Phase transformations in neutron-irradiated Zircaloys Page: 4 of 38
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characteristics of vacancy and interstitial loops in several neutron-
irradiated Zr alloys, Jostsons et al.^ concluded that an association of
nonmetallic elements, in particular oxygen, with vacancies is very likely.
Lee and Koch examined the diffraction characteristics of the aligned defect
structures in Zircaloy-2 irradiated with Ni ions and suggested that the
aligned structure may consist of precipitates of solute atoms or a second
phase. However, no conclusive evidence was given. Small precipitates ^10 nm
in size were observed in a recent TEM-HVEM microstructural investigation of
Zircaloy-2 and -4 spent-fuel cladding irradiated in commercial power
reactors. The precipitates had the same structure and lattice parameters as
the a-Zr matrix and frequently gave rise to double diffraction. Since
radiation-induced defects would not produce double diffraction, the
precipitates were deduced to be Zr^O; a structure identical to that of an a-Zr
matrix except for oxygen ordering in the unit cell. Superlattice reflections
associated with the oxygen ordering were also reported.
It appears, however, that detection of bulk precipitates of ^10 nm in
size in irradiated Zr alloys has been greatly complicated by the presence of
artifact surface hydride and oxide particles^which are formed during
thin-foil preparation or in the microscope. Because of the ever-present
artifact surface phases, nonmatrix reflections that are frequently detected in
diffraction patterns appear to have been categorically assumed by most
investigators to be "spurious" reflections originating from the artifact
phases. Apparently, the validity of this assumption for irradiated Zr alloys
has not been critically examined. However, in view of the above-mentioned
background, it was considered, necessary to reexamine the origin of the
nonmatrix reflections with a specific purpose of determining whether a
radiation-induced phase transformation is involved in the evolution of the
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Chung, H.M. Phase transformations in neutron-irradiated Zircaloys, article, April 1, 1986; United States. (https://digital.library.unt.edu/ark:/67531/metadc1094084/m1/4/: accessed March 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.