Showing papers on "Grain boundary strengthening published in 1998"

Softening of nanocrystalline metals at very small grain sizes

Abstract: Nanocrystalline solids, in which the grain size is in the nanometre range, often have technologically interesting properties such as increased hardness and ductility. Nanocrystalline metals can be produced in several ways, among the most common of which are high-pressure compaction of nanometre-sized clusters and high-energy ball-milling1,2,3,4. The result is a polycrystalline metal with the grains randomly orientated. The hardness and yield stress ofthe material typically increase with decreasing grain size, a phenomenon known as the Hall–Petch effect5,6. Here we present computer simulations of the deformation of nanocrystalline copper, which show a softening with grain size (a reverse Hall–Petch effect3,7) for the smallest sizes. Most of the plastic deformation is due to a large number of small ‘sliding’ events of atomic planes at the grain boundaries, with only a minor part being caused by dislocation activity in the grains; the softening that we see at small grain sizes is therefore due to the larger fraction of atoms at grain boundaries. This softening will ultimately impose a limit on how strong nanocrystalline metals may become.

Nanocrystals: The strongest size

Abstract: In polycrystalline materials with grain sizes in the micrometre range, strength increases with decreasing grain size This is because dislocations pile up at the grain boundaries and, as the grains become smaller, the effect of dislocation blocking increases, thereby strengthening the material But with grains in the nanometre range, the opposite behaviour is found Why? Computer simulations show that the reverse effect arises primarily from sliding motions at grain boundaries

Theory for intermetallic phase growth between Cu and liquid Sn-Pb solder based on grain boundary diffusion control

Abstract: Kinetics of phase formation during interdiffusion in solid-liquid diffusion couples are influenced by the morphology of the intermediate compound layer. In some cases, an intermediate compound layer is formed which has very fine grain size. This condition favors grain boundary diffusion as the predominant mechanism for transport through the layer. In systems where grain coarsening occurs, the coarsening kinetics will influence the interdiffusion kinetics. In addition, for some solid-liquid systems, a grain boundary grooving effect is observed which leads to a highly nonuniform layer thickness; the layer is thinner where the liquid phase penetrates the grain boundaries. As a consequence of the grooving effects, the diffusion path through the layer is shorter along the grain boundaries. This differs from standard interdiffusion models which assume that the diffusion distance is equal to the average layer thickness. A model for growth kinetics of an intermediate compound layer is presented for the case where grain boundary diffusion is the predominant transport mechanism. The model includes the geometric effects caused by grain boundary grooving. The model predicts layer growth which follows a t1/3 dependence on time t. Experimental data for intermetallic growth between copper and 62Sn-36Pb-2Ag solder exhibit a t1/4 dependence on time t. If experimental data are interpreted in terms of the grain boundary diffusion control model presented in this paper, the activation energy for grain boundary diffusion is 27 kJ/mole.

Mechanical behavior of a bulk nanostructured iron alloy

Abstract: Bulk, fully dense materials were prepared from Fe-10Cu with grain diameters between 45 nm and 1.7 µm. The materials were prepared by ball milling of powders in a glove box, followed by hot isostatic pressing (hipping) or powder forging. Larger grain sizes were obtained by thermal treatment of the consolidated powders. The bulk materials were relatively clean, with oxygen levels below 1500 wpm and other contaminants less than 0.1 at. pct. The mechanical behavior of these materials was unique. At temperatures from 77 to 470 K, the first and only mechanism of plastic deformation was intense shear banding, which was accompanied by a perfectly plastic stress-strain response (absence of strain hardening). There was a large tension-compression asymmetry in the strength, and the shear bands did not occur on the plane of maximum shear stress or the plane of zero extension. This behavior, while unusual for metals, has been observed in amorphous polymers and metallic glasses. On the other hand, the fine-grained Fe-10Cu materials behaved like coarse-grained iron in some respects, particularly by obeying the Hall-Petch equation with constants reasonably close to those of pure iron and by exhibiting low-temperature mechanical behavior which was very similar to that of steels. Transmission electron microscopy (TEM) studies found highly elongated grains within shear bands, indicating that shear banding occurred by a dislocation-based mechanism, at least at grain sizes above 100 nm. Similarities and differences between the fine-grained Fe-10Cu and metals, polymers, metallic glasses, radiation-damaged metals, and quench-damaged metals are discussed.

The atomic origins of reduced critical currents at [001] tilt grain boundaries in YBa2Cu3O7−δ thin films

Abstract: Grain boundaries have long been known to have a deleterious and irreproducible effect on the transport properties of high- T c oxide superconductors, particularly in the high-angle regime where an exponential decrease in critical current has been reported. We demonstrate, through a combination of atomic resolution Z-contrast imaging and bond valence sum analysis, that it is the atomic structure of the grain boundary that dominates this behavior. [001] tilt grain boundaries in thin-film YBa 2 Cu 3 0 7− δ are composed of arrays of dislocations in defined sequences. The resulting strain fields seriously perturb the local electronic structure, leading to a non-superconducting zone at the grain boundary. The width of this zone increases linearly with misorientation angle, naturally explaining the observed exponential decrease in critical current. In addition, the widely varying J c measurements for a given grain boundary misorientation can be naturally explained by the facetting of the grain boundary plane.

Grain Boundary Migration in Metals: Recent Developments

Abstract: Current research on grain boundary migration in metals is reviewed. For individual grain boundaries the dependence of grain boundary migration on misorientation and impurity content are addressed. Impurity drag theory, extended to include the interaction of adsorbed impurities in the boundary, reasonably accounts quantitatively for the observed concentration dependence of grain boundary mobility. For the first time an experimental study of triple junction motion is presented. The kinetics are quantitatively discussed in terms of a triple junction mobility. Their impact on the kinetics of microstructure evolution during grain growth is outlined.

Grain Boundary Migration in Ceramics

Abstract: During ceramic fabrication, densification processes compete with coarsening processes to determine the path of microstructural evolution. Grain growth is a key coarsening process. This paper examines grain boundary migration in ceramics, and discusses the effects of solutes, pores, and liquid phases on grain boundary migration rates. An effort is made to highlight work in the past decade that has contributed to and advanced our understanding of solute drag effects, pore-boundary interactions, and the role of liquid phases in grain growth and microstructural evolution. Anisotropy of the grain boundary mobility, and its role in the development of anisotropic (anisometric) microstructures is discussed as it is a central issue in recent efforts to produce ceramic materials with new combinations of properties and functionality.

The role of grain size and selected microstructural parameters in strengthening fully lamellar TiAl alloys

Abstract: More than 5 years ago, wrought processing was first used to produce fully lamellar (FL) microstructures in TiAl alloys having grain sizes less than ≈400 µm. These alloys exhibit an improvement in overall balance of properties, especially at high temperatures. More recently, such microstructural forms led to exceptional yield strengths (500 to 1000 MPa at low temperatures) while maintaining attractive high-temperature properties. The improvements appeared to be related to an unusually high apparent sensitivity of strength to grain size. Studies reported an apparent value for the slope of the Hall-Petch (HP) plot approaching 5 MPa√m for FL gamma alloys, while that for single-phase or duplex microstructures is near unity. The present investigations examine the slope of the HP plot for FL microstructures, paying particular attention to the lamellar microstructural variables. Results show that the α 2 lamellar thickness and spacing and the γ lamellar thickness can vary over more than two orders of magnitude with typical process methods. These spacings influence the value of k y in the HP (grain size) relationship. Since they often change concomitantly with grain size in processing, they can give rise to a large scatter in the HP plot. The investigations also examine the flow behavior, glide barriers, and slip multiplicity for polysynthetically twinned (PST) crystals (the single-grain analogue of FL material), and then map this behavior into an explanation of the yield behavior of high-strength FL gamma alloys.

A TEM investigation of M23C6 carbide precipitation behaviour on varying grain boundary misorientations in 304 stainless steels

Abstract: Transmission electron microscopy (TEM) along with electrochemical potentiokinetic reactivation (EPR) testing was performed on different grades of 304 stainless steel (0.01, 0.025, 0.05, and 0.07%C) in order to assess the sensitization and precipitation behaviour on different grain boundary misorientations. The materials were heat treated at 670°C for 50 h to subject the materials to the sensitization regime. The EPR data and TEM observations revealed that when the amount of carbon was increased the degree of sensitization increased along with the density of precipitates. Large angle misorientations (Θ>15°) were prevalent in all the carbon content materials and the {1 1 0} grain surface orientation was found to be the major texturing orientation. The steels with lower carbon contents nucleated a few small precipitates on high angle grain boundaries, while larger amounts of carbides were observed on lower angle grain boundaries for the higher carbon contents. It was deemed that higher carbon contents required lower energies to nucleate and grow precipitates. A carbon content threshold was found (above 0.05% C) in which precipitates fully saturate the grain boundary. Precipitation followed the energies of different types of boundaries. The highest energy boundary (general random grain boundary) nucleated precipitates first, then precipitation followed on non-coherent twin boundaries, and was not observed on coherent twin boundaries. A “critical nucleation energy”, γgb(crit.), was therefore found to exist at which precipitation will occur on a boundary. This value was found to be in the range of 16 mJ m-2<γgb(crit.)<265 mJ m-2 which corresponds to the energies of special boundaries (coherent and non-coherent portions of twins respectively) at the ageing temperature of 670 °C. © 1998 Chapman & Hall

Atomistic Simulation of Curvature Driven Grain Boundary Migration

Abstract: We present two dimensional molecular dynamics simulations of grain boundary migration using the half-loop bicrystal geometry in the experiments of Shvindlerman et al. We examine the dependence of steady-state grain boundary migration rate on grain boundary curvature by varying the half-loop width at constant temperature. The results confirm the classical result derived by absolute reaction rate theory that grain boundary velocity is proportional to the curvature. We then measure the grain boundary migration rate for fixed half-loop width at varying temperatures. Analysis of this data establishes an Arrhenius relation between the grain boundary mobility and temperature, allowing us to extract the activation energy for grain boundary migration. Since grain boundaries have an excess volume, curvature driven grain boundary migration increases the density of the system during the simulations. In simulations performed at constant pressure, this leads to vacancy generation during the boundary migration, making the whole migration process jerky.

Scaling theory of the hall-petch relation for multilayers

Abstract: A scaling theory describing the strength of lamellar materials is developed. The theory is based on the premise that dislocation pileups are responsible for the yielding behavior of the lamellar material. The theory is developed in terms of four parameters: The pileup length, the exponent characterizing the divergence of the dislocation density at the interface, a critical distance between the lead dislocation and the interface, and a pinning stress at the interface which acts as an additional hindrance to dislocation motion.

Molecular-Dynamics Simulation of Grain-Boundary Diffusion Creep

Abstract: Molecular-dynamics (MD) simulations are used, for the first time, to study grain-boundary diffusion creep of a model polycrystalline silicon microstructure. Our fully dense model microstructures, with a grain size of up to 7.5 nm, were grown by MD simulations of a melt into which small, randomly oriented crystalline seeds were inserted. In order to prevent grain growth and thus to enable steady-state diffusion creep to be observed on a time scale accessible to MD simulations (of typically 10-9s), our input microstructures were tailored to (i) have a uniform grain shape and a uniform grain size of nm dimensions and (ii) contain only high-energy grain boundaries which are known to exhibit rather fast, liquid-like self-diffusion. Our simulations reveal that under relatively high tensile stresses these microstructures, indeed, exhibit steady-state diffusion creep that is homogenous (i.e., involving no grain sliding), with a strain rate that agrees quantitatively with that given by the Coble-creep formula.

Growth of an initially sharp crack by grain boundary cavitation

Abstract: A new computational model is presented to analyze intergranular creep crack growth in a polycrystalline aggregate in a discrete manner and based directly on the underlying physical micromechanisms. A crack tip process zone is introduced in which grains and their grain boundaries are represented discretely, while the surrounding undamaged material is described as a continuum. Special-purpose finite elements are used to represent individual grains and grain boundary facets. The constitutive description of the grain boundary elements accounts for the relevant physical fracture mechanisms, i.e. viscous grain boundary sliding, the nucleation of grain boundary cavities, their growth by grain boundary diffusion and local creep, until coalescence of cavities leads to microcracks. Discrete propagation of the main crack occurs by linking up of neighbouring facet microcracks. Assuming small-scale damage conditions, the model is used to simulate the initial stages of growth of an initially sharp crack under C∗ controlled, mode I loading conditions. Material parameters are varied so as to lead to either ductile or brittle fracture, thus elucidating creep constrained cavitation near cracks. The role of the stress state dependence of cavity nucleation on the crack growth direction is emphasized.

Grain boundary misorientation effects on creep and cracking in ni-based alloys

Abstract: The effect of altered grain boundary character distributions on the creep and cracking behavior of polycrystalline Ni-16Cr-9Fe at 360°C was studied by comparing the creep and intergranular cracking behavior of solution-annealed material containing mostly high-angle boundaries to material that was thermomechanically processed to enhance the proportion of coincident-site lattice boundaries. In parallel with mechanical testing, the modification of dislocation structures in the grain boundary resulting from reactions with run-in lattice dislocations was studied using transmission electron microscopy. Observations were concerned with the ability of grain boundaries to act as sinks for lattice dislocations in which the kinetics depend on the grain-boundary structure. A mechanism based on dislocation annihilation was proposed to account for the observed effect of the coincident-site lattice boundaries on creep.

Factors influencing the flow and hardness of materials with ultrafine grain sizes

Abstract: Ultrafine grain sizes were introduced into an Al-3wt%Mg solid solution alloy and a commercial Al-Mg-Li-Zr alloy through intense plastic straining by equalchannel angular (ECA) pressing at room temperature and at 673 K respectively. Tensile testing of pressed samples at room temperature revealed markedly different stress-strain curves for these two alloys, with the Al-3wt%Mg alloy exhibiting a high yield stress with little subsequent strain hardening and the Al-Mg-Li-Zr alloy exhibiting a lower yield stress and extensive strain hardening after yielding. These and other experimental results are interpreted in terms of the nature of the microstructure introduced by the ECA pressing procedure. It is concluded that significant variations may occur in the mechanical properties of nominally similar ultrafine-grained materials depending upon the pressing conditions and the extent of any relaxation which may occur during the straining process.

An SEM study of porosity and grain boundary microstructure in quartz mylonites, Simplon Fault Zone, Central Alps

Abstract: A backscattered and secondary electron SEM study of the grain boundary microstructure in quartz mylonites sampled along the length of the retrograde Simplon Fault Zone established three characteristic components. (1) Fine isolated pores (≤ 1 μm diameter) are scattered across two-grain interfaces, preferentially concentrated on surfaces in extension. Pores are uncommon on three-grain junctions and there is no evidence for fluid interconnectivity along three- and four-grain junctions. The fine porosity may develop by accumulation of original, mainly intragranular fluid inclusions to the grain boundary during deformation and recrystallization and by cavitation of grain boundaries during grain boundary sliding. Dynamic cavitation implies that the “ductile” mylonitic deformation is at least locally dilatant and therefore pressure sensitive. (2) Large “vug”-like pores (up to mm-scale) extend along multi-grain boundaries. Observed in all samples, they are most common in the higher initial temperature, coarse-grained samples with a microstructure dominated by grain boundary migration recrystallization. Grains bordering this connected porosity develop perfect crystal faces, undecorated by fine pores or pits. The irregular “lobate” optical microstructure of many migrating grain boundaries actually consists of a series of straight crystal faces. The coarse porosity is probably due to accumulation during dynamic recrystallization of (CO2-rich ?) fluid with a high wetting angle against quartz. (3) In one sample, interconnected sinuous ridges, ≤ 0.2 μm high, are observed to follow three- and four-grain junctions and disjoint into more isolated worms and spheroidal globules. On two-grain interfaces, these are transitional to more branching vein-like or convoluted brain-like forms. The brain-like and globular forms have been observed, with varying frequency, through the range of samples, with the globules attaining sizes of up to 60 μm. Vein structures have also been observed on intragranular fractures. These topologies do not match across adjoining surfaces and must have developed into free space. The ridge-vein-brain-spheroid structure is distinctly different to that previously observed on experimentally healed microcracks and its origin is not unequivocally established. They could represent unstable meniscus necking of a thin grain-boundary phase of low viscosity, developed due to quasi-adiabatic shear and/or local stress-induced dilatancy during microcracking.

Continuum analysis of dislocation pile-ups: Influence of sources

Abstract: One explanation of the Hall-Petch relationship is that dislocation pile-ups serve to enhance the stress felt at grain boundaries. This dislocation pile-up model results in a d −½ dependence of the yield strength, with d the grain size. In fact, this dependence is observed often. However, the traditional pile-up picture neglects two important aspects of pile-up formation: firstly the existence of a threshold stress for dislocation production, and secondly the necessity of a finite-sized dislocation-free region in which a source may operate. In this paper, both of these aspects are addressed within a continuum model of the dislocation pile-ups. A closed-form expression is obtained for the dependence of yield stress on grain size and source characteristics. The continuum model agrees closely with the corresponding discrete dislocation model even when the pile-up contains as few as ten dislocations.