Grain boundary diffusion in severely deformed Al-based AA5024 alloy is investigated. Different st... more Grain boundary diffusion in severely deformed Al-based AA5024 alloy is investigated. Different states are prepared by combination of equal channel angular processing and heat treatments, with the radioisotope 57 Co being employed as a sensitive probe of a given grain boundary state. Its diffusion rates near room temperature (320 K) are utilized to quantify the effects of severe plastic deformation and a presumed formation of a previously reported deformation-modified state of grain boundaries, solute segregation at the interfaces, increased dislocation content after deformation and of the precipitation behavior on the transport phenomena along grain boundaries. The dominant effect of nano-sized Al3Sc-based precipitates is evaluated using density functional theory and the Eshelby model for the determination of elastic stresses around the precipitates.
Nanomaterials by severe plastic deformation: review of historical developments and recent advances
Materials Research Letters, 2022
Severe plastic deformation (SPD) is effective in producing bulk ultrafine-grained and nanostructu... more Severe plastic deformation (SPD) is effective in producing bulk ultrafine-grained and nanostructured materials with large densities of lattice defects. This field, also known as NanoSPD, experienced a significant progress within the past two decades. Beside classic SPD methods such as high-pressure torsion, equal-channel angular pressing, accumulative roll-bonding, twist extrusion, and multi-directional forging, various continuous techniques were introduced to produce upscaled samples. Moreover, numerous alloys, glasses, semiconductors, ceramics, polymers, and their composites were processed. The SPD methods were used to synthesize new materials or to stabilize metastable phases with advanced mechanical and functional properties. High strength combined with high ductility, low/room-temperature superplasticity, creep resistance, hydrogen storage, photocatalytic hydrogen production, photocatalytic CO2 conversion, superconductivity, thermoelectric performance, radiation resistance, corrosion resistance, and biocompatibility are some highlighted properties of SPD-processed materials. This article reviews recent advances in the NanoSPD field and provides a brief history regarding its progress from the ancient times to modernity. Abbreviations: ARB: Accumulative Roll-Bonding; BCC: Body-Centered Cubic; DAC: Diamond Anvil Cell; EBSD: Electron Backscatter Diffraction; ECAP: Equal-Channel Angular Pressing (Extrusion); FCC: Face-Centered Cubic; FEM: Finite Element Method; FSP: Friction Stir Processing; HCP: Hexagonal Close-Packed; HPT: High-Pressure Torsion; HPTT: High-Pressure Tube Twisting; MDF: Multi-Directional (-Axial) Forging; NanoSPD: Nanomaterials by Severe Plastic Deformation; SDAC: Shear (Rotational) Diamond Anvil Cell; SEM: Scanning Electron Microscopy; SMAT: Surface Mechanical Attrition Treatment; SPD: Severe Plastic Deformation; TE: Twist Extrusion; TEM: Transmission Electron Microscopy; UFG: Ultrafine Grained GRAPHICAL ABSTRACT IMPACT STATEMENT This article comprehensively reviews recent advances on development of ultrafine-grained and nanostructured materials by severe plastic deformation and provides a brief history regarding the progress of this field.
High entropy alloy nanocomposites produced by high pressure torsion
Acta Materialia, 2021
Abstract High-pressure torsion was applied to join two disks of single-phase equiatomic fcc CoCrF... more Abstract High-pressure torsion was applied to join two disks of single-phase equiatomic fcc CoCrFeMnNi and bcc HfNbTaTiZr high entropy alloys (HEAs). After 15 revolutions a bulk nanocomposite had developed with alternating nano-lamellae of elongated nanocrystalline CoCrFeMnNi and mixed amorphous-nanocrystalline HfNbTaTiZr, exhibiting complex microstructures with numerous vortex-like regions. While the fcc phase retains a rather homogeneous elemental distribution, the former bcc high entropy alloy experiences a chemical separation towards Ta-rich and Ta-poor phases. The joining of dissimilar HEAs under constrained conditions (cold shut) enables the design of novel HEA-based composites by non-equilibrium processing.
Short-circuit diffusion in fine-grained Ni samples processed by Spark Plasma sintering has been i... more Short-circuit diffusion in fine-grained Ni samples processed by Spark Plasma sintering has been investigated by the radiotracer technique. Ni grain boundary self-diffusion is measured in samples sintered from commercial as-received powder and from a powder processed by mechanical milling (MM). Both samples displayed high penetration of the radiotracer and ultrafast diffusion rates, which exceed the diffusivity along general high-angle grain boundaries as they are present in pure polycrystalline Ni. A distinct profile was observed for each sample, dependent on the precursor powder. A stable penetration profile after pre-annealing at 773 K was observed when using commercial powder whereas a decrease in the grain boundary diffusion coefficient was depicted for the sample prepared from MM powders. The latter observation was interpreted in terms of partial relaxation of non-equilibrium grain boundaries generated by MM. Sample preparation by focused ion beam enabled the observation of interconnected porous paths in the sample prepared from commercial powders, which represent the main ultrafast diffusion path. A measurement of the surface diffusion coefficient through the pores was attempted considering a B -C type kinetic regime. The isolated pores observed in the sample prepared from MM powder, suggest a complex hierarchy of diffusion paths.
Variable-resolution fluctuation electron microscopy (VR-FEM) data from measurements on amorphous ... more Variable-resolution fluctuation electron microscopy (VR-FEM) data from measurements on amorphous silicon and PdNiP have been obtained at varying experimental conditions. Measurements have been conducted at identical total electron dose and with an identical electron dose normalized to the respective probe size. STEM probes of different sizes have been created by variation of the semi-convergence angle or by defocus. The results show that defocus yields a reduced normalized variance compared to data from probes created by convergence angle variation. Moreover, the trend of the normalized variance upon probe size variation differs between the two methods. Beam coherence, which affects FEM data, has been analyzed theoretically using geometrical optics on a multi-lens setup and linked to the illumination conditions. Fits to several experimental beam profiles support our geometrical optics theory regarding probe coherence. The normalized variance can be further optimized if one determines the optimal exposure time for the nanobeam diffraction patterns.
Intrinsic heterogeneity of shear banding: Hints from diffusion and relaxation measurements of Co micro-alloyed PdNiP-based glass
Journal of Applied Physics, 2020
The influence of Co micro-alloying (1 at. %) on the shear band diffusion and the relaxation proce... more The influence of Co micro-alloying (1 at. %) on the shear band diffusion and the relaxation processes in a model PdNiP bulk metallic glass is investigated. The shear bands are induced by one-pass cold-rolling. In addition to a fast shear band diffusion branch (Dsb≃10−16m2/s at 473 K), with the diffusivity being similar to that observed for the cold-rolled standard Pd40Ni40P20 composition, an ultrafast diffusion branch (Dsb≃10−14m2/s at the same temperature) is found to exist in the micro-alloyed glass. Combined with previously reported observations of faster relaxation of both the Boson peak height and the fictive temperature, the results indicate that Co micro-alloying affects the excess free volume distribution and thus changes the potential energy landscape of the glass, introducing a higher number of local atomic arrangements prone to the formation of shear transformation zones under plastic deformation.
Comparative study of structure and phase transitions in Fe-(25–27)%Ga alloys
Journal of Alloys and Compounds, 2019
Abstract A wide range of complementary research technique - XRD, SEM, TEM, MFM, HV, and low tempe... more Abstract A wide range of complementary research technique - XRD, SEM, TEM, MFM, HV, and low temperature heat capacity, in situ neutron diffraction, resistivity, vibrating sample magnetometry, internal friction, dilatometry, and differential scanning calorimetry - is applied to study structure and phase transitions at heating and cooling of two Fe–Ga functional alloys with 25 and 27 at.% Ga. Using high resolution neutron diffraction, it is unambiguously proven that the initial state of both as-cast samples at room temperature is the D03 phase which results from ordering of the high temperature A2 phase during cooling. Heating of as-cast samples and their subsequent cooling leads to a cascade of phase transitions which change the samples’ structural, mechanical and physical properties. These transitions and properties are discussed in the paper. Transition from metastable D03 to stable L12 phase at heating or isothermal annealing leads to significant changes in macro (grain size) and micro (ordering) structure, hardness and magnetic properties (magnetostriction, magnetization). According to our structural studies (ND, XRD), composition Fe-25.5 at.%Ga below ∼600 °C belongs to a single phase (L12) range of the equilibrium phase diagram, while in the Fe-26.9Ga alloy some amount of the D019 phase is also present.
Analysis of medium-range order based on simulated segmented ring detector STEM-images: amorphous Si
Ultramicroscopy, 2019
Properties of amorphous materials are connected to the local structure at the nanoscale, which is... more Properties of amorphous materials are connected to the local structure at the nanoscale, which is typically described in terms of short- and medium-range order (SRO, MRO). Variable resolution fluctuation electron microscopy (VR-FEM) is a sensitive method to characterize the underlying characteristic length scale of MRO of amorphous samples (Voyles, Gibson and Treacy, J. Electron Microsc. 49 (2000) 259). VR-FEM data was acquired using scanning transmission electron microscopy (STEM), collecting a large number of nano-beam diffraction patterns (NBDPs) with various probe sizes. Here we present an advanced method to accelerate the calculation of simulated FEM normalized variance profiles using a newly developed simulation and analysis approach with segmented ring detectors using the program STEMcl (Radek et al., Ultramicroscopy 188 (2018) 24). VR-FEM simulations are based on structures obtained from molecular dynamics (MD) simulations. A comparison between simulated and experimental VR-FEM profiles with respect to peak position, ratio and shape (and intensity) show good agreement. Moreover, a crystalline cluster of 1 nm in size was embedded into the MD box to test the validity of the paracrystalline approximation with the pair-persistence analysis suggested by Gibson et al. (Gibson, Treacy and Voyles, Ultramicroscopy 83 (2000) 169). The corresponding VR-FEM simulation and calculation of MROs yield close results to the size of the initially embedded crystalline cluster, which supports both the paracrystalline approach and the validity of the segmented detector simulation. Additionally, we conclude that continuous random network (CRN) amorphous silicon models contain a higher degree of MRO than experimentally expected.
Stability and Performance of Nanostructured Perovskites for Light‐Harvesting Applications
Small Methods, 2019
Halide perovskites in general and organometal‐halide perovskites in particular have become an int... more Halide perovskites in general and organometal‐halide perovskites in particular have become an intensely researched topic due their excellent optical properties, cheap production costs, and easy fabrication procedures. A high absorption coefficient, paired with a direct bandgap and a wide variety of accessible deposition method, makes these perovskites an excellent materials class for use in a plethora of applications, ranging from solar cells to light‐emitting diodes, and image sensors with architectures of varying length scales ranging from simple thin‐film solid‐state solar cells to highly ordered nanostructured solar cells and sensors. A downside of perovskites is their poor stability when subjected to ambient conditions as a consequence of poor encapsulation which, can be countered by using a nanostructured as additional encapsulation. In this work, different approaches to fabricate nanostructured perovskite‐based light harvesting devices are outlined and the influences of nanos...
Characterization of Special Grain Boundaries and Triple Junctions in CuxNi1‐x Alloys upon Deformation and Annealing
Advanced Engineering Materials, 2019
We compare two quantities to describe a microstructure: the length fraction of 3/9‐grain boundari... more We compare two quantities to describe a microstructure: the length fraction of 3/9‐grain boundaries and the number fraction of 3‐x‐x/3‐3‐9‐triple junctions using Cu, Ni and four of their alloys in several microstructural states. The fractions of 3‐grain boundaries show similar tendencies as the respective fractions of 3‐x‐x‐triple junctions in relation to the grain size upon deformation and annealing. However, the fraction of 9‐grain boundaries stagnates at certain grain sizes, while there is still a considerable change of 3‐3‐9‐triple junctions during grain growth, meaning that the 3‐3‐9‐triple junction microstructure is still evolving. To analyze the evolution of the triple junction microstructure, a program, such as pythorient, is necessary.
The impact of Co micro-alloying on low temperature specific heat capacity and the relaxation proc... more The impact of Co micro-alloying on low temperature specific heat capacity and the relaxation processes in a model Pd40Ni40P20 bulk metallic glass in both as-cast and cold-rolled states was investigated. In addition to an excess (Boson) peak at low temperatures of about 12 to 14 K, a magnetic heat capacity anomaly in the Co alloyed glass below 10 K was observed. The measured Boson peak heights and the fictive temperatures both advocate a much faster β relaxation in the micro-alloyed glass. Yet, the micro-alloying does not measurably affect the enthalpy associated with α-relaxation. The impact of shear bands on the thermodynamic properties of the micro-alloyed glass is discussed.
An ultra-fine-laminated (UFL) Ni sample with a high fraction of low-angle grain boundaries (LAGBs... more An ultra-fine-laminated (UFL) Ni sample with a high fraction of low-angle grain boundaries (LAGBs) was produced by dynamic plastic deformation. And self-diffusion behavior was studied in this material by the radio-tracer technique. Significantly enhanced interfacial diffusivity was detected in the UFL Ni in comparison with coarse-grained Ni. Analyses indicated that diffusion along LAGBs in the UFL sample is unexpectedly faster than the typical diffusion rates along conventional grain boundaries in the coarse-grained sample. This behavior is explained by the interaction of LAGBs with numerous extrinsic dislocations, as revealed by high-resolution transmission electron microscopy. This work is the first to demonstrate that low-angle grain boundaries in ultra-fine-laminated Ni possess extremely fast diffusion rate, which contributes to understanding the role of interfaces in nanostructured metals.
Grain boundary engineering parameters for ultrafine grained microstructures: Proof of principles by a systematic composition variation in the Cu-Ni system
Acta Materialia, 2018
Abstract The main principles of grain boundary engineering of an ultrafine grained microstructure... more Abstract The main principles of grain boundary engineering of an ultrafine grained microstructure are studied via a systematic variation of the stacking fault energy ( γ S F E ), solid solution effects (SSEs) and the homologous deformation/annealing temperatures choosing the Cu-Ni system as a model case for non-segregating alloys. Cu and Ni are completely miscible and the γ S F E varies strongly with the alloy composition. Ultrafine grained microstructures are produced by high-pressure torsion and their evolution upon annealing is investigated. The thermal stability and the saturation grain sizes after deformation are determined by SSEs. The fraction of deformation twins varies in accordance with the γ S F E . The increase of the length fraction of Σ 3 grain boundaries versus the grain size is found to be governed by the SSEs, whereas the increase of the length fraction of Σ 9 grain boundaries versus the grain size depends on the γ S F E . The results indicate that grain boundary engineering may lead to an optimum ultrafine grained structure, i.e. with a high length fraction of Σ 3 and Σ 9 grain boundaries of approximately 40%. This is a significantly large fraction in comparison to a not-engineered microstructure and achieved for grain sizes below 1000 nm in alloys close to the equiatomic composition having a low γ S F E and a high melting temperature. This high fraction of Σ 3n grain boundaries (including their conjunctions) proved itself most effective for microstructure stabilization in this virtually non-segregating system. Thus, for Cu50Ni50 and Cu65Ni35, a narrow grain size distribution of small grains, a high hardness and a high fraction of special grain boundaries can be adjusted.
Nanodot deposition on a side surface of a rectangular sample and digital image correlation are us... more Nanodot deposition on a side surface of a rectangular sample and digital image correlation are used to quantify the in-plane strain fields associated with the propagation of a shear band in a PdNiP bulk metallic glass,induced by rolling. Within the resolution of the method related to an average inter-dot distance of 100 nm, deformation is found to be highly localized at the shear bands, while alternating areas with a size of 100 -400 nm with opposite local shear strains are found.This phenomenon substantiates a local stick-slip nature of shear band propagation during the metallic glass deformation, even during rolling.
Al3(Sc,Zr)-based precipitates in Al–Mg alloy: Effect of severe deformation
Acta Materialia, 2017
Abstract Precipitate evolution in an AA5024 Al-based alloy containing Sc and Zr is investigated i... more Abstract Precipitate evolution in an AA5024 Al-based alloy containing Sc and Zr is investigated in different states: as-cast, extruded and severely deformed. An overview of the microstructure is given and the evolution of particle size and morphology as a function of mechano-thermal processing is investigated, with an emphasis on the Al3(Sc,Zr)-based precipitates. The precipitates are found to maintain their coherence with the matrix after the interaction with dislocations and low-angle grain boundaries as it is revealed by atomistic simulations and by the so-called Geometric Phase Analysis of the local strain fields. It is established that the precipitates can loose their coherency with the matrix during the severe plastic deformation, that decreases generally grain boundary pinning effect and promotes recrystallization.
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Papers by Gerhard Wilde