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This research deals with the synthesis and characterization (transmission electron microscopy and optical spectroscopy) of composite nanocondensates produced by pulse laser ablation (PLA) on Zn, Zn-Cu and Cu-Au targets in liquid or vacuum. First, wurtzite-type (W)-ZnO and

The present thesis deals with synthesis of free alloy nanoparticles in immiscible alloy systems by the process of laser ablation under a liquid. In this process the alloy target is submerged in a liquid and the plume formed by the laser beam interaction with the target is confined in the liquid. The nanoparticles formed inside this plume and get quenched by the surrounding liquid yielding suspension of nanoparticles in the liquid. By the addition of suitable surfactants, these nanoparticles can be protected from other reactions and their size can be controlled by preventing further growth. We have selected immiscible alloys for the present study. These alloys tend to phase separate in melt as well as in solid depending on the value of the positive heat of mixing. We have used two binary alloys for the present study. These are alloys in Ag-Cu system and Fe-Cu system. In both these systems, there are reports of formation of extended solid solution due to kinetic factors during nonequilibrium processing like rapid solidification and mechanical alloying. In the present thesis we report synthesis of alloy nanoparticles of different compositions and sizes in these two systems and explore the nature of the phases that form in the small (nano) particles and their evolutionary pathways leading to the final microstructure. Microscopic techniques, especially transmission electron microscope, were used for characterization of these nanoparticles. The phase evolution was further studied using in situ microscopic techniques. After introducing the thesis in the Chapter 1, we describe briefly the relevant literatures in Chapter 2. The experimental details, in particular the experimental set up for laser ablation with targets under liquid are described in chapter 3. This chapter also includes the experimental details of the characterization. Transmission electron microscopy was used as primary characterization tool in the present study. The Chapter 4 presents the result of our study of alloy nanoparticles in Fe-Cu system. This system exhibits a submerged liquid miscibility gap. Although we have studied alloy targets of different compositions, the results of alloy nanoparticles obtained from targets with compositions Cu-40at.%Fe and Cu-60at.%Fe were primarily presented in this chapter. The nanoparticles that were synthesized had a size range of approximately 40nm to more than 100 nm. These particles have spherical morphology. The measurements of local compositions of different locations in the particle indicate the presence of a layer of Fe3O4 oxide at the spherical surface. This layer is devoid of copper. Most of the copper exist in the core of the particle. Fe rich spherical particles of much smaller size (~15 nm) are found to be embedded in the copper rich core. The copper formed solid solution with Fe and a copper concentration gradient exists in the particle below oxide layer due to oxidation of Fe. In contrast the nanoparticles obtained from alloy target with composition Fe-40at.% Cu have a spherical morphology. These have a composite structure with a Fe core in addition to Fe3O4 oxide layer at the surface. We have attempted to explain the phase evolution taking into account under cooling of the melt condensate that forms in the plume and their subsequent solidification through submerged miscibility gap. The chapters 5-7 deals with alloys of Ag-Cu system. In Chapter 5, we have carried out a detailed study of morphological evolution of the nanoparticles of Ag-Cu system. After optimizing the ablation parameters using pure Ag and Cu targets, we have synthesized alloy nanoparticles using different target compositions over the entire range of compositions with sizes having a mode of 25 nm. The evolution of the two phase structure is shown to be composition dependent with particles near equiatomic composition exhibit solid solution with uniformly distributed segregations of composition (Cu & Ag rich) while copper rich alloys exhibit a core shell structure with outer layer being Ag rich. The isothermal experiments again reveal emergence of core-shell morphology at intermediate time for particles with equiatomic composition. In order to compare the results of Ag-Cu nanoparticles with particles produced by other techniques we have synthesized Ag-Cu nanoparticles of near equiatomic composition by chemical route using nitrate salts and NaBH4 as reducing agent. PVP was used as capping agent. The results are presented in chapter 6. Depending on time of reaction, it is possible to synthesis free alloy particles from 2-3 nm to a network of chains. The nanoparticles contain Ag rich and Ag deficient region with Ag tends to segregate near surface. We have also presented mechanism for the formation of chain structure with prolonged reaction. The thermodynamic basis of phase formation in the immiscible system and evolution of phases under nonequilibrium situation have been discussed in chapter 7. This also includes a model to estimate size dependent surface energy. The analysis presented allows a discussion of possible pathways for phase evolution observed in the present work. The thesis ends with a final chapter that discussed the critical issues remains to be addressed and possible future work.

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A liga binária Al-Cu é a base para todas as ligas da série 2xxx que são de grande importância em diversas aplicações como na indústria aeronáutica, transporte, máquinas e equipamentos. No entanto, pouco se conhece sobre os efeitos do tipo de solidificação desta liga e da deformação plástica sobre sua resistência à corrosão em meio aquoso. O escopo deste trabalho foi procurar correlacionar microestruturas da liga Al-4,5%p.Cu solidificada de maneira convencional e unidirecional vertical ascendente (brutas de fusão, deformadas plasticamente por forjamento rotativo a frio, e tratadas termicamente visando recristalização), orientações preferenciais de grãos com as características de resistência à corrosão. Foram utilizadas técnicas de microscopia eletrônica de varredura e óptica, difratometria de raios X e ensaios eletroquímicos em soluções de NaCl 0,6 M e Na2SO4 0,1M por polarização potenciodinâmica e espectroscopia de impedância eletroquímica. A taxa de corrosão foi maior em solução de NaCl 0,6 M do que em Na2SO4 0,1M para todas as amostras. O aumento da redução por deformação plástica conduziu a uma diminuição da resistência à corrosão da liga devido ao aumento das tensões internas e ao aparecimento de orientações preferenciais dos grãos de menor densidade planar como (200) e (220). As amostras recristalizadas apresentaram de uma forma geral uma resistência maior quando comparada às amostras não recristalizadas, comportamento atribuído ao alívio de tensões internas e ao desaparecimento da orientação preferencial do plano menos denso (220). / The binary Al-Cu alloy is the basis for all AA2xxx alloys, being important in several applications as aeronautical industry, transportation, machines and equipments. However, few is known about the effect of the type of solidification of this alloy and plastic deformation on the corrosion resistance in aqueous medium. The scope of this work was to correlate microstructures of the Al-4.5 % wt. Cu alloy solidified under conventional and upward direct chilling conditions (as cast condition, plastic formed by cold swaging, and heat treated seeking recristallyzation), crystallographic orientations and corrosion resistance characteristics. Scanning electron microscopy and optical, X ray diffractometry techniques and electrochemical measurements in NaCl 0,6 M and Na2SO4 0,1 M potentiodynamic polarization and electrochemical impedance spectroscopy were used. The corrosion rate in NaCl 0,6 M was higher than in Na2SO4 0,1 M for all samples. The increase of the plastic deformation led to alloy corrosion resistance decrease, due to the internal stresses increase and the arising of less dense grain preferred orientations of lower planar density as (200) and (220) orientations. The recristallyzed samples presented in a general way a higher corrosion resistance, when compared to the non recristallyzed samples, being imputed to the internal tension relieve and the extinction of the plane (220) of lower planar density to a structure of random orientation.

A liga binária Al-Cu é a base para todas as ligas da série 2xxx que são de grande importância em diversas aplicações como na indústria aeronáutica, transporte, máquinas e equipamentos. No entanto, pouco se conhece sobre os efeitos do tipo de solidificação desta liga e da deformação plástica sobre sua resistência à corrosão em meio aquoso. O escopo deste trabalho foi procurar correlacionar microestruturas da liga Al-4,5%p.Cu solidificada de maneira convencional e unidirecional vertical ascendente (brutas de fusão, deformadas plasticamente por forjamento rotativo a frio, e tratadas termicamente visando recristalização), orientações preferenciais de grãos com as características de resistência à corrosão. Foram utilizadas técnicas de microscopia eletrônica de varredura e óptica, difratometria de raios X e ensaios eletroquímicos em soluções de NaCl 0,6 M e Na2SO4 0,1M por polarização potenciodinâmica e espectroscopia de impedância eletroquímica. A taxa de corrosão foi maior em solução de NaCl 0,6 M do que em Na2SO4 0,1M para todas as amostras. O aumento da redução por deformação plástica conduziu a uma diminuição da resistência à corrosão da liga devido ao aumento das tensões internas e ao aparecimento de orientações preferenciais dos grãos de menor densidade planar como (200) e (220). As amostras recristalizadas apresentaram de uma forma geral uma resistência maior quando comparada às amostras não recristalizadas, comportamento atribuído ao alívio de tensões internas e ao desaparecimento da orientação preferencial do plano menos denso (220). / The binary Al-Cu alloy is the basis for all AA2xxx alloys, being important in several applications as aeronautical industry, transportation, machines and equipments. However, few is known about the effect of the type of solidification of this alloy and plastic deformation on the corrosion resistance in aqueous medium. The scope of this work was to correlate microstructures of the Al-4.5 % wt. Cu alloy solidified under conventional and upward direct chilling conditions (as cast condition, plastic formed by cold swaging, and heat treated seeking recristallyzation), crystallographic orientations and corrosion resistance characteristics. Scanning electron microscopy and optical, X ray diffractometry techniques and electrochemical measurements in NaCl 0,6 M and Na2SO4 0,1 M potentiodynamic polarization and electrochemical impedance spectroscopy were used. The corrosion rate in NaCl 0,6 M was higher than in Na2SO4 0,1 M for all samples. The increase of the plastic deformation led to alloy corrosion resistance decrease, due to the internal stresses increase and the arising of less dense grain preferred orientations of lower planar density as (200) and (220) orientations. The recristallyzed samples presented in a general way a higher corrosion resistance, when compared to the non recristallyzed samples, being imputed to the internal tension relieve and the extinction of the plane (220) of lower planar density to a structure of random orientation.

AA6061 Al alloys modified with addition of Mn, Cr and Cu were homogenized at temperatures between 350 ºC and 550 ºC after casting. STEM experiments revealed that the formation of α-Al(MnFeCr)Si dispersoids during homogenization were strongly affected by various factors such as heating rate, concentration of Mn, low temperature pre-nucleation treatment and homogenization temperature. Through analysis of the STEM results using an image software Image-Pro, the size distributions and number densities of the dispersoids formed during different annealing treatments were quantitatively measured. It was revealed that increasing the heating rate or homogenization temperature led to a reduction of the number density and an increase in size of the dispersoids. The number density of dispersoids could be markedly increased through a low temperature pre-nucleation treatment. A higher Mn level resulted in the larger number density, equivalent size and length/width ratio of the dispersoids in the alloy. Upsetting tests on two of these Mn and Cr-containing AA6061 (Al-Mg-Si-Cu) Al alloys with distinctive Mn contents were carried out at a speed of 15 mm s-1 under upsetting temperature of 450 ºC after casting and subsequent homogenization heat treatment using a 300-Tone hydraulic press. STEM experiments revealed that the finely distributed α-Al(MnFeCr)Si dispersoids formed during homogenization showed a strong pinning effect on dislocations and grain boundaries, which could effectively inhibit recovery and recrystallization during hot deformation in the two alloys. The fractions of recrystallization after hot deformation and following solution heat treatment were measured in the two alloys with EBSD. It was found that the recrystallization fractions of the two alloys were less than 30%. This implied that the finely distributed α-dispersoids were rather stable against coarsening and they stabilized the microstructure by inhibiting recovery and recrystallization by pinning dislocations during deformation and annealing at elevated temperatures. By increasing the content of Mn, the effect of retardation on recrystallization were further enhanced due to the formation of higher number density of the dispersoids. STEM and 3-D atom probe tomography experiments revealed that α-Al(MnFeCr)Si dispersoids were formed upon dissolution of lathe-shaped Q-AlMgSiCu phase during homogenization of the modified AA6061 Al alloy. It was, for the first time, observed that Mn segregated at the Q-phase/matrix interfaces in Mn-rich regions in the early stage of homogenization, triggering the transformation of Q-phase into strings of Mn-rich dispersoids afterwards. Meanwhile, in Mn-depleted regions the Q-phase remained unchanged without segregation of Mn at the Q-phase/matrix interfaces. Upon completion of α-phase transformation, the atomic ratio of Mn and Si was found to be 1:1 in the α-phase. The strengthening mechanisms in the alloy were also quantitatively interpreted, based on the measurements of chemical compositions, dispersoids density and size, alloy hardness and resistivity as a function of the annealing temperature. This study clarified the previous confusion about the formation mechanism of α-dispersoids in 6xxx series Al alloys. Four-point bend fatigue tests on two modified AA6061 Al alloys with different Si contents (0.80 and 1.24 wt%, respectively) were carried out at room temperature, f = 20 Hz, R = 0.1, and in ambient air. The stress-number of cycles to failure (S-N) curves of the two alloys were characterized. The alloys were solution heat treated, quenched in water, and peak aged. Optical microscopy and scanning electron microscopy were employed to capture a detailed view of the fatigue crack initiation behaviors of the alloys. Fatigue limits of the two alloys with the Si contents of 0.80 and 1.24 wt% were measured to be approximately 224 and 283.5 MPa, respectively. The number of cracks found on surface was very small (1~3) and barely increased with the applied stress, when the applied stress was below the yield strength. However, it was increased sharply with increase of the applied stress to approximately the ultimate tensile strength. Fatigue crack initiation was predominantly associated with the micro-pores in the alloys. SEM examination of the fracture surfaces of the fatigued samples showed that the crack initiation pores were always aspheric in shape with the larger dimension in depth from the sample surface. These tunnel-shaped pores might be formed along grain boundaries during solidification or due to overheating of the Si-containing particles during homogenization. A quantitative model, which took into account the 3-D effects of pores on the local stress/strain fields in surface, was applied to quantification of the fatigue crack population in a modified AA6061 Al alloy under cyclic loading. The pores used in the model were spherical in shape, for simplicity, with the same size of 7 μm in diameter. The total volume fraction of the pores in the model were same as the area fraction of the pores measured experimentally in the alloy. The stress and strain fields around each pore near the randomly selected surface in a reconstructed digital pore structure of the alloy were quantified as a function of pore position in depth from the surface using a 3-D finite element model under different stress levels. A micro-scale Manson-Coffin equation was used to estimate the fatigue crack incubation life at each of the pores in the surface and subsurface. The population of fatigue cracks initiated at an applied cyclic loading could be subsequently quantified. The simulated results were consistent with those experimentally measured, when the applied maximum cyclic stress was below the yield strength, but the model could not capture the sudden increase in crack population at UTS, as observed in the alloy. This discrepancy in crack population was likely to be due to the use of the spherical pores in the model, as these simplified pores could not show the effects of pore shape and their orientations on crack initiation at the pores near surface. Although it is presently very time-consuming to calculate the crack population as a function of pore size and shape in the alloy with the current model, it would still be desirable to incorporate the effects of shape and orientation of the tunnel-shaped pores into the model, in the future, in order to simulate the fatigue crack initiation more accurately in the alloy.

Nous étudions la fragilisation par l'hydrogène de l'alliage d'aluminium 7108. Une technique expérimentale spécifique a été développée : Un pré-chargement en hydrogène des échantillons, à travers un dépôt de nickel de quelques dizaines de microns, qui empêche la dissolution du substrat d'aluminium, est utilisé. Il permet la comparaison de la résistance à la fragilisation de différentes microstructures modèles. Nous étudions l'effet du traitement thermique et de la précipitation sur la sensibilité à l'hydrogène pour des vitesses de déformation macroscopiques imposées variables. Différents modes de rupture sont observés ainsi que des transitions entre eux. Au moyen de simulations numériques à l'échelle mésoscopique, l'effet de taille des précipités intergranulaires pré-fragilisés sur la ténacité des joints de grains est estimé, en utilisant un modèle de zone cohésive. Nous analysons la compétition entre la diffusion de l'hydrogène vers la pointe de la fissure et la vitesse de fissuration par un couplage mécanique - diffusion basé sur la diffusion de l'hydrogène assistée par la contrainte hydrostatique. Une vitesse critique au-delà de laquelle l'hydrogène ne peut plus suivre la fissure, est mise en évidence. L'influence de la microstructure du joint de grains sur cette vitesse est analysée. La valeur est comparée à une estimation des vitesses de propagation expérimentales obtenues pour différentes vitesses de déformation macroscopiques. Nous analysons l'effet du piégeage de l'hydrogène par les précipités intergranulaires et la désorption sur la répartition de l'hydrogène le long du joint de grains en imposant un flux au niveau de l'interface précipités - matrice. / We study the hydrogen embrittlement of the 7108 aluminum alloy. A specific experimental technique was developed : A hydrogen pre-charging, through few tens of microns of deposit of nickel, which prevents the dissolution of the aluminum substrate is used. It allows a comparison of the resistance to embrittlement of different model microstructures. We study the effect of heat treatment and intergranular precipitation on the susceptibility to hydrogen embrittlement for several macroscopic strain rates. Different failure modes and transitions between them are observed. Through numerical simulations, at the mesoscopic scale, the effect of the size of pre-weakened intergranular precipitates on the grain boundary toughness is estimated using a cohesive zone model. We further analyze the competition between the hydrogen diffusion toward the crack tip and crack velocity. For this purpose, a mechanical – diffusion coupling based on the hydrogen diffusion assisted by hydrostatic stress is elaborated. A critical crack velocity, beyond which hydrogen can no longer follow the crack, is highlighted. The influence of the grain boundary microstructure on this critical crack velocity is evaluated and its value is compared with an estimate of velocities obtained for different experimental macroscopic strain rates. We analyze the effect of hydrogen trapping by intergranular precipitates and hydrogen desorption by imposing a flux at the precipitates – matrix interfaces.

Investigations on size dependent phase stability and transformations in isolated nanoparticles have gained momentum in recent times. Size dependent phase stability generates size specific particle microstructure which consequently yields size specific functionality. One important prerequisite for conducting studies on nanoparticles is their synthesis. A substantial amount of research effort has therefore been focused on devising methodologies for synthesizing nanoparticles with controlled shapes and sizes. The present thesis deals with both these two aspects: (a) synthesis of nanoparticles and (b) phase transformations in nanoparticles. The system chosen in this study is AuCu intermetallic nanoparticles. The choice of AuCu nanoparticle was due to the fact that the literature contains abundance of structural and thermodynamic data on Au–Cu system which makes it a model system for investigating size dependence of phase transformations. With respect to synthesis, the present thesis provides methodologies for synthesizing alloyed Au–Cu nanoparticles of different sizes, Au–Cu nano-chain network structures and uniform Au–Cu2S hybrid nanoparticles. For every type, results are obtained from a detailed investigation of their formation mechanisms which are also presented in the thesis. With respect to phase transformation, the thesis presents results on the size dependence of fcc to L10 transformation onset in Au–Cu nanoparticles under isothermal annealing conditions. The present thesis is divided into eight chapters. A summary of results and key conclusions of work presented in each chapter are as follows. The ‘introduction’ chapter (chapter I) describes the organization of the thesis. Chapter II (literature study) presents a review of the research work reported in the literature on the various methodologies used for synthesizing Au–Cu based nanoparticles of different shapes and sizes and on ordering transformation in AuCu nanoparticles. The chapter also presents a brief discussion on the reaction variables that control the process of nucleation and growth of the nanoparticles in solution. Chapter III titled ‘experimental details and instrumentation’ describes the synthesis procedures that were used for producing various nanoparticles in the present work. The chapter also briefly describes the various characterization techniques that were used to investigate the nanoparticles. The fourth chapter titled ‘synthesis and mechanistic study of different sizes of AuCu nanoparticles’ provides two different methodologies for synthesis, referred as ‘two-stage process’ and ‘two-step process’ that have been used for producing alloyed AuCu nanoparticles of different sizes (5, 7, 10, 14, 17, 25 nm). The ‘two-stage’ process involved sequential reduction of Au and Cu precursors in a one pot synthesis process. Whereas, the ‘two-step’ process involved a two-pot synthesis in which separately synthesized Au nanoparticles were coated with Cu to generate alloyed AuCu nanoparticles. In the two-stage synthesis process it was observed that by changing the total surfactant-to-metal precursor molar ratio, sizes of the alloyed AuCu nanoparticles can be varied. ‘Total surfactants’ here include equal molar amounts of oleic acid and oleylamine surfactants. Interestingly, it was observed that there exists a limitation with respect to the minimum nanoparticle size that can be achieved by using the two-stage process. The minimum AuCu nanoparticle size achieved using the two-stage synthesis process was 14 nm. Mechanism of formation of AuCu nanoparticles in the two-stage synthesis process was investigated to find out the reason for this size limitation and also to determine how the synthesis process can be engineered to synthesize alloyed AuCu nanoparticles with smaller (<14nm) sizes. Studies to evaluate mechanism of synthesis were conducted by investigating phase and size of nanoparticles present in the reaction mixture extracted at various stages of the synthesis process. Their studies revealed that (a) the nanoparticle formation mechanism in the two-stage synthesis process involves initial formation of Au nanoparticles followed by a heterogeneous nucleation and diffusion of Cu atoms into these Au rich seeds to form Au–Cu intermetallic nanoparticles and (b) by increasing the relative molar amount of the oleylamine surfactant, size of the initial Au seed nanoparticles can be further reduced from the minimum size that can be achieved in the case when equal molar amounts of oleylamine and oleic acid surfactants are used. The information obtained from the mechanistic study was then utilized to design the two-step synthesis process. In the two-step process, Au nanoparticles were synthesized in a reaction mixture containing only the oleylamine surfactant. Use of only oleylamine resulted in production of pure Au nanoparticles with sizes that were well below 10 nm. These Au nanoparticles were washed and dispersed in a solution containing Cu precursor. Introduction of a reducing agent into this reaction mixture led to the heterogeneous nucleation of Cu onto the Au seed particles and their subsequent diffusion into them to form alloyed AuCu nanoparticles with sizes of ~5, 7 and 10 nm. The study present in this chapter essentially signified that the surfactants used in the reaction mixture not only prevent nanoparticles from agglomerating in the final dispersion but also control their nucleation and growth and therefore can be used as a tool to tune nanoparticle sizes. The fifth chapter titled ‘size dependent onset of FCC-to-L10 transformations in AuCu alloy nanoparticles’ illustrates the effect of AuCu nanoparticle size on the onset of ordering under isothermal annealing conditions. Nanoparticles in this study were annealed in-situ in a transmission electron microscope. Samples were prepared by drop drying a highly dilute dispersion of as-synthesized nanoparticles onto an electron transparent TEM grid. Nanoparticles sitting on the TEM grid were well separated from each other to minimize particle sintering during the annealing operation. It was however observed that during the isothermal annealing, particle coarsening due to atomic diffusion was appreciable for 5 nm particles but negligible for 7 and 10 nm particles. Therefore for this study only 7 nm and 10 nm sized particles were considered. Onset of ordering was determined from the time when first sign of the diffraction spot, corresponding to the ordered phase, appears in the selected area electron diffraction pattern from a region containing large number of AuCu nanoparticles. Through a series of isothermal experiments it was observed that the time for onset of ordering increased with decrease in size of the nanoparticles. It is speculated that the delay in onset of ordering may be due to the fact that with a decrease in nanoparticle size the probability of a nanoparticle containing a fluctuation that shall generate a thermodynamically stable nuclei of the ordered phase decreases. A sharp interface between the ordered and the disordered phase inside the particle was also observed which suggested that the ordering transformation in as-synthesized fcc AuCu nanoparticles is a first order transformation. The sixth chapter titled ‘synthesis and characterization of Au1-xCux–Cu2S hybrid nanostructures: morphology control by reaction engineering’ provides a modified polyol method based synthesis strategy for producing uniform Au–Cu2S hybrid nanoparticles. Detailed compositional and structural characterization revealed that the hybrid nanoparticles are composed of cube shaped Au-rich, Au–Cu solid solution phase and hemispherical shaped Cu2S phase. Interestingly, the hemispherical Cu2S phase was attached to only one facet of the cube shaped phase. A study on the formation mechanism of hybrid nanoparticles was also conducted by characterizing specimens extracted from the reaction mixture at different stages of the synthesis process. The study revealed that the mechanism of formation of hybrid nanoparticles involved initial formation of isolated cube shaped pure Au nanoparticles and Cu–thiolate complex with a sheet morphology. With increase in time at 180°C, the Cu–thiolate complex decomposed and one part of the Cu atoms that were produced from the decomposition were utilized in forming the spherical Cu2S and other part diffused into the Au nanoparticles to form Au–Cu solid solution phase. The chapter also presents a study on the effect of dodecanethiol (DDT) on achieving the hemisphere-on-cube hybrid morphology. In this study it is illustrated that an optimum concentration of dodecanethiol is required both for achieving size and morphological uniformity of the participating phases and for their attachment to form a hybrid nanoparticle. The seventh chapter titled ‘synthesis of Au–Cu nano-chains network and effect of temperature on morphological evolution’ provides methodology for synthesizing fcc Au– Cu nano-chain network structures using polyvinylprrolidone (PVP) surfactant. It was observed that with increase in the molar amount of PVP in the reaction mixture, morphology of the as-synthesized product gradually changed from isolated nanoparticles to branched nano-chain like. The nano-chains contained twins which indicated an absence of continuous growth and possibility of growth by oriented attachment of initially formed Au–Cu nanoparticles. Both in-situ and ex-situ annealing of the nano-chains led to their decomposition into isolated nanoparticles of varying sizes. Annealing also caused fcc-to¬L10 phase transformation. Investigation of the wave length of perturbation leading to breaking of a nano-chain into particles indicated that the surface energy anisotropy affects the splitting of nano-chain network structure into nano-sized particles. The thesis ends with a last chapter where we have presented possible future extension of current work.

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