The Characteristic of Nonequilibrium Boron Segregation at Grain Boundaries During Isothermal Holding

The behaviour of grain boundary segregation and precipitation of boron in low carbon MO-B steels (with or without Mn) and Fe-30%Ni (B) alloys during isothermal holding have been investigated with Particle Tracking Au-toradiography (PTA).Specimens heated to 1350 or 120013 are quenched and isothermally held at 550-1000℃ for different periods.It is shown that the nonequilibrium grain boundary segregation of boron occurs as a dynamic Process and the magnitude of nonequilibrium grain boundary segregation of boron may be many times of that of equilibrium segregation.
After cooling, the boron segregation at grain boundaries,on isothermal holding at a relatively high temperature, probably bring there by complex of boron atoms with supersaturated vacances, and increases with time to the annihilation of vacancies,then it becomes supersaturated with respect to equilibrium segregation concentration. Thus it decrease by diffusing back into the grain interior, giving a peak On the segregaton degree-isothermal holding time curve.The height and position of the peak depend on the initial heating and subsequent holding temperature.On the other hand,when the holding temperature is low, the nonequilibriumly segregated boron becomes supersaturating and will be Precipitated at the grain boundaries, owing to the difficulty in the back diffusion at low temperature. Thus a curve increasing at the first stage and then reaching a steady segregation (including Precipitated boron) is given.