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Grain refinement of cast niobium via equal channel angular extrusion/annealing

by Bryant, Don O.

Abstract (Summary)
This research investigated the effectiveness of equal channel angular extrusion (ECAE) and annealing to improve the grain morphology and mechanical properties of electron-beam remelted pure niobium. Extrusions were performed at room temperature with a 90o die. Routes 1A, 2C, 4E and 8E were investigated with duplicate billets undergoing routes 1A, 2C and 4E to determine reproducibility. Niobium proved to be very workable during the ECAE process. Hardness increased most dramatically after the first pass and leveled off thereafter. Recrystallization temperatures for the Reference Metals material ranged from a low of 800o C for routes 2C, 4E and 8E to a high of 1000o C for route 1A. For the Wah Chang material, the recrystallization temperatures ranged from a low of 800o C for the 4E ECAE processed material to a high of 950o C for the asreceived material. The initial grain size and orientation have been hypothesized to be influential on recrystallized grain morphology for ECAE/annealing processed niobium. Smaller initial grains produce a smaller and more homogeneous recrystallized grain microstructure. The average grain diameters for the recrystallized 4E ECAE processed Wah Chang material are 13 �± 6.3 mm for the 1000o C annealed state (no banding) and are 21 �± 9.5 mm for the 1100o C annealed state (no banding). Reference Metals material that underwent route 4E and annealed at 1000o C resulted in an average grain diameter of 28 mm for billet 2 (banding) and an average grain diameter of 32 mm for billet 19 (slight banding). Reference Metals material that underwent route 8E (no banding) annealed at 1000o C resulted in an average grain diameter of 36 mm. Reference Metals material that underwent route 4E samples annealed at 1100o C resulted in an average grain diameter of 26 mm for billet 2 (banding) and an average grain diameter of 43 mm for billet 19 (slight banding). Route 8E (significant banding) annealed at 1100o C resulted in an average grain diameter of 29 mm. Strain failure decreases from the as-cast material to the worked/recrystallized material while the 0.2% yield stress and ultimate tensile strength increase from the as-cast material to the worked/recrystallized material. Long sub-grains are created in the as-worked material after one and two passes and become more broken up after four passes. Sub-grain boundary angles increase with increasing strain. To produce a fine and homogeneous microstructure from large grained niobium, intermediate annealing should be employed.
Bibliographical Information:

Advisor:Hartwig, Karl T.; Griffin, Richard B.; Naugle, Donald G.

School:Texas A&M University

School Location:USA - Texas

Source Type:Master's Thesis

Keywords:niobium ecae

ISBN:

Date of Publication:12/01/2005

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