Spinodal Decomposition and Order-Disorder Transformation in a Water-Quenched U-6wt%Nb Alloy Page: 3 of 30
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SPINODAL DECOMPOSITION AND ORDER-DISORDER TRANSFORMATION
IN A WATER-QUENCHED U-6wt%Nb ALLOY
Luke Hsiung and Jikou Zhou
Materials Science and Technology Division, L-352
Objectives and accomplishments
A combinative approach of microhardness testing, tensile testing, and TEM microstructural analysis has
been employed to study phase stability and aging mechanisms of a water-quenched U-6wtNb (WQ-
U6Nb) alloy subjected to different aging schedules that include artificial aging of WQ-U6Nb at 200 C,
natural aging of WQ-U6Nb at ambient temperatures for 15 tol8 years, and accelerative aging of the
naturally aged (NA) alloy at 200 C. During the early stages of artificial aging at 200 C, the
microhardness values continuously increase as a result of the development of a fine-scale compositional
modulation (wavelength: 3 nm) caused by spinodal decomposition. Coarsening of the modulated
structure occurs after prolonged aging of WQ-U6Nb at 200 C for 16 hours, which leads to a decrease of
microhardness. Phase instability has also been found to occur in the NA alloy, in which the formation of
partially ordered phase domains resulting from an atomic-scale spinodal modulation (wavelength: 0.5
nm) renders the appearance of antiphase domain boundaries (APBs) in TEM images. Although 18-year
natural aging does not cause a significant change in hardness, it affects dramatically the aging
mechanism of WQ-U6Nb subjected to the accelerative aging at 200 C. The result of microhardness
measurement shows that the hardness values continuously increase until after aging for 239 hours, and
the total hardness increment is twice in magnitude than that in the case of the artificial aging of water-
quenched alloy at 200 C. The anomalous increment of hardness for the accelerative aging of NA alloy
can be attributed to the precipitation of an ordered U3Nb phase. It is accordingly concluded that the
long-term natural aging at ambient temperatures can detour the transformation pathway of WQ U-6Nb
alloy; it leads to the order-disorder transformation and precipitation of ordered phase in the alloy.
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Hsiung, L & Zhou, J. Spinodal Decomposition and Order-Disorder Transformation in a Water-Quenched U-6wt%Nb Alloy, report, September 12, 2006; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc881270/m1/3/: accessed February 20, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.