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Title: Creep deformation in niobium at intermediate temperatures under cyclic loading conditions

Abstract

Comparisons of creep rates in pure niobium under static and cyclic loading conditions have been made at temperatures of 0.3 to 0.4 T/sub m/ at various stress levels. Internal stresses and apparent activation energies for creep have also been measured for static as well as cyclic loading conditions. The dislocation substructure formed under static and cyclic loading have been examined in transmission electron microscopy studies. Results are discussed in terms of dislocation rearrangements and change in obstacle structure under cyclic loading conditions. The cyclic creep acceleration increased with an increase in maximum applied stress. The cyclic creep rate increases with an increase in fractional unloading or with an increase in frequency at high values of maximum stress, but does not change significantly low values of maximum stress. Cyclic loading resulted in a significant lowering of average internal stress levels. The softening occurs due to strain-induced recovery (rearrangement and annihilations of dislocations) at high maximum stress values. At low values of maximum stress, recovery occurs by means of cross slip. The apparent activation energy for creep is lower than the activation energy for self-diffusion in niobium. There is no signficant difference in the activation energy values for static and cyclic creep.more » A significant difference in static and cyclic dislocation structures is seen in TEM studies. Well-developed cell structure is observed for the cyclic loading condition. Ragged and irregular cell structure with a high density of tangled dislocations is observed for the static loading condition. The present results suggest that bcc metals are not subject to large cyclic creep acceleration as found in close-packed metals at intermediate temperatures. For niobium, the cyclic creep rate exceeded the static creep rate by a factor of 3.5 under the most favorable experimental conditions.« less

Authors:
Publication Date:
OSTI Identifier:
5414365
Resource Type:
Thesis/Dissertation
Resource Relation:
Other Information: Thesis (Ph. D.)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; NIOBIUM; CREEP; COMPARATIVE EVALUATIONS; STRESSES; TEMPERATURE EFFECTS; ELEMENTS; MECHANICAL PROPERTIES; METALS; TRANSITION ELEMENTS; 360103* - Metals & Alloys- Mechanical Properties

Citation Formats

Singh, S. Creep deformation in niobium at intermediate temperatures under cyclic loading conditions. United States: N. p., 1980. Web.
Singh, S. Creep deformation in niobium at intermediate temperatures under cyclic loading conditions. United States.
Singh, S. 1980. "Creep deformation in niobium at intermediate temperatures under cyclic loading conditions". United States.
@article{osti_5414365,
title = {Creep deformation in niobium at intermediate temperatures under cyclic loading conditions},
author = {Singh, S},
abstractNote = {Comparisons of creep rates in pure niobium under static and cyclic loading conditions have been made at temperatures of 0.3 to 0.4 T/sub m/ at various stress levels. Internal stresses and apparent activation energies for creep have also been measured for static as well as cyclic loading conditions. The dislocation substructure formed under static and cyclic loading have been examined in transmission electron microscopy studies. Results are discussed in terms of dislocation rearrangements and change in obstacle structure under cyclic loading conditions. The cyclic creep acceleration increased with an increase in maximum applied stress. The cyclic creep rate increases with an increase in fractional unloading or with an increase in frequency at high values of maximum stress, but does not change significantly low values of maximum stress. Cyclic loading resulted in a significant lowering of average internal stress levels. The softening occurs due to strain-induced recovery (rearrangement and annihilations of dislocations) at high maximum stress values. At low values of maximum stress, recovery occurs by means of cross slip. The apparent activation energy for creep is lower than the activation energy for self-diffusion in niobium. There is no signficant difference in the activation energy values for static and cyclic creep. A significant difference in static and cyclic dislocation structures is seen in TEM studies. Well-developed cell structure is observed for the cyclic loading condition. Ragged and irregular cell structure with a high density of tangled dislocations is observed for the static loading condition. The present results suggest that bcc metals are not subject to large cyclic creep acceleration as found in close-packed metals at intermediate temperatures. For niobium, the cyclic creep rate exceeded the static creep rate by a factor of 3.5 under the most favorable experimental conditions.},
doi = {},
url = {https://www.osti.gov/biblio/5414365}, journal = {},
number = ,
volume = ,
place = {United States},
year = {1980},
month = {1}
}

Thesis/Dissertation:
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