A symmetry-violating precipitation process induced under irradiation

TiC precipitates in austenitic steel have been found to reveal a very unusual precipitation behaviour /l,2/. As a consequence, nucleation initiated by dislocation recovery or growth induced during recrystallization can be used to create predesired modifications of the microstructure and the mechanical properties /2—3/. The extraordinary high resistance to precipitate growth and coarsening /l/ appears to be the key property in these applications and also appears to be the origin for a unique precipitate phenomenon which was observed in the present irradiation experiment.A 1.4970 type steel (15Ni—15Cr austenite, containing Ti and C) was neutron irradiated at 845 K to a dose of 2.6 x 1025 m-2 thermal neutrons (helium production) and 3.0 x 1025 fast neutrons (1.5dpa). Before irradiation the steel was solution annealed leaving about half of the Ti and C in supersaturated solution. Defect cascades introduced by the neutron irradiation acted as nucleation centres for MC (M=Ti, Nb, Mo...) precipitates as has already previously been demonstrated in Ti- and Nb-stabilized steels /4/. In addition to the common, irregularly shaped MC precipitates, however, two further precipitate variants were created during the present high temperature neutron irradiation. One precipitate variant was star—shaped with cube—on—cube orientation relationship with the matrix. The other variant was nail—shaped. Analysis of crystal structure and orientation relationship of the nail—shaped precipitates was made difficult by their small shape and by double diffraction effects. Fig. la shows a (001) SAD pattern of matrix (heavy spots) and precipitates (fine spots). The quadruplets in the vicinity of <110> are created from the new precipitates. Each of the four quadruplet spots actually consists of three closely spaced spots (Fig. lb), and the eight precipitate spots surrounding each of the matrix spots in Fig. la each consist of two spots. After the double diffraction effects had been assigned (schematic in Fig. lb for example) they were reduced by specified specimen tilting around two axes (Fig. lc). It was found by combined dark- field and diffraction analyses that all diffraction spots originated from TiC precipitates and that four orientation variants of the TiC precipitates were distinguishable. They are imaged in Fig. 1d in bright field and in Fig. 2 in four corresponding dark field images by using reflections 0 to 3 of Fig. lc. 0 shows precipitates of the common cube—on— cube orientation relationship. 1 to 3 shows nail—shaped precipitates with nail axes pointing along the three <110> directions. The diffraction analysis revealed that the lattice of the nail precipitates is rotated by 45 degrees around the nail axes from cube-on-cube orientation, thus giving rise to the three orientation variants.