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Student Number : 9102778K - MSc(Eng) dissertation - School of Chemical and Metallurgical Engineering - Faculty of Engineering and the Built Environment / Strain ageing anisotropy, a surprising property of iron, implies that interstitials, which are non-lattice obstacles, can give non-symmetrical opposition to glide. Al- though this has been investigated by others, it is shown that it is di±cult to eliminate extraneous residual stresses during testing. An Avery torsion machine was adapted for the study of strain ageing anisotropy through the design of a torque and twist measurement system. This required the optimization of sensitivity, stiffness and me- chanical stability criteria, while ensuring practicality. When a ¯ne-grained sample with circumferential groove was tested a sharp yield point and lack of yield after ageing in the reverse direction was observed. Although more testing is required, it con¯rms the results of other researchers. In testing it was found that the quality of the sample machining was critical in achieving an accurate yield, and the groove design must be reviewed and improved. While the system measured to the required torque resolution, the strain measurement system could be improved by redesign and better calibration statistics.

The mechanical behaviour of the three advanced high strength steel grades, Docol 600DP, Docol 1200M and HyTens 1000, has been experimentally investigated under various types of deformation, and material models have been developed, which account for the experimentally observed behaviour. Two extensive experimental programmes have been conducted in this work. In the first, the dual phase Docol 600DP steel and martensitic Docol 1200M steel were subjected to deformations both under linear and non-linear strain paths. Regular test specimens were made both from virgin materials, i.e. as received, and from materials pre-strained in various directions. The plastic strain hardening, as well as plastic anisotropy and its evolution during deformation of the two materials, were evaluated and modelled with a phenomenological model. In the second experimental program, the austenitic stainless HyTens 1000 steel was subjected to deformations under various proportional strain paths and strain rates. It was shown experimentally that the material is sensitive both to dynamic and static strain ageing. A phenomenological model accounting for these effects was developed, calibrated, implemented in a Finite Element software and, finally,validated. Both direct methods and inverse analyses were used in order to calibrate the parameters in the material models. The agreement between the  numerical and experimental results are in general very good. This thesis is divided into two main parts. The background, theoretical framework and mechanical experiments are presented in the rst part. In the second part, two papers are appended.

L'objectif de cette thèse est d'étudier expérimentalement les phénomènes de vieillissement dus à la diffusion des atomes en solution dans les alliages d’aluminium et les instabilités qui leur sont associées comme le phénomène Portevin-Le Châtelier ou les bandes de Lüders et de proposer une modélisation de ces phénomènes dans le cadre de la thermodynamique des processus irréversibles. Une étude expérimentale détaillée est alors entreprise sur les alliages d’aluminium AA5083-H116 et AA5182-O. Le comportement du premier présente l’effet PLC de façon prononcée et les deux types d’instabilités sont observées simultanément pour le second. La corrélation d'images numériques et la thermographie infrarouge sont essentiellement employées pour détecter et caractériser les aspects spatiotemporels des instabilités observées. La déformation non homogène due à l’apparition et la propagation de bandes de localisation est mise en évidence. Ces bandes de déformation sont visualisées, permettant à leurs diverses caractéristiques (vitesse, orientation, largeur, vitesse de déformation à l'intérieur des bandes, augmentation de la température à l'intérieur des bandes) d'être mesurées sur des éprouvettes lisses (plates, cylindriques ou prismatiques). Dans le cas des éprouvettes plates, l’effet des épaisseurs de l’éprouvette a aussi été examiné. Les caractéristiques des bandes sont aussi analysées sur d’autres géométries d’éprouvette (entaillées avec divers types d’entailles) à des vitesses de déformation différentes pour exhiber leur morphologie en présence de chargements multiaxiaux. Des simulations non linéaires et tridimensionnelles ont été effectuées en utilisant le modèle de McCormick pour montrer comment la prise en compte des phénomènes de vieillissement, même partielle, permet de décrire les hétérogénéités et le mouvement des bandes de déformation ainsi que de prévoir leurs différentes caractéristiques. Enfin, en se basant sur les mécanismes physiques à la base des phénomènes de vieillissement et en soulignant les limites du modèle de McCormick, un modèle élasto-viscoplastique prenant en considération les phénomènes de vieillissement est proposé dans le cadre de la thermodynamique des processus irréversibles. / The objective of this thesis is to study experimentally the ageing phenomena due to the atoms diffusion in solid solutions in aluminium alloys and associated instabilities as the Portevin-Le Châtelier phenomenon or the Lüders bands and to propose a model of these phenomena within the framework of thermodynamics of irreversible processes. Digital Image Correlation (DIC) and Digital Infrared Thermography (DIT) are essentially employed to capture and characterize the spatio-temporal aspects of these instabilities. Inhomogeneous deformation, due to the appearance and the propagation of various localization bands are observed. These deformation bands are visualized, allowing their various characteristics (velocity, orientation, width, strain rate inside the bands, temperature increase inside the bands) to be measured on smooth specimens (flat, cylindrical or prismatic). In the case of flat test, the effect of the specimen thickness was also examined. The band characteristics are also analyzed on other specimen geometries (notched with various kinds of notches) at different strain rates to exhibit their morphology in the presence of multiaxial loadings. Some nonlinear and three-dimensional simulations were carried out with McCormick model to show how the inclusions of ageing phenomena, even partial, makes possible to describe heterogeneities and the deformation bands but also reproducing qualitatively their various characteristics. Finally, based on the physical mechanisms of aging phenomena and underlying the limitations of the McCormick model, an elasto-viscoplastic model taking into account the aging phenomena is proposed in the framework of thermodynamics of irreversible processes.

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.