Ea. Trillo et Le. Murr, A TEM INVESTIGATION OF M23C6 CARBIDE PRECIPITATION BEHAVIOR ON VARYING GRAIN-BOUNDARY MISORIENTATIONS IN 304 STAINLESS-STEELS, Journal of Materials Science, 33(5), 1998, pp. 1263-1271
Transmission electron microscopy (TEM) along with electrochemical pote
ntiokinetic reactivation (EPR) testing was performed on different grad
es of 304 stainless steel (0.01, d0.025, 0.05, and 0.07% C) in order t
o assess the sensitization and precipitation behaviour on different gr
ain boundary misorientations. The materials were heat treated at 670 d
egrees C for 50 h to subject the materials to the sensitization regime
. The EPR data and TEM observations revealed that when the amount of c
arbon was increased the degree of sensitization increased along with t
he density of precipitates. Large angle misorientations (Theta > 15 de
grees) were prevalent in all the carbon content materials and the {110
} grain surface orientation was found to be the major texturing orient
ation. The steels with lower carbon contents nucleated a few small pre
cipitates on high angle grain boundaries, while larger amounts of carb
ides were observed on lower angle grain boundaries for the higher carb
on contents. It was deemed that higher carbon contents required lower
energies to nucleate and grow precipitates. A carbon content threshold
was found (above 0.05% C) in which precipitates fully saturate the gr
ain boundary. Precipitation followed the energies of different types o
f boundaries. The highest energy boundary (general random grain bounda
ry) nucleated precipitates first, then precipitation followed on non-c
oherent twin boundaries, and was not observed on coherent twin boundar
ies. A ''critical nucleation energy'', gamma(gb(crit.)), was therefore
found to exist at which precipitation will occur on a boundary. This
value was found to be in the range of 16 mJ m(-2) < gamma(gb(crit.)) <
265 mJ m(-2) which corresponds to the energies of special boundaries
(coherent and non-coherent portions of twins respectively) at the agei
ng temperature of 670 degrees C. (C) 1998 Chapman & Hall.