High-Temperature Creep of Nb-Al-V Alloys
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with previous investigations on this alloy [6]. The origins of this orientation relationship have been considered elsewhere [7]. Diffraction contrast images obtained from the samples indicated that there were virtually no dislocations in the A 15 precipitates. The density of dislocations in the B2 matrix was, however, rather inhomogeneous with quite high densities close to A 15/1B2 interfaces and lower densities away from these areas. The dislocations at the phase boundaries were presumably introduced during precipitation of the A15 due mismatch with the B2 matrix. Mechanicaldata
All of the creep curves showed predominantly steady-state creep commencing shortly after loading. The variation of the steady state creep rate with temperature at an applied stress of 120 MPa KK8.28.2
is shown in Fig. 2. Interestingly, the refined samples exhibited a slightly higher creep rate than the unrefined ones. The differences between them became less significant with decreasing temperature and at 1000°C, they exhibit a similar creep rate. From these data we estimate that the activation energy is approximately 430kJ/mol for both sets of samples. 1.OOE-3
0 Refimed samples
1473K 0
A Ustefine
I.OOE-4-
samples
1423K 1373K
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1.OOE-6•
I
I-
I.OOE-7
i
7.4E-4
6.4E-4
8.4.-4
I/Temperature (K) Figure 2. Plot of steady-state creep-rate (on a logarithmic scale) against 1/T for tests performed at 120 MPa.
IIRM-Z • M•
A
I
1.00&5-
0 ReS• suamples A Unreflne sample
I
L1OOE-41 11.ODE 4ý
1.OOE-6
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.
50
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100
.
tress (M)
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Figure 3. Plot of steady-state creep-rate against stress (both on logarithmic scales) for tests performed at 12000C.
The variation of creep rate with applied stress at a constant temperature of 1200'C is shown in Fig. 3. Here again, it is clear that the refined samples exhibit higher creep rates, particularly at lower KK8.28.3
stresses, i.e. 80 MPa. We note that since the creep rate does not vary monotonically with stress, the stress exponent n is changing from higher to lower values as the stress rises. From the limited data in Fig. 3 it would appear that for the unrefined samples n changes from 5.5 to 2 as the stress rises from 80 to 240 MPa whereas for the refined samples n changes from 3.5 to 1.5. Deformation microstructure Examination of the deformation microstructures using TEM revealed that the samples con-
tained high densities (109 cm-2) of dislocations which were distributed rather inhomogeneously. Most
of the dislocations were present in the B2 matrix and, particularly at the lower stresses (i.e. 80 MPa -
100 MPa), the density of dislocations in the A 15 precipitates was extremely low even at creep strains of 10%. At higher stresses (i.e. 120 and 240 MPa) occasional dislocations were observed within the A15 precipitates but in all cases the densities of these were very much lower than those in the B2 matrix. A typical micrograph of the deformation microstructure is shown in Fig. 4 which was obtained from an unrefined sample which had been deformed to 11% plastic strain
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