Microstructural Evolution in a 17-4 PH Stainless Steel after Aging at ...

(a)
(b)
110
110
lated with the depth scale of the analysis, and the total
depth of this analysis is estimated to be approximately
60 nm. This data shows that the martensite in the solution
heat-treated specimen is a supersaturated solid solution
containing all solute atoms homogeneously.
C. Tempered microstructure
(c)
100nm
110
011
Fig. 5 (a) TEM bright field image, (b) [001] and (c) [111] SAD
pattern of the martensite phase tempered at 580°C for 4 h.
Figure 5 shows a bright field TEM image of the
martensite phase after aging for 4 h at 580°C. Brightfield
image does not give any clear contrast corresponding
to fine coherent Cu precipitates expected in this
stage. This suggests that Cu precipitates, if any, is still
coherent and they do not cause large strain contrast due
to the small strain field around the precipitate. In fact,
the SADP taken from a martensite lath does not show
any evidence for a secondary phase, suggesting that
there is no precipitates with the distinct structure different
from the bcc matrix. Figure 6 shows 3DAP elemental
mapping of Cu and Cr obtained from the martensite
phase. The Cu mapping clearly shows that there
is a small spherical particle enriched with Cu. On the
other hand, the Cr mapping shows that the distribution
of Cr atoms is uniform martensite phase. The Cu el-
Published in Metall. Mater. Trans. A. Vol. 30A, pp. 345-353. 1999
emental map clearly shows that there is Cu enriched
precipitate in this stage. As the SADP (Figure 5(b) and
(c)) does not show any evidence for presence of the
secondary phase, the Cu enriched precipitate observe
in the 3DAP data is believed to be fully coherent bcc-
Cu. In order to quantify the concentration of the particle,
concentration depth profile was measured from
the selected region near the Cu-rich precipitate as shown
in Fig. 6 (c). The chemical composition of the Cu-rich
precipitates has been found to be 55 at.pct Cu, 30 at.pct
Fe, 10 at. pct Cr, 5 at. pct Ni. It is seen that Cr and Ni
are rejected from the Cu-enriched particle slightly. It
should also be noted that the concentration of Cu in the
particle is significantly lower than that expected from
the equilibrium e-Cu.
D. Aging for 100 h at 400°C
Figure 7 (a) and (b) show 3DAP elemental mapping
of Cu and Cr obtained from the martensite phase in the
specimen aged for 100 h at 400°C. The Cu mapping
shows that a high density of Cu-rich precipitates of approximately
3 nm in diameter is present. The Cr mapping
shows that the distribution of Cr atoms is no longer
uniform and fluctuations of Cr concentration occur.
Concentration depth profiles of Fe, Cr, Ni, Si and Cu
were measured from a selected region cutting two Curich
precipitates as shown in Figure 6(c). The concentration
of Cu in the Cu-rich precipitates is approximately
70 at. pct Cu, which is much higher than that observed
in the tempered specimen, but it is still lower than the
equilibrium concentration of the ε-Cu. The presence of
fluctuations in Cr concentration indicates that the phase
decomposition occurs in the martensite phase, and it
decomposes to Fe-rich a and Cr-enriched α’ phases. The
Cr concentration in the α’ phase is only 25 at. pct and
this is significantly lower than that of equilibrium α’
phase, suggesting that it is still in an initial stage of the
decomposition process. It should be noted that there is
no indication of partitioning of Ni.
E. Aging for 5000 h at 400°C
Figure 8 (a) shows a dark field image excited using
the 1/2(110) reflection and Figures 8 (b) – (d) shows
[001], [011], [111] zone SAD patters, respectively obtained
from the martensite phase aged for 5000 hours
at 400°C. Extra reflections indicate an ordered phase
having a cube-on-cube orientation relationship precipitates
after prolonged aging. The dark field image which
was taken using the 1/2(110) reflection shows a high
density of fine, ordered particles are dispersed in the
martensite matrix. The diffraction pattern was found to
be consistent with the G phase (structure type D8 , space a
group Fm3m ) reported in aged duplex stainless steels
[23,26,27] [19] and type 308 stainless steels .
Figure 9 shows atom probe concentration depth profiles
obtained from the martensite phase in the alloy