[Truncated abstract. Please see pdf format for complete text.] Over the years much work has been done on studying sequence effects under multilevel loading. Yet, the underlying fatigue mechanisms responsible for such interactions are not fully understood. The study of fatigue under complex loading histories begins by investigating strain interaction effects arising from simple 2-step loading sequences. Fatigue for all investigations were conducted under uniaxial push-pull mode in strain-control. Fatigue is traditionally classified as either low or high cycle fatigue (LCF and HCF respectively). The boundary for LCF and HCF is not well-defined even though the fatigue life of LCF is typically dominated by crack “initiation”, while for HCF, fatigue life is usually dominated by stable crack growth. The terms LCF and HCF, apart from referring to the low and high number of fatigue cycles required for failure, also bear little physical meaning in terms of describing the state of fatigue imposed. As a result, conventional definitions of the two distinct regimes of fatigue are challenged and a new method of classifying the boundary between the two regimes of fatigue is proposed. New definitions are proposed and the terms plastically dominant fatigue (PDF) and elastically dominant fatigue (EDF) are introduced as suitable replacements for LCF and HCF respectively. PDF refers to the condition of a material undergoing significant reverse plasticity during cyclic loading, while for EDF, minimal reverse plasticity is experienced. Systematic testing of three materials, 316 L stainless steel, 6061-T6 aluminium alloy and 4340 high strength steel, was performed to fully investigate the cycle ratio trends and “damage” accumulation behaviour which resulted from a variety of loading conditions. Results from this study were carried over to investigate more complex multilevel loading sequences and possible mechanisms for interaction effects observed both under 2-step and multi-step sequences were proposed. Results showed that atypical cycle ratio trends could result from loading sequences which involve combinations of strain amplitudes from different fatigue regimes (i.e. PDF or EDF). Mean strain effects on fatigue life were also studied. The objective of this study was to identify regimes of fatigue which are significantly influenced by mean strains. Results indicated that mean strains affected EDF but not PDF. 2-step tests, similar to those performed in earlier studies were conducted to investigate the effects of mean strain on variable amplitude loading. Again, atypical cycle ratio trends were observed for loading sequences involving combinations of PDF and EDF. It is understood that fatigue crack growth interaction behaviour and mean stress effects are two dominant mechanisms which can be used to explain cycle ratio trends observed. The significance and importance of proper PDF/EDF definition and specification are also stressed. The study of fracture mechanics is an important component of any fatigue research. Fatigue crack growth in 4140 high strength steel CT specimens, under conditions of plane stress and plane strain were studied. In this investigation, the effects of R and overload ratios were also studied for both plane stress and plane strain conditions. Results indicate that differences in the point of crack “initiation” under both plane stress and plane strain conditions decrease with increasing load range, while the extent of crack retardation as a result of overloading, is greater under plane stress than plane strain conditions. The extent of crack growth retardation increases with decreasing R ratios and increasing overload ratios. The final phase of this project involves the proposal of two practical models used to predict cumulative “damage” and fatigue crack propagation in metals. The cumulative “damage” model proposed takes the form of a power law and the exponent which governs “damage” accumulation can easily be calculated by knowing the failure life, Nf, for a given strain or load level. Predictions for the “damage” model performed better when compared to other popular cumulative “damage” models. The second model proposed predicts fatigue crack growth behaviour from known monotonic and smooth specimen fatigue data. There are several benefits of having a model that can predict fatigue crack growth from monotonic and smooth specimen fatigue data: a) traditionally, engineers had to rely on expensive and time-consuming crack propagation tests to evaluate and select materials for maximum fatigue resistance, and b) monotonic and smooth specimen fatigue data are readily available. The crack propagation model is proposed to alleviate the material selection process by providing engineers a means to rapidly eliminate and narrow down selections for possible material candidates.

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.

Hot sections of gas engine turbines require specialized materials to withstand extreme conditions present during engine operation. Nickel-base superalloys are typically used as blades and disks in the high pressure turbine section because they possess excellent fatigue strength, creep strength and corrosion resistance at elevated temperatures. Components undergo thermomechanical fatigue conditions as a result of transient engine operation. Sharp geometric features, such as cooling holes in blades or fir-tree connections in disks, act as local stress raisers. The material surrounding these features are potential sites of localized inelastic deformation and crack formation. To reduce customer costs associated with unnecessary overhauls or engine down-time, gas turbine manufacturers require accurate prediction methods to determine component endurances. The influence of stress concentration severity on thermomechanical fatigue crack formation is of particular importance as cracks often initiate in these hot spots. Circumferentially notched specimens were utilized to perform thermomechanical fatigue experiments on blade material CM247LC DS and disk material PM IN100. A parametric study on CM247LC DS was performed utilizing four notched specimens. Experimental results were coupled with finite element simulations utilizing continuum based constitutive models. The effects of applied boundary conditions on crack initiation life was studied in both alloys by performing experiments under remotely applied force and displacement boundary conditions. Finite element results were utilized to develop a life prediction method for notched components under thermomechanical fatigue conditions.

This research establishes a physics-based life determination model for the second generation single crystal superalloy PWA 1484 experiencing out-of-phase thermomechanical fatigue (TMF). The life model was developed as a result of a combination of critical mechanical tests, dominant damage characterization and utilization of well-established literature. The resulting life model improves life prediction over currently employed methods and provides for extrapolation into yet unutilized operating regimes. Particularly, the proposed deformation model accounts for the materials' coupled fatigue-environment-microstructure response to TMF loading. Because the proposed model is be based upon the underlying deformation physics, the model is robust enough to be easily modified for other single crystal superalloys having similar microstructure. Future use of this model for turbine life estimation calculations would be based upon the actual deformation experienced by the turbine blade, thereby enabling turbine maintenance scheduling based upon on a "retirement for a cause" life management scheme rather than the currently employed "safe-life" calculations. This advancement has the ability to greatly reduce maintenance costs to the turbine end-user since turbine blades would be removed from service for practical and justifiable reasons. Additionally this work will enable a rethinking of the warranty period, thereby decreasing warranty related replacements. Finally, this research provides a more thorough understanding of the deformation mechanisms present in loading situations that combine fatigue-environment-microstructure effects.

Bulk metallic glasses (BMGs) are a new class of structural materials and exhibit unique combinations of mechanical properties. As a result, their mechanical behavior has been an active area of scientific pursuit in the recent past and considerable emphasis has been paid to understand plastic deformation in them. It is now well accepted that shear transformation zones (STZs), aided by free volume, are the fundamental carriers of plasticity. At a microscopic level, deformation at low temperatures and high stresses tends to localize into shear bands. Most BMGs posses high fracture toughness despite high yield strengths and poor global ductility. However, the micro-mechanisms of fracture and fatigue in this new class of materials are not fully understood yet. The overall objective of this study is to provide insights into the fracture and fatigue response of amorphous alloys, which is important both from scientific and technological perspectives. The key questions we seek to answer through this study are the following. Do amorphous alloys undergo a ductile-brittle transition (DBT), and if so what are the reasons for it? What are the parameters that influence fatigue crack initiation in amorphous alloys and whether fatigue life can be improved by surface treatments? A related question is whether the BMGs are susceptible to deformation-induced crystallization (DIC). A Zr-based BMG, Zr41.2Ti13.75Cu12.5Ni10Be22.5 was utilized to conduct this study. By comparing the fracture and fatigue behaviors in the as-cast and annealed states {annealing was carried out below the glass transition temperature (Tg) because of established embrittlement effects}, we seek to provide answers for the questions posed above. We begin by examining the influence of temperature on the toughness of BMGs. Impact toughness measurements show that the annealed samples, which are brittle at room temperature, recover the lost toughness beyond a critical temperature (TDB) and exhibit a sharp DBT. However, the hardness remains unaffected across the TDB. Fractography reveals nano-scale patterning and cleavage fracture in the brittle state, while the formation of thick vein-patterns and shear fracture are characteristics of the ductile state of the annealed samples. We explore various micro-mechanistic possibilities for explaining the features of this transition, including a critical Poisson’s ratio-toughness correlation. Next, to understand the origins of fatigue crack initiation, we study the un-notched fatigue response of as-cast and sub-Tg annealed Zr-based BMG specimens. Because of embrittlement and nano-crystallization at the crack initiation region, the annealed specimens exhibit a lower fatigue life than the as-cast specimens. Shot-peening of the as-cast specimens did not exhibit significant improvement in their fatigue performance because of competing effects between the compressive residual stress field (CRSF) and deformation-induced softening. To further investigate surface and repeated loading effects, the tribological response of the as-cast Zr-based BMG was compared with specimens annealed above and below the Tg. A good correlation between the hardness (increasing as a function of the annealing temperature) and wear rate was obtained. The formation and peeling of the oxide layer formed during testing was the primary wear mechanism in all the specimens. Lastly, crystallization was observed within the deformed region of the as-cast Zr-based BMG repeatedly scratched with a sharp diamond indenter. But, transmission electron microscopy (TEM) does not reveal any evidence of crystallization within the indents formed within an electron transparent film formed by laser deposition of the as-cast Zr-based BMG. Absence of crystallization in deformed regions obtained by designing critical experiments, which avoid artifacts generated during sample preparation, suggests that the occasional observation of DIC might be an exception rather than the rule in BMGs.

Metal fatigue is a recurring problem for metallurgists and materials engineers, especially in structural applications. It has been responsible for many disastrous accidents and tragedies in history. Understanding the micro-mechanisms during cyclic deformation and combating fatigue failure has remained a grand challenge. Environmental effects, like temperature or a corrosive medium, further worsen and complicate the problem. Ultimate design against fatigue must come from a materials perspective with a fundamental understanding of the interaction of microstructural features with dislocations, under the influence of stress, temperature, and other factors. This research endeavors to contribute to the current understanding of the fatigue failure mechanisms. Cast aluminum alloys are susceptible to fatigue failure due to the presence of defects in the microstructure like casting porosities, non-metallic inclusions, non-uniform distribution of secondary phases, etc. Friction stir processing (FSP), an emerging solid state processing technique, is an effective tool to refine and homogenize the cast microstructure of an alloy. In this work, the effect of FSP on the microstructure of an A356 cast aluminum alloy, and the resulting effect on its tensile and fatigue behavior have been studied. The main focus is on crack initiation and propagation mechanisms, and how stage I and stage II cracks interact with the different microstructural features. Three unique microstructural conditions have been tested for fatigue performance at room temperature, 150 °C and 200 °C. Detailed fractography has been performed using optical microscopy, scanning electron microscopy (SEM) and electron back scattered diffraction (EBSD). These tools have also been utilized to characterize microstructural aspects like grain size, eutectic silicon particle size and distribution. Cyclic deformation at low temperatures is very sensitive to the microstructural distribution in this alloy. The findings from the room temperature fatigue tests highlight the important role played by persistent slip bands (PSBs) in fatigue crack initiation. At room temperature, cracks initiate along PSBs in the absence of other defects/stress risers, and grow transgranularly. Their propagation is retarded when they encounter grain boundaries. Another major finding is the complete transition of the mode of fatigue cracking from transgranular to intergranular, at 200 °C. This occurs when PSBs form in adjacent grains and impinge on grain boundaries, raising the stress concentration at these locations. This initiates cracks along the grain boundaries. At these temperatures, cyclic deformation is no longer microstructure- dependent. Grain boundaries don’t impede the progress of cracks, instead aid in their propagation. This work has extended the current understanding of fatigue cracking mechanisms in A356 Al alloys to elevated temperatures.