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ADVANCED MATERIALS & PROCESSES •
SEPTEMBER 2014
24
useful toughness (Figs. 2 and 3a). The processing routes to
synthesize this microstructure are based on innovative pow-
der processing and directional solidification
[14-16]
. Recent de-
velopments include incorporation of TiC particle
dispersions that enhance high temperature mechanical
properties, reduce density to levels comparable to levels
for Ni-base superalloys, and facilitate casting
[17]
. To achieve
full density in powder processing, minor additions of ele-
ments such as Fe promote rapid sintering. The borosilica
scale that develops during high temperature oxidation pro-
vides some protection, but the recession rate must be re-
duced, and at low temperature (i.e., 700°C) a pesting
reaction develops before the borosilica can provide full
coverage
[18]
. Both low and high temperature oxidation re-
actions can be inhibited by a pack cementation coating in-
volving codeposition of B and Si that also resists calcia-
magnesia-alumina-silica (CMAS) and water vapor at-
tack
[19,20]
.
For Nb silicides, the main alloy constitution is based on
the Nb-Si-Ti system, but also includes additions of Cr and
Hf. The main microstructure phases include Nb(ss) and
Nb
5
Si
3
with minor amounts of an NbCr
2
Laves phase (Fig.
3b)
[10]
. The Nb
5
Si
3
phase features a structure of either the
T
1
or T
2
phase in the Mo-Si-B system (Fig. 2) depending on
temperature and composition. Again, this microstructure
can be achieved through either powder processing or by
solidification. However, inadequate oxidation resistance
remains a significant issue with Nb-base alloys. Recent
progress reveals that silicide surface layers formed due to
oxidation of an Nb
3
Fe
3
CrSi
6
phase offer the potential to
Fig. 3 —
SEM backscat-
tered image of as-cast
Mo-14.2Si-9.6B (a),
directionally solidified
Nb alloy (Nb-19Ti-2Hf-
13Cr-2Al-4B-16Si) with
dispersions of (Nb)
5
Si
3
and
(Nb)Cr
2
(b).
(a)
Mo (white)
Mo
3
Si (grey)
T
2
(dark)
10 µm
(b)
Laves
(Nb)
5
Si
3
(Nb)
100 µm