Superplasticity

In 1986, Wakai et al. (1986) in Japan discovered that ultra-fine-grained ceramics can also be superplastically deformed they may be brittle with respect to dislocation 

This outline case-history is an excellent example of a parepisteme which began as a metallurgical curiosity and developed, at a leisurely pace, into a well-understood phenomenon, from which it became, at a much accelerated pace, an important industrial process. 

Parepistemes grow from an individual s curiosity, which in turn ignites curiosity in others if a piece of research is directly aimed at solving a specific practical problem, then it is part of mainline research and not a parepisteme at all. However, the improvement of a technique used for solving practical problems constitutes a parepisteme. 

Curiosity-driven research, a term 1 prefer to fundamental or basic , involves following the trail wherever it may lead and, in Isaac Newton s words (when he was asked how he made his discoveries) by always thinking unto them. 1 keep the subject constantly before me and wait until the first dawnings open little by little into full light . The central motive, curiosity, has been rendered cynically into verse by no less a master than A.E. Housman  

A further motive is the passion for clarity, which was nicely illustrated many years ago during a conversation between Dirac and Oppenheimer (Pais 1995). Dirac was astonished by Oppenheimer s passion for Dante, and for poetry generally, side by side with his obsession with theoretical physics. Why poetry Dirac wanted to 

Institute of Metallurgy and Materials Sciences, Polish Acad. Sci. Krakow, Poland. 

A ductile material can be stretched uniformly only when stable flow occurs. The stable flow of materials has been investigated by Hart who described the transition from the stable to unstable flow. The beginning of geometrical instability and localisation of strain is the limit of the stable flow. At temperatures above 0.5 T (at equilibrium between recovery and hardening) the strain rate sensitivity parameter m may be derived from the expression  

The comparison of fractions of these mechanisms is possible only at comparable deformation conditions, for instance for the optimum strain rate (10 - 10 s ). The share of the gbs mechanism depending on strain rate, grain size and shape has a maximum for the middle 

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. 

Scanning microscopy investigation and quantitative image analysis 

The morphology of globular type is the most favourable when superplastic deformation is to occur in A178wt%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. 

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It exhibits superplasticity. Neither zinc nor zirconium is ferromagnetic but ZrZnz exhibits... 

AJ—Zn. Aluminum-rich binary ahoys (Fig. 18) are not age hardenable to any commercial significance, and 2inc [7440-66-6] Zn, additions do not significantly increase the abhity of aluminum to strain harden. Al—Zn ahoys find commercial use as sacrificial claddings on high strength Al—Cu—Mg—Zn aircraft ahoy sheet. The eutectoid composition near 78% Zn has found use as a superplastic sheet ahoy. 

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. 

Superplastic Metal Manual, The New Jersey Ziac Company, Paknerton, Pa., 1970. 

Headley, T.J., Kalish, D. and Underwood, E.E. (1970) The current status of applied superplasticity, in Ultrafine-Grain Metals, ed. Burke, J.J. and Weiss, V. (Syracuse University Press, Syracuse, NY) p. 325. 

Nieh, T.G., Wadsworth, J. and Sherby, O.D. (1997) Superplasticity in Metals and Ceramics (Cambridge University Press, Cambridge). 

Not all fracture is by crack propagation. Highly ductile materials stressed at high temperature will eventually break by the growth, through absorption of lattice vacancies, of plastic voids. This shades into the phenomenon of superplasticity, which was examined in Section 4.2.5. 

In this work, we present calculated SFE using the LKKR-CPA method for Al-Cu and Al-Mg which are of interest from the point of view of superplasticity. We use the SFE to validate the rigid band model which allows a deeper insight into the electronic structure and its implication on the nature of inter-atomic potentials. 

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... 

Fig. 12. The scheme of the press for backward forming of cups m one action by means of the superplastic deformation mechanism. The deformation was done at temperature T = 523 K and at stamp velocity of about 1 mm/min.

The globular shape of a and p grains of size 1 - 2 pm (allowing the best superplastic de-... 

Financial support by grant No 14830/C-SO/94 " The application of the superplasticity phenomenon in production for military industry" is acknowledged... 

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

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