In addition to the fine-structure superplasticity (FSS) described in the previous chapters, there is another type of superplasticity known as internal-stress superplasticity (ISS). In these materials, in which internal stresses can be developed, considerable tensile plasticity can take place under the application of a low, externally applied stress. This is because internal-stress superplastic materials can have a strain-rate-sensitivity exponent as high as unity; i.e., they can exhibit ideal Newtonian viscous behavior. Such superplastic materials are believed to be deformed by a slip-creep mechanism.
There are many ways in which internal stresses can be generated. These include thermal cycling of composite materials, such as whisker- and particulate-reinforced composites, in which the constituents have different thermal expansion coefficients; thermal cycling of polycrystalline pure metals or single-phase alloys that have anisotropic thermal expansion coefficients; and thermal cycling through a phase change. In addition, pressureinduced phase changes have been cited as a possible source of superplastic flow in geological materials. For example, there is a phase transformation in the earth's upper mantle, because of pressure, from orthorhombic olivine to a spinel phase at a depth of about 400 km below the earth's surface. And it is believed that internal-stress superplasticity, arising from the transformation stresses through pressure cycling (analogous to temperature cycling), leads to a mixed-phase region of low effective viscosity.