Low-Temperature Aging and Phase Stabiliy of U6Nb
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LOW-TEMPERATURE AGING AND PHASE STABILIY OF U6Nb L. M. Hsiung1, C. L. Briant2, and K. R. Chasse2 1 Lawrence Livermore National Laboratory, Chemistry and Materials Science Directorate, P.O. Box 808, L-352, Livermore, CA 94551-9900. 2Brown University, Division of Engineering, Providence, RI 02912. ABSTRACT Aging behavior and phase stability of a water-quenched U-6wt%Nb (U-14at%Nb) alloy artificially aged at 200°C and naturally aged at ambient temperature for 15 years have been investigated using Vickers hardness test, X-ray diffraction analysis, and transmission electron microscopy techniques. Age hardening/softening phenomenon is observed from the artificially aged samples according to microhardness measurement. The age hardening can be rationalized by the occurrence of spinodal decomposition, or fine scale of Nb segregation, which results in the formation of a nano-scale modulated structure within the artificially aged samples. Coarsening of the modulated structure after prolonged aging leads to the age softening. The occurrence of chemical ordering (disorder-order transformation) is found in the naturally aged sample based upon the observations of antiphase domain boundaries (APB′s) and superlattice diffraction patterns. A possible superlattice structure is accordingly proposed for the chemically ordered phase observed in the naturally aged alloy sample. INTRODUCTION It is well known that U-6wt%Nb (U6Nb hereafter) alloy is exploited for a variety of engineering applications with a microstructure containing martensitic phases supersaturated with Nb, which can be obtained by rapid quenching the alloy from γ (bcc)-field solid solution to room temperature [1, 2]. The high cooling rate forces the γ-phase solid solution to transform martensitically to a variant of the low temperature α (orthorhombic) phase in which Nb is retained in supersaturated solid solution. Because the variant phase is supersaturated and its lattice parameters differ from the equilibrium α phase, it has been designated α′ martensite. Two additional variant phases, a monoclinic distortion of α′, named α′′ martensite or a tetragonal distortion of γ, named γo martensite, can also be formed within the as-quenched material. The resulting alloy has improved mechanical properties (ductility and toughness) and excellent corrosion resistance because of the uniform distribution of Nb in solid solution that suppresses the diffusional decomposition reaction to form two-phase (an α phase and a Nb-enriched γ phase) cellular microstructures, which are undesirable for engineering applications. It has been reported that the water-quenched (WQ) U6Nb alloy containing extensively twinned α′′ martensitic microstructure results in low yield strength (∼200 MPa) and high tensile ductility (∼30%) [2]. It has also been demonstrated that the U-Nb alloy containing α′′ martensite reveals sound shape memory effect [3], and the shape memory behavior is controlled mainly by the twin structures within the alloy [4]. Aging of the α′′ martensite in the 150°C to 400°C range
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