The mechanical properties of the superplastic AI- 33 Pct Cu eutectic alloy
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I.
INTRODUCTION
THE mechanical properties of superplastic alloys are usually characterized by a relationship of the form tr = Bk m
[I]
where o- is the steady-state flow stress, k is the imposed strain rate, B is a constant which incorporates the dependence on microstructure and temperature, and m is termed the strain rate sensitivity. It is a standard procedure in superplasticity to investigate the value of m by logarithmically plotting o- against ~. In general, the experimental data usually divide into three distinct regions, as illustrated schematically in Figure 1. At high strain rates in region III the value of m is low and typically ~0.5. A difficulty arises at low strain rates in region I because different sets of experimental data tend to show different trends. Thus, there are various reports for superplastic alloys of either a low value of m ~ 0.3 (curve A in Figure 1), an extension of region II to very low strain rates with m -~ 0.5 (curve B), or a high value of m --~ 1.0 (curve C). The problem of region I has been discussed in some detail for the Zn-22 pct A1 eutectoid alloy, Ill and it was concluded that the experimental evidence favored curve A in Figure 1 with m ~< 0.3. This conclusion was subsequently supported by three independent sets of experiments on the Zn-22 pct A1 alloy t2"3:] and also by calculations on the effect of concurrent grain growth on experimental data at low strain rates. [5.6] The problem of the precise value of m in region I has not been resolved for the superplastic A1-33 pct Cu eutectic alloy. Although many experiments have been conducted to
investigate various aspects of the superplastic behavior of this alloy, t7-351there is no u n a m b i g u o u s report of a region I at low strain rates with m ~< 0.3. On the contrary, early experiments by Rai and Grant, f~71using an as-extruded A133 pct Cu alloy with an initial grain size of 1.5/xm, showed that an erroneous region I with a low value of m may occur at low strain rates due to substantial grain growth in the relatively unstable microstructure. More recently, Kashyap and Tangri t35] attributed the appearance of an apparent region I with low m at 673 K to the occurrence of significant strain hardening in the stress-strain curves of AI-33 pct Cu at this temperature. Thus, there appears to be a significant difference in the behavior at low strain rates between the Zn-22 pct AI and the A1-33 pct Cu alloys. The present investigation was conducted to obtain detailed information on the mechanical properties of the A133 pct Cu eutectic alloy in an annealed condition, and especially to determine whether there is a genuine region I with a low strain rate sensitivity when there is very little grain growth.
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METALLURGICAL TRANSACTIONS A
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I I
z
I
J I
rr
m value - -
b
A 8
0.3
s
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-0,5
c ~t.o /.~/~'/~
B,, " ATUL H. CHOKSHI is Postdoctoral Research Engineer, Division of Materials Science and Engineering, Department of Mechanical Engineering, University of California, Davis, CA 95616. TERENCE G. LANGDON is Professor, Dep
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