On the Common Origin of Structural Superplasticity in Different Classes of Materials

The present model starts with an assumption that grain/ interphase boundary sliding (GBS) that is dominant during optimal superplastic flow is slower than the accommodation processes of dislocation emission from sliding boundaries, highly localized diffusion in the boundary regions and/ or grain rotation that are present as a concomitant of the GBS process. When boundary sliding develops to a mesoscopic scale (of the order of a grain diameter or more), by the alignment of contiguous boundaries, plane interface formation/ mesoscopic boundary sliding is observed. Significant and simultaneous sliding along different plane interfaces and their interconnection can lead to large scale deformation and superplasticity. The accommodation steps, being faster than GBS, do not enter the strain rate equation. Mathematical development of these ideas using transition state theory results in a transcendental strain rate equation for steady state optimal superplastic flow, which when solved numerically helps one to describe the phenomenon quantitatively in terms of two constants, the activation energy for the rate controlling process, Delta F-0 and the threshold stress needed to be overcome for the commencement of mesoscopic boundary sliding, sigma(0). The analysis also explains quantitatively texture randomization as a function of superplastic strain. It is also pointed out, without going into details, that recently the problem has been reduced to FOUR "universal" constants, viz. the mean strain associated with a unit boundary sliding event gamma(0), specific grain boundary energy gamma(B), which is assumed to be isotropic, N the average number of boundaries that align to form a plane interface during mesoscopic boundary sliding and "a" a grain-size- and shape- dependent constant that obeys the condition 0 更多