NASA Technical Reports Server (NTRS) 19980202467: Thermodynamics, Solubility, and Diffusivity of Oxygen in Titanium and Ti-Al Alloys

12 Pages·0.3 MB·English

by  NASA Technical Reports Server (NTRS)

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THEP_ODYNAMICS,SOL_ILITY, XNDDIFFUSIVITY OF OXYGEN
IN TITANI1]I( ANDTi-A1 ALLOYS o r/7"-
Oopal _. _ehrotra
Department of Mechanical and Materials Engineering
Wright State University
Dayton, Ohio
Titanium aluminides and titanium aluminide-based composites are attractive
candidate materials for high-temperature structural applications, is these
materials may be exposed to oxidizing environments during their use at elevated
temperatures, it is essential thz_ they possess a good oxidation resistance.
Previous studies (ref. I} have shown that the oxidation resistance of Al-rich
alloys in the Ti-Al system is superior to that of the Ti-rich alloys. The
scales formed on the surface of the Al-ricb and Ti-rich alloys have been
reported (ref. I) to be predominantly AltOs and TiO_, respectively. Since the
relative stabilities of the oxides of _i and Ti ar various temperatures and
oxygen pressures can be assessed from their thermodynamic data, it is possible,
with the help of thermodyua_ic calculations, to determine the compositions o_
the alloys _hich would form scales of AltOs, TiOx or a ternary oxide such as
Ti_l_O_ during oxidation at a given temperature. The thermodynamic calculations
require reliable activity data for the Ti-AI system. These data have not been
determined for the entire composition and temperture range of interest. _sing
the da_ avzilabte in the literature, Ita_el and Spencer (re_. 2) recently
performed thermodynamic calculations and concluded that the stable oxide changed
from TiO to AI_O_ in the existence region of the TiAI phase.
In the case of titanium aluminid_based composites, another major concern
is the mutual chemical compatibility of the matrix material _ith the
reinforcement phase. Fibers of SiC, TiB: and AI_O_ are currently being
investigated for reinforcement of titanium aluminide matrices. Recent studies
(re_. 3) have shown that SiC is incompatible with TidAl. No detectable reaction
h_s been observed between TiB_ and Ti-43 at.Z A1 (a_*7) alloy at 1473_ (ref. 4).
7-TiAI has been repor$ed to be compatible with AI_O_. Eovever, _n a recent
study by _isra (ref. 5), a chemical reaction between ll_O_ and titanium
aluminides contzznzng less _han 50 at.Z A1 has been observed. _isra (re_. 5)
has found that the reaction between a Ti-AI alloy, (wish < 50 at.Z AI) and kl_O_
is tha_ of the dissolution of A1 and atomic oxygen from kl_Oz:
AI_O_ = 2AI + 3_ (i)
The above reaction results in an increase in the concentration of A1 in :he
surface of the Ti-AI alloy. The _ormation of 7-Till phase thus takes place.
Diffusion of A1 and atomic oxygen a_ay from the in_erface cruses the reaction to
proceed further.
For reaction (1) _o be _easible _he product {a_ 0 . a_A1 } (_here a_l is the
activity of AI and a0 is the activity of oxygen in the alloy), should be less
0O4
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than the equilibrium constant, Kt, for reaction (I). At high values of sTi
and/or a0 in the alloy, the following reaction may also take place:
Z£ + _ = TiO (2)
Thus, the feasibility and the extent of reactions between a Ti A1 alloy and
Al_Oa are influenced by the thermodynamic activities of various species in the
alloy, As there is little information available on the thermodynamics,
solubility and diffusivity of oxygen in Ti-Al alloys, it is important to
investigate the Ti-AI-O system.
The thermodynamics o( the Ti-O system has been studied by Kubaschevski and
Dench (ref. 6), and Miyazaki, et. al (ref. 7). The solubility of oxygen at
partial pressures of oxygen established by Ca-CaO and Mg-_gO equilibria has also
been determined in these studies. The phase diagra_ for the system shows that
the maximum solubility of oxygen in fl-Ti is about 8 at.Z and that in o-Ti is
31.9 at.Z. The diffusivity of oxygen in titanium has been estimated from
oxidation and internal friction studies. The activzzion energy for the
diffusion process has been reported (reg. 8) to be about 203 kJ/mole for a-Ti
and about 246 kJ/mole for fl-Ti.
It has been reported (ref. 9) that the additions of A1 to Ti result in a
decreased solubility of oxygen. However, the information on the thermodynamics,
solubility and diffusivity of oxygen in Ti-AI alloys is very limited. Attempts
are therefore being made to investigate the thermodynamics, solubility and
diffusivity of oxygen in Ti-A1 alloys usin_ electrockemical cells with
yttris-doped thoria as a solid electrolyte. H_gh reactivity of these alloys
with oxygen is, however, s problem in these measurements. Other approaches,
such as oxidation kinetics studies, may also therefore have to be tried to
determine the solubility and diffusivity of oxygen in Ti-AI alloys of various
compositions.
Acknowledgement
This project is supported by _ASA Craft No. NAG3-1233.
R_ferences
I. O. Velsch and A.I. Kehveci: Oxidazion Behavior of Titanittm Aluminum
Alloys. Oxidation of High Temperature Intermetallics, T. GrobsSein and J.
Doychsk, eds., The Kinerals, Metals and Materials Society, 1989, pp.
207-218.
2. A. RaVel and P.J. Spencer: Thermodynamic Aspects of TiAI and TiSi_
Oxidation: The AI-Ti-O and Si-Ti-O Phase Diagrams. Oxidation of Metals,
Vol. 35, Nos. I/2_ 1991, pp. 53-67.
3. J.-N. Yang and _ Jeng: Interracial Reaction Kinetics of SiC
Fiber-Reinforced TisAl Matrix Composites. Scripts _et., Yol. 23, 1989, pp.
1559-1564,
4. M. Saqib, I. Weiss, G.X. lehrotra, E. Clevenger, A.O. Jackson, and H.A.
Lipsitt: Microstructural and Therm_l Stability of a Ti-43AI Alloy
Containing Dispersoids of Titanium Diboride. To appear £n Met. Trans.,
1991.
5. j'4_'K. Misra: Reaction of Ti and Ti-Al Alloys _ith Alumina. Met. Trans.,
Vol. 22A, _srch 1991, pp. 715-721.
6. O. Kubzsche_ski"_'_ V.A. Dench: The _ree _nergy Dia_ra_ o_ the System
Titanium-Oxygen. J. Inst. Metals, Vol. 82, 1953-54, pp. S7-91.
U_-_u
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a
1 liyazaki, T. Oishi and K. Ono: Thermodynamic Properties of _;_en in
_he Alpha and Beta Titanium-Oxygen Alloys at 800-1200°C: 5th
International Conference on Titanium, Sept. I0-14, 1984, Munich. Titanium
Science and TechnoioEy, Vol. 4, G. Luetjering, ¥. Zwicker and V. Bunk,
eds., Deutsche Gesellschaft fuer _etallkunde, 1984, pp. 2657-2663.
C.J. Rosa: Oxygen Diffusion in Alpha and Beta Titanium in the Temperature
1
II_ge of 932o to 1142°C. _et. Trans.. Vol. 1, 1970, pp. 2517-2522.
9. A.W. Cbaze and C. Coddet: Influence of Alloying Elements on the
DissoluZiou of Oxygen in the _et_llic Phase during the Oxidation of
Titanium Alloys. I. _at. Sci., Vol. 22, 1987, pp. 1206-1214.
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OBJECTIVES
- THERMODYNAMIC STUDY OF TI-AL-O ALLOYS
- DETERMINE THE SOLUBILITY OF OXYGEN IN
TI-AL ALLOYS
RATIONALE FOR _ORK
Tz-AL-0
- THERMODYNAHIC DATA ON SYSTEH
USEFUL ZN PREDICTING THE PHASES FORMED
DURING 0XIDATZON
- CHEHZCAL COHPATIBILZTY OF AL,0 3
REINFORCEMENT PHASE WITH T_TANZUH
ALUHZNZDE MATRICES
(MZSRA)
ALz0 _ = 2AJ. + 3Q
+O=Tz0
APPROACH
- ELECTROCHEHICAL HEASUREMENTS USING
YTTRIA-DOPED THORIA AS SOLID
ELECTROLYTE
- 0XIDATZON STUDIES
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SIMPLIFIED Tz-AL-0 PHASE DIAGRAM. ISOTHERMAL
SECTION AT 1173 K. (FROM RAHMELAND SPENCER)
_008
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PHASE DIAGRAM FOR THE SYSTEM Tz-AL-0. ISOTHERMAL
SECTION AT 1273 K. (FROM GLAZOVA)
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SOLUBILITY OF OXYGEN IN TITANIUM
0-31.9
0-4
AL AND SI LOWER THE SOLUBILITY OF OXYGEN
(CHAZE AND CODDET)
- AT 700°C, ZN THE PRESENCEOF AL, THE
SOLUBILITY OF OXYGENDECREASESFROH
31.9 AT_ TO 0.3 AT_
- CR HAS NEGLIGIBLE EFFECT ON THE OXYGEN
SOLUBILITY
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ZN TZTANZUM AS A FUNCTION OF"
SOLUBILITY OF OXYGEN PRESSURE. (FROM MZYAZAKZ, ET AL.)
TEMPERATURE AND OXYGEN
Oll
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SOLUBILITY OF OXYGEN IN TITANIUH AS A FUNCTION
OF TEHPERATURE AND OXYGEN PARTIAL PRESSURE*
T(°C)
E(IUZLZBRZUM P0z(ATM) 0 ZN TZ (ATe)
POz ESTABLISHED BY
800 CA - CA0 1.2 X 10"sz 0.04
MG - MG0 1.9 x 10"s° 0.16
1000 CA0 - CA0 6.4 X 10 -42 0.05
2.7 0.98
HG - MG0 x 10 "3g
1200 CA - CA0 8.7 X 10-_s 0.08
MG - MG0 3.7 X 10 "31 5.34
TI - TI0 1.7 x 10"z9 36.2
ALPHA: 1/2 LN P0z = LN(_ O) + 10.3 - 70.500 (1150-1340K)
T
BETA: 1/2 LN P0z = LN(_ Q) + 12.1 - (1150-1600K)
T
* MIYAZAKI, ET AL.
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F V T OF 0X G N N TZTA U
0UACH-KAHZHURA, ET AL.
0 = 0,45 Exp {-_00 RTKj/MOL_)
r D= = 0.778 £xp (932-11420C)
I -48,600 CAL/_LS CH:S'_ i
RT
ROSA I
i
L
DB = 3.3 x 102 ExP (932-1142°C)
(-58,800 CAL/HOLER-T )
DECHAMPS
DO= 0.408 EXP {-47.040 CAL/MOL, E/
t RT /
er ..., _ TRANSZT]:0N AT 882°C
QUACH-KAHIHURA ROSA DECHAMPS
700°C 8.Z4 x 10-12 - - 1.11 x 1011
800 8.25 x 1011 - - 1.07 x 10"1°
900 5.58 x 10"_° 6.84 x 10"1° 3.65 x 10.9 1.7Z x 10.9
1000 2.79 x 10.9 3.52 x 10.9 2.65 x 10a 3.4Z X i0 .9