Showing papers on "Grain boundary strengthening published in 2002"

High tensile ductility in a nanostructured metal.

Abstract: Nanocrystalline metals--with grain sizes of less than 100 nm--have strengths exceeding those of coarse-grained and even alloyed metals, and are thus expected to have many applications. For example, pure nanocrystalline Cu (refs 1-7) has a yield strength in excess of 400 MPa, which is six times higher than that of coarse-grained Cu. But nanocrystalline materials often exhibit low tensile ductility at room temperature, which limits their practical utility. The elongation to failure is typically less than a few per cent; the regime of uniform deformation is even smaller. Here we describe a thermomechanical treatment of Cu that results in a bimodal grain size distribution, with micrometre-sized grains embedded inside a matrix of nanocrystalline and ultrafine (<300 nm) grains. The matrix grains impart high strength, as expected from an extrapolation of the Hall-Petch relationship. Meanwhile, the inhomogeneous microstructure induces strain hardening mechanisms that stabilize the tensile deformation, leading to a high tensile ductility--65% elongation to failure, and 30% uniform elongation. We expect that these results will have implications in the development of tough nanostructured metals for forming operations and high-performance structural applications including microelectromechanical and biomedical systems.

Grain Boundaries and Dislocations

Abstract: The hardness of coarse-grained materials is inversely proportional to the square root of the grain size. But as Van Swygenhoven explains in her Perspective, at nanometer scale grain sizes this relation no longer holds. Atomistic simulations are providing key insights into the structural and mechanical properties of nanocrystalline metals, shedding light on the distinct mechanism by which these materials deform.

Atomic mechanism for dislocation emission from nanosized grain boundaries

Abstract: The present work deals with the atomic mechanism responsible for the emission of partial dislocations from grain boundaries (GB's) in nanocrystalline metals. It is shown that in 12 and 20 nm grain size samples GB's containing GB dislocations can emit a partial dislocation during deformation by local atomic shuffling and stress-assisted free volume migration. The free volume is often emitted or absorbed in a neighboring triple junction. It is further suggested that the degree of delocalization surrounding the grain boundary dislocation determines whether atomic shuffling can associate displacements into the Burgers vector necessary to emit a partial dislocation. Temporal analysis of atomic configurations during dislocation emission indicates that creation and propagation of the partial might be separate processes.

Crystal plasticity model with enhanced hardening by geometrically necessary dislocation accumulation

Abstract: A strain gradient dependent crystal plasticity approach is used to model the constitutive behaviour of polycrystal FCC metals under large plastic deformation. Material points are considered as aggregates of grains, subdivided into several fictitious grain fractions: a single crystal volume element stands for the grain interior whereas grain boundaries are represented by bi-crystal volume elements, each having the crystallographic lattice orientations of its adjacent crystals. A relaxed Taylor-like interaction law is used for the transition from the local to the global scale. It is relaxed with respect to the bi-crystals, providing compatibility and stress equilibrium at their internal interface. During loading, the bi-crystal boundaries deform dissimilar to the associated grain interior. Arising from this heterogeneity, a geometrically necessary dislocation (GND) density can be computed, which is required to restore compatibility of the crystallographic lattice. This effect provides a physically based method to account for the additional hardening as introduced by the GNDs, the magnitude of which is related to the grain size. Hence, a scale-dependent response is obtained, for which the numerical simulations predict a mechanical behaviour corresponding to the Hall–Petch effect. Compared to a full-scale finite element model reported in the literature, the present polycrystalline crystal plasticity model is of equal quality yet much more efficient from a computational point of view for simulating uniaxial tension experiments with various grain sizes.

Microstructure evolution and thermal properties in nanocrystalline Cu during mechanical attrition

Abstract: The microstructural evolution and thermal properties of nanocrystalline (nc) Cu during mechanical attrition were investigated by using quantitative x-ray-diffraction and thermal analysis techniques. Upon milling of the Cu powders with coarse grains, the grain sizes are found to decrease gradually with the milling time, and remain unchanged at a steady-state value (about 11 nm) with continued milling. The microstrain and the stored enthalpy increase to maximum values during the grain refinement, and decrease then increase to the second maxima and decrease again within the milling stage of steady-state grain size, while the lattice parameter remains unchanged during the entire milling process. The grain boundary (GB) enthalpy of the nc Cu was estimated, showing a GB softening-hardening-softening cyclic variation within the steady-state milling. The present work indicated with clear experimental evidence that even within the milling stage of steady-state grain size, the microstructure (both the GB's and the crystallites) of nc materials is still changing, which may result from the GB sliding.

Segregation effects at grain boundaries in fluorite-structured ceramics

Abstract: The atomic-scale structure, composition, and chemistry of grain boundaries in two fluorite-structured ceramic materials were characterized by a combination of Z-contrast imaging and electron energy-loss spectroscopy (EELS). In the case of a symmetric 24° [001] tilt bicrystal of yttria-stabilized-zirconia (YSZ), a shift in the zirconium M-edge onset and a change in the yttrium and zirconium M-edge ratios at the boundary indicate an increase in the number of electrons in the boundary plane. A detailed study of the structure and composition indicates that this is caused by an increase in the number of oxygen vacancies in the grain boundary core that is partially compensated by yttrium segregation. Studies of grain boundaries in an industrial Gd-doped ceria ceramic reveals similar changes in vacancy/dopant profiles indicating that these effects may be generic to grain boundaries in fluorite-structured materials.

Scaling behavior of grain-rotation-induced grain growth.

Abstract: Recent investigations of grain growth in nanocrystalline materials have revealed a new growth mechanism: grain-rotation-induced grain coalescence. Based on a simple model employing a stochastic theory and using computer simulations, here we investigate the coarsening of a polycrystalline microstructure due solely to the grain-rotation coalescence mechanism. Our study demonstrates that this mechanism exhibits power-law growth with a universal scaling exponent. The value of this universal growth exponent is shown to depend on the assumed mechanism by which the grain rotations are accommodated.

Intergranular fracture in nanocrystalline metals

Abstract: Crack propagation studies in nanocrystalline Ni samples with mean grain sizes ranging from 5 to 12 nm are reported using atomistic simulations. For all grain sizes pure intergranular fracture is observed. Intergranular fracture is shown to proceed by the coalescence of microvoids formed at the grain boundaries ahead of the crack. The energy released during propagation is higher than the Griffith value, indicating an additional grain-boundary accommodation mechanism.

Cooperative processes during plastic deformation in nanocrystalline fcc metals: A molecular dynamics simulation

Abstract: Large scale molecular dynamics (MD) simulations are used to simulate the plastic deformation of a nanocrystalline model Ni sample with an average grain size of 5 nm containing 125 grains at 800 K up to 4.0% plastic strain. For the first time in MD simulation, emerging shear planes involving several grain boundaries have been observed indicating cooperative plastic deformation activity between grains. It is found that three mechanisms have been involved in the development of such shear planes: grain-boundary migration, continuity of shear plane via intragranular slip, and rotation and coalescence of grains.

Effects of the grain size on the corrosion behavior of refined AISI 304 austenitic stainless steels

Abstract: Although there have been many studies on fine grained ferritic steels, only a few research reports are available on refined austenitic stainless steels and, in particular, on the influence of the grain size on the corrosion resistance of this class of material [1, 2]. The grain size of ferritic steels can be easily induced by phase transformation, but in austenitic alloys, following the absence of a phase transformation, the grain diameter is usually controlled by recrystallization after cold working [3]. This method is mainly affected by the working temperature, amount of deformation and recrystallization temperature. Recrystallization after hot rolling is reported to have the effect of grain refining [4] but this method seems to be limited. In a previous paper [5] we examined the effect of subzero working on the grain refining of austenitic stainless steels. In particular, ultrafine grained AISI 304 stainless steel of ca. 1 μm average grain size was obtained by applying the reverse transformation of martensite to austenite on subzeroworked steel annealed at low temperatures. Up to now, the corrosion behavior of such ultrafinegrained austenitic stainless steels has not been reported. This paper deals with the corrosion behavior, especially general corrosion (GC), intergranular corrosion (IGC) and pitting corrosion (PC) of ultrafine-grained AISI 304 stainless steel. Results are compared with those of similar measurements on standard AISI 304 steel. The chemical composition of the AISI 304 stainless steel, obtained from a commercial batch, is shown in Table I. After subzero working down to 90% thickness reduction, the material was subjected to the following four heat treatments in order to obtain different microstructures: annealing at 800 ◦C for 160 s and 900 s (specimens A and B respectively) and at 1000 ◦C for 10 s and 600 s (specimens C and D respectively). The grain sizes corresponding to the above specimens, as measured by automatic image analyzer, are shown in Table II. The typical microstructures of the 1 μm and 50μm specimens are shown in Fig. 1. Tensile properties of the specimens are shown in Fig. 2. Ultimate tensile stress and 0.2% yield stress increase with decreasing grain size, according to the Hall Petch relation [6]. Steel materials were machined into corrosion test specimens of 15 × 15 × 1 mm. The specimen surface was polished by using increasingly finer abrasive papers, starting with a 300 grit paper and finishing up with