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.     

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.     

Competition among basal,prism, and pyramidal slip modes in hcp metals

The competition of slip among , and slip modes of hcp metals has been analyzed geometrically in terms of a critical resolved shear stress, CRSS, criterion. Under the action of an applied stress slip systems of one or more modes may be activated depending on the value of the CRSS and on the orientation of the slip systems with respect to the applied stress. If the CRSS of a given slip mode should exceed a limiting value relative to the CRSS of the other modes, however, the given mode becomes inoperative even under the most favorably stressed conditions. It is found by an examination of the yield loci that basal slip is inoperative if α₂ < cos θ; prism slip is inoperative if α₂ < α₁ sin θ; and pyramidal slip is inoperative if α₂ > cos θ + α₁ sin θ where and are, respectively, the ratios of CRSS for prism and pyramidal slips relative to basal slip, and ϕ is the angle between the (0001) and normals. Since the value of ϕ is a function ofc/a, the limiting values of α₁ and α₁ depend on thec/a ratio of the crystal structure.     

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.     

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. 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.     

Stress-substructure relationships in cyclically and monotonically deformed wavy slip mode metals

Polycrystalline aluminum was cyclically deformed under strain control at room temperature in the as-extruded form and different annealed conditions after forging. The dislocation substructure developed on saturation was examined by transmission electron microscopy. An inverse relationship was found between the saturation stress and dislocation cell size. Comparison of these results with those for aluminum, copper, and iron obtained under both monotonie and cyclic conditions at various temperatures indicated that the same form of equation with a different constant was characteristic of each of the three wavy slip mode metals. The relationship appears to be dependent upon the stacking fault energy of the particular material and independent of the type of deformation(i.e., monotonie or cyclic).     

Examination gloves as barriers to hand contamination in clinical practice

OBJECTIVE: To test the effectiveness of vinyl and latex gloves as barriers to hand contamination with gram-negative organisms and enterococci during routine hospital procedures. DESIGN AND INTERVENTIONS: We studied 137 procedures during which a health care worker's gloved hand contacted a patient's mucous membrane and was thus potentially contaminated with gram-negative rods or enterococci. Quantitative hand cultures were obtained from each health care worker before and after the gloved contact using a modified glove juice method, and the exterior glove surface was also quantitatively cultured after patient contact. Used gloves were then tested for leaks using the American Society for Testing and Materials' watertight test. SETTING: Harborview Medical Center, a 330-bed city-county hospital and level I regional trauma and burn center, is both a teaching facility affiliated with the University of Washington and the major provider of care to indigent and uninsured persons in Seattle-King County, Washington. PATIENTS AND OTHER PARTICIPANTS: Respiratory therapists performing endotracheal tube care on intubated intensive care unit patients, registered nurses performing digital rectal stimulation for bowel training on patients with spinal cord injury in the rehabilitation ward, and dentists performing routine dental examinations and procedures on healthy outpatients in the dental clinic. MAIN OUTCOME MEASURE AND RESULTS: Eighty-six of the 135 gloves cultured had gram-negative rods or enterococci on the external surface after use and were thus sources of potential hand contamination. Microbial contamination of the health care worker's hands occurred in 11 (13%; 95% confidence interval, 6% to 20%) of these 86 events, and was more frequent with vinyl (10 of 42) than latex (one of 44) gloves (P < .01). After use, glove leaks were also more frequent in vinyl gloves (26 of 61) than with latex gloves (six of 70) (P < .001). Even when leaks were present, gloves prevented hand contamination in 77% of instances and quantitative counts of microorganisms contaminating hands were 2 to 4 logs less than counts on external glove surfaces. Health care workers reported awareness of the presence of glove leaks in only seven (22%) of the 32 events in which leaks were subsequently demonstrated. CONCLUSIONS: Under conditions of routine use, gloves effectively function as a protective barrier even when leaks are present. Latex gloves were less frequently associated with leaks and hand contamination. Since hand contamination occurred after 13% of exposures and cannot be readily identified by health care workers, routine hand washing should be done after each patient contact.     

AnIn Situ transmission electron microscope deformation study of the slip transfer mechanisms in metals

The slip transfer mechanisms across grain boundaries in 310 stainless steel, high-purity aluminum, and a Ni-S alloy have been studied by using thein situ transmission electron microscope (TEM) deformation technique. Several interactions between mobile lattice dislocations and grain boundaries have been observed, including the transfer and generation of dislocations at grain boundaries and the nucleation and propagation of a grain boundary crack. Quantitative conditions have been established to correctly predict the slip transfer mechanism. This paper is based on a presentation made in the symposium “Interface Science and Engineering” presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26–29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee.     

Creep and plasticity of hexagonal polycrystals as related to single crystal slip

The role of slip on basal, prismatic and pyramidal systems of hexagonal single crystals in determining inelastic polycrystalline behavior is studied using a uniform strain-rate upper bound and a self-consistent method. Steady power-law creep is considered. Included as a limiting case is rigid-perfectly plastic behavior, for which the upper bound to the yield stress of the polycrystal coincides with the Bishop-Hill bound for these materials. When the resolved shear stress needed to produce a given level of slip on the pyramidal systems is large compared to that on the other systems the upper bound lies well above the self-consistent estimate. Scif-consistent theory indicates that overall inelastic deformation of a polycrystal is possible without pyramidal slip. Implications for hexagonal materials, including ice, are discussed.     

Tektins as structural determinants in basal bodies

Tektins, present as three equimolar 47-55 kDa protein components, form highly insoluble protofilaments that are integral to the junctional region of outer doublet microtubules in cilia and flagella. To identify and quantify tektins in other compound microtubules such as centrioles or basal bodies, a rabbit antiserum was raised against tektin filaments isolated from Spisula solidissima (surf clam) sperm flagellar outer doublets and affinity-purified with nitrocellulose blot strips of tektins resolved by SDS- or SDS-urea-PAGE. These antibodies recognized analogous tektins in axonemes of organisms ranging from ctenophores to higher vertebrates. Quantitative immunoblotting established that outer doublet tektins occur in a 1:17 weight ratio to tubulin. Cilia and basal apparatuses were prepared from scallop gill epithelial cells; cilia and deciliated cells were prepared from rabbit trachea. Tektins were detected by immunoblotting in basal body-enriched preparations while tektins were localized to individual basal bodies by immunofluorescence. Supported by greater fluorescence in basal bodies than in adjacent axonemes in tracheal cells, analysis of basal apparatuses demonstrated both a proportionately greater ratio of tektin to tubulin (approximately 1:13) and two distinct solubility classes of tektins, consistent with tektins comprising the B-C junction of triplets in addition to the A-B junction as in doublets.     

An analysis of constrained deformation by slip and twinning in hexagonal close packed metals and alloys

A procedure for predicting the isostrain behavior of hcp metals and alloys which deform by basal, prism, and pyramidal slip as well as twinning is developed. This procedure assumes that deformation will occur on four slip systems and one or more twinning system(s). Stress states are derived for the simultaneous operation of all combinations of four slip systems for allc/a ratios as well as all values of the three critical-resolved shear stresses for slip. A numerical procedure for obtaining the remaining kinemati-cally required deformation by twinning is outlined for the case of impure titanium. Formerly a Graduate Student at Carnegie-Mellon University     

An analysis of constrained deformation by slip and twinning in hexagonal close packed metals and alloys

A procedure for predicting the isostrain behavior of hcp metals and alloys which deform by basal, prism, and pyramidal slip as well as twinning is developed. This procedure assumes that deformation will occur on four slip systems and one or more twinning system(s). Stress states are derived for the simultaneous operation of all combinations of four slip systems for allc/a ratios as well as all values of the three critical-resolved shear stresses for slip. A numerical procedure for obtaining the remaining kinemati-cally required deformation by twinning is outlined for the case of impure titanium.