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The flow properties of Zr-2.5Nb were determined in the (α + β) range at 998 and 1148 K, which corresponds to about 15 and 85 volume % β. The flow stresses were determined in axisymmetric compression at constant true strain rates ranging from 3.1 × 10−5 s−1 to 3.1 × 10−2 s−1.
The flow curves were characterized by a peak stress followed by considerable flow softening and, for strain rates less than 10−3 s−1, the flow stress for the large β-fraction materials dropped by only 25 to 30% of the peak stress as compared with 50 to 70% for the predominantly α-materials. The amount of flow softening also decreased with increasing coarseness of the prior α-plate morphology.
The α-plates rotated from an initially random to a distinctly transverse orientation, and at strain rates less than 10−3 s−1, the α-plate structure broke up to produce an equiaxed grain morphology. The strain rate sensitivity concurrently increased from 0.22 to 0.4.
Significant anneal hardening was observed in the predominantly or fully α-region after a β-heat treatment, the amount of which increased with time of holding in the β-region. The activation energy was approximately 140 kJ/mole, which is similar to that for the diffusion of niobium in β-zirconium. The anneal hardening is ascribed to the formation of niobium-rich clusters in the β-phase. These clusters seem to be retained in the α-phase on cooling. Part of the flow softening in the anneal-hardened material can be ascribed to declustering by deformation.
flow softening, anneal hardening, declustering, plate rotation, flow stress, substructure, proeutectoid, habit plane, rate sensitivity
Technical University of Nova Scotia, Halifax, N.S.
Combustion Engineering Superheater Ltd., Moncton, N.B.
McGill University, Montreal, P.Q.