Plastic deformation of bcc metal thin foils without dislocation

Heavy plastic deformation of fcc metal thin foils to fracture has been found recently to proceed without involving dislocations, and it results in the formation of high density of vacancy clusters. Thin foil specimens of bcc metals such as V and Mo were plastically deformed to fracture in in situ elongation experiments under an electron microscope. Morphology of thinning and fracture was found to be similar to fcc metals, and no dislocation was observed during heavy deformation. Electron diffraction analysis at the tip of a crack during deformation confirmed a large elastic deformation of up to 5%. Unlike in fcc metal thin foil specimens, point defect clusters were not observed near fractured tips. This difference is attributed to the difference in vacancy reaction, though the deformation in bcc metals without dislocation most likely does produce vacancies.     

Dislocation Behaviour near the Crack Tip in Bulk Fe-3% Si Single Crystal

An investigation has been made of the disloca-tion distribution and dislocation free zone near thecrack tip in bulk Fe-3% Si single crystal duringdeformation in SEM.It has been found that anumber of dislocations were emitted from the cracktip during deformation.After that,the dislocationsmoved rapidly away from the crack tip,which indi-cated that they were strongly repelled by the stressfield at the crack tip.Between the crack tip and theplastic zone there is a region of dislocation-free,which is referred to as dislocation-free zone (DFZ).The length of DFZs is roughly estimated 100μm which is much longer than that found in thinfoil specimen.The variation of dislocation densityas a function of the distance from the crack tip wasmeasured,which showed that the dislocations areinversely piled up in the plastic zone.The length ofDFZs increased with both the length of pre-crackand the amplitude of applied stress.     

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.     

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.     

超声波振动对钛箔拉伸性能及位错分布的影响

以研究超声波振动条件下钛箔的塑性变形特征和位错分布为目的,通过超声波辅助单向拉伸实验和微观组织分析研究不同超声波振动施加方式对钛箔塑性拉伸变形过程中的应力-应变、伸长率及位错分布的影响规律。结果表明:超声波振动过程中钛箔的流动应力最大降幅可以达到约80%,材料的伸长率从未施加超声时的40.33%最大增加至54.46%。通过TEM可以发现超声波振动条件下位错呈现平行分布的趋势且无大量缠结出现,而未施加超声拉伸的试样中位错的分布则显得杂乱无章且缠结严重。     

NANOCRYSTALLIZATION OF METALS AND COMPOSITES PROCESSED MECHANICALLY IN THE SOLID STATE

Extreme plastic deformation encountered in a variety of mechanical processes may yield nanostructured and nanophase materials with a broad range of chemical composition and microstructure. This paper discusses several such examples including the deformation induced nanophase formation in powder particles, thin foil sandwich structures, and at the surface of metals and alloys exposed to friction induced wear conditions. The deformation processes occurring within powder samples during mechanical attrition are important for fundamental studies on the effect of extreme mechanical deformation on the microstructure and its stability.     

Generation of vacancies in high-speed plastic deformation

In a recent experiment, crystalline metals were subjected to high-speed plastic deformation, and subsequently a number of vacancy clusters were observed without any trace of dislocations. In an effort to explain this result, in the present study fluid-like behavior of solid in ultra-high-speed deformation is considered, and the possibility of spontaneous generation of vacancies analogous to cavitation in high-speed fluid flow is discussed. Similar to a large velocity gradient that induces turbulence in a high-speed fluid flow, large shear stress induced in a solid material during the course of high-speed deformation may generate vacancies instead of dislocations, if the dislocations cannot follow the deformation speed. In this paper, similarities between dislocation in solid and vortex in a fluid discussed, along with similarities between vacancy in a solid and cavitation in fluid, and a mechanism of vacancy production under high-speed plastic deformation of crystalline materials is proposed.     

Evaluation of subgrain formation in Al2O3–SiC nanocomposites

Both theoretical analysis and transmission electron microscopy (TEM) complementary studies have been conducted to evaluate the possible role of subgrain formation as a strengthening mechanism in a nanocomposite consisting of Al2O3 and 5 vol % 0.15 m SiC particles. The theoretical calculation predicted that the residual stresses due to thermal expansion mismatch between Al2O3 and SiC are insufficient to induce the extensive plastic deformation required for subgrain formation upon annealing. This prediction was consistent with TEM observations that the bulk of the material was completely free from subgrains, and that only a low density of dislocations was present in isolated areas. The results suggest, therefore, that microstructure refinement through subgrain formation cannot account for the superior mechanical behaviour of the nanocomposite reported in previous studies.TEM examination of the ground surfaces revealed significant plastic deformation in both single phase Al2O3 and the nanocomposite. Upon annealing at 1300°C for 2 h, dislocation-free subgrains were formed in Al2O3, whereas a high density of tangled dislocations were present in the nanocomposite. These observed differences are consistent with the fact that during annealing, residual stress relaxation is more difficult in the nanocomposite than in Al2O3.     

Analysis of high-speed-deformation-induced defect structures using heterogeneity parameter of dislocation distribution

An appropriate parameter, named the heterogeneity parameter, is introduced for quantitatively representing the nature of distribution of deformation-induced dislocations. The parameter assumes values ranging from 0 to 1, the extremes corresponding to homogeneous and heterogeneous distribution, respectively. The parameter can represent distribution of intermediate nature, typified by stochastically random distribution which has a heterogeneity parameter of about 0.35. Heterogeneity parameter is used to analyze variation in dislocation structure in many kinds of metals for a wide range of deformation speed from 10⁻² to 10⁷ s⁻¹ in strain rate. The parameter has large values at slow-speed-deformation, and decreases with increasing strain rate, reaching the level of random distribution at a deformation speed of around 10⁴ s⁻¹. Above this strain rate, formation of vacancy clusters increases remarkably. On the basis of these results of analysis, occurrence of dislocation-free plastic deformation during high-speed-deformation is proposed.     

Dynamic transitions from smooth to rough to twinning in dislocation motion

The motion of dislocations in response to stress dictates the mechanical behaviour of materials. However, it is not yet possible to directly observe dislocation motion experimentally at the atomic level. Here, we present the first observations of the long-hypothesized kink-pair mechanism in action using atomistic simulations of dislocation motion in iron. In a striking deviation from the classical picture, dislocation motion at high strain rates becomes rough, resulting in spontaneous self-pinning and production of large quantities of debris. Then, at still higher strain rates, the dislocation stops abruptly and emits a twin plate that immediately takes over as the dominant mode of plastic deformation. These observations challenge the applicability of the Peierls threshold concept to the three-dimensional motion of screw dislocations at high strain rates, and suggest a new interpretation of plastic strength and microstructure of shocked metals.     

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.     

Deformation of thin foil of fcc and bcc metals containing pre-introduced He bubbles

Thin foil of fcc and bcc metals subjected to tensile deformation has been found to exhibit an anomalously high density of small vacancy clusters, probably in the absence of dislocations. Deformation of fcc Au and bcc Fe containing pre-introduced He bubbles is carried out, at strain rates ranging from 10⁻³ to 10⁵ s⁻¹ to a 10²% strain at −180 and 25 °C. Microstructures in the deformed regions are examined by transmission electron microscopy. Rows of bubbles are formed due to extreme elongation of bubbles under stress and its subsequent division into smaller pieces in response to vacancy diffusion around the bubble surfaces. The bubble rows are parallel to the low-index crystallographic directions, 001, 011, and 012 for Au and 011 and 001 for Fe, which can be resolved into ‘slip directions’. The results indicate that displacement of atoms in these thin-foil specimens during tensile deformation progresses while conforming to the nature of the crystal, even in the absence of dislocations.     

Positron annihilation study of vacancy-type defects in high-speed deformed Ni, Cu and Fe

Vacancies and vacancy clusters in Ni, Cu, and Fe induced by high- and low-speed deformations are studied systematically by positron annihilation techniques and are compared with those induced by the conventional-rolling. To clarify the nature of the defects, the experimental results are compared with our superimposed-atomic-charge calculations of the positron lifetimes in the vacancy clusters as a function of their size. It is found that the deformation-induced defects in the fcc and bcc metals are significantly distinct. In the fcc metals of Ni and Cu, monovacancies with high number densities are induced by the high- and low-speed deformations and by heavy conventional-rolling (>10% in Ni and >40% in Cu). Vacancy clusters are observed after the high- and low-speed deformation for Ni and after the conventional-rolling for Cu. On the contrary, dislocations and vacancy clusters are introduced in bcc Fe regardless of the type or degree of deformation.     

一维钛纳米材料的SEM和TEM形貌研究

在10mol／L氢氧化钠溶液中，以溶胶状纳米TiO2为钛源，分别于150℃和180℃采用水热法合成了钛纳米晶须和钛纳米线；以金红石型纳米TiO2为原料在150℃水热合成了钛纳米管。用扫描隧道显微镜（SEM）、电子透射电镜（TEM）对产物进行了表征，结果表明钛酸钠纳米晶须扫描形貌为球形颗粒状，透射形貌为直径l～3nm针尖...     

Dislocations at ductile/plastic crack tips: in-situ TEM observations

 In-situ observations of dislocation structures ahead of crack tips in TEM metal foils are reviewed. Two cases are compared in particular: Structure development during in-situ straining to failure of (i) electron-transparent foils ahead of the tip of a growing crack that spreads from the thinnest regions or perforations and (ii) initially non-transparent thick foils. In the latter case cracks formed only after substantial in-situ straining, and they propagated along dislocation cell walls via repeated stimulated crack nucleation ahead of the tip. This behavior was shown to adequately simulate bulk behavior and such cracks do not exhibit dislocation-free zones at their tips. By contrast, dislocation-free regions along ligaments formed by crack propagation and observed in thin (e.g. about 100 nm or less) TEM foils are found to be artifacts due to strong dislocation image forces. These image forces at the same time limit mutual dislocation interactions to the thickness of the foil, and rotate the dislocations to be normal to the foil plane, meanwhile straightening them. This behavior has no correspondence to conditions at real cracks in bulk materials. Theoretical expressions are derived for the dislocation densities ahead of crack tips that give rise to long-range and shorter range stress fields in mode I crack tip configurations, respectively. Received:19 December 1997 / Accepted: 22 December 1997     

Two-internal variable thermodynamics modelling of severe plastic deformation: dislocation and flow stress evolutions

Two-internal variable thermodynamics model is presented to investigate the evolution of microstructure and flow stress during severe plastic deformation. Previous studies have shown that due to heterogeneous distribution of dislocations during severe plastic deformation, the use of multivariable models is needed. In this regard, a two-internal variable model is presented. In the present paper, the dislocation densities in the subgrain boundaries and interiors are considered as internal variables. The model uses general laws of thermodynamics and describes the evolution of the dislocation densities on the basis of parameters such as the self-diffusion activation energy and the stacking fault energy. The model predicts the dislocation density, subgrain size and strength. To verify the model, the achieved results are compared with the experimental data.     

Distribution of dislocations at a mode I crack tip and their shielding effect

Distribution of dislocations at a finite mode I crack tip is formulated. Closed form solutions for the dislocation distribution function, the dislocation-free zone (DFZ), the local stress intensity factor and the crack tip stress field are obtained. The dislocation distribution has similar features to a mode III crack model. Under a given applied stress, there may exist different configurations of plastic zone and DFZ. Crack tip shielding by dislocations depends on both applied stresses and the configuration.     

The Behavior of Screw Dislocations Emitted from a Star Crack with a Central Hole

The behavior of screw dislocations emitted from a star crack with a central hole was investigated using discrete dislocation modeling. Cracks are uniformly distributed along the circumference of a circular hole. Dislocations are assumed to be emitted one by one from the crack tips along the radial direction. Each emitted dislocation moves along the radial direction and its velocity is proportional to the third power of the effective shear stress. A dislocation-free zone exists based on the assumption that the crack tip must overcome an energy barrier to emit a dislocation. The effects of the central hole, slit crack, number of cracks and applied stress on the plastic zone, total number of dislocations emitted from all crack tips and the first crack tip, and the dislocation-free zone were studied for a given friction stress. This revised version was published online in July 2006 with corrections to the Cover Date.     

Flow mechanisms in ultrafine-grained metals with an emphasis on superplasticity

An analysis was conducted to examine the flow behavior of ultrafine-grained (UFG) metals produced by severe plastic deformation (SPD) processing in equal-channel angular pressing. The results reveal two distinct types of behavior. At elevated temperatures, the analysis shows that superplastic flow is accurately described by the theoretical mechanism developed for coarse-grained metals so that flow in UFG materials may be interpreted using conventional flow mechanisms. By contrast, localized small-scale grain boundary sliding is observed during deformation at low temperatures and this is attributed to the movement of extrinsic dislocations in the non-equilibrium grain boundaries produced by SPD processing.     

Characterization of nanostructured metals produced by plastic deformation

Nanostructured metals produced by plastic deformation often exhibit characteristic structural features such as elongated morphology, a bimodal misorientation distribution and the presence of interior dislocations. The characterization of these parameters is demonstrated by results of TEM and EBSD analyses of pure Ni processed by high pressure torsion (HPT) and pure Al processed by accumulative roll bonding (ARB). Care needed in selecting sample plane and characterization technique is discussed.