Overview of the development of FeAl intermetallic alloys Page: 5 of 14
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[38,39]. The poor as-cast properties were attributed to a
combination of factors that included a) grain size being too
large (>0.5 mm), b) weak grain boundaries, c) coarse NbC
particles and d) the wrong matrix phase (100% ordered DO3).
Consistently, the best room-temperature ductility and fatigue
crack growth resistance found in this same Fe,AU/FA-129 alloy
and other similar alloys occurred in wrought material which
retained the B2 instead of the DO, phase [6,38-42]. Studies of
scale-up and industrial melt-practice variables at ORNL for
both Fe3AI and FeAI showed that a) various melting processes
could easily control target chemistries, b) air-induction-melting
(AIM) introduces moisture and porosity whereas vacuum or
inert gas processes did not, and c) an Al20, crucible is better
than MgO [8,11,12]. A new method of loading and drying the
elemental charge for aluminides to take advantage of the
exothermic heat of reaction, termed Exo-Melt'", has been
developed at ORNL to further improve IM technology for
these intermetallic alloys [43,44].
The as-cast properties of the 132-phase FeAI/FA-385
and B-modified alloys from vacuum-induction-melted (VIM),
33 kg rectangular ingots have only recently been determined
at ORNL [261, and are shown in Figs. 2, 7 and 8. The room-
temperature ductility of FeAI/FA-385 is 1.8-2.4% in air, and
is similar with and without heat-treatment. The smaller
difference between tests run in air and oxygen (Fig. 2a)
indicates that moisture embrittlement has less of an effect in
the as-cast compared to hot-rolled material, despite the coarse
grain size of the as-cast structure (0.25-0.67 mm, Fig. 4b).
The YS and UTS of the cast FA-385 are not much different
than wrought properties (Fig. 3b). Figure 7 shows the effects
of B-micro-alloying additions on the room-temperature
strength and ductility of as-cast material. The B-doped FeAI
alloys have twice the ductility in air and show better resistance
to the moisture effect compared to the FA-385 without B. The
B-doped alloys are also stronger, with 50 wppm (210 appm)
B having more effect (Fig. 7b). At higher temperatures, FA-
385 retains its YS (about 400 MPa) to about 600*C while the
B-doped alloys retain their YS to about 750*C (Fig. 8a).
These FeAI alloys become significantly weaker and much more
ductile at 800*C, and make a transition from transgranular
cleavage to ductile-dimple failure at 750-800*C (Fig. 8d). The
B-micro-alloying addition also significantly enhances the
creep-rupture strength of these FeAl alloys at 600*C/207 MPa
(fig. 9). TEM analysis indicates that B-micro-alloying
additions enhance the formation of fine ZrC particles, so that
precipitation-strengthening is the main reason for the better
high-temperature strength of the B-modified FA-385 alloys.
While all the as-cast FeAI alloys show much higher YS
relative to type 316 austenitic stainless steel at temperatures
below 750*C, the B-doped alloys also have a UTS advantage
over 316 at 750-800*C. The as-cast FeAI alloys show better
strength at 600-750*C than was found in previous work on
wrought FeAI (FA-362) or Fe3AI (FA-129) alloys at those
temperatures [12,38]. The high-temperature strength of these
cast FeAI alloys is much better than binary Fe-(40-45) at.% Al
alloys doped with 500 appm B , and their creep strength
at 600*C is better than ODS Fe-40 Al (with Zr and B added)
(Powder Processing of FeAI) - Recent work on FeAI l:as
shown that PM (including mechanically-alloyed [MA] and
oxide-dispersion-strengthened [ODS] materials) can enhance
strength, ductility and toughness of FeAI and Fe3AI alloys
[20.46]. Moreover, powders enable iron-aluminide coating
technologies to produce coextruded composite tubing  or
thermal spray coatings. A commercial heat (880 kg) of
nitrogen-atomized FeAI/FA-385 (-100 mesh, spherical) powder
was produced by Ametek. Most particles ranged from 70-130
pm in diameter, with a small volume fraction of fine (<5-20
m) particles also present. Some of this powder was
consolidated by direct extrusion (12:1 reduction ratio) at 950-
I100*C. The FA-385 powder extruded at I100*C showed
good consolidation with no voids and a grain size <20 pm.
The PM FA385 showed >9% ductility in air (>11% in 02) and
more strength (YS-500 MPa, UTS> 1000 MPa, 02) at room-
temperature than IM processing of that same alloy (Fig. 3).
Tests of the PM alloys show a YS of 155-230 MPa at 800*C.
These good properties of extruded FeAI powders indicate that
extruded cladding on steel tubes or piping should also behave
well. Components produced by extrusion or direct forging of
powders should also be feasible for fabrication from FeAI.
MA and ODS processing should further enhance the high-
temperature strength of these particular FeAI alloys, in order
to exploit the better oxidation/sulfidation resistance of these
alloys relative to conventional high-temperature alloys like
FeCrAI, 800H and MA956 [20,35].
Recently, the fine-sized component of these FeAV/FA-
385 powders was used to produce plasma-spray coatings on
conventional steels and nickel-based superalloys. FeAI
coatings of 0.13-0.2 mm thickness were produced directly on
a type 316 austenitic stainless steel substrate using particle
velocities ranging from subsonic to Mach II . These
coatings showed good density and adherence to the steel
substrate interface (Fig. 10). Higher velocity spraying will
further increase the FeAI coating density.
(Industrial Component Testing of FeAI) - To take advantage of
the good mechanical properties observed in the as-cast FeAI,
a 1.320 kg heat of FA-385M2 (200 appm B) was melted using
the Exo-MeltT' technology by Alloy Engineering & Casting
Company. Radiant heating tubes were centrifugally cast (Fig.
11) and U-bends were sand cast. The radiant heating tubes are
95.25 mm in outer diameter, 2.44 m long, with different wall
thicknesses varying from 6.35 to 19.05 mm. The target alloy
chemistry was easily achieved, and the tubes were pressure-
checked and showed no leaks. Properties tests of these FeAI
tubes is now in progress. These radiant tubes are of direct
interest to large automakers and others with large heat-treating
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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/m1/5/: accessed May 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.