Superplasticity by internal frictional heat under biased cyclic loading

A new damping method was developed not only as a testing tool to investigate in situ deformation under stress, but also as a processing method to superplastically deform ceramics. The specific damping capacity (SDC) at low frequencies (<0.2 Hz) decreased with increasing frequencies, which matched previous internal friction results. However, at higher frequencies (0.2–5 Hz) SDC increased with frequencies, which was explained by a new internal frictional heat mechanism. Three different ceramics: a non-superplastic one and two superplastic ones with different activation energies, showed the same behavior at the high frequency damping tests (1–5 Hz). From these results, it was deduced that a cyclic load at high frequencies, superimposed on a static one, has a great potential to enhance superplasticity by specifically heating up grain boundaries from internal frictional heat.