Overview of the development of FeAl intermetallic alloys Page: 3 of 14
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ever reported for bare FeAI specimens. Preoxidation at 700*C
further increased the tensile ductility to 14.7% . Boron
additions had also been found to have an important beneficial
effect on the room-temperature ductility of an Fe-40 at.% Al
alloy , and Mo, Zr, and B are all important additives for
high-temperature strength in FeAI alloys as well [20-22]. The
initial weldability tests of these FeAl alloys showed that all
alloys with 0.24 at.% B additions were unweldable due to hot-
cracking, and that the B-free FeAI alloys showed even
marginal weldability . However, the best weldability was
found in the FA-385 and FA-388 alloys that contained 0.13-
0.24 at.% C instead of B . Therefore, a second phase of
FeAI alloy development at ORNL focused on improving the
weldability of these alloys.
DEVELOPMENT OF WELDABLE FeAI
WELDABILITY - The goal of the second phase of the
program was to improve the weldability of the FA-385/FA-388
based alloys without further degrading the good mechanical
properties achieved in the FA-3621FA-372 type alloys. The
FA-385 alloy was chosen as the base FeAI alloy for further
composition modifications because while carbon improved
weldability, it was also already known that too much carbon
had a deleterious effect on the room-temperature ductility and
high-temperature strength of both FeAI and Fe3AI type alloys
[7,12,17]. The modified alloy compositions (FA-385-M I thru
-Ml 1) are also shown in Table 1. The Sigmajig test  was
used to quantitatively measure the threshold stress for hot-
cracking in thin-sheet specimens of all these FeAl alloys (Fig.
2b) . From the standpoint of hot-cracking resistance, the
best FeAI alloys were FA-385 modified with micro-additions
of boron (0.01-0.021 at.%, FA-385M1 and -385M2,
respectively, Table 1). For perspective, threshold hot-cracking
stresses above 100 MPa are considered to be good, with 300
series austenitic stainless steels falling in the range of 100-280
MPa . There are very few weldability studies of FeAI
iron-aluminides. Of the Fe3AI alloys developed at ORNL, the
most weldable (FA-129) has a threshold hot-cracking stress of
172 MPa while others fall in the range of 100-140 MPa .
The modified FeAI alloys with Cr, Nb, and with Cr+Nb+Ti
also showed good weldability, but did not have good
mechanical behavior. By contrast, FA-385 alloys modified
with small amounts of boron showed the best strength at
600*C (Fig. 2a), and room-temperature properties roughly
similar to the base FA-385 alloy. From these results, further
FeAI alloy development focused on the FA-385 and the B-
micro-alloyed FA-385 alloys for microstructure/processing
optimization for improved mechanical behavior. A parallel
effort began in 1992 to explore FeAI weld-overlay cladding on
structural steels to try to-take immediate advantage of the
newly discovered weldability and the inherent FeAI
oxidation/corrosion resistance for industrial applications.
ENVIRONMENTAL EFFECTS AND ROOM-
TEMPERATURE DUCTILITY - One difficult problem was
the fact that most of the hot-rolled sheet-specimens of the FeAI
alloys showed relatively low ductility in air (<1%), with some
showing only a fracture stress of 300 MPa or less. One typical
example (e,<0.1%) is shown for a FA-385 specimen punched
from sheet that was hot-rolled at 900*C and then heat-treated
for lh at 900*C and tensile tested in air (Fig. 3a and 3b).
After Liu and co-workers found that environmental
embrittlement was a major cause for low ductility in FeAI
alloys tested in air at room temperature in 1989 , an
identical FA-385 specimen was tested in oxygen at room
temperature and showed 10.5% elongation (fig. 3a) and much
higher UTS. A higher heat-treatment of Ih at 1000*C
produced 1.8% elongation in air and 14.7% in oxygen for the
same hot-rolling condition. The hot-rolled sheet material had
a relatively large recrystallized grain size of >250 pm. Hot-
extrusion at 900*C of the same FA-385 alloy produced a much
finer grain size of 25-80 pm (similar to FA-385M1 shown in
Fig. 4a), which increased the ductility in air (to 8.4%), and the
strength (UTS-760 MPa) relative to the hot-rolled material.
'The much smaller difference between the elongation of the
hot-extruded material in air and in oxygen clearly shows that
the finer-grained microstructure also made the FA-385 alloy
more resistant to moisture-induced hydrogen embrittlement.
The oxide present from the stress-relief heat-treatment in air
also contributes to this effect, as observed by Liu for
preoxidation of FeAI , and analogous to the beneficial effect
of oil observed by Sikka et al  in Fe3AI alloys. These
results demonstrate the importance of moisture/hydrogen
effects on the mechanical behavior of FeAI at room-
temperature in air, and the importance of
microstructure/processingcontrol in minimizing or eliminating
such embrittlement effects.
PHYSICAL PROPERTIES - The lack of physical properties
on FeAI prompted an effort in 1993-1994 to better determine
them for the FA-385 alloy to a) aid in selecting engineering
applications appropriate for FeAI, b) help select and determine
substrate behavior for FeAI weld-overlay cladding, and c) help
optimize processing parameters. FeAI with 36 at.% Al has a
density of about 6 gm/cm3 [27,28], has the ordered-B2 phase
structure from room-temperature until it disorders at about
1100-1125*C, has a solidus of 1402*C , and is non-
magnetic. It has a relatively high Young's modulus of 180
GPa. The B2-+a transition can be seen in the plot of specific
heat versus temperature shown in Fig. 5a. The thermal
expansion and thermal conductivity of FeAI/FA-385 are shown
in Figs. 5b and 5c, respectively [29,30]. The thermal
expansion of FeAI is somewhat similar to type 316 austenitic
stainless steel up to about 1000*C, but its thermal conductivity
is significantly lower than that found in austenitic stainless
steel. The combination of higher thermal expansion and low
conductivity can easily lead to high near-surface stresses if
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Maziasz, P.J.; Liu, C.T. & Goodwin, G.M. Overview of the development of FeAl intermetallic alloys, article, September 1, 1995; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc623434/m1/3/: accessed April 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.