Superplasticity

Permanent deformation of a material through flow, e.g., creep, viscosity, viscoplasticity, gets easier as the grain size in the material gets smaller. In the most spectacular cases, relative extensions greater than 100% (nominal strain > 1) can be obtained at relatively low temperatures compared with the temperatures usually required to observe creep in materials: this is the effect known as superplasticity. Typically, superplasticity only occurs in fine-grained dense materials (grains < 0.01 mm for metals, < 1 μm for ceramics), barely affected by strain localisation effects, or striction, at temperatures > 0.5T _melting, when such a temperature has any meaning (materials sometimes decomposing before melting). Even in ancient times, smiths made good use of this remarkable property to forge tough, hard steel blades. The steel used by the Persians at the time of the crusades, and by Saladin’s armies, or Damascus steel, is one of the greatest achievements of metallurgy and the forge, where the choice of alloy at the outset (in this case a steel with a high carbon content, known as wootz, from India) and the masterly control of a judicious forging cycle (the thickness of the initial ingot was first reduced by a factor of about 10 by hammering) produced a material with ideal fine microstructure for making sharp cutting blades that could also resist mechanical shocks. Figure 9.1 illustrates the phenomenon of superplastic behaviour for a steel containing 1.6% carbon (ultrahigh carbon steel), with a fine microstructure, close to Damascus steel, which seems to have been produced first in India in the fourth century BC.