Long-term phase instability in a water-quenched uranium alloy
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The U–6 wt% Nb (U6Nb) alloy in the 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 paper, the water quenched U–6 wt% Nb (WQ-U6Nb) alloy in service for 15 years at ambient temperatures was studied using an analytical transmission electron microscopy (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 discussed 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.
I. INTRODUCTION
Uranium alloys have a variety of engineering applications that require a combination of high strength, good ductility, and corrosion resistance.1–6 The martensitic phase transformation and shape memory effects exhibited by U–Nb alloys have also gained increasing interests.7–9 To obtain desired physical and mechanical properties, the alloys are generally subject to various heat treatments, including annealing, solution treatment, water quenching, and artificial aging at elevated temperatures.10–13 Water quenching is one of the most important heat treatments. A water quenched U–6 wt% Nb (WQ-U6Nb) from ␥ body-centered cubic (bcc)-field solid solution has a microstructure containing martensitic phases that are supersaturated with Nb.6,12 In the stressinduced martensitic transformation, a variant of the lowtemperature ␣ (orthorhombic) phase is usually formed. This phase is designated ␣⬘ martensite since its lattice parameters differ from the equilibrium ␣ phase. Two additional variant phases, a monoclinic distortion of ␣⬘, named ␣⬙ martensite, or a tetragonal distortion of ␥, named ␥o martensite, may also be formed within the water-quenched alloy. All these variant phases are Nb
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0101 904
supersaturated. The uniformly distributed Nb in solid solution suppresses the diffusional decomposition reaction, resulting in improved mechanical properties (ductility and toughness) and excellent corrosion resistance.12–14 These supersaturated, unstable phases are prone to decompose. Numerous studies have been carried out to investigate aging behavior of the alloy at different temperatures.12–14 These studies provide important insights to understand the effects of aging at elevated temperatures on the mechanical properties. However, what is still missing is the aging behavior of the alloy at a
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