Dislocation Modeling Of Orientation Gradients In Phase-Transformed Tantalum Thin Films

Ari Kestenbaum
Tantalum thin films are used in a wide range of applications, from micro-and nanoelectronics to military and aerospace applications, to medical devices. Tantalum has two phases, the stable bulk α phase and the metastable β phase found only in thin films. In a recent discovery, it has been shown that when tantalum is phase-transformed from β to α, it can form a unique microstructure with continuous changes in orientation and discontinuous grain boundaries. Dislocation structures must be present
more » ... n order to account for the lattice curvature associated with orientation gradients. To identify the dislocation arrays that give rise to this microstructure, we first analyzed the electron backscatter diffraction (EBSD) data of the orientation gradients using geometrically necessary dislocations (GNDs), but we found that our EBSD data are too noisy and do not have sufficient resolution for this approach. In this work, two models are presented in an attempt to describe the dislocation structures that must be present to account for the orientation gradients. In the first, a method of generating smooth, noise-free orientation data that match key features of the actual phase-transformed microstructure is developed and validated by comparing model data with film data. In the second model, a genetic algorithm approach is used to generate dislocation structures that can account for the orientation gradients produced using the first model. The genetic algorithm model consistently generates dislocation structures that produce orientation maps with an average misorientation of less than 2 • from the smooth orientation gradients generated by the first model. The generated dislocation structure is consistent across multiple runs, with similar shapes and Burgers vectors. The genetic algorithm model selects two types of dislocations, mostly (∼80%) [1 1 1] and the rest [1 1 1]. The dominance of these two slip systems provides a starting point to search for a mechanism in the β-to-α phase transformation that is capable of producing these dislocations. Biographical Sketch Ari Kestenbaum was born and raised in Westchester, NY just outside New York City. He attended Cornell University for his undergraduate education and received a B.S. in materials science with a minor in computer science. He liked Cornell so much he decided to stay for a fifth year. iii First and foremost I want to thank my parents for their tremendous love and support. I would also like to thank my advisor, Shefford Baker, for his advice and guidance, as well as his assistance in seeing the bigger picture whenever I got too wrapped up in the details of whatever I was doing. I also want to thank him for giving me the opportunity to do research as an undergraduate and for giving me a real problem to work on. I also want to thank Michael Thompson, who manages to pack more into less time than anyone else I know; each conversation with him turned into months of research. I would also like to thank him for his suggestion to use a genetic algorithm for creating dislocation structures. Last, but certainly not least, I am very grateful to Betsy Ellis, who has been there to help me every time I needed it during the last three years of research. iv vii
doi:10.7298/x4h41pdb fatcat:6rten76lfvc6nipht2x7niampe