Point defect configurations in irradiated iron-carbon
alloys* ~nsider&ble exper~ent~l data has been abt~~ed relating to the migration of carbon in irradiated iron-carbon ~lloys.o-~) The model which was proposed to interpret the data was one in which the carbon atoms migrate above room temperature and are trapped by an immobile point defect, presumably the vacancy. At a higher temperature the carbon atoms are released from the traps and precipitate, and at a still higher temperature the point defect anneals. In the course of these studies a resistivity decay near -30Β°C was observed and reported but was not inveatiga~d in detail. It was suggested that this decay step could arise from the annealing of the iron interstitial, the iron interstitial trapped by a carbon atom or other impurity, or the divacancy. This low temperature region was also examined with internal friction following irradiation in an attempt to observe the strain associated with such defects, but with negative: results.c5) Further information related to this interpretation has been obtained in the course of a study by computer techniques of iron and of carbon in iron. These calculations yielded configurations of in~rstitial-carbon and vacancy-carbon complexes, which, combined with estimates of migration and binding energies, allow some estimates to be made of temperature of their formation and dissociation and of the expectation of internal friction arising from their reorientation (not from their ability to pin dislocations).
The point defect calculations were made using a high-speed computer to minimize the energy of an array of lattice atoms, a technique by now wellestablished.(e,7) Specifically, in the present computation, a lattice array of about 500 atoms WELS used for iron in a manner quite similar to that used and described by lJohnson and Brown(') for copper. The precise potential used for the iron-iron interaction will be described elsewhere.(8) The potential for the iron-car'bon interaction was constructed from a cubic equation with parameters chosen such that the measured migmtion energy of carbon in iron and the activation volume for such migration were given correctly by the calculation.(g) In addition, out of a, variety of iron-carbon potentials which matched these experimental results, the one selected also gave 0.41 eV for the binding energy of a carbon atom to a