FCC-phase In Nuclear Grade Pure Zirconium

A unique FCC structure has been revealed in Ti,Zr,Hf and their alloys with HCP structure.The formation mechanism of the FCC(labeled as ?)phase is still inconclusive.The ? phase will affect the service performance of zirconium and its alloys as the cladding materials in nuclear reactor.Therefore,it is urgent to carry out relevant research work.In this thesis,firstly,according to the reported results of ? phase,the crystallographic relationship between ? and ? phases in three-dimensional space was reconstructed.Secondly,the crystallographic characteristics of ? phase in solidified,solid solution and annealed states were studied by means of EBSD,TEM and TKD techniques and the corresponding mechanism models were proposed.Furthermore,the crystallographic characteristics of ? phase and ? matrix under the condition of variable morphology are studied,and the mechanism of stress induced ? phase is proposed Finally,an overall consideration of the crystallographic characteristics of ? phase in different states was built.In addition,the evolution of ? phase from solidification to deformation was concluded.According to the reported results of crystallographic orientation relationship between ? phase and a matrix,composite stereographic projection,composite electron diffraction patterns and the number(12)and distribution of theoretical variants of ?phase were simulated based on the path of ??? phase transition.Similarly,composite stereographic projection of ? and ? phases and the number(24)of theoretical variants of ? phase were simulated with the path of ??? phase transition.The area fraction of ? phase in the solidified,solid solution,annealed and deformed samples was about 7.50%,6.68%,1.50%and 11.7%,respectively.In all states,the major axes of the same type ? phase in a single ? grain were parallel to each other.The end of B-type ? phases exhibited zig-zag morphologis and crystal structure near the zig-zag interface was a 12R long period stacking ordered structure with a stacking sequence of...ABCACABCBCAB....The Whole period spacing is ahout 3.324nm.In a matrix,Shockley partial dislocations with Burgers vector of 1/3<0110>glide on every four {0001} basal planes,resulting in the 12R structure.In addition,an intermediate state with stacking sequence…AAA…was detected.If the intermediate state undergoes further partial dislocation slip with Burgers vector of 1/6<0110>,making born of the ? phase.The sum Burgers vector during the whole process(??intermediate state??)was 1/3<0110>.The intermediate state also provides direct evidence for B-type ? phase via partial dislocation.The formation mechanism of B-type ? was proposed by combining the results of solidified and solid solution states.Firstly,the lattice expands from 2.575 A to 2.92 A along the[0001]direction.Then Shockley partial dislocations with Burgers vector of 1/3[0110]glide on every other {0001} basal planes,which makes the ??? phase transition.In a single ? grain,the major axes of B-type and P-type ? phases are perpendicular to each other.If B-type and P-type ? phases meet,two B/P interfaces will induced,one of which is parallel to(331)B plane,along[116]B direction and the otheris parallel to(116)B plane,along[331]B direction.In the annealed samples,the crystallographic orientation relationship between P-type ? phase and a matrix was reveal along[110]? ?[2110]? direction and a new mechanism of the P-type ? phase was proposed.Firstly,? matrix expands along[0110]? direction and then atoms move further undergo a shuffle displacement along[0001]? direction.When the shuffle displacement reaches a critical value,a column of atoms(misfit dislocation)will be induced on the side of ? phase to coordinate the shuffle process.In addition,a rare new orientation relationship between ? and a matrix has been exposed.That is:<2110>??<100>?,{0001}??{002}?.The formation mechanism of this kind of ? phase involves lattice expansion and slip of partial dislocation.In the bending deformed samples,stress-induced {1012}<1011>and{1021}<1126>twins can be observed.It is found that there is a local strain region in a matrix at the end of ? phase by TKD technique and this result may accelerate the corrosion of zirconium and its alloys.IMS between B-type and P-type ? phases was studied.The modulation wave vector is q=-0.129b*+0.318c*.The IMS was caused by the strain fluctuation during ??? phase transition.The direct evidence of stress-induced ? phase was proved by Vickers indentation method.In addition,three-dimensional morphology of the ? phase is lamella.