Review: Processing of metals by equal-channel angular pressing

Equal-channel angular pressing (ECAP) is a processing method in which a metal is subjected to an intense plastic straining through simple shear without any corresponding change in the cross-sectional dimensions of the sample. This procedure may be used to introduce an ultrafine grain size into polycrystalline materials. The principles of the ECAP process are examined with reference to the distortions introduced into a sample as it passes through an ECAP die and especially the effect of rotating the sample between consecutive presses. Examples are presented showing the microstructure introduced by ECAP and the consequent superplastic ductilities that may be attained at very rapid strain rates.     

Electronic Conductivity of La-Doped Ceria Ceramics

Lanthanum-doped ceria powder with a composition Ce_0.8La_0.2O_1.9 was prepared by heating the oxalate solid solution (Ce_0.8La_0.2)₂(C₂O₄)₃ at 873 K in air. As-prepared powder was densified to 96%–97% relative density by sintering in air at 1773 K for 4 h. The electronic current of the disk sample was measured in a temperature range from 773 to 1113 K by the direct current polarization method using a Hebb–Wagner ion-blocking cell. A linear relationship, which was theoretically predicted, was measured between log σ_e (electronic conductivity) and E (applied voltage) in the applied voltage range of 0.2–1.0 V. The σ_e was proportional to P _O2^−1/4.3≈ P _O2^−1/4.6 in the oxygen partial pressure range of 10⁻²–10⁻⁸ Pa, and to P _O2^−1/6.7≈ P _O2^−1/7.1 in the oxygen partial pressure range of 10^−7.5–10⁻²² Pa. The apparent activation energy of the electronic conduction was 1.87–1.94 eV. The hole conduction was also measured in the P o₂ range of 10²–10⁵ Pa. The transport number of oxide ion was 0.96–1.00 at 773–1113 K under an oxygen partial pressure of 10⁻⁵ Pa.     

Grain-Boundary Diffusion of Strontium in (La,Ca)CrO3 Perovskite-Type Oxide by SIMS

The grain-boundary-diffusion coefficient ( D _gb) of strontium in La_0.9Ca_0.13CrO_3-δ was determined by secondary-ion mass spectrometry (SIMS), using two different measurement modes: depth profiling from the surface and line scanning on the fracture surface. The depth profiles that were sputtered by an O₂⁺-primary-ion beam gave two slopes of strontium concentration profiles, which corresponded to grain (bulk) and grain-boundary diffusion. The depth profiles were fitted to an appropriate equation that gave the grain- (bulk-) and grain-boundary-diffusion coefficients ( D _bulk= 6.5 × 10^-20 m²/s and D _gb= 1.6 × 10^-15 m²/s in air at 1273 K, respectively). Initially, to obtain the D _gb value via the SIMS line-scanning measurement, the fracture surface of La_0.9Ca_0.13CrO_3-delta was measured by a low-energy O₂⁺-primary-ion beam. The line-scanning measurement enabled us to successfully determine the strontium concentration profiles around the grain boundary. However, the D _gb value that was obtained via the line-scanning mode was 6.0 10⁻¹³ m²/s, which was a factor of 100 greater than that which was obtained by the depth-profile mode. Comparison between the depth-profile and line-scanning modes will require additional careful examination.     

High-pressure torsion of SiO2 quartz sand: Phase transformation,optical properties,and significance in geology

Phase transformation and optical properties of silica (silicon dioxide, SiO₂) quartz sand under high pressure/temperature has been of interest in geology and optical physics for many years. In this study, besides high pressure/temperature, high plastic strain is simultaneously applied to the quartz sand by high-pressure torsion (HPT) processing. The material shows oxygen vacancy formation and transformation to (a) a denser nanocrystalline quartz phase, (b) a high-temperature amorphous phase and (c) a high-pressure coesite phase. These structural and microstructural changes lead to light absorbance, electron spin resonance, photoluminscence and photocatalytic activity, while these changes are enhanced by increasing strain. This study introduces a possible pressure-temperature-strain-based mechanism for the formation of naturally observed vacancies and coesite phase in SiO₂-based minerals and sands.     

Chromium poisoning for prolonged lifetime of electrodes in solid oxide fuel cells - Review

Solid Oxide Fuel Cells (SOFCs) offer low carbon emission and high efficient energy conversion systems. For the wide commercial distribution of this system, one of the technological issues and challenges is prolonged durability: the SOFC systems should have a long lifetime of more than 10 years. The volatile chromium species poisoning is the key degradation factor to overcome at the functional ceramics of air electrode (cathode)/interlayer/electrolyte interfaces in the SOFC system among many degradation factors. This paper reports recent degradation mechanisms, especially on the chromium (Cr) vapors poisoning at the perovskite oxide cathode. The Cr-concentration levels at cathodes were evaluated from the reported data at small cells and practical cell-stacks. The interactions of volatile Cr species and perovskite oxide cathode surface were evaluated by the chemical reaction of cathode materials with Cr-vapors to form SrCrO₄ and the electrochemical induced Cr-vapors reduction (Cr⁶⁺ to Cr³⁺) to form Cr₂O₃ at (La,Sr,Ca)MnO₃-based and (La,Sr)(Co,Fe)O₃-based materials. Recovery mechanism from Cr-poisoning was reanalyzed at the (La,Sr)(Co,Fe)O₃/ceria-based interlayer/YSZ electrolyte interfaces by Cr-cleaning reaction with the evaporation of Cr₂O₃/SrCrO₄ and nano-meter level cation migration/rearrangement effects with phase separation and new phases formation. This paper is covering not only the elucidation of degradation mechanism but also the fundamentals of physical and chemical analyses on perovskite oxide cathode surface and interfaces. An insight for new materials combination for the next-generation SOFCs is also included.     

Effects of ball milling and high-pressure torsion for improving mechanical properties of Al–Al2O3 nanocomposites

Pure Al powders were mixed with a 30 % volume fraction of Al₂O₃ powders having particle sizes of ~30 nm. The mixed powders were first subjected to ball milling (BM) and thereafter consolidated by high-pressure torsion (HPT) at room temperature under a pressure of 3 GPa for 10 turns. The Al–Al₂O₃ composite produced by BM and HPT (BM + HPT) had a more uniform dispersion of the nano-sized Al₂O₃ particles in the Al matrix. Hardness values of the BM + HPT composites were higher than those of the composites without BM. It is shown that the use of BM powders for HPT is more effective in achieving a uniform dispersion of the nano-sized Al₂O₃ particles and in improving mechanical properties of the Al–Al₂O₃ nanocomposites.     

Incremental Feeding High-Pressure Sliding for Grain Refinement of Large-Scale Sheets: Application to Inconel 718

Metallurgical and Materials Transactions A - This study updates a process of high-pressure sliding (HPS) recently developed as a severe plastic deformation process under high pressure for grain...