P1REPARATION AND PROPERTIES OF TITANIUM-BASE ALLOYS P. H. Brace, W. J. Hurford, and T. H. Gray' Westinghouse Electric Corporation, East Pittsburgh, Pa.
. T h i s paper presents the chief results of a n investigation of the preparation and the mechanical properties of a number of titanium-base alloys, with particular reference to their behavior in cast form a t high temperatures. The alloys were prepared by melting and casting in vacuum or under an atmosphere of highly purified argon in a specially constructed furnace system which, by rotation about a horizontal axis, transferred the metal from the inductively heated melting chamber to a mold t h a t was preheated by means of a n electrical resistance unit. The special apparatus for the purification of argon is describedin general terms and results are given of mass-spectrographic analyses of the purified argon. Experiences with the various refractories are outlined with particular reference to reaction between the melts and the refractories and the wetting of the latter by the molten alloys. Thorium oxide was the material of choice for crucibles. Various
materials were used for molds including graphite, alumina, and magnesium oxide. The design and manufacture of molds for producing cast specimens for tensile test are described. Quantitative tensile data for as-cast specimens are given for tests b y constant strain rate and creep-rupture methods a t several elevated tempera tures and some information is given concerning oxidation characteristics. Alloys containing more than 95% titanium were found t h a t exhibited surprisingly good resistance to oxidation in air u p to approximately 900" C. Among the more highly alloyed compositions the stronger showed yield strengths a t 950" C. comparable to those of such high-temperature alloys as No. 31 Stellite; because of the relatively low density of the titanium alloys, the ratios of yield-strength to weight were in some cases favorable. Many of the stronger alloys were brittle at room temperature, a disadvantageous characteristic.
T
of molten titanium when in contact with what were believed to be the more promising of the refractory oxides-namely, the oxides of aluminum, beryllium, and thorium. The results of that investigation were reported in 1948 (I), but are recapitulated here because of their importance to the present investigations. The general scheme of the tests was t o melt small slabs of titanium in vacuo while in contact with prefired compacts made from the respective oxides. Heating was effected in a specially constructed inductiontype vacuum furnace whose construction is indicated by Figure 1. The vacuum enclosure consisted of a fused-silica tube, A , approximately 5 inches (12.7 cm,) in inside diameter by 36 inches (91 cm.) long with smoothly ground end surfaces held between water-cooled brass end pieces, B and C. Neoprene gaskets fitted into grooves in the end plates formed effective vacuum seals under the axial compression exerted by the tie rods, E, that connected the end pieces. The heating chamber comprised a covered capsule, F, and sight-tube GI machined from compacts of sintered molybdenum. A filling of granular electrically fused thorium oxide occupied the annular space between the heating chamber and the fusedsilica tube, supporting the former and providing much-needed thermal insulation. Heating of the molybdenum was effected by induction from an axially-movable coil, H,outside the silica vacuum tube. Power was supplied at 9.6 kilocycles by a Westinghouse motor-alternator set. The temperature was controlled by adjusting the excitation of the alternator and measured by means of a Leeds & Northrup optical pyrometer sighted through a heat-resistant gless window at the top of the furnace. The furnace was evacuated by a Cenco Hypervac 20 mechanical pump in series with Westinghouse 4-inch diffusion booster and high vacuum pumps. Pressures obtained ranged from 1 X 10+ when cold to as much as 1 X 10-1 during rapid heating. The procedure was t o make disks, 1inch (2.54 cm.) in diameter and about 0.375 inch (0.95cm.) thick, of the oxides by moisten-
HERE has been a spectacular rise of interest in the metals
I
titanium and zirconium following the remarkable and extremely valuable results of the development work done under the sponsorship of the Bureau of Mines that has gone far toward making those metals available in quantity in a relativery pure state and as articles of commerce. It seems likely t h a t the advent of titanium as a commercial structural material may have far-reaching effects upon design, engineering, and performance, particularly in the fields of aeronautics and of rotating and reciprocating machinery, especially where high operating temperatures must be endured. The authors have been interested in the development of titanium alloys for structural purposes and this paper is a n attempt to summarize the more intriguing results of their exploratory investigations, with particular reference to alloys suitable for use at elevated temperatures. Because their interest was in hightemperature capabilities they have been more concerned with titanium alloys than with the metal itself, for as is now generally recognized, high-temperature strength of pure titanium is disappointingly low, although the hot ductility is amazing. At high temperatures the oxidation characteristics of the pure metal are not particularly favorable.
Refractories When a n alloy program involving temperatures as high as 1800" C. or more is undertaken it mn be anticipated that the problem of refractories will be a difficult one. And so it proved, because both the melting point and the heat of oxidation of titanium are relatively high and, furthermore, molten titanium has the property of dissolving large percentages of oxygen. It was known that the usual silicate-bonded refractory bodies would be quite useless; preliminary experiments had shown magnesium oxide to be unsuitable because of its volatility and, therefore, before undertaking the melting and casting program, experiments were made to learn something about the behavior 1
Present address, Boeing Aircraft Corporation, Seattle, Wash.
227
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 42, No. 2
!
Table I.
Figure 1. InductionTj-pe Vacuum Furnace
Relative Malleability of Specimens of VacuumFused Titanium
Mark Specimen A Original B Suspended globule
C D
Melted in contact with A1203 hlelted in contact with B e 0
E
Melted in contact with ThOz
Remarks Strong and malleable Strong and malleable but perhaps slightly leas ductile than A Extremely brittle, weak, and friable uite brittle, hut showed some 'strength Koticeablp harder and b u t slightly lass malleable than A b u t , nevertheless, comparable as to mechanical properties
TITANIUM
I
iOXIDE DISK
from thorium oxide for melting titanium and titanium-base alloys. From the theoretical angle the oxygen dissociation pressures were calculated and Figure 2 was prepared showing graphically the oxygen dissociation pressures as functions of temperature for a number of oxides. TEMPERATURE,^.
COURTESY E L E C T R O C H E M I C A L SOCIETY
ECU 20W153313031100 900 I
ing the finely divided materials, compacting in a steel die, and prefiring them in vacuum a t approximately 1700" C. in the vacuum system described above. The titanium, in the form of a rectangular block approximately 3 / i b inch (0.48 em.) thick was fitted loosely into a cavity machined in one face of an oxide disk. With the disk in place the system was evacuated and the temperature slowly raised until the titanium melted. Power was shut off shortly after the titanium melted while vacuum was maintained until the furnace was cold. By properly controlling the heating rate the pressures within the furnace could be kept below approximately 5 X 10-3.mm. of mercury as measured by a vacuum thermocouple gage. As a check on possible effects due to the residual atmosphere in the furnace, a short rectangular bar of material was suspended within the capsule and heated until the lower end melted and formed a drop held by surface tension to the lower end of the bar. Thus specimens were obtained that had been melted without contact with contaminants other than the furnace atmosphere. The mechanical properties of these were enough like those of the original material to justify the conclusion that the furnace atmospheres were so tenuous that any important changes in the properties of the specimens melted in contact with the oxides were due primarily to reaction between the molten titanium and the respective oxides. I t was found t h a t molten titanium reacted exothermally with aluminum oxide to produce a glass-brittle alloy that bore little resemblance to either of the metals concerned. In the case of beryllium oxide the reaction was less vigorous and the resultant alloy, although less fragile than t h a t resulting from the reaction of titanium with aluminum oxide, was quite a different substance than the titanium used for the test blocks. With thorium oxide the reaction was relatively slight although perceptible, as evidenced by the wetting of the oxide by the titanium while the latter was molten. However, the resemblance between the mechanical properties of the titanium melted on thorium oxide and those of the original material (Table I ) were close enough to encourage using crucibles made
I
I
l
l
8
700
500
I
I
Mo I
Figure 2. Calculated Theoretical Dissociation Pressures of -Metal Oxides Curves terminate at either boiling point of metal or melting point of oxide 0 Melting point of metal Boiling point of metal A Melting point or decomposition temperature of oxide
February 1950
INDUSTRIAL A N D ENGINEERING CHEMISTRY
229
Table 11. Calculated Equilibrium Concentration in Titanium at 1900' C. of Metals from Oxides Noted Oxide
Alios
Be0 ThOn
*
Equilibrium Concentration in Titanium, Mole Fraction 0.16 1 . 7 3 x 10-4 74 x 10-0
On the basis of those data it is surprising that titanium and aluminum oxide react so vigorously. However, the relative positions of the dissociation pressure curves for aluminum oxide, beryllium oxide, and thorium oxide are concordant with the experimental results noted above in that thorium oxide appears t o have the lowest oxygen dissociation p r e s s u r e of t h e three oxides and the least reactivity with respect to titanium. Table I1 shows the calculated equilibrium concent r a t i o n s of t h e metals from the oxides noted in titanium at 1900' when in c o n t a c t with the respective oxides. On the basis of Figure 4. Titanium Alloy Soaking Figure 2 and Table 11 it is surprising through Thoria Crucible into Thoria Refractory Sand that titanium had TCW-23A, 30Ti45Cr16.4Mo-8.6W; poursuch a strong reing temp., 1885' C.; scale, 1 inch ducing action on the oxides tested unless i t be postulated that the respective metals or the corresponding oxygen were being removed from the scene of action as by vaporization or otherwise. Although vaporization cannot be ruled out, i t is believed t h a t it played no important part even in the most spectacular case-namely, t h a t of aluminum oxideinasmuch as the action was so rapid t h a t a visible rise of temperature occurred as soon as the titanium melted, and the titanium became so highly alloyed t h a t the product bore no resemblance t o either of the metals concerned. Perhaps aluminum-titanium intermetallic compounds were formed with sufficient energy of reaction to remove effectively from circulation the reduced aluminum and thus give rise t o a trend opposite to that expected from the oxygen dissociation pressure data of Figure 2. The embrittlement of the specimen melted in contact with beryllium oxide suggests t h a t some transfer of beryllium occurred and that intermetallic compound formation may have played a part in this case also. With thorium oxide the situation appears t o be more nearly in line with what would be expected from the oxygen dissociation pressure data, although even here more thorium appears t o have passed into the titanium than might have been anticipated. However, i t was decided to base the melting program on crucibles made from high-purity thorium oxide. The authors' first successful crucibles were made by plastering the inner surfaces of cylindrical molybdenum capsules with a paste made from ground, electrically fused thorium oxide plasticized with a n acid-treated slip made by webgrinding fused thorium oxide in a porcelain ball mill. After air-drying and baking at approximately 300" C., firing was accomplished by inductively heating the molybdenum capsule in vacuo at approxi-
c.
4
Figure 4. Thoria Crucible (Wrapped with Molybdenum Sheet) after 12 Melting Runs
mately 2000" C. for a few hours. Such crucibles were used for vacuum melting a number of high-titanium alloys and were fairly satisfactory, provided the temperatures of the melts were kept within reason. Later on thoria crucibles were obtained from commercial sources. Some bodies resisted attack fairly well while others behaved toward the titanium alloy melts as a blotter t o ink (Figure 3). Although the authors succeeded in preparing approximately 50 titanium alloys in thoria crucibles, i t cannot be said t h a t the refractories problem is solved; only that the best of the crucibles were fairly satisfactory while the worst were quite hopeless. It appears t h a t the best results are to be expected from a dense body of high-purity thorium oxide fired at rather high temperatures, say 2000' C. (see Figure 4). Some attempts were made to use graphite crucibles, but under the conditions employed by the authors t h a t material was unsatisfactory.
Figure 5.
Vacuum Casting Furnace and "CrossCountry" Pumping System
AB meen f r o m pump end; after rehabilitation
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INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL
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42, No. 2
ALUMINA D I S K
~
COIL TERMINAL BLOCKS
N S P G R IS ,>,
=ii=JJ
BRASS S U P P O R T RING B R A S S TIE ROD MOLU S P E C I M E N P L U G S SILICA TUBE A L U M I N A REFRACTORY S A N D P R E H E A T I N G COIL
. INSULATING ROD MOLYBDENUM SHlELD -'MOLD
-MOLD
SPECIMEN
CUP
A L U M I N A DISK BRASS SUPPORT RING
Figure 8.
Melthag and Casting Apparatus and Procedares The melting program comprised two phases. I n the first a number of exploratory alloys were prepared by melting weighed charges in vacuo and simply allowing them to freeze in the crucible. From hot-hardness and oxidation tests inferences were d r a m as to the potentialities of the various nominal compositions with respect to serviceability a t temperatures in the neighborhood of 900" C. in air. I n order to obtain some more accurately quantitative information concerning the high-temperature tensile characteristics of a variety of titanium-base alloys the second phase of the program was undertaken and a considerable number of tensile specimens were c a s t f r o m titanium alloy melts by m e a n s of a n o v e l vacuum a n d cont r o 11e d - a t m o s p here induction-melting and casting system. The general arrangements of the system are shown in Figure 5. A twopart ' ' c l a m.-s h e 11' ' vacuum casing, approximately 30 inches in diameter, was attached to a header, g, arranged to allow rotation about a horizontal axis, while connection to the stationary parts of the vacuumsystem was maintained by the rotatable vacuum seal, h. T h a t casing enclosed t h e i n d u c t o r coil, melting assemblr, and mold a s s e m b l y . The outboard half of the casing ( 9 , Figure 6) w a s c a r r i e d o n it wheeled dolly so that free access could be had to the interior arrangements by s e p a r a t i n g Figure 7. Thermosiphon Arthe two elements of the gon Purifier with Sampling casing a t the doubleBulb in Plare
Mold Setup for Vacuum Casting Furnace
gasketed seal ( d , Figure 6 ) . The exhaust system comprised tb 4-inch multiple-stage metal-oil vapor high vacuum pump, d, a n oil-vapor booster pump, c, and a two-stage Kinney mechanical! forepump, a. When the interior of the vacuum casing was clean and a t room temperature the pumps would keep the pressure below l X 10-6 mm. of mercury as measured by a Philips-RCA ion gage. During melting and casting the pressures ranged< from approximately 1 X to 30 X mm. of mercury. With some metals, notably chromium, considerable vaporization occurred when vacuum conditions were maintained, particularly after melting was complete. By outgassing for a time without melting and then melting and casting under argon a t a pressure of a few millimeters of mercury the vaporization, with its attendant losses and uncertainties, could largely be avoided without having enough gas in the system to cause interference with the flow of metal into the mold when casting. An important adjunct to the melting system was a thermosiphon purification system, Figure 7, for providing a supplv of
L3 1 0%
12.9
27.4
0 24:, 0 805
6 2 20.1
0.6 0 35
0 60
1.5.0
1.0
From these results the conclusion can be drawn t,lla,t it is possible t o produce an alloy that, ns compared wit,h KO,31 d-tellite. has approximately the same hot ductility, only z/!3 the creep rate, nearly 5OC$ better strength-weight quotient (ratio), and more than 30 times the rupture life, and that from as-czst malerial! The effect of temperat,ure on one o f the titanium alloys is illiist,ra,t,edby t,he following t,abulat,ion for TCTa-1 and TCTa,-2.
Table V.
1500
..
*
12.7
I600 1750
5,7 4.3
R 5
8.61
~ u p i u r ostrain
Extension c/u TC'ra-1 TCTa-2 81.1 2.2.3 .. 33.7 99.6 41.0 7.1 2 46.3
niictilitv Oirotient. ?irijitiYreNtrain
2.8 4.G 8.2
Figures 19 and 23 show [,he appcar;-tnce of the tensile spccirnerir after fracture at elevated tempciatmcs in the L'OUIXC of the constant strain-rate tests. Noteworthy is {,he very great ductilit,p of the alloys TCW-2 and TCW-8 (Figurc 19)) for example, made evident by the large elongations and re:iuct,i?ns of area clearly visible in the picture. However, a t room t,nmperature, none of the alloys showed any great ductility. Perhaps it, can be said that, by virtue of the peculiarities of the titani-im crystal, an unusually large temperature coefficient of ductilit,y is characterist,ic of titanium and many of its alloys.
Cmdasion 1. Some 50 alloys based on titanium Tvere nielted by induction and cast into tensile t8estspecimens in a novel type of vacuum-casting furnace. 2. Thorium oxide was found the lnost satisfactory of the variQUS refractories tried and the only one of these t.hat could be used at all under the authors' melting routine. 3. Molten titanium and many titanium alloys were found to wet thorium oxide and in some cases to soak through the crucible
Mo
W
(-49receimd frnin
A G R A A A A I3
Ni
Fr
Co
Otherh
20 35 30
37.5 27.5 25 25 24.5 24.5 25
a
27.5
.4 A R
R
-1
A A
iC A A -4 A A .A A
TCTa-1 TCTa-2 TCTa-3
B a R
10 10
15
.
1.5 6.6 12.2 6
8.4
16.1
i3.1
11.5 16.4 16.4
8.6 8.6
16.1 19.7
l6:4
16.4 3 1 . 6 16.75 31.6 16.75 16.4 45 16.8 2 4 . 7 25 5 25 5
-4
Titanium Alloy .\If&! Co.)
10
,
30 30
~\
TCZ-1 TCZ-8 TCZ-4 TCZ-5 TCZ-6
. ... . . , .
20
.A C B R B
30
10
40 40 30 35 27.5
10 10
2% Bo 2% Be
8.4 10.3 17.5 8.6 8.6 7.95 7.95 8.6 11.9
... ..
..
15 15
17.5
..\
. ,
.
TC-4
B
60
TCCb-1
.A
70
20
'I'CCO- 1 TCCo-2
A
60 50
30 30
TCF-1
A
60
30
TCS-5
.4
65
30.
'I'CV-2
A
63
'30
TCTlr-1 TCTh-2
A
-z
10 Cb in 20
10 5
5v 1 Th 1 Th
A
.1
60
..
10
TTW- 1
d
60
..
..
30 Ta
Titanium Uloy ilIanufacturing C o . Titanium h s r . Bilreau of Mine8.
Table V I ,
30 TR
10
e Except a e otherwise noted, titanirirn powder, Bureau C
10Ta 10Ta 10Ta
40
TTLI-1
0 . 2 % Yi9ld 3trs-z TCTn-I TCTa-2 7 9s 13 1 2
17.4 17.0
Cr
Titanium Alloys
1R
T(T.4; T-3 TCW-2 T C W-2 TCW-8 TCW-9 T C W-10 TCWII T C W- 12 TCW-14 TCIT- 15 TCI5'- 15 T C W- 16 TCV-17 T C W-18 T C W-19 TCW-20 TCW-21 TCW-22 TCW-22 TCW-23 TCTV-26 ~cni-3 rCRI-8 TCM-10 TChI-11 TChI-11 'r c AT- 12 TCM-13 T ClI- 14
b
Test T ~ ~ , ~ 0.27, ~ , ,Yield Htresb a I?. TCTa-1 TCTa-2 L400 10.2 18.94
Nominal Compositions of Tension-Tested Light IBIetal Alloys
Specimen Mark Tia
Alloy No.
Vol. 42, No. 2
of Mincs.
Calculated Densities of Tension-Tested Light
Metal 4llogs \liov
(
Vo
T(T.4) T-3 T C I\'- 2
rcn7-s
TCW-9 C 10
w-
rcw-i 1
r c w-12 T C Y 14 ~~1~7.15 TCK-I 6 TCK-17 T C W- 18 T C W -19 TCW-20 TCW-2 1 TCW-22 TCW-23 TCW-26 TChI-3 TCM-8 TCM-10 TCM-11 Mild steel Copper
d
