An examination of creep data for an Al-Mg composite
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G LI, Research Assistant Professor, and TERENCE G. LANGDON, Professor, are with the Departments of Materials Science and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1453. Manuscript submitted January 20, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
straight, rather than curved, lines. However, it has been demonstrated that this procedure may be inconclusive in those experiments where the experimental data cover only a limited range of strain rates.[11,12,13] Specifically, Cˇadek and Sˇustek[12] noted that an unambiguous determination of n requires data covering not less than five orders of magnitude of strain rate. Very recently, an alternative approach was suggested for those sets of data in which it can be shown, by making a limited extrapolation, that the plots of εz vs s are essentially vertical at a strain rate of 10210 s21.[14] This procedure, which sets s0 equal to the level of stress extrapolated at 10210 s21, has the dual advantages of requiring no a priori selection of the value of n and permitting a subsequent determination of n by logarithmically plotting the data in the form of εz vs (s 2 s0). In order to use this alternative procedure with the present data for the Al-Mg composite, it is first noted, by inspection of Figure 1, that there is a distinct change in slope in the plot of εz vs s for a temperature of 573 K at strain rates below ;3 3 1028 s21. Therefore, it appears appropriate to attribute these points to region I. Furthermore, this classification of the data is justified when it is noted that the apparent stress exponent at 573 K changes from ;25 at strain rates slightly above 3 3 1028 s21 to ;4 at strain rates slightly below this strain rate. The creep datum points attributed to region II in the present analysis are replotted in
Fig. 1—Creep data for Al-4 wt pct Mg-10 vol pct SiC(p) composite,[1] showing division into two regions. VOLUME 28A, MAY 1997—1271
Fig. 2—Strain rate vs stress for Al-4 wt pct Mg-10 vol pct SiC(p) in region II, showing extrapolation to 10210 s21.
Fig. 3—Strain rate vs effective stress for Al-4 wt pct Mg-10 vol pct SiC(p) in region II.
Table I. Estimated Values of the Threshold Stresses for Al-4 Wt Pct Mg-10 Vol Pct SiC( p) Using Two Different Procedures
s0 (MPa) T (K)
Stress at 10210 s21
Linear Extrapolation (n 5 3)
573 623 673
57.0 47.5 41.8
58.0 47.4 42.3
Figure 2, and the lines are extrapolated vertically to a strain rate of 10210 s21 to give the values of s0 documented in the second column of Table I. Using these values of s0, Figure 3 shows a logarithmic plot of εz vs (s 2 s0), thereby demonstrating that the datum points at each temperature are well fitted by straight lines that have slopes lying in the range of ;2.8 to 3.1 and with an average slope close to 3.0, which is similar to the value of n reported for creep by the mechanism of viscous glide in dilute Al-Mg alloys.[15,16] Then, taking n 5 3.0, the true activation energy for creep may be determined by constructing, for a selected constant value of (s 2 s0
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