Alloy superplastic

Source C.H. Hamilton, Superplasticity in Titanium Alloys, Superplastic Fanning, ASM InItBmal onali, 1965,p13-22... 

Zinc rolling slabs have been cast successfully by semicontinuous direct-chill casting methods. This is the preferred method for superplastic zinc alloys which, because of their large freezing range, display unacceptable surface shrinkage when cast in open molds. 

Another commercial development of the 1970s is the appHcation of superplasticity which is exhibited by a number of zinc alloys (135—138). Under the right conditions, the material becomes exceptionally soft and ductile and, under low stresses, extensions exceeding 1000% can be obtained without fracture. The grain size must be extremely small (about 1 micrometer) and stable. This grain size is less than one tenth that of common metals in the wrought condition. 

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. 

The morphology of globular type is the most favourable when superplastic deformation is to occur in AI78wt%Zn alloy. This type of structure is formed by decomposition of the a solid solution a -> a + P However, plates usually dominate in the structure of this alloy. To obtain the non-plate or globular type, a special heat treatment is neccesary i.e. the optimal cooling rate as well as the temperature and time of ageing. 

More distinct regularities observed in the Al-phase allow us to distinguish dislocation slips expressed by the movement of orientation peaks along the fibres from the grain boundary sliding responsible for the smoothing effects and thus for the superplasticity of the alloy... 

H. Inagaki, "Enhanced superplasticity in high strength titanium alloys" Z. Metallkde 86, 643,1995... 

S.C. Chang, J, W, Jeh, D C. Luu," The superplasticity of aluminium - lithium alloys" Sixth International Aluminium - Lithium Conference, Garmisch - Partenkirchen, aluminhium -Lithium vol. 1. 1047-1052, 1992, Publ Deutsche Gesellschaft fiir Materialkunde e.V. Oberursel, Germany. 

S.W. Lim, Y. Nishida, "Superplasticity of whisker reinforced 2024 aluminium alloy composites fabricated by squeeze casting" Scr. Metall Mater. 32, 1911, 1995... 

K.N. Melton, J.W. Edington, "Crystallographic slip during superplastic deformation of the Zn - A1 eutectoid alloy" Scipta Met. 6, 1141, 1974... 

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. 

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. 

There apparently exists a critical amount of liquid phase for the optimization of grain/interface boundary sliding during superplastic deformation. The optimum amount of liquid phase may depend upon the precise material composition and the precise nature of a grain boundary or interface, such as local chemistry (which determines the chemical interactions between atoms in the liquid phase and atoms in its neighboring grains) and misorientation. The existence of an equilibrium thickness of intergranular liquid phase in ceramics has been discussed [14]. This area of detailed study in metal alloys has not been addressed. 

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). 

J. Koike, M. Mabuchi, and K. Higashi, "In-Situ Observation of Partial Melting in Superplastic Aluminum Alloy Composites at x.igh Temperatures," Acta Metall Mater., 43(1) 199 (1994). 

K. Higashi, "Deformation Mechanisms of Positive Exponent Superplasticity in Advanced Aluminum Alloys with Nano or Near-Nano Scale Grained Structures," in Materials Science Forum Vols. 170-172, pp. 131-140, T.G. Langdon ed., Trans Tech Publications, Switzerland, (1994). 

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. 

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