The grain boundary segregation of boron during isothermal holding

Abstract

Boron segregation to grain boundaries was investigated by means of particle tracking autoradiography (PTA) in a low carbon and two Mo-bearing steels and in an Fe-30% Ni alloy. Non-equilibrium segregation took place after rapid cooling to a degree which increased as the temperature difference between the austenitization and subsequent holding temperatures was increased. The amount of deformation during isothermal holding had a similar effect. The maximum segregation produced in this way can be many times higher than that associated with equilibrium segregation. The observations support the view that boron atoms are transported to the boundaries by forming complexes with vacancies which migrate to the boundaries prior to annihilation. During holding at different temperatures, the boron intensity curves produced by the PTA method exhibit three types of time dependence. Two of these involve the appearance of a segregation peak, after which there is either complete or partial disappearance of the segregation. The former is associated with the back diffusion of boron into the depleted zone, a process that takes place at the highest holding temperatures and in the absence of precipitation. The latter involves the conversion of temporary segregation into grain boundary precipitates and is observed at intermediate temperatures. The third type is observed at the lowest temperatures; in this case the development of non-equilibrium segregation is converted into precipitation prior to the appearance of segregation peak. The temperature range associated with each type of curve depends on the relation between the kinetics of non-equilibrium segregation on the one hand and that of precipitation on the other.