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Title: Long-Term Phase Instability in Water-Quenched U-6Nb

Abstract

A combinative approach of microhardness testing, tensile testing, and TEM microstructural analysis was employed to study the microstructure and mechanical instability of a water-quenched U-6wt.% Nb (WQ-U6Nb) alloy subjected to different aging schedules including artificial aging at 200 C, 15-year natural aging at ambient temperatures, and 15-year natural aging followed by accelerative aging at 200 C. The changes in mechanical property during and after the aging processes were examined using microhardness and tensile-testing methods. During the early stages of artificial aging at 200 C, the microhardness of WQ-U6Nb alloy increased, i.e., age hardening, as a result of the development of nanoscale modulation caused by spinodal decomposition. Coarsening of the modulated structure occurred after a prolonged aging at 200 C for 16 hours, and it led to a decrease of microhardness, i.e., age softening. Phase instability was also found to occur in WQ-U6Nb alloy that was subjected to a 15-year natural aging at ambient temperatures. The formation of partially ordered domains resulting from a spinodal modulation with an atomic-scale wavelength rendered the appearance of swirl-shape antiphase domain boundaries (APBs) observed in TEM images. Although it did not cause a significant change in microhardness, 15-year natural aging has dramatically affected the agingmore » mechanisms of the alloy isothermally aged at 200 C. Microhardness values of the NA alloy continuously increased after isothermal aging at 200 C for 96 hours as a result of the phase decomposition of partially ordered domains into Nb-depleted {alpha} phase and Nb-enriched U{sub 3}Nb ordered phase in the alloy. It is concluded that the long-term natural aging changes the transformation sequence of WQ-U6Nb, and it leads to order-disorder transformation, precipitation hardening, and ductility embrittlement of WQ-U6Nb alloy.« less

Authors:
;
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
895078
Report Number(s):
UCRL-CONF-218278
TRN: US200702%%599
DOE Contract Number:
W-7405-ENG-48
Resource Type:
Conference
Resource Relation:
Conference: Presented at: 15th Biennial Conference on Nuclear Explosive Design Physics Conference, Livermore, CA, United States, Oct 17 - Oct 21, 2005
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; 45 MILITARY TECHNOLOGY, WEAPONRY, AND NATIONAL DEFENSE; AGE HARDENING; AGING; ALLOYS; AMBIENT TEMPERATURE; DUCTILITY; EMBRITTLEMENT; INSTABILITY; MICROHARDNESS; MICROSTRUCTURE; MODULATION; NUCLEAR EXPLOSIVES; ORDER-DISORDER TRANSFORMATIONS; PHYSICS; PRECIPITATION HARDENING; TRANSFORMATIONS; WAVELENGTHS

Citation Formats

Hsiung, L L, and Zhou, J. Long-Term Phase Instability in Water-Quenched U-6Nb. United States: N. p., 2006. Web.
Hsiung, L L, & Zhou, J. Long-Term Phase Instability in Water-Quenched U-6Nb. United States.
Hsiung, L L, and Zhou, J. Mon . "Long-Term Phase Instability in Water-Quenched U-6Nb". United States. doi:. https://www.osti.gov/servlets/purl/895078.
@article{osti_895078,
title = {Long-Term Phase Instability in Water-Quenched U-6Nb},
author = {Hsiung, L L and Zhou, J},
abstractNote = {A combinative approach of microhardness testing, tensile testing, and TEM microstructural analysis was employed to study the microstructure and mechanical instability of a water-quenched U-6wt.% Nb (WQ-U6Nb) alloy subjected to different aging schedules including artificial aging at 200 C, 15-year natural aging at ambient temperatures, and 15-year natural aging followed by accelerative aging at 200 C. The changes in mechanical property during and after the aging processes were examined using microhardness and tensile-testing methods. During the early stages of artificial aging at 200 C, the microhardness of WQ-U6Nb alloy increased, i.e., age hardening, as a result of the development of nanoscale modulation caused by spinodal decomposition. Coarsening of the modulated structure occurred after a prolonged aging at 200 C for 16 hours, and it led to a decrease of microhardness, i.e., age softening. Phase instability was also found to occur in WQ-U6Nb alloy that was subjected to a 15-year natural aging at ambient temperatures. The formation of partially ordered domains resulting from a spinodal modulation with an atomic-scale wavelength rendered the appearance of swirl-shape antiphase domain boundaries (APBs) observed in TEM images. Although it did not cause a significant change in microhardness, 15-year natural aging has dramatically affected the aging mechanisms of the alloy isothermally aged at 200 C. Microhardness values of the NA alloy continuously increased after isothermal aging at 200 C for 96 hours as a result of the phase decomposition of partially ordered domains into Nb-depleted {alpha} phase and Nb-enriched U{sub 3}Nb ordered phase in the alloy. It is concluded that the long-term natural aging changes the transformation sequence of WQ-U6Nb, and it leads to order-disorder transformation, precipitation hardening, and ductility embrittlement of WQ-U6Nb alloy.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Jan 16 00:00:00 EST 2006},
month = {Mon Jan 16 00:00:00 EST 2006}
}

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  • A combinative approach of microhardness testing, tensile testing, and TEM microstructural analysis was employed to study the microstructure and mechanical instability of a water-quenched U-6wt.% Nb (WQ-U6Nb) alloy subjected to different aging schedules including artificial aging at 200 C, 15-year natural aging at ambient temperatures, and 15-year natural aging followed by accelerative aging at 200 C. The changes in mechanical property during and after the aging processes were examined using microhardness and tensile-testing methods. During the early stages of artificial aging at 200 C, the microhardness of WQ-U6Nb alloy increased, i.e., age hardening, as a result of the development ofmore » nanoscale modulation caused by spinodal decomposition. Coarsening of the modulated structure occurred after a prolonged aging at 200 C for 16 hours, and it led to a decrease of microhardness, i.e., age softening. Phase instability was also found to occur in WQ-U6Nb alloy that was subjected to a 15-year natural aging at ambient temperatures. The formation of partially ordered domains resulting from a spinodal modulation with an atomic-scale wavelength rendered the appearance of swirl-shape antiphase domain boundaries (APBs) observed in TEM images. Although it did not cause a significant change in microhardness, 15-year natural aging has dramatically affected the aging mechanisms of the alloy isothermally aged at 200 C. Microhardness values of the NA alloy continuously increased and no age softening was found after isothermal aging at 200 C for 96 hours as a result of the phase decomposition of partially ordered domains into Nb-depleted {alpha} phase and Nb-enriched U{sub 3}Nb ordered phase in the alloy. It is concluded that the long-term natural aging changes the transformation pathway of WQ-U6Nb, and it leads to order-disorder transformation, precipitation hardening, and ductility embrittlement of WQ-U6Nb alloy.« less
  • Microhardness testing and transmission electron microscopy are used to study the effects of long-term service on the aging behavior of a water-quenched U{sub 6} wt.% Nb alloy when subjected to isothermal aging at 200 deg. C. The original {alpha}'' phase in the WQ-U{sub 6}Nb alloy is found to become partially ordered over 18 years of aging at ambient temperatures, i.e., natural aging, forming a microstructure that is featured by antiphase domain boundaries (APBs). When subsequently aged at 200 deg. C, an ordered phase U{sub 3}Nb is precipitated through a nucleation-and-growth mechanism, suppressing spinodal decomposition that occurs when the water-quenched alloymore » is artificially aged at the same temperature. The different phase transformation paths lead to different microhardness changes during artificial aging: the naturally aged alloy is more slowly hardened, but to a greater microhardness peak value. (authors)« less
  • The U-6 wt.% Nb (U6Nb) alloy in water-quenched (WQ) state has been in service for a number of years. Its long-term reliability is affected by the changes of the alloy microstructure and mechanical properties during service. In this communication, the water quenched U-6 wt.% Nb (WQ-U6Nb) alloy in service for 15 years at ambient temperatures was studied using an analytical TEM analysis. We found that the long-term natural aging resulted in a disorder-order phase transformation, leading to the formation of anti-phase boundaries (APBs). The newly-found ordered phase was then identified by proposing two phase transform schemes, which were also discussedmore » with regards to the potential subsequence of the microstructural evolution for the alloy in further service. The initial study also provides convincing evidence for the disorder-order transformation, which has been predicted by numerous studies to be a transient thermodynamic event before spinodal decomposition. This suggests that the long-term naturally aged WQ-U6Nb is a good model alloy to study thermodynamic and kinetic phenomena requiring chronic processes.« less
  • A study was undertaken at Lawrence Livermore National Laboratory to characterize uranium, 6% niobium ingots produced via electron beam melting, hearth refining and continuous casting and to compare this material with conventional VIM/skull melt/VAR material. Samples of both the ingot and feed material were analyzed for niobium and trace metallic elements, carbon, oxygen and nitrogen. This material was also inspected metallographically and via microprobe analysis.
  • This study focuses on the ductility evaluation of low-temperature (100 and 200 C) aged U-6Nb alloy. The objective is to develop a ductility-based aging model to improve lifetime prediction for weapon components in the stockpile environment. Literature review shows that the work hardening n-value and the strain-rate hardening mvalue are the two most important metallurgical factors for the uniform and the post-uniform (necking) ductility control, respectively. Unfortunately, both n and m values of the U-6Nb alloy are lacking. The study shows that the total ductility of U-6Nb is dominated by the uniform ductility, which deteriorates in both 100 C andmore » 200 C aging. Further analysis shows that the uniform ductility correlates well with the work hardening n-value of the later stage deformation in which dislocation-slip is the mechanism. The kinetics of the loss of uniform ductility and the associated reduction in work-hardening n-value in low temperature aging will be used for the development of a ductility-based aging model. The necking ductility appears to be a minor but significant factor in the total ductility of U-6Nb. It does not show a clear trend due to large data scatter. The uncertain nature of necking failure may always hinder a reliable measurement of necking ductility. Consequently, a precise measurement of strain-rate hardening m-value could be a viable alternative to model the metallurgical contribution to the necking ductility. The conventional strain rate step-change method and the ABI (Automated-Ball-Indentation) test both show promising result in m-value measurement.« less