Aim of the present work was to explore the possibility of improving strength of cast Mg by alloying additions, viz., Si and Zn+Al. All the alloys were produced by squeeze casting technique using squeeze pressure of 12MPa and their microstructure, tensile and corrosion properties were studied. Mg-Si system was chosen because the intermetallic compound Mg2Si possesses many desirable properties, such as, low density, high hardness, high melting point. Hence, there is scope for improving the strength of Mg by dispersion of primary Mg2Si particles. Addition of Si to Mg resulted in the formation of �-Mg, particles of primary Mg2Si and eutectic as microstructural constituents. The morphology of primary Mg2Si changed from polyhedral shaped particles to dendrites as Si content was increased from 3.57 to 5.5 wt%. Volume fraction of primary Mg2Si increased with increase in Si content. Particle size of primary Mg2Si also increased with increase in silicon content but at the same time it was found to be dependent on melt temperature, i.e., a lower particle size was obtained at higher melt temperatures. Addition of Al and Sr was made to Mg-2Si alloy in order to further increase the strength by solid solution strengthening and refinement/modification of primary Mg2Si particles/eutectic. Addition of 1.2 wt% Al to Mg-2Si alloy resulted in irregular type of morphology of Mg2Si particles and increased particle size. Addition of 0.2 wt% SrtoMg-2Si-1.2Al alloy resulted in slight refinement of primary Mg2Si particles and modification of eutectic. Addition of 0.4 wt% Sr resulted in both refinement and restoration of morphology of Mg2Si particles from irregular to polyhedral shape. This was accompanied by destruction of eutectic, and rods containing Mg, Si, Al and Sr were observed. The addition of 1.33 wt%Si to Mg resulted in improvement in 0.2%PS by about 80 MPa,UTS by about 40MPa and these values did not change much till the addition of 3.57 wt% Si. A drop in the strength values was observed at Si content of 5.5 wt%,where transition in morphology of primary Mg2Si occurred from polyhedral to dendrite. Addition of Si resulted in reduction in % elongation by about 2%. The addition of Al and Sr did not change the tensile properties of binary Mg-2Si alloy much. It was concluded that the volume fraction and size of primary Mg2Siparticles obtained with Si addition up to 3.57 wt% did not contribute much to strength and the strengthening mainly came from the eutectic present in the matrix. As Si content was increased to 5.5 wt% in order to increase the volume fraction of primary Mg2Si particles, the morphology of Mg2Si changed to dendritic type resulting in reduction in strength. Thus, the maximum increase in strength is achieved at near eutectic composition,i.e.,intheMg-1.33Sialloy,andfurtherincreaseinstrengthdoesnotseem to be feasible with this alloy system. The ductility of all the Mg-Si based alloys was also low, i.e, 0.5% elongation to fracture or less. Regarding the corrosion behaviour, the addition of Si to Mg deteriorated the corrosion resistance and the addition of Al and Sr further worsened it. Since further improvement in tensile properties did not seem feasible with Mg-Si alloy system, the focus was shifted to Mg-Zn-Al alloy system. There is scope for improvement in strength in Mg-Zn-Al alloy system by solid solution strengthening, grain refinement and precipitation hardening. It was observed that the addition of Zn and Al resulted in microstructure containing α-Mg grains and secondary phase at the grain boundary. XRD analysis showed the secondary phase to be Al5Mg11Zn4 but EDS analysis did not match with this composition. Therefore, the nature of this phase remains uncertain. Addition of 6 wt% Zn and 1 wt% Al resulted in improvement in strength as well as ductility: 0.2%PS improved by about 70 MPa, UTS by about 100 MPa and % elongation by about 7%. Addition of small amounts of Caresultedinrefinementofmicrostructurecausingimprovementinstrengthwithout much decrease in % elongation. Increase in Al content from 1 to 4 wt% resulted in increase in 0.2%PS but UTS slightly decreased, as % elongation reduced. Alloys subjected to T6 heat treatment showed improvement in strength but slight reduction in % elongation. ZA64 alloy in T6 condition gave 130 MPa 0.2%PS, 225 MPa UTS and 4.9% elongation, which are much higher tensile properties as compared Mg-Si alloys. All the three mechanisms mentioned above contribute to the strengthening. There is scope for further improvement in strength by employing a more suitable heat treatment. Regarding corrosion behaviour, addition of 6 wt% Zn and 1 wt% of Al to Mg did not deteriorate its corrosion resistance much. Addition of small amounts of Ca was found to be beneficial for corrosion resistance, whereas an increase in Al content lowered the corrosion resistance. Heat treatment also reduced the corrosion resistance.

A large number of advanced structural materials are based on metallic materials where alloying additions play a key role in imparting the required properties. Most of the commercially important aluminium alloys are classified by the nature of the alloying additions. Among them the 2219, 2618, 5086, and 7075 are important class of lightweight alloys that plays critical role in modern engineering application. However, despite having a series of commercially useful aluminum alloys for commercial applications the increasing need of improved performance requires newer development in particular for applications that require high strength at elevated temperatures and performance at extreme environments. Precipitations of the intermetallic compounds containing copper during thermal treatments play a very important role in developing high strength aluminium alloys. Although,these precipitates are stable at fairly high temperatures, the rapid coarsening of these second phase precipitates (e.g. Al2Cu), leads to loss of strength at elevated temperature. Several approaches are explored to overcome this problem. One of them is to utilize non-equilibrium solidification route, which can increase solid solubility and hence increasing the precipitate density. Nonequilibrium processing can also alter the selection pathway of the competitive phases and evolution of the microstructure. Recently, non–equilibrium solidification by suction casting technique is becoming increasingly popular for casting of metallic materials of any shape. In this technique solidification is effected by sucking the molten alloy into water cooled copper mold using a suction force resulting from the differences between the melting chamber in Argon gas pressure and casting chamber under vacuum. The present thesis aims to develop a set of newer alloys with small amount of alloying additions primarily based on nickel that can retain reasonable strength at high temperature by utilizing the non-equilibrium solidification route. In addition to Ni (≤ 0.10at.%), the thesis present results of the effect of minor addition of Sc and Zr as ternary and quaternary additions. Following a short review in chapter 2, Chapter 3 presents the experimental techniques adopted for both preparation of alloys and their characterization. Chapter 4 deals with the results of alloying of aluminum with minor amount of nickel. The Ni in the range of 0.05-0.20at% was used to develop a high temperature template, containing a set of hardening intermetallic compounds to increase the strength of the host matrix. The microstructural investigations of the suction cast alloys reveal a characteristic feathery microstructure. At higher magnification the microstructure reveals the presence of fine dispersions of a second phase. Both x-ray and transmission electron microscopy confirms the phase in the dispersions to be primarily crystalline Al9Ni2 phase having a monoclinic crystal structure. This phase does not exist in equilibrium phase diagram. Only at higher concentration one can observe equilibrium Al3Ni (Orthorhombic) particles. The size of the particle ranges from 50-200nm. Beyond~0.5at%Ni, the microstructure changes to normal cellular type solidification morphology with interdendritic space decorated by the eutectic network of Al-Al3Ni having a rod eutectic morphology. A careful observation of alloys with small amount of Ni reveals that the feathery structure is associated with the thin cells, which have grown by continuously splitting the tip yielding a fractal like dendritic morphology. The dispersoids form at the intercellular regions. We have presented clear evidence of their origin from the interdendritic liquid, which most likely underwent Rayleigh instability. The random distribution reflects the nature of the dendritic growth. We have argued that these inter-dendritic liquid droplets, which are enriched with Ni, get undercooled. The metastable Al9Ni2 phase nucleates and grows in this liquid. In order to confirm this scenario, we have carried out a phase field simulation for dendritic growth of aluminium solid solution in the alloy melt both under the condition of constraint growth and free growth. The observed distribution of the dispersoid is well reflected in the phase field simulation. The chapter also report the response of effect of direct ageing of suction cast alloy as one expect an extension solid solubility of Ni in Al. A small increase in hardness could be observed during ageing treatment. In order to determine the thermal stability of the intermetallic particles, the samples of the suction cast alloys were exposed at 200°C for 200h and 500°C for 100h respectively. No change in the microstructure could be observed excepting a slight coarsening indicating the dispersed particles are thermally stable. After exposure at two different temperatures the maximum retained hardness was measured to be 350MPa. We have also attempted to correlate the hardness with coarsening behavior of particles. The feathery morphology of the cast structures and fine dispersion of the intermetallic phase is expected to improve the tensile strength of the alloy. The tensile yield strength of cast alloys was determined to be 150MPa ± 20 for Al- 0.09at%Ni alloy. We have tried to estimate the expected strength of the alloy from quantitative microstructural parameters using possible hardening mechanism. The estimates are in good agreement to the observed values. The chapter 5 reports attempts to develop thermally stable precipitation strengthened aluminum alloys by retaining the dispersion template developed earlier alloyed with Ni. Then, the binary alloys were added with extremely low diffusivity element Zr. The element Zr is traditionally added in the aluminium alloys as grain refiner and as a powerful agent for inhibiting recrystallization especially for high strength aluminium alloys. However, in this work we have alloyed Zr for imparting precipitation hardening. An amount of 0.15at%Zr was added to the suction cast alloys of Al-0.05, 0.09 and 0.20at%Ni. The first two alloys exhibit the formation of metastable phase Al9Ni2 during solidification stage. Increase the concentration of the alloy to Al-0.20at% Ni with 0.15at%Zr additions exhibits combination of both stable Al3Ni and Al9Ni2 metastable phases. Microstructures of these alloys show columnar cells of ~200μm with dispersions of spherical nodules of Al9Ni2 and Al3Ni with varying size ranges from 200-500nm. Particle size distribution of Zr containing aluminium alloys with 0.05at% Ni is 595nm ± 20 while the alloy having the 0.09 at% Ni has the optimum size of 290nm. Further increase of Zr composition above 0.20at % led to columnar to equiaxed transition. The as cast alloys containing Zr does not show the improvement with limited yield strength of the order of 150MPa. The equivalent hardness of the samples has been measured to be about 370-420MPa. Heat-treated alloys however show the presence of Al3Zr (L12) precipitates with ~20nm size that are coherent with the matrix. Binary suction cast Al-0.15at%Zr alloy after ageing exhibits tensile yield strength of ~200MPa. With ternary aluminium alloy with minor additions Ni and Zr, The strength increases to ~300MPa. Additionally, the alloy continue retain a maximum hardness of 870-920MPa even after long hours of aging. The Zr containing alloys were proved to be stable. When the tests were carried out on a nominally alloyed sample of Al-0.09at%Ni-0.15at%Zr peak aged and exposed to 250°C for 200h, the yield strength under compression tests was found to be 280MPa. The chapter 6 of the thesis discusses the role of Sc with the ternary Al-alloys with Ni and Zr. Addition of small quantities 0.1 and 0.2at%Sc substantially reduces the inter-particle distance of precipitates by increasing volume fraction and number of nano-sized particles. It has been observed and presented in this thesis that the Sc addition provides the highest incremental strengthening per atom percent of any alloying element. Chill-suction cast samples show equiaxed cells in the samples with dispersions of particles inside and some segregated particles at the cell boundaries. To achieve a further increase in the number density of precipitates we processed the suction cast alloys with additional heat treatment at 375 and 450°C. All the suction cast alloys with varying Ni content and keeping the Sc and Zr constant at 0.10 and 0.15at% respectively exhibit formation of Al9Ni2 phase. The alloy Al-0.20at%Ni-0.10at%Sc-0.15at%Zr also contain stable phase of Al3Ni with an eutectic morphology. The DSC experiments in the dynamic mode with heating rate of 10°C min-1 exhibit two distinct exothermic peaks due to precipitates from solution at 375 and 450°C. The TEM analysis using STEMEDX has further confirmed the existence of nano-sized particles 30-50 nm of both phases of Al3Sc and Al3 (Sc, Zr). The tensile yield strength of the as cast alloy show 200MPa while after precipitation treatment, we observe improved yield strength 350-450MPa. Thermal stability of the alloys were tested after peak aged condition and exposed to 200°C for 250h. The results show that the yield strength is unaffected implying the coarsening resistance of the precipitate particles. Overall the thesis establishes that with minimum alloying additions, it is possible to design alloys that are expected to perform for high temperature applications by the formation of set of dispersions of Al9Ni2 (monoclinic) and precipitates of ordered cubic phases of (L12) structure of Al3Zr, Al3Sc and Al3 (Sc, Zr) with required number density of particles.

Dendritic solidification has been the subject of continuous research, also because of its high importance in metal production. The challenge of predicting macroscopic material properties due to complex solidification processes is complicated by the multiple physical scales and phenomena involved. Practical modeling approaches are still subject to significant limitations due to remaining gaps in the systematic understanding of dendritic microstructure formation. The present work investigates some of these problems at the microscopic level of interfacial morphology using phase-field simulations. The employed phase-field models are implemented within a finite-element framework, allowing efficient and scalable computations on high-performance computing facilities. Particular emphasis is placed on the evolution and interaction of dendrite sidebranches in the broader context of dendrite fragmentation, varying and dynamical solidification conditions.

Microstructure changes in uranium and uranium/metal alloys due to radiation damage are of great interest in nuclear science and engineering. Titanium has attracted attention because of its similarity to Zr. It has been proposed for use in the second generation of fusion reactors due to its resistance to radiation-induced swelling. Aluminum can be regarded as a standard absorbing material or backing material for irradiation targets. Initial study of thin aluminum films irradiation by �������Cf fission fragments and alpha particles from source has been conducted in the Radiation Center, Oregon State University. Initial study of thin aluminum films irradiation by �������Cf fission fragments and alpha particles from source has been conducted in the Radiation Center, Oregon State University. Aluminum can be regarded as a standard absorbing material or backing material for irradiation targets. The AFM investigation of microstructure damages of thin aluminum surfaces revealed that the voids, dislocation loops and dislocation lines, formed in the thin aluminum films after bombardment by �������Cf fission fragments and alpha particles, depends on the irradiation dose. The void swelling and diameter and depth of voids increase linearly with the fluence of particles and dose; however, the areal density of voids decreased when formation of dislocation loops began. Study of deposition of uranium on titanium backing material by molecular plating and characterization of produced U/Ti film has been performed. The U/Ti film has smooth and uniform surfaces but the composition of the deposits is complex and does not include water molecules which probably involve the presence of U (VI). A possible structure for the deposits has been suggested. X-ray diffraction pattern of U/Ti films showed that The U/Ti film has an amorphous structure. Uranium films (0.500 mg/cm��) and stack of titanium foils (thickness 0.904 mg/cm��) were used to study the microstructural damage of the uranium film and its backing material. Irradiation of U/Ti film and Ti foils with 1 MeV/u (136 MeV) �������Xe�������� ions in was performed in the Positive Ion Injector (PII) unit at the Argonne Tandem Linear Accelerator System (ATLAS) Facility at Argonne National Laboratory, IL. Pre- and post- irradiation of samples was analyzed by X-ray diffraction, Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS) and Atomic Force Microscopy (AFM). The irradiation of U/Ti films results in the formation of a crystalline U���O��� phase and polycrystalline Ti phase. Annealing of the thin uranium deposit on a titanium backing at 800��C in the air atmosphere condition for an hour produced a mixture of UO���, U���O���, Ti, TiO and TiO��� (rutile) phases; meanwhile, annealing at 800oC for an hour in the argon environment produced a mixture of ��-U���O���, Ti and TiO��� (rutile) phases. These phenomena indicate that the damage during irradiation was not due to foil heating. Microstructural damage of irradiated uranium film was dominated by void and bubble formation. The microstructure of irradiated titanium foils is characterized by hillocks, voids, polygonal ridge networks, dislocation lines and dislocation networks. Theory predicts that titanium undergoes an allotropic phase transformation at 882.5 ��C, changing from a closed-packed hexagonal crystal structure (��-phase) into a body-centered cubic crystal structure (��- phase). When the titanium foils were irradiated with 136MeV �������Xe�������� at beam intensity of 3 pnA corresponding to 966��C, it was expected that its structure can change from hexagonal-close packed (hcp) to body-centered cubic (bcc). However, in contrast to the theory, transformation from ��-Ti (hcp) phase to fcc-Ti phase was observed. This phenomenon indicates that during irradiation with high energy and elevated temperature, the fcc-Ti phase more stable than the hcp-Ti Phase. / Graduation date: 2012

No description available.

Al-Zn-Mg-Cu-Zr based AA7010 aluminum alloy belongs to the class of heat treatable alloys and the semi-finished products are generally produced by hot rolling, forging or extrusion processes. It is well known that the thermo-mechanical processing parameters strongly influence both the evolution of texture as well as microstructure in the material. As a result, the semi-finished products exhibit anisotropy in mechanical properties causing legitimate concerns on the applicability of the alloys. In the present thesis, a systematic study on the evolution of texture and microstructure and its implications on the mechanical properties anisotropy of AA7010 alloy has been attempted. A brief introduction on the development of texture and its influence on the anisotropy of the mechanical properties of 7xxx series aluminum alloys is presented first with a view to explore the scopes for further investigation. An overview of the relevant literature is described subsequently. The development of texture and microstructure in an Al-Zn-Mg-Cu-Zr based 7010 aluminum alloy during uneven, hot cross-rolling is presented. Materials processing involves three different types of uneven cross-rolling. The variations in relative intensity of the β-fibre components as a function of cross rolling modes have been investigated. It has been shown that the main attributes to the texture evolution in the present study are (a) cross-rolling and inter-pass annealing that reduce the intensity of Cu component following each successive pass, (b) recrystallization resistance of Bs oriented grains, (c) stability of Bs texture under cross-rolling, and (d) Zener pinning by Al3Zr dispersoids. The stability of the unique single, rotated Brass-{110}(556) component developed in the present alloy, during long term thermal annealing and cold rolling deformation has been systematically investigated further. Subsequently, the influence of development of microstructure and texture on the in-plane anisotropy (AIP) of yield strength, work hardening behavior and yield locus anisotropy has been presented. The AIP and work hardening behavior are evaluated by tensile testing at 0o, 45o and 90o to the rolling direction, whilst yield loci have been generated by Knoop hardness method. It has been observed that in spite of having strong rotated Brass texture, the specimens show low AIP especially in peak aged temper. The in-plane anisotropy (AIP) of yield strength, and work hardening behavior of a heat treated 7010 aluminum alloy sheet having strong, rotated Brass-{110}556 component with different texture intensity and volume fraction of recrystallization has been further evaluated. It is observed that the AIP increases with increase in texture intensity and volume fraction of recrystallization. In the subsequent chapter, the tensile flow and work hardening behavior are described using constitutive equations. Room temperature tensile properties have been evaluated as a function of tensile axis orientations in as-hot rolled as well as peak aged conditions. It has been found that both the Ludwigson and a generalized Voce-Bergström relation adequately describe the tensile flow behavior in all conditions compared to the Hollomon relation. The Voce-Bergström parameters define the slope of - plots in the stage-III regime when the specimens show a classical linear decrease in hardening. Further analysis of work hardening behavior throws light on the effect of texture on the dislocation storage and dynamic recovery. An overall summary of the experimental results and the scopes for future studies have been presented at the end.