Superplasticity applications

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

Alloy Composition Temperature of Applicability (°C) Superplasticity (% Elongation) m... 

For high-pressure applications in the hydrocarbon and chemical processing industries, a titanium compact heat exchanger has been developed by Rolls-Laval. This heat exchanger consists of diffusion-bonded channels that are created by superplastic forming of titanium plates (18). This heat exchanger can handle high pressure and corrosive fluids and is suitable for marine applications. 

In summary, my view is that the fundamental cause for superplasticity is electronic in origin which has to do with the probability curves for the formation of compounds. This in turn creates the instability of the compounds and results in the ultra small grain size. Then, on the application of tensile stress, the plastic deformation is purely mechanical and has nothing to do with electrons. This is completely different from that observed in the normal plasticity as described above. The cause and mechanism for super-plasticity and normal plasticity are therefore fundamentally different. The phenomenon of superplasticity therefore can be viewed stepwise as follows ... 

Superplastic ceramics have several obvious potential advantages for commercial application. These include net size and shape forming and the possibility of forming complex components from initially flat sheets. Whilst the practical problems of forming at temperatures in excess of 1200°C obviously... 

The application of ceramics has infiltrated almost all fields in the last 20 years, because of their advantages over metals due to their strong ionic or covalent bonding. But it is just this bonding nature of ceramics that directly results in their inherent brittleness and difficulty in machining. In other words, ceramics show hardly any macroscopic plasticity at room temperature or at low temperatures like metals. Hence, superplasticity at room temperature is a research objective for structural ceramics. In recent years, many researches have been carried out to investigate nanophase ceramic composites. 

Superplasticity is a very promising property, not only because, like in metals, the superplastic formation opens a way for the manufacturing of complex ceramic pieces for industrial applications, but also because the combination of GBS and diffusional processes makes superplasticity an interesting tool for joining ceramic pieces in shorter times and lower temperatures than the diffusional joining technique. 

In order to outline the future trends in superplasticity in ceramics, first of all it is necessary to give an answer to the following question why is superplasticity in ceramics so important The potential use of these materials in more and more severe applications makes superplasticity in ceramics an important tool for their processing, as happened with metals at the beginning of the 1960s. 

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. 

A recent successful application of carbon/carbon composites is the tool for superplastic forging of titanium illustrated by Figure 6 tubes up to 1.5 m in length can be forged at temperatures up to 1000°C, thus offering a rapid alternative fabrication technique to present production methods, e.g., riveted tubes (15). Contact brushes for electrical commutators, made with carbon fibers and carbon/carbon composites (16), are opening another new field of application. Furthermore, pistons in diesel engines have been proposed to be made from carbon/carbon composites (17). 

Warm rolling of preform with a submicrocrystalline structure can be produced at much lower temperatures than generally used. Submicrongrained sheets demonstrate mechanical isotropy and high superplastic properties at reduced temperature range of 650-750°C. The Ti-64 sheets with SMC structure can be successfully used for SPF and SPF/DB applications with significantly decrease in operations temperature (by 200°C). 

Structural modifications of engineered materials are caused by the incorporation of nanoparticles as passive basic building blocks and lead, for example, to superplastic ceramics or extremely hard metals. Functional applications, on the other hand, rely on the transformation of external signals, such as the filtering of light, the change of electrical resistance in different environments, or the occurrence of luminescence when electrically activated (Tab. 11.1). 

Studies performed at the University of Missouri-Rolla in conjunction with Rockwell Scientific have shown FSP to produce a hne-grain-size material and create low-temperature, high-strain-rate superplasticity in aluminum and titanium alloys. The PNNL is currently investigating the application of this FSP-induced superplasticity in the fabrication of large, integrally stiffened structures. 

A tracking shot on titanium alloys and their composites will close this section. Titanium is recognised as the most important metal in aerospace applications in the range between 200 C and 450 °C. Its position in the market has been further strengthened by the development of superplastic forming/diffusion bonding manufacturing techniques, which allow the production of complex shapes at reduced costs. 

There are extensive studies centering on the fabrication of nanostructured metals in order to improve their mechanical properties. Since mechanical performance of orthopedic implant is critical to its applications, liability and lifetime, superior mechanical properties are always wanted. Depending on clinical settings, the wanted properties include, but are not limited to, enhanced mechanical strengths, toughness, ductility, wear resistance, corrosion resistance, and special characteristics such as superplasticity and shape-memory effect. Due to space limitations, only the typical aspects and examples of implant mechanical properties enhanced by nanotechnology are introduced here. 

In this chapter, the macroscopic and microscopic aspects of superplasticity, the accommodation processes, the applications and the future prospects of ceramic superplasticity vdll be addressed. 

In conclusion, although this mechanism has been used to account for superplasticity in metals and metaUic alloys, at presentthere is no evidence of its applicability in ceramics. 

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