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

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 delta-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 delta-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 Widmansta spacing diaeresis tten after 1273 K anneal while corresponding modulus, hardness and yield strength elevated to a comparable level with delta-hydrides due to the increase of oxygen content.

Automated summary

The microstructure evolution and mechanical properties of hydrogenated Zircaloy-4 tubes during the thermal decomposition process of precipitated hydrides were investigated. Nanoindentation tests were used to determine the mechanical properties in different states, showing that the modulus, hardness, and yield strength were the highest for the delta-hydrides and increased at 1273K.