NUMERICAL STUDY OF PHASE TRANSITION PROBLEMS USING STRING METHOD

Ph.DDOCTOR OF PHILOSOPH

Effect of nonlinear and noncollinear transformation strain pathways in phase-field modeling of nucleation and growth during martensite transformation

The phase-field microelasticity theory has exhibited great capacities in studying elasticity and its effects on microstructure evolution due to various structural and chemical non-uniformities (impurities and defects) in solids. However, the usually adopted linear and/or collinear coupling between eigen transformation strain tensors and order parameters in phase-field microelasticity have excluded many nonlinear transformation pathways that have been revealed in many atomistic calculations. Here we extend phase-field microelasticity by adopting general nonlinear and noncollinear eigen transformation strain paths, which allows for the incorporation of complex transformation pathways and provides a multiscale modeling scheme linking atomistic mechanisms with overall kinetics to better describe solid-state phase transformations. Our case study on a generic cubic to tetragonal martensitic transformation shows that nonlinear transformation pathways can significantly alter the nucleation and growth rates, as well as the configuration and activation energy of the critical nuclei. It is also found that for a pure-shear martensitic transformation, depending on the actual transformation pathway, the nuclei and austenite/martensite interfaces can have nonzero far-field hydrostatic stress and may thus interact with other crystalline defects such as point defects and/or background tension/compression field in a more profound way than what is expected from a linear transformation pathway. Further significance is discussed on the implication of vacancy clustering at austenite/martensite interfaces and segregation at coherent precipitate/matrix interfaces.National Science Foundation (U.S.). Division of Materials Research (DMR-1410322)National Science Foundation (U.S.). Division of Materials Research (DMR-1410636

Physics Informed Reinforcement Learning for Power Grid Control using Augmented Random Search

Wide adoption of deep reinforcement learning in energy system domain needs to overcome several challenges , including scalability, learning from limited samples, and high-dimensional continuous state and action spaces. In this paper, we integrated physics-based information from the generator operation state formula, also known as Swing Equation, into the reinforcement learning agent's neural network loss function, and applied an augmented random search agent to optimize the generator control under dynamic contingency. Simulation results demonstrated the reliability performance improvements in training speed, reward convergence, and future potentials in its transferability and scalability

Accelerating ferroic ageing dynamics upon cooling

Once a structural glass is formed, its relaxation time will increase exponentially with decreasing temperature. Thus, the glass has little chance of transforming into a crystal upon further cooling to zero Kelvin. However, a spontaneous transition upon cooling from amorphous to long-range ordered ferroic states has been observed experimentally in ferroelastic, ferroelectric and ferromagnetic materials. The origin for this obvious discrepancy is discussed here conceptually. We present a combined theoretical and numerical study of this phenomenon and show that the diffusive and displacive atomic processes that take place in structural glass and amorphous ferroics, respectively, lead to markedly different temperature-dependent relaxation behaviors, one being ‘colder is slower’ and the other being ‘colder is faster’.National Basic Research Program of China (2012CB619402)National Basic Research Program of China (2014CB644003)National Key Basic Research Program of China (51671156)National Basic Research Program of China 111 Project (B06025)National Science Foundation (U.S.). Division of Materials Research (DMR-1410322)National Science Foundation (U.S.). Division of Materials Research (DMR-1410636

TACE with Ar-He Cryosurgery Combined Minimal Invasive Technique for the Treatment of Primary NSCLC in 139 Cases

Background and objective TACE, Ar-He target cryosurgery and radioactive seeds implantation are the mainly micro-invasive methods in the treatment of lung cancer. This article summarizes the survival quality after treatment, the clinical efficiency and survival period, and analyzes the advantages and shortcomings of each methods so as to evaluate the clinical effect of non-small cell lung cancer with multiple minimally invasive treatment. Methods All the 139 cases were nonsmall cell lung cancer patients confirmed by pathology and with follow up from July 2006 to July 2009 retrospectively, and all of them lost operative chance by comprehensive evaluation. Different combination of multiple minimally invasive treatments were selected according to the blood supply, size and location of the lesion. Among the 139 cases, 102 cases of primary and 37 cases of metastasis to mediastinum, lung and chest wall, 71 cases of abundant blood supply used the combination of superselective target artery chemotherapy, Ar-He target cryoablation and radiochemotherapy with seeds implantation; 48 cases of poor blood supply use single Ar-He target cryoablation; 20 cases of poor blood supply use the combination of Ar-He target cryoablation and radiochemotheraoy with seeds implantation. And then the pre- and post-treatment KPS score, imaging data and the result of follow up were analyzed. Results The KPS score increased 20.01 meanly after the treatment. Follow up 3 years, 44 cases of CR, 87 cases of PR, 3 cases of NC and 5 cases of PD, and the efficiency was 94.2%. Ninety-nine cases of 1 year survival (71.2%), 43 cases of 2 years survival (30.2%), 4 cases with over 3 years survival and the median survival was 19 months. Average survival was (16±1.5)months. There was no severe complications, such as spinal cord injury, vessel and pericardial aspiration. Conclusion Minimally invasive technique is a highly successful, micro-invasive and effective method with mild complications. To non-small cell lung cancer, we can improve the middle and long term clinical effect by using the different combination of multiple minimally invasive treatments according to the patient's condition