Microstructural evaluation and crystallographic texture modification of heat-treated zirconium Excel pressure tube material

Abstract

Dual phase (α − β) Zr-Excel (Zr-3.5 wt% Sn-0.8 wt% Mo-0.8 wt% Nb) Pressure Tube (PT) material was solution treated at a range of temperatures in the (α + β)Zr or βZr-phase field, and cooled in water or air to generate various microstructures. TEM and neutron diffraction revealed significant alterations during the heat-treatments in both the microstructure and the initial strong transverse texture of the As-Received (ASR) PT material. The proportion of transformed β-phase (β → α) or martensitic/acicular α-phase increased progressively with increasing solution temperature. The morphologies of the transformed β products are different in the water-quenched material (martensitic plates) than in the air-cooled material (relatively wide, parallel plates). The room temperature texture of the heat-treated material shows that: the extent to which the initially strong transverse texture is modified (basal poles shifting towards the axial direction) increases with increasing solution temperature, yielding a relatively random texture at higher solution treatment temperature (quenching from ∼930 °C). This texture modification through heat-treatment is more effective by water-quenching than by air-cooling. Although, a randomized texture is developed in the α → β → α phase transformation, variant selection is observed. It is suggested that this selection occurs primarily during cooling (β → α), whereas no (or very weak) variant selection occurs during heating (α → β), as indicated by the identical room temperature β-phase (remnant) textures in the heat-treated and ASR materials.