Orientation Effect on the Stress Response by Strain-Rate Change at 400K in Ni 3 Al Single Crystals
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KK10.4.1 Mat. Res. Soc. Symp. Proc. Vol. 552 © 1999 Materials Research Society
Table 1. Al contents, tensile axes, and Schmid factors for (111) [101] of specimens
Specimen A B C D
Al content (at.%) 24.8 24.9 25.2 24.7
Tensile axis T 1.08 8.59 T 4.03 7.08 T 33.3 85 T 1.89 2.83
Schmid factor for (111)[ 1011 0.447 0.495
E
24.6
T 5.38 5.46
0.434
0.494
0.462
Tensile tests were performed with an Instron-type machine in a vacuum of 10-4 Pa. The test temperature was 400K at which the stress response by the strain-rate change was most typical among the temperatures previously studied [7,8]. After yielding, the cross-head speed was repeatedly changed between 0.5 mm/min and 0.005 mm/min that give initial strain rates of 8.3x10-4 sec-1 and 8.3x10-6 sec-, respectively. The load data were collected at a frequency of 2.5Hz or 5Hz with a computer. RESULTS Yielding behavior in all the orientations was similar to that previously reported [7,8]. That is, yielding gradually occurred without a yield drop, followed by linear work-hardening. In this linear work-hardening region after yielding, the cyclic strain-rate change tests were performed. Figure 1 shows the part of the stress response by strain-rate change for the different orientations. The stress and strain here are resolved on [T01](1 11). In the figure, the downward and upward arrows represent an increase (up change) and a decrease (down change) in strain rate, respectively. The stress response is peculiar, as we previously reported [7,8]. When strain rate is down-changed, the stress drops sharply. After showing a minimum, it spontaneously starts to increase. Note that it returns to the former stress level that is extrapolated from the curve before the down change (the dashed line in fig. 1). Then, it increases according to the dashed line as if there was no strain-rate change, which can be assumed a steady state. In this steady state the next up change test was performed. After the up change, the stress increases rapidly and then it gradually returns to the dashed line, similar to the down change. From the results, it turns out that the steady-state flow stress is independent of strain rate and that there is a temporary stress change right after the strain-rate change. This repeated itself in the wide range of strain. As shown in fig. 1, this stress response is observed in all the orientations, confirming the strainrate independence of the flow stress. We next estimate the orientation effect on the temporary stress change. For this purpose, we tried to express the stress change from the steady-state level, AT, as a simple function of time, t. Figure 2 shows a representative curve of Ar versus t for the down change. As mentioned above, after showing a minimum, A" returns to the steady state (the dashed line). Among some simple functions attempted, an exponential function is found to be the best fit to the curve, as shown by the dotted line in fig. 2. The function used is that AT= AT" exp (- ) where fitting parameters, Ar0 (MPa) and A (sec), represent initial stress
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