Review on Recent Advancements in Severe Plastic Deformation of Oxides by High‐Pressure Torsion (HPT)

Abstract

Severe plastic deformation (SPD) of oxides has been of interest in mineralogy and geology for many decades, but oxides have ionic or covalent bonds with brittle nature and can hardly be deformed at ambient temperature. SPD processing of oxides is first realized in 1930s, when Percy W. Bridgman introduce the principles of high-pressure torsion (HPT) method. Although the method is widely used nowadays to produce ultrafine-grained (UFG) metals, the application of method for SPD processing of oxides is quite limited. This article reviews some of the recent publications on the application of HPT method to oxide ceramics () and summarizes their major findings: powder compaction and partial consolidation, nanograin formation, formation of different kinds of lattice defects such as dislocations and oxygen vacancies, strain and grain size dependence of phase transformations, and improvement of functional properties such as bandgap narrowing, photoluminescence, photocatalytic activity, and dielectricity. Early Studies on HPT Processing of OxidesAs mentioned earlier, SPD processing of oxides has been of interest for many decades in mineralogy and geology. Boeker is

High-Pressure Phase Transformations under Severe Plastic Deformation by Torsion in Rotational Anvils

“…in oxides ZrO 2 , Y 2 O 3 , BaTiO 3 , Al 2 O 3 , TiO 2 , and iron oxides. 29) From the macroscopic point of view, the pressure hysteresis for pressureand stress-induced PT is proportional to the current value (under current plastic strain) of the yield strength. 7,24,25,51) Plastic strain and grain refinement increase (until saturation) the yield strength and, consequently, pressure hysteresis, which may prevent the reverse PT.…”

“…28) (3) Plastic deformation may substitute a reversible PT with irreversible PT. 1,3,7,8,21,27,29) This allows one to utilize these metastable HPPs in various applications at ambient pressure. (4) Plastic straining may reduce the pressure hysteresis (i.e.…”

Impact of TiO2-II phase stabilized in anatase matrix by high-pressure torsion on electrocatalytic hydrogen production

“…The sample with 18 wt.% TiO 2 -II phase shows the highest activity for water splitting, however, the anatase powder shows the best activity for the electrocatalytic reduction of oxalic acid. Since the HPT method partially consolidates the oxide powders and reduces their surface area [35,36], the catalytic activity data in Figure 4(a) are possibly influenced by reduced surface area effect. To examine this effect, surface areas were measured using the Brunauer-Emmett-Teller (BET) method and the electrocatalytic data were plotted per unit area in Figure 4(b).…”

Gradient bandgap narrowing in severely deformed ZnO nanoparticles

“…Though oxides are brittle under normal conditions and possess much less formability than their metallic counterparts, substantial ductility can be gained under high hydrostatic confining pressure, which facilitates the transformation from micro-cracking to plastic slip and inelastic relaxation, and reduces the critical resolved shear stress for activation of slip systems [9][10][11]. For example, extensive plasticity of α-alumina under hydrostatic pressure of about 3 GPa was reported by Percy Bridgman already in the year 1947 [12].…”

“…The SPD techniques such as high-pressure torsion (HPT) have been widely employed to manipulate the microstructure of oxides (see Ref. [11] for review). In addition to grain refinement and phase transformations, formation of oxygen vacancies during HPT processing has been observed in a range of oxides nanoparticles including BaTiO 3 [13], TiO 2 [14], ZnO [15], Al 2 O 3 [16], Y 2 O 3 [17], and GaN-ZnO [18].…”