Applications of superplasticity

Using this technique, a multilayer made of four different layers 0.5 mm thick of 3YTZP with grain sizes of 0.3, 0.5, 0.8 and 1.0 pm, obtained by annealing the as-sintered ceramics (0.3 pm), labelled from a to d , have 

6 SEM micrographs of junctions of (a) two layers with the same grain size (0.3 pm), and (b) two layers with different grain sizes (0.3 and 1 pm). 

3 mol% YTZP (3Y) and 6 mol% YTZP (6Y) (b). Optical micrograph of the cross-section of the 3Y-6Y junction with a Vickers indentation and the crack pattern, (c) Stress-strain curve of the 3Y-6Y junction deformed at 1400°C with the stress parallel (isostrain) and perpendicular (isostress) to the interface. 

The use of nano-ceramics as interlayers can reduce drastically the temperature of joining. A good example can be observed in layers of YTZP, which were joined with an interlayer of 20 nm of YTZP at 1150°C, 200°C lower than the temperature used without the nano-layer.76 

8 (a) Schema of the composite obtained with four different layers labelled a to d. Stress-strain curve of the composite deformed at 1400°C in the isostrain and isostress regime. 

When accommodated by some of the mechanisms involving dislocation movement or the diffusion of point defects, GBS forms the basis of the structural superplastic behavior of these materials (see Section 15.2). By taking advantage of the processes involved in superplasticity, it is possible to join ceramics super-plastically. For example, when two pieces of the same ceramics in contact are deformed within a superplastic regime (i.e., as soon as GBS is activated), the grains of one part interpenetrate those of the other part. This produces a rapid and perfect junction of the two, in such a way that a shorter time and a lower temperature can be used than are commonly required in other conventional process for ceramics joining [90]. 

When using the creep model for duplex microstructure, as developed by French et al. [98], it is possible to fabricate a composite with a defined creep resistance that will control the grain size, the width of the layers, and the compression axis. In the case of isostrain, where the strain and strain rate are the same for each phase, and assuming the creep equation in a compact form as, e = Ajo the stress for this configuration, o, is given as  

For the isostress case, the stress is the same for each layer and the strain rate for this configuration can be written as  

In this case, the mechanical behavior of the laminated compound will be controlled by the softer layer deformed at the strain rate imposed by the testing machine. 

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The SPF/DB process has greatly extended the applicability of superplastic forming. Using SPF/DB, a sheet can be formed onto pre-placed details and difib-sion bonded, or two or more sheets can be formed and bonded at selected locations. Figure 18 illustrates the SPF/DB process for three-sheet parts. 

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... 

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 ... 

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. 

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. 

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). 

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. 

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. 

Not unlike the case of superplastic ceramics, ductility and strength relations are influenced by strain rate. The conditions of the experiment must be above the DBT to observe plastic flow, which is different for various ceramics. An illustration of the effect of strain rate and temperature on the strain (ductility) at some stress level can be seen in monolithic Si-C-N. Silicon-nitride-based ceramics are quite promising candidates for mechanical applications at elevated temperatures. Specimens were prepared by hot isostatic pressure (henceforth HIP) of pyrolyzed powder compact at 1500 °C and 950 MPa, without any sintering additives. These compression tests were conducted at temperatures from 1400 to 1700 °C in a nitrogen atmosphere with a servo-hydraulic-type testing machine at constant crosshead speed in an induction heating furnace. In Fig. 2.5, stress-strain curves... 

The above data, on the strength of superplastic materials, indicate that the plastic deformation of 3Y-TZP by the application of compressive stress (e.g., by forging) may be accomplished without too much difficulty and without... 

Various jet engine and car engine components have been identified for application of TiAl-based alloys. It is particularly noteworthy that large sheets, which are suitable for superplastic forming and joining by diffusion bonding, can be produced from these intrinsically brittle intermetallic compound alloys. 

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. 

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... 

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 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. 

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). 

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