Allotropic FCC Phase In Nuclear Grade Hafnium

As an important nuclear material,nuclear-grade hafnium(Hf)plays an important role in regulating the reactivity of the core operation.In the past decade,stress-induced the allotropic face-centered cubic(FCC)phase has been discovered in Hf.The emergence of FCC-Hf brought new challenges to the application of Hf in nuclear reactors.Therefore,it is an urgent task to explore the crystallographic aspects HCP to FCC phase transformation in Hf.This thesis is divided into four parts:first,the type,size,content and morphology of FCC phase in Hf were analyzed;second,the source of dislocations in FCC phase was studied and the related mechanism was explored;third,the spatial relationship of different types of FCC phases was considered;fourth,the crystallographic characteristics of the modulated structure in Hf and its electronic structure were explored.Theoretically,there could be six types of orientation relationships between FCC phase and HCP matrix in Hf,and the categories of FCC phase were redefined and symbolized as Bα,Bβ,Bγ,Pα,Pβ and Pγ types.Among them,Bβ and Bγ-types FCC phases were undiscovered types and their orientation relationship with matrix could be expressed as:[1120]HCP//[110]FCC and(0001)HCP//(110)FCC,[1121]HCP//[100]FCC and(0001)HCP//(010)FCC.In cold-rolled Hf,the total area fraction of FCC phase was～11.37%and that of Bα,Pα and Pβ FCC phases were～10.08%,～0.24%and～0.05%,respectively.The formation of Bα-type FCC phase was related to the gliding of the partial dislocations on the(0001)planes.The morphology profiles of the FCC phase could be divided into three types:type-Ⅰ,type-Ⅱ and type-Ⅲ;and the partial dislocations forming the FCC phases with these three profiles originated from the decomposition of basal plane<a>dislocations within the grains.The FCC phase nucleated at the E interface(edge)and the BP(basal plane)and PB(prismatic plane)interfaces of {1102} twins.Under appropriate stress conditions,on the one hand,the partial dislocations at the sharp corner of FCC phase would continue to slip on(0001)plane;on the other hand,the<a>type dislocation at the sharp corner would decompose thereby providing partial dislocation sources for the nucleation of FCC phase.This caused the FCC phase grew along its longitudinal axis and thickened along its transverse axis.Similarly,under suitable stress conditions,the basal<a>type dislocations at the BP and PB interfaces of the {1102}twins decomposed into partial dislocations.If the partials slipped separately along the(0001)plane of the matrix and the twin,the FCC phase was formed at the BP and PB interfaces of the twin.In case of concurrent slip,two FCC phases with a longitudinal axis angle of 85° were simultaneously formed at the BP and PB interfaces of the twins.The spatial relationship of the FCC phases in Hf could be classified into three categories:the contact interface of the Bα-type FCC phases,90° crossover between Bα and Pβ-type FCC phases,and the 120° crossing between the Pα-type variants.The contact interface between Bα-type FCC phases could further be divided into three types,viz.SS(side-side),EE(edge-edge)and EESS(side-side,edge-edge)interfaces.Among them,9R and 12R’long-period structures were observed at the EE and EESS interfaces.Formation of these long-period structures was related to the gliding of dislocation on the close-packed planes of the FCC phase.For the 90°cross-interfaces of the Bα and Pβ-type FCC phases,there could be divided into two categories:the zigzag interface composed of(110),(111)and(111)planes and a V-shaped interface with an included angle of 157.50 consisting of the(113)and(110)interfaces.Two Pα-type FCC phases with an included angle of 120°in the longitudinal axis were formed in the matrix after the atoms undergo sheared along the[1210]and[1120]directions,respectively.Under the action of applied stress,changes in α-Hf lattice occurred due to the shift of atoms among different layers which made the Fermi surface tangent to the Brillouin zone.This split the Brillouin zone and consequently reduced energy of the system thereby stabilizing the modulated structure.