Low temperature age hardening in U-13 at.% Nb: An assessment of chemical redistribution mechanisms |
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Authors: | A.J. Clarke R.E. Hackenberg D.J. Thoma D.F. Teter K.F. Russell G. Beverini |
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Affiliation: | a Materials Science and Technology Division, Mail Stop G770, Los Alamos National Laboratory, Los Alamos, NM 87545, USA b Materials Science and Technology Division, Microscopy, Mail Stop 6136, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA c School of Process, Environmental and Materials Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom |
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Abstract: | Low temperature aging (<350 °C) of U-13 at.% Nb martensite results in increased strength levels accompanied by significant ductility loss. To determine the decomposition mechanism(s) responsible for these mechanical property changes, atom probe tomography was used to examine the niobium and impurity distributions after aging at 200 or 300 °C for times ranging from 2 h to 70 days. No patterns of niobium or impurity atoms were observed that would indicate segregation to the martensitic twin interfaces, making this hardening mechanism unlikely. Phase separation into roughly equiaxed regions of high and low niobium concentration was clearly observed after aging at 300 °C for 70 days. However, only subtle niobium concentration changes were observed after aging at 200 °C relative to the as-quenched condition, indicating that conventional phase separation is an unlikely explanation for the dramatic mechanical property changes at 200 °C. Therefore, consideration of aging mechanisms other than segregation and phase separation may be warranted. |
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