Orientador: Eugênio José Zoqui / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-24T12:59:54Z (GMT). No. of bitstreams: 1 Campo_KaioNiitsu_M.pdf: 7361224 bytes, checksum: c6570a6666ed0e98563f7345bc5c6734 (MD5) Previous issue date: 2014 / Resumo: Este trabalho investigou o uso da extrusão em canal angular (ECA) na produção de matéria-prima para tixoconformação visando à obtenção de uma rota simples e eficiente na formação de microestruturas globulares. Para tanto, uma liga de alumínio A356 foi submetida a um único passe ECA e, em seguida, reaquecida ao estado semissólido a 580°C. Dessa forma, pôde-se determinar a evolução morfológica e os mecanismos envolvidos na formação da pasta metálica, a influência do tratamento isotérmico na evolução microestrutural no estado semissólido, o comportamento de deformação durante a compressão a quente e o comportamento mecânico em tração do material tixoconformado. Os resultados indicaram que apenas um passe ECA foi suficiente para gerar uma pasta metálica refinada e globular apenas com o reaquecimento da matéria-prima. O engrossamento da microestrutura durante o tratamento isotérmico foi controlado principalmente pelo mecanismo de Ostwald ripening, fato comprovado pelo baixo valor da constante taxa de engrossamento K, o que mostrou a estabilidade dessa pasta no estado semissólido. As amostras exibiram comportamento tixotrópico típico com baixos valores de viscosidade aparente, na faixa de 10^2 a 10^3 Pa.s para as taxas de cisalhamento testadas. Por fim, as amostras tixoconformadas apresentaram valores de ductilidade e resistência à tração superiores aos das amostras fundidas e deformadas por um passe ECA / Abstract: This work investigated the use of equal channel angular pressing (ECAP) for the production of raw materials for thixoforming in order to obtain a simple and efficient processing route to generate globular microstructures. Therefore, an A356 aluminum alloy was processed by one ECAP pass, and then reheated to the semisolid state up to 580°C. Thus, it was possible to determine the morphological evolution and mechanisms involved in the formation of the semisolid slurry, the influence of isothermal treatment on microstructural evolution in the semisolid state, the deformation behavior during hot compression tests and the tensile properties of the thixoformed material. The results indicated that a single ECAP pass was sufficient to promote a refined and globular semisolid slurry. Microstructure coarsening during the isothermal treatment was mainly controlled by Ostwald ripening, which was verified by the low rate constant K, showing the stability of the reheated material in the semisolid state. The samples exhibited typical thixotropic behavior with low apparent viscosity ranging from 10^2 to 10^3 Pa.s over the applied shear rates. Finally, the thixoformed samples exhibited values of ductility and tensile strength superior to the as-cast and ECAPed samples / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

It is well known that texture and microstructure play a crucial role in determining properties of metallic materials. The evolution of microstructure and texture during deformation and annealing of copper and some copper base alloys have been investigated to some extent. However, the knowledge about the role of the mode of deformation, particularly different variants of rolling deformation, is still very limited for deformation and annealing texture of two-phase copper alloys. Therefore it appeared important to study the influence of deformation path (in the present case, mode of rolling) on texture and microstructure in two-phase copper alloy Cu-40Zn alloy. Hot rolled bar of Cu-40Zn alloy (as-received material) was subjected to unidirectional and cross rolling ( multi-step cross-rolling) at room temperature with strain per pass (true strain) being constant for each step. In multi-step cross-rolling, the rolling direction was altered (rotated by 90°)after each pass. Strains corresponding to rolling reduction of 50% and 80% were given to as-received material through each of the different mode of rolling. In a second route, the as-received material was solution treated at 800°C for 4 hours first and then subjected to rolling in the same manner as above. A piece was cut from each of the as-rolled materials and was subjected to annealing at 560°C for one hour for recrystallization. The bulk textures were determined by measuring the pole figures at the center of the rolled as well as the annealed specimen using X-ray texture goniometer based on Schultz reflection geometry. Three dimensional texture analyses were carried out using the method of orientation distribution function(ODF). Micro-textures and associated microstructural parameters were determined using a Field Emission Gun Scanning Electron Microscope(FEG-SEM) operated at 20KV, equipped with Electron back scattering detector(EBSD). In the experimental material, texture was examined for both the α (fcc) and the β (ordered cubic) phases. In the present investigation, α phase of unidirectional rolled as-received material had Bs {011}<112> orientation as the strongest component whereas for multi-step cross rolled material P(BND) {011}<111> orientation had the maximum intensity, which could be obtained by rotating the Bs orientation and about ND.The texture development of β phase of as-received unidirectional rolled sample could be understood in terms of relaxed constraints Taylor model. The initial texture had a pronounced effect on texture development of α phase for solution treated alloy during deformation. This material exhibited very strong P(BND) {011}<111> orientation for unidirectional as well as for multi-step cross rolling. For cross rolled alloys, this orientation is promoted by two factors simultaneously, (i) initial texture and (ii) special attributes of cross rolling process. The volume fraction of cube oriented grains was very low for all recrystallized samples because of dominance of Bs orientation in the deformation texture plus formation of shear bands in the microstructure. The texture of β phase for unidirectionally rolled solution treated alloy got sharpened on annealing. However, strength of texture decreased with increasing deformation. Grain boundary (and CSL boundary) analyses were carried out with EBSD data. These analyses indicated that all the recrystallized samples had a high number of Σ3 boundaries. The proportion of Σ3 boundaries was higher in multi-step cross-rolled annealed material. The deformed material had higher number fraction of low angle boundary than any other special boundary. Solution treated material had an average grain size of α phase smaller than the as-received material. Another dimension of the present investigation was to characterize the microstructural features in three dimension(3D) in order to examine the morhphology of constituent phases using serial sectioning. In the present work, 3D studies were carried out on the alloy after post deformation annealing. The alignment of serial section images and generation of 3D image out of the stack of 2D images was carried out through standard software. The same was used to measure the suitable 3D microstructural parameters from the 2D sections. Three dimensional microstructural parameters like mean caliper diameter of β particle, number of β particles per unit volume ‘Nv’, surface to volume ratio for β phase particles (α- β interface) ‘Sv’, were calculated. Number of β particle intercepts per unit area ‘NA’ was determined by measuring number of β phase particle in each section. The volume of a β particle as calculated from the caliper diameter using three-dimensional microstructural analysis, which could not get directly determined with conventional two-dimensional microscopy.

Superplastic deformation behaviour of AZ31 magnesium alloy having initial grain sizes 8, 11 and 17μm alloy was investigated at 673 K with initial strain rates ranging from 1x10-2 to 1x10-4 s-1. Mechanical data on fine grained AZ31 alloy with grain sizes 8 and 11 μm indicated a transition in deformation mechanisms. The strain rate sensitivity, m ~ 0.5 at low strain rates and m ~ 0.2 at high strain rates which suggest GBS and dislocation slip as the corresponding deformation mechanism. For coarse grained alloy having grain size 17 μm, m < 0.4 at low strain rates and ~ 0.2 at high strain rates, suggesting dislocation slip as the deformation mechanism. A superplastic maximum elongation of ~ 475% was observed for 8 μm alloy at low rate of deformation. At high strain rates, the deformation was non-superplastic for fine and coarse grained alloys. The contribution of GBS to total strain, ξ in the low strain rate regime was evaluated to be 50 – 60%, for both low and high elongation. EBSD studies indicated the maintenance of high fraction of high angle boundaries up to true strain of ~ 0.88 and a reduction in texture intensity. These observations show GBS as the dominant deformation mechanism for fine grained alloy. At higher strain rate, ξ was estimated to be 30%. Fraction of high angle boundaries was reduced and [0001] direction of grains was found to be rotated towards the tensile direction, suggesting dislocation slip. Based on mechanical data, flow localization and cavitation studies; the failure of the material during high rates of deformation was mainly due to flow localization. Extensive cavitation along with more uniform flow at a lower strain rate regime suggests the failure due to the cavity interlinkage and coalescence. The present GBS data are consistent with the previous relevant data in fine grained Mg based alloys in the low strain rate regime. The GBS data obtained in the dislocation regime in the present study are also in agreement with that of the previous investigations in fine grained Mg alloys.

The aim of this thesis is to provide microstructural and mechanical characterisation of α-β titanium alloys exposed to a range of thermo-mechanical conditions, in particular under-going high rate deformation at elevated temperatures, representative of the Linear Friction Welding (LFW) manufacturing process. Three α-β titanium alloys provided by Rolls-Royce are studied: Ti-64 blade, disc and Ti-6246 disc. Ti-64 and Ti-6246 show complex deformation behaviour with strain, strain rate and temperature, especially near the transus temperature, where the low temperature α phase is transformed into the high temperature β phase. The microstructure and mechanical properties evolve in an interconnected fashion, and understanding this mutual influence is necessary to better predict the behaviour of these alloys. Characterisation of the mechanical properties was performed through uniaxial compression tests at strain rates from 0.001 to 3000 s^-1, using an Instron screw-driven machine at quasi-static rates, a servo-hydraulic machine at medium rates and a Split-Hopkinson Pressure Bar and a drop-weight tower at high strain rates. The tests were performed over a range of temperatures from room temperature to 1300 °C. The main focus was on high strain rate and high temperature tests, with the development of a gravity driven direct impact Hopkinson bar, referred as a drop-weight system, which is intended to evaluate the mechanical response of metals to high strain rate loading at temperatures up to c. 1300 °C. The design and principles of operation of the system are presented, along with calibration and validation data. Preliminary tests were performed on stock Ti-64, heated at two rates: 1 and 20 °C s^-1. The evolution of the mechanical properties was analysed, focussing on the strain rate, temperature and phases dependencies. Characterisation of the microstructure was realised by performing interrupted compression tests, first at room temperature, three plastic strains, 4%, 10% and 20%, and two different strain rates, 0.001 and 2000 s^-1; then at 4% plastic strain, a strain rate of 2000 s^-1 and three elevated temperatures, 700, 900 and 1100 °C. A better understanding of the microstructure evolution with strain, strain rates and temperature, including the macrotexture and microtexture of the specimens, was obtained using Electron Backscatter Diffraction (EBSD) to characterise the texture of the undeformed and deformed materials. The better understanding of the flow stress and microstructural evolution of both Ti-64 and its individual α and β phases with various strain rates and temperatures is intended to be used in the development of more accurate models representing the behaviour of these alloys. Predicting the microstructure evolution and then the mechanical properties of a material is essential to optimise the final mechanical properties of the alloys when welded by manufacturing processes such as the LFW process.

No description available.

Crystallographic texture in polycrystalline materials are known to play an important role in tailoring suitable properties for various technological applications. In addition, the evolution of texture provides a profound basis to develop scientific understanding of physical processes occurring in the material during deformation and annealing. Between the two, the understanding of deformation texture is much broader. However, certain issues pertaining to the evolution of deformation texture evolution are yet to be explored or not uniquely agreed upon. A few notable examples are the effects of extreme grain sizes and strain rates. Moreover, most of the studies are pertaining to single phase metals and alloys. While many engineering alloys consist of two phase microstructures, the effect of second phase in the microstructure on the evolution of texture in the individual phases has not been studied in a comprehensive manner. The present thesis is an attempt to addresses these issues in a more generic manner. The studies have been specifically aimed at examining the aforesaid issues in the close packed Face Centre Cubic (FCC) and Hexagonal Close Packed (HCP) metals and alloys. In brief, this thesis addresses the following problems pertaining to deformation texture: (i) the effect of extreme grain sizes, (ii) the effect of extreme strain rates and (iii) the effect of a second ductile phase. Chapter 1 of the thesis gives a detailed survey of literature pertaining to the evolution of deformation textures in different metals and alloys, while chapter 2 includes the details of the experimental techniques and simulation procedures, which are mostly common for the entire work. The issue of grain size is addressed in chapter 3. In the present investigation, the evolution of deformation texture in nickel (FCC) and titanium (HCP) with the extreme grain sizes (nanometre and millimetre) has been studied. Nanocrystalline nickel with the grain size ~ 20 nm was obtained by pulse electro-deposition while the other extreme of the grain size in nickel was obtained by annealing of a cold rolled sheet at 1373 K. The rolling texture in nanocrystalline nickel had a higher volume fraction of Brass component than in nickel with normal grain size. These results have been explained on the basis of inhibition of cross slip in small grain sizes and the operation of planar slip. This has been validated by viscoplastic self-consistent simulations. The texture of coarse grain nickel samples (typified as oligocrystalline, owing to the lesser number of grains in the thickness direction) also had higher Brass component like the nanocrystalline sample. A detailed analysis was performed by examining misorientation development in the grain interior and in the vicinity of the grain boundaries. The similarity at the two extreme length scales has been explained on the basis of lower “Grain Boundary Affected Zone” at the extreme length scales. To examine the effect of grain size in the case of HCP materials, commercially pure titanium with ultra-fine (500 nm) and normal grain size (~50 μm), was investigated. A monotonic evolution of texture was observed in the former, which has been attributed to the absence of twinning, a situation that could arise due to the lack of coordinated movement of twinning partials in the sub-micron grain size regime. Thus, a reasonable understanding of the evolution of deformation texture in hitherto unexplored regime of grain sizes was developed for the two materials. The chapter 4 of the thesis is dedicated to the study of strain rate effects in both FCC and HCP materials. The issue of strain rate has been addressed by two ways: (a) deforming the materials at extreme strain rates, namely 10-3 s-1 to 10+3 s-1 under compression up to a reasonable strain, and (b) deforming the materials under torsion within a reasonable range of strain rates, but up to large strains. In this case, in addition to nickel, copper was also investigated owing to the different strain hardening behaviour of the two materials. The compression texture in nickel and copper was characterized by the presence of <101> component at low strain rates. At high strain rate, ~10+3 s-1, there was a decrease in the intensity of the <101> component for nickel but it strengthened for copper. This has been explained on the basis of continuous dynamic recrystallization in copper. The torsion texture evolution in nickel and copper was similar at low strain rate (10-3 s-1) and was characterized by the presence of important shear texture components. At high strain rate (1 s-1), texture weakened for nickel, while for copper a rotated cube component was observed which has been attributed to dynamic recrystallization. The effect of strain rate was studied more comprehensively in hexagonal titanium by adding one more variable, that is, the initial texture. Extreme strain rates were imparted using static and dynamic compression tests. It was found that different initial textures led to different mechanical response in terms of yield strength and strain hardening as well as microstructural response in terms of twin fractions. The samples deformed at high strain rate showed increased twinning that led to some scatter in the texture components compared to low strain rate deformed samples. VPSC simulations were able to successfully capture the evolution of texture as well as microstructural evolution in terms of twin activity in the deformed samples at the extreme strain rates. Torsion tests on titanium at different strain rates indicated evolution of inhomogeneous nature of fibre texture components with increase in strain rate. Thus, weakening of texture was observed irrespective of the strain path (compression or torsion) and crystal structure (FCC or HCP) unless additional restoration mechanism like recrystallization (continuous or discontinuous) intervened. In chapter 5, the evolution of rolling texture in two phase FCC + BCC (Ni-Fe-Cr alloys) and HCP + BCC (Ti-13Nb-13Zr ) alloys has been studied. This study was aimed at examining the effect of second deforming phase on the texture evolution in the primary phase. The effect of various parameters like volume fraction and morphology of the second phase on deformation texture evolution was studied experimentally as well as by VPSC simulations. A reduction in the Brass component of texture was observed in the austenite phase due to the presence of harder ferrite phase while a characteristic rolling texture evolved in the ferrite phase. It has been established that the softer austenite phase carried maximum strain at low volume fractions of ferrite while the harder ferrite phase carried the maximum strain at higher volume fractions of ferrite. In case of the two phase HCP+BCC alloy Ti-13Nb-13Zr, both the hexagonal α and the cubic β phases showed a characteristic rolling texture irrespective of two different morphologies. For both the equiaxed and colony microstructures, the softer β phase carried the maximum strain. VPSC simulations were able to model the deformation texture evolution as well as microstructural parameters like strain partitioning and twin fraction satisfactorily for both the microstructural conditions. It was found that the deformation mechanism in one phase could be affected by the presence of the second phase and that a characteristic change in deformation texture could be produced in the presence of the second phase. Thus, a comprehensive perspective has been developed pertaining to the evolution of texture in FCC and HCP phases in the presence of a second ductile phase. The overall findings of the three investigations carried out for the thesis are summarised in chapter 6.

Dilute Mg alloys are exclusively identified for wrought structural applications in automotive industry. Any improvement in mechanical properties of alloys is possible only by grain size refinement and by the development of suitable texture. The grain size, grain size anisotropy and texture in these alloys affect the compatibility stresses in a very complex manner. To launch a full scale study towards understanding the complex deformation mechanisms operating in these alloys, it is necessary to understand the effect of grain size and texture on the mechanical behavior of Mg alloys in a broad or semi-quantitative manner first. Current literature lacks such broad study. In this present study, the effect of grain size, grain size anisotropy and texture evolution on the mechanical properties are examined in order to develop an understanding of the deformation mechanism that control the mechanical properties of a dilute conventionally extruded Mg alloy, AZ21. The approach adopted was to first study the microstructure and texture evolution in this conventionally extruded alloy. Since the grain sizes in these alloys vary over a wide range, it is hence necessary to study the microstructure evolution in a highly quantitative manner. In understanding texture, the present study is only limited to qualitatively evaluating the evolution of fibre component of texture using X-Ray Diffraction spectra. For truly quantitative microstructure evolution results in materials were grains sizes are spread over a wide range, it is critical to study a statistically enough no. of grains. Hence to avoid any sampling error, large montages (about 0.3 sq. mm) were constructed out of a series of high resolution images captured using an optical microscope. The montages so constructed are subjected to extensive image enhancement and various other operations are performed to convert these coloured to binary montages. Information like grain size, diameter etc., can be easily extracted from these binary montages and used for further analysis. Fibre texture in these conventionally extruded dilute Mg alloys generally develops due to alignment of basal planes along the direction of extrusion. The Critical Resolved Shear Stress for basal slip is very low when compared to that of non-basal planes. And also since there are very limited primary slip systems in these dilute Mg alloys, the development of strong fibre texture drastically changes the compatibility stresses and hence the mechanical properties . To broadly study the effect of microstructure-texture on mechanical proerties, after post extrusion annealing, heat treatments representing typical microstructure-texture combinations were identified. Effect of each microstructure-texture combination on the tensile and completely reversed cyclic fatigue properties are studied and qualitatively interpreted. The fibre texture showed pronounced effect on tensile ductility but it hardly affected the yield strength. With just 10% reduction in BPI, the ductility reduced by about 50%. A small change in average grain size did not alter the yield strength. Unlike tensile ductility, fatigue endurance stress was not altered drastically by the change in grain size or texture. But there appeared to be a significant effect of residual stress. In ending, a small change in microstructure-texture combination in these conventionally extruded alloys has a pronounced effect on ductility or in other words plastic properties. But a it has minimal effect on yield strength and fatigue endurance stress.

Titanium alloys are important engineering alloys that are extensively used in various industries. This is due to their unique combination of mechanical and physical properties such as low density combined with high strength and toughness as well as outstanding corrosion resistance. An additional benefit associated with Ti alloys, in general, is that their properties are relatively temperature-insensitive between cryogenic temperature and ~500 °C. Amongst the Ti alloys, Ti-6Al-4V (referred as Ti64) is a widely used alloy. Conventionally cast Ti64 possesses classical Widmanstätten microstructure of (hcp) α and (bcc) β phases. However this microstructure suffers from large prior β grain size, which tends be in the order of a few mm. Such large grain sizes are associated with poor processability as well as inferior mechanical performance. The necessity to break this coarse as-cast microstructure down, through several successive thermo-mechanical processing steps, adds considerably to the cost of finished Ti alloy products, making them expensive vis-à-vis other competing alloys. The addition of small amount of B (~0.1%) to Ti64 alloys, on the other hand reduces the cast grain size from couple of mm to ~200 µm. Moreover, addition of B to Ti alloys produces the intermetallic TiB needles during solidification by an in situ chemical reaction. The overall objective of this work is to gain insights into the role of microstructural modifications, induced by B addition to Ti64, on the mechanical performance of the alloys, in particular the room temperature damage tolerance (fracture toughness and fatigue crack growth) characteristics. The key questions we seek to answer through this study are the following: (a) What role does the microstructural refinement plays on the quasistatic as well as fracture and fatigue behavior and high temperature deformability of the alloys? (c) A hierarchy of microstructural length scales exist in Ti alloys. These are the lath, colony and grain sizes. Which of these microstructural parameters control the mechanical performance of the alloy? (b) What (possibly detrimental) role, if any, do the TiB needles play in influencing the mechanical performance of Ti64 alloys? This is because TiB being much stiffer, strain incompatibility between the matrix and the TiB phase could lead to easy nucleation of cracks during cyclic loading as well as can pose problems during dynamic deformation. (d) What is the optimum amount of B that can be added to Ti64 such that the most desirable combination of properties can be achieved? Five B-modified Ti64 alloys with B content varying from 0.0 to 0.55 wt.% were utilised to answer the above questions. Marked prior β grain size reduction was noted with up to 0.1 wt.% B addition. Simultaneous refinement of α/β colony size has also been observed. The addition of B to Ti64, on the other hand increases the α lath size. The TiB needles that form in-situ during casting are arranged in a necklace like structure surrounding the grain boundaries for higher B added Ti64 alloys. An anomalous enhancement in elastic modulus, E, of the alloy with only 0.04 wt.% B to Ti64 was found. E has been found to follow the same trend of variation with B content at higher temperatures (up to 600 °C) as well. Nanoindentation experiments were conducted to evaluate the moduli of the various phases present in the microstructure and then rationalize the experimental trends within the framework of approximate models. Marginal but continuous enhancement in strength of the alloys with B addition was observed. It correlates well with the grain size refinement according to Hall-Petch relationship. Ductility on the other hand increases initially with up to 0.1 wt.% B addition followed by a reduction. While the former is due to the microstructural refinement, the latter is due to the presence of significant amount of brittle TiB phase. Room temperature fracture toughness decreases with B addition to Ti64. Such reduction in fracture toughness with the refinement of prior β grain size has been justified with Ritchie-Knott-Rice model. Contradictory roles of microstructural refinement have been observed for notched and un-notched fatigue. While reduction in length scale has a negative role in crack propagation, it enhances the fatigue strength of the alloy owing to better resistance to fatigue crack initiation. TiB needles on the other hand act as sites for crack initiation and hence limit the enhancement in fatigue strength of alloys with 0.30 and 0.55 wt.% B. An investigation of the high temperature deformability of the alloys has been performed over a wide range of temperature (within the two phase α+β regime) and strain rate windows. Results show that microstructural refinement does not alter the high temperature deformation characteristics as well as optimum processing conditions of the alloys. TiB needles, however act as sites for instability owing to differences in compressibility between the matrix and the whisker phase. In summary, this study suggests that the addition of ~0.1 wt.% B to Ti64 can lead to the elimination of certain thermo-mechanical processing steps that are otherwise necessary for breaking the as-cast structure down and hence make finished Ti components more affordable. In addition, it leads to marginal enhancement in the quasi-static properties and significant benefits in terms of high cycle fatigue performance.

Background and Objective: Nickel-base superalloys, strengthened by a high volume fraction of Ni3Al precipitates, have been the undisputed choice for turbine discs in gas turbines as they exhibit the best available combination of elevated temperature tensile strength and resistance to low cycle fatigue (LCF), which are essential for a disc alloy. Alloy 720LI is a wrought nickel-base superalloy developed for disc application and exhibit superior elevated temperature tensile strength and LCF properties. It is distinct from contemporary disc alloys because of its chemistry, (especially Ti, Al and interstitial (C and B) contents), processing and heat treatment. However, literature available in open domain to develop an understanding of these properties in alloy 720LI is rather limited. This study was taken up in this background with an objective of assessing the tensile and LCF properties exhibited by alloy 720LI within a temperature regime of interest and understand the structure-property correlations behind it. Tensile Behavior: The effect of temperature and strain rate on monotonic tensile properties were assessed at different temperature in the range of 25 – 750°C (0.67 Tm) at a strain rate of 10-4 s-1 and strain rate effects were explored in detail at 25, 400, 650 and 750°C at different strain rates between 10-5 s-1 and 10-1 s-1. Yield and ultimate tensile strength of the alloy remains unaffected by temperature till about 600°C (0.58Tm) and 500°C (0.51Tm), respectively, beyond which both decreased drastically. Negligible strain rate sensitivity exhibited by the alloy at 25 and 400°C indicated that flow stress is a strong function of strain hardening rather than strain rate hardening. However at 650 and 750°C, especially at low strain rates, strain rate sensitivity is relatively high. TEM studies revealed that heterogeneous planar slip involving shearing of precipitates by dislocation pairs was prevalent under strain rate insensitive conditions and more homogeneous slip was evident when flow stresses were strain rate sensitive. The planarity of slip is also considered responsible for the deviation in experimental data from the Ludwick–Hollomon power-law at low plastic strains in regimes insensitive to strain rate. Irrespective of strain rate sensitivity and degree of homogeneity of slip, fracture mode remained ductile at almost all the conditions studied. Dynamic Strain Ageing: Alloy 720LI exhibits jerky flow in monotonic tension at intermediate temperatures ranging from 250-475°C. After considering all known causes for serrated flow in materials, the instability in flow (Portevin-LeChatelier (PLC) effect) is considered attributable to dynamic strain ageing (DSA), arising from interactions between diffusing solute atoms and mobile dislocations during plastic flow. As the temperature range of DSA coincided with typical bore and web temperatures of turbine discs, its possible influence on tensile properties is considered in detail. No significant change in tensile strength, ductility, or work hardening is observed, due to DSA, with increase in temperature from smooth to serrated flow regime. However strain rate sensitivity, which is positive in smooth flow regime turned negative in the serrated flow regime. Analysis of serrated flow on the basis of critical plastic strain for onset of serrations revealed that in most of the temperature-strain rate regimes studied, alloy 720LI exhibits ‘inverse’ PLC effect which is a phenomenon that has not been fully understood in contrast to ‘normal’ PLC effect observed widely in dilute solid solutions. Other characteristics of serrated flow viz., stress decrement and strain increment between serrations are also analyzed to understand the mechanism of DSA. Though the activation energy determined using stress decrements suggest that carbon atoms could be responsible for locking of dislocations, based on its influence on mechanical properties and also on its temperature regime of existence, weak pinning of dislocations by substitutional solute atoms are considered responsible for DSA in alloy 720LI. LCF Behavior: LCF studies were carried out under fully reversed constant strain amplitude conditions at 25, 400 and 650°C with strain amplitudes ranging from 0.4-1.2%. Different cyclic stress responses observed depending on the imposed conditions are correlated to the substructures that evolved. Low level of dislocation activity and interactions observed in TEM is considered the reason behind stable cyclic stress response at low strain amplitudes at all temperatures. TEM studies also show that secondary γ’ precipitates that are degraded through repeated shearing are responsible for the continuous softening, observed after a short initial hardening phase, at higher strain amplitudes. Studies at 400°C show manifestation of DSA on LCF behavior at 400°C in the form increased cyclic hardening which tends to offset softening effects at higher strain amplitudes. Plastic strain dependence of fatigue lives exhibited bilinearity in Coffin-Manson plots at all temperatures. TEM substructures revealed that planar slip with deformation concentrated on slip bands is the major deformation mode under all the conditions examined. However, homogeneity of deformation increases with increase in strain and temperature. At 25°C, with increasing strain, increased homogeneity manifested in the form of increased number of slip bands. At 650°C, with increase in strain, increased dislocation activity in the inter-slip band regions lead to increased homogeneity. It is also seen that fine deformation twins that form at 650°C and low strain amplitudes play a role in aiding homogenization of deformation. Unlike other alloy systems where an environmental effect or a change in deformation mechanism leads to bilinearity in Coffin – Manson (CM) plots, our study shows that differences in distribution of slip is the reason behind bilinear CM plots. While the properties and behavior of alloy 720LI under monotonic and cyclic loading conditions over a range of temperatures could be rationalized on the basis of deformation substructures, the thesis opens up the door for further in-depth studies on deformation mechanisms in 720LI as well as other disc alloys of similar microstructure.

Amorphous alloys possess attractive combinations of mechanical properties (high elastic limit, ~2%, high fracture toughness, 20-50 MPa.m1/2, etc.) and exhibit mechanical behavior that is different, in many ways, from that of the crystalline metals and alloys. However, fundamental understanding of the deformation and fracture mechanisms in amorphous alloys, which would allow for design of better metallic glasses, has not been established on a firm footing yet. The objective of this work is to understand the deformation and fracture mechanisms of amorphous materials at various length scales and make connections with the macroscopic properties of glasses. Various experimental techniques were employed to study the macroscopic behavior and atomistic simulations were conducted to understand the mechanisms at the nano level. Towards achieving these objectives, we first study the toughness of a Zr-based bulk metallic glass (BMG), Vitreloy-1, as a function of the free volume, which was varied by recourse to structural relaxation of the BMG through sub-Tg annealing treatment. Both isothermal annealing at 500 K (0.8Tg) for up to 24 h and isochronal annealing for 24 h in the temperature range of 130 K (0.65Tg) to 530 K (0.85Tg) were conducted and the impact toughness, Γ, values were measured. Results show severe embrittlement, with losses of up to 90% in Γ, with annealing. The variation in Γ with annealing time, ta, was found to be similar to that observed in the enthalpy change at the glass transition, ΔH, with ta, indicating that the reduction of free volume due to annealing is the primary mechanism responsible for the loss in Γ with annealing. Having established the connection between sub-atomic length scales (free volume) and macroscopic response (toughness), we investigated further the affects of relaxation on intermediate length scale behavior, namely deformation induced by shear bands, by employing instrumented indentation techniques. While the Vickers nano-indentation response of the as-cast and annealed glasses do not show any significant difference, spherical indentation response shows reduced shear band activity in the annealed BMG. Further, relatively high indentation strain was observed to be necessary for shear band initiation in the annealed glass, implying an increased resistance for the nucleation of shear bands when the BMG is annealed. In the absence of microstructural features that allow for establishment of correlation between properties and the structure, we resort to atomistic modeling to gain further understanding of the deformation mechanisms in amorphous alloys. In particular, we focus on the micromechanisms of strain accommodation including crystallization and void formation during inelastic deformation of glasses. Molecular dynamics simulations on a single component system with Lennard-Jones-like atoms suggest that a softer short range interaction between atoms favors crystallization. Compressive hydrostatic strain in the presence of a shear strain promotes crystallization whereas a tensile hydrostatic strain was found to induce voids. The deformation subsequent to the onset of crystallization includes partial re-amorphization and recrystallization, suggesting important mechanisms of plastic deformation in glasses. Next, a study of deformation induced crystallization is conducted on two component amorphous alloys through atomistic simulations. The resistance of a binary glass to deformation-induced-crystallization (deformation stability) is found to increase with increasing atomic size ratio. A new parameter called “atomic stiffness” (defined by the curvature of the inter-atomic potential at the equilibrium separation distance) is introduced and examined for its role on deformation stability. The deformation stability of binary glasses is found to increase with increasing atomic stiffness. For a given composition, the internal energies of binary crystals and glasses are compared and it is found that the energy of glass remains approximately constant for a wide range of atomic size ratios unlike crystals in which the energy increases with increasing atomic size ratio. This study uncovers the similarities between deformation and thermal stabilities of glasses and suggests new parameters for predicting highly stable glass compositions.