Atomic-scale modeling of dislocations and related properties in the hexagonal-close-packed metals

Metals with the hcp crystal structure have a wide variety of mechanical and physical properties, and understanding the links between atomic processes, microstructure, and properties can open the way for new applications. Computer modeling can provide much of the information required. This article reviews recent progress in atomic-scale computer simulation in three important areas. The first is the core structure of dislocations responsible for the primary slip modes, where modeling has revealed the variety of core states that can arise in pure, elemental metals and ordered alloys. While most research has successfully employed many-body, central-force interatomic potentials, they are inadequate for metals which have an unfilled d-electron band, such as α-Ti and α-Zr, and the resulting noncentral character of the atomic bonding is shown to have subtle yet significant effects on dislocation properties. Deformation twinning is an important process in plasticity of the hcp metals, and modeling has been used to investigate the factors that control the structure and mobility of twinning dislocations. Furthermore, simulation shows that twinning dislocations are actually generated, in some cases, following the interaction of crystal dislocations with twin boundaries; this can lead to the very mobile boundaries observed experimentally. The final area concerns the nature and properties of the defects created by radiation damage. Computer simulation has been used to determine the number and arrangement of defects produced in primary, displacement-cascade damage in several hcp metals. The number is similar to that found in cubic metals and is considerably smaller than that expected from earlier models. Many self-interstitial atoms cluster in cascades to form highly glissile dislocation loops, and, so, contribute to two-dimensional material transport in damage evolution.     

Interfacial deformation mechanisms in hexagonal-close-packed metals

Deformation processes involving interfacial dislocation mechanisms in twin boundaries of hexagonal-close-packed (hcp) metals are described. The topological properties of individual defects, namely their Burgers vectors, b, and step heights, h, are defined rigorously, and the magnitude of the diffusional flux of material required for motion of a defect along an interface is expressed quantitatively in terms of b, h, and the material’s density. This framework enables interactions between defects to be treated and, in particular, enables identification of processes that are conservative. Using these topological arguments, it is shown that sessile interfacial defects in twins need not block further twinning and that the recently discovered Serra-Bacon (S—B) twinning mechanism is conservative. The possible wider significance of the S—B-type mechanism that causes localized lateral growth of twins is also considered briefly in the context of the deformation of hcp and martensitic materials. This article is based on a presentation made in the symposium entitled “Defect Properties and mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Lousiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Interfacial deformation mechanisms in hexagonal-close-packed metals

Deformation processes involving interfacial dislocation mechanisms in twin boundaries of hexagonal-close-packed (hcp) metals are described. The topological properties of individual defects, namely their Burgers vectors, b, and step heights, h, are defined rigorously, and the magnitude of the diffusional flux of material required for motion of a defect along an interface is expressed quantitatively in terms of b, h, and the material’s density. This framework enables interactions between defects to be treated and, in particular, enables identification of processes that are conservative. Using these topological arguments, it is shown that sessile interfacial defects in twins need not block further twinning and that the recently discovered Serra-Bacon (S-B) twinning mechanism is conservative. The possible wider significance of the S-B-type mechanism that causes localized lateral growth of twins is also considered briefly in the context of the deformation of hcp and martensitic materials. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Effect of pressure on zone-center phonons in hexagonal-close-packed metals

We report on studies of several hexagonal-close-packed (hcp) metals by Raman scattering techniques in the diamond anvil cell for pressures up to 60 GPa. The pressure response of the observed transverse-optical (TO) zone-center phonon mode includes positive pressure shifts as well as anomalies, such as mode softening in connection with phase transitions. It is shown that the phonon frequencies and their pressure dependences are related to macroscopic elastic parameters. More general, these results show that the measurement of Raman-active phonons provides a direct probe of bonding in metals, and agreement with theoretical models gives additional confidence in ab initio techniques. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Effect of pressure on zone-center phonons in hexagonal-close-packed metals

We report on studies of several hexagonal-close-packed (hcp) metals by Raman scattering techniques in the diamond anvil cell for pressures up to 60 GPa. The pressure response of the observed transverse-optical (TO) zone-center phonon mode includes positive pressure shifts as well as anomalies, such as mode softening in connection with phase transitions. It is shown that the phonon frequencies and their pressure dependences are related to macroscopic elastic parameters. More general, these results show that the measurement of Raman-active phonons provides a direct probe of bonding in metals, and agreement with theoretical models gives additional confidence in ab initio techniques. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Twins as barriers to basal slip in hexagonal-close-packed metals

The boundary structure of , , , and twins in hexagonal-close-packed (hcp) metals and the interaction of crystal dislocations with the first two twin types have been studied previously using atomic-scale computer simulation. The interaction of crystal dislocations with and twin boundaries is described here and compared with the results for and twins. These four twins are found to create barriers to the motion of crystal dislocations gliding on the basal plane, and the strength of the barrier depends in a relatively complex manner on crystallographic parameters and details of the atomic structures of the interfaces. In some circumstances, crystal dislocations can be transmitted through the twin boundary, thereby creating twinning dislocations. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Twins as barriers to basal slip in hexagonal-close-packed metals

The boundary structure of {10 1}, {10 2}, {11 1} and, {11 2} twins in hexagonal-close-packed (hcp) metals and the interaction of crystal dislocations with the first two twin types have been studied previously using atomic-scale computer simulation. The interaction of crystal dislocations with {11 1} and {11 2} twin boundaries is described here and compared with the results for {10 1} and {10 2} twins. These four twins are found to create barriers to the motion of crystal dislocations gliding on the basal plane, and the strength of the barrier depends in a relatively complex manner on crystallographic parameters and details of the atomic structures of the interfaces. In some circumstances, crystal dislocations can be transmitted through the twin boundary, thereby creating twinning dislocations. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

The crystallography and atomic structure of line defects in twin boundaries in hexagonal-close-packed metals

The crystallographic analysis of line defects in interfaces is discussed and applied to the particular case of twinning dislocations in hexagonal-close-packed (hop) metals, which have been studied here by atomistic simulation. Two crystallographic approaches are used; first, the concept of bicrystal structure maps is developed for the case of interfaces between crystals having multiple-atom bases, and second, the topological theory of line defects based on symmetry theory is used. On the basis of the atomistic calculations, some general conclusions concerning the relative contribution to the total energy of dislocations made by their elastic fields and core structures are presented. This paper is based on a presentation made in the symposium “The Role of Ledges in Phase Transformations” presented as part of the 1989 Fall Meeting of TMS-MSD, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Phase Transformations Committee of the Materials Science Division, ASM INTERNATIONAL.     

Analytical modeling of the rheological behavior of semisolid metals and composites

The thixotropic nature of semisolid alloys and composites has been shown to be advantageous in semisolid metal processing. This effect can be highlighted by hysteresis experiments in the semisolid state where the shear rate is increased from zero to a maximum value and then decreased back to zero in a specific time period. The results presented in this article show that the rheological behavior of a semisolid material subjected to such a cycle depends strongly on the initial state of its microstructure. It is shown that the rheological behavior of the dissociated structure is reversible, while that of the agglomerated state is strongly thixotropic. The agglomeration of the structure is favored by a rest time and a low initial shear rate prior to the shear cycle. A model based on the assumptions of Moore was developed to describe our experimental results. It takes into account all the variables that influence the form of the hysteresis loops, i.e., the time to reach the maximum shear rate, the maximum shear rate attained during a cycle of shearing, the initial state of the microstructure, and the number of shearing cycles imposed. The results of our simulations show that we are able to reproduce the experimental results under various conditions in an analytical model, which take into account a set of experimentally determined constants and by varying the value of a structural parameter describing the initial state of agglomeration of the semisolid metal slug.     

A framework for modeling creep in pure metals

The process of creep in pure metals is modeled as the cooperative interaction of three phenomena: the thermally activated, force-dependent release of dislocation segments from obstacles; the substructural refinement of the microstructure due to plastic deformation; and the diffusion-controlled coarsening of the substructure. Key parameters are given as approximate generic values which can be varied. It is shown that for a wide range of parameters, the model reproduces the key features of the creep of pure metals: a steady-state stress exponent near 5 is recovered, and the key microstructural-length scale is related by a power law close to the reciprocal of stress (this dependence is not a strong function of temperature at a given stress). In addition, the activation energy of steady-state creep is nearly that of self-diffusion. Thus, the model reproduces the well-known phenomenology of puremetal steady-state creep. However, the present model is based on separate microstructural phenomena, which can be independently refined and studied. This article is based on a presentation made in the workshop entitled “Mechanisms of Elevated Temperature Plasticity and Fracture,” which was held June 27–29, 2001, in San Diego, CA, concurrent with the 2001 Joint Applied Mechanics and Materials Summer Conference. The workshop was sponsored by Basic Energy Sciences of the United States Department of Energy.     

Application of nonlocal elasticity to the energetics for solute atoms in body-centered cubic transition metals with dislocations

The self-energy of interstitial atoms and the interaction energies between edge or screw dislocations and interstitial atoms at octahedral or tetrahedral interstitial positions in body-centered cubic transition metals are calculated analytically by using nonlocal linear elasticity. All quan- tities calculated,e.g., the distances, sites, and volume changes, are unambiguous. The results show that the energies can be calculated within an error of 0.1 eV by applying nonlocal elastic theory.