THE EFFECT OF THE GRAIN-BOUNDARY MISORIENTATION ON THE KINETICS OF THE DISCONTINUOUS ORDERING REACTION IN FE-50 AT-PERCENT-CO

The kinetics of the discontinuous ordering reaction in the Fe-50 at,% Co alloy has been studied in the temperature range 280-400 degrees C. The misorientation parameters for 61 grain boundaries have been determ ined with the help of the electron backscattering diffraction techniqu e. It is shown that for low-angle grain boundaries with a misorientati on angle below 15 degrees and for grain boundaries which were identifi ed as close to the special Sigma 3[111], Sigma 9[011], Sigma 11[011] a nd Sigma 17[001] misorientations (here Sigma is the reciprocal density of the coincidence sites in the two misoriented lattices) the reactio n rate is very low. Contrarily, the GBs close to the special Sigma 319 a[011] and Sigma 19b[111] exhibit high reaction rate. The reaction rat es for the other large-angle grain boundaries vary considerably from b oundary to boundary. This is discussed in terms of the effect of the g rain boundary inclination on the observed reaction rate. The migration rate of large-angle random grain boundaries obeys the Arrhenius relat ionship: nu = 2 x 10(14) exp(-132 kJ.mol(-1)/RT) m/s, where R and T ar e the gas constant and the absolute temperature, respectively. A model connecting the grain boundary velocity with the relaxation time of th e order parameter in the grain boundary and its mobility is suggested. (C) 1998 Acta Metallurgica Inc.