Abstract
This article overviews the mechanical characterization of nanocrystalline ceramics synthesized by consolidating MA amorphous powder without additive. Berkovich indentation with depth-sensing enables to obtain a yield stress (σy) under constrained compression and the strain rate sensitivity exponent (m) as an accepted parameter of micro-plasticity; for nanocrystalline (ZrO2)80(Al2O3)20, the σy is a low level of 4.4 GPa when non-strain hardening is assumed, and the m has a considerably low value of 0.014 suggesting the appearance of non-homogeneous flow in a nearly ideal plastic solid. For monolithic tetragonal (ZrO2-3mol%Y2O3)80 (Al2O3)20 with the longitudinal crystallite size of 40 nm, the flexural strength shows a size effect with the aspect ratio of sample width (W) to height (H) having nearly 2 GPa at the maximum in the case of W/H≈1, and a proportionality to the fracture toughness as obtained from the indentation-microfracture method, being apart from a trade off relationship. Then, the isothermal superplastic forming up to a compressibility of 0.75 is used to derive the constitutive equation; m is given as a function of strain rate reduced by Zener-Hollomon parameter (ZH) and reciprocal stress, εσ−1exp(Q/kT) with the activation energy (Q) of 312 kJ·mol−1, having the constancy of 0.7 and a decreasing value to 0.3 at its higher and lower ZH respectively.
