Plastic deformation and allotropic phase transformations in zirconia ceramics during high-pressure torsion

Abstract

Severe plastic deformation using high-pressure torsion (HPT) is successfully applied at room temperature to partially stabilized ZrO 2. X-ray diffraction and Raman spectroscopy analyses reveal that appreciable strain is introduced in the sample and a phase transformation with a coherent interface occurs from the metastable tetragonal phase to the monoclinic phase during HPT. The fraction of the stress-induced monoclinic phase increases with straining, reaches a saturation level. Transmission electron microscopy shows that nanograins of high dislocation density but with no twins are formed during HPT.

Strain-induced phase transformations under compression, unloading, and reloading in a diamond anvil cell

“…However, the case p d e < p r e is at least of the same importance, for which both direct and reverse PTs occur in the pressure range p d e < p < p r e . This leads to a stationary value of concentration 0 < c < 1 at very large plastic strains, which was observed experimentally for various pressure-shear loading, e.g., under high pressure torsion [23][24][25] and ball milling. [26][27][28][29] Therefore, new PT features and phenomena may appear.…”

“…They are caused both by heterogeneous stress redistribution and TRIP. This PT requires reconsideration of quantitative values of phase concentrations in experiments on the unloaded sample, like in high pressure torsion 24,25 and ball milling. [26][27][28][29] The reverse PT may potentially be reduced or even avoided if a gasket with specially designed parameters will be used, 19 which creates quasi-homogenous pressure distribution under loading.…”

“…Characterization of PT processes under pressure based on the results of measurements after complete pressure release [23][24][25][26][27][28][29] is based on the strong assumption that there are no PTs during unloading. In this section, the unloading is studied and a surprising result is obtained: for a fast kinetics, k ¼ 30, and r y2 r y1 , unloading is accompanied by plastic flow, which causes first a small increase in c above p d e followed by much stronger reverse PT below p r e .…”

“…Obtained results require reconsideration of the reported values of concentration in experiments, [24][25][26][27][28][29] which are based on the measurements of concentration after unloading. To avoid this problem for DAC, one can try to find loading, which will minimize reverse PT during unloading.…”

“…However, if there is no reverse PT under hydrostatic loading-unloading, this does not mean that reverse PT will not occur during the reduction of the load to zero after strain-induced PT. In many cases (excluding in situ studies), concentration of phases is determined after unloading [23][24][25][26][27][28][29] but is related to the loading process with a salient assumption that it does not change during unloading because of the absence of plastic deformation. As we will show, in contrast, unloading is accompanied in many cases by plastic flow and reverse PT, which should be taken into account in experiments.…”