In situ magnetic measurements under neutron radiation in Fe metal and low carbon steel

Magnetic minor hysteresis loops of low carbon steel and Fe metal have been measured during neutron radiation at 563 K in a 50 MW nuclear reactor. For investigation of nucleation mechanism of copper precipitates and dislocation loops during neutron radiation, special attention was paid to minor-loop coefficients, which are deduced from simple relations between minor-loop parameters and are very sensitive to lattice defects such as dislocations, copper precipitates, and grain boundaries. We found that with increasing neutron fluence, the minor-loop coefficients of low carbon steel sharply increase and show a maximum at the fluence of 1x10(19) cm(-2), followed by a slow decrease. The appearance of the maximum suggests the presence of two mechanisms of internal stress; while copper precipitates and dislocation loops in the matrix make the internal stress increase, those grown in the vicinity of dislocations compensate the internal stress of dislocations so as to minimize the elastic energy. On the other hand, the minor-loop coefficients show a monotonic increase with neutron fluence in Fe metal where dislocation loops play an important role for the brittleness.(c) 2006 American Institute of Physics.