Effects Of Hf Addition On The Thermal Stability Of FeCrAl ODS Steels

Oxide dispersion strengthened?ODS?steels have been considered as hopeful candidates for cladding materials in the next generation nuclear reactors due to their superior high-temperature mechanical properties and good resistance to coolants corrosion and neutron irradiation swelling.In the FeCrAl ODS steels,due to the addition of Al element,formation of the coarser Y-Al-O oxides decreases the mechanical property of the ODS steel.To improve the thermal stability of the FeCrAl ODS steels during high temperature service,we study the effect of Hf addition on the thermal stability of the microstructure and tensile properties of the aged 16Cr-ODS steels in this paper.In this paper,the effects of hafnium?Hf?addition on the thermal stability of the microstructure and tensile properties of Al-containing 16Cr-ODS steels were investigated.High temperature aging tests of the ODS steels with and without Hf were conducted at 750°C,950°C and 1150°C for 100 h respectively.And the microstructure evolution after thermal aging was investigated using transmission electron microscopy?TEM?and electron backscatter diffraction?EBSD?.The results indicated that Hf addition could improve the coarsening resistance of the oxide particles and the grains during aging treatment due to the formation of stable Y₂Hf₂O₇particles.Tensile tests on the aged-steels showed that the ultimate tensile strength and yield strength of 16Cr-Hf-ODS steel were higher than those of 16Cr-ODS steel.It was considered that the excellent thermal stability and tensile properties of 16Cr-Hf-ODS steel were related with the refined microstructure.It is found that the superior thermal stability of the oxides in 16Cr-Hf-ODS benefits from the formation of Y₂Hf₂O₇ particles.Because of the better coherent relationship between Y₂Hf₂O₇ and the matrix,there is a smaller lattice misfit and less misfit dislocations at the Y₂Hf₂O₇/matrix interface,which can lower the interfacial energy and enhance bonding force thus resulting in a better stability of Y₂Hf₂O₇ oxides relative to Y-Al-O oxides.During the aging treatment at high temperature,stable Y₂Hf₂O₇ oxides with smaller sizes can produce stronger pinning effect on the migration of grain boundaries,thus enhancing the thermal stability of the grains in16Cr-Hf-ODS steel.Hf addition can improve the stability of both oxides and grains,so that 16Cr-Hf-ODS steel exhibits better tensile properties than 16Cr-Hf-ODS steel.