Deformation Characteristics Of The SPPs In Nuclear Grade Zirconium Alloys

Second phase particles（SPPs）,which were roughly believed to be refined gradually along with the matrix during various kinds of deformation processes,can greatly affect the microstructure,mechanical properties,corrosion and oxidation resistance of zirconium alloys.Hence,the deformation and fracture characteristics of SPPs associated with their size refinement mechanism,which are of great significance to establish the relationship between thermo-mechanical treatment and microstructures,needs to be explored.In this thesis,firstly,the substructures of Zr₂Si and 2r2Fe nanoprecipitates in the solidified and solution-treated Zr-4-0.4wt.%Si alloy and Zr-4 alloy,were separately determined by combined uses of EBSD（Backscatter Electron Diffraction）,TKD（Transmission Kikuchi Diffraction）and TEM（Transmission Electron Microscopy）techniques.And the crystallography of core-shell Ge-containing precipitation in the solution treated Zr-4-0.4wt.%Ge alloy has also been analyzed.Then,based on the deformation characteristics of the α-Zr matrix,the deformation of the SPPs in the zirconium alloy subjected to compression tests at room temperature,600℃,800℃,and 1000℃ were studied successively.Among them,interactions between different precipitates and the effect of these interaction on the deformation of SPPs were the most interesting research.Finally,the deformation and fracture behavior regularity of these SPPs was summarized,and their deformation mechanisms were proposed.In addition,some theoretical models were extracted to describe the size refinement mechanisms of these SPPs.The most common precipitates observed in solidified Zr-4-0.4wt.%Si alloy and Zr-4 alloy were body-centered tetragonal（BCT）structured Zr₂Si phase and Zr₂Fe phase,respectively.Both of the above two precipitation phases contained similar 60°rotation twins with their axes along the[110]direction,the twinning plane and direction were determined to be {112} and[332],respectively.U-shaped stacking faults（SFs）were observed,for the first time,within the Zr₂Si phase precipitated in the solution-treated Zr-4-0.4wt.%Si alloy.These SFs consists of one arc segment and two straight segment stacking faults,in which the formation mechanism of the arc segment one can be ascribed to the alternate occurence and connections of two non-coplanar stacking faults on（100）and（010）planes.In addition,unique precipitations with core-shell structure were observed in the solution-treated Zr-4-0.4wt.%Ge alloy,the core was identified as BCT structured 2r3Ge nanoprecipitate,while the shell was determined to be Zr（Fe,Cr）₂ Laves phase with Hexagonal-Closed packed（HCP）structure.A large number of slip traces and lamellar FCC-Zr precipitates were observed in the Zr-4 alloy that subjected to micro Vickers indentation tests,revealing dislocation slip and stress-induced HCP→FCC phase transformation were the predominant plastic deformation mechanism of the α-Zr matrix.It was found that most of the FCC-Zr precipitates in the Zr-4 alloy was B-type FCC-Zr phase,which obeyed the[2110]_α-Zr‖[110]_FCC-Zr,（0001）_α‖（111）_FCC-Zr crystallographic orientation relationships with the α-Zr matrix.The strength of SPPs at room temperature was much higher than that of the α-Zr matrix,therefore,the dislocations in matrix generally pile-up at precipitate interfaces or just bypass the nanoprecipitates.The Zr₂Fe SPPs were generally surrounded by a layer of {0001}α-Zr stacking faults transition zone,and FCC-Zr precipitates were also observed in some part of the transition zone.At 600℃,the interactions between the SPPs and dislocations in the matrix were intensified.The growth of B-type FCC-Zr phase along its major axis would exert mechanical stress on the nearby Zr₃Ge phase in close contact.The synchroshear process on the {110} planes of Zr₃Ge phase would be activated when the magnitude and orientation of applied stress reached the thershold conditions,thereby produced stacking faults in the SPPs.The core-shell structured precipitate were deformed under the external forces exerted by the nearby α-Zr grains and FCC-Zr phases,in which the C 14-type Zr（Fe,Cr）₂ phase deformed via shear-induced stacking faults while the Zr₃Ge phase exhibited brittle fracture.In addition,the nearby C15 Zr（Fe,Cr）₂ phase was also brittle fractured under the pressure of the core-shell precipitate.At 800℃,although the C15-type Zr（Fe,Cr）₂ SPPs still exhibited obvious brittle fracture behaviors e.g.shear fracture and bending fracture,micro-scaled plastic deformation characteristics including local shear on {112} planes and shear-induced faults on{110} planes,were observed in some local areas of them.At 1000℃,the deformation characteristics of SPPs were significant and simple,most of the Fe-Cr containing precipitates were significantly refined with their average sizes less than 150 nm.And the size refinement effect of Fe-Cr containing precipitates in the Zr-4 alloy was better than that in the Zr-4-0.4wt.%Ge alloy.The Zr₃Ge phases has not been refined,but their shapes have changed to some extent.