Dynamic observation of dislocation-free plastic deformation in gold thin foils

Ductile fracture of metals produces a thin foil portion, which is observable by transmission electron microscopy, at the fractured edge. The thin foil portion shows unusual deformation microstructure, which contains no dislocations, but contains vacancy-type point defect clusters at extraordinarily high density. Dynamic observation of the deformation process revealed that these defect clusters are produced in the portion of local heavy deformation; however, no dislocation motion was observed during the course of the heavy plastic deformation, constituting direct evidence that the unusual deformation microstructure is produced by plastic deformation without dislocations. Also, the deformation was found to involve 14% elastic deformation, indicating that the dislocation-free plastic deformation occurs under an extraordinarily high internal stress level of more than 10 GPa, which is comparable to the ideal strength of metals. Furthermore, during the dislocation-free plastic deformation, equal-thickness fringes were found to disappear temporarily, suggesting that instability of crystalline state under extraordinarily high internal stress level is a key factor for the mechanism of dislocation-free plastic deformation.     

Dynamic observation of dislocation-free deformation process in Al, Cu, and Ni thin foils

Dislocation-free plastic deformation, which occurs under extraordinarily high internal stress comparable to ideal strength of metals, was discovered in thin foil portion produced by ductile fracture of fcc Au by dynamic observation of the deformation process [1, 2, 3, 4 and 5]. In the present study, the deformation process of thin foil portion in other fcc metals (Al, Cu, Ni) was examined in the same manner. In all these fcc metals, production of vacancy-type point defect clusters was confirmed during deformation without dislocations. Also, the dislocation-free deformation was found to progress under extraordinarily high internal stress levels corresponding to 14% elastic deformation in Ni, 12% in Cu, and 4% in Al. Especially in Al, as temperature decreased, the number density of stacking fault tetrahedra produced during deformation increased, along with increasing of the detected elastic deformation. These results indicate that internal stress level is a key factor in generalizing the new theory regarding dislocation-free plastic deformation.     

Dislocation structures introduced by high-speed deformation in bcc metals

Systematic experiments were carried out over a wide range of strain rate, 10⁰–10⁶ s⁻¹, so as to reveal the deformation mode in bcc crystals, especially at high strain rate. Dislocation structure showed heterogeneous distribution at low strain rates in all three bcc metals examined. At higher strain rates exceeding 10³ s⁻¹, distribution of dislocations was random, and the formation of small dislocation loops was observed in V and Nb. In Mo, small dislocation loops were not formed by deformation, even at high strain rates. However, post-deformation annealing of an Mo specimen that had been deformed by 20% at 5×10⁵ s⁻¹ produced dislocation loops. The inside–outside contrast method identified these loops to be of vacancy type. These results reveal that in Mo vacancy clusters are not formed directly from the interaction of dislocations, but by the aggregation of vacancies. In V and Nb, the same formation process is believed to occur at high strain rates. These results suggest that the different mode of plastic deformation at high strain rates accompanied by production of vacancies also occurred in bcc metals.     

Plastic deformation of copper thin foils

According to one suggested model, bending of a single crystal introduces edge dislocations of the same sign. In the present study, this model is examined by computer simulation using molecular dynamics. When a notch is present on the tension surface, Heidenreich-Shockley partial dislocations are created near the tip of the notch. In the compression surface, partial dislocations are created due to wrinkling of the crystal plane. The results of simulation shows that dislocations are more easily created in a compressive bending region than in a tension bending region or simple tension region. For shear deformation, partial dislocations are created on the highest resolved shear stress slip plane {1 1 1} and slip in the direction of highest resolved shear stress.     

Defect structure of gold introduced by high-speed deformation

The effect of deformation speed on defect structures introduced into bulk gold specimens at 298 K has been investigated systematically over a wide range of strain rate from ′=10⁻² to 10⁶ s⁻¹. As strain rate increased, dislocation structure changed from heterogeneous distribution, so-called cell structure, to random distribution. Also, stacking fault tetrahedra (SFTs) were produced at anomalously high density by deformation at high strain rate. The anomalous production of SFTs observed at high strain rate is consistent with the characteristic microstructure induced by dislocation-free plastic deformation, which has been recently reported in deformation of gold thin foils. Thus, the results of the present study indicate that high-speed deformation induces an abnormal mechanism of plastic deformation, which falls beyond the scope of dislocation theory. Numerical analysis of dislocation structure and SFTs revealed that the transition point of variation of deformation mode is around the strain rate of 10³ s⁻¹.     

Phase composition of mixed ZnS-EuS thin films grown by metal organic chemical vapor deposition

The phase composition of the mixed ZnS-EuS films deposited from volatile dithiocarbamates has been studied using differential dissolution technique (chemical method of the phase analysis) and electron microscopy. Phase composition was found to depend on the Eu content in the films, that in turn depends on a flow density ratio of the Eu and Zn volatile precursors. A single-phase solid solution, Zn_0.998Eu_≤0.002S, was observed only for films with Eu content≤1 mol%, other films were found to be two-phase. For films with the Eu content between 2 and 16% and above 80%, impurity phases, EuS and ZnS, respectively, were detected by differential dissolution technique. They evolved as low-sized sulfide precipitates encapsulated in an organic coat. No impurity phases in the films of the same Eu content were noticed by X-ray technique and Raman spectroscopy. For the films with the Eu content between 16 and 80%, sulfide phases, ZnS and EuS, were found to be free from any organic coat, and structural methods as with differential dissolution technique were also capable of observing the phases. Conditions are given to prepare Eu doped ZnS films of good quality by MOCVD technique.     

In situ high resolution transmission electron microscopy investigation of deformation mechanism in sub-10-nm Au crystals

Abstract

In situ high resolution transmission electron microscopy investigations were performed on sub-10-nm Au crystals. The effects of tensile loading direction and crystal size on the deformation mechanism of Au crystals were analysed. For the Au crystals with a width below 2 nm, the surface atom diffusion with a phenomenon of layer by layer peeling is the main deformation mechanism and the tensile loading direction plays negligible effect. For the Au crystals with a width over 7 nm, the dislocations generated form surface and gliding into crystal dominate the plastic deformation and the tensile loading direction plays important role. Lomer dislocations are produced and destructed by dislocation reaction during tensile strain process in <001> oriented Au crystal. The Schmid law is the key intrinsic issue controlling the deformation mechanism for the nanowires with a size larger than 7 nm.     

Formation of vacancy clusters in deformed thin films of Al–Mg and Al–Cu dilute alloys

In the present study, vacancy clusters in elongated Al–Mg and Al–Cu thin films (Mg/Cu CONCENTRATION=0.05–1.70 at.%) were examined by electron microscopy. No dislocations were observed in these films. In Al–Mg thin films deformed at room temperature, a large number of stacking fault tetrahedra (sft) were observed alongside a few vacancy loops. The opposite was true for Al–Cu thin films, where well-grown loops predominated, and only a few sft were observed. The Al–Cu film results show that the majority of vacancies form loops larger than sft. We also deformed Al–0.05at.% (Mg or Cu) alloys in liquid nitrogen and cold-transferred to an electron microscope. In Al–Mg, a large number of dotted defects (possibly sft) were observed, while very few such defects were observed in Al–Cu. This indicates that loops observed in Al–Cu thin films deformed at room temperature, grew during/after deformation. The likely contribution of strain-induced vacancies in deformed Al thin films to the voiding in VLSI interconnect wires due to electro-migration were discussed.