Observations of Orientation Dependent Creep of Ni 3 Al
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KK9.11.1 Mat. Res. Soc. Symp. Proc. Vol. 552 0 1999 Materials Research Society
EXPERIMENT Single crystal rods of single phase Ni 3A1 with the composition Ni 75.0, Al 22.0, Ti 1.0 and Ta 2.0 (at.%) were grown with a seeding technique (for further details see [10]) in different orientations near [001], [0111, [111], [557] and [012] (see Figure 1). The specimen orientations were verified using Laue back-reflection X-ray technique. From the rods tensile creep specimens with threads were prepared. The creep tests were performed in air under a constant load of 350 MPa at 1123K. In order to preserve the microstructure the tests were interrupted in different stages of creep by cooling under load. Transmission electron microscope (TEM) analysis was carried out using a Jeol 200C microscope and a Philips CM 30 microscope. Cross sectional and longitudinal TEM-foils were prepaired by cutting the rods and thinning by electro-polishing. RESULTS Figure I shows the creep curves of the different orientations. It can be seen, that the creep rates are initially high with a sharp decrease to a minimum at very small strains (dO. 1%). Thereafter a grad-
ual increase in strain rate within a small region of less than 1%creep strain is observed. This is known as inverse primary creep. For strains higher than 1%all orientations achieve a steady-state region. The [557] and the [012] directions differ slightly from the other orientations in the primary regime, the first is an orientation favoring single cube glide, the latter is close to a single octahedral glide orientation. The [001] orientation has the highest steady-state creep rate, whereas [111] has the slowest in agreement with Shah [5] and Hemker and Nix [6]. The [557] and the [012] steady-state strain rates are in between. We can conclude that the strain rate is decreasing over the orientation triangle from [001] to the [011]-[ 111]-line as indicated in Figure 1. Ni 3(A1TiTa)
1123 K, 35OMPa
[001] 10*20* 30* 40° t011]
S.[012
badb
10.[
0.0
strain
rat steady state
[111]
1.0
2.0
3.0
4.0
Snrain [%]
Figure 1. Creep curves of Ni3AI single crystals plotted as strain rate vs plastic strain. Orientations of the load axis are indicated inthe orientation triangle. Previously published work [10] shows the dislocation configurations of the [001], [011] and [1 11] orientations in the primary and secondary creep regime. In the primary regime mostly long straight screw dislocations glide on only few planes for all three load axis orientations. With higher deformation dislocation reactions take place, leading to the formation of nodes. The distribution of the dislocations for all orientations and creep stages is homogeneous throughout the gauge volume. Figure 2 shows the analysis of the slip systems for the [001] orientation. Due to the slight misorientation away from [001] two systems are preferred at the beginning of deformation (primary creep). At the secondary creep stage all possible slip systems are activated and some < 110>{ 110} -systems too. The number of passive systems (Sch
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