Showing papers on "Grain boundary strengthening published in 1999"

Atomic-scale simulations of the mechanical deformation of nanocrystalline metals

Abstract: Nanocrystalline metals, ie, metals in which the grain size is in the nanometer range, have a range of technologically interesting properties including increased hardness and yield strength We present atomic-scale simulations of the plastic behavior of nanocrystalline copper The simulations show that the main deformation mode is sliding in the grain boundaries through a large number of uncorrelated events, where a few atoms (or a few tens of atoms) slide with respect to each other Little dislocation activity is seen in the grain interiors The localization of the deformation to the grain boundaries leads to a hardening as the grain size is increased (reverse Hall-Petch effect), implying a maximum in hardness for a grain size above the ones studied here We investigate the effects of varying temperature, strain rate, and porosity, and discuss the relation to recent experiments At increasing temperatures the material becomes softer in both the plastic and elastic regime Porosity in the samples result in a softening of the material; this may be a significant effect in many experiments

Grain Boundary Migration in Metals: Thermodynamics, Kinetics, Applications

Abstract: Thermodynamics of Grain Boundaries Introductory Remarks Thermodynamics of Surfaces Experiments Applications of Grain Boundary Thermodynamics The Equilibrium Shape of Grain Boundaries Structure of Grain Boundaries Terminology and Definitions Atomic Structure of Grain Boundaries Grain Boundary Motion Fundamentals Driving Forces for Grain Boundary Migration Drag Effects During Grain Boundary Motion Measurement of Grain Boundary Mobility Experimental Results Effect of Wetting Phase Transitions on Grain Boundary Migration Compensation Effect in Grain Boundary Motion Mechanisms of Grain Boundary Migration Thermodynamics and Kinetics of Connected Grain Boundaries Microstructural Elements of Polycrystals Thermodynamics of Triple Junctions Motion of a Grain Boundary System with Triple Junctions Triple Junctions Motion in the Presence of Impurities Experimental Investigations of Triple Junction Motion Triple Junction Drag and Grain Growth in 2D Polycrystals Grain Growth in 3D Systems Kinetics of Grain Growth Inhibited by Vacancy Generation Computer Simulation of Grain Boundary Motion Introduction Driving Force Concepts Migration of [001] Twist Grain Boundaries Motion of Tilt Boundaries Compensation Effect Comparison with Experiments Grain Boundary Diffusion Atomic Mechanisms Simulation of Triple Junction Motion Applications Characterization of Microstructure and Texture Recrystallization and Grain Growth On Precipitation Controlled Grain Size Mechanisms on Retardation of Grain Growth Grain Boundary Junction Engineering Appendices Solutions References

Mechanisms for microstructure evolution in electroplated copper thin films near room temperature

Abstract: We present a model which accounts for the dramatic evolution in the microstructure of electroplated copper thin films near room temperature. Microstructure evolution occurs during a transient period of hours following deposition, and includes an increase in grain size, changes in preferred crystallographic texture, and decreases in resistivity, hardness, and compressive stress. The model is based on grain boundary energy in the fine-grained as-deposited films providing the underlying energy density which drives abnormal grain growth. As the grain size increases from the as-deposited value of 0.05–0.1 μm up to several microns, the model predicts a decreasing grain boundary contribution to electron scattering which allows the resistivity to decrease by tens of a percent to near-bulk values, as is observed. Concurrently, as the volume of the dilute grain boundary regions decreases, the stress is shown to change in the tensile direction by tens of a mega pascal, consistent with the measured values. The small ...

Effects of Grain Size on the Initiation and Propagation Thresholds of Stress-induced Brittle Fractures

Abstract: The microstructure of rock is known to influence its strength and deformation characteristics. This paper presents the results of a laboratory investigation into the effects of grain size on the initiation and propagation thresholds of stress-induced brittle fracturing in crystalline rocks with similar mineralogical compositions, but with three different grain sizes. Strain gauge and acoustic emission measurements were used to aid in the identification and characterization of the different stages of crack development in uniaxial compression. Results indicate that grain size had only a minor effect on the stress at which new cracks initiated. Crack initiation thresholds were found to be more dependent on the strength of the constituent minerals. Grain size did have a significant effect, however, in controlling the behaviour of the cracks once they began to propagate. The evidence suggests that longer grain boundaries and larger intergranular cracks, resulting from increased grain size, provide longer paths of weakness for growing cracks to propagate along. This promoted degradation of material strength once the longer cracks began to coalesce and interact. Thus, rock strength was found to decrease with increasing grain size, not by inducing crack initiation at lower stresses, but through a process where longer cracks propagating along longer planes of weakness coalesced at lower stresses.

Thermal stability of ultrafine-grained aluminum in the presence of Mg and Zr additions

Abstract: Superplastic deformation occurs at high temperatures and requires the presence of a very small grain size. Experiments demonstrate that equal-channel angular (ECA) pressing is capable of introducing ultrafine grain sizes in pure Al, Al–Mg and Al–Zr alloys, with grain sizes lying in the sub-micrometer range for the alloys. It is shown by static annealing that these ultrafine grain sizes are not stable in pure Al and the Al–Mg alloys at elevated temperatures but in the Al–Zr alloys the grains remain small up to temperatures of ≈600 K.

The effect of grain refinement and silicon content on grain formation in hypoeutectic Al–Si alloys

Abstract: The effect of increasing the amount of added grain refiner on grain size and morphology has been investigated for a range of hypoeutectic Al-Si alloys. The results show a transition in grain size at a silicon concentration of about 3 wt% in unrefined alloys; the grain size decreasing with silicon content before the transition, and increasing beyond the transition point. A change in morphology also occurs with increased silicon content. The addition of grain refiner leads to greater refinement for silicon contents below the transition point than for those contents above the transition point, while the transition point seems to remain unchanged. The slope of the grain size versus silicon content curve after the transition seems to be unaffected by the degree of grain refinement. The results are related to the competitive processes of nucleation and constitutional effects during growth and their impact on nucleation kinetics. (C) 1999 Elsevier Science S.A. All rights reserved.

The impedance of ceramics with highly resistive grain boundaries: validity and limits of the brick layer model

Abstract: Impedance spectroscopy is an important tool to investigate the electrical properties of grain boundaries. For the analysis of the impedance spectra cubic grains, laterally homogeneous grain boundaries and identical properties of all grain boundaries are usually assumed (brick layer model). However, in real ceramics these assumptions are generally violated. Using the finite element method we calculated the impedance of several polycrystals exhibiting highly resistive grain boundaries with microstructures and grain boundary properties deviating from the simple brick layer model. Detours around highly resistive regions (e.g. due to high grain boundary density or enhanced grain boundary resistivity) can play an important role and lead to grain-boundary semicircles depending on bulk properties and even to additional semicircles. Conditions are discussed within which the brick layer model allows for a reasonable evaluation of the spectra.

On hardness and toughness of ultrafine and nanocrystalline hard materials

Abstract: The availability of nanocrystalline hard material powders offers a possibility to study the mechanical properties of extremely fine-grained sintered materials and polycrystals. In the micrometer range the hardness of polycrystalline materials usually increases with decreasing grain size. For the nanometer range this relation suggests extraordinarily high hardness values. But, at small crystallite size interfaces begin to play a growing role in deformation. Whole crystallites may be displaced instead of being plastically deformed due the huge number of atoms arranged in interfaces of a low atomic order and an increasing number of cracks along grain boundaries is formed. The hardness of hard metals and sintered carbides and carbonitrides with a grain size

Influence of temperature, grain size and cobalt content on the hardness of WC–Co alloys

Abstract: The Vickers hardness of WC–Co alloys has been measured at temperatures ranging from −196 to 900°C. The cobalt content of the alloys ranged from 10 to 24 vol% and the grain size from 0.5 to 2.3 μm. It was found that, at all cobalt contents and all temperatures, the decrease in hardness with increasing grain size can be approximated by a Hall–Petch type relationship. Up to about 600°C the decrease in hardness with increasing temperature appears to be due mostly to the decrease in the intrinsic hardness of the individual phases. Above 600°C the decrease in hardness appears to be due mostly to easier slip transfer across grain boundaries. Finer grained alloys have been found to preserve their hardness at high temperature better than coarser grained alloys, at all cobalt contents.

Microstructural analysis on boundary sliding and its accommodation mode during superplastic deformation of Ti–6Al–4V alloy

Abstract: A study was carried out to investigate the mechanisms of superplastic deformation in two phase Ti-6Al–4V alloy. Tensile tests were carried out using fine (3 μm) and coarse (11 μm) grained microstructures at 600 and 900°C with a strain rate of 10−3s−1, and the quenched microstructures were analyzed by transmission electron microscopy. In the coarse grain microstructure, the planar arrays of dislocations were developed well in the α-phase both at 600 and 900°C. However, in the fine grain microstructure, dislocations were rarely observed in the α-phase at 600°C, while a few dislocations were observed near the α/β boundaries at 900°C. The results indicate that most of deformation is imposed on the β-phase in the fine grain microstructure, and on the α-phase as well as the β-phase in the coarse-grain microstructure. It is also considered that accommodation processes for phase/grain boundary sliding in the α and β phase are closely associated with the dislocation motion.

Pinning effect of the grain boundaries on magnetic domain wall in FeCo-based magnetic alloys

Abstract: We have studied the dynamics of grain growth and the pinning effect of grain boundaries on magnetic domain walls in FeCo soft magnetic alloys. It has been found that grain growth takes place at temperatures above 600 °C. The activation energy for grain growth in a disordered state at 820 °C is about 57.4±0.5 kcal/mole. The effect of grain size on magnetic properties has been singled out by keeping the same ordering parameter (S=0 and 0.88) for all samples studied. Microstructural characterization and magnetic measurements indicate that the grain size significantly affects the magnetic coercivity. A linear relationship between the coercivity and the reciprocal of the grain size has been universally found regardless of the heat-treatment histories. Lorenz microscopic observation demonstrates that grain boundaries act as pinning sites for the magnetic domain wall movement.

Grain boundary strengthening in austenitic nitrogen steels

Abstract: The effect of nitrogen and carbon on the strengthening of the austenitic steel Cr18Ni16Mn10 by grain boundaries is studied. It is established in accordance with previous results that, in contrast to carbon, nitrogen markedly increases the coefficient k in the Hall-Petch equation. Because of a pronounced planar slip induced by nitrogen and the absence of any noticeable segregation of nitrogen atoms at the grain boundaries, nitrogen austenite presents an excellent object for testing different existing models of grain boundary strengthening (pile-up of dislocations, grain boundary dislocation sources, work hardening). Based on the analysis of available data and measurements of interaction between nitrogen or carbon atoms and dislocations it is shown that the nitrogen effect can be attributed to a strong blocking of dislocation sources in grains adjacent to those where the slip started.

Grain Refinement under Multiple Warm Deformation in 304 Type Austenitic Stainless Steel

Abstract: The dynamic process of fine grain evolution as well as deformation behaviour under warm working conditions was studied in compression of a 304 type austenitic stainless steel. Multiple compression tests were carried out at a strain rate of 10-3 s-1 to produce high cumulative strains, with changing of the loading direction in 90o and decreasing temperature from 1223 to 873 K (0.7-0.5Tm) in each pass. The steel exhibits two types of deformation behaviours with different mechanical and structural characteristics. In the deformation region where flow stresses are below about 400 MPa, conventional dynamic recrystallization takes place accompanied mainly by bulging of serrated grain boundaries. The dynamic grain size evolved can be related to the high temperature flow stress through a power law function with a grain size exponent of -0.72. On the other hand, in the region of higher stresses above 400 MPa the flow stresses show small strain rate and temperature dependence, and so it is suggested to be in an athermal deformation region. The stress-strain curves show a steady state like flow without any strain softening, while the multiple deformation to high cumulative strains brings about the evolution of fine grained structures with grain sizes less than one micron. The relationship between the warm temperature flow stresses and the grain sizes evolved also can be expressed by a unique power law function of grain size with an exponent of -0.42. The interrelations between the mechanisms of plastic deformation and microstructure evolution at warm and high temperatures are analysed in detail and also the multiple compression method for obtaining ultra fine grained structure is discussed as a simple thermomechanical processing.

Plastic deformation of nanocrystalline Cu and Cu–0.2 wt.% B

Abstract: Plastic deformation behavior of nanocrystalline copper with and without trace boron was studied. Fully dense defect-free compacts were produced and mechanical properties were measured. The Hall–Petch (H–P) relationship was followed down to the 25 nm, smallest grain size investigated. The H–P slopes were smaller in nanocrystalline copper than in conventional copper. The role of boron in altering grain growth, strain hardening, recovery, and recrystallization was investigated by comparison of results in nanocrystalline copper with and without trace boron.

Minimal surfaces, screw dislocations, and twist grain boundaries

Abstract: Large twist-angle grain boundaries in layered structures are often described by Scherk's first surface whereas small twist-angle grain boundaries are usually described in terms of an array of screw dislocations. We show that there is no essential distinction between these two descriptions and that, in particular, their comparative energetics depends crucially on the core structure of their screw-dislocation topological defects.