The relationship between yield stress and the distribution of microscopic plastic deformation was numerically investigated by using a crystal plasticity finite element method (CP-FEM) in the model where particles were randomly distributed. It was in order to reveal which particle spacing. i.e., the maximum, minimum or average particle spacing, can be taken as the representative length which controls yielding. The critical resolved shear stress for the onset of the slip deformation in any element was defined under the extended equation in the Bailey-Hirsch type model. The model includes the term of the Orowan stress obtained from the local values of the representative length. Each particle spacing was distributed with a standard deviation of approximately 2 to 3 times larger than the average particle spacing. The macroscopic mechanical properties obtained with CP-FEM were in good agreement with those experimentally obtained. The onset of microscopic slip deformation depended on the particle distribution. Plastic deformations started first in the area where the particle size is larger, then the plastic region grows in the areas where the particle spacing is smaller. Slip deformation had occurred in 90% of the matrix phase by the macroscopic yield point. The length factor in the Orowan equation was the average spacing of the particles in the model, which is in good agreement with Foreman and Makin. The CP-FEM indicated that in dispersed hardened alloys, microscopic load transfer occurred between the areas where the large particles spacing and the small one at the yielding.
TEM specimen preparation: ion milling by 6 keV Ar at R.T. (JEOL Cross Section Polisher ), focused ion beam by 30 keV Ga at R.T. (HITACHI FB 2000) TEM utilized: JEM 1300NEF with EELS analyzer (1250 kV, zero loss image) Fatigue condition: crack growth rate (da/dN ), 0.25 mm/cycle, in helium gas
Fundamental mechanism governing the fracture toughness of materials is reviewed in terms of a concept of the interaction between a crack and dislocations. The mechanism of brittle-to-ductile transition (BDT) is demonstrated using a simple model based on dislocation dynamics and the theory of crack-tip shielding by dislocations. The effects of dislocation mobility as well as dislocation sources on the BDT behavior are discussed, which enables us to understand the various factors such as grain refinement influencing fracture toughness.
Body-centred cubic (bcc) high-entropy alloys exhibit high strength. However, their yield behaviour and controlling mechanisms are still ambiguous. In this study, the temperature dependence of the yield stress, effective stress, activation volume, and activation enthalpy in a polycrystalline bcc refractory high-entropy alloy of TiZrNbHfTa were measured by tensile tests at 77–750 K. At temperatures above 650 K, the temperature dependence of the yield stress disappeared, and serration appeared in stress–strain curves. By extrapolating the effective shear stress to 0 K, the Peierls stress was estimated to be 580 MPa. The value was compared with different crystals using the relationship τpμ and hb, where τp is the Peierls stress, μ is the shear modulus, h is the distance between the slip planes, and b is the Burgers vector. The τpμ value in this study was slightly higher than that of other bcc crystals. The activation enthalpy below 260 K corresponds to that of other bcc crystals with high Peierls potentials, suggesting kink-pair nucleation is the controlling mechanism of the dislocation glide in this temperature range. Meanwhile, the activation enthalpy above 260 K deviated from the trendline, indicating the changes in the dislocation glide from the kink-pair nucleation to other one.
We investigated the electromagnetic wave transmission property of a strontium titanate $(\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3})$ hole array (SHA) in the terahertz region and found that the resonance peak appears at the same frequency as metal hole arrays (MHA). The resonant transmission phenomenon observed in MHAs has been now understood in terms of the resonant excitation of surface plasmon polaritons (SPPs) on metal surfaces. The existence of the SPPs, or more generally surface waves, requires the real part of the permittivity ${\ensuremath{\epsilon}}_{m}^{\mathrm{Re}}$ of constitutive materials to be negative, and therefore the existence of a surface wave cannot be expected for $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ because of the positive value of ${\ensuremath{\epsilon}}_{m}^{\mathrm{Re}}$ at relevant frequencies. However, when the imaginary part of the permittivity is very high, the attenuation constant of the electromagnetic wave in the direction perpendicular to the surface becomes a complex number, indicating that the electromagnetic wave can be localized in the vicinity of the surface. The resonant transmission phenomenon observed in SHA is attributed to such a complex surface wave excited on the $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ surface.
Microscopic deformation and fracture behavior of a ferrite-bainite dual phase steel was investigated by the micro-grid method and FE analysis to understand the inherent conditions of plastic instability and ductile fracture. The micro-grid method, which the microscopic strain is measured by the displacement of grids with 500 nm intervals drawn on the specimen surface, clearly revealed that the shear deformation along the lath structure in the bainite phase was seen before reaching the maximum loading point. Then, voids were observed in the ferrite phase adjacent to the ferrite/bainite boundary, where showing higher strain concentration. From the FE analysis with the model simulating actual ferrite-bainite microstructure, stress distribution was seen in the bainite phase, and high stressed regions could cause the shear deformation of the bainite phase. The local shear deformation in the bainite phase decreased strain hardenability and triggered the macroscopic plastic instability. It is considered that the macroscopic plastic instability accelerates the strain localization, and promotes the void nucleation and growth. Ductile fracture path was also visualized by the micro-grids in the ferrite phase along the shear deformation bands which is connecting the high strain regions. Development of shear deformation bands inside the ferrite phase was well simulated with the FE analysis, same as the development of high stressed region in the bainite phase in the early stage. It can be stated, therefore, that plastic instability and ductile fracture of dual phase steel is a structure dependent phenomenon which is strongly controlled by the morphologies of each constituent phases.
