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use of any fºrmatºes, awareºue, methºd, sº presses élesleesd is tºe report. AGING OF COLUMBIUM ALLOY D-43 ºf on Elie IN THE RECEIVING SECTION:
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** -
Metals and Ceramics Division º
Oak Ridge National Laboratory
Oak Ridge, Tennessee
A study of the aging characteristics of columbium alloy D-43 is in
progress at Oak Ridge National Laboratory. D-43 is a second-generation
**s-, a
its relatively high carbon content benefits from a carbide dispersion
for strengthening. At least two carbide phases have been reported: one,
& high-temperature modification occurring above approximately 2900°F
identified as hexagonal Cb 20 and present in the form of platelets, and
the other occurring below this temperature identified as a cubic colum-
bium-zirconium carbide and present as a finely dispersed phase.
To illustrate the two carbide morphologies, Fig. l shows the micro-
structures developed in the alloy on heating for 2 hr over the tempera-
ture range of 2880–3300°F. At 2880°F it can be seen that the carbide
phase is predominantly the columbium-zirconium carbide with only slight
evidence of Cb2C platelets. On raising the temperature, the platelets
become predominant and the grain size increases appreciably just above
3OOO°F. • e
Earlier heat treating studies on the alloy by other investigators
strongly indicated an aging reaction associated with the conversion of
the high-temperature to the low-temperature carbide. Because of the
*Research sponsored by the U. S. Atomic Energy Commission under
contract with the Union Carbide Corporation.
profound influence of aging reactions on the properties of materials,
and our interest in this alloy as a structural material, we have under-
taken an investigation of the aging characteristics of the alloy over
the temperature range of l800–2600°F following high-temperature annealing.
Our interest here is in the kinetics of the aging reaction. We have been
using as the principal criterion for aging room-temperature elongation of
Vacuum heat-treated tensile specimens with a 1.0-in. long × 0.230-in.
wide x 0.040-in. thick gage section. All tests have been conducted at a
strain rate of 0.05 min".
To establish reference data prior to aging, we have determined the
effect of high-temperature annealing on the ductility of the material as
the microstructure goes through the change from the low-temperature to the
high-temperature form of the carbide. The results were found to be ex-
tremely dependent upon the cooling rate from the annealing temperature as
shown in Fig. 2. For a "fast" cooling rate the total elongation only de-
creased from about 32 to 20% between 2800 and 3350°F which can be attri-
buted to the increase in grain size and the presence of large amounts of
Cb 2C in the microstructure of specimens annealed at temperatures greater
than approximately 2900°F. On the other hand, a "slow" cooling rate
caused a sharp drop in ductility at about 3000°F which decreased to an
average value of 8.6% between 3150 and 3350°F. This sharp drop in duc-
tility has been attributed to aging of the material during the "slow" cool
from the annealing temperature. e
The microstructures developed in D-43 for both a "fast" and "slow"
cool from 3300°F are presented in Fig. 3. There is only a slight differ-
ence in these structures (optically) as compared to the relatively wide
difference in ductility. -
Figure 4 shows the furnace cooling curves responsible for the
results presented in Fig. 2. Based on microstructures developed as a
function of annealing temperature, that is, the presence of large amounts
of Cb2C for annealing temperatures greater than approximately 2900°F, and
the aging kinetics to be described later, the temperature range
2400–2900°F can be referred to as the "rapid-aging" range. The additional
time of approximately l min spent in this range during "slow" cooling has
caused aging to occur.
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Two techniques were used to prove the Sensitivity of the alloy to
aging during "slow" cooling. First, the "fast" cooling furnace was
**. * § ~3 º & Q º *
manually programmed to closely match the cooling rate of the "slow"
! !!
cooling furnace through the "rapid-aging" range in an effort to em-
brittle the alloy. This produced the expected result as indicated in
Table i. Second, specimens were "fast" cooled and then aged for 1/2 hr
at 2250 and 2300°F to determine the extent of any aging embrittlement.
These results presented in Table 2 show that the probable minimum in
ductility was reached at 2250°F, and overaging has taken place at 2300°F.
Figure 5 shows the overaging kinetics for D-43 over the tempera-
ture range of l800–2600°F. The specimens were "slow" cooled from the
annealing temperature of 3300°F thereby embrittling the specimens prior
to aging. As would be expected, the rate of recovery in ductility of the
alloy increased with increasing aging temperature. Initiation of recov-
ery required approximately 200, 50, and more than 5 hr at l800, 2000,
and 2200°F, respectively. At 2400, 2500, and 2600°F initiation of
recovery was rapid, and ductility was restored within 2 hr at these
temperatures. 25 e
Figures 6 and 7 include microstructures of the alloy "slow" cooled.
from 3300°F then aged at 2200°F. At this aging temperature the presence
Cf the columbium-zirconium carbide was apparent prior to the recovery in
ductility, and the ductility plateau was reached before stabilization of
the microstructure. º
As can be seen, this work has not completed the effort necessary
for satisfying the original objective of the investigation; but it has
defined parameters which must be considered in understanding the problem.
The key to characterizing the aging behavior of the alloy is in appre-
ciating the very significant influence of cooling rate from the annealing
temperature on the subsequent kinetics of the aging reaction.
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Table 2. Effect of Aging on the Room-Temperature
Tensile Properties of D-43
Heat Treatment: "fast" cooled from 3300°F (1815°C) followed by
l/2-hr aging treatments at indicated temperatures.
Aging Temperature Tensile Strength Yield Strength Total
- - - Elongation
(°F) (°C) (psi) (psi) - (%)
As-annealed - 74,400 54,000 21.4
2250 l232 72,000 57,100 6.7
2300 . l260 79,700 57,600 23.9
*-
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7.
. Y-6072l
. ORNL-Dºg 63-772
Y-6O722
ORNL-DWG 65-773
ORNL-DWG 65-774
Y-6O723
Y-6O724.
FIGURES
Microstructures of D-43 Annealed 2 hr at Indicated
Temperatures.
Effect of Cooling Rate from the Annealing Tempera-
ture Range of 2800–3350°F on the Room-Temperature
Ductility of D-43.
Microstructures of D-43 Annealed 2 hr at 3300°F
Followed by "Fast" and "Slow" Cooling.
Observed Cooling Rates from Approximately 3300°F
for Two Heat Treating Furnaces.
Recovery in Room-Temperature Ductility of D-43 by
Aging Between 1800 and 2600°F. After "Slow" Cooling
from 3300°F.
Microstructures of D-43 "Slow" Cooled from 3300°F
Followed by Aging at 2200°F for Indicated Times.
Microstructures of D-43 "Slow" cooled from 33OO°F
Followed by Aging at 2200°F for Indicated Times.
;