No abstract available

[en] EUROFER'97 was processed by equal channel angular pressing (ECAP) at 823 K for a total of 4 or 8 passes, using a die angle of 105^o, and its microstructure and tensile behavior in temperature range 568-873 K investigated. A single ECAP pass developed a deformation texture {1 1 0} <0 1 1> that was practically stable during subsequent ECAP passes. The materials processed by 1 or 2 passes exhibited a fine microstructure of recovered subgrains and tensile behavior very similar to that for the tempered material in the as-received condition. The materials processed using 4 or 8 passes exhibited nearly equiaxial submicron grained structures with a high density of high-angle grain boundaries. These materials became softer than the as-received material at a testing temperature of ∼823 K. EUROFER ECAP processed under the present conditions exhibited hardening ratio somewhat higher than that of the as-received material.

[en] A polycrystal undergoes microstructural changes to reach a lower energy state. In particular, the system evolves so as to reduce the total grain boundary energy. A simple two-dimensional model of a polycrystal comprised of randomly oriented crystalline grains suggests that energy minimization reduces or eliminates any spatial correlation among high-energy grain boundaries. Thus grain boundary engineering not only reduces the density of high-energy boundaries, but it prevents their organization into a coarse, albeit discontinuous, network

[en] Most theories developed to explain interaction of impurities with a moving grain boundary involve a uniform excess impurity concentration distributed along a planar grain boundary. As boundary velocity increases, the excess impurities exert a net drag force on the boundary until a level is reached whereat the drag force no longer can balance the driving force and breakaway of the boundary from these impurities occurs. In this investigation, assumptions of a uniform lateral impurity profile and a planar grain boundary shape are relaxed by allowing both forward and lateral diffusion of impurities in the vicinity of a grain boundary. It is found that the two usual regions (drag of impurities by, and breakaway of a planar grain boundary) are separated by an extensive region wherein a uniform lateral impurity profile and a planar grain boundary shape are unstable. It is suspected that, in this unstable region, grain boundaries assume a spectrum of more complex morphologies and that elucidation of these morphologies can provide the first definitive description of the breakaway process and insight to more complex phenomena such as solid-solution strengthening, grain growth and secondary recrystallization

No abstract available

[en] Microstructural changes in nanocrystalline materials are driven by their high grain boundary density and greater mobility at elevated temperatures. For instance, segregation, precipitation, and grain growth become active as the temperature increases. The potential of nanometallic multilayers as a route to synthesize nanostructures is explored examining the microstructural evolution of Hf-Ti nanometallic multilayers through differential scanning calorimetry and characterization at different temperatures. Thermodynamic and kinetic calculations were performed to understand the observed transitions. This study presents a guide to microstructural changes in nanometallic multilayers which can be used to develop new nanocrystalline systems.

[en] The control of segregation-induced grain boundary (GB) strengthening is a strategy for improving the ductility and intergranular fracture of metals. In this work, by first-principles calculations we study the strengthening effect of transition metals on a series of tungsten GBs to uncover its dependence on the GB structures and the radius of solute itself. The results show that the GB strengthening depends strongly on the GB structures. The solutes tend to enhance cohesion for GBs with larger GB energies, while they decrease cohesion for those with smaller GB energies. In addition, it is generally found that for all GBs the strengthening energies of elements correlate positively with their metallic radii, implying that the size effect plays a major role in the GB strengthening in tungsten. By analyzing the strengthening effect of solutes at different segregated sites, we find that the oversized solutes (Zr, Nb, Hf, Ta) act as cohesion enhancers when segregated in the GB plane, while the undersized elements (Ru, Rh, Re, Os, Ir) strengthen the GBs at the nearest neighbor positions. Experimentally, we show that the presence of Zr/Ta/Re impurity, which leads to an increase of intergranular fracture, increases the theoretical tensile strength and the GB cohesion. These results suggest that the improved fracture resistance by Re, Ta and Zr originates mainly from the GB strengthening due to their segregation.

[en] In the present work, the influence of subgrain boundaries on the coarsening kinetics of individual grains embedded in an average environment as well as within a grain structure is investigated. It is found that a specific introduction of subgrain boundaries not only influences the speed with which grains shrink or grow, but in contrast to the von Neumann-Mullins-law, a distinct manipulation of the location of the subgrain boundaries allows even grains with few edges to grow, while grains with many edges shrink. During these circumstances one fact stays the same: the area of the individual grains is a linear function of annealing time as long as the environment does not change. (paper)

[en] Nabarro-Herring diffusional creep (DC) is of importance at low stresses and high temperatures in fine grained materials. In a previous analysis, the entire DC strain was considered to be arising from a single step of each of diffusion and sliding. It is more realistic to consider the overall strain of DC in terms of several infinitesimal steps of diffusion and sliding. Making use of a two-dimensional array of hexagonal grains and treating the DC as a multistep process involving discrete sliding and diffusion steps of varying number of magnitude, the relative contributions of the two processes are assessed as a function of total DC strain. Our analysis is further based on conservation of mass or area (two dimensional case) of grains and invariance of grain boundary angles because of deformation. (orig./MM)

[en] Numerical simulations of the time-dependent Swift-Hohenberg equation are used to test the predictions of Cross [Phys. Rev. A 25, 1065 (1982)] that Rayleigh-Benard convection in the form of straight rolls or of an array of dislocations may be observed in an annular domain, depending on the values of inner radius r₁, outer radius r₂, reduced Rayleigh number ε, and initial states. As r₁ is decreased for a fixed r₂ and for different choices of ε and of symmetric and random initial state, we find that there are indeed ranges of these parameters for which the predictions of Cross are qualitatively correct. However, when the radius difference r₂-r₁ becomes larger than a few roll diameters, a new pattern is observed consisting of stripe domains separated by radially oriented grain boundaries. The relative stabilities of the various patterns are compared by evaluating their Lypunov functional densities