D.A. Hughes

Microstructure and strength of nickel at large strains

A quantitative microstructural analysis is presented for pure polycrystalline nickel (99.99%) cold rolled to reductions from 70 to 98% (evM 1.4–4.5). Applying transmission electron microscopy (TEM) techniques, key structural parameters, such as spacing between dislocation boundaries and high angle boundaries, as well as the misorientations across their boundaries, have been measured and analyzed. Application of scaling and similitude hypotheses to these microstructural parameters and their distributions revealed that the structures maintain a similar character with increasing strain. This similarity indicates that the measured parameters capture the important features of the structure. Scaling and similitude provide governing principles for structure formation. Based on this structural information and a detailed description of the morphology, structural parameters are identified, strength determining parameters chosen, and strength–structural relationships discussed. The suggestion is then made that two strengthening contributions should be considered: (i) dislocation strengthening due to the presence of low angle boundaries and (ii) grain boundary strengthening due to medium to high angle boundaries. The calculated individual strength contributions evolve differently with the strain and their addition leads to flow stress values and hardening rates in good agreement with those observed experimentally. No saturation of the calculated or experimental flow stress was observed.

Geometrically necessary boundaries, incidental dislocation boundaries and geometrically necessary dislocations

Abstract The concept of statistically stored and geometrically necessary dislocations is discussed in terms of observed deformation microstructures subdivided by incidental dislocation and geometrically necessary boundaries. This discussion is illustrated by examples of microstructures formed under conditions of homogeneous and nonhomogeneous deformation, respectively.

Microstructural evolution and hardening parameters

The evolution of dislocation structures in polycrystals with increasing strain is described within a framework of grain subdivision by dislocation boundaries and high angle boundaries. The evolving microstructures are characterized with emphasis on morphology and the changes in the misorientation angle across and spacing between deformation induced boundaries. Also the relationship between the slip pattern and the deformation microstructure is analyzed based on experimental observations showing a correlation between grain orientation in a polycrystal and the deformation microstructure. Next the changes in structural parameters are analyzed over a large strain range using a scaling hypothesis and the principle of similitude. The observations and analyses lead to a discussion of the effect of microstructural parameters on the flow stress behavior. Plastic deformation of metals has been a long lasting interest of Ali Argon and we are very glad on this occasion to thank him for many inspiring papers, scientific discussions and friendly companionship.

Strain hardening and substructural evolution in NiCo solid solutions at large strains

Abstract Torsion deformation was used to investigate dislocation substructure evolution at large strains in high purity nickel and NiCo solid solutions. Observations of small strain dislocation structures formed in stage III revealed that the laminar dislocation structure observed after large strains in stage IV develops from short paired dislocation sheets within the tangled dislocations of an equiaxed cell wall. The development of these short paired dislocation sheets into long microbonds occurs gradually by a multiple-slip process in accordance with the principles of low energy dislocation structures and without the occurrence of a shear instability. The plane of these sheets and /or microbands does not correspond to a {111} slip plane. As these microbands form, a misorientation between the interior of the paired sheets and the surrounding matrix develops and increases with increasing strain.

On boundary misorientation distribution functions and how to incorporate them into three-dimensional models of microstructural evolution

The fundamental difficulties of incorporating experimentally obtained boundary misorientation distributions (BMDs) into three-dimensional microstructural models are discussed. An algorithm is described which overcomes these difficulties. The boundary misorientations are treated as a statistical ensemble which is evolved toward the desired BMD using a Monte Carlo method. The application of this algorithm to a number of complex arbitrary BMDs shows that the approach is effective for both conserved and non-conserved textures. The algorithm is successfully used to create the BMDs observed in deformation microstructures containing both incidental dislocation boundaries (IDBs) and geometrically necessary boundaries (GNBs). The application of an algorithm to grain boundary engineering is discussed.

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.

A large deformation atomistic study examining crystal orientation effects on the stress–strain relationship

Abstract We performed large deformation molecular dynamics calculations using the embedded atom method in order to examine yield and plastic flow of single crystal nickel. A parametric study using an analysis of variance technique was used to efficiently determine the influence of the deformation path, temperature, size scale, strain rate, and crystal lattice orientation with respect to yield and the plastic energy. The analysis of variance study revealed that orientation had a primary influence on the volume averaged shear stress, demonstrating the continued importance of slip within the crystal lattice for small sized metals and high applied strain rates. Consequently, other orientations were examined with a focus on averaged shear stress, axial stresses, and stress distributions. Results are presented for crystals oriented for single, double, quadruple, and octal slip, as well as a pseudopolycrystal. One last interesting point is that orientation effects on the stress-strain curves could be only approximately normalized by application of the Schmid factor.

Current issues in recrystallization: A review

Summary The authors have agreed that recrystallization is theformation of a new grain structure in a deformed material by the formation and migration of high angle grain boundaries driven by the stored energy of deformation. High angle grain boundaries are those with greater than a 10 to 15° misorientation. Recovery can be defined as all annealing processes occurring in deformed materials that occur without the migration of a high angle grain boundary. Grain coarsening can in turn, be defined as processes involving the migration of grain boundaries when the driving force for migration is solely the reduction of the grain boundary area itself. These definitions are consistent with some earlier definitions.

A microstructurally based method for stress estimates

Abstract A method is described in which quantitative microstructural analysis is used to estimate the local stress and strain states occurring within near-surface layers due to frictional contact. Quantitative estimates of local stress and strain have applications in friction and wear models, in finite element analysis of sliding interfaces, and as a basis for formulating and evaluating models on a local scale. This method is illustrated for three cases of dry sliding on nominally flat surfaces. Sliding tests were performed on a flat plate friction tester, developed at Sandia, which used copper friction samples and a steel test platen. The evolving friction coefficients were measured as a function of normal load and sliding speed. Microstructural analyses included both scanning and transmission electron microscopy (SEM and TEM) of the cross-sectioned friction samples. The sliding-induced dislocation substructures were quantitatively characterized and measured as a function of subsurface depth and normal load. Two simple relationships between the size scale of the dislocation substructure and the flow stress were used to estimate the material properties and the stress state as a function of depth and normal load.

Multi-scale modeling of polycrystal plasticity: a workshop report

Abstract The workshop on multi-scale modeling of polycrystal plasticity was held on April 9–11, 1997 at the Institute for Mechanics and Materials at the University of California, San Diego in La Jolla, CA. This workshop addressed length-scale issues associated with developing a predictive capability in the modeling of the plastic deformation of polycrystals by the incorporation of more physically based information in the models. The goals of the workshop were to: (1) establish a model system that is well suited to the multi-scale modeling methodology; (2) explore a set of discrete simulation methods at the continuum-scale, meso-scale, micro-scale, and atomic-scale; and (3) identify critical links connecting the length scales which will allow information to be passed among scales and allow the end goal of predictive models at the continuum scale. This paper presents the technical summary of the topics covered by the speakers at the workshop and a discussion of critical issues at each length scale.