Oscillations in load observed during
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Oscillations in Load Observed during High-Temperature Low Strain-Rate Testing of Superplastic Materials
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F. LI and P.L. B L A C K W E L L The phenomenon of superplasticity is associated with materials which have a flow stress with a high strainrate sensitivity. Such high strain-rate sensitivities are generally obtained at high homologous temperatures (>0.5T~) and slow strain rates. Researchers working in this area have occasionally reported a rapid oscillation of the load during deformation of materials in the superplastic regime. Various proposals have been put forward to explain this effect. In an investigation on aluminum alloy 7475 (A1-Zn-Mg-Cu), Ghosh m considered that the load serrations were related to dynamic recrystallization which was observed within the superplastic temperature and strain-rate regime, the effect being more visible when the grain size became larger. With the same alloy, Ghosh and Raj 12j studied the oscillations observed at 516 ~ within the strain-rate range from 1 0 -3 S - t to 5 • 10 -3 S - I and developed a model to describe the phenomenon. They commenced with the premise that a polycrystal contains a range of grain sizes. The finer grain size volume fraction was assumed to deform by diffusional creep and to undergo strain-assisted grain growth. The coarser grains were taken to deform by dislocation creep and eventually undergo refinement by dynamic recrystallization. The serrations in the stressstrain curve were attributed to changes in the grain size distribution produced by concomitant grain refinement and grain growth during deformation. Similar experimental observations were made by Hamilton et al. ,[31 again using AA7475. Serrations in the stress-strain curves were observed at 482 ~ and 516 ~ at strain rates of 10 -2 S - l and 5 x 10 -3 S - I , although they were not observed at lower strain rates. The authors in this instance attributed the effect to dynamic strain aging. Unfortunately, none of the above authors included details of the testing equipment used in their investigations. A further possible cause, however, is the test machine itself. During research concerned with the hightemperature ( > 5 0 0 ~ slow strain-rate ( < 10 2 s ~) deformation of AI-Li and A1-Mg alloys, it was observed that the load exhibited rapid fluctuations. The amplitude of the fluctuations was found to vary according to the strain-rate sensitivity of the material under testing (see for comparison Figures l(a) and (b)). The machine used for these tests was a twin screw-driven Instron 1195. The possibility of dynamic recrystallization in such aluminum alloys deformed under these conditions was F. LI, Research Student, and P.L. BLACKWELL, Research Fellow, are with the IRC in Materials for High Performance Applications, University of Birmingham, Birmingham B15 2TT, United Kingdom. Manuscript submitted December 4, 1991. METALLURGICAL TRANSACTIONS A
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(b) Fig. 1 - - L o a d oscillations obtained during testing of an AI-Mg alloy (a) under conditions where m = 0.
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