The effects of boron, aluminium, and rare earth metals (REM) on the graphitization at 700°C in 0.53% C steel were investigated in order to clarify a mechanism of graphite nucleation on the precipitates using a field emission transmission electron microscopy.
The graphitization is promoted by the addition of boron, aluminium, and REM, since the precipitates such as BN, AlN, and REM-sulfide play the role as nucleation sites of graphite.BN increases the number of graphite particles and shortens the time required for the completion of graphitization more effectively than AlN and REM-sulfides.
BN in steel consists of several single crystals whose [000 2] axes radiate from the center of particles to the surface.Therefore, BN particle exposes its basal plane of (0001).The (0001) planes of a graphite particle epitaxially nucleate on each (0001) plane of BN, and each [0001] axis of graphite also radially grows to the surface.
BN and graphite have a similar crystal structure to each other, and the morphologies are also very similar.It seems that the graphite particles easily nucleate and grow epitaxially on the surface of BN.
