Particulate reinforced Al alloy composites (AlMCs) are recognized as important structural materials due to their lightweight, high modulus and strength and high wear resistance. In order to understand the effect of matrix, reinforcement and secondary processing techniques on the microstructure development and mechanical properties of AlMCs produced by powder metallurgy routes, Al alloy composites reinforced with three types of reinforcements by different secondary processing techniques have been produced and examined. Fabrication of Al or 6061Al alloy based composites reinforced with nano-sized SiC particles (~500nm), micro-sized (<25µm) quasicrystalline alloy particles (hereinafter referred to as “NQX”) and micro-sized Nb particles (~130µm) has been carried out by powder metallurgy routes followed by extrusion or cold rolling. After extrusion, a homogeneous distribution of secondary particles has been obtained with rare interfacial reaction products. The 6061Al/SiC composites exhibit superior mechanical properties than either monolithic alloys or composites reinforced with micro-sized particles with retained ductility while the 6061Al/NQX and 6061Al/Nb composites show limited improvement in tensile strength mainly due to their reinforcement size and poor interfacial bonding. After cold rolling, the evolution in microstructure, texture and strength has been analysed. A typical near β fibre texture with highest intensities near Copper and Brass orientations has been developed for 6061Al/NQX and 6061Al/Nb composites. For 6061Al/SiC composites, a randomized texture with very small grains has achieved due to the presence of the non-deformable SiC particles. Mechanical property tests including microhardness, three-point bending tests and tensile tests have been carried out on cold rolled samples and the results exhibit some level of improvement when compared with as-extruded samples due to work hardening. Finally, the work moves on to the general discussion based on the previous result chapters. The microstructural development related to reinforcement, matrix and interfacial areas during extrusion and cold rolling has been summarised and the correlation between microstructure and mechanical properties has been discussed. The thesis provides a thorough understanding of AlMCs produced by powder metallurgy routes in terms of matrix, reinforcement and processing techniques. It can provide reference to the future development of AlMCs for high strength applications.

Superalloys have performed well as blade and disc materials in turbine engines due to their exceptional elevated temperature strength, high resistance to creep, oxidation and corrosion as well as good fracture toughness. This study explores the use of a relatively new technique of strain measurement, high resolution electron backscatter diffraction (HR-EBSD) to measure local deformation fields. The heart of the HR-EBSD technique lies in comparing regions in EBSD patterns from a strained region of a sample to those in a pattern from an unstrained region. This method was applied to study the elastic strain fields and geometrically necessary dislocation density (GND density) distribution near hard carbide particles in a nickel-based superalloy MAR-M-002. Significant thermal strains were initially induced by thermal treatment, which included a final cooling from the ageing temperature of 870°C. Elastic strains were consistent with a compressive radial strain and tensile hoop strain that was expected as the matrix contracts around the carbide. The mismatch in thermal expansion coefficient of the carbide particles compared to that of the matrix was sufficient to have induced localized plastic deformation in the matrix leading to a GND density of 3 x 10¹³ m^–2 in regions around the carbide. These measured elastic strain and GND densities have been used to help develop a crystal plasticity finite element model in another research group and some comparisons under thermal loading have also been examined. Three-point bending was then used to impose strain levels within the range ±12% across the height of a bend bar sample. GND measurements were then made at both carbide-containing and carbide-free regions at different heights across the bar. The average GND density increases with the magnitude of the imposed strain (both in tension and compression), and is markedly higher near the carbide particles. The higher GND densities near the carbides (order of 10¹⁴ per m²) are generated by the large strain gradients produced around the plastically rigid inclusion during monotonic mechanical deformation with some minor contribution from the pre-existing residual deformation from thermal loading. A method was developed of combining the local EBSD measurements with FE modelling to set the average residual strains within the mapped region even when a good strain free reference point was unavailable. Cyclic loading was then performed under four point loading to impose strain levels of about ±8% across the height of bend bar samples. Similar measurements as in the case of monotonic deformation were made at several interruptions to fatigue loading. Observations from the cyclic loading such as slip features, carbide cracking, GND density accumulation have been explored around carbide particles, at regions away from them and near a grain boundary.

The factors controlling the reliability of Pb-free solders when subject to thermomechanical regimes relevant to the harsh aerospace environment have been studied. Ball grid array (BGAs) typical of microelectronic devices have been manufactured in-house and subjected to isothermal ageing and thermal cycling. The BGAs comprised both Cu and Ni-Au metallizations, Pb-free Sn-Ag-Cu 400 and 600&mu;m solder balls, FR4 and Al₂O₃ boards, and included circuits to measure resistance changes due to damage in the joints during thermal cycling. Microstructural evolution within the solders balls and complex interfacial reactions were studied in all configurations using various types of electron microscopy. The mechanical properties of the different phases formed within solder joints were studied using nanoindentation at room and elevated temperatures up to 175°C for the first time. Intermetallic compounds (IMCs) were stiff, hard and brittle with very low creep rates, while the softer primary Sn, eutectic regions and Cu metallization readily underwent creep. Two-dimensional finite element analysis (FEA) of nanoindentation was used to understand better the physical meaning of nanoindentation creep data. Reliability experiments comprised both thermal cycling and FEA of BGAs. The difference in coefficient of thermal expansion (CTE) in the BGA materials caused interfacial fatigue damage in the solder joints, which was detected primarily at the solder/metallization interface of the outermost, most strained solder joint. Accumulated creep strain per cycle at this interface was evaluated using 3D FEA of the stress-strain state of the BGA and results calibrated against experimental BGA mean lifetimes using the Coffin-Mason relationship. Nanoindentation combined with FEA has been shown to be a viable route for the rapid assessment of creep performance and lifetime in lead-free solders under aerospace thermal cycles.

Currently, the maintenance and repair of civil engineering infrastructures (especially bridges and highways) have become increasingly important, as these structures age and deteriorate. Interface between two different mixes or strengths of concrete also appear in large concrete structures involving mass concreting such as dams, nuclear containment vessels, cooling towers etc., since joints between successive lifts are inevitable. These joints and interfaces are potential sites for crack formation, leading to weakening of mechanical strength and subsequent failure. In case of a bi-material interface, the stress singularities are oscillatory in nature and the fracture behavior of a concrete-concrete bi-material interface is much more complicated. A comprehensive experimental work has been undertaken for characterization of the behavior of different concrete-concrete interfaces under static and fatigue loading. The effect of specimen size on the concrete-concrete interfaces is studied and the non-linear fracture parameters such as fracture energy, mode I fracture toughness, critical crack tip opening displacement, critical crack length, length of process zone, brittleness number, size of process zone, crack growth resistance curve and tension softening diagram. These parameters are required for modeling the concrete-concrete interfaces in non-linear finite element analysis. Presently, the advanced non-destructive techniques namely acoustic emission, digital image correlation and micro-indentation have great capabilities to characterize the fracture behavior. The damage in plain concrete and concrete interface specimens is characterized both qualitatively and quantitatively using acoustic emission technique by measuring the width of fracture process zone and width of damage zones. The DIC technique is used to obtain the fracture parameters such as mode I and mode II fracture toughness and critical energy release rate. The micro-mechanical properties are obtained by performing depth-sensing micro-indentation tests on the concrete-concrete interfaces. Civil engineering structures such as long-span bridges, offshore structures, airport pavements and gravity dams are frequently subjected to variable-amplitude cyclic loadings in actual conditions. Hence, in order to understand the fracture behaviour under fatigue loading, the fatigue crack growth in plain concrete and concrete-concrete interface is also studied using the acoustic emission technique. An attempt is made to apply the Paris’ law, which is applicable to mechanical behaviour of metals, for acoustic emission count data. All these studies show that, as the difference in the compressive strength of concrete on either side of the interface increases, the load carrying capacity decreases and the fracture parameters indicate the increase in the brittleness of the specimens. It is concluded that the repair concrete should be selected in such a way that its elastic properties are as those of the parent concrete.

This thesis has two parts. In the first part, we use the results of new fatigue experiments conducted with variable load levels as well as variable stress ratios to critically assess three (two old and one relatively new) cumulative fatigue damage models. These models are deterministic. Such models are usually tested using multiple blocks of periodic loading with differing amplitudes. However, available data pertains to zero-mean loading, and does not investigate the role of variable stress ratio (Smin/Smax). Here, we present experimental results for variable stress ratios. Two specimen geometries and two materials (Al 2014and Al 2024)are tested. Manson’s double linear damage rule (DLDR)gives the highest accuracy in predicting the experimental outcome, even in the presence of variable stress ratios, whereas predictions of the newer model (“A constructive empirical theory for metal fatigue under block cyclic loading,” Proceedings of the Royal Society A, 464 (2008), 1161-1179) are slightly inferior. The widely used Miner’s rule is least accurate in terms of prediction. The merits and drawbacks of these models, in light of the experimental results, are as follows. The DLDR, though accurate, has minor scientific inconsistencies and no clear generalization. The constructive model has possible generalizability and more appealing scientific consistency, but presently has poorer accuracy. Miner’s rule, least accurate, lies within the constructive approach for special parameter values. The DLDR can guide the new (constructive)approach through new parameter fitting criteria. In the second part of this thesis, we consider the scatter in fatigue life and use the Weibull distribution to describe ‘S-N-P’ curves. We first assume homoscedasticity (load-independent or constant variance) and present a way to draw a p-percentile line on a log-log load-life plot. Then heteroscedasticity (load-dependent variance) in fatigue life is incorporated and a simple statistical model is proposed, to obtain a straight line percentile plot at a pre-specified probability of survival ps. The proposed method is illustrated for Al 2014-T6 and Al 2024-T4 data sets (extracted manually) from MMPDS-01 (a data handbook).

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.

The ability to predict fatigue crack growth rates is essential in safety critical systems. The discovery of fatigue crack closure in 1970 caused a flourish of research in attempts to simulate this behaviour, which crucially affects crack growth rates. Historically, crack tip plasticity models have been based on one-dimensional rays of plasticity emanating from the crack tip, either co-linear with the crack (for the case of plane stress), or at a chosen angle in the plane of analysis (for plane strain). In this thesis, one such model for plane stress, developed to predict fatigue crack closure, has been refined. It is applied to a study of the relationship between the apparent stress intensity range (easily calculated using linear elastic fracture mechanics), and the true stress intensity range, which includes the effects of plasticity induced fatigue crack closure. Results are presented for all load cases for a finite crack in an infinite plane, and a method is demonstrated which allows the calculation of the true stress intensity range for a growing crack, based only on the apparent stress intensity range for a static crack. Although the yield criterion is satisfied along the plastic ray, these one-dimensional plasticity models violate the yield criterion in the area immediately surrounding the plasticity ray. An area plasticity model is therefore required in order to model the plasticity more accurately. This thesis develops such a model by distributing dislocations over an area. Use of the model reveals that current methods for incremental plasticity algorithms using distributed dislocations produce an over-constrained system, due to misleading assumptions concerning the normality condition. A method is presented which allows the system an extra degree of freedom; this requires the introduction of a parameter, derived using the Prandtl-Reuss flow rule, which relates the magnitude of slip on complementary shear planes. The method is applied to two problems, confirming its validity.