Copyright©JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume 46.
285
ANNEALING STUDIES OF PURE AND ALLOYED TANTALUM
EMPLOYING ROCKING CURVES
David W. Richards1, Michael P. Kramer1, Joel W. House1
and Robert J. De Angelis2
1
Air Force Research Laboratory, Eglin AFB, Florida, 32542
2
University of Florida/GERC, Shalimar, Florida, 32579
Abstract
To control the mechanical response of pure and alloyed tantalum requires tailoring the microstructure
by carefully controlling thermo-mechanical processing. In this investigation X-ray rocking curves were
employed to characterize the microstructure and further the understanding of the mechanisms of
annealing. Plate material produced from the alloyed tantalum demonstrated annealing stages 300ºC
higher than the pure tantalum. Activation energies determined from the rocking curve data were onehalf to one third of the self-diffusion activation energy.
Introduction
Controlling texture and grain structure in unalloyed tantalum has been difficult [1,2]. Sources of the
inhomogeneous structure are ingot chemistry, ingot grain size and mechanical processing. Introduction
of high levels of plastic strain by rolling and forging prior to annealing only partially homogenize the
grain structure. Heterogeneous grain structures in components generate local variations in mechanical
properties and are potential sources for unanticipated failures.
The annealing response of forged tantalum, reported by O’Brien et al. [1], demonstrated that
significant differences in grain size, size distribution, and morphology existed in microstructures
obtained after the annealing of forged tantalum. In a recent study the effects of an intermediate
annealing cycle (before forging) on texture and the microstructure formation in forged and annealed
tantalum plates were quantified [3]. The current investigation examines the benefits of X-ray rocking
curves in studying the processes of annealing in tantalum and tantalum 2.5% tungsten.
Material and Thermo-Mechanical Processing
Chemical composition of the pure tantalum studied is listed in Table I. The material was purchased
from H.C. Starck Inc., Newton, MA. The composition of the tantalum 2.5% tungsten alloy was the
same except for the additional tungsten. Two 165 mm (6.5 inch) diameter ingots were produced at
Starck utilizing a process consisting of vacuum arc remelting of a dual electron beam processed
electrode ingots.
Table I. Composition of the tantalum. (PPM by weight)
O
72
N
11
C
13
H
1
Fe
24
Ni
29
Cr
11
Cu
1
Si
8
Ti
5
Mo
12
W
117
Nb
43
The cast ingots were turned and cold swaged to 63 mm (2.5 inch) diameter bars. The swaged bars
were sectioned into 86 mm (3.4 inch) long billets. The billets were upset forged in multiple steps with
the application of lubrication between each step to 228 mm (9-inch) diameter plates by 6.35 mm (0.25
inch) thick.
This document was presented at the Denver X-ray
Conference (DXC) on Applications of X-ray Analysis.
Sponsored by the International Centre for Diffraction Data (ICDD).
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286
The post-forge annealing study was performed on samples from two plates, one of each composition.
Employing a water-jet cutter, twenty specimens approximately two by two centimeters were sectioned
from each plate. These specimens were cut from a circle of material centered on a 64 mm (2.5 inch)
radius of both plates.
Hardness measurements were made on the surface of the plate. X-ray measurements and optical
observations were made on the mid-plane of specimens that were annealed for one hour in vacuum at
intervals of 50ûC at temperatures between 750 and 1300ûC for tantalum and 900 and 1500ûC for
tantalum 2.5% tungsten. The specimens were wrapped in tantalum foil and annealed in a furnace
under a vacuum of <10-5 torr.
Experimental
After heat treatment, hardness data were collected from the annealed specimens of tantalum and
tantalum 2.5% tungsten. Rockwell B hardness numbers as a function of annealing temperature are
shown in Fig. 1. Rapid softening of pure tantalum set in at 900ûC and annealing was complete at
1150ûC. For the alloyed tantalum softening started at temperatures above 1050ûC, was most rapid in
the 1150ûC temperature range and was complete at 1350ûC. The hardness increase observed in
specimens annealed at temperatures above 1350ûC indicated oxygen contamination occurred at these
higher temperatures.
The annealed specimens were sectioned on their mid-plane using a diamond saw. The split specimens
were mounted and metallographically polished using the technique described by Kelly et al. [4]. All of
the X-ray data was collected from the mid-plane surface of the annealed specimens employing copper
Kα radiation. The X-ray data were collected on a Siemens diffractometer outfitted with a split cradle
and a graphite diffracted beam monochromator. The X-ray beam geometry on the specimen at sixty
degrees two theta was one centimeter by one millimeter.
Rocking curve data were collected in steps of 0.02 degrees and two second counting times. Texture
data were collected in five degree steps of phi and chi to describe the {110}, {200} and {211} pole
figures. The pole figure data was processed employing the analysis software package popLA [5] to
calculate the inverse pole figure. Inverse pole figure data were analyzed using the techniques reported
in [6,7] to give the fraction of grains in the plane of the plate that have normals with orientations
within 12.5 degrees of the <111>, <110> and <100> directions. These quantitative texture results
Ta-W
Ta
80
60
40
20
750
850
950
1050
1150
1250
1350
1450
Annealing Temperature, C
Fig. 1. Rockwell B Hardiness of Tantalum and
Tantalum 2.5% Tungsten Versus Annealing
Temperature.
10
Density {hkl}, Times Random
Hardness, Rockwell B
100
{100}
8
6
4
{111}
2
0
700
800
900
1000
1100
1200
1300
Annealing Temperature, C
Fig. 2. Densities of Planes within 12.5 Degrees of
<111> and <100> in Tantalum Versus Annealing
Temperature.
10
600
8
500
Intensity, CPS
Density {hkl}, Times Random
Copyright©JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume 46.
6
{111}
4
287
As Forged
400
300
200
2
100
{100}
0
900
1000
1100
1200
1300
1400
1500
850 C
0
30
Annealing Temperature, C
35
40
45
50
55
60
65
70
75
80
Omega, Degrees
Fig.3. Densities of Planes within 12.5 Degrees of
<111> and <100> in Tantalum 2.5% Tungsten
Versus Annealing Temperature.
Fig. 4. (222) Rocking Curves of Tantalum in the
“As Forged” Condition and After Annealing at
850ºC.
from the pure tantalum and alloy plates were plotted versus annealing temperature and are shown in
Figs. 2 and 3 respectively. Texture strengths of the <110> components were less than 0.01 times
random and were not included in these figures.
During the initial stage of annealing of the pure tantalum (see Fig. 2) in the temperature range of
750°C to 900°C, where the softening rate is at its maximum, there is an increase in the {100)
component. In this same temperature range the {111} texture strength decreases by about a factor of
four. Annealing at temperatures of 900°C to 1100°C decreases the fraction of grains with {100}
orientation and increased the {111} component more than a factor of four. At annealing temperatures
above 1100°C preferred growth of the {111} grains occurs producing texture strengths greater than
eight times random and the magnitudes of the {100} components are below 0.3 times random.
The texture changes demonstrated by the alloyed tantalum (see Fig.3) during annealing were in general
similar to those observed in pure tantalum. The strength of the <111> component increases at higher
annealing temperatures and the <100> component decreases. However the magnitude of the changes
in texture strength during annealing were lower in the alloyed material.
2500
600
2000
950 C
Intensity, CPS
Intensity, CPS
500
400
1500
300
1000 C
1000
200
500
100
0
30
35
40
45
50
55
60
65
70
75
80
Omega, Degrees
Fig. 5. (222) Rocking Curve of Tantalum After
Annealing at 950ºC.
0
30
35
40
45
50
55
60
65
70
75
80
Omega, Degrees
Fig. 6. (222) Rocking Curve of Tantalum After
Annealing at 1000ºC.
Copyright©JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume 46.
288
Rocking curves of the (222) obtained from the pure tantalum in the “as forged” and after annealing at
850ûC are shown in Fig. 4. The (222) rocking curves for the tantalum after annealing at 950ûC and
1000ûC are shown in Figs. 5 and 6. Intensity spikes were observed to form on the rocking curve
obtained from the material annealed at 850ûC. These spikes are from poligonized regions of the
structure. These occurred in an annealing temperature range in which the hardness was just starting to
decrease. After annealing at 950ûC and 1000ûC the spikes increased in intensity, as the scale of the
ordinate in Figs. 5 and 6 indicate, signifying the formation of larger diffracting crystallites.
(222) Rocking curves from the tantalum 2.5% tungsten alloy in the “as forged” condition and after
annealing at 1050ûC are shown in Fig. 7. The (222) rocking curves for the alloyed material after
annealing at 1150ûC and 1500ûC are shown in Figs. 8 and 9. Intensity spikes of about one-tenth of
those at 1150ûC were observed to form on the rocking curve obtained from the alloyed material
annealed at 1050ûC. These spikes indicated that poligonization was occurring in the alloyed material
at a temperature 300ûC higher than in the pure tantalum.
7000
2000
Intensity, CPS
Intensity, CPS
6000
1500
1050C
1000
500
As Forged
1150 C
5000
4000
3000
2000
1000
0
0
30
35
40
45
50
55
60
65
70
75
80
Omega, Degrees
Fig. 7. (222) Rocking Curves of Tantalum 2.5%
Tungsten in the “As Forged” Condition and After
Annealing at 1050ºC.
30
35
40
45
50
55
60
65
70
75
80
Omega, Degrees
Fig. 8. (222) Rocking Curve of Tantalum 2.5%
Tungsten After Annealing at 1150ºC.
The averages of the intensity spikes present in the (222) rocking curves from the pure and alloyed
tantalum were determined at each of the annealing temperature. The averaging was accomplished by
subtracting the background intensity from a pattern, performing a peak search to identify seventy to
one hundred peaks, summing the intensity values at the peak maximums and dividing by the number of
peaks. Plots of the logarithms of the magnitude of the intensity spike averages versus the reciprocal of
the annealing temperature are shown in Figs. 10 and 11. Activation energies for the mechanism
controlling poligonization were calculated from the slopes of these plots. The activation energy was
determined to be 29,200 calories per mole for pure tantalum and 55,800 calories per mole for tantalum
2.5% tungsten. The energy value for pure tantalum, about one-third of the activation energy of selfdiffusion, is in the range of activation energy values for boundary diffusion [8]. The much greater
activation energy for boundary migration observed for the tantalum 2.5% tungsten indicates the
additional tungsten solute atoms reduce the mobility of the migrating boundaries significantly.
Ln of Average Intensity of Rocking
Curve Spikes
Copyright©JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume 46.
7000
Intensity, CPS
6000
1500 C
5000
4000
3000
2000
1000
0
30
35
40
45
50
55
60
65
70
75
8
6
29,200 Cal / Mole
4
2
0
0.6
80
289
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
1000 / Annealing Temperature, K
Omega, Degrees
Fig. 9. (222) Rocking Curve of Tantalum 2.5%
Tungsten After Annealing at 1500ºC.
Fig. 10. Logarithm of the Average Magnitude of
the Intensity Spikes on the (222) Rocking Curve
of Tantalum Versus the Inverse of the Annealing
Temperature.
Ln of Average Intensity of
Rocking Curve Spikes
8
6
55,800 Cal / Mole
4
2
0
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
1000 / Annealing Temperature, K
Fig. 11. Logarithm of the Average Magnitude of the
Intensity Spikes on the (222) Rocking Curve of
Tantalum 2.5% Tungsten Versus the Inverse of the
Annealing Temperature.
Discussion
There are two advantages in employing rocking curves to investigate the annealing behavior of
tantalum and its alloys. The most apparent advantage was the appearance of intensity spikes in the
rocking curve patterns. The lowest annealing temperature that was associated with the appearance of
the intensity spikes defined the annealing temperature that produced significant recovery of the
deformed substructure. Annealing at higher temperatures provided a method to study the crystallite
size growth kinetics. The second and less obvious advantage of the rocking curve technique is the
data it provides on the grain orientation relationships between the deformed structure and the
recovered or recrystallized material. Nucleation of the recrystallized grains in both tantalum materials
investigated took place within ten degrees of the center of the intensity distribution of the rocking
curves. The magnitude of the intensity spikes in this angular range is proportional to the strength of
the <111> component of texture. This is easily seen by comparing the intensities of the intensity scales
on the plots of the rocking curves with the texture strengths of the <111> shown in Figs. 2 and 3.
Copyright©JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume 46.
Acknowledgements
The financial support from the United States Air Force for RJD under contract FO8635-98-D-0016 is
acknowledged.
References
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