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Oxide Dispersed Strengthened (ODS) ferritic stainless steels present well-known fine grains microstructures where dislocation movement is hindered by a dense precipitation of nano-oxides particles. Previous research, on the thermomechanical behavior at high temperature and strain rates, was focused on torsion tests (Karch in J Nucl Mater 459:53-61, 2014). Considering texture evolution and grain shape as indicators of the intragranular dislocation glide activity, it was shown that, for high temperature and strain rate, intragranular deformation was in competition with intergranular accommodation. The latter phenomenon was related to early damaging at grain boundaries. The occurrence of a transition phenomenon from an intragranular to an intergranular deformation mechanism, with increasing temperature, was recently confirmed by neutron diffraction spectroscopy (Stoica in Nature Commun 5:5178, 2014). In the present paper, hot extrusion (HE) tests are performed, avoiding damage due to the high stress triaxiality, and allowing further investigation of intragranular and intergranular plasticity at large strains. Three ferritic steels exhibiting various precipitation size and density were hot extruded. Microstructure evolution at different stages of deformation is investigated using the Electron Back-Scattered Diffraction (EBSD) technique. After extrusion at 1373 K (1100 °C), the microstructure of ODS steels consists of a mixture of small round shape grains and larger elongated grains containing low-angle grain boundaries. Texture measurements show the appearance of the α-fiber (<110>//extrusion direction) and an increase in its intensity during the extrusion process in the larger grains. The fragmentation of the large elongated grains by Continuous Dynamic Recrystallization (CDRX) partially occurs in ODS materials depending on precipitation reinforcement. For smaller grains, plastic deformation has no effect on crystallographic orientation and grain shape, indicating a grain boundary accommodation phenomenon as the major deformation mechanism. Precipitation density not only impacts the intragranular dislocation glide activity, but also reduces CDRX kinetics in coarse grains. [ABSTRACT FROM AUTHOR]

Strain-rate cycling tests associated with ultrasonic oscillation were conducted at 80–239 K for two kinds of KCl:Sr2+ (0.05 mol% in the melt) single crystals: quenched and annealed specimens. Examining the relationship of temperature and dislocation velocity-effective stress exponent, m*, estimated from the data obtained in this series, we could find the suitable force-distance relation between a dislocation and the impurity. The force-distance relation for the quenched specimen approached to the Fleischer's model taking account of the Friedel relation rather than the Fleischer's model. As for the annealed specimen, the SQ was the most appropriate of the three models: the SQ, the PA, and the TR indicate a square, a parabolic, and a triangular force-distance profile respectively. The three force-distance relations are taken account of the Friedel relation. By annealing the quenched specimen, m* became low at a given temperature. This may have been caused by the following two phenomena. First, the concentration of weak obstacles to dislocation motion decreased after the heat treatment. Secondly, the resistance to movement of a dislocation in the quenched specimen was weakened by annealing it, e.g., F0 was reduced to about one-third and ϕ0 increased from 154 to 172 degrees. F0 and ϕ0 are the force acted on the dislocation and the bending angle of dislocation by the weak obstacle such as the impurity. Therefore, it may be deduced that the dislocation velocity in the quenched specimen is more sensitive to the effective stress due to the impurities than that in the annealed specimen at the temperature. [ABSTRACT FROM AUTHOR]

The mechanisms of macroscopic ductile deformation of brittle ceramics during grinding operations are still unclear although the microcracking-free processes of grinding have been used widely in industry. This paper analyses the dislocation structure in the plastic zones of scratched alumina. It is shown that there exist three independent slip systems (two basal and one prism) and two twin systems even far below the critical onset temperatures of these systems that is usually expected. This new finding explains the formation mechanism of the macroscopic deformation induced by scratching and indicates that it is the stress field around the cutting edge which plays an important role in the activation of the slip systems. The present study offers a new insight into the mechanism understanding of material removal in ductile-regime grinding of ceramic materials. [ABSTRACT FROM AUTHOR]

A microgrid extensometry method has been developed and used to obtain information about intragranular and intergranular creep mechanisms. An oxide grid was deposited on a creep specimen using an electron lithography technique. This oxide grid offers high backscattered electron contrast and can withstand long duration creep tests under vacuum in the 700–850 °C range without degradation. Specific methods were used to measure in-plane displacements at the grid nodes or at the grain boundaries using correlation of grid images taken before and after the creep test. The local strain and grain boundary sliding (GBS) data were then calculated. Combined information about grain boundary crystallography and GBS has been obtained by superimposing the electron backscattered diffraction (EBSD) map on the deformation maps. To illustrate the potential of this set of processes, two examples of application on a nickel-base disc superalloy are presented. The first one concerns the influence of the creep temperature on the local strain and the GBS. The second application quantitatively shows the influence of grain boundary character on GBS of this material. [ABSTRACT FROM AUTHOR]

The theory of thermally-activated slip is used to derive a crystal-plasticity materials constitutive model for deformation of OFHC copper single crystals. The mechanical response of the polycrystalline material is next determined from the single-crystalline materials constitutive relations using the classical Taylor approximation for apportionment of the deformation gradient between grains. Simulations of the deep drawing of cylindrical cups from as-rolled OFHC-copper blanks are next carried out using an explicit finite element formulation. The results obtained show that the crystallographic texture in as-rolled sheets, which can be accounted for through the use of crystal-plasticity, gives rise to rim-earing in fully-drawn cups. It is further shown that the extent of rim-earing can be greatly reduced by properly modifying the shape of the blank. A procedure is next proposed for optimization of the blank shape. [ABSTRACT FROM AUTHOR]