The metastable beta-Ti alloy Beta-21S, Ti-15Mo-2.7Nb-3Al-0.2Si (weight percent), has been proposed as a candidate for use in metal matrix composites in future hypersonic vehicles. The present study investigated the oxidation behavior of Beta-21S over the temperature range 600 C to 800 C. Oxidation weight gain was evaluated using thermogravimetric analysis. Oxidized specimens were evaluated using x ray diffraction techniques, scanning electron microscopy, energy dispersive x ray analysis, and electron microprobe analysis to identify oxidation products and evaluate oxidation damage to the alloy

Clark, Ronald K.

Wallace, Terryl A.

Wiedemann, Karl E.

English

NASA Technical Reports Server

NASA Technical Memorandum 104217
/I"/ - ;
Oxidation Characteristics of Beta-21S
Temperature Range 600 to 800°C
in Air in the
Terryi A. Wallace
Ronald K. Clark
Karl E. Wiedemann
(NASA-TM-104217) OXI_AT[ON CHARACTERISTICS
OF BETA-21S IN AIR IN [HE TEMPERATURE RANGE
600 TO 800 C (NASA) 10 p CSCL IIF
G3/26
N? 2-2240 /'
Uncles
008fi185
March 1992
N/L A
National Aeronautics and
Space Administration
Langley Research Center
Hampton, Virginia 23665
https://ntrs.nasa.gov/search.jsp?R=19920013164 2020-03-17T12:03:18+00:00Z

OXIDATION CHARACTERISTICS OF BETA-21S IN AIR
IN THE TEMPERATURE RANGE 600 TO 800°C
T. A. Wallace*, K. E. Wiedemann**, and R. K. Clark*
*NASA Langley Research Center
Mail Stop 194, Hampton, Virginia 23665-5225
**Analytical Services and Materials, Inc.
107 Research Drive, Hampton, Virginia 23666
Abstract
The metastable _-Ti alloy Beta-21S , Ti-15Mo-2.7Nb-3A1-.2Si (weight percent), has been
proposed as a candidate for use in metal matrix composites in future hypersonic vehicles.
The present study investigated the oxidation behavior of Beta-21S over the temperature
range 600 to 800°C. Oxidation weight gain was evaluated using thermogravimetric analysis.
Oxidized specimens were evaluated using x-ray diffraction techniques, scanning electron
microscopy, energy dispersive x-ray analysis, and electron microprobe analysis to identify
oxidation products and evaluate oxidation damage to the alloy.
Introduction
Current designs for future hypersonic vehicles call for the use of titanium-matrix composites
for many components, consituting as much as fifty percent of the structural weight. This
will require a titanium alloy which has good oxidation resistance, is easily producible as
foil, and has good high temperature properties in the temperature range 600 to 800°C.
This has required the development of new titanium alloys with an acceptable balance of
these properties. One alloy developed for use in this application is the metastable _-Ti
alloy Beta-21S, Ti-15Mo-2.TNb-3A1-.2Si (wt. %).
The purpose of the present st.udy was to characterize the oxidation behavior of Beta-
21S in the temperature rangc 600 to 800°C. Oxidation weight gain was determined by
oxidizing samples in air using a thermogravimetric apparatus (TGA). Selected samples
were analyzed metallographically to characterize the oxide composition and morphology
and the microstructural changes in the metal.
Experimental Procedures
Specimens and Materials. The as-received Beta-21S material was produced from a triple
melted ingot by forging to a 12.7 cm (5 inch) slab, hot rolling to a nominal thickness of
0.406 cm (0.160 inch), and cold rolling to 0.051 cin (0.02 inch) thickness using intermediate
vacuum anneal steps. The chemical analysis of the Beta-21S materials in weight percent
was 15.8 Mo, 2.95 A1, 2.88 Nb, 0.23 Si, 0.026 C, 0.114 O, and 0.005 N with the balance
being Ti.
TGA sampleswere machined to a width of 1 cm, and a length of 1.5 cm with a 0.16 cm
diameter hole through the thickness at one end for suspending the sample during the test.
Sample surfaces were ground to a uniform finish using silicon carbide paper through 1200
grit. Samples were first detergent cleaned, then ultrasonically cleaned in acetone and ethyl
alcohol, and finally air dried. The dimensions of finished samples were measured to the
nearest 0.0001 inch and weights were recorded before and after exposure to the nearest
0.01 mg.
Oxidation Tests. Oxidation tests were conducted in high purity air (true hydrocarbon
content less than 0.1 ppm) from 600 to 800°C for times up to 100 hr. The sample weight
change was recorded continuously during exposure using a Cahn C2000 microbalance with
an accuracy of I _tg. Samples were suspended by quartz fibers in a mullite tube inside a
vertical tubular furnace, and the temperature was monitored continuously with an R-type
thermocouple located just below the suspended specimen. A low flow of air (1 cc/sec)
through the mullite tube was maintained during the test. Duplicate or triplicate tests
were performed at each temperature.
Characterization Studies. Specimens were examined before and after oxidation using scan-
ning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray
diffraction (XRD) to characterize the microstructure and identify oxide phases. The distri-
bution and morphology of the oxide phases were determined by examining cross-sectioned
specimens using SEM, EDS, and electron microprobe analysis. These specimens were
cross-sectioned after mounting in epoxy, then polished and lightly etched using Kroll's
reagent (2 % HNO3, 1% HF), and finally coated with a thin layer of gold to minimize
charging.
Results
Oxidation Kinetics. Table 1 summarizes the results for the oxidation exposures of Beta-
21S over the temperature range 600 to 800°C. The total weight gains were obtained from
comparing initial and final specimen weights. Analysis of the weight gain data obtained
from the TGA showed that the weight gain increased parabolically with respect to the
time of exposure, as would be expected for a titanium alloy in this temperature range (1).
This means that the weight gain can be expresscd as w 2 = kpt where w is the weight gain
per unit area, kp is the parabolic rate coefficient, and t is the time of exposure. Calculated
values of k_ for Beta-21S are presented in table 1.
Figure 1 is an Arrhenius plot of the parabolic rate coefficients calculated for Beta-21S.
The activation energy for oxidation was calculated to be 41.2 kcal/mol from the parabolic
rate coefficients. Also shown for reference is data for commercially pure (c.p.) titanium
(2), and the Wi3Al-based alloy Wi-14A1-21Nb (wt %) (3). This analysis shows that over
the temperature range investigated, Beta-21S has better oxidation resistance than c.p.
titanium but oxidizes more rapidly than Ti-14A1-21Nb.
Oxidation Products. The results from XRD analysis of as-received and oxidized Beta-21S
specimens are shown in table 2. The as-received material consisted exclusively of _-Ti,
while some a-Ti was identified after all oxidation exposures. TiO2 (rutile) was the primary
Table 1. Thermogravimetric Analysis Results For Beta 21-S
Exposure Conditions
Temp,°C Time,hr
Total Weight
Gain,
mg/cm 2
Parabolic Rate
Coefficient
mg2/cml/hr
600 72.5 0.204 0.000575
650 70.8 0.364 0.00187
700 71.8 0.762 0.00801
700 111.5 1.01 0.00922
750 24.1 0.809 0.0271
800 29.8 1.55 0.0808
800 100.1 2.92 0.0852
oxide identified after exposure at all temperatures examined. Trace amounts of A12 03 were
also identified after exposure at temperatures above 600°C.
Figure 2 is an SEM micrograph of the cross-section of a specimen exposed 100 hr at 800°C.
The surface oxide consists of two regions; a discontinuous top layer, and a relatively thick
compact (void-free) layer. EDS analysis of the top layer showed the presence of Ti, A1, and
O; while analysis of the middle layer showed the presence of Ti, O, and a small amount
of Nb. Combined with the XRD analysis results (table 2), these scans suggest that the
surface oxide is primarily TiO2, with a mixture of A1203 and TiO2 at the surface.
The oxidation of titanium alloys, however, is known to proceed by the dual processes of
oxygen dissolution and oxide formation. The total weight gain, therefore, represents the
sum of the oxygen consumed in the formation of the surface oxide and the oxygen that
diffuses into the metal. Metallurgical analysis of the oxidized specimens is necessary to
allocate the total weight gain between these two processes.
Measurement of oxide thicknesses can be used to estimate the amount of oxygen consumed
in oxide formation. Since the oxide on Beta-21S has been shown to be primarily TiO2, the
oxide thickness can be used to estimate a weight gain per unit area using the theoretical
density of oxygen in TiO2 (1.7 g/cm3). Subtracting this weight from the total measured
oxygen weight gain also allows an estimation of the amount of oxygen that has diffused
into the substrate.
The results of this analysis for several oxidized Beta-21S specimens are presented in table
3. These results show that there is measureable weight gain due to oxide formation.
Comparison with the total weight gain data, however, shows that oxide formation only
accounts for 70 percent of the total oxygen reacting with the metal. This means that there
is also an appreciable amount of the oxygen diffusing into the metal.
Metallurgical examination of the oxidized Beta-21S specimens should confirm the fact that
3
!iiiii}iiiiiii:ili_!ii!
Substrate
Figure 2 -SEM Micrograph of B_ta-21S Exposed at 800_C for i00 hr
4
ORIGINAL ""
BLACK AND WHITE PHOTOGRAPH
Table 2. Oxide Composition Of Beta-21S From XRD
Measured Intensities 1
Exposure /_-Ti _-Ti TiO2 A1203
as-received VS -
600°C, 72.5 hr VS VS W
700°C, 71.8 hr S S VS
800°C, 29.8 hr W W VS
800°C, 100.1 hr W W VS
W
W
W
I VS=very strong, S=strong, W=weak
Table 3. Oxide Thickness and Partitioning of Oxygen
Exposure Condition
Temperature, °C Time, hr
Oxide Weight Gain,
Thickness, mg/cm 2
#m Oxide Metal
600 72.5 0.84 0.14 0.061
700 71.8 3.2 0.54 0.22
800 29.8 5.8 0.99 0.56
I VS=very strong, S=strong, W=weak
oxygen diffusion is occuring since oxygen is a known a-Ti stabilizer and should produce
some microstructural changes. Figure 3 shows the resulting microstructure after various
oxidation exposures. Figure 3(a) shows the microstructure after exposure at 800°C for 30
hr, (b) 800°C for 100 hr, (c) 700°C for 70.8 hr, and (d) 600°C for 72.5 hr. All specimens
consist of a mixture of a-Ti and fl-Ti, with an increase in the tendency to form Wid-
manstatten a-Ti as the temperature is decreased. Separation of the affects of oxygen and
thermal exposure in this temperature range is difficult, however, since the _-Ti transus for
Beta-21S is roughly 815°C (4).
Close examination of these cross-sections, however, does indicate an increased concentra-
tion of c_-Ti near the metal surface, particularly after exposures at 700°C and 800°C.
Figure 4 plots the fraction percent _-Ti (calculated using the point count method: ASTM
E562) as a function of distance from the oxide-metal interface for specimen exposed 30 hr
(c) (d)
SEM ___ ofmt,,-21SZ_:,_ ,,t(,,)SO0°C_o,30_,_,
1® _,(c)_,_Cfo,Z2_,_a (a)soo°cfor-12.5h_.
6 ORIG IN.."_L _''i "__-"
E]LACK AND WH_rE FHOI'OGRAP_
Q 100 hr50-
\ x
20 •
10
r v v _
0
Z
0
I--
0
<
ri-
LL
0 30hr
(p _ _ _...•_ • •
• .......
0 -0
Specimen CenterXx_
I .... I .... I .... I ' '
50 100 150 200
DISTANCE FROM OXIDE-METAL INTERFACE, p.m
r
250
Figure 4 - Fraction Percent er-Ti as a Functions of Distance from the Oxide-
Metal Interface for Beta-21S Exposed at 800°C.
and 100 hr at 800°C. For the specimen exposed for 30 hr there is approximately 50 percent
a-Ti at the specimen surface decreasing to a constant value of approximately 10 percent
100/_m from the oxide-metal interface. After 100 hr exposure at 800°C, however, the per-
centage of er-Ti is higher throughout the entire specimen, decreasing to about 20 percent
a-Ti at the center of the specimen. Although percent a-Ti is not a direct measurement
of oxygen content, the increased levels of ot-Ti near the surface do indicate that there is
a concentration gradient of oxygen into the metal, extending across the entire specimen
aftcr longer term exposures.
Discussion
The oxidation behavior of Beta-21S, like that of other titanium alloys, is difficult to define
precisely due to the number and complexity of operative oxidation mechanisms. For in-
stance, for c.p. titanium it has been shown that oxidation proceeds by simultaneous growth
of a surface oxide of TiO2 and oxygen dissolution into the metal. At low temperatures
(below 650°C) the oxidation rate is controlled by the growth rate of the TiO2, producing
an activation energy between 25.4 kcal/mol and 27.4 kcal/mol. At higher temperatures,
the activation energy changes to that required for diffusion of oxygen into the metal (2,5).
Analysis of the oxidation kinetics and metallurgical characterization for Beta-21S, however,
does allow some important insights. For instance, if the surface oxide layer of TiO2 that
forms on Beta-21S was rate controlling, the activation energy should be similar to that
for c.p. titanium and other titanium alloys. The activation energy for the oxidation of
Beta-21S, however, was calculated from the parabolic rate coefficient to be 41.2 kcal/mol.
This indicates that oxide formation does not control the oxidation kinetics, and suggests
that oxygen dissolution into the metal also must be considered.
The fact that measureable oxygen dissolution occurs is confirmed by metallurgical exami-
nation of the oxidized Beta-21S specimens. Figure 3 and figure 4 indicates that there there
is an increase in the percent of er-Ti near the specimen surface after oxidation exposure,
suggesting the presence of an oxygen concentration gradient in the metal. In fact, after 100
hr exposure at 800°C, the percent alpha is increased even at the center of the specimen,
suggesting that oxygen has diffused throughout the entire specimen thickness.
This analysis of the oxidation behavior of Beta-21S has shown that, although oxidation
rates for that alloy are lower than that for other titanium alloys, there are still measureable
oxidation weight gains in the investiaged temperature range. While some of this weight
gain is due to oxide formation, an appreciable amount of this oxygen weight gain is due to
oxygen diffusing into the metal. The increase in amount of a-Ti as a result of this oxygen
diffusion could have significant effects on the mechanical properties of Beta-21S.
Conclusions
Analysis of the oxidation behavior of Beta-21S over the temperature range 600 to 800°C
shows that it does have better oxidation resistance than c.p. titanium, but poorer resis-
tance that titanium aluminides. While Beta-21S forms a compact layer of TiO2, the data
suggests that oxygen dissolution into metal is appreciable. Examination of the microstruc-
tures after exposure showed an increase in the amount of tr-Ti with length of exposure,
which should be linked directly,with the diffusion of oxygen into the metal. This means
that in the temperature range 600 to 800°C, exposure to air may severely degrade the
mechanical properties of Beta-21S.
References
1. Kofstad, P.; Anderson, P. B.; and Krudtaa, O. J.; Oxidation of Titanium in the Tem-
perature Range 800-1_00 ° C. J. Less-Common Metals, vol. 3, no. 2, Apr. 1961, pp. 89-97.
2. Unnam, J.; Shenoy, R. N.; and Clark, R. K.; Oxidation of Commercial Purity Titanium.
Oxidation of Metals, vol. 26, Nos. 3/4, 1986, pp. 231-252.
3. Sankaran, S. N.; Clark, R. K.; Unnam, J.; and Wiedemann, K. E.: Oxidation Charac-
teristics of Ti-14Al-_1Nb Alloy. NASA-TP 3012, 1990.
4. Bania, P. J.: Beta-21S : A High Temperature Metastable Beta Titanium Alloy. Pro-
ceedings of the 1990 International Conference on Titanium Products and Applications,
Titanium Development Association, 1990.
5. Stringer, J.: The Oxidation of Titanium in Oxygen at High Temperatures. Acta
MetaUurgica, vol. 8, Nov. 1960, pp. 758- 766.

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Oxidation Characteristics of Beta-21S in Air in the
Temperature Range 600 to 800°C
6. AUTHORS)
Terryl A. Wallace
Ronald K. Clark
Karl E. Wiedemann
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
NASA Langley Research Center
Hampton, VA 23665-5225
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National Aeronautics and Space Administratlon
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Wallace and Clark: NASA Langley Research Center, Hampton, VA
Wiedemann: Analytical Services & Materials, Inc., Hampton, VA
Presented at 7th World Conf. on Titanium, San Die_o, CA, June 28-Jul_ 2, 1992
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13. ABSTRACT(Maximum2_ words)
The metastable B-Ti alloy Beta-21S, Ti-15Mo-2.7Nb-3Al-.2Si (weight percent), has
been proposed as a candidate for use in metal matrix composites in future hyper-
sonic vehicles. The present study investigated the oxidation behavior of Beta-21S
over the temperature range 600 to 800°C. Oxidation weight gain was evaluated using
thermogravimetric analysis. Oxidized specimens were evaluated using x-ray diffrac-
tion techniques, scanning electron microscopy, energy dispersive x-ray analysis,
and electron microprobe analysis to identify oxidation products and evaluate oxida-
tion damage ot the alloy.
14. SU_ECTTERMS
titanium alloys
oxidation
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OF REPORT OF THIS PAGE OF ABSTRACT
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Oxidation characteristics of Beta-21S in air in the temperature range 600 to 800 C

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