The strengthening mechanism of the metallic material is related to the hindrance of the dislocation motion, and it is possible to achieve superior strength by maximizing these obstacles. In this ...study, the multiple strengthening mechanism-based nanostructured steel with high density of defects was fabricated using high-pressure torsion at room and elevated temperatures. By combining multiple strengthening mechanisms, we enhanced the strength of Fe-15 Mn-0.6C-1.5 Al steel to 2.6 GPa. We have found that solute segregation at grain boundaries achieves nanograined and nanotwinned structures with higher strength than the segregation-free counterparts. The importance of the use of multiple deformation mechanism suggests the development of a wide range of strong nanotwinned and nanostructured materials via severe plastic deformation process.
Creation of nanometer‐scale conductive filaments in resistive switching devices makes them appealing for advanced electrical applications. While in situ electrical probing transmission electron ...microscopy promotes fundamental investigations of how the conductive filament comes into existence, it does not provide proof‐of‐principle observations for the filament growth. Here, using advanced microscopy techniques, electrical, 3D compositional, and structural information of the switching‐induced conductive filament are described. It is found that during in situ probing microscopy of a Ag/TiO2/Pt device showing both memory‐ and threshold‐switching characteristics, a crystalline Ag‐doped TiO2 forms at vacant sites on the device surface and acts as the conductive filament. More importantly, change in filament morphology varying with applied compliance currents determines the underlying switching mechanisms that govern either memory or threshold response. When focusing more on threshold switching features, it is demonstrated that the structural disappearance of the filament arises at the end of the constricted region and leads to the spontaneous phase transformation from crystalline conductive state into an initial amorphous insulator. Use of the proposed method enables a new pathway for observing nanosized features in a variety of devices at the atomic scale in three dimensions.
Phase transformation on the nanometer scale determines the electrical, compositional, and structural features of conductive filaments in a Ag/TiO2/Pt device. During in situ probing, a nanometer‐sized crystalline Ag‐doped TiO2 acts as a conductive filament. Focusing more on threshold switching feature, the structural disappearance of the filament resembles phase transformation from crystalline conductive state into an initial amorphous insulator.
Multi-material joining is attracting the attention of the automotive industry given its potential to realize lighter vehicles, and therefore fuel savings and reduced emissions. The aim of the present ...study was to understand the bonding mechanisms whereby metal and plastic/composites are joined and to improve the multi-material joint strength. In this study, the effect of thermal oxidation of Mg alloy sheets on the strengths of Mg–CFRP (carbon-fiber-reinforced plastic) lap joints prepared using laser-assisted metal and plastic joining technique was investigated. Characterization techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), micro-computed tomography (MCT), x-ray photoelectron spectroscopy (XPS), and atom probe tomography (APT) were used to study the underlying mechanisms of the thermal oxidation. The formation of bubbles, mechanical interlocking and chemical reactions at the joint interface were found to be the three key factors influencing the strength of joints. Thermal oxidation increased the joint strength significantly through the suppression of bubble formation, CFRP decomposition and the creation of mechanical interlocking effects at the joint interface. Moreover, MgCO3, MgO1+x, and Mg(OH)2 phases were detected by XPS analysis at the joints prepared using thermally oxidized Mg alloy sheets. The presence of the high O/Mg ratio phases was also confirmed by APT analysis. The formation of these phases confirmed the occurrence of chemical reactions between the MgO and CFRP matrix at the nanometer level, which are regarded as contributing to the increase in the joint strength.
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We study the structure and chemical composition of the κ-carbide formed as a result of isothermal transformation in an Fe–3.0Mn–5.5Al–0.3C alloy using transmission electron microscopy and atom probe ...tomography. Both methods reveal the evolution of κ-particle morphology as well as the partitioning of solutes. We propose that the κ-phase is formed by a eutectoid reaction associated with nucleation growth. The nucleation of κ-carbide is controlled by both the ordering of Al partitioned to austenite and the carbon diffusion at elevated temperatures.
•Pd and Pt/C nanoparticles were embedded in Ni films by means of electrophoresis and electrodeposition.•Atom Probe Tomography (APT) specimens were fabricated from the embedded nanoparticles using ...focused ion beam milling.•While Pt/C specimens showed uneven field evaporation and reconstruction artifacts, mass spectra and APT reconstructions of high data integrity could be obtained for the Pd nanoparticle specimens.
We present a new method of preparing needle-shaped specimens for atom probe tomography from freestanding Pd and C-supported Pt nanoparticles. The method consists of two steps, namely electrophoresis of nanoparticles on a flat Cu substrate followed by electrodeposition of a Ni film acting as an embedding matrix for the nanoparticles. Atom probe specimen preparation can be subsequently carried out by means of focused-ion-beam milling. Using this approach, we have been able to perform correlative atom probe tomography and transmission electron microscopy analyses on both nanoparticle systems. Reliable mass spectra and three-dimensional atom maps could be obtained for Pd nanoparticle specimens. In contrast, atom probe samples prepared from C-supported Pt nanoparticles showed uneven field evaporation and hence artifacts in the reconstructed atom maps. Our developed method is a viable means of mapping the three-dimensional atomic distribution within nanoparticles and is expected to contribute to an improved understanding of the structure-composition-property relationships of various nanoparticle systems.
The 5% pre-straining of a Ni-based solid-solution-strengthened superalloy containing serrated grain boundaries promotes the formation of nano-sized precipitates, that is, the L12-ordered Ni3Al phase, ...during creep at 950 °C, instead of the Ti(C,N) phase, as predicted by computational thermodynamics. Atom probe tomography revealed that the nano-sized precipitate was an uncommon γ′ phase partitioned by Cr atoms of more than 14 at%.
•Superior creep resistance at 950 °C was achieved by multi-strengthening.•The unique processing route induced an unprecedented Cr-rich γʹ for the first time.•High amount of Cr atoms partitioning into γʹ favored in coherency energy and thermal stability.
We report on the effects of Cr and Al on the microstructure and elemental partitioning behavior of γ/γ′ forming Co-Ti-W-based alloys. Additions of Cr to a Co-15Ti-3W (at.%) base alloy at the expense ...of Ti led to a decrease in γ′ volume fraction, whereas Al additions at the expense of Co led to substantially increased γ′ volume fractions. Such alloy modifications resulted in reduced γ/γ′ lattice misfits. Using atom probe tomography, we could show that Ti and W strongly partitioned to the γ′ phase, whereas Al and Cr partitioned to γ. While Cr appeared to have only minor effects on the partitioning behavior of the other alloying elements, Al enhanced partitioning of Ti and in particular W to γ′, thus causing the observed increases in γ′ volume fraction.
•A two-phase γ/γ′ microstructure was observed for Cr and Al-added Co-Ti-W-based alloys.•W was found to partition to γ′, reducing the γ/γ′ lattice misfit and increasing the γ′ volume fraction and solvus temperature as compared to the previously reported Co-Ti alloy.•Cr and Al additions caused further reduction of the γ/γ′ lattice misfit and increases in γ′ volume fraction and solvus temperature.•Al enhances partitioning of Ti and in particular W to γ′.
Abstract
Chemical short-range order in disordered solid solutions often emerges with specific heat treatments. Unlike thermally activated ordering, mechanically derived short-range order (MSRO) in a ...multi-principal-element Fe
40
Mn
40
Cr
10
Co
10
(at%) alloy originates from tensile deformation at 77 K, and its degree/extent can be tailored by adjusting the loading rates under quasistatic conditions. The mechanical response and multi-length-scale characterisation pointed to the minor contribution of MSRO formation to yield strength, mechanical twinning, and deformation-induced displacive transformation. Scanning and high-resolution transmission electron microscopy and the anlaysis of electron diffraction patterns revealed the microstructural features responsible for MSRO and the dependence of the ordering degree/extent on the applied strain rates. Here, we show that underpinned by molecular dynamics, MSRO in the alloys with low stacking-fault energies forms when loaded at 77 K, and these systems that offer different perspectives on the process of strain-induced ordering transition are driven by crystalline lattice defects (dislocations and stacking faults).
WC-Co coatings with low degree of WC decomposition were deposited from electroless Ni-plated micro- and nano-structured feedstock powders using high velocity oxygen fuel (HVOF) spraying. Dry sliding ...friction and wear behavior of the resultant coatings, referred to as Ni/mc-WC and Ni/nc-WC, were investigated by using sintered alumina (Al2O3) as the mating material at 700°C. For the purpose of comparison, similar experiments were carried out on conventional micro- and nano-structured coatings, denoted as mc-WC and nc-WC. The worn scars were examined by field emission scanning electron microscopy (FESEM) equipped with energy dispersive spectroscopy (EDS). The wear resistance of Ni/mc-WC and Ni/nc-WC coatings at 700°C was found to be 45% and 72% greater than that of mc-WC and nc-WC coatings, respectively. Moreover, Ni/mc-WC and Ni/nc-WC coatings exhibited a significantly lower friction coefficient with negligible fluctuations as compared to mc-WC and nc-WC. The microscopic analyses of mc-WC and nc-WC worn surface revealed the presence of plastically deformed and discontinuous tribofilms, made up of a mixture of MWO4/WO3 (M=Co) and Al2O3, severe cracking and delamination especially at the interface of tribofilm and the underlying surface. In contrast, examination of Ni/mc-WC and Ni/nc-WC substantiated that a large area of the coatings surface is covered by a dense and adhered MWO4-type oxide layer (M=Co, Ni), which provides a more protection against wear at elevated temperature.
•Wear resistance of Ni/mc-WC at 700°C was enhanced by 45% compared to mc-WC coating.•Ni/nc-WC showed 72% improvement in wear resistance at 700°C relative to nc-WC.•Type of oxide layer formed on wear track at 700°C has a key role on wear resistance.•For mc-WC and nc-WC, tribofilm was comprised of a mixture of CoWO4/WO3 and Al2O3.•For Ni/mc- and Ni/nc-WC, dense and adhered MWO4 oxide formed on wear track at 700°C.
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