Showing papers on "Grain boundary strengthening published in 2003"

Dynamic continuous recrystallization characteristics in two stage deformation of Mg-3Al-1Zn alloy sheet

Abstract: Dynamic recrystallization (DRX) characteristics of a Mg/3Al/1Zn (AZ31) alloy sheet were investigated at temperatures ranging from 200� /450 8C and constant strain rates of 1/10 4 � /2/10 4 s 1 . The average grain size of the as-received alloy was 12 mm and can be refined to 6 mm via deformation at 250 8C, 1/10 4 s 1 to a strain level of 60%. Grain refinement was less effectiv ea t higher temperatures due to rapid grain growth. The grain refinement was attributed to dynamic continuous recrystallization which involves progressive increase in grain boundary misorientation and conversion of low angle boundaries into high angle boundaries. During DRX, subgrains were developed through the conversion of dislocation cell walls into subgrain boundaries. The presence of precipitates was not essential for dynamic recrystallization in the magnesium alloy being investigated because of its limited slip systems, low stacking fault energy and high grain boundary diffusion rate. # 2003 Elsevier Science B.V. All rights reserved.

Deformation mechanism in nanocrystalline Al: Partial dislocation slip

Abstract: We report experimental observation of a deformation mechanism in nanocrystalline face-centered-cubic Al, partial dislocation emission from grain boundaries, which consequently resulted in deformation stacking faults (SFs) and twinning. These results are surprising because (1) partial dislocation emission from grain boundaries has not been experimentally observed although it has been predicted by simulations and (2) deformation stacking faults and twinning have not been reported in Al due to its high SF energy.

Hall–Petch relationship in friction stir welds of equal channel angular-pressed aluminium alloys

Abstract: The effect of grain size on hardness in the stir zones of friction stir (FS) welds of equal channel angular (ECA)-pressed Al alloys 1050 and 5083 was examined. The hardness was found to be essentially related to grain size through the Hall–Petch relationship in the stir zone of Al alloy 1050. The kH slope of the Hall–Petch equation for the stir zone of Al alloy 1050 was different from the previously reported ones, which was attributed to dynamic recrystallisation during friction stir welding (FSW). On the other hand, the relationship between hardness and grain size in the stir zone of Al alloy 5083 was expressed by the Hall–Petch equation with a change in slope. The change in slope was attributed to the homogeneous distribution of many fine particles.

Grain size dependence of the plastic deformation kinetics in Cu

Abstract: Data on the effect of grain size d in the range of nm to mm on the plastic deformation kinetics of Cu at 77–373 K are analyzed to determine the influence of grain size on the strain rate-controlling mechanism. Three grain size regimes were identified: Regimes I (d≈10−6–10−3 m), II (d≈10−8–10−6 m) and III (d<∼10−8 m). A dislocation cell structure characterizes Regime II, which no longer occurs in Regime II. The absence of all intragranular dislocation activity characterizes Regime III. The following mechanisms were concluded to be rate-controlling for : (a) Regime I, intersection of dislocations; (b) Regime I, grain boundary shear promoted by dislocation pile-ups; and (c) Regime III, grain boundary shear. The major effect of grain size on the intersection mechanism in Regime I is on the mobile and forest dislocation densities; the effect in Regime II is on the number of dislocations and on the number of grain boundary atom sites; the effect in Regime III is on the number of grain boundary atom sites. The transition grain size from one regime to another depends on the strain rate and temperature. Crystallographic texture is also important.

Creating and destroying vacancies in solids and nonequilibrium grain boundary segregation

Abstract: How the vacancies in excess of equilibrium concentration are created and destroyed in solids is crucial for understanding many of their physical characteristics and processes. Grain boundaries are known as sources and sinks for bulk vacancies, but what exchange will occur between the grain boundary and the bulk under a stress is still obscure. In the present paper, we show that grain boundaries will work as sources to emit vacancies when a compressive stress is exerted on them and as sinks to absorb vacancies when a tensile stress is exerted. This is the sixth method of introducing and annihilating vacancies in solids in addition to the five methods mentioned in literature. At the same time, this physical process will produce solute nonequilibrium grain-boundary segregation/dilution. A set of kinetic equations is established to describe such a physical process. Additionally an attempt has been made to simulate Misra’s experimental data with the kinetic equations to justify the physical process.

Synthesis and characterization of electrodeposited nanocrystalline nickel–iron alloys

Abstract: Nickel–iron nanocrystalline alloys with different compositions and grain sizes were fabricated by electrodeposition. The iron content of the deposits was changed by varying the Ni/Fe ion ratio in the electrolyte. X-ray diffraction (XRD) analysis was applied for measuring the lattice parameter, grain size and the level of internal strain of the deposits. The strength of the alloys was evaluated by microhardness testing. The results of this study revealed that at a constant grain size of approximately 11 nm the hardness depends strongly on the iron content. The hardness showed a maximum around 20% Fe. The grain size of the alloys with 4–6% iron was very sensitive to the deposition conditions. The hardness of these alloys followed the Hall–Petch relationship. The alloys with equal or more than 35% Fe cracked upon deposition. The tendency to cracking was correlated with the level of internal stresses in the deposits.

Deformation mechanism crossover and mechanical behaviour in nanocrystalline materials.

Abstract: We use molecular dynamics simulations to elucidate the transition with decreasing grain size from a dislocation- to a grain-boundary-based deformation mechanism in nanocrystalline fcc metals. Our simulations reveal that this crossover is accompanied by a pronounced transition in the mechanical behaviour of the material; namely, at the grain size where the crossover occurs (the 'strongest size'), the strain rate under tensile elongation goes through a minimum. This simultaneous transition in both the deformation mechanism and the corresponding mechanical behaviour offers an explanation for the experimentally observed crossover in the yield strength of nanocrystalline materials, from Hall-Petch hardening to 'inverse Hall-Petch' softening.

Microstructural evolution and its effect on Hall-Petch relationship in friction stir welding of thixomolded Mg alloy AZ91D

Abstract: Microstructural evolution of a thixomolded magnesium (Mg) alloy AZ91D during friction stir welding was investigated. Friction stir welding resulted in a homogeneous microstructure consisting of fine recrystallised α-Mg grains in the thixomolded material. The microstructural homogenisation and refinement was attributed to dynamic recrystallisation accompanied by the dissolution of the eutectic structure during the welding. The grain refinement in the stir zone was effective in increasing the hardness, as predicted by the Hall-Petch equation. The effect of grain size on hardness was smaller than that in conventional and rapidly solidified AZ91. This phenomenon may be explained as being due to the microstructure of the stir zone which consisted of fine equiaxed grains with a high density of dislocations.

Grains, Growth, and Grooving

Abstract: We report the in situ investigation of grain growth and grain boundary migration, performed with a variable-temperature scanning tunneling microscope (STM) on a polycrystalline gold film. Atomic step resolution allowed us to identify the individual grains and, thus, also the grain boundaries. Our special, thermal-drift-compensated STM design made it possible to follow the same sample area over large temperature intervals. In this way, we have directly observed grain boundary migration and grain growth. In a first quantitative analysis we correlate the observed, unexpected changes in surface roughness with the evolution of the grain and grain boundary configuration.

The Nature and Consequences of Coherent Transformations in Steel

Abstract: Advanced research on structural steels has recently focused on the improvement of properties through the control of grain size. Grain refinement increases strength via the Hall-Petch relation, lowers the ductile-brittle transition by increasing resistance to transgranular cleavage, and reduces hydrogen embrittlement by minimizing interfacial fracture along grain or lath boundaries. However, given their different mechanisms, these properties require slightly different measures of the effective grain size. When the grains are smooth and random, all measures of the effective grain size are roughly equivalent. However, transformations in steel are often crystallographically coherent, producing a martensitic, bainitic or ferritic product that has either a Kurdumov-Sachs (KS) or a Nishiyama-Wasserman (NW) relation to the parent austenite. The 24 KS variants and 12 NW variants divide into three sets of eight, corresponding to the three Bain variants of the fcc→bcc transformation. Grain, packet or block boundaries that separate different Bain variants have signifi-cant misorientations of the {100} cleavage planes, but may have only slight misorientations of the {110} slip planes. It follows that grain refinement through coherent transformation is very effective in improving resistance to cleavage fracture and, if the boundary facets are small, to hydrogen embrittlement, but is often relatively ineffective in increasing strength. For this reason, grain refinement for increased strength is best done with incoherent transformations (such as the strain-induced ferrite transformation) while grain refinement for low-temperature toughness or hydrogen resistance is best done with coherent transformations that refine the effective grain size without overstrengthening to unacceptably low ductility.

Modeling the microstructure–mechanical property relationship for a 12Cr–2W–V–Mo–Ni power plant steel

Abstract: For a 12Cr–2W–V–Mo–Ni power plant steel, the microstructure was characterized and the 0.2% proof strength was quantitatively related to its microstructure and chemical composition. The microstructural features including martensite lath width, carbide size and dislocation density were measured from transmission electron microscopy and the equilibrium concentrations of alloying elements dissolved in the matrix after tempering were calculated using the ‘Metallurgical Thermodynamic Data Bank’ (MTDATA). The strengthening contribution to the 0.2% proof stress was evaluated from each individual strengthening mechanism by using measured microstructural parameters. The modeled 0.2% proof stress is compared with the experimental result and a reasonable agreement is reached by root mean-square summation, while the simple linear summation gives an overestimated result. The accuracy and limitation of the present modeling is also discussed.

Influence of grain size evolution on convective instability

Abstract: [1] Grain size, one of the most important microstructural properties of materials, evolves during creep deformation to minimize the free energy of polycrystalline aggregates. We apply a model of grain size evolution to the study of convective instability of cooling boundary layers. The grain size evolution model is coupled to a composite rheology where the deformation rate is the sum of that due to dislocation creep and grain size sensitive diffusion creep. The onset of convection is sensitive to grain growth rates and the initial grain size. The formation of convective instabilities is enhanced by stresses induced by plate motions; therefore small-scale convection is more likely to occur beneath fast-moving plates. In finite amplitude convection, grain size evolution leads to high viscosity in regions where convective stresses are low and can induce viscosity contrasts exceeding one order of magnitude. Such viscosity contrasts are sufficient to influence the dynamics of convection, often leading to domains which remain isolated from the well-mixed convecting fluid. A composite viscosity including diffusion creep, which has a lower activation energy than dislocation creep, reduces the effective temperature dependence of viscosity.