Superplasticity strain rate

Figure 7 shows the dependence of optimum superplastic strain rate on the inverse grain size for a large variety of superplastic aluminum alloys produced by different routes [14], These dependencies suggest that the strain rates up to 10 1 s 1, limited by the testing equipment, were out of the optimum conditions to get superplastic behavior. 

Figure 7. Dependence of optimum superplastic strain rate on the grain size for superplastic Al alloys produced by different routes.

Fig. 1 The relationship between optimum superplastic strain rate and grain size for superplastic aluminum alloys (data from Ref. [4]). Subscripts "w" and p" stand for whisker and particulate, respectively.

Fic 14 2 The influence of grain size on (a) optimal strain rate of aluminum alloys (Source Ref 5) and (b) the superplastic temper- ature (Source Ref 7, 8). Note the range of grain sizes obtainable by friction stir processing and the corresponding superplastic strain rate and temperature that are possible. 

The aim of this study is to determine the structure and texture of the initial sample and the temperature and strain rate parameters, at which the superplastic deformation in AlZn78, AlZn76Cu2 and AlZn78 Mg0.02 alloys is the most likely to occur. 

EFFECT OF LIQUID PHASE ON SUPERPLASTICITY AT HIGH STRAIN RATES IN METALS AND THEIR COMPOSITES... 

An overview of the superplastic behavior of aluminum alloys to demonstrate the grain-size effect is depicted in Fig. 1, in which the quantitative relation between the logarithm of the optimum strain rate for superplastic flow and the grain size (plotted as the logarithm of reciprocal grain size) is clearly shown [4]. The slope of the curve in Fig. 1 is noted to be about 3. 

It was pointed out by Nieh and Wadsworth [5] that fine grain size is a necessary but insufficient condition for HSRS. This conclusion resulted from the observation that many fine-grained composites are not superplastic at high strain rates. Evidently, in addition to grain size, microstructural factors, such as detailed structure and chemical composition at the reinforcement-matrix interfaces and grain boundaries, may play important roles. 

For the purpose of discussion. Table 2 summarizes HSRS data obtained from a number of Al alloys and composites. It was first noted by Nieh et al [5] that the optimum temperature for high strain rate superplasticity in an alloy is either above or close to the solidus temperature. This led them to suggest that the presence of a liquid phase might have contributed to the observed HSRS. 

T.G. Nieh, J. Wadsworth, and T. Imai, "A Rheological View of High-Strain-Rate Superplasticity in Metallic Alloys and Composites," Scr. Metall. Mater., 26(5) 703 (1992). 

T. G. Nieh, J. Wadsworth, and K. Higashi, "High Strain Rate Superplasticity in Metals and Composites," in Transaction of the Materials Research Society of Japan Vol 16B - Composites, Grain Boundaries and Nanophase Materials, pp. 1027-1032, M. Sakai, M. Kobayashi, T. Suga, R. Watanabe, Y. Ishida, and K. Niihara ed., Elsevier Science, Netherland, (1994). 

B.Q. Han, K.C. Chan, T.M. Yue, and W.S. Lau, "A Theoretical Model for High-Strain-Rate Superplastic Behavior of Particulate Reinforced Metal Matrix Composites," Scr. Metall. Mater., 33 925 (1995). 

Observations of what appeared to be superplastic behavior were initially made in the late 1920s with a maximum of 361% for the Cd-Zn eutectic at 20°C and strain rates of 10 /s and 405% at 120°C and a strain rate of 10 /s. Jenkins [2] also reported a maximum of 410% for the Pb-Sn eutectic at room temperature but at strain rates of 10 /s. However, the most spectacular of the earlier observations was that by Pearson in 1934 [3]. While working on eutectics, he reported a tensile elongation of 1950% without failure for a Bi-Sn alloy. 

Superplasticity was exhibited only on deformation at above approximately half the melting point and over a specific range of strain rate, 10"2 to 10-4 sec 1. 

Several parameters can influence strongly the superplastic behaviour of ceramics, i.e. the strain rate at which the material can be superplastically deformed. Between them can be mentioned the grain size, second phases and segregation of impurities at the grain boundaries, etc. 

The commonly used equation for the steady-state strain rate e characterizing superplastic behaviour is written as ... 

In the immediate future, the main objective in ceramic superplasticity will be the search of the right conditions to achieve high strain rate superplasticity (HSRS) ((e > 1CT2 s 1). Although this phenomenon has been found in several ceramic compounds and several inputs have been outlined to achieve it, we are still far from knowing what to do to obtain this effect systematically. This HSRS will enlarge the applications for ceramics. 

Morita, K., Hiraga, K., and Sakka, Y., High-strain rate superplasticity in Y203-stabilized tetragonal Zr02 dispersed with 30 vol% Mg A1204 spinel , J. Am. Ceram. Soc, 2002, 85, 1900-2. 

Komura Sh., Berbon P.B., Furukawa M., Horita Z., Nemoto M., Langdon T.G. (1998) High strain rate superplasticity in Al-Mg alloy containing scandium, Scripta Materialla 38, No. 12,1851-1856. 

HIGH STRAIN RATE SUPERPLASTIC BEHAVIOR OF Al-Li-Mg-Cu-Sc ALLOY SUBJECTED TO SEVERE PLASTIC DEFORMATION... 

High strain rate superplastic behavior of an Al-Li-Mg-Cu-Sc alloy... 

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