Low Temperature Ageing Behaviour of U-Nb $\gamma^{o}$ Phase Alloys

TL;DR

This study investigates the microstructural and mechanical changes in U-7%wtNb alloy subjected to long-term ageing at 150°C, revealing a stress-induced martensitic transformation that causes thermal hardening.

Contribution

It provides detailed insights into the ageing mechanisms of U-Nb gamma phase alloys, highlighting a stress-induced martensitic transformation as the key process.

Findings

Ageing causes microstructural changes detectable by advanced microscopy.

Thermal hardening correlates with ageing time and temperature.

Transformation from gamma to alpha'' phase occurs without chemical redistribution.

Abstract

Ageing mechanisms of the U-7\%wtNb alloy have been investigated on samples exposed to temperatures of 150$^{o}$C for up to 5000\,hours. A variety of surface and bulk analytic techniques have been used to investigate microstructural, chemical and crystallographic changes. Characterisation of the alloy's evolving behaviour was carried out through secondary electron microscopy, energy dispersive x-ray spectroscopy, electron backscatter diffraction, transmission electron microscopy and x-ray diffraction. Vickers hardness testing showed evidence of a strong thermal hardening relationship with ageing. The mechanism responsible for these changes is thought to be a stress-induced isothermal martensitic transformation; a displacive reaction, in which correlated shuffling of atoms creates a phase change from $\gamma^{o}$ to $\alpha''$ without chemical species redistribution.