Effects Of Mechanical Alloying On The Microstructural Evolution Of Y₂O₃ And Properties Of Oxide Dispersion Strengthened Steel

Oxide dispersion strengthened?ODS?steel is characterized by favorable void swelling resistance under neutron irradiation,high-temperature mechanical properties,and thermal stability,thus promising a bright prospect to be used in nuclear reactors and superalloy.However,the particle size and distribution of oxides cannot be controlled effectively when preparing ODS steel using the traditional powder metallurgy.Besides,powder metallurgy is a complex and costly process,has little production in a single batch,and is likely to introduce defects such as inclusions and pores in the production process.For this reason,the traditional powder metallurgy fails to be widely used to prepare structures and materials for use in nuclear reactors.The research prepared ODS alloy steel by using the mechanical alloying,in combination with the induction melting,thus providing an alternative to traditional powder metallurgy.At first,the Fe-10%Y₂O₃-5%Ti?wt.%?mixed powder was used to prepare alloy powder containing nano-sized Y₂Ti₂O₇matrix-strengthening particle phases by all milling and annealing.Meanwhile,the intermediate alloy was produced by sintering the intermediate alloy powder ball-milled at 1200°C for 30 h.Then,the intermediate alloy was added in the as-cast melts,to obtain the x%?x=3?5wt%?FA-ODS steel.Finally,the heat treatment of the ODS steel was further optimized by systematically studying influences of dispersion strengthening effects of Y₂O₃on the microstructures,mechanical properties,and corrosion resistance of the steel.The main contents and results are as follows:?1?The microstructural evolution of the Fe-10%Y₂O₃-5%Ti mixed powder experiencing mechanical alloying and annealing treatment was investigated.The experimental results showed that the powder particles underwent alloying transformation with the increase of the ball milling time.After being ball-milled for10-30 h,the Oswald ripening occurred,that is,small particles were dissolved while large ones grew constantly.After ball milling the powder for 30 h,the X-ray diffraction?XRD?patterns were found to display disappearance of the Y₂O₃peak and amorphization of the Fe peak,and a tiny amount of the Y₂Ti₂O₇phase was produced.During the mechanical ball milling,the alloying elements were subjected to solid solution in Fe lattices,leading to amorphization of the Fe peak.Then,the amorphous Fe phase was crystallized after annealing treatment of the alloy powder at 736.3°C.As the temperature of heat treatment rose,little amounts of Y and O elements dissolved in Fe lattices were precipitated,followed by precipitation of Y₂O₃phase as the temperature reached 1000°C.When the sintering temperature reached 1200°C,large amounts of Y₂O₃and Ti O₂phases were subjected to phase transformation,forming the Y₂Ti₂O₇phase.?2?The ODS steel was prepared by virtue of the intermediate alloy process.Moreover,the effects of the Y₂Ti₂O₇particles and the high-temperature and high-pressure treatment on the microstructures,mechanical properties,and corrosion resistance of the OSD steel were explored.The research results revealed that3%FA-ODS steel showed superior mechanical properties:the ultimate tensile strength?UTS?and the yield strength?YS?were 950.78 MPa and 678.12 MPa,respectively,and the elongation?TE?was 9.07%;its strength was significantly higher than that of the control,mainly attributed to the addition of the intermediate alloy that contained nano-sized dispersion strengthening phases.Microstructure analysis indicated that the dispersed phases in the matrix mainly appeared as face-centered cubic Y₂Ti₂O₇particles with the average size of 2.1 nm;there were a little amount of Y₂O₃particles,which had an average size of 5.3 nm and were uniformly distributed in the matrix,showing a superior dispersion strengthening effect.Therefore,addition of an appropriate amount of the intermediate alloy was able to effectively improve the corrosion resistance of the ODS steel,and the 3%FA-ODS steel was found to have the best corrosion resistance.Afterwards,the 3%FA-ODS steel was subjected to high-temperature and high-pressure treatment using a cubic press,which inhibited growth of the grain size,facilitated grain refinement,and enlarged areas of grain boundaries.Whereas,the particle size of the Y₂Ti₂O₇strengthening phase remained in the range of 1.3-2.5 nm,so that the added nano-sized particles could exert a better pinning effect at the grain boundaries,which favored the dispersion strengthening effect.As a result,the YS of the 3%FA-ODS increased,so did the plasticity.