ABSTRACT
Earliest observations of very high ductilities in metals were made in the late 1920s and early 1930s [1-4], but the term “superplasticity” was ‰rst introduced by Bochvar and Sviderskaya in 1945 [5]. Superplasticity, as a phenomenon that provides elongations exceeding 800% at hot-working temperatures (according to Pilling and Ridley [6], the most spectacular elongation observed was 4850% in a Pb-Sn alloy [7]), was thus discovered more recently than the establishment of dynamic restoration mechanisms. Moreover, it was surprising in that the basic mechanism, GBS, had always been associated with failure at high temperature (Section 9.7). As already explained, the inherent hot-work fracture mechanism is cracking initiated at triple junctions by differential sliding, even though it constituted less than 1% total strain (Section 4.9). Creep research had shown that such sliding climbed to over 30% of the strain as strain rate was reduced [8]. Creep life was extended by introducing larger grains; great success was attained in directionally solidi‰ed turbine blades either with axial columnar grains or later monocrystalline. For superplasticity, the microstructure is carried to the opposite extreme; grains of less than 10 μm reduce the problem of differential sliding by facilitating accommodation by intragrain slip. Nevertheless, the failure mechanism is still related to cavity formation. Although superplastic behavior of Al alloys is commonly found in the range 10−4-10−3 s−1 and 500-550°C [9-18] this range is constantly moving toward higher strain rates and lower temperatures. The activation energy for sliding is much less than it is for dislocation climb, and so provides an advantage in lowering the temperature. The signi‰cant mechanical attribute is a strain-rate sensitivity m > 0.4, that is linked to a power-law exponent n about 2-2.5, thus much lower than that of dislocation creep. Such sensitivity prevents neck formation by raising the strength as the strain rate increases locally. In contrast to SRPD attained by GBS, in Al-Mg alloys where m = 0.33, very high strain to rupture arises primarily from solute drag as described in Section 5.10.2.
