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

Defects exist in almost all materials and defect engineering at the atomic level is part of modern semiconductor technology Defects and their long-range strain fields can have a negative impact on the host materials In materials with confined dimensions, the influence of defects can be even more pronounced due to the enhanced relative volume of the 'defective' regions Here we report the dislocation-induced polarization instability of (001)-oriented Pb(Zr(052)Ti(048))O(3) (PZT) nanoislands, with an average height of approximately 9 nm, grown on compressive perovskite substrates Using quantitative high-resolution electron microscopy, we visualize the strain fields of edge-type misfit dislocations, extending predominantly into a PZT region with a height of approximately 4 nm and width of approximately 8 nm The lattice within this region deviates from the regular crystal structure Piezoresponse force microscopy indicates that such PZT nanoislands do not show ferroelectricity Our results suggest that misfit engineering is indispensable for obtaining nanostructured ferroelectrics with stable polarization

Impact of misfit dislocations on the polarization instability of epitaxial nanostructured ferroelectric perovskites.

Recent improvements in strength and ductility of 5083 aluminum alloys have been obtained through the development of complex microstructures containing either reduced grain sizes (ultra-fine and nano-grained materials), grain size distributions (bimodal microstructures), particle reinforcements, or combinations of the above. Optimization of such microstructures requires an understanding of the conventional, coarse-grained basis alloy. We present here a complete experimental data set for conventional Al-5083 H-131, with primary alloying element Mg (4.77 wt%) and secondary element Mn (0.68 wt%) in compression over a range of strain rates (10−4–6000 s−1) at room temperature. The various strengthening mechanisms in Al-5083 are explored, including solute strengthening, precipitate hardening, strain hardening, strain rate hardening, and strengthening due to dislocation sub-structures. Previous experiments found in the literature on Al–Mg binary alloys allow us to calculate the solute strengthening due to Mg in solid solution, and TEM analysis provides information about precipitate hardening and dislocation cell structures. A basic strength model including these strengthening mechanisms is suggested.

Strengthening mechanisms in an Al–Mg alloy

The propagation of seismic waves in the Earth's mantle can be significantly affected by relaxation processes, causing attenuation and velocity dispersion (reduction). This paper reviews the solid-state mechanisms of relaxation processes based on the theory of defect microdynamics in solids together with some experimental observations on defects in minerals (particularly in olivine). For a given mechanism to have a significant effect on seismic wave propagation, both the density and the mobility of the defects must be in an appropriate range. The examination of the densities (and geometry) and mobilities of defects in olivine shows that dislocation and/or grain boundary mechanisms can have a significant effect on seismic wave propagation, although wide distributions of geometrical factors (such as spacing of pinning points) and of mobilities are required to explain all available data. Point defect mechanisms, however, are unlikely to be important because their densities are too small and/or their mobilities are too large. Since the dislocation density and/or grain size are determined in most cases by the long-term tectonic stress, seismic wave attenuation and velocity dispersion (reduction) involving these defects are likely to depend on the magnitude of the tectonic stress as well as the temperature. Theoretical considerations suggest a wide range of dependence of seismic wave attentuation (and velocity dispersion) on the long-term tectonic stress. This is particularly the case for dislocation mechanisms and warrants careful experimental investigation. Dislocations and/or grain boundaries cause anelastic behavior (relaxation peaks) when they are pinned or blocked at some points. Pinning or blocking becomes ineffective at high temperatures and/or low frequencies, causing a transition to viscoelastic behavior. Both laboratory and seismological observations of internal friction are dominated by the “high-temperature background” where internal friction increases monotonically with temperature, which can be interpreted as a gradual transition to viscoelastic behavior or to a wide distribution of relaxation times. However, in most experimental studies to date, the dislocation densities or the grain sizes were not well controlled, making it difficult to identify the attenuation mechanisms and preventing any quantitative applications to Earth. The need for better characterization of defect microstructures in experimental specimens is emphasized.

Defect microdynamics in minerals and solid state mechanisms of seismic wave attenuation and velocity dispersion in the mantle

Kinetics of dislocations in pure Fe. Part II. In situ straining experiments at low temperature

Dislocation dynamics and plasticity

Copper (and its alloys) and iron or steel form two groups of metals that have been utilized since the ancient times. More recently, aluminium and its alloys have found many applications, thus forming three important metallic groups. When we compare the low-temperature flow stresses of these three kinds of metals, we find that that of Fe is much higher than the other two: at liquid nitrogen temperature usual steels can no longer be plastically deformed without fracture. From the fact that Fe has body-centered cubic (bcc) structure while Cu and Al have face-centered cubic (fcc) structure, such a difference in low-temperature plasticity is considered to be due to the difference in crystal structure. In fact, while all pure fcc metals exhibit weak temperature dependence of the yield stress, metals with bcc structure show, without exception, a strong temperature dependence. In this chapter, we describe characteristic plastic behavior of bcc metals and its interpretation.

Dislocations in bcc Metals and Their Motion

Deformation rates of crystals, according to many experimental and theoretical studies, are controlled by thermal activation processes for surmounting various barriers. So far, the problems have been discussed using the following implicit solutions: (1) the processes are quasi-static and do not involve any dynamical aspects; (2) the frictional forces produced in materials are neither extremely large nor small. If the frictional forces due to the material are very small, the hypothesis (1) does not hold any more. This is the case of deformation of superconductors as described in the next chapter. Weertman [3.1] dissussed creep of ice on the basis of visco-elastic deformation theory developed by Eshelby [3.2]. According to ultrasonic attenuation experiments [3.3], B is 107 times as large as that for metals. In this case, although the frictional force is still proportional to the velocity of dislocations, the time of motion in the area between barriers becomes longer than that needed to surmount each of them. In other words, the barriers are of no importance for the discussion of the rate of deformation. Accordingly, such a linear visco-elastic deformation may be put aside in the present discussion. As for studies of plastic deformation of crystalline materials, the past treatments of the case, where the frictional forces are very small, should be carefully reconsidered. In the present chapter, we discuss problems within the above framework, (1) and (2), and those outside of this will be discussed in the next chapter.

Dislocation Motion in the Field of a Random Distribution of Point Obstacles: Solution Hardening

The special aspect of high-temperature deformation is that the effect of diffusion is pronounced. The dislocations can climb as well as glide so that recovery due to dislocation rearrangement and pair annihilation proceeds quickly. Sometimes recrystallization occurs during deformation. For polycrystalline metals, grain boundary sliding and migration also play an important role. Under a very low stress, the deformation occurs not by dislocation motion, but exclusively by diffusion. In this chapter, the characteristics of high-temperature deformation are shown by using the deformation mechanism map. The problems special to high-temperature deformation, and the experimental techniques to settle them, are given to help the understanding of the deformation mechanism described in the following chapters.

High-Temperature Deformation of Metals and Alloys

Reflecting the periodicity of the crystal lattice, any dislocation in any crystal has its own intrinsic resistance to glide, because the self-energy of the dislocation changes with the periodicity of the lattice This periodic potential energy with respect to the dislocation position is called the Peierls potential, after the researcher who first estimated this potential theoretically for a simple model [51] The stress necessary for the dislocation to surmount this potential, without the aid of thermal energy, is called the Peierls stress This chapter is concerned with theoretical treatments of the dislocation glide controlled by the Peierls potential

Shin Takeuchi

Papers

Dislocation dynamics and plasticity

Dislocations in bcc Metals and Their Motion

Dislocation Motion in the Field of a Random Distribution of Point Obstacles: Solution Hardening

High-Temperature Deformation of Metals and Alloys

Dislocation Motion Controlled by the Peierls Mechanism