The influence of an electric field or corresponding current on the plastic deformation of metals and ceramics is reviewed. Regarding metals, the following are considered: (a) the effects of high density electric current pulse on the flow stress at low to intermediate homologous temperatures; and (b) the effects of an external electric field on superplasticity at high temperatures. The major effect of the current pulses was to reduce the thermal component of the flow stress. This resulted from the combined action of an electron wind force, a decrease in the activation enthalpy for plastic deformation and an increase in the pre-exponential, the last making the largest contribution. Besides giving a reduction in the flow stress during superplastic deformation, an external electric field reduced cavitation and grain growth. The influence of the external field appears to be on the migration of vacancies or solute atom-vacancy complexes along grain boundaries to the charged surface. In the case of ceramics, the effects of an internal electric field on the plastic deformation of polycrystalline NaCl at 0.28–0.75TM and on the superplasticity of fine-grained oxides (MgO, Al2O3 and ZrO2) at T>0.5TM are considered. Regarding NaCl, at T≤0.5TM an electric field E≥10 kV cm−1 is needed to enhance dislocation mobility in single crystals. However, a field of only 1 kV cm−1 significantly reduced the flow stress in polycrystals, which is concluded to result from an enhancement of cross slip. At T>0.5TM, there occurred a decrease in the flow stress of polycrystalline NaCl along with a reduction in the rate-controlling diffusion activation energy. Regarding the fine-grained oxides at T>0.5TM, an internal electric field E≤0.3 kV cm−1 gave an appreciable, reversible, reduction in the flow stress by an enhancement of the rate-controlling diffusion process. Limited work suggests that a field may also retard grain growth and cavitation in ceramics.

Electroplasticity in metals and ceramics

Radiation hardening revisited: role of intracascade clustering

Experimental observations related to the initiation of plastic deformation in metals and alloys irradiated with fission neutrons have been analyzed. The experimental results, showing irradiation-induced increase in the upper yield stress followed by a yield drop and plastic instability, cannot be explained in terms of conventional dispersed-barrier hardening because (a) the grown-in dislocations are not free, and (b) irradiation-induced defect clusters are not rigid indestructible Orowan obstacles. A new model called ‘cascade-induced source hardening’ is presented where glissile loops produced directly in cascades are envisaged to decorate the grown-in dislocations so that they cannot act as dislocation sources. The upper yield stress is related to the breakaway stress which is necessary to pull the dislocation away from the clusters/loops decorating it. The magnitude of the breakaway stress has been estimated and is found to be in good agreement with the measured increase in the initial yield stress in neutron irradiated copper.

Radiation hardening revisited: Role of intracascade clustering

Bulk, fully dense materials were prepared from Fe-10Cu with grain diameters between 45 nm and 1.7 µm. The materials were prepared by ball milling of powders in a glove box, followed by hot isostatic pressing (hipping) or powder forging. Larger grain sizes were obtained by thermal treatment of the consolidated powders. The bulk materials were relatively clean, with oxygen levels below 1500 wpm and other contaminants less than 0.1 at. pct. The mechanical behavior of these materials was unique. At temperatures from 77 to 470 K, the first and only mechanism of plastic deformation was intense shear banding, which was accompanied by a perfectly plastic stress-strain response (absence of strain hardening). There was a large tension-compression asymmetry in the strength, and the shear bands did not occur on the plane of maximum shear stress or the plane of zero extension. This behavior, while unusual for metals, has been observed in amorphous polymers and metallic glasses. On the other hand, the fine-grained Fe-10Cu materials behaved like coarse-grained iron in some respects, particularly by obeying the Hall-Petch equation with constants reasonably close to those of pure iron and by exhibiting low-temperature mechanical behavior which was very similar to that of steels. Transmission electron microscopy (TEM) studies found highly elongated grains within shear bands, indicating that shear banding occurred by a dislocation-based mechanism, at least at grain sizes above 100 nm. Similarities and differences between the fine-grained Fe-10Cu and metals, polymers, metallic glasses, radiation-damaged metals, and quench-damaged metals are discussed.

Mechanical behavior of a bulk nanostructured iron alloy

Microstructural processes of plastic instabilities in strengthened metals

Dislocation channels cleared of radiation‐produced defect clusters were observed by transmission electron microscopy in polycrystalline niobium irradiated to 4×1018 neutrons/cm2 (E>1 MeV) and then deformed in tension to 6.6% strain. From electron diffraction patterns and diffraction contrast (g·b) analysis, the {110} plane was determined to be the plane on which the dislocation channels formed in most cases. The strain due to the motion of slip dislocations in the channels was deduced from the offset at intersections between the channels and other microstructural features. The strain corresponded to the passage of 1–3 slip dislocations per slip plane. Several mechanisms for the clearing of defect clusters by slip dislocations are discussed. These include: annealing due to the heat of plastic deformation, chopping up or sweeping up of defect clusters by slip dislocations, and annihilation by antidefects.

Dislocation Channeling in Neutron‐Irradiated Niobium

The effect of low-temperature annealing on the yield stress of niobium following neutron irradiation was studied. The initial increase in yield stress upon irradiation was sensitive to the interstitial carbon content. A further increase in the yield stress (“radiation-anneal hardening”) was observed after two-hour anneals near 150 and 300 °C. When the annealing temperature was raised above 400 °C, the yield stress gradually recovered toward the preirradiation value. Changes in the density and size of the radiation-produced defect clusters were determined by transmission electron microscopy following post-irradiation anneals. An analysis of the observed hardening based on a dispersed barrier model and the density and size of defect clusters indicated that post-irradiation annealing strengthened the clusters as barriers to dislocation motion by as much as a factor of two. Previous resistivity and internal friction measuremements have shown that interstitial oxygen and carbon in irradiated niobium migrate to the defect clusters at the temperatures near 150 and 300 °C, respectively. Thus, the radiation-anneal-hardening peaks at these temperatures are attributed to the trapping of oxygen and carbon, respectively, at defect clusters. There is a reduced tendency for dislocation channeling (i.e., defect cluster removal by slip dislocations) in radiation-anneal-hardened niobium, suggesting that indeed the clusters have been strengthened.

Radiation-anneal hardening in niobium: an effect of post-irradiation annealing on the yield stress.

The electron microscope image contrast of dislocation loops under two-beam dynamical diffraction conditions is calculated using the Howie-Whelan equations and the displacement fields of the dislocation loops of pure edge, as well as shear, orientation in an infinite isotropic medium. The results indicate that the deviation of the black-white direction, measured from the diffraction vector, is directly proportional to the sum of the deviations of Burgers vector and loop normal projected onto the image plane. The rule governing the black-white direction, which is also consistent with first-order analytical theories of image contrast, will be useful in determining the crystallographic nature of dislocation loops from contrast analysis.



Der elektronenmikroskopische Bildkontrast wird mit Hilfe der dynamischen Beugungstheorie (Zwei-Strahl-Fall) fur Versetzungsringe mit Hilfe der Howie-Whelan-Gleichungen berechnet. Zugrunde gelegt werden hierbei die Verzerrungsfelder von Versetzungsschleifen mit reiner Stufen- wie auch mit Scher-Orientierung in einem unendlichen, isotropen Medium. Die Ergebnisse zeigen, das die Abweichung von der Schwarz-Weis-Richtung, vom Beugungsvektor aus gemessen, direkt proportional der Summe der Abweichungen des Burgersvektors und der Schleifen-Normalen ist, beide projiziert auf die Bildebene. Diese Beziehung, die die Schwarz-Weis-Richtung bestimmt, ist nutzlich bei der Ermittlung der kristallographischen Natur von Versetzungsschleifen mittels der Kontrast-Analyse. Das Ergebnis ist ebenfalls konsistent mit den „first-order”-Bildkontrast-Theorien.

Direction of the black‐white contrast of dislocation loops

The electron microscope images of large dislocation loops lying perpendicular to the electron beam are calculated by applying the image simulation technique. It is found that the residual double arc contrast arising from the g · b × u term exhibits asymmetry which is depth dependent. The predictions of the theory are verified experimentally for a[001] dislocation loops observed in single crystals of MgO. The depth oscillation of the residual contrast can be used to determine the vacancy or interstitial nature of dislocation loops.



Die elektronenmikroskopischen Abbildungen groser Versetzungsringe, die senkrecht zum Elektronenstrahl liegen, werden unter Anwendung der Abbildungssimulations-Technik berechnet. Es wird gefunden, das der vom g · b × u-Term stammende Doppelbogen-Restkontrast eine Asymmetrie aufweist, die tiefenabhangig ist. Die Voraussagen der Theorie werden experimentell bestatigt fur in MgO-Einkristallen beobachtete a[001]-Versetzungsringe. Die Tiefen-Oszillationen des Restkontrastes konnen zur Bestimmung der Leerstellen- oder Zwischengitteratom-Natur der Versetzungsringe benutzt werden.

Residual image contrast of dislocation loops

The electron microscope image contrast of pure edge dislocation loops in cubic crystals is calculated by taking into account elastic anisotropy. In copper and nickel, the black—white contrast of a Frank loop is stretched considerably and is aligned exactly along the 〈111〉 direction of the Burgers vector. In niobium, the black—white contrast direction of a perfect loop is approximately halfway between the Burgers vector and the diffraction vector. These reslts are in good agreement with the anisotropic elastic properties of these crystals.



Der elektronenmikroskopische Bildkontrast von rein stufenartigen Versetzungsringen in kubischen Kristallen wird unter Berucksichtigung der elastischen Anisotropie berechnet. In Kupfer und Nickel ist der Schwarz—Weis-Kontrast eines Frank'schen Versetzungsringes deutlich gestreckt und exakt parallel zur 〈111〉-Richtung des Burgers-Vektors ausgerichtet. In Niob liegt die Richtung des Schwarz—Weis-Kontrastes eines vollstandigen Versetzungsringes annahernd in der Mitte zwischen Burgers-Vektor und Beugungsvektor. Diese Ergebnisse befinden sich in guter Ubereinstimmung mit den anisotropen elastischen Eigenschaften dieser Kristalle.

S. M. Ohr

Papers

Dislocation Channeling in Neutron‐Irradiated Niobium

Radiation-anneal hardening in niobium: an effect of post-irradiation annealing on the yield stress.

Direction of the black‐white contrast of dislocation loops

Residual image contrast of dislocation loops

Black—white contrast of dislocation loops in anisotropic cubic crystals†