Papers by Szilvia Kalácska
EBSP shift on the detector screen ρ dislocation density σ ij stress tensor elements τ shear stres... more EBSP shift on the detector screen ρ dislocation density σ ij stress tensor elements τ shear stress Θ, Θ 0 half of the scattering angle, half of the Bragg angle Θ B Bragg angle v k (q) k th order restricted moment
IOP conference series, Oct 18, 2018
In crystalline materials a detailed understanding of the effects of external deformation is essen... more In crystalline materials a detailed understanding of the effects of external deformation is essential to develop new materials. The mechanical properties are highly influenced by the collective motion of lattice defects (mainly dislocations). The primary task of the research is to gain more experimental knowledge on the stress and strain distribution evolving in the crystal due to external deformation. In the reported investigation an indenter was pressed into the sample to a depth of a few micrometers. After the removal of the head high resolution electron backscatter diffraction (HR-EBSD) measurements were carried out in the vicinity of the mark, giving the 3D depth distribution of stress values.
arXiv (Cornell University), Jun 14, 2023
In metals geometrically necessary dislocations (GNDs) are generated primarily to accommodate stra... more In metals geometrically necessary dislocations (GNDs) are generated primarily to accommodate strain gradients and they play a key role in the Bauschinger effect, strain hardening, micron-scale size effects and fatigue. 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 pileups 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 to provide an in-depth explanation of the Bauschinger-effect and strain hardening at this scale. We conclude that the most dominant role in this behaviour is played by short-range dislocation interactions.
Physical Review Materials, Mar 30, 2023
arXiv (Cornell University), Mar 27, 2019
Zircaloy-4 is used extensively as nuclear fuel cladding materials and hydride embrittlement is a ... more Zircaloy-4 is used extensively as nuclear fuel cladding materials and hydride embrittlement is a major failure mechanism. To explore the effect of δ-hydride on plastic deformation and performance of Zircaloy-4, in situ high angular resolution electron backscatter diffraction (HR-EBSD) was used to quantify stress and geometrically necessary dislocation (GND) density during bending tests of hydride-free and hydride-containing single crystal Zircaloy-4 microcantilevers. Results suggest that while the stress applied was accommodated by plastic slip in the hydride-free cantilever, the hydride-containing cantilever showed precipitationinduced GND pileup at hydride-matrix interface pre-deformation, and considerable locallyincreasing GND density under tensile stress upon plastic deformation.
For the large majority of materials science studies a good quality surface is extremely important... more For the large majority of materials science studies a good quality surface is extremely important. Especially, it is of great importance in case of electron backscatter diffraction (EBSD) where the information depth is as shallow as some tens of nanometres and this is the reason why this analytical method requires a damage- and oxide-free sample surface. The SC-1000 SEMPrep dual Ar ion beam workstation developed by Technoorg Linda Ltd., Hungary, is suitable for both surface polishing and slope cutting of solid state samples, preparing high-quality surfaces. These surfaces allow several types of SEM investigations including the surface sensitive EBSD analysis. The present paper demonstrates the operating principle of the SC-1000 SEMPrep apparatus and its outstanding abilities. Here we present Ar ion polishing of different conductive and nonconductive materials followed by high-resolution EBSD measurements. The average image quality (IQ) number of the Kikuchi patterns has been studied as a function of polishing angle and time in order to find the optimal polishing conditions. It has also been shown that using the optimal operation parameters high-quality surfaces can be obtained on different metallic and non-metallic materials.
Social Science Research Network, 2022
In this work, 99.999% pure magnesium (Mg) single microcrystals have been deformed in [0001] compr... more In this work, 99.999% pure magnesium (Mg) single microcrystals have been deformed in [0001] compression (caxis compression) and [1010] tension (c-axis contraction) conditions at room temperature and under loading rates ranging from 5 × 10 − 4 up to ~590 s − 1. The strain rate sensitivity and apparent activation volume of prismatic and pyramidal slip systems were evaluated. In c-axis contraction, at strain rates of 45 s − 1 , the formation of a new grain whose crystallographic characteristics do not correspond to those of well-known twin systems could be observed. An explanation was found but it requires the breakdown of the invariant plane strain condition, and a unit cell reconstruction via pyramidal II to basal plane transformation. This unconventional twin is at 2.1 • far from a classical simple shear twin on {1015} planes. In c-axis compression, at the highest applied strain rate, no twin could be detected in the 5 μm sized pillars of 2:1 (height to width) aspect ratio. Plasticity is thus purely mediated by slip. However, the appearance of newly oriented grains was observed by lowering the sample size or by reducing the aspect ratio. Their crystallographic features suggest a mechanism of unit cell reconstruction through the transformation from pyramidal I to basal plane. The results presented in this study impose to consider twinning as a reorientation mechanism not necessarily limited to a simple shear.
arXiv (Cornell University), Nov 20, 2022
Slow strain rates tests (SSRT) were conducted on hydrogen-containing specimens of PH13-8Mo maragi... more Slow strain rates tests (SSRT) were conducted on hydrogen-containing specimens of PH13-8Mo maraging stainless steel. Hydrogen-assisted subcritical quasi-cleavage cracking was shown to take place during SSRT, thus accelerating material failure. Fractographic analysis showed that quasi-cleavage is composed of flat brittle areas and rougher areas. Using crosssectional electron backscatter diffraction (EBSD) analysis of a secondary subcritically grown crack, we observed brittle cracks propagated across martensite blocks ahead the main crack tip. These cracks were stopped at high-angle boundaries. The crack direction was consistent with propagation along {100} type planes. High-resolution EBSD showed significant crystal lattice rotation, hence consequential plastic deformation, concentrated between the main crack tip and the cracks located ahead it. It is concluded that quasi-cleavage in the material investigated here consists of {100} cleavage cracks connected by ductile ridges. A discontinuous mechanism, involving re-initiation of new cleavage cracks ahead the main crack tip is suggested.
arXiv (Cornell University), Jul 21, 2022
The dislocation microstructure developing during plastic deformation strongly influences the stre... more The dislocation microstructure developing during plastic deformation strongly influences the stressstrain properties of crystalline materials. The novel method of high resolution electron backscatter diffraction (HR-EBSD) offers a new perspective to study dislocation patterning. In this work copper single crystals deformed in uniaxial compression were investigated by HR-EBSD, X-ray line profile analysis, and transmission electron microscopy (TEM). With these methods the maps of the internal stress, the Nye tensor, and the geometrically necessary dislocation (GND) density were determined at different load levels. In agreement with the composite model long-range internal stress was directly observed in the cell interiors. Moreover, it is found from the fractal analysis of the GND maps that the fractal dimension of the cell structure is decreasing with increasing average spatial dislocation density fluctuation. It is shown that the evolution of different types of dislocations can be successfully monitored with this scanning electron microscopy based technique.
SSRN Electronic Journal
Slow strain rate tests on hydrogen-containing specimens of PH13-8Mo maraging stainless steel show... more Slow strain rate tests on hydrogen-containing specimens of PH13-8Mo maraging stainless steel showed Hassisted subcritical quasi-cleavage cracking, accelerating material failure. Fractographic analysis revealed that quasi-cleavage is composed of flat brittle areas and rougher areas. Electron backscatter diffraction (EBSD) was performed on a secondary subcritically grown crack. High resolution EBSD showed significant crystal lattice rotation, hence consequential plastic deformation, concentrated between the main crack tip and the cracks located ahead of it. Quasi-cleavage consisted of {100} cleavage cracks connected by ductile ridges, suggesting a discontinuous mechanism, involving re-initiation of new cleavage cracks ahead the main crack tip.
arXiv: Materials Science, Mar 30, 2020
Atomic force microscopy (AFM) is a well-known tool for studying surface roughness and to collect ... more Atomic force microscopy (AFM) is a well-known tool for studying surface roughness and to collect depth information about features on the top atomic layer of samples. By combining secondary ion mass spectroscopy (SIMS) with focused ion beam (FIB) milling in a scanning electron microscope (SEM), chemical information of sputtered structures can be visualized and located with high lateral and depth resolution. In this paper, a high vacuum (HV) compatible AFM has been installed in a TESCAN FIB-SEM instrument that was equipped with a time-of-flight secondary ion mass spectroscopy (ToF-SIMS) detector. To investigate the crater's depth caused by the ToF-SIMS sputtering, subsequent AFM measurements were performed on a multilayer vertical cavity surface emitting laser (VCSEL) sample. Surface roughness and milling depth were used to aid accurate 3D reconstruction of the sputtered volume's chemical composition. Achievable resolution, surface roughness during sputtering and surface oxidation issues are analysed. Thus, the integration of complementary detectors opens up the ability to determine the sample properties as well as to understand the influence of the analysis method on the sample surface during the analysis.
Nature Communications
Compression experiments on micron-scale specimens and acoustic emission (AE) measurements on bulk... more 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-sc...
Materials Science Forum, 2015
Surface quality development on series of metal samples was investigated using a new Ar ion millin... more Surface quality development on series of metal samples was investigated using a new Ar ion milling apparatus. The surface quality of samples was characterized by the image quality (IQ) parameter of the electron backscatter diffraction (EBSD) measurement. Ar ion polishing recipes have provided to prepare a surface appropriate for high quality EBSD mapping. The initial surfaces of samples were roughly grinded and polished. High quality surface smoothness could be achieved during the subsequent Ar ion polishing treatment. The optimal angles of Ar ion incidence and the polishing times were determined for several materials.
High angular resolution electron backscatter diffraction (HR-EBSD) was coupled with focused ion b... more High angular resolution electron backscatter diffraction (HR-EBSD) was coupled with focused ion beam (FIB) slicing to characterize the shape of the plastic zone in terms of geometrically necessary dislocations (GNDs) in W single crystal in 3 dimensions. Cantilevers of similar size with a notch were fabricated by FIB and were deformed inside a scanning electron microscope at different temperatures (21^∘C, 100^∘C and 200^∘C) just above the micro-scale brittle-to-ductile transition (BDT). J-integral testing was performed to analyse crack growth and determine the fracture toughness. At all three temperatures the plastic zone was found to be larger close to the free surface than inside the specimen, similar to macro-scale tension tests. However, at higher temperature, the 3D shape of the plastic zone changes from being localized in front of the crack tip to a butterfly-like distribution, shielding more efficiently the crack tip and inhibiting crack propagation. A comparison was made betw...
Based on the cross correlation analysis of the Kikuchi diffraction patterns high-resolution EBSD ... more Based on the cross correlation analysis of the Kikuchi diffraction patterns high-resolution EBSD is a well established method to determine the internal stress in deformed crystalline materials. In many cases, however, the stress values obtained at the different scanning points have a large (in the order of GPa) scatter. As it was first demonstrated by Wilkinson and co-workers this is due to the long tail of the probability distribution of the internal stress (P(σ)) generated by the dislocations present in the system. According to the theoretical investigations of Groma and co-workers the tail of P(σ) is inverse cubic with prefactor proportional to the total dislocation density <ρ>. In this paper we present a direct comparison of the X-ray line broadening and P(σ) obtained by EBSD on deformed Cu single crystals. It is shown that <ρ> can be determined from P(σ). This opens new perspectives for the application of EBSD in determining mesoscale parameters in a heterogeneous s...
SSRN Electronic Journal, 2022
Applied Materials Today, 2022
Microscale dynamic testing is vital to the understanding of material behavior at application rele... more Microscale dynamic testing is vital to the understanding of material behavior at application relevant strain rates. However, despite two decades of intense micromechanics research, the testing of microscale metals has been largely limited to quasi-static strain rates. Here we report the dynamic compression testing of pristine 3D printed copper micropillars at strain rates from ∼ 0.001 s −1 to ∼ 500 s −1. It was identified that microcrystalline copper micropillars deform in a single-shear like manner exhibiting a weak strain rate dependence at all strain rates. Ultrafine grained (UFG) copper micropillars, however, deform homogenously via barreling and show strong rate-dependence and small activation volumes at strain rates up to ∼ 0.1 s −1 , suggesting dislocation nucleation as the deformation mechanism. At higher strain rates, yield stress saturates remarkably, resulting in a decrease of strain rate sensitivity by two orders of magnitude and a four-fold increase in activation volume, implying a transition in deformation mechanism to collective dislocation nucleation.
Atomic force microscopy (AFM) is a well-known tool for studying surface roughness and to collect ... more Atomic force microscopy (AFM) is a well-known tool for studying surface roughness and to collect depth information about features on the top atomic layer of samples. By combining secondary ion mass spectroscopy (SIMS) with focused ion beam (FIB) milling in a scanning electron microscope (SEM), chemical information of sputtered structures can be visualized and located with high lateral and depth resolution. In this paper, a high vacuum (HV) compatible AFM has been installed in a TESCAN FIB-SEM instrument that was equipped with a time-of-flight secondary ion mass spectroscopy (ToF-SIMS) detector. To investigate the crater's depth caused by the ToF-SIMS sputtering, subsequent AFM measurements were performed on a multilayer vertical cavity surface emitting laser (VCSEL) sample. Surface roughness and milling depth were used to aid accurate 3D reconstruction of the sputtered volume's chemical composition. Achievable resolution, surface roughness during sputtering and surface oxida...
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Papers by Szilvia Kalácska