Compression experiments on micron-scale specimens and acoustic emission (AE) measurements on bulk samples revealed that the dislocation motion resembles a stick-slip process - a series of ...unpredictable local strain bursts with a scale-free size distribution. Here we present a unique experimental set-up, which detects weak AE waves of dislocation slip during the compression of Zn micropillars. Profound correlation is observed between the energies of deformation events and the emitted AE signals that, as we conclude, are induced by the collective dissipative motion of dislocations. The AE data also reveal a two-level structure of plastic events, which otherwise appear as a single stress drop. Hence, our experiments and simulations unravel the missing relationship between the properties of acoustic signals and the corresponding local deformation events. We further show by statistical analyses that despite fundamental differences in deformation mechanism and involved length- and time-scales, dislocation avalanches and earthquakes are essentially alike.
We study the properties of strain bursts (dislocation avalanches) occurring in two-dimensional discrete dislocation dynamics models under quasistatic stress-controlled loading. Contrary to previous ...suggestions, the avalanche statistics differ fundamentally from predictions obtained for the depinning of elastic manifolds in quenched random media. Instead, we find an exponent τ=1 of the power-law distribution of slip or released energy, with a cutoff that increases exponentially with the applied stress and diverges with system size at all stresses. These observations demonstrate that the avalanche dynamics of 2D dislocation systems is scale-free at every applied stress and, therefore, cannot be envisaged in terms of critical behavior associated with a depinning transition.
Mechanical testing of micropillars is a field that involves new physics, as the behaviour of materials is non-deterministic at this scale. To better understand their deformation mechanisms we applied ...3-dimensional high angular resolution electron backscatter diffraction (3D HR-EBSD) to reveal the dislocation distribution in deformed single crystal copper micropillars. Identical micropillars (6 μm ×6μm ×18μm in size) were fabricated by focused ion beam (FIB) and compressed at room temperature. The deformation process was stopped at different strain levels (≈1%, 4% and 10%) to study the evolution of geometrically necessary dislocations (GNDs). Serial slicing with FIB and consecutive HR-EBSD mapping on the (100) side was used to create and compare 3-dimensional maps of the deformed volumes. Average GND densities were calculated for each deformation step. Total dislocation density calculation based on X-ray synchrotron measurements were conducted on the 4% pillar to compare dislocation densities determined by the two complementary methods. Scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM) images were captured on the 10% pillar to visualize the actual dislocation structure. With the 3D HR-EBSD technique we have studied the geometrically necessary dislocations evolving during the deformation of micropillars. An intermediate behaviour was found at the studied sample size between bulk and nanoscale plasticity: A well-developed dislocation cell structure built up upon deformation but with significantly lower GND density than in bulk. This explains the simultaneous observation of strain hardening and size effect at this scale.
In single crystals, plastic deformations are predominantly governed by dislocation movement and interactions. The group of dislocations that creates strain gradients, known as geometrically necessary ...dislocations (GNDs), also deterministically contributes to strain hardening, micron-scale size effects, fatigue, and Bauschinger effect. During bending large strain gradients naturally emerge which makes this deformation mode exceptionally suitable to study the evolution of GNDs. Here we present bi-directional bending experiment of a Cu single crystalline microcantilever with in situ characterisation of the dislocation microstructure in terms of high-resolution electron backscatter diffraction (HR-EBSD). The experiments are complemented with dislocation density modelling to provide physical understanding of the collective dislocation phenomena. We find that dislocation pile-ups form around the neutral zone during initial bending, however, these do not dissolve upon reversed loading, rather they contribute to the development of a much more complex GND dominated microstructure. This irreversible process is analysed in detail in terms of the involved Burgers vectors and slip systems. We conclude that at this scale the most dominant role in the Bauschinger effect and corresponding strain hardening is played by short-range dislocation interactions. The in-depth understanding of these phenomena will aid the design of microscopic metallic components with increased performance and reliability.
•Cyclic bending of a copper single crystalline microcantilever revealed a non-reversible evolution of dislocation structures.•Microstructures obtained by high-resolution EBSD measurements are recovered by continuum dislocation density simulations.•TEM confirmed that the dislocation pile-ups formed during the initial bending do not dissolve upon reserved loading.•Upon cyclic loading a rather complex dislocation microstructure develops that changes its polarisation in every cycle.•The strong strain hardening and the Bauschinger-effect was mainly caused by the short-range dislocation interactions.
Understanding the spontaneous emergence of dislocation patterns during plastic deformation is a long standing challenge in dislocation theory. During the past decades several phenomenological ...continuum models of dislocation patterning were proposed, but few of them (if any) are derived from microscopic considerations through systematic and controlled averaging procedures. In this paper we present a two-dimensional continuum theory that is obtained by systematic averaging of the equations of motion of discrete dislocations. It is shown that in the evolution equations of the dislocation densities diffusionlike terms neglected in earlier considerations play a crucial role in the length scale selection of the dislocation density fluctuations. It is also shown that the formulated continuum theory can be derived from an averaged energy functional using the framework of phase field theories. However, in order to account for the flow stress one has in that case to introduce a nontrivial dislocation mobility function, which proves to be crucial for the instability leading to patterning.
Micro-deformation testing has recently gained far-reaching scientific importance as it provides intrinsic information on the dynamics of plastic deformation which is concealed when bulk materials are ...tested. In this work, single-crystal Mg micropillars favorably oriented for mechanical twinning were tested in compression with concurrent scanning electron microscopy imaging. The experimental data were complemented by the finite element modeling in order to reveal the underlying physical background of the observed twinning dynamics. It was shown that the thickness of a twin should reach a critical value before triggering the nucleation of another twin to accommodate further strain. Nucleation and growth are repeated until the twins form throughout the whole micropillar, from top to bottom. Afterwards, the thickening and coalescence of all these twins take place until the entire micropillar volume is twinned. In addition, a line-by-line analysis of the scanning electron microscopy images was employed to reveal the twin lateral growth rates, which were shown to be on the order of 10−5–10−4 m/s.
Display omitted
•Single-crystal Mg micropillars (10 × 10 × 30 μm3) favorably oriented for mechanical twinning were tested in compression.•High-precision compression data and in-situ SEM observation were supplemented by finite element modeling.•Gradual nucleation of twins from top to bottom across the micropillar was observed, followed by their subsequent thickening.•The existence of critical width (around 3 μm here) was proved responsible for such nucleation and growth dynamics.•Detailed examination of SEM images and stress drops indicated twin lateral growth rates on the order of 10−5–10−4 m/s.
The microstructural evolution in randomly oriented Mg–Al samples is investigated in situ during compression by X‐ray diffraction as a function of Al concentration. The diffraction data are evaluated ...by the variance method, which provides information about the dislocation density and spatial distribution of the dislocations. The dislocation density increases with increasing alloying content. Since the increment of the dislocation density above the yield point is linear, the mutual dislocation interaction type is determined from the Taylor equation. The results indicate the dominance of basal–basal dislocation interactions, but at higher alloying content the share of the basal–non‐basal interactions increases. It is shown that the dynamics of dislocation wall formation also depend on Al content. Transmission electron microscopy observations are in agreement with the results obtained by X‐ray line profile analysis.
Statistical properties of the dislocation microstructure developing in Mg alloys deformed by compression have been determined by the variance method of X‐ray line profile analysis.
Display omitted
•Compression experiments were carried out on pristine and irradiated micropillars.•Acoustic emission was recorded concurrently during deformation.•Irradiation induced dislocation ...loops were found by transmission electron microscopy.•Strain bursts caused by intermittent cooperative dislocation motion were detected.•The detected acoustic events were found to match the strain bursts in both cases.•Irradiation was found to decrease strain burst sizes and acoustic activity.•Interaction of mobile dislocations and loops are undetectable by acoustic emission.
Plastic deformation of microsamples is characterised by large intermittent strain bursts caused by dislocation avalanches. Here we investigate how ion irradiation affects this phenomenon during single slip single crystal plasticity. To this end, in situ compression of Zn micropillars oriented for basal slip was carried out in a scanning electron microscope (SEM). The unique experimental setup also allowed the concurrent recording of the acoustic emission (AE) signals emitted from the sample during deformation. It was shown that irradiation introduced a homogeneous distribution of basal dislocation loops that lead to hardening of the sample as well as strain softening due to dislocation channeling at larger strains. Under the loading conditions imposed in the present work, the intensity of strain bursts was found to decrease during channeling. The concurrently recorded AE events were correlated with the strain bursts and their analysis provided additional information of the details of collective dislocation dynamics. It was found that the rate of AE events decreased significantly upon irradiation, however, other statistical properties did not change. This was attributed to the appearance of new type of dislocation avalanches which is dominated by short-range dislocation-obstacle interactions that cannot be detected by AE sensors.
During neutron irradiation of metals, owing to the enhanced number of vacancies and interstitial atoms, the climb motion of dislocations becomes significant at room temperature, leading to a ...recrystallization of the material. Moreover, the vacancies and interstitial atoms tend to form prismatic dislocation loops that play a crucial role in the plastic properties of the materials. X‐ray peak profile analysis is an efficient nondestructive method to determine the properties of dislocation microstructure. In the first half of this article, the foundation of the asymptotic peak broadening theory and the related restricted‐moments peak‐evaluation method is summarized. After this, the microstructural parameters obtained by X‐ray peak profile analysis are reported for irradiated E110 and E110G Zr alloys used as cladding material in the nuclear industry.
The dislocation microstructure developing during neutron irradiation is determined by X‐ray line profile analysis.
In this work we study the microstruture evolution of a newly developed 12% Cr martensitic/ferritic steel in as-received condition and after creep at 650°C under 130MPa and 80MPa. The microstructure ...is described as consisting of mobile dislocations, dipole dislocations, boundary dislocations, precipitates, lath boundaries, block boundaries, packet boundaries and prior austenitic grain boundaries. The material is characterized employing light optical microscopy (LOM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). TEM is used to characterize the dislocations (mobile+dipole) inside the subgrains and XRD measurements are used to the characterize mobile dislocations. Based on the subgrain boundary misorientations obtained from EBSD measurements, the boundary dislocation density is estimated. The total dislocation density is estimated for the as-received and crept conditions adding the mobile, boundary and dipole dislocation densities. Additionally, the subgrain size is estimated from the EBSD measurements. In this publication we propose the use of three characterization techniques TEM, XRD and EBSD as necessary to characterize all type of dislocations and quantify the total dislocation densty in martensitic/ferritic steels.
•Creep properties of a novel 12% Cr steel alloyed with Ta•Experimental characterization of different types of dislocations: mobile, dipole and boundary•Characterization and interpretation of the substructure evolution using unique combination of TEM, XRD and EBSD
PDF
