Microstructure and mechanical properties evolution of hydrogenated Zircaloy-4 tube during thermal decomposition process of precipitated δ-hydride

Abstract

Microstructure evolution and mechanical properties, i.e., Young's modulus, hardness and yield strength variations were investigated for hydrogenated Zircaloy-4 tubes during thermal decomposition process of precipitated hydrides. Firstly, the embedded δ-hydrides were synthesized through hydrochemical method in autoclave. Then the dehydrogenation temperature and corresponding activation energy were detected via DSC as 832–869 K and 185.4 KJ/mol when heated up to 1273 K which simulated the loss of coolant accident (LOCA) temperature. Finally, nanoindentation technique was performed on samples in as-received state, hydrides, surrounding matrix, post-hydriding samples after 869 and 1273 K anneal to determine the mechanical properties. The Young's modulus and hardness calculation were carried out by Oliver-Pharr theory and corrections were implemented by P-U model meanwhile the yield strength was figured out using empirical equations. The results showed that the modulus, hardness and yield strength of δ-hydrides were the largest of all. Mechanical properties of as-received material were highly similar to the surrounding matrix. Different from 869 K anneal, the microstructure changed from equiaxed grain to Basket-weave Widmanstätten after 1273 K anneal while corresponding modulus, hardness and yield strength elevated to a comparable level with δ-hydrides due to the increase of oxygen content.