Showing papers on "Grain boundary strengthening published in 2001"

Grain-boundary sliding in nanocrystalline fcc metals

Abstract: Molecular-dynamics computer simulations of a model Ni nanocrystalline sample with a mean grain size of 12 nm under uniaxial tension is reported. The microscopic view of grain-boundary sliding is addressed. Two atomic processes are distinguished in the interfaces during sliding: atomic shuffling and stress-assisted free-volume migration. The activated accommodation processes under high-stress and room-temperature conditions are grain-boundary and triple-junction migration, and dislocation activity.

Grain size reduction by dynamic recrystallization: can it result in major rheological weakening?

Abstract: It is widely believed that grain size reduction by dynamic recrystallization can lead to major rheological weakening and associated strain localization by bringing about a switch from grain size insensitive dislocation creep to grain size sensitive diffusion creep. Recently, however, we advanced the hypothesis that, rather than a switch, dynamic recrystallization leads to a balance between grain size reduction and grain growth processes set up in the neighborhood of the boundary between the dislocation creep field and the diffusion creep field. In this paper, we compare the predictions implied by our hypothesis with those of other models for dynamic recrystallization. We also evaluate the full range of models against experimental data on a variety of materials. We conclude that a temperature dependence of the relationship between recrystallized grain size and flow stress cannot be neglected a priori. This should be taken into account when estimating natural flow stresses using experimentally calibrated recrystallized grain size piezometers. We also demonstrate experimental support for the field boundary hypothesis. This support implies that significant weakening by grain size reduction in localized shear zones is possible only if caused by a process other than dynamic recrystallization (such as syntectonic reaction or cataclasis) or if grain growth is inhibited.

A few remarks on the kinetics of static grain growth in rocks

Abstract: Static grain growth is a relatively simple transformation in which grain size increases under driving forces caused by grain and interphase boundary curvature. Given the relative simplicity of the protocol for grain growth experiments, measurements of grain boundary mobility show surprising variations. Boundary mobilities during grain growth are affected by solute and impurity chemistry, chemical fugacity of trace and major elements, pore size and number, pore fluid chemistry, the presence of melts, and the presence of solid second phases, as well as temperature and pressure. All of these factors may exert influence on grain growth of rocks in natural situations and many are also present during the laboratory experiments. Provided that the necessary kinetics parameters are known, bounds may be placed on the interface mobility when pores, partial melts, or solutes are present. To predict the rate of grain growth in natural situations will require improved laboratory data and careful consideration of the thermodynamic conditions likely to be encountered in nature.

Mechanisms of grain growth in nanocrystalline fcc metals by molecular-dynamics simulation.

Abstract: To elucidate the mechanisms of grain growth in nanocrystalline fcc metals, we have performed fully three-dimensional molecular-dynamics simulations with a columnar grain structure and an average grain diameter of 15 nm. Based on the study of coarse-grained materials, the conventional picture is that grain growth is governed by curvature-driven grain-boundary migration. However, our simulations reveal that in a nanocrystalline material grain rotations play an equally important role, at least during the early stages of grain growth. By eliminating the grain boundary between neighboring grains, such rotations lead to grain coalescence and the consequent formation of highly elongated grains. A detailed analysis exposes an intricate coupling between this mechanism and the conventional grain-boundary-migration dominated mechanism. Incorporation of these insights into mesoscopic models should enable more realistic mesoscopic simulations of grain growth in nanocrystalline materials. (A short movie showing the overall evolution of the grain microstructure can be viewed at http://www.msd.anl.gov/im/movies/graingrowth.html.)

Cyclic behavior of ultrafine-grain titanium produced by severe plastic deformation

Abstract: The cyclic behavior is investigated of commercial purity massive ultrafine-grained titanium obtained by severe plastic deformation through equal channel angular pressing (ECAP). Both stress- and strain-controlled experiments were carried out to access the fatigue performance in terms of Wohler diagram, fatigue limit, Coffin–Manson plot, cyclic hardening curves and cyclic stress-strain curves in a range of plastic strain amplitudes from 7.5×10 −4 to 10 −2 . A significant enhancement of fatigue limit and fatigue life in the ultrafine-grained state is found under constant stress testing. No cyclic softening and degradation in the strain-controlled experiments is noticed in titanium contrary to wavy slip materials such as Cu subjected to ECAP. A simple one-parameter dislocation-based model is proposed to account for experimental results. It is shown that many cyclic properties of severely predeformed materials with fine grains can be rationalized in terms of Hall–Petch grain boundary hardening and dislocation hardening.

Correlation of the M23C6 precipitation morphology with grain boundary characteristics in austenitic stainless steel

Abstract: The relationship between grain boundary characteristics and the formation of grain boundary carbides in AISI 304 stainless steel have been investigated by using the electron backscattered diffraction (EBSD) technique. It was observed that an increase in the misorientation between two adjacent grains resulted in a change in the carbide morphology from a plate-like to an acute triangular form, where carbides preferentially maintained coherency with the grain for which the {111} planes made the smallest angle with the grain boundary plane. The carbides grew into the other grain at a later stage, having the lowest interfacial energy, which subsequently resulted in the triangular carbide morphology. After low cycle fatigue with a hold time at tensile peak strains, it was observed that cavity formation was more pronounced at random boundaries than at coincidence site lattice (CSL) boundaries. This result provides a good explanation that acute triangular carbides, which predominantly precipitate at random boundaries, are more likely to lead to cavity nucleation than the plate-like carbides precipitate at CSL boundaries.

Grain Size Control of Commercial Wrought Mg-Al-Zn Alloys Utilizing Dynamic Recrystallization

Abstract: Dynamic recrystallization (DRX) behavior was systematically examined in two commercial Mg-Al-Zn alloys in order to clarify the relationship between deformation conditions and the resulting microstructure. The materials were deformed by upset forging at temperatures ranging from 473 to 673 K at an initial strain rate of 3.3 x 10 -2 s -1 . Grain refinement was observed during deformation. It was found that the dynamically recrystallized grain size decreases with an increasing Zener-Hollomon parameter and/or a decreasing initial grain size. A phenomenological constitutive equation was developed in order to provide a guideline for the control of the grain size of hot deformed AZ61 alloy.

A molecular dynamics study of polycrystalline fcc metals at the nanoscale: grain boundary structure and its influence on plastic deformation

Abstract: Molecular dynamics computer simulation of nanocrystalline Ni and Cu show that grain boundaries in nanocrystalline metals have the short range structure of most grain boundaries found in conventional polycrystalline materials. The simulations also indicate the presence of a critical grain size below which all plastic deformation is accommodated in the grain boundary and no intra-grain deformation is observed.

Grain refinement of AZ31 and ZK60 Mg alloys — towards superplasticity studies

Abstract: Low temperature superplastic (SP) behavior (mechanical and deformation mechanisms) of two commercial Mg-based alloys (AZ31 and ZK60) was characterized. The two alloys were tested in the as extruded condition with initial grain size of 15 μm (AZ31) and fine (2 μm) and coarse (25 μm) grains mixed randomly for the ZK60. Strain rate was activated in the range 10−5–1 s−1 at 450 K (0.49Tm) in order to determine the deformation capacity curves (elongation to failure vs. strain rate), and to evaluate the strain rate sensitivity coefficient, m, from the stress versus strain rate curves. Optical, scanning and transmission electron microscopy observations (SEM and TEM) were performed to elaborate on the dynamic recrystallization (DRX) grain growth, fracture modes and deformation mechanisms at the SP mode. In addition, X-ray diffraction was utilized to track for microstructural classification. Although low temperature was applied, the ZK60 exhibited superplastic-like behavior and the maximum peak of elongation (220%) was detected at 1×10−5 s−1 with m equal to 0.2. In AZ31 SP behavior was suppressed due to grain growth, while for ZK60, DRX was detected. However, for the latter alloy, it was observed that the coarse/fine grain interface was the trigger for microcracking initiation. Actually, this phenomenon reduces the SP capacity of the ZK60 alloy. Surface observations and TEM findings indicate that grain boundary sliding with homogeneous character is the controlling SP deformation mode. Typical dislocation features have supported this deformation mode, mainly by grain boundary dislocation pile-ups. More sophisticated extrusion processes as equal angular channel extrusion (EACE) is likely to be considered in the future as a mean to improve grain homogeneity and produce ultra fine grain microstructure.

Tensile behavior and fracture in nickel and carbon doped nanocrystalline nickel

Abstract: The potential engineering applications of nanocrystalline materials need more detailed study on deformation and fracture mechanisms at room and elevated temperatures under tensile loading. This paper reports results of a series of experiments carried out on nickel and carbon doped nanocrystalline nickel with different carbon concentrations from 500 to 1000 ppm at room temperature to 300°C. Grain growth was observed in nanocrystalline nickels as the testing temperature increases. A fast grain growth was noticed at 300°C. Pure nanocrystalline nickel experienced an abnormal grain growth at 500°C and its tensile properties reduced to a very low level. The addition of carbon exerted a potential effect to enhance the stability of the microstructure in nanocrystalline nickel at intermediate temperatures. However, carbon doped nickels exhibited lower tensile properties. Nanocrystalline nickels displayed a conventional Hall–Petch relationship. The results are discussed in relation to microstructural characteristics by using TEM and SEM.

A computational model of ceramic microstructures subjected to multi-axial dynamic loading

Abstract: A model is presented for the dynamic finite element analysis of ceramic microstructures subjected to multi-axial dynamic loading. This model solves an initial-boundary value problem using a multi-body contact model integrated with interface elements to simulate microcracking at grain boundaries and subsequent large sliding, opening and closing of microcracks. An explicit time integration scheme is adopted to integrate the system of spatially discretized ordinary differential equations. A systematic and parametric study of the effect of interface element parameters, grain anisotropy, stochastic distribution of interface properties, grain size and grain morphology is carried out. Numerical results are shown in terms of microcrack patterns and evolution of crack density, i.e., damage kinetics. The brittle behavior of the microstructure as the interfacial strength decreases is investigated. Crack patterns on the representative volume element vary from grains totally detached from each other to a few short cracks, nucleated at voids, except, for the case of microstructures with initial flaws. Grain elastic anisotropy seems to play an important role in microfracture presenting higher values of crack density than the isotropic case. The computational results also show that decreasing the grain size results in a decrease in crack density per unit area at equal multiaxial dynamic loading. Histograms of crack density distribution are presented for the study of the stochasticity of interface parameters. Finally, a strong dependency with grain shape is observed for different microstructures generated using Voronoi Tessellation. The micromechanical model here discussed allows the study of material pulverization upon unloading. The qualitative and quantitative results presented in this article are useful in developing more refined continuum theories on fracture properties of ceramics.

Grain growth and strain release in nanocrystalline copper

Abstract: Grain growth and strain release processes in the electrodeposited nanocrystalline (nc) Cu specimen with a high purity were investigated by means of differential scanning calorimetry, x-ray diffraction, electrical resistance measurement, and high-resolution transmission electron microscopy. It was found that for the as-deposited nc Cu, the grain growth started at about 75 degreesC, at which the microstrain in (111) plane (e(111)) began to release, while the mean microstrain and that in (100) plane (e(100)) began to release at a higher temperature (150 degreesC). With an increment in microstrain in the nc Cu introduced by cold rolling, the grain growth onset temperature increased while the strain release onset temperature dropped obviously. These results showed an evident correlation between the grain size stability and the microstrain in the nc materials. The activation energy for the grain growth was determined by using Kissinger analysis and isothermal kinetics analysis, being about 86 kJ/mol, implying that the grain growth process is dominated by grain boundary diffusion. (C) 2001 American Institute of Physics.

Combined effect of special grain boundaries and grain boundary carbides on IGSCC of Ni–16Cr–9Fe–xC alloys

Abstract: Susceptibility to intergranular stress corrosion cracking in Ni–16Cr–9Fe– x C alloys in 360°C primary water is reduced with increasing fraction of special grain boundaries, i.e. coincident site lattice boundaries (CSLB) and low angle boundaries, and grain boundary carbides. Intergranular stress corrosion cracking (IGSCC) was investigated using interrupted constant extension rate tensile tests in a primary water environment at 360°C. Thermal–mechanical treatments were used to increase the fraction of special boundaries from approximately 20–25% to between 30 and 40%. In a carbon-doped heat, further heat treating was used to precipitate grain boundary carbides preferentially on high-angle boundaries (HAB). Orientation imaging microscopy was used to determine the relative grain misorientations and scanning electron microscopy (SEM) was used to identify specific grain boundaries after each interruption. After each strain increment, the same regions in each sample were examined for cracking. Results showed that irrespective of the microstructure condition, CSLBs always cracked less than HABs. Results also showed that IGSCC is reduced with increasing solution carbon content, and for the same amount of carbon in solution, the addition of grain boundary carbides reduced IGSCC still further. The best microstructure was the one consisting of an enhanced CSLB fraction and chromium carbides precipitated preferentially on high-angle boundaries.

Fine-grained alloys by thermomechanical processing

Abstract: Grain refinement during thermomechanical processing is conventionally achieved by discontinuous recrystallization. However, techniques involving severe deformation which increase the grain boundary area and can result in a micron-scale grain size by a process of continuous recrystallization have been developed. Various methods which are based on control of the deformation conditions and cooling rate during hot rolling of steels have utilised the α–γ phase transformation to produce micron-grained ferrite. Ultra-fine-grain structures exhibit interesting mechanical properties, and in certain cases a large amount of strengthening may be achieved with little or no reduction in ductility.

Influence of α layers at β grain boundaries on mechanical properties of Ti-alloys

Abstract: Tensile and fatigue properties were evaluated for two β-processed Ti-alloys, β-Cez and Ti-6246, both with a pancake shaped grain structure. Tests were performed in L- and ST-direction and under various inclined angles. The orientation dependence observed for tensile ductility and HCF strength will be associated with an influence of the soft α layer regions along the β grain boundaries.

Grain Boundaries: Their Microstructure and Chemistry

Abstract: Boundaries and interfaces in materials - introduction principles governing grain shape and size size and orientation of grains energy polycrystalline materials diffusion structure of boundaries and interfaces composition of boundaries and interfaces electronic structure strength of grain boundaries grain boundary composition: equilibrium segregation non-equilibrium segregation interphase boundaries composition changes in materials: introduction role of crystallography on composition relationship between different surfaces interfaces and boundaries iron alloys and steels non-ferrous alloys non-metal systems measurement of composition - introduction indirect methods direct methods electron energy loss spectrometry contrast analysis auger X-ray photoelectron spectroscopy secondary ion mass spectroscopy field ion and atom probe microscopy scanning probe microscopy and atom force microscopy mechanical properties: engineering requirements fracture in materials intergranular fracture strength of grain boundaries temper embrittlement higher temperature and liquation embrittlement of steels creep and creep fatigue stress relief and reheat cracking environmentally assisted cracking irradiation damage others.

Microstructure evolution, slip patterns and flow stress

Abstract: The development of deformation microstructures in medium to high stacking fault energy fcc metals is reviewed illustrating that the basic microstructural element is a cell block. The elongated cell blocks are defined by extended nearly planar boundaries that lie on special planes and enclose a group of approximately equiaxed cells. This duplex structure persists throughout Stage II–IV. What is known about the origins of these structures and the role of the slip pattern is reviewed. Taking all the measurable structural parameters with respect to the cell blocks: spacing, misorientation angle, and dislocation density, and assuming additive strength contributions, flow stress predictions are derived from Hall–Petch and dislocation strengthening. These predictions are in good agreement with the stress values and hardening rates observed experimentally.

Atomic force microscopic study on surface morphology of ultra-fine grained materials after tensile testing

Abstract: The surface morphology of tensile tested ultra-fine grained copper and nickel produced by severe plastic deformation is investigated on different scale with a help of atomic force microscopy. A key role of grain boundaries in surface patterning and plastic flow is emphasized. The presence of two mechanisms contributing to a total strain was found (1) microscopic dislocation slip limited to the grain interior and (2) grain boundary shearing (sliding) of the mesoscopic scale in the plane of maximum shear stresses. This grain boundary sliding which appears at room temperature, results in a macroscopically observed characteristic shear band relief that is often observed on the surface of deformed ultra-fine grain metals.

Low-temperature recrystallization in calcite: Mechanisms and consequences

Abstract: Vein-calcite–dominated fault rocks collected from several locations show evidence for intense intracrystalline plasticity and interface (twin and grain boundary) mobility, leading to dynamic recrystallization of calcite at temperatures (150–250 °C) significantly below those at which these features are commonly anticipated. These observations require a reappraisal of calcite deformation at low temperature, particularly the capability for dynamic recrystallization in the apparent absence of significant, thermally activated recovery processes. The cyclic introduction of coarse-grained calcite veins is observed to be essential for the initiation of intracrystalline deformation and associated dynamic recrystallization. The introduction of veins generates an essentially monomineralic rock of a grain size larger than the protolith. As a result, the mylonitization does not occur within a given protolith, but rather in the introduced secondary calcite. Through Hall-Petch–type grain- size–dependent dislocation interactions, stress is locally increased, and the resulting increase in dislocation densities promotes grain-boundary migration. The recognition that nominal high-temperature creep processes and associated microstructures can occur outside their expected temperature range has implications for fault rheology (strength) and fault permeability and porosity.