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.