Precipitation of Nb(CN) during high strain rate compression testing of a 0.07 Pct Nb-bearing austenite
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vacuum melted as a 22 kg laboratory heat and rolled to 16 mm diameter rod from which cylindrical compression specimens were machined. The specimens were reheated to 1300 ~ for 30 minutes in an inert atmosphere, and water quenched to room temperature. TEM studies indicated that all of the Nb was in solution after this treatment. The hot compression tests were run as follows: The specimens were first reheated again in the testing furnace for four minutes at 1300 ~ and deformed in uniaxial compression in the range of temperatures between 1250 and 800 ~ and at constant true strain rates of 2 and 12 s -j, using a modified MTS machine. 2~ True strains up to 0.9 were imposed and uniform deformation was obtained by using glass lubricants and a modified Rastagaev's technique. 22 Following deformation, the specimens were quenched in agitated iced brine. In all cases, temperatures below 600 ~ were reached within 0.75 second of the completion of the deformation. These times were measured by means of a thermocouple embedded in each specimen. In addition, selected specimens were exposed to the two successive reheating treatments and allowed to cool down to the deformation temperature but were quenched without being deformed. TEM observations in these reheated but undeformed specimens failed to show evidence of precipitation in the undeformed austenite. The specimens for TEM studies were sectioned parallel to the compression axis and thinned both mechanically and chemically before the final jet electropolishing. The foils were studied in a JEOL 100/120 U electron microscope operated at 120 kV. Precipitates at least as small as 2 nm could be detected in this study. That the precipitates had formed in the austenite was confirmed by using the orientation relationships. Details of this technique are given elsewhere. 23.24,25 The flow curves of the specimens were measured between 850 and 1250 0(2 at a strain rate of 2 s -L and are shown in Figure 1. Some measurements at 12 s -~ were also performed. The flow curves shown in Figure 1 indicate that a TRUE STRESS, MPa 280 r 0.07 Nb Reheated 1300~ ~ = 2 s -1 21 175 140
lO5 t
METALLURGICAL TRANSACTIONS A
/J
-- 975~ 1000~
j j
lo5o c
11oo c 1,5ooc 2oooc
70 1 35 r ~ - 1 2 4 5 ~ 0
G. FITZSIMONS, K. TIITTO, R. FIX, and A. J. DeARDO are all with the Department of Metallurgical and Materials Engineering, University of Pittsburgh, 848 Benedum Hall, Pittsburgh, PA 15261. Manuscript submitted June 7, 1983.
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0.25
0.50 0.75 1.0 TRUE STRAIN
1.25
Fig. I - - F l o w curves at a strain rate of 2 s -t for the 0.07 Nb-0.025 N austenite. Note continuous hardening below 1100 ~ VOLUME 15A, JANUARY 1984--241
Table I. Deformation Temperature, ~ 800,925 850 850 850
Strain (e) 0.9 0.9 0.6 0.9
Summary of Precipitation Observations Strain Rate (k) s ~ 2 2 2 12
well-developed peak occurred only in the specimen tested at 1245 ~ A peak in the flow curve is often interpreted to mean that dynamic recrystallization has occurred during the deformation. The presence of a peak is also suggested by th
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