Materials Week '97: Monday AM Session

Focusing on physical metallurgy and materials, Materials Week '97, which incorporates the TMS Fall Meeting, features a wide array of technical symposia sponsored by The Minerals, Metals & Materials Society (TMS) and ASM International. The meeting will be held September 14-18 in Indianapolis, Indiana. The following session will be held Monday morning, September 15.

DEFORMATION AND MICROSTRUCTURE: General Abstract Session I

Session Chair: Sriram Seshagiri, Wright Patterson Air Force Base, OH

CHARACTERIZING SUPERDISLOCATIONS IN Ll₂ STRUCTURES WITH TEM OBSERVATIONS AND IMAGE SIMULATIONS: EXAMPLES WITH APB- AND SISF DISSOCIATIONS: Mukul Kumar, Kevin J. Hemker, Dept. of Mechanical Engineering, John Hopkins University, Baltimore, MD 21218-2686

The nature of dissociated superlattice dislocation cores in Ll₂-modified Al₆₆Ti₂₅Cr₉, deformed at room temperature, has been characterized by weak-beam transmission electron microscopy and image matching by computer simulation techniques. The displacement fields associated with APB- and SISF- dissociated <110> superdislocations were calculated to account for the asymmetry in dislocation contrast, and to provide a better understanding of the formation of images from the cores of narrowly dissociated superdislocations. These techniques have been utilized in the present study to enhance the descriptive and quantitative information gathered from experimental weak-beam TEM observations of dissociated superdislocations prevalent on compressive deformation of cubic Al₃Ti at room temperature. The results indicate that suitable caution must be exercised in the interpretation of weak beam images from dislocations with large Burgers vectors, particularly when the spacing between the individual partial dislocations is on the order of a few nanometers, and small deviations from exact Bragg condition are employed.

8:50 am

DISLOCATION SUBSTRUCTURE IN NiAl SINGLE CRYSTALS: X. Shi, T.M. Pollock, S. Mahajan, V.S. Arunachalam, Department of Materials Science & Engineering, Carnegie Mellon University, Pittsburgh, PA 15213

NiAl single crystals, oriented for single slip, were deformed in compression over a range of temperatures (77K-673K). The resulting dislocation structures were examined using weak-beam transmission electron microscopy. Results indicate that the activated slip system is [001](110). The substructure consists mainly of near-edge dislocations and their density decreases with increasing temperature. These observations suggest that the edge dislocations have a low mobility in comparison to the screw dislocations. Another salient feature of the dislocation structure is the presence of a high density of jogs on both near edge and near screw zigzag dislocations. In this presentation, we will discuss the origins of the observed substructural features and comment on the relative mobilities of the edge and screw dislocations. The above work was supported by Dr. G. Yoder of ONR and the authors gratefully acknowledge the support.

9:10 am

USE OF THE ELECTRON CHANNELING CONTRAST IMAGING (ECCI) TECHNIQUE TO IMAGE DISLOCATIONS NEAR CRACK TIPS AND CRACK EDGES IN BULK SAMPLES: B-C. Ng, B.A. Simkin, M.A. Crimp, Department of Materials Science and Mechanics, Michigan State University, East Lansing, MI 48824-1226

The electron channeling contrast imaging (ECCI) technique has been developed in recent years to image near surface crystalline defects. This talk will review the basis of ECCI image formation and then discuss application of the technique to studies of fracture behavior. In our present studies, the technique is being used to image dislocations near crack tips and crack edges of bulk specimens. Examination of stable cracks shows that dislocations are generated ahead of the crack tips resulting in blunting of the cracks. Dislocations are also observed along the crack edges, left in the wake of the propagating cracks. Changes in dislocation density ahead of cracks have been correlated with material toughness. Interpretation of the images as a function of channeling conditions will be discussed. For example, the contrast of the dislocation images changes when the channeling conditions change from a positive (+) g vector to negative (-) g vector. This change of contrast channeling condition also serve to distinguish between dislocations and microcracks. This work was supported by the Office of Naval Research (Grant No. N00014-94-1-0204) and the National Science Foundation (Grant No. DMR#9257826).

9:30 am

STUDY OF DISLOCATION MOTION IN NiAl SINGLE CRYSTALS: B. Ghosh, M.A. Crimp, Department of Materials Science and Mechanics, College of Engineering, Michigan State University, East Lansing, MI 48824-1226

Dislocation mobility in commercial and high purity stoichiometric NiAl single crystals has been studied by in-situ straining transmission electron microscopy. Pre-existing dislocations, whether isolated or tangled, did not move at any time, even through the initiation and propagation of cracks. Dislocations generated upon straining glide past/through pre-existing dislocations. Images of mobile dislocations were captured dynamically by a wide angle camera coupled with a video cassette recorder. Straight slip traces left by the dislocations suggest easy glide and wavy slip traces present in the wake of dislocations indicate cross-slip of screw dislocations. Dislocation motion was found to be much slower in commercially pure crystals than in high purity crystals, suggesting solute drag. This retarded motion was characterized by small jumps of portions of individual dislocations, implying motion through kink migration. In hard oriented <001> crystals, <110> or <001> dislocations were found to be mobile on {110}planes. In the soft <110> orientation, <111> and <110> dislocations were observed to be mobile on {121}planes. This research has been supported by the Office of Naval Research (Grant # N 00014-94-1-0204).

9:50 am BREAK

10:00 am

AN ANALYSIS OF THERMALLY ACTIVATED DEFORMATION IN B₂ ALUMINIDES: T.M. Pollock, D. Lu, K. Eow, Carnegie Mellon University, Pittsburgh, PA 15213

Intermetallics with the B2 structure have recently been of interest for high temperature structural applications. While the limited ductility of many of these compounds is of concern, it is difficult to identify solutions to this problem, since ductility is a material property that is very sensitive to the details of the microstructure and processing. In this study, a variety of B2 aluminides have been investigated with the use of strain rate change experiments, in order to obtain phenomenological deformation parameters which may be capable of distinguishing intrinsic vs. extrinsic and short-range vs. long range barriers to deformation. Materials investigated to date include: RuAl, Ru₅₂Al₄₈, RuAl+0.5%B, Ru₅₃Al₄₇+0.5%B, Ru_50.5Al_44.5Cr₅, Fe₆₀Al₄₀+0.2%B, and single crystal and polycrystalline NiAl. The results of deformation experiments and accompanying electron microscopy studies will be presented.

10:20 am

CORRELATING YIELDING BEHAVIOR WITH DISLOCATION CORE STRUCTURES IN Ni₃GeFe₃Ge INTERMETALLIC ALLOYS: Mukul Kumar, T. John Balk, Kevin J. Hemker, Dept. of Mechanical Engineering, Johns Hopkins University, Baltimore, MD 21218-2686

The transition from positive to negative temperature dependence of 0.2% yield stress is investigated in the model pseudo-binary Ni₃Ge-Fe₃Ge system. Ni₃Ge and Fe₃Ge both L1₂ intermetallic alloys-show complete solid solubility as Fe is continuously substituted for Ni across the composition range. However, Ni₃Ge exhibits the yield stress anomaly, whereas the yield stress of Fe₃Ge shows a normal decline with temperature. Mechanical testing has verified this behavior, with the anomalous behavior gradually disappearing with increasing Fe content. It is proposed, and will be shown through weak-beam observations, that this transition is related to changes in the core structure of dissociated superdislocations. Results from mechanical testing will be presented and correlated with TEM observations of deformation structures. These results will be discussed in the light of planar fault energies calculated through computer simulations of images.