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Ultra fine grained production in RRR niobium for RF cavities ECAE. by Trever Howard Carnes

Ultra fine grained production in RRR niobium for RF cavities

ECAE. by Trever Howard Carnes

Published
ISBN : 9781109363333
NOOKstudy eTextbook
111 pages
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 About the Book 

Efficient fabrication of seamless RF cavities by hydroforming requires the use of very fine microstructure. In this study, grain refinement of commercial purity (99.99%) niobium was carried out by equal channel angular extrusion (ECAE) for up to 8 passes via route BC using a die with a channel intersection angle of 90° at a rate of 0.25mm/sec. Samples were sectioned (top, middle and bottom) from the as received material and the billets of each pass and then analyzed using various analytical tools, which includes Vickers microhardness, X-ray diffraction and Orientation Imaging Microscopy (OIM).-In the as-received condition, the material was characterized by equiaxed microstructure and and duplex texture at the top, middle and bottom sections. In spite of the billet rotation employed in the BC route, the bottom section showed a consistently different microstructure and texture from the top and middle sections upon processing. Both the top and middle sections of the 1 to 6-pass billets were found to retain most of the and duplex texture, while the bottom region was essentially with weak and . The 8-pass exhibited primarily texture in all the three sections. Microstructural evolution resulting from the processing can be classified into two regimes: cell band formation in the early stages (passes 1-3) of processing and orientation splitting, which occurred during passes 4 to 8. Grain size analysis showed that the grains were reduced in diameter in the middle section from an average of 32.6mum in the AR sample to 0.60mum for the 8P sample. The Nb hardened significantly after the first pass and then gradually increased thereafter. A plot of the hardness value vs. d-1/2 revealed that Hall-Petch relation was valid for the top, middle (up to 6-pass) and bottom sections, with the top and middle section utilizing the same friction stress and locking parameter constants. The simulated texture of the middle section using a combination of ABAQUS FEM and Lapp softwares was found to be in good agreement with the experimental macro- and micro-texture. The top and bottom sections were not accurately predicted by this simulation.