Showing papers on "Grain boundary strengthening published in 2007"

Influence of friction stir welding parameters on grain size and formability in 5083 aluminum alloy

Abstract: Friction stir welding (FSW) has received a great deal of attention as a new solid-state welding technique. In the present study, the relationship between the microstructure of stir zone and the mechanical property of FS-welded 5083 aluminum alloy was investigated. The microstructures of the stir zones consisted of fine equiaxed grains at various FSW conditions in FS-welded 5083 Al alloy. However, the grain size of the stir zone decreased with the decrease in friction heat flow during FSW. The ductility in FS-welded 5083 Al alloy increased with the decrease in friction heat flow. It was indicated that the formability in FS-welded 5083 Al alloy was improved by the refinement of grain size of the stir zone.

Paleowattmeters: A scaling relation for dynamically recrystallized grain size

Abstract: During dislocation creep, mineral grains often evolve to a stable size, dictated by the deformation conditions. We suggest that grain-size evolution during deformation is determined by the rate of mechanical work. Provided that other elements of microstructure have achieved steady state and that the dissipation rate is roughly constant, then changes in internal energy will be proportional to changes in grain-boundary area. If normal grain-growth and dynamic grain-size reduction occur simultaneously, then the steady-state grain size is determined by the balance of those rates. A scaling model using these assumptions and published grain-growth and mechanical relations matches stress–grain-size relations for quartz and olivine rocks with no fitting. For marbles, the model also explains scatter not rationalized by assuming that recrystallized grain size is a function of stress alone. When extrapolated to conditions typical for natural mylonites, the model is consistent with field constraints on stresses and strain rates.

Dislocation structures. Part I. Grain orientation dependence

Abstract: To clarify the effect of grain orientation on the evolution of dislocation structures in metals of medium-to-high stacking fault energy, detailed TEM characterization of structures was carried out for more than 350 individual grains in Al and Cu deformed in tension or by cold rolling up to moderate strain levels (ϵvM ≤ 0.8). Efforts were made to obtain a precise description of the three-dimensional arrangement of the dislocation structures and to determine the crystallographic plane of extended dislocation boundaries (geometrically necessary boundaries). A universal pattern of structural evolution characterized by a formation of three types of structure was found in both metals, irrespective of material parameters (stacking fault energy, grain size and impurity) and deformation conditions (deformation mode, strain and strain rate). The key parameter controlling the formation of the different structural types was found to be grain orientation with respect to the deformation axis (axes) and a clear relation...

Higher-order stress and grain size effects due to self-energy of geometrically necessary dislocations

Abstract: The higher-order stress work-conjugate to slip gradient in single crystals at small strains is derived based on the self-energy of geometrically necessary dislocations (GNDs). It is shown that this higher-order stress changes stepwise as a function of in-plane slip gradient and therefore significantly influences the onset of initial yielding in polycrystals. The higher-order stress based on the self-energy of GNDs is then incorporated into the strain gradient plasticity theory of Gurtin [2002. A gradient theory of single-crystal viscoplasticity that accounts for geometrically necessary dislocations. J. Mech. Phys. Solids 50, 5–32] and applied to single-slip-oriented 2D and 3D model crystal grains of size D. It is thus found that the self-energy of GNDs gives a D - 1 -dependent term for the averaged resolved shear stress in such a model grain under yielding. Using published experimental data for several polycrystalline metals, it is demonstrated that the D - 1 -dependent term successfully explains the grain size dependence of initial yield stress and the dislocation cell size dependence of flow stress in the submicron to several-micron range of grain and cell sizes.

A systematic study of hcp crystal orientation and morphology effects in polycrystal deformation and fatigue

Abstract: Elastically anisotropic, physically based, length-scale- and rate-dependent crystal plasticity finite element investigations of a model hcp polycrystal are presented and a systematic study was carried out on the effects of combinations of crystallographic orientations on local, grain-level stresses and accumulated slip in cycles containing cold dwell. It is shown that the most damaging combination is the one comprising a primary hard grain with c -axis near-parallel to the loading direction and an adjacent soft grain having c -axis near-normal to the load and a prismatic slip plane at approximately 70° to the normal to the load. We term such a combination a rogue grain combination. In passing, we compare results with the Stroh model and show that even under conditions of plasticity in the hcp polycrystal, the Stroh model qualitatively predicts some of the observed behaviours. It is shown that under very particular circumstances, a morphological – crystallographic interaction occurs which leads to particularly localized accumulated slip in the soft grain and the penetration of the slip into the adjacent hard grain. The interaction effect occurs only when the (morphological) orientation of the grain boundary in the rogue grain combination coincides (within approximately ±5°) with the (crystallographic) orientation of an active slip system in the soft grain. It is argued that the rogue grain combination and the morphological–crystallographic interaction are responsible for fatigue facet formation in Ti alloys with cold dwell, and a possible mechanism for facet formation is presented. The experimental observations of fatigue facet formation have been reviewed and they provide considerable support for the conclusions from the crystal plasticity modelling. In particular, faceting was found to occur at precisely those locations predicted by the model, i.e. at a rogue grain combination. Some experimental evidence for the need for a crystallographic–morphological interaction in faceting is also presented.

Hall–Petch Behavior in Ultra-Fine-Grained AISI 301LN Stainless Steel

Abstract: An ultra-fine-grained AISI 301LN austenitic stainless steel has been achieved by heavy cold rolling, to induce the formation of martensite, and subsequent annealing at 800 °C, 900 °C, and 1000 °C, from 1 to 100 seconds. The microstructural evolution was analyzed using transmission electron microscopy and the yield strength determined by tension testing. Ultra-fine austenite grains, as small as ∼0.54 μm, were obtained in samples annealed at 800 °C for 1 second. For these samples, tensile tests revealed a very high yield strength of ∼700 MPa, which is twice the typical yield strength of conventional fully annealed AISI 301LN stainless steels. An analysis of the relationship between yield strength and grain size in these submicron-grained stainless steels indicates a classical Hall–Petch behavior. Furthermore, when the yield dependence on annealing temperature is considered, the results show that the Hall–Petch relation is due to an interplay between fine-grained austenite, solid solution strengthening, precipitate hardening, and strain hardening.

Multiscale modelling of dislocation/grain boundary interactions. II. Screw dislocations impinging on tilt boundaries in Al

Abstract: The interaction of dislocations with grain boundaries (GBs) determines a number of important aspects of the mechanical performance of materials, including strengthening and fatigue resistance. Here, the coupled atomistic/discrete-dislocation (CADD) multiscale method, which couples a discrete dislocation continuum region to a fully atomistic region, is used to study screw-dislocations interacting with Σ3, Σ11, and Σ9 symmetric tilt boundaries in Al. The low-energy Σ3 and Σ11 boundaries absorb lattice dislocations and generate extrinsic grain boundary dislocations (GBDs). As multiple screw dislocations impinge on the GB, the GBDs form a pile-up along the GB and provide a back stress that requires increasing applied load to push the lattice dislocations into the GB. Dislocation transmission is never observed, even with large GBD pile-ups near the dislocation/GB intersection. Results are compared with experiments and previous, related simulations. The Σ9 grain boundary, composed from a more complex set of str...

Dislocation nucleation from bicrystal interfaces and grain boundary ledges: Relationship to nanocrystalline deformation

Abstract: Molecular dynamics simulations are used to evaluate the primary interface dislocation sources and to estimate both the free enthalpy of activation and the critical emission stress associated with the interfacial dislocation emission mechanism. Simulations are performed on copper to study tensile failure of a planar Σ5 {2 1 0} 53.1° interface and an interface with the same misorientation that contains a ledge. Simulations reveal that grain boundary ledges are more favorable as dislocation sources than planar regions of the interface and that their role is not limited to that of simple dislocation donors. The parameters extracted from the simulations are utilized in a two-phase composite mesoscopic model for nanocrystalline deformation that includes the effects of both dislocation emission and dislocation absorption mechanisms. A self-consistent approach based on the Eshelby solution for grains as ellipsoidal inclusions is augmented by introduction of stress concentration in the constitutive law of the matrix phase to account for more realistic grain boundary effects. Model simulations suggest that stress concentration is required in the standard continuum theory to activate the coupled grain boundary dislocation emission and absorption mechanisms when activation energy of the dislocation source is determined from atomistic calculation on grain boundaries without consideration of impurities or other extrinsic defects.

The influence of silicon on the strength and fracture toughness of molybdenum

Abstract: Mo–Si alloys containing up to 1 wt.% Si were fabricated by powder-metallurgical processing and their lattice parameters, elastic constants, densities, grain sizes, strengths, ductilities, and fracture toughness values were measured. The yield strength was insensitive to the grain size, i.e., a Hall–Petch relationship was not observed. Generally, Si additions caused pronounced solid solution strengthening. However, for small Si concentrations (≤0.1 wt.%) solid solution softening was observed at room temperature and below. With increasing Si concentration, the room temperature ductility and fracture toughness dropped precipitously. This is attributed to the increase in strength and a transition from transgranular to intergranular fracture.

A boundary cohesive grain element formulation for modelling intergranular microfracture in polycrystalline brittle materials

Abstract: In this paper, a cohesive grain boundary integral formulation is proposed, for simulating intergranular microfracture evolution in polycrystalline brittle materials. Artificially generated polycrystalline microstructures are discretized using the proposed anisotropic boundary element method, considering the random location, morphology and material orientation of each grain. Each grain is assumed as a single crystal with general elastic orthotropic mechanical behaviour. Crack initiation and propagation along the grain boundaries interfaces are modelled using a linear cohesive law, considering mixed mode failure conditions. Furthermore, a non-linear frictional contact analysis is performed over cracked grain interfaces to encounter cases where crack surfaces come into contact, slide or separate. The effect of randomly located pre-existing flaws on the overall behaviour and microcracking evolution of a polycrystalline material is also investigated for different Weibull moduli. The stochastic effects of each grain morphology-orientation, internal friction and randomly distributed pre-existing flaws, under different loading conditions, are studied probabilistically by simulating various randomly generated microstructures. Copyright © 2006 John Wiley & Sons, Ltd.

The influence of soft, precipitate-free zones at grain boundaries in Ti and Al alloys on their fatigue and fracture behavior

Abstract: This paper discusses the effect of the presence of soft zones along grain boundaries in high-strength Al and β-Ti alloys. Soft zones are deformed preferentially and therefore have a strong effect on fracture related properties. The presentation focuses on the ductility, the high-cycle fatigue strength, and the fracture toughness. For microstructures with pancake-shaped grains, the anisotropy of the mechanical properties is discussed on the basis of the effective slip length, determined by the length of the soft grain boundary zones. In high-strength Al alloys, the presence of hard inclusions additionally affects the fracture behavior. For β-Ti alloys, the role of the strength difference between the age hardened matrix and the soft grain boundary layers is discussed.

Plasticity and strength of micro- and nanocrystalline materials

Abstract: This review is devoted to the effect of grain boundaries on the deformational and strength properties of poly-, micro-, and nanocrystalline materials (predominantly metals). The main experimental facts and mechanisms concerning the dislocation structure and mechanical behavior of these materials over wide ranges of temperatures and grain sizes are presented. The experimentally established regularities are analyzed theoretically in terms of equations of dislocation kinetics taking into account the properties of grain boundaries as barriers, sources, and sinks for dislocations and as places where dislocations annihilate. The origin of the Hall-Petch relations for the yield stress and the flow stress as functions of the grain size, as well as the deviations from these relations observed in nano- and microcrystalline materials, is discussed in detail in terms of the dislocation-kinetics approach. Embrittlement of micro- and nanocrystalline materials at low temperatures and superplasticity of these materials at elevated temperatures are also analyzed in terms of the dislocation-kinetics approach.

Grain boundaries and pinning in bulk MgB2

Abstract: We report the grain size dependence of critical current and grain boundary pinning in bulk MgB2. By combining polarized optical microscopy and electron backscatter diffraction, we obtain evidence of special grain boundaries with a high density of dislocations that are able to provide high critical current in MgB2 polycrystals. We argue that reduction of grain size to the nanoscale level is sufficient to provide the critical current densities required for large-scale applications at the boiling temperature of liquid hydrogen.

Prediction for the austenite grain size in the presence of growing particles in the weld HAZ of Ti-microalloyed steel

Abstract: A model to predict the austenite grain size in a Ti-microalloyed steel weld heat affected zone (HAZ) was developed. Grain boundary mobility for the austenite grain growth was expressed as a function of aging temperature and alloying elements. By analyzing isothermal austenite grain growth behavior, the Zener coefficient of cubic TiN particle was measured. From quantification of the effect of grain boundary pinning by TiN particle and alloying elements on the grain boundary mobility, an isothermal grain growth model of Ti-microalloyed steel is presented. The predicted austenite grain sizes from the proposed model were in agreement with the experimental results. Finally, combining with the additivity rule, a general austenite grain growth model during the continuous welding thermal cycle was developed.

Relationship between hardness and grain size in electrodeposited copper films

Abstract: Copper films were electrodeposited under various conditions and the relationship between the hardness and grain size of the films was investigated. The Vickers hardness depended on the processing conditions. This is because the processing conditions affected the grain size. In particular, nanocrystalline Cu film with a grain size of 31 nm was obtained by optimizing the electrodeposition conditions. The hardness of the nanocrystalline film deviated from the Hall–Petch relationship because the grain size dependence of hardness is smaller for the grain sizes of 100 nm. Also, the constants Hv 0 and k Hv in the Hall–Petch relationship for films processed with thiourea were different from those for films processed with gelatin. The differences may not be related to texture but to superabundant vacancies generated in the process of electrodeposition.

Thin film compressive stresses due to adatom insertion into grain boundaries.

Abstract: Atomic simulations of the growth of polycrystalline Ni demonstrate that deposited atoms incorporate into the film at boundaries, resulting in compressive stress generation. Incorporated atoms can also leave the boundaries and thus relieve compressive stress. This leads to a complex interplay between growth stress, adatom incorporation, and surface structure. A simple, theoretical model that accounts for grain size effects is proposed and is in good agreement with simulation results.