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Spinodal ordering and precipitation in U-6wt%Nb

Published online by Cambridge University Press:  26 February 2011

Luke Lieh-Ming Hsiung  and
Jikou Zhou
Luke Lieh-Ming Hsiung
Affiliation:
hsiung1@llnl.gov, Lawrence Livermore National Laboratory, Chemistry and Materials Science, 7000 East Avenue, P.O. Box 808, L-352, Livermore, California, 94551, United States, 925-424-3125, 925-424-3815
Jikou Zhou
Affiliation:
zhou5@llnl.gov, Lawrence Livermore National Laboratory, Chemistry and Materials Science, United States
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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 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 α phase and Nb-enriched U3Nb 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 and precipitation hardening of WQ-U6Nb alloy.

Keywords

phase transformationcrystallographic structuretransmission electron microscopy (TEM)
Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 893: Symposium JJ – Actinides 2005 – Basic Science, Applications and Technology , 2005 , 0893-JJ04-08
Copyright
Copyright © Materials Research Society 2006

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References

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