite billets were processed with each containing Ti powder with a different processing history. Up to 20vol%Ti was successfully incorporated, however due to the cooling effect from powder injection, porosity was significant. The quenching effect provided a finer Al-4Cu grain structure in the region of Ti injection, and also promoted precipitation of θ'-Al2Cu precipitates. A Ti/Al-4Cu interfacial reaction was more prominent in the billet spray formed at 850°C than those spray formed at 750°C. Angular Ti processed by a hydride-dehydride route had better deformation characteristics than spherical gas atomised Ti. Deformation processing by extrusion and rolling was investigated for Al-4Cu+20vol%Ti using SEM, EBSD and FIB. After extrusion to a strain of 5, the composite contained elongated reinforcing fibres characteristic of metal-metal composites. The microstructure studied by EBSD revealed equiaxed polygonal Al-4Cu matrix grains. Rolling was not as efficient as extrusion in producing elongated Ti fibres and was attributed to a lower deformation processing temperature. The rolled composites consisted of elongated Al-4Cu grains 1-5μm in thickness. An UTS of 339MPa at a strain of 3 was attributed to texture strengthening in the α-Al.

This study examines the production parameters of bulk nanoquasicrystalline Al-Fe-Cr-Nb alloys.

Conventional aluminium alloys rapidly lose mechanical strength above 150°C, limiting their elevated temperature usage in engineering applications. The rapidly solidified (RS) nanoquasicrystalline (NQX) Al93Fe3Cr2Ti2 alloy has the potential to be used at elevated temperatures (250-350°C) as it retains high strength at those temperatures. There have been some microstructural studies and limited work on the mechanical properties of the alloy. However, a detailed study of both the microstructure and the mechanical properties (elastic constants, tensile behaviour, and fatigue behaviour) has not been carried out for the bulk alloy. RS NQX Al93Fe3Cr2Ti2 alloy powder in 25-50μm fraction size was consolidated using extrusion to produce bars. The microstructure of the extruded bars was characterised using X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, scanning transmission electron microscopy, and electron backscattered diffraction. The microstructure was observed to contain approximately 44% volume fraction of nano size quasicrystalline icosahedral (I-phase) particles embedded in submicron size aluminium grains. The alloy bar samples were annealed at elevated temperatures (250, 300, 350, 400, and, 450°C) for long hours (100h) and the microstructural evolution was studied using aforementioned characterisation techniques. As a result of the annealing at 350°C for 100h, the I-phase starts to transform into other stable and metastable intermetallics which grow with the increase in the annealing temperature. Room temperature mechanical properties such as hardness, Young's modulus, and shear modulus were measured, and tensile tests for the alloy samples were carried out in the 'as extruded' and 'annealed' conditions. The alloy retains its microstructure and mechanical properties after annealing at 300°C for 100hrs. The alloy results are exceptional, as conventional Al alloys suffer >45% strength loss after the same heat treatment. Orowan and Hall-Petch mechanisms were found the dominant strengthening mechanism in the alloy at room temperature. Tensile tests were performed at 23°C as well as elevated temperature (150-500°C) at four strain rates (10-2 to 10-5 s-1). Tensile strength values at ambient temperature are comparable to the high strength conventional Al alloys; however, elevated temperatures (∼300°C) tensile strength of the NQX Al93Fe3Cr2Ti2 alloy is 3 to 8times higher than conventional Al alloys. High values of the apparent stress exponent and the apparent activation energy of plastic flow were observed for all test temperatures. At intermediate temperatures (150-250°C), several manifestations of dynamic strain ageing were observed in the alloy. Plastic flow models were used to explain the elevated temperature behaviour, and it was deduced that 250-350°Ctemperature range experienced a transition from power law break down to power lawcreep, whereas a climb controlled mechanism was operative at 425-500°C temperature range. Fatigue testing was carried out using conventional and ultrasonic frequencies at roomtemperature and only conventional frequency at 300°C. The alloy exhibits type II fatigue behaviour. The room temperature fatigue performance is comparable with the high strength conventional alloys, and far superior to any conventional Al alloy at 300°C.

Particulate reinforced Al alloy composites (AlMCs) are recognized as important structural materials due to their lightweight, high modulus and strength and high wear resistance. In order to understand the effect of matrix, reinforcement and secondary processing techniques on the microstructure development and mechanical properties of AlMCs produced by powder metallurgy routes, Al alloy composites reinforced with three types of reinforcements by different secondary processing techniques have been produced and examined. Fabrication of Al or 6061Al alloy based composites reinforced with nano-sized SiC particles (~500nm), micro-sized ( After cold rolling, the evolution in microstructure, texture and strength has been analysed. A typical near β fibre texture with highest intensities near Copper and Brass orientations has been developed for 6061Al/NQX and 6061Al/Nb composites. For 6061Al/SiC composites, a randomized texture with very small grains has achieved due to the presence of the non-deformable SiC particles. Mechanical property tests including microhardness, three-point bending tests and tensile tests have been carried out on cold rolled samples and the results exhibit some level of improvement when compared with as-extruded samples due to work hardening. Finally, the work moves on to the general discussion based on the previous result chapters. The microstructural development related to reinforcement, matrix and interfacial areas during extrusion and cold rolling has been summarised and the correlation between microstructure and mechanical properties has been discussed. The thesis provides a thorough understanding of AlMCs produced by powder metallurgy routes in terms of matrix, reinforcement and processing techniques. It can provide reference to the future development of AlMCs for high strength applications.

The main aim of this work is to study the bonding behaviour of Al alloys as a function of temperature, time and processing conditions from a perspective of oxidation so as to gain a comprehensive knowledge of the oxidation issues during processing and to design better bonding approaches for different alloy systems. Two major parts of work have been carried out during the study: (a) the use of a stacking approach and a double pouring approach to potentially join two Al alloys and their effectiveness are assessed; and (b) a precise investigation of the oxidation mechanisms for Al-Cu and Al-Mg alloys using a combination of theoretical analysis and experimental characterization. The project started with the use of a stacking approach to try to bond two stacked Al alloys. The stacked sample can be viewed as a bi-metal which has an oxide bi-film layer at the bond interface. It was found that the bi-film layer was a physical barrier preventing direct metallic bonding. How this bi-film layer evolves during the bonding process was then investigated. The work then moved on to investigate the oxidation mechanisms of Al-Cu alloys and Al-Mg alloys in greater detail. The thermodynamics, kinetics of oxidation, chemistry and morphology of the oxide scale were particularly studied. Briefly speaking, in the case of the Al-Cu-O2 system, the oxidation proceeds in the order of amorphous γ-Al2O3 - to - crystalline γ-Al2O3 - to - α-Al2O3; in the case of the Al-Mg-O2 system, the oxidation proceeds as amorphous γ-Al2O3 to MgO to MgAl2O4 and the morphology of the oxide scale develops from a protective layer to a porously structured composite layer. A double pouring approach was finally developed to bond Al and Al-5Cu but the method has still not been perfected due to the formation of bi-film defects along the bond interface. Induction melting, squeeze casting, and extrusion bonding were therefore studied as an attempt to reduce the harmful effect of bi-film defects. Although all of the approaches exhibit some limitations, they have potential for future development.

Rapidly solidified nanoquasicrystalline Al93Fe3Cr2Ti2 at% alloy has previously shown outstanding mechanical performance and microstructural stability up to elevated temperatures. Despite this, no in-depth study had previously been performed assessing the active strengthening mechanisms, the long term microstructural stability and the effect of plastic deformation at elevated temperature to simulate the production methods utilised for engineering applications. The current project analysed eight bars consisting of a nanoquasicrystalline Al93Fe3Cr2Ti2 at% alloy matrix with varying amounts of pure Al fibres, produced through gas atomisation and warm extrusion. Microstructural characterisation and thermal analysis of the as-atomized powder was carried out to assess whether microstructural changed were likely to occur at the extrusion temperature. A microstructure made primarily of nanometre-sized icosahedral particles in an FCC-Al matrix was observed through a combination of SEM, TEM (and CBDP), EDX, XRD. Thermal analysis of the powders performed by DSC showed that no change was expected to occur at the extrusion temperature. Five bars were extruded during the course of this project: one bar of pure Al-Fe-Cr-Ti alloy, two composite bars with 10 vol% added pure Al and two bars with 20 vol% added Al. Three more bars were received from a previous project and analysed. Warm extrusion caused the powder particles to become well bonded and elongated in the extrusion direction introducing a preferred orientation in the FCC-Al grains. A bimodal distribution of grain size was observed after extrusion. Several low angle (5-15 °) grain boundaries were also identified by EBSD along the extrusion direction. No obvious change in size or shape was observed by TEM in the icosahedral phase (a bimodal distribution of hard, incoherent precipitates was observed after extrusion), or any change in the amount of solutes in solid solution in the Al matrix. Mechanical properties at room temperature were tested by Vickers microhardness, quasi-static tensile tests, dynamic tensile tests and dynamic compression tests. A theoretical model correlating the microstructures observed with the various active strengthening mechanisms was applied in order to predict an estimate of the yield strength of the material produced. It was found that the strength of the Al93Fe3Cr2Ti2 alloy came primarily from a combination of the effect of the hard, incoherent nanoparticles, the small grain size and work hardening. The fibre addition to this alloy caused a linear decrease in mechanical strength with increasing vol% pure Al. This work represents the first quantitative estimate of which strengthening mechanisms are active and how much they influence the mechanical strength of Al93Fe3Cr2Ti2 alloy and composites. An understanding of the yield strength is essential as engineering components would only be safe to use within the elastic region. To investigate the thermal stability of the alloy and composites, thermal analyses involving DSC and long heat treatments (up to a maximum of 1000 hours) were performed at various temperatures along with microstructural characterisation by XRD, SEM and TEM and microhardness tests. No microstructural change was detected, however a 2-5% decrease in microhardness was observed. Compression tests were performed across a range of temperatures and strain rates to simulate the behaviour of these composites under typical conditions necessary to process them into useful engineering components. Phase changes occurring during plastic deformation at high temperature were investigated by XRD. 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 at 500 °C into more stable intermetallic phases. Serrated flow was also observed in some of the tests. The high temperature compressive data was then used for the first time in a semi-quantitative analysis to determine which species in solid solution (Fe, Cr or Ti) was likely to cause the serrations. A dynamic strain ageing model, which calculates the diffusion coefficients at the minimum in ductility and strain rate sensitivity, suggested that the Ti in solid solution in the matrix could be the most likely candidate.

Nel secondo Cinquecento, con l’affermarsi di alcune sperimentazioni di pesca d’altura, frutto di un’evoluzione di antichi sistemi di cattura nelle acque costiere e lagunari si avvia la formazione di marinerie organizzate che alimentano poi nel corso del Seicento una sorta di “rivoluzione piscatoria” mirata ad un maggiore sfruttamento delle risorse ittiche del Mediterraneo. Precedentemente erano in uso pratiche di pesca lungo le coste gestite da gruppi di pescatori organizzati in “compagnie”. La limitatezza degli spazi costieri atti allo sfruttamento delle risorse ittiche obbligava le comunità ad un severo controllo delle acque territoriali, con la messa a punto di apposite norme inserite negli statuti cittadini che comportava comunque la necessità di regolamentare l’accesso alle risorse con concessioni dei permessi di pesca (emblematici e assai rappresentativi sono i casi di Noli, Gaeta, Comisa, Lussino, isole dalmate soggette al dominio veneto e a questi si aggiungono i laghi salsi e altre aree lagunari vocate a questo sfruttamento). Salvo particolari eccezioni, fino a tutto il Cinquecento le pratiche piscatorie si svolgono dunque pressoché esclusivamente lungo i litorali. Le acque costiere perciò risultano necessariamente monitorate con una possibilità di sfruttamento regolamentata dagli statuti comunali al fine di scongiurare situazioni di sovraffollamento ed il depauperamento delle riserve. La ristrettezza dei campi acquei in cui dar pratica ai pescatori obbligava infatti le autorità di molte località rivierasche, ad una ripartizione equa delle zone operative con un sistema di turni a rotazione che disponeva un utilizzo nominativo a tempo determinato delle diverse zone (“poste”) individuabili nelle rispettive giurisdizioni, assegnate per graduatoria. Si assiste insomma ad una sorta di “gara”, termine espressamente utilizzato a questo proposito anche da Paolo Giovio in un passo del suo libellus sui pesco edito nel 1524. Una “gara”, concentrata soprattutto durante la buona stagione, sorvegliata dalle autorità locali al fine di scongiurare l’insorgenza di possibili controversie fra i pescatori. I conflitti fra pescatori risultano comunque una costante e si accendono anche quando i gruppi più intraprendenti si avventurano al di fuori degli ambiti geografici di appartenenza, invadendo gli spazi territoriali di altre comunità con emigrazioni marittime stagionali. Se da un lato gli spostamenti delle flottiglie pescherecce maggiormente evolute alimentano preoccupazioni di tipo protezionistico, dall’altro, contribuiscono a volte a sopperire meglio all’approvvigionamento dei mercati delle città portuali mancanti di pescatori di mestiere. E non mancano neanche poi i casi di insediamento di pescatori professionisti forestieri che, nell’arco di una, due generazioni vanno a formare nuove compagini marinare nella località di adozione. D’altro canto l’osservazione della superiorità tecnologica di strumentazioni differenti rispetto a quelle normalmente in uso, dà spunto ai pescatori locali per cimentarsi in strategie di pesca alternative attraverso un’emulazione personalizzata delle pratiche “forestiere” che permette loro di azzardare anche il salto di qualità. Proprio in virtù di questi contatti si produrranno le due importanti svolte nella storia delle attività alieutiche del mare interno, individuate la prima con l’introduzione e diffusione della pesca a tartana, la seconda con la pesca a strascico effettuata con coppie o gruppi barche.