Balachandran, Shreyas (2015-05). Microstructure Development in Bulk Niobium Following Severe Plastic Deformation and Annealing. Doctoral Dissertation.
Niobium (Nb) finds application in the fabrication of Nb3Sn superconducting wires for superconducting magnet applications, and SRF cavity applications. These applications need a favorable starting microstructure in precursor Nb to increase manufacturing reliability, reduce costs, and advance technology. The main problem with traditional Nb processing is that pure Nb is obtained by electron beam melting (EBM), which produces initial polycrystals tens to hundreds of centimeters in width/length, and can be comparable to the dimensions of the initial ingot. Breakdown of these initial cast microstructures, and to obtain consistent, fine grain microstructures in bulk product is a processing challenge. Generally, pure Nb, traditional thermo-mechanical processing methodologies such as: rolling, swaging, forging have been unable to effectively breakdown initial cast structures. An objective of the thesis is to develop thermo-mechanical processing strategies involving severe plastic deformation (SPD) by Equal Channel Angular Extrusion (ECAE) for bulk Nb. ECAE enables grain refinement without appreciable crossectional area changes to the workpiece, which is a hallmark of ECAE. The microstructure refinement process by ECAE is not well understood for the breakdown of large grain Nb. We have shown that ECAE produces continuous orientation splitting, and hence grain refinement even when processed under a strain path that is perceived to be structure conserving (route 2C). Initial bi-crystal results indicate that significant convergence in microstructure is possible within two ECAE passes for extremely large grain Nb. Processing strategies have been developed for effective control of grain size and texture in polycrystalline microstructures. The main results include effective grain size control leading to uniform recrystallized grain sizes in the range of tens of microns, and texture control in Nb bars. Nb bars processed by this technique have better co-deformation characteristics as observed in Cu-Nb monocore wire experiments where a circularity greater than 0.85 is obtained for several textures , as compared to circularities of 0.5-0.7 using traditional processing. The methodology to adopt precursor Nb processed by thermomechanical processing by SPD to traditional tube development techniques such as area reduction extrusion are presented. The tubes formed by this approach show sufficient ductility (greater than 30%) along the circumferential and extrusion directions with tensile properties being very similar in perpendicular directions. The development of tube ECAE (tECAE) process produces textures very close to f111g parallel to tube normal directions that are considered to be favorable for hydroforming applications. This is the first report of such textures in a seamless tube. The processing strategies developed are able to homogenize and converge the microstructures in Nb leading to similar deformation behavior irrespective of the starting microstructure. The grain refinement strategies for Nb presented in this thesis have the potential to be scaled up, and similar performance are expected irrespective of the Nb workpiece dimensions. The same processing strategies can be adapted to other bcc refractory metals including: tantalum (Ta), molybdenum (Mo), and tungsten (W) where the processing needs to be done at similar homologous temperatures/conditions where similar deformation mechanisms are active. \indent An objective of the thesis is to develop thermo-mechanical processing strategies involving severe plastic deformation (SPD) by Equal Channel Angular Extrusion (ECAE) for bulk Nb. ECAE enables grain refinement without appreciable crossectional area changes to the workpiece, which is a hallmark of ECAE. The microstructure refinement process by ECAE is not well understood for the breakdown of large grain Nb. We have shown that ECAE produces continuous orientation splitting, and hence grain refinement even when processed under a st