Showing papers in "Materials research letters in 2014"
TL;DR: In this article, a review of critical aspects of high-entropy alloys, including core effects, phases and crystal structures, mechanical properties, high-temperature properties, structural stabilities, and corrosion behaviors are discussed.
Abstract: High-entropy alloys (HEAs) are alloys with five or more principal elements. Due to the distinct design concept, these alloys often exhibit unusual properties. Thus, there has been significant interest in these materials, leading to an emerging yet exciting new field. This paper briefly reviews some critical aspects of HEAs, including core effects, phases and crystal structures, mechanical properties, high-temperature properties, structural stabilities, and corrosion behaviors. Current challenges and important future directions are also pointed out.
2,005 citations
TL;DR: In this paper, the authors report that gradient structures in engineering materials such as metals produce an intrinsic synergetic strengthening, which is caused by macroscopic stress gradient and the bi-axial stress generated by mechanical incompatibility between different layers.
Abstract: Gradient structures are characterized with a systematic change in microstructures on a macroscopic scale. Here, we report that gradient structures in engineering materials such as metals produce an intrinsic synergetic strengthening, which is much higher than the sum of separate gradient layers. This is caused by macroscopic stress gradient and the bi-axial stress generated by mechanical incompatibility between different layers. This represents a new mechanism for strengthening that exploits the principles of both mechanics and materials science. It may provide for a novel strategy for designing material structures with superior properties.
405 citations
TL;DR: In this paper, the authors describe the discovery of giant magnetoresistance (GMR) in magnetic multilayers and their applications in fundamental science as well as applications such as medical applications.
Abstract: Layered magnetic materials are fascinating from the point of view of fundamental science as well as applications. Discoveries such as giant magnetoresistance (GMR) in magnetic multilayers have revo ...
120 citations
TL;DR: In this article, two co-zone twins are activated and interact with each other, resulting in two types of tilt boundaries that have habit planes (0001) and (101¯0) and prevent twin-across-twin transmission but facilitate the propagation of a basal slip band.
Abstract: Using in situ optical microscopy, electron backscatter diffraction analysis, and atomistic simulations, we studied co-zone {1¯012} twin interaction in magnesium single crystal under compression–tension along the [101¯0] direction. Two co-zone twins are activated and interact with each other, resulting in two types of tilt boundaries that have habit planes (0001) and (101¯0) and prevent twin-across-twin transmission but facilitate the propagation of a basal slip band. Upon strain reversal, the unfavorable dissociation of dislocations in the formed tilt boundaries hinder de-twinning.
98 citations
TL;DR: In this paper, the growth, deformation, and extrinsic faults in binary Mg-X alloys were investigated via first-principles calculations, and it was observed that rod-like directional bonds in non-fault planes transform into tetrahedral morphologies in fault planes.
Abstract: The growth, deformation, and extrinsic faults in binary Mg–X alloys are investigated via first-principles calculations. Here, the alloying elements X include Al, Ca, Cu, Fe, K, La, Li, Mn, Na, Nd, Pr, Si, Sn, Sr, Y, Zn, and Zr. In addition to stacking fault energies, the effect of the elements on the bond structure of Mg are studied in term of electron localization morphology. It is observed that rod-like directional bonds in non-fault planes transform into tetrahedral morphologies in fault planes and are strengthened by addition of Zn and Al, but weakened by Na.
95 citations
TL;DR: In this paper, the aluminum-containing solid solution Mn+1AXn phases were synthesized using Rietveld analysis of powder X-ray diffraction patterns to calculate the lattice parameters and phase fractions.
Abstract: We synthesized the following previously unreported aluminum-containing solid solution Mn+1AXn phases: (Ti0.5, V0.5)3AlC2, (Nb0.5, V0.5)2AlC, (Nb0.5, V0.5)4AlC3 and (Nb0.8, Zr0.2)2AlC. Rietveld analysis of powder X-ray diffraction patterns was used to calculate the lattice parameters and phase fractions. Heating Ti, V, Al and C elemental powders—in the molar ratio of 1.5:1.5:1.3:2—to 1, 450°C for 2 h in flowing argon, resulted in a predominantly phase pure sample of (Ti0.5, V0.5)3AlC2. The other compositions were not as phase pure and further work on optimizing the processing parameters needs to be carried out if phase purity is desired.
94 citations
TL;DR: In this article, the sink strength of low-angle GBs can exceed that of high angle GBs due to the effect of GB stress fields, which provides a novel opportunity to enhance the radiation resistance of nc materials through GB engineering.
Abstract: A fundamental understanding of the interactions between point defects and grain boundaries (GBs) is critical to designing radiation-tolerant nanocrystalline (nc) materials. An important consideration in this design is sink strength, which quantifies the efficiency of a sink to annihilate point defects. Contrary to the common belief that random high-angle GBs provide the upper limit for rate of defect annihilation, here we show that the sink strength of low-angle GBs can exceed that of high-angle GBs due to the effect of GB stress fields. This surprising finding provides a novel opportunity to enhance the radiation resistance of nc materials through GB engineering.
53 citations
TL;DR: In this paper, the authors present evidence for the coexistence of a Mn-rich ferromagnetic (FM) state and a Mnpoor reentrant cluster glass state in bulk, polycrystalline, layered (Cr1−x,Mnx)2GeC samples, where x is varied between 0.01 and 0.1.
Abstract: Herein we present evidence for the coexistence of a Mn-rich ferromagnetic (FM) state and a Mn-poor reentrant cluster glass state in bulk, polycrystalline, layered (Cr1−x,Mnx)2GeC samples, where x is varied between 0.01 and 0.1. The Mn-poor regions form a reentrant cluster glass state below ∼30 K. The Mn-rich regions become FM at Curie temperatures that increase with increasing Mn content. The interface coupling between these two regions gives rise to exchange anisotropy and a change in sign at 20 K resulting in, rarely observed, inverted hysteresis loops.
43 citations
TL;DR: In this paper, a phase field model was used to investigate the impact of temperature gradients on isotropic grain growth in 2D UO2 polycrystals, and it was shown that the temperature gradient does not significantly impact the average grain growth behavior because the curvature driving force is dominant.
Abstract: Grain boundaries (GBs) are driven to migrate up a temperature gradient. In this work, we use a phase field model to investigate the impact of temperature gradients on isotropic grain growth. GB motion in 2D UO2 polycrystals is predicted under increasing temperature gradients. We find that the temperature gradient does not significantly impact the average grain growth behavior because the curvature driving force is dominant. However, it does cause significant local migration of the individual grains. In addition, the temperature dependence of the GB mobility results in larger grains in the hot portion of the polycrystal.
42 citations
TL;DR: In situ X-ray synchrotron tomography was used to investigate the stress corrosion cracking behavior of under-aged Al-Zn-Mg-Cu alloy in moisture.
Abstract: In situ X-ray synchrotron tomography was used to investigate the stress corrosion cracking behavior of under-aged Al–Zn–Mg–Cu alloy in moisture. The discontinuous surface cracks (crack jumps) mentioned in the literature are actually a single continuous and tortuous crack when observed in three dimension (3D). Contrary to 2D measurements made at the surface which suggest non-uniform crack growth rates, 3D measurements of the crack length led to a much more accurate measurement of crack growth rates.
42 citations
TL;DR: In this article, a molecular dynamics simulation framework augmented to include the electronic excitations of the SHIs was used to predict that the reshaping of spherical particles into nanorods occurs continuously during consecutive ion impacts by a dynamic crystal-liquid-crystal phase transition of metal particle with the flow of liquid phase into an underdense track core in silica.
Abstract: Swift heavy ion (SHI) irradiation of amorphous SiO2 that contains metal nanocrystals can be used to transform the shape of the particles into peculiar asymmetric ones not easily achievable by other means. Using a molecular dynamics simulation framework augmented to include the electronic excitations of the SHIs, we predict that the reshaping of spherical particles into nanorods occurs continuously during consecutive ion impacts by a dynamic crystal–liquid–crystal phase transition of metal particle with the flow of liquid phase into an underdense track core in silica. The simulated nanocrystals are shown to have a saturation width that agrees with experiments.
TL;DR: In this article, the deformation behavior of nano-sized Ni3Al cubes with {100} side surfaces is investigated under uniaxial compression using constant-temperature molecular dynamics simulations at 300 K.
Abstract: The deformation behaviour of nano-sized Ni3Al cubes with {100} side surfaces is investigated under uniaxial compression using constant-temperature molecular dynamics simulations at 300 K. The simulations reproduce key features of recently performed nanocompression experiments, namely the lack of strain hardening, homogeneous deformation of the entire sample and overall high stress levels of the order of 3–5 GPa. According to the simulations, the critical initial step is the formation of a pseudo-twin structure, which then further deforms by Shockley partial dislocations. These deformation mechanisms differ significantly from bulk Ni3Al and are rationalized in terms of generalized stacking fault energies and resolved shear stresses.
TL;DR: Using Raman and X-ray photoelectron spectroscopy, the authors delineate the bond and defect structures in nuclear block graphite (NBG-18) under neutron and ion irradiation.
Abstract: Using Raman and X-ray photoelectron spectroscopy, we delineate the bond and defect structures in nuclear block graphite (NBG-18) under neutron and ion irradiation. The strengthening of the defect (D) peak in the Raman spectra under irradiation is attributed to an increase in the topological, sp2-hybridized defects. Using transmission electron microscopy, we provide evidence for prismatic dislocations as well as a number of basal dislocations dissociating into Shockley partials. The non-vanishing D peak in the Raman spectra, together with a generous number of dislocations, even at low irradiation doses, indicates a dislocation-mediated amorphization process in graphite.
TL;DR: In this paper, the atomic-scale elementary properties of the twin interfaces and their disconnections were investigated and it was shown that the transverse propagation of twins is hindered by the absence of low-energy mobile interfaces, whereas twins benefit from prismatic-basal interfaces.
Abstract: The two most frequently observed twins in hexagonal close-packed (HCP) Mg, and twins, have surprisingly different properties and morphologies, with twins appearing under higher stresses and being much thinner than twins. By considering the atomic-scale elementary properties of the twin interfaces and their disconnections, we show that (1) the transverse propagation of twins is hindered by the absence of low-energy mobile interfaces, whereas twins benefit from prismatic-basal interfaces and (2) the thickening of twins is slowed by higher energy barriers against both the nucleation and propagation of disconnections along their interfaces.
TL;DR: In this article, a novel technique was proposed and demonstrated for processing interpenetrating ceramic-metal composites by the infiltration of molten metals/alloys into ceramic foams utilizing a current-activated, pressure-assisted technique.
Abstract: A novel technique is proposed and demonstrated for processing interpenetrating ceramic–metal composites by the infiltration of molten metals/alloys into ceramic foams utilizing a current-activated, pressure-assisted technique. Ti2AlC foams were prepared and used as the infiltration preform. Aluminum alloy 6061 (AA6061) has been infiltrated into the Ti2AlC foams, resulting in AA6061/Ti2AlC composites. The microstructure, composition, and distribution of phases in the composites were studied. The AA6061/Ti2AlC composites were studied under compression testing, and the results were compared with those of the pure components of the composites. Advantages of this method in comparison to other methods for processing metal–ceramic composites are discussed.
TL;DR: In this paper, the authors report that mechanically assisted triple junction motion is an important contributor to dynamic recovery, leading to an almost steady state, which rationalizes both a decreasing work hardening rate and the approach to a dynamic equilibrium of structural refinement at large strains.
Abstract: Plastic deformation of metals refines the microstructure and increases the strength through work hardening, but this effect of deformation is counterbalanced by dynamic recovery. After large strain, the microstructure typically shows a lamellar morphology, with finely spaced lamellar boundaries connected by triple junctions. Here, we report that mechanically assisted triple junction motion is an important contributor to dynamic recovery, leading to an almost steady state. Triple junction motion replaces two boundaries by one, while maintaining the structural morphology. The observation rationalizes both a decreasing work hardening rate and the approach to a dynamic equilibrium of structural refinement at large strains.
TL;DR: Most deformation twins in nanocrystalline face-centered cubic (NC fcc) metals are reported to produce zero-macrostrain, which is attributed to either random activation of partials or cooperative slip of three partials (CSTP) as discussed by the authors.
Abstract: Most deformation twins in nanocrystalline face-centered cubic (NC fcc) metals are reported to produce zero-macrostrain, which is attributed to either random activation of partials (RAP) or cooperative slip of three partials (CSTP). Here, we report that when the RAP mechanism is suppressed, ∼44% twins in NC Cu produced zero-macrostrain via the CSTP mechanism. This indicates that both RAP and CSTP are major mechanisms to generate zero-macrostrain twins. In addition, our results also indicate that stress state affects the twinning mechanism in NC fcc metals, and monotonic activation of partials with the same Burgers vector dominates twin formation under monotonic stress.
TL;DR: In this paper, a metric for quantifying the degree of dispersion of carbon nanotubes in polymeric and other matrices was proposed, motivated by the technological importance of nanostructure-based composites.
Abstract: We seek to provide a metric for quantifying the degree of dispersion of nanostructures, such as carbon nanotubes in polymeric and other matrices, motivated by the technological importance of nanostructure-based composites. Our proposed measure of dispersion uses a quadrat-based sampling algorithm and a metric d(P| | Q), to correlate randomness to the dispersion. This allows a quantitative comparison of the given distribution (say, P) to a preferred distribution or pattern (say, Q). We provide examples from our own studies and those from the literature on the application of the metric.
TL;DR: In this paper, single-crystal Cu micro-pillars were self-ion irradiated up to 190 displacements per atom, a level commensurate with damage expected after long exposure in a reactor environment.
Abstract: Single-crystal Cu micro-pillars were self-ion irradiated up to 190 displacements per atom, a level commensurate with damage expected after long exposure in a reactor environment. Compression experiments performed along the ⟨ 110 ⟩ to 10% strain were compared against un-irradiated Cu. Two specimen configurations were explored: large 10 μm tall and small 4 μm tall pillars. Compared to un-irradiated Cu, the small irradiated pillars exhibited a flow stress increase of more than 500 MPa and were able to attain peak stresses approaching 1 GPa. These results are discussed in the context of an end of range effect, a damage gradient effect, and size effects.
TL;DR: In this paper, the defect mediated electrical and magnetic properties of transition metal-doped perovskites have been investigated and Mn4+ is found to substitute Ti in bulk SrTiO3, but Mn2+ segregates inside grain boundaries at both Sr and interstitial sites.
Abstract: Mn-doped SrTiO3 shows promising magnetic and electrical properties, but the doping mechanism remains unclear. In this research Mn4+ is found to substitute Ti in bulk SrTiO3, but Mn2+ segregates inside grain boundaries at both Sr and interstitial sites. Mn interstitial doping has never been reported, but is found possible with the formation of Sr vacancies. This finding is significantly different from the amphoteric doping of Mn2+ substituting Sr and Mn4+ substituting Ti sites, therefore leads to different understanding on the defect mediated electrical and magnetic properties of transition metal-doped perovskites.
TL;DR: In this paper, size effects on the compressive strength of nanostructured Mg-micropillars were investigated and the deformation mechanisms of the compressed pillars were characterized using transmission electron microscopy.
Abstract: Size effects on the compressive strength of nanostructured Mg-micropillars were investigated. Mg–10Al alloy micropillars with diameters ranging from 1.5 to 8 μm were prepared by focused-ion-beam and tested under micro-compression. A significant improvement in strength was found by reducing the pillar diameter<3.5 μm. The deformation mechanisms of the compressed pillars were characterized using transmission electron microscopy. We attribute the size-induced strengthening to a less number of dislocation sources along with a higher activity of non-basal deformation mechanisms.
TL;DR: In this paper, a successful fabrication of nanopatterned graphene (NPG) using a PS-b-P4VP polymer, which was never used previously for the graphene patterning, was demonstrated.
Abstract: We demonstrate a successful fabrication of Nanopatterned Graphene (NPG) using a PS-b-P4VP polymer, which was never used previously for the graphene patterning. The NPG exhibits homogeneous mesh structures with an average neck width of ∼19 nm. Electronic characterization of single and few layers NPG FETs (field effect transistors) were performed at room temperature. We found that the sub-20 nm neck width creates a quantum confinement in NPG, which has led to a bandgap opening of ∼0.08 eV. This work also demonstrates that BCP (block co-polymer) lithography is a pathway for low-cost, high throughput large-scale production of NPG with critical dimensions down to the nanometer regime.
TL;DR: In this paper, the atomic structure of metadislocation cores in the phase o-Al13Co4 was investigated using aberration corrected scanning transmission electron microscopy, and a partial atomic model for metadelication glide motion, taking Cobalt atom jumps into account, was developed.
Abstract: The atomic structure of metadislocation cores in the phase o-Al13Co4 is investigated using aberration corrected scanning transmission electron microscopy. Metadislocations with Burgers vectors of -b/τ4 [0 1 0] and b/τ3 [0 1 0] having six and four stacking faults, respectively, are found. They are associated with separate phason defects, which escort the movement of the metadislocations through the material. A first partial atomic model for metadislocation glide motion, taking Cobalt atom jumps into account, is developed. Atom jumps take place in pairs along various crystallographic directions. Typical jumps occur between atomic columns separated by 1.5 A.
TL;DR: In this article, large-scale molecular dynamics simulations were used to predict that knock-on damage in irradiated metallic glasses yields spontaneous anisotropic deformation, i.e. shape change in the absence of externally applied loads.
Abstract: Using large-scale molecular dynamics simulations, we predict that knock-on damage in irradiated metallic glasses yields spontaneous anisotropic deformation, i.e. shape change in the absence of externally applied loads. The root cause of this behavior is anisotropic plastic deformation around non-spherical thermal spikes. Such thermal spike-induced plasticity (TSIP) does not depend on electronic excitations and is distinct from the ‘ion hammer’ effect. Macroscale TSIP is predicted to occur under unidirectional heavy ion and neutron irradiation. The consequences of TSIP for potential applications of metallic glasses under irradiation are discussed.
TL;DR: In this paper, the formation of nanocrystals occurred at an ultra-low stress of 0.25 GPa in the elastic deformation regime, accompanied by load-drops without evidence of shear bands.
Abstract: Stress driven nucleation of nanocrystals in amorphous alloys has been a subject of intensive debate in the past decade. It has long been postulated that nanocrystals form succeeding the occurrence of shear bands in deformed amorphous alloys. In this study, we show, via in situ nanoindentation of amorphous Cu44Zr44Al12 alloy in a transmission electron microscope that the formation of nanocrystals occurred at an ultra-low stress of 0.25 GPa in the elastic deformation regime, accompanied by load-drops without evidence of shear bands. Furthermore, during successive loading, repetitive nanocrystal nucleation events were observed, and the stress required for nucleation kept on increasing to ∼0.54 GPa, implying the occurrence of a ‘hardening’ effect in the amorphous alloy. This study provides direct evidence to advance our understanding on deformation-induced nanocrystallization of amorphous alloys.
TL;DR: In this article, an atomic-scale investigation of late-stage void evolution, including growth, coalescence and shrinkage, under electron irradiation was performed, and surface diffusion of adatoms was observed to be largely responsible for the void coalescence.
Abstract: We report in situ atomic-scale investigation of late-stage void evolution, including growth, coalescence and shrinkage, under electron irradiation. With increasing irradiation dose, the total volume of voids increased linearly, while the nucleation rate of new voids decreased slightly and the total number of voids decreased. Some voids continued to grow while others shrank to disappear, depending on the nature of their interactions with nearby self-interstitial loops. For the first time, surface diffusion of adatoms was observed to be largely responsible for the void coalescence and thickening. These findings provide fundamental understanding to help with the design and modeling of irradiation-resistant materials.
TL;DR: In this article, the fracture behavior of Fe−25Mn−3Si−3Al steels was investigated and the authors obtained much attention because of their excellent mechanical properties.
Abstract: Fe–25Mn–3Si–3Al steels obtained much attention because of their excellent mechanical properties. However, the fracture behavior of the steels remains unclear. In this study, Fe–25Mn–3Si–3Al steels ...
TL;DR: In this paper, a brief historical review about how graphene logically fits into the nanoscience field from my own perspective in contrast to the actual history of graphene is given, and a brief review of the history of the development of graphene can be found.
Abstract: I start this perspectives article with a brief historical review about how graphene logically fits into the nanoscience field from my own perspective in contrast to the actual history of graphene r...
TL;DR: In this article, a real-time investigation of Bragg angle shift during volume hologram formation is presented in an organic cationic ring-opening polymerization material and positive and negative values of optical shrinkage are found, assumedly related to mechanical deformations of the volume and a change of the average refractive index.
Abstract: Real-time investigation of Bragg angle shift during volume hologram formation is presented in an organic cationic ring-opening polymerization material. Positive as well as negative values of optical shrinkage are found, assumedly related to mechanical deformations of the volume and a change of the average refractive index, respectively. Ruled by the interplay of polymerization and diffusion, the originate grating formation mechanisms prove to represent competing effects regarding the contribution to the optical shrinkage. The influence of sample preparation and holographic exposure procedure on the effects observed is investigated and the usability for minimization of total Bragg resonance detuning is considered.
TL;DR: In this article, a computational approach was utilized to design the framework and to identify potential guest atoms for stabilizing nitrogen-substituted carbon and silicon clathrates, which were discovered as potential candidate materials.
Abstract: A computational approach was utilized to design the framework and to identify potential guest atoms for stabilizing nitrogen-substituted carbon and silicon clathrates. Two new series of N-substituted carbon and silicon clathrates compounds were discovered as potential candidate materials. One of the hybrid C\bond N clathrates was successfully synthesized using an industrial arc-melting technique. The theoretical bulk moduli of these N-substituted carbon and silicon clathrates were computed and they are comparable to those of C3N4, Si3N4, and SiC. Some N-substituted carbon clathrates may be suitable for application as hard materials.