Technical University of Nova Scotia, Halifax, N.S.
Combustion Engineering Superheater Ltd., Moncton, N.B.
McGill University, Montreal, P.Q.
Pages: 24 Published: Jan 1984
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
Paper ID: STP34470S