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.     

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.     

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.     

Microjoining of Ni3Al based intermetallic thin foils

Abstract

A preliminary analysis of the joining ability of thin Ni₃Al (Zr, B) based foils (below 150 μm thickness) into 'honeycomb' structures by various techniques is described in this work. Resistance welding, CO₂ laser welding and microplasma arc welding, as well as explosive welding, were used in the investigations. The examinations conducted in this study of the joint microstructures obtained by each technique do not reveal the existence of a heat affected zone. Columnar grains with a longer axis, as determined by the direction of heat flow, were observed along the fusion lines. Changes in microhardness were also examined along with a microanalysis of the chemical composition of the joint cross-section. Preliminary tests of the joining of thin Ni₃Al foil into 'honeycomb' experimental structures were carried out.     

Electron tomography: An imaging method for materials deformation dynamics

The combination of in-situ and three-dimensional (3D) in transmission electron microscopy (TEM) is one of the emerging topics of recent advanced electron microscopy research. However, to date, there have been only handful examples of in-situ 3D TEM for material deformation dynamics. In this article, firstly, the authors briefly review technical developments in fast tilt-series dataset acquisition, which is a crucial technique for in-situ electron tomography (ET). Secondly, the authors showcase a recent successful example of in-situ specimen-straining and ET system development and its applications to the deformation dynamics of crystalline materials. The system is designed and developed to explore, in real-time and at sub-microscopic levels, the internal behavior of polycrystalline materials subjected to external stresses, and not specifically targeted for atomic resolution (although it may be possible). Technical challenges toward the in-situ ET observation of 3D dislocation dynamics are discussed for commercial structural crystalline materials, including some of the early studies on in-situ ET imaging and 3D modeling of dislocation dynamics. A short summary of standing technical issues and a proposed guideline for further development in the 3D imaging method for dislocation dynamics are then discussed.     

Precipitation of Cu in Fe–Cu alloys by high-speed deformation

The differences between defect structures in Fe–Cu alloys deformed at the high (4.3×10⁵ s⁻¹) and the low strain rate (67 s⁻¹) were studied. Positron lifetime and coincidence Doppler broadening (CDB) measurements were carried out to investigate the formation of vacancy clusters and Cu precipitates. Both the size of vacancy clusters and the total amount of vacancy-type defects were larger after high-speed deformation at room temperature. Cu precipitation in the specimen deformed at the high-speed stopped for 10 h after annealing at 400 °C, while that in the specimen deformed at the low-speed continued for 100 h. Transmission electron microscopy (TEM) observations showed a heterogeneous distribution of dislocations in the case of low-speed deformation but a homogeneous distribution in the case of high-speed deformation. These results suggested that the sink efficiency for defects was higher in the specimen deformed at the high-speed.     

In situ straining: crack development in thin foils of Ni3Al

Foils of stoichiometric Ni₃Al were deformedin situ in a transmission electron microscope. Under plane stress conditions the crack propagated along slip planes, i.e. along {1 1 1} planes. This is in contrast to the intergranular fracture mode of bulk material. In the direction of the crack path, directly in front of the crack tip (but in the plastic zone), inverse dislocation pileups developed during straining. These dislocations are screw dislocations with Burgers vectorsb=a110 andb=a/2110, respectively. Owing to their extremely low Peierls stresses, these dislocations are highly mobile on {1 1 1} planes. Because the slip plane of these screw dislocations is coplanar to the crack plane, the plastic part of the crack development corresponds to shear cracking of the mode III type. Calculation of the local stress intensity factor,k _IC, confirmed that cleavage fracture occurs in mode I deformation, which is typical of the crack characteristics of Ni₃Al foils. Crack behaviour of Ni₃Al is similar to that of simple b c c metals because of the comparable relations of thek values.     

Cyclic deformation response and dislocation structure of Cu single crystals oriented for double slip

Cyclic deformation behavior of double-slip oriented Cu single crystals with a stress axis in the [034] direction was investigated under plastic strain control mode for a shear strain amplitude range of 1 × 10⁻⁴ to 5 × 10⁻³. Dislocation structures in the tested samples were observed using a transmission electronic microscope. It has been found that the effect of the operation of critical slip in these [034] crystals on cyclic responses and dislocation structures is nearly the same as that of increase in strain amplitude. The nucleation stress and number of cycles for PSB formation at each specific strain amplitude in the double-slip oriented crystals were found to be both considerably lower than those observed in single-slip oriented crystals. This observation is in a good agreement with the Kuhlmann-Wilsdorf and Laird analysis, in that the formation of PSBs is associated with glide behavior on the secondary slip system. A dislocation “cord” structure has also been observed and is believed to be caused by the operation of the cross-slip system during cyclic deformation. Labyrinth wall structures were found to form with increase in strain amplitude by the operation of critical slip and cross-slip systems. However, the formation of labyrinth structure was suppressed by the coplanar slip at high strain amplitudes.     

Quantifying the Microstructures of Pure Cu Subjected to Dynamic Plastic Deformation at Cryogenic Temperature

A pure Cu (99.995 wt%) has been subjected to dynamic plastic deformation at cryogenic temperature to a strain of 2.1. Three types of microstructures that are related to dislocation slip, twinning and shear banding have been quantitatively characterized by transmission electron microscopy (TEM) assisted by convergent beam electron di?raction (CBED) analysis. Microstructures originated from dislocation slip inside or outside the shear bands are characterized by low angle boundaries (<15°) that are spaced in the nanometer scale, whereas most deformation twins are deviated from the perfect Σ3 coincidence (60°/<111>) up to the maximum angle of 9°. The quantitative structural characteristics are compared with those in conventionally deformed Cu at low strain rates, and allowed a quantitative analysis of the flow stress-structural parameter relationship.     

Nanoindentation characterised plastic deformation of a Al0.5CoCrFeNi high entropy alloy

Abstract

The plastic deformation of a high entropy alloy Al_0.5CoCrFeNi was investigated by instrumented nanoindentation over a broad range of strain rates at room temperature. Results show that the creep behaviour depends on the strain rate remarkably. In situ scanning images showed a significant pile up around the indents, demonstrating that a highly localised plastic deformation occurred in the process of nanoindentation. Under different strain rates, contact stiffness and elastic modulus basically remain unchanged. However, the hardness decreases as indentation depth increases due to indentation size effect. For the same maximum load, serrations became less prominent as the loading rate of indentation increased. Similar serrations have been observed in the current alloy upon quasi-static compression.     

Vertical dislocations in Ge films selectively grown in submicron Si windows of patterned substrates

The structural properties of straight screw dislocations extended in the [001] direction formed in squared- and line shaped- Ge(001) films selectively grown on submicron regions of Si(001) substrates were investigated by transmission electron microscopy. The screw dislocations propagating as a result of spiral surface growth were redirected toward the SiO₂ sidewalls. This redirection is linked to the formation of facets such as {111} facets in the growing Ge films. In the process of strain relaxation upon annealing, the screw dislocations were dissociated into dislocations with Burgers vectors of the a/2<110> type, which glided on the {111} surfaces and disappeared.     

Deformation mechanisms in tensile fracture of Al foils containing Si precipitates

Recently, Kiritani et al. proposed a new mechanism of plastic deformation without involving dislocations in tensile fracture of metal foils. The paper reports transmission electron microscopy (TEM) study of tensile fracture of Al containing hard precipitates (Si) that are considered to act as obstacles to dislocation motion. In sawtooth-shaped thin-foils formed at the fracture tip (‘sawtooth portion’), tensile strain was as high as 10³, but only a few dislocations were pinned to precipitates. Instead, voids were formed at precipitate/matrix interface, elongated in the direction of tension, and broke up into several smaller voids, due to stress concentration around hard precipitates. The thicker area of the specimen (‘base portion’), where tensile strain was 30, did not contain voids but showed a dislocation cell structure. In tensile fracture of pre-thinned specimen, voids were formed in the sawtooth portion, despite the tensile strain also being 30. These results suggest that the sawtooth portion is formed by a new mechanism that does not involve dislocations.     

Fatigue and fracture properties of thin metallic foils

Metallic thin foils are essential structural parts in microsystems, which may be subjected to fatigue loading caused by thermal fluctuations and mechanical vibrations influencing their reliability in numerous engineering applications. It is well known that the fatigue properties of bulk material cannot be adopted for small scaled structures. For a better understanding of the `size-effect' in the present investigation fatigue crack growth near threshold in the high cycle fatigue regime and associated fracture processes were studied. Free-standing rolled and electrodeposited Cu-, Mo- and Al foils of thickness from 20 m to 250 m in different conditions have been tested in a special experimental set up operating at R=–1 and a testing frequency of 20 kHz. At a given constant strain value the fatigue crack growth behaviour has been recorded accompanied by intermittent observation of the change of the dislocation structure in the vicinity of the growing crack by use of the electron channeling contrast imaging (ECCI)-technique in a scanning electron microscope (SEM). In a load shedding technique fatigue threshold stress intensity factor values have been derived and compared with data of bulk material. Typical crack growth features were detected depending on thickness and grain sizes of the foils. Various criteria (compliance, extent of plastic zones and plastic strain gradients) were selected for the explanation of this anomalous behaviour. Additionally fractomicrographs of uniaxial strained and fatigued foils have been studied to obtain further insight of the effect of dimensional constraint.     

Fatigue and fracture properties of thin metallic foils

Metallic thin foils are essential structural parts in microsystems,which may be subjected to fatigue loading caused by thermal fluctuations and mechanical vibrations influencing their reliability in numerous engineering applications. It is well known that the fatigue properties of bulk material cannot be adopted for small scaled structures. For a better understanding of the `size-effect' in the present investigation fatigue crack growth near threshold in the high cycle fatigue regime and associated fracture processes were studied. Free- standing rolled and electrodeposited Cu-, Mo- and Al foils of thickness from 20 m to 250 m in different conditions have been tested in a special experimental set up operating at R=–1 and a testing frequency of 20 kHz. At a given constant strain value the fatigue crack growth behaviour has been recorded accompanied by intermittent observation of the change of the dislocation structure in the vicinity of the growing crack by use of the electron channeling contrast imaging (ECCI)-technique in a scanning electron microscope (SEM). In a load shedding technique fatigue threshold stress intensity factor values have been derived and compared with data of bulk material. Typical crack growth features were detected depending on thickness and grain sizes of the foils. Various criteria (compliance, extent of plastic zones and plastic strain gradients) were selected for the explanation of this anomalous behaviour. Additionally fractomicrographs of uniaxial strained and fatigued foils have been studied to obtain further insight of the effect of dimensional constraint.     

Dynamic observation of the phase transformation in Cu–Al–Ni alloys using an optical reflectivity technique

The colour change of Cu-13.8 mass% Al-4.0 mass% Ni and Cu-14.2 mass% Al-3.1 mass % Ni alloys was measured after annealing at 500 and 620°C, and quenching from 900°C by means of a recording spectrophotometer equipped with an integrating sphere. A large colour change of these alloys occurred on heat treatment, depending on the different phases of γ′, β₁, (γ₂+β₁) and (α+γ₂). The phase transformation between β₁ phase with the DO₃ structure, and γ′ martensite with the Cu3Ti-type structure, occurred in the alloys. The phase transformation was dynamically observed by the change in the spectral reflectivity on the surface of these alloys during heating and cooling. A great hysteresis in the phase transformation was noticed in the spectral reflectivity-temperature curves and the premonitory phenomenon of the martensitic transformation was observed in the curves. This revised version was published online in November 2006 with corrections to the Cover Date.     

Evolution of deformation structures under varying loading conditions followed in situ by high angular resolution 3DXRD

With high angular resolution three-dimensional X-ray diffraction, individual subgrains are traced in the bulk of a polycrystalline specimen and their dynamics is followed in situ during varying loading conditions. The intensity distribution of single Bragg reflections from an individual grain is analyzed in reciprocal space. It consists of sharp high-intensity peaks arising from subgrains superimposed on a cloud of lower intensity arising from dislocation walls. Individual subgrains can be distinguished by their unique combination of orientation and elastic strain. The responses of polycrystalline copper to different loading conditions are presented: during uninterrupted tensile deformation, formation of subgrains can be observed concurrently with broadening of the Bragg reflection shortly after onset of plastic deformation. With continued tensile deformation, the subgrain structure develops intermittently. When the traction is terminated, stress relaxation occurs and number, size and orientation of subgrains are found to be constant. The subgrain structure freezes and only a minor clean-up of the dislocation structure is observed. When changing the tensile direction after pre-deformation in tension, a systematic correlation between the degree of strain path change and the changes in the dislocation structure quantified by the volume fraction of the subgrains is established. For obtaining the subgrain volume fraction, a new fitting method has been developed for partitioning the contributions of subgrains and dislocation walls.     

Parametric variations of the interatomic potential in atomistic analysis of nano-scale metal plasticity

Atomistic simulations of plastic deformation in nano-scale copper crystals are carried out. Attention is devoted to adjusting interatomic potential parameters with the objective of gaining fundamental insight into the crystal defect processes in FCC metals. An initial point defect is utilized in the molecular statics model to trigger plasticity in a controlled manner. Two different potential models have been employed: Morse and Embedded Atom Method. With the Morse potential, the interaction range has been parameterized to view dislocation slip behavior and/or phase transformation without the influence of an unstable surface state of the specimen. We focus on tensile loading along a low-symmetry orientation where single slip prevails upon yielding. When the Morse interaction distance is small, dislocation slip is seen to be the dominant deformation mechanism. A slight increase in the interaction range results in phase transition from the FCC structure to a BCC structure when using the Morse potential. Re-orientation of the BCC lattice also occurs at later stages of the deformation via a twinning operation. When the atomic interaction range is increased further, the effect of surface stress becomes increasingly important. Plastic yielding occurs in the form of partial slip which creates stacking faults. The initial point defect plays a less significant role and phase transition during deformation is suppressed. Detailed mechanisms of these atomistic features, as well as comparisons between the two potentials, are discussed.     

Evaluation of Stored Energy from Microstructure of Multi-component Nanostructured Cu

A polycrystalline Cu of 99.995% purity has been deformed by dynamic plastic deformation at liquid nitrogen temperature to a strain of 2.1 (LNT-DPD Cu). Three distinct regions that are dominated by dislocation slip, shear banding and nanotwinning, form a multi-component nanostructure. The microstructure of each region has been quantified by transmission electron microscopy assisted by Kikuchi line analysis. Based on the structural parameters the stored energy of each region was evaluated, and the total energy can be assumed to be a linear additivity of that in each region weighted by the respective volume fraction. A microstructure based evaluation of the stored energy of multi-component nanostructure has been proposed.     

Diffusion during annealing of Al/Cu/Fe thin films

The Al-Cu-Fe system is interesting due to the existence of the quasicrystalline phase Al_62.5Cu₂₅Fe_12.5 as well as its approximant phases. A two-step procedure of thin film preparation is considered: deposition of a multilayer structure of individual elements and consequential annealing. To analyze the diffusion processes trilayers of individual elements were deposited by sputtering with a total thickness of about 400 nm. Afterwards, the samples were annealed in tube furnace in inert atmosphere. Rutherford backscattering spectrometry, Auger electron spectrometry and X-ray diffraction were used to quantify the depth profiles. The results point out to a three-stage process as a function of rising temperature: first Al and Cu form the γ-Al₄Cu₉ compound layer; second the aluminium spreads throughout the film with copper and iron mainly divided. The β-Al(Cu,Fe) phase is observed. Complete homogenization is followed afterwards.     

Effects of shot peening on fatigue properties of 0Cr13Ni8Mo2Al steel

Abstract

The influence of shot peening on the fatigue properties of 0Cr13Ni8Mo2Al steel has been studied. Changes in surface roughness, surface topography and residual compressive stress field were determined by experiments. The experimental results show that shot peening improves the fatigue property and the fatigue crack sources are pushed to the region beneath the hardened layer. Low Almen intensities should be used when 0Cr13Ni8Mo2Al steel is shot peened because of its sensitiveness to the surface roughness.