Rheological and metallurgical discussion of superplastic behaviour

Abstract

The analysis of the deformation of superplastic materials shows that the structure tends to evolve towards an equiaxed state and to undergo grain growth. This behaviour influences the constitutive equations : apparent strain-hardening, sigmoidal variation of the Log stress- Log strain rate curve, low apparent activation energy values at intermediate strain rates. This deformation behaviour can be explained when the structural change is taken into account. The analysis described leads to a constitutive equation which presents a metallurgical basis since it takes into account the structural changes. By contrast, the generally proposed relationship is rheological and can be used only within a strictly mechanical approach. An hydrodynamic analogy of superplastic flow is then presented. It shows that superplasticity occurs, on the one hand when the hardest phase is not continuous, and on the other hand when the most ductile phase has a mean free path no greater than a few microns. The analogy also demonstrates that the textural evolution is different for the two phases and accounts for the observed interface sliding. Furthermore, this study suggests that superplastic behaviour with elevated values of the coefficient m can be observed, although large elongations cannot be obtained in cases where the material undergoes cavitation resulting from difficulties in interface sliding

T-shaped equi-channel angular pressing of Pb–Sn eutectic and its tensile properties

“…6 show first an increase and then decrease in stress with increasing strain. Due to high temperature condition of deformation, the increase in stress with increasing strain is attributed to grain growth whereas the decrease is attributed to either recrystallization or breaking up of elongated grains into equiaxed grains [22]. However, the microstructural observation made here does not support these reasons.…”

“…With increased mechanical working this microstructure breaks up and undergoes recrystallization to give fine equiaxed grains (Fig. 3), which is known to give superplasticity [22]. The material thus shows softening, with increasing number of passes and reaches a stable state of constant hardness value.…”

High-temperature plastic flow behaviour in the binder of WC-Co cemented carbides

“…However, the deformation mechanism in the low strain rate region i.e. region I in cemented carbides has been somewhat in ambiguity as well as in superplastic metals in which it is still controversial because of the various viewpoints [8][9][10][11][12][13][14][15][16][17][18]. Recently, the present author has proposed a temperature-dependent threshold stress to give an explanation for the presence of region I in WCCo cemented carbides [7].…”

“…The situation is the same with that in superplastic metals. Several suggestions have so far been made to provide an explanation for the origin of region I; (a) a consequence of static grain growth [8][9][10], (b) a different deformation mechanism from region II [11][12][13][14], (c) the plastic flow of binder phase [24] and (d) the presence of a threshold stress [15][16][17][18]. Among the above suggestions, then, let me consider a possible rate-controlling mechanism working actually in region 1 of cemented carbides.…”

Anisotropy in flow and microstructural evolution during superplastic deformation of a layered-microstructured AA8090 Al–Li alloy

“…5). The elongated grains, like the one present in CL, are known to be unfavorable for high temperature deformation whereas the material with equiaxed grains, like the one present in SL, is known to deform at lower stress [2].…”