Note: Descriptions are shown in the official language in which they were submitted.
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TWO PHASE TITANIUM ALUMINIDE ALLOY
Field of the Invention
The invention relates generally to two-phase titanium aluminide alloy
compositions useful for resistive heating and other applications such as
structural
applications.
Background of the Invention
Titanium aluminide alloys are the subject of numerous patents and
publications including U.S. Patent Nos. 4,842,819; 4,917,858; 5,232,661;
5,348,702; 5,350,466; 5,370,839; 5,429,796; 5,503,794; 5,634,992; and
5,746,846, Japanese Patent Publication Nos. 63-171862; 1-259139; and 1-42539;
European Patent Publication No. 365174 and articles by V.R. Ryabov et al
entitled
"Properties of the Intermetallic Compounds of the System Iron-Aluminum"
published in Metal Metalloved, 27, No.4, 668-673, 1969; S.M. Barinov et al
entitled "Deformation and Failure in Titanium Aluminide" published in
Izvestiya
Akademii Nauk SSSR Metally, No. 3, 164-168, 1984; W. Wunderlich et al
entitled "Enhanced Plasticity by Deformation Twinning of Ti-Al-Base Alloys
with
Cr and Si" published in Z. Metallkunde, 802-808, 11/1990; T. Tsujimoto
entitled
"Research, Development, and Prospects of TiAl Intermetallic Compound Alloys"
published in Titanium and Zirconium, Vol. 33, No. 3, 19 pages, 7/1985; N.
Maeda entitled "High Temperature Plasticity of Intermetallic Compound TiAI"
presented at Material of 53d Meeting of Superplasticity, 13 pages, 1/30/1990;
N.
Maeda et al entitled "Improvement in Ductility of Intermetallic Compound
through
Grain Super-refinement" presented at Autumn Symposium of the Japan Institute
of
Metals, 14 pages, 1989; S. Noda et al entiitled " Mechanical Properties of
TiAI
Intermetallic Compound" presented at Autumn Symposium of the Japan Institute
of
Metals, 3 pages, 1988; H.A. Lipsitt entitled "Titanium Aluminides - An
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Overview" published in Mat. Res. Soc. Symp. Proc. Vol. 39, 351-364, 1985; P.L.
Martin et al entitled "The Effects of Alloying on the Microstructure and
Properties
of Ti3 Al and TiAI" published by ASM in Titanium 80, Vol. 2, 1245-1254, 1980;
S.H. Whang et al entitled "Effect of Rapid Solidification in Llo TiAl Compound
Alloys" ASM Symposium Proceedings on Enhanced Properties in Structural
Metals Via Rapid Solidification, Materials Week, 7 pages, 1986; and D. Vujic
et
al entitled "Effect of Rapid Solidification and Alloying Addition on Lattice
Distortion and Atomic Ordering in L1o TiAI Alloys and Their Ternary Alloys"
published in Metallurgical Transactions A, Vol. 19A, 2445-2455, 10/1988.
Methods by which TiAI aluminides can be processed to achieve desirable
properties are disclosed in numerous patents and publications such as those
mentioned above. In addition, U.S. Patent No. 5,489,411 discloses a powder
metallurgical technique for preparing titanium aluminide foil by plasma
spraying a
coilable strip, heat treating the strip to relieve residual stresses, placing
the rough
sides of two such strips together and squeezing the strips together between
pressure bonding rolls, followed by solution annealing, cold rolling and
intermediate anneals. U.S. Patent No. 4,917,858 discloses a powder
metallurgical
technique for making titanium aluminide foil using elemental titanium,
aluminum
and other alloying elements. U.S. Patent No. 5,634,992 discloses a method of
processing a gamma titanium aluminide by consolidating a casting and heat
treating
the consolidated casting above the eutectoid to form gamma grains plus
lamellar
colonies of alpha and gamma phase, heat treating below the eutectoid to grow
gamma grains within the colony structure and heat treating below the alpha
transus
to reform any remaining colony structure a structure having oci laths within
ganuna
grains.
Still, in view of the extensive efforts to improve properties of titanium
aluminides, there is a need for improved alloy compositions and economical
processing routes.
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Invention
Summ= of the
According to a first embodiment, the invention provides a two-phase
titanium aluminum alloy having a lamellar microstructure controlled by colony
size. The alloy can be provided in various forms such as in the as-cast, hot
extruded, cold and hot worked, or heat treated condition. As an end product,
the
alloy can be fabricated into an electrical resistance heating. element having
a
resistivity of 60 to 200 ,uS)=cm. The alloy can include additional elements
which
provide fme particles such as second-phase or boride particles at colony
boundaries. The alloy can include grain-boundary equiaxed structures. The
additional alloying elements can include, for example, up to 10 at% W, Nb
and/or
Mo. The alloy can be processed into a thin sheet having a yield strength of
more
than 80 ksi (560 MPa), an ultimate tensile strength of more than 90 ksi (630
MPa),
and/or tensile elongation of at least 1.5%. The aluminum can be present in an
amount of 40 to 50 at%, preferably about 46 at%. The titanium can be present
in
the amount of at least 45 at%, preferably at least 50 at%. As an example, the
alloy can include 45 to 55 at% Ti, 40 to 50 at% Al, 1 to 5 at% Nb, 0.5 to 2
at%
W, and 0.1 to 0.3 at% B. The alloy is preferably free of Cr, V, Mn and/or Ni.
Brief Ih-scrintion of the TZrawingc
Figures la-d are optical micrographs at 200X of PMTA TiAI alloys hot
extruded at 1400 C and annealed for 2 hours at 1000 C. Figure la shows the
microstructure of PMTA-1, Figure lb shows the microstructure of PMTA-2,
Figure lc shows the microstructure of PMTA-3 and Figure ld shows the
microstructure of PMTA-4;
Figures 2a-d show optical micrographs at 500X of PMTA alloys hot
extruded at 1400 C and annealed for 2 hours at 1000 C. Figure 2a shows the
microstructure of PMTA-1, Figure 2b shows the microstructure of PMAT-2,
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Figure 2c shows the microstructure of PMAT-3 and Figure 2d shows the
microstructure of PMTA-4;
Figure 3 shows ghost-pattern bands observed in a back-scattered image of
PMTA-2 hot extruded at 1400 C and annealed for 2 hours at 10000C wherein the
non-uniform distribution of W is shown;
Figure 4 shows a back-scattered image of PMTA-2 hot extruded at 1400 C
and annealed for 2 hours at 1000 C;
Figure 5a is a micrograph at 200X of PMTA-3 hot extruded at 1400 C and
annealed for one day at 1000 C and Figure 5b shows the same microstructure at
500X;
Figure 6a shows the microstructure at 200X of PMTA-2 hot extruded at
1400 C and annealed for 3 days at 1000 C and Figure 6b shows the same
microstructure at 500X;
Figure 7a is an optical micrograph of TiAI sheet (Ti-45A1-SCr, at%) in the
as-received condition and Figure 7b shows the same microstructure after
annealing
for 3 days at 1000 C, both micrographs at 500X;
Figure 8a shows a micrograph of PMTA-6 and Figure 8b shows a
micrograph of PMTA-7, both of which were hot extruded at 1380 C
(magnification 200X);
Figure 9a is a micrograph of PMTA-6 and Figure 9b is a micrograph of
PMTA-7, both of which were hot extruded at 1365 C (magnification 200X);
Figure 10 is micrograph showing abnormal grain growth in PMTA hot
extruded at 1380 C;
Figures 11 a-d are micrographs of PMTA-8 heat treated at different
conditions after hot extrusion at 1335 C, the heat treatments being two hours
at
1000 C for Figure lla, 30 minutes at 1340 C for Figure llb, 30 minutes at
1320 C for Figure llc, and 30 minutes at 1315 C for Figure lld (magnification
200X);
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Figure 12 is a graph of resistivity in microhms versus temperature for
samples 1 and 2 cut from an ingot having a PMTA-4 nominal composition;
Figure 13 is a graph of hemispherical total emissivity versus temperature
for samples 1 and 2;
5 Figure 14 is a graph of diffusivity versus temperature for samples 90259-1,
80?,59-2 and 80259-3 cut from the same ingot as samples 1 and 2;
Figure 15 is a graph of specific heat versus temperature for titanium
= aluminide in accordance with the invention; and
Figure 16 is a graph of thermal expansion versus temperature for samples
80259-1H, 80259-1C, 80259-2H, 80259-3H, and 80259-3C cut from the same
ingot as samples 1 and 2.
Detailed Description of the Preferred Embodiments
The invention provides two-phase TiAl alloys with thermo-physical and
mechanical properties useful for various applications such as resistance
heater
elements. The alloys exhibit useful mechanical properties and corrosion
resistance
at elevated temperatures up to 1000 C and above. The TiAI alloys have
extremely
low material density (about 4.0 g/cm3), a desirable combination of tensile
ductility
and strength at room and elevated temperatures, high electrical resistance,
and/or
can be fabricated into sheet material with thickness.< 10 mil. One use of such
sheet material is for resistive heating elements of devices such as cigarette
lighters.
For instance, the sheet can be formed into a tubular heating element having a
series of heating strips which are individually powered for lighting portions
of a
cigarette in an electrical smoking device of the type disclosed in U.S. Patent
Nos
5,591,368 and 5,530,225. In addition, the alloys can be free of elements such
as
Cr, V, Mn and/or Ni.
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Compared to TiAI alloys containing 1 to 4 at% Cr, V, and/or Mn for
improving tensile ductility at ambient temperatures, according to the present
invention, tensile ductility of dual-phase TiAl alloys with lamellar
structures can
be mainly controlled by colony size, rather than such alloying elements. The
invention thus provides high strength TiAI alloys which can be free of Cr, V,
Mn
and/or Ni.
Table 1 lists nominal compositions of alloys investigated wherein the base
alloy contains 46.5 at% Al, balance Ti. Small amounts of alloying additions
were
added for investigating effects on mechanical and metallurgical properties of
the
two-phase TiAI alloys. Nb in amounts up to 4% was examined for possible
effects
on oxidation resistance, W in amounts of up to 1.0% was examined for effects
on
microstructural stability and creep resistance, and Mo in amounts of up to 0.5
%
was examined for effects on hot fabrication. Boron in amounts up to 0.18 % was
added for refmement of lamellar structures in the dual-phase TiAI alloys.
Eight alloys identified as PMTA-1 to 9, having the compositions listed in
Table 1, were prepared by arc melting and drop casting into a 1" diameter x 5"
long copper mold, using commercially-pure metals. All the alloys were
successfully cast without casting defects. Seven alloy ingots (PMTA -1 to 4
and 6
to 9) were then canned in Mo cans and hot extruded at 1335 to 1400 C with an
extrusion ratio of 5:1 to 6:1. The extrusion conditions are listed in Table 2.
The
cooling rate after extrusion was controlled by air cooling and quenching the
extruded rods in water for a short time. The alloy rods extruded at 1365 to
1400 C showed an irregular shape whereas PMTA-8 hot-extruded at 1335 C
exhibited much smoother surfaces without surface irregularities. However, no
cracks were observed in any of the hot-extruded alloy rods.
The microstructures of the alloys were examined in the as-cast and heat
treated conditions (listed in Table 2) by optical metallography and electron
superprobe analyses. In the as-cast condition, all the alloys showed lamellar
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structure with some degree of segregation and coring. Figures 1 and 2 show the
optical micrographs, with a magnification of 200X and 500X, respectively, for
hot
extruded alloys PMTA-1 to 4 stress-relieved for 2 hours at 1000 C. All the
alloys
showed fully lamellar structures, with a small amount of equiaxed grain
structures
at colony boundaries. Some fine particles were observed at colony boundaries,
which are identified as borides by electron microprobe analyses. Also, there
is no
apparent difference in microstructural features among these four PMTA alloys.
Electron microprobe analyses reveal that tungsten is not uniformly
distributed even in the hot extruded alloys. As shown in Figure 3, the ghost-
pattern bands in a darker contrast are found to be depleted with about 0.33
at% W.
Figure 4 is a back-scattered image of PMTA-2, showing the formation of second-
phase particles (borides) in a bright contrast at colony boundaries. The
composition of the borides was determined and listed in Table 3 together with
that
of the lamellar matrix. The second-phase particles are essentially (Ti,W,Nb)
borides, which are decorated and pinned lamellar colony boundaries.
Figures 5 and 6 show the optical microstructures of hot extruded PMTA-3
and 2 annealed for 1 day and 3 days at 1000 C, respectively. Grain-boundary
equiaxed structures are clearly observed in these long-term annealed
specimens,
and the amount increases with the annealing time at 1000 C. A significant
amount
of equiaxed grain structures exists in the specimen annealed for 3 days at
1000 C.
For comparison purposes, a 9-mil thick TiAI sheet (Ti-45A1-SCr, at%) was
evaluated. Figure 7 shows the optical microstructures of the TiAlCr sheet in
both
as-received and annealed (3 days at 1000 C) conditions. In contrast to the
dual-
phase lamellar structure of the alloys according to the invention, the TiAlCr
sheet
has a duplex structure, and its grain structure shows no significant
coarsening at
1000 C.
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Tensile sheet specimens with a thickness of 9-20 mils and a gage length of
0.5 in were sectioned from the hot extruded alloys rods after annealing for 2
hours
at 1000 C, using a EDM machine. Some of the specimens were re-annealed up to
3 days at 1000 C prior to tensile testing. Tensile tests were performed on an
Instron testing machine at a strain rate of 0.1 inch/second at room
temperature.
Table 4 summarizes the tensile test results.
All the alloys stress-relieved for 2 hours at 1000 C exhibited 1% or more
tensile elongation at room temperature in air. The tensile elongation was not
affected when the specimen thickness varied from 9 to 20 mils. As indicated in
Table 4, among the 4 alloys, alloy PMTA-4 appears to have the best tensile
ductility. It should be noted that a tensile elongation of 1.6% obtained from
a 20-
mil thick sheet specimen is equivalent to 4% elongation obtained from rod
specimens with a gage diameter of 0.12 in. The tensile elongation appears to
increase somewhat with annealing time at 1000 C, and the maximum ductility is
obtained in the specimen annealed for 1 day at 1000 C.
All the alloys are exceptionally strong, with a yield strength of more than
100 ksi (700 MPa) and ultimate tensile strength more than 115 ksi (800 MPa) at
room temperature. The high strength is due to the refmed fully lamellar
structures
produced in these TiAI alloys. In comparison, the TiAlCr sheet material has a
yield strength of only 61 ksi (420 MPa) at room temperature. Thus, the PMTA
alloys are stronger that the TiAICr sheet by as much as 67 %. The PMTA alloys
including 0.5 % Mo exhibited significantly increased strengths, but slightly
lower
tensile elongation at room temperature.
Figures 8a-b and 9a-b show the optical micrographs of PMTA-6 and 7 hot
extruded at 1380 C and 1365 C, respectively. Both alloys showed lamellar grain
structures with little intercolony structures. Large colony grains (see Figure
10)
were observed in both alloys hot extruded at 1380 C and 1365 C, which probably
resulted from abnormal grain growth in the alloys containing low levels of
boron
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after hot extrusion. There is no significant difference in microstructural
features in
these two PMTA alloys.
Figures 11a-d show the effect of heat treatment on microstructures of
PMTA-8 hot extruded at 1335 C. The alloy extruded at 1335 C showed much
fmer colony size and much more intercolony structures, as compared with those
hot extruded at 1380 C and 1365 C. Heat treatment for 2 h at 1000 C did not
produce any significant change in the as-extruded structure (Figure 11a).
However, heat treatment for 30 mins at 1340 C resulted in a substantially
larger
colony structure (Figure 11b). Lowering the heat-treatment temperature from
1340 C to 1320-1315 C (a difference by 20-25 C) produced a sharp decrease
in
colony size, as indicated by Figures llc and lld. The annealing at 1320-1315 C
also appears to produce more intercolony structures in PMTA-8. The abnormal
grain growth is almost completely eliminated by hot extrusion at 1335 C.
Tensile sheet specimens of PMTA-6 to 8 with a thickness varying from 8 to
22 mils and with a gage length of 0.5 inch were sectioned from the hot
extruded
alloy rods after giving a final heat treatment of 2 h at 1000 C or 20 min at
1320-
1315 C, using an EDM machine. Tensile tests were performed on an Instron
testing machine at a strain rate of 0.1 in/s at temperatures up to 800 C in
air. All
tensile results are listed in Tables 5 to 8. The alloys PMTA-4, -6 and -7 heat
treated for 2 h at 1000 C showed excellent strengths at all temperatures,
independent of hot extrusion temperature. The hot extrusion at 1400-1365 C
gives low tensile ductilities (<49b) at room and elevated temperatures. A
significant increase in tensile ductility is obtained at all temperatures when
hot
extruded at 1335 C. PMTA-8 which was hot extruded at 1335 C exhibited the
highest strength and tensile ductility at all test temperatures. There did not
appear
to be any systematical variation of tensile ductility with specimen thickness
varying
from 8 to 22 mils.
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Tables 7 and 8 also show the tensile properties of PMTA- 6 and 7 heat
treated for 20 min. at 1320 C and 1315 C, respectively. As compared with the
results obtained from heat treatment at 1000 C, the heat treatment at 1320-
1315 C
resulted in higher tensile elongation, but lower strength at the test
temperatures.
5 Among all the alloys and heat treatments, PMTA-8 hot extruded at 1335 C and
annealed for 20 min at 1315 C exhibited the best tensile ductility at room
and
elevated temperatures. This alloy showed a tensile ductility of 3.3% and 11.7%
at
room temperature and 800 C, respectively. PMTA-8 heat treated at 1315 C
appears to be substantially stronger than known TiAI alloys.
10 In an attempt to demonstrate the bend ductility of TiAI sheet material,
several pieces of 11 to 20 mil PMTA-7 and PMTA-8 alloy sheets, produced by hot
extrusion and heat treated at 1320 C, were bent at room temperature. Each
alloy
piece did not fracture after a bend of 42 . These results clearly indicate
that
PMTA alloys with a controlled microstructure is bendable at room temperature.
The oxidation behavior of PMTA-2, -5 and-7 was studied by exposing
sheet samples (9-20 mils thick) at 800 C in air. The samples were periodically
removed from furnaces for weight measurement and surface examination. The
samples showed a very low weight gain without any indication of spalling. It
appears that the alloying additions of W and Nb affect the oxidation rate of
the
alloys at 800 C, and W is more effective in improving the oxidation resistance
of
TiAI alloys. Among the alloys, PMTA-7 exhibits the lowest weight gain and the
best oxidation resistance at 800 C. Oxidation of PMTA-7 indicated that oxide
scales are fully adherent with no indication of microcracking and spalling.
This
observation clearly suggests that the oxide scales formed at 800 C are well
adherent to the base material and are very protective.
Figure 12 is a graph of resistivity in microhms versus temperature for
samples 1 and 2 which were cut from an ingot having a nominal composition of
PMTA-4, i.e. 30.8 wt% Al, 7.1 wt% Nb, 2.4 wt% W, and 0.045 wt% B.; Figure
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13 is a graph of hemispherical total emissivity versus temperature for samples
1
and 2; Figure 14 is a graph of diffusivity versus temperature for samples
80259-1,
80259-2 and 80259-3 cut from the same ingot as samples 1 and 2; Figure 15 is a
graph of specific heat versus temperature for titanium aluminide in accordance
with the invention; and Figure 16 is a graph of thermal expansion versus
temperature for samples 80259-1H, 80259-1C, 80259-2H, 80259-3H, and 80259-
3C cut from the same ingot as samples 1 and 2.
In summary, the hot PMTA alloys extruded at 1365 to 1400 C exhibited
mainly lamellar structures with little intercolony structures while PMTA-8
extruded at 1335 C showed much fmer colony structures and more intercolony
structures. The heat treatment of PMTA-8 at 1315-1320 C for 20 min. resulted
in
fme lamellar structures. The alloys may include (Ti,W,Nb) borides formed at
colony boundaries. Moreover, tungsten in the hot-extruded alloys is not
uniformly
distributed, suggesting the possibility of high electrical resistance of TiAI
alloys
containing W additions. The inclusion of 0.5 at. % Mo significantly increases
the
yield and ultimate tensile strengths of the TiAl alloys, but lowers the
tensile
elongation to a certain extent at room temperature. Among the four hot
extruded
alloys PMTA 1-4, PMTA-4 with the alloy composition Ti-46.5 A1-3 Nb-0.5 W-
0.2 B (at%) has the best combination of tensile ductility and strength at room
temperature. In comparison with the TiAlCr sheet material (Ti-45 AI-5Cr),
PMTA-4 is stronger than the TiAlCr sheet by 67%. In addition, the TiAlCr sheet
showed no bend ductility at room temperature while PMTA-4 has an elongation of
1.4%. The tensile elongation of TiAl alloys is independent of sheet thickness
in
the range of 9 to 20 mils. The alloys PMTA 4, 6 and 7 heat treated at 1000 C
for
2h showed excellent strength at all temperatures up to 800 C, independent of
hot
extrusion temperature. Hot extrusion temperatures of 1400-1365 C, however,
provides lower tensile ductilities (<4%) at room and elevated temperatures. A
significant increase in tensile ductility is obtained at all temperatures when
the
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extrusion temperature is 1335 C. PMTA-8 (Ti-46.5 Al-3 Nb-iW-0.5B) hot
extruded at 1335 C and annealed at 1315 C for 20 min. exhibited the best
tensile
ductility at room and elevated temperatures (3.3% at room temperature and
11.7%
at 800 C).
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Table 1. Nominal Alloy Compositions
~oRn~s~tions
Alloy
number Ti Al Cr Nb Mo W B
PMTA-1 50.35 46.5 0 2 0.5 0.5 0.15
PMTA-2 50.35 46.5 0 2 -- 1.0 0.15
PMTA-3 49.85 46.5 0 2 0.5 1.0 0.15
PMTA-4 49.85 46.5 0 3 -- 0.5 0.15
PMTA-5 47.85 46.5 0 4 -- 0.5 0.15
PMTA-6 49.92 46.5 0 3 -- 0.5 0.08
PMTA-7 49.92 46.5 0 3 -- 1.0 0.08
PMTA-8 49.40 46.5 0 3 - 1.0 0.10
PMTA-9 49.32 46.5 0 3 - 1.0 0.18
Table 1 (cont'd) c6~osicious (vvt%a)
Alloy
number Ti Al Cr Nb Mo W B
PMTA-1 60.46 31.36 0 4.64 1.20 2.30 0.04
PMTA-2 59.80 31.02 0 4.60 -- 4.54 0.04
PMTA-3 58.86 30.83 0 4.57 1.18 4.52 0.04
PMTA-4 59.55 31.19 0 6.93 -- 2.29 0.04
PMTA-5 57.71 30.85 0 9.14 - 2.26 0.04
PMTA-6 59.56 31.20 0 6.93 -- 2.29 0.02
PMTA-7 57.98 30.68 0 6.82 -- 4.50 0.02
PMTA-8 57.98 30.68 0 6.82 -- 4.50 0.02
PMTA-9 57.97 30.67 0 6.82 -- 4.49 0.05
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Table 2. Fabrication and Heat Treatment Condition Used for PMTA Alloys
Alloy number Hot extrusion Heat treatment (C /time)
temperature (C
PMTA-1 1400 1000 C for up to 3days
PMTA-2 1400 1000 C for up to 3days
PMTA-3 1400 1000 C for up to 3days
PMTA-4 1400 1000 C for up to 3days
PMTA-5
PMTA-6 1380, 1365 1000 C/2 hours
PMTA-7 1380, 1365 1000 C/2hr, 1320 C/20 min
PMTA-8 1335 1000 C/2hr, 1315 C/20 min
Table 3 Phase Compositions in PMTA-2 Alloy Determined
by Electron Microprobe Analyses
~t~y ~le~er~ts (at~a) '
Phase Ti Al W Nb
Matrix phase Balance 44.96 0.82 1.32
(dark contrast)
Matrix phase Balance 44.70 1.15 1.32
(bright contrast)
Borides* 77.69 8.66 9.98 3.67
*metal elements only
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Table 4. Tensile Properties of PMTA Alloys
Hot Extruded at 1400 C and Tested at Room Temperature
+~on~~v
``ensdi'
,AilQ~ elaogaQ~
(a~~b) ~~~ (ksi} (ksi)
2hours/1000 C
5 PMTA-1 2/0.5/0.5 1.0 114 118
PMTS-2 2/0/1.0 1.2 104 117
PMTA-3 2/0.5/1.0 1.1 123 132
PMTA-4 3/0/0.5 1.4 102 115
lday/1000 C
10 PMTA-3 2/0.5/ 1.0 1.4 115 131
3days/1000 C
PMTA-2 2/0/1.0 0.8 105 109
Table 5. Tensile Properties of PMTA-4 Hot Extruded at
1400 C and Annealed for 2h at 1000 C
15 Test temperature Yield strength Ultimate tensile Elongation
(C ) (ksi) strength (ksi) (%)
22 102.0 115 1.4
600 101.0 127 2.4
700 96.5 130 2.7
800 97.8 118 2.4
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Table 6. Tensile Properties of PMTA-6 Hot Extruded at
1365 C and Annealed at 1000 C for 2 h
Test temperature Yield strength Ultimate tensile Elongation
(C ) (ksi) strength (ksi) (%)
22 121.0 136 1.3
300 101.0 113 1.2
700 93.6 125 2.7
800 86.5 125 3.9
Table 7. Tensile Properties of PMTA-7 Hot Extruded at 1365 C
Test temperature Yield strength Ultimate tensile Elongation
(C ) (ksi) strength (ksi) ( %)
Annealed for 2 h at 1000 C
22 116.0 122 1.0
300 101.0 116 1.5
700 105.0 131 2.7
800 87.2 121 3.1
Annealed for 20 min at 1320 C
84.5 106.0 3.0
300 71.4 89.8 2.5
20 700 68.5 97.2 4.5
800 63.5 90.2 4.5
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Table 8. Tensile Properties of PMTA-8 Hot Extruded at 1335 C
Test temperature Yield strength Ultimate tensile Elongation
(C ) (ksi) strength (ksi) (%)
Annealed for 2 h at 1000 C
22 122.0 140 2.0
300 102.0 137 4.3
700 95.0 131 4.7
800 90.2 124 5.6
Annealed for 20 min at 1315 C
20 96.2 116 3.3
300 79.4 115 6.1
700 72.2 112 7.5
800 72.0 100 11.7
The foregoing titanium aluminide can be manufactured into various shapes
or products such as electrical resistance heating elements. However, the
compositions disclosed herein can be used for other purposes such as in
thermal
spray applications wherein the compositions could be used as coatings having
oxidation and corrosion resistance. Also, the compositions could be used as
oxidation and corrosion resistant electrodes, furnace components, chemical
reactors, sulfidization resistant materials, corrosion resistant materials for
use in
the chemical industry, pipe for conveying coal slurry or coal tar, substrate
materials for catalytic converters, exhaust walls and turbocharger rotors for
automotive and diesel engines, porous filters, etc.
With respect to resistance heating elements, the geometry of the heating
element blades can be varied to optimize heater resistance according to the
CA 02319505 2000-08-01
WO 99/51787 PCT/US99ro2212
18
formula: R = p (L/W x T) wherein R= resistance of the heater, p = resistivity
of the heater material, L = length of heater, W = width of heater and T =
thickness of heater. The resistivity of the heater material can be varied by
changes
in composition such as adjusting the aluminum content of the heater material,
processing or by incorporation of alloying additions. For instance, the
resistivity
can be significantly increased by incorporating particles of alumina in the
heater
material. The heater material can optionally include ceramic particles to
enhance
creep resistance and/or thermal conductivity. For instance, the heater
material can
include particles or fibers of electrically conductive material such as
nitrides of
transition metals (Zr, Ti, Hf), carbides of transition metals, borides of
transition
metals and MoSiz for purposes of providing good high temperature creep
resistance up to 1200 C and also excellent oxidation resistance. The heater
material may also incorporate particles of electrically insulating material
such as
A1203, Y203, Si3N4, ZrO2 for purposes of making the heater material creep
resistant at high temperature and also improving thermal conductivity and/or
reducing the thermal coefficient of expansion of the heater material. The
electrically insulating/conductive particles/fibers can be added to a powder
mixture
of Fe, Al, Ti or iron aluminide or such particles/fibers can be formed by
reaction
synthesis of elemental powders which react exothermically during manufacture
of
the heater element.
The foregoing has described the principles, preferred embodiments and
modes of operation of the present invention. However, the invention should not
be
construed as being limited to the particular embodiments discussed. Thus, the
above-described embodiments should be regarded as illustrative rather than
restrictive, and it should be appreciated that variations may be made in those
embodiments by workers skilled in the art without departing from the scope of
the
present invention as defined by the following claims.
