INTRINSIC STRENGTH, PRECIPITATION HARDENING AND TRIBOLOGICAL BEHAVIOR OF HIGH-ENTROPY ALLOYS

Recently, concentrated multicomponent alloys, or high-entropy alloys (HEAs) consisting of several principal components in approximately equiatomic proportions, have increasingly attracted research interest because of their unusual intrinsic characteristics, such as severe lattice distortion and sluggish diffusion. These features confer HEAs promising mechanical properties, making HEAs potential for structural applications. However, limited knowledge on intrinsic strength, precipitate stability and tribological behavior of HEAs is available.In Chapters 2 and 3, we systematically analyzed yield strengths of a series of fcc Ni-based and bcc Nb-based equiatomic alloys. By subtracting all possible strengthening contributions, the intrinsic strength (or lattice friction stress) of each alloy was extracted. It was found that lattice friction stress scaled linearly with the lattice distortion in these equiatomic alloys. A simplified model was developed to interpret this result. It was demonstrated that the enhanced strength in HEAs was mainly attributed to the lattice distortion. In Chapter 4, I reported the study of the coarsening of L12 precipitates in an fcc (NiCoFeCr)94Ti2Al4 HEA. Temporal evolutions of the morphology, size, and volume fraction of these coherent precipitates were examined and evaluated using electron microscopes. Treating the fcc-(NiCoFeCr)94Ti2Al4 alloy as a pseudo ternary Ni-Ti-Al alloy, I analyzed the coarsening kinetics of the precipitates. One of the major findings was that, owing to the relatively slow atomic diffusion in HEAs, coarsening of L12 precipitates in the current HEA was found to be slower than that in the conventional Ni-based alloys. This result demonstrated good thermal stability of the L12 precipitates. In Chapter 5, I investigated the tribological behavior of an amorphous Zr20Ti20Cu20Ni20Be20 HEA (a-HEA). Nanoscratch tests were carried out to measure the coefficient of friction (COF) and wear resistance of the amorphous alloy. The morphology of scratched surface and subsurface was further examined using electron microscopes. Due to high hardness and large elastic recovery, the current a-HEA exhibited good wear resistance and low COF, suggesting that a-HEAs are probably good candidate materials for tribological applications.Finally, a conclusion of all my studies was given. In addition, a future perspective based on my research results were presented in Chapter 6

Automatic Curriculum Learning With Over-repetition Penalty for Dialogue Policy Learning

Dialogue policy learning based on reinforcement learning is difficult to be applied to real users to train dialogue agents from scratch because of the high cost. User simulators, which choose random user goals for the dialogue agent to train on, have been considered as an affordable substitute for real users. However, this random sampling method ignores the law of human learning, making the learned dialogue policy inefficient and unstable. We propose a novel framework, Automatic Curriculum Learning-based Deep Q-Network (ACL-DQN), which replaces the traditional random sampling method with a teacher policy model to realize the dialogue policy for automatic curriculum learning. The teacher model arranges a meaningful ordered curriculum and automatically adjusts it by monitoring the learning progress of the dialogue agent and the over-repetition penalty without any requirement of prior knowledge. The learning progress of the dialogue agent reflects the relationship between the dialogue agent's ability and the sampled goals' difficulty for sample efficiency. The over-repetition penalty guarantees the sampled diversity. Experiments show that the ACL-DQN significantly improves the effectiveness and stability of dialogue tasks with a statistically significant margin. Furthermore, the framework can be further improved by equipping with different curriculum schedules, which demonstrates that the framework has strong generalizability

Hessian-based occlusion-aware radiance caching

Simuler efficacement l'éclairage global est l'un des problèmes ouverts les plus importants en infographie. Calculer avec précision les effets de l'éclairage indirect, causés par des rebonds secondaires de la lumière sur des surfaces d'une scène 3D, est généralement un processus coûteux et souvent résolu en utilisant des algorithmes tels que le path tracing ou photon mapping. Ces techniquesrésolvent numériquement l'équation du rendu en utilisant un lancer de rayons Monte Carlo. Ward et al. ont proposé une technique nommée irradiance caching afin d'accélérer les techniques précédentes lors du calcul de la composante indirecte de l'éclairage global sur les surfaces diffuses. Krivanek a étendu l'approche de Ward et Heckbert pour traiter le cas plus complexe des surfaces spéculaires, en introduisant une approche nommée radiance caching. Jarosz et al. et Schwarzhaupt et al. ont proposé un modèle utilisant le hessien et l'information de visibilité pour raffiner le positionnement des points de la cache dans la scène, raffiner de manière significative la qualité et la performance des approches précédentes. Dans ce mémoire, nous avons étendu les approches introduites dans les travaux précédents au problème du radiance caching pour améliorer le positionnement des éléments de la cache. Nous avons aussi découvert un problème important négligé dans les travaux précédents en raison du choix des scènes de test. Nous avons fait une étude préliminaire sur ce problème et nous avons trouvé deux solutions potentielles qui méritent une recherche plus approfondie.Efficiently simulating global illumination is one of the most important open problems in computer graphics. Accurately computing the effects of indirect illumination, caused by secondary bounces of light off surfaces in a 3D scene, is generally an expensive process and often solved using algorithms such as path tracing or photon mapping. These approaches numerically solve the rendering equation using stochastic Monte Carlo ray tracing. Ward et al. proposed irradiance caching to accelerate these techniques when computing the indirect illumination component on diffuse surfaces. Krivanek extended the approach of Ward and Heckbert to handle the more complex case of glossy surfaces, introducing an approach referred to as radiance caching. Jarosz et al. and Schwarzhaupt et al. proposed a more accurate visibility-aware Hessian-based model to greatly improve the placement of records in the scene for use in an irradiance caching context, significantly increasing the quality and performance of the baseline approach. In this thesis, we extended similar approaches introduced in these aforementioned work to the problem of radiance caching to improve the placement of records. We also discovered a crucial problem overlooked in the previous work due to the choice of test scenes. We did a preliminary study of this problem, and found several potential solutions worth further investigation

INNOVATION STRATEGIES OF LATECOMER FIRMS FROM EMERGING ECONOMIES

Ph.DDOCTOR OF PHILOSOPH

Characterization of Ductile Crack Propagation by Fractal Energy Dissipation Rate

Because of its geometry dependence and loss of physical meaning, the incremental crack resistance curve cannot characterize ductile fractures with large crack extensions and plastic deformations. Therefore, the energy dissipation rate R is employed to overcome these deficiencies, even though specimen size effects still exist. In the study, considering the fractal crack path and concomitant plastic dissipation in the fractal domain, a scale‑invariant energy dissipation rate, γp*, is proposed in the context of renormalization group theory. Some experiments in the literature have validated this approach. The fitted fractal energy dissipation rate is independent of the specimen size and initial crack length; moreover, as the specimen size increases, progressive fractality vanishing is found consistently with geometrical multifractality