Showing papers in "Materials research letters in 2015"
TL;DR: A low-density, nanocrystalline high-entropy alloy, Al20Li20Mg10Sc20Ti30 was produced by mechanical alloying as discussed by the authors, which formed a single phase fcc structure during ball milling and transformed to single phase hcp upon annealing.
Abstract: A low-density, nanocrystalline high-entropy alloy, Al20Li20Mg10Sc20Ti30 was produced by mechanical alloying. It formed a single-phase fcc structure during ball milling and transformed to single-phase hcp upon annealing. The alloy has an estimated strength-to-weight ratio that is significantly higher than other nanocrystalline alloys and is comparable to ceramics. High hardness is retained after annealing.
448 citations
TL;DR: In this paper, the discovery of a hexagonal high-entropy alloy (HEA) with hexagonal crystal structure was reported, and the results of electron diffraction investigations and high resolution scanning transmission electron microscopy are consistent with an Mg-type hexagonal structure.
Abstract: We report on the discovery of a high-entropy alloy (HEA) with a hexagonal crystal structure. Equiatomic samples in the alloy system Ho–Dy–Y–Gd–Tb were found to solidify as homogeneous single-phase HEAs. The results of our electron diffraction investigations and high-resolution scanning transmission electron microscopy are consistent with an Mg-type hexagonal structure. The possibility of hexagonal high-entropy alloys in other alloy systems is discussed.
215 citations
TL;DR: In this article, a 100-fold capacitance increase in MoS2-based supercapacitors was achieved via optimizing the in-plane 1T-2H phase hybridization of the monolayers.
Abstract: We report on a 100-fold capacitance increase in MoS2-based supercapacitors achieved via optimizing the in-plane 1T-2H phase hybridization of the monolayers. Chemically exfoliated MoS2 monolayers were annealed at low temperature to tune their 1T content from 2% to 60%. The obtained hybridization states were confirmed by X-ray photoelectron and Raman spectroscopies. After optimizing the hybridization degree, the electrode based on MoS2 monolayers with 40% of the 1T phase exhibited outstanding performance with a resistance as low as 0.68 kΩ sq−1, specific capacitance of 366.9 F g−1, and retention ratio of 92.2% after 1000 cycles at current densities of 0.5 A g−1.
142 citations
TL;DR: In this paper, a five-element alloy, CrCoCuFeNi, was deposited via radio frequency magnetron sputtering and confirmed to be a single-phase solid solution through the high-energy synchrotron X-ray diffraction, energy-dispersive spectroscopy, wavelength dispersive spectrum analysis, and transmission electron microscopy.
Abstract: The concept of high configurational entropy requires that the high-entropy alloys (HEAs) yield single-phase solid solutions. However, phase separations are quite common in bulk HEAs. A five-element alloy, CrCoCuFeNi, was deposited via radio frequency magnetron sputtering and confirmed to be a single-phase solid solution through the high-energy synchrotron X-ray diffraction, energy-dispersive spectroscopy, wavelength-dispersive spectroscopy, and transmission electron microscopy. The formation of the solid-solution phase is presumed to be due to the high cooling rate of the sputter-deposition process.
108 citations
TL;DR: In this article, a grain refinement from several millimeters in as-received (AR) condition to the range of 0.35-15μm was achieved by friction stir processing (FSP).
Abstract: Grain refinement from several millimeters in as-received (AR) condition to the range of 0.35–15 μm was achieved by friction stir processing (FSP). Due to the sluggish nature of atomic diffusion in high entropy alloys (HEAs), the FSP region exhibited an immense variation in microstructure which was directly attributed to the accumulated plastic strain during FSP. In accordance with the Hall–Petch relationship, yield strength (YS) has increased by a factor of four after grain refinement while maintaining large uniform elongation (UE). The Kocks–Mecking plot indicated different deformation mechanisms operative in both FSP and AR conditions.
95 citations
TL;DR: In this paper, an ideal ultrafine-grained (UFG) microstructure for high strength and high ductility should have short dislocation-slip path to impede dislocation slip and very low dislocation density to ensure more room for dislocation accumulation.
Abstract: An ideal ultrafine-grained (UFG) microstructure for high strength and high ductility should have short dislocation-slip path to impede dislocation slip and very low dislocation density to ensure more room for dislocation accumulation. Such a microstructure is hard to produce, especially for UFG metals produced by severe plastic deformation techniques. Here, we report an ideal UFG structure produced by reverse transformation of deformation-induced martensite in 304 L austenitic stainless steel. It produced small grains and a high density of nanotwins for both high strength and high ductility. This approach is applicable to face-centered cubic metals with low stacking fault energy.
95 citations
TL;DR: In this paper, the thermal expansion dependency of temperature for nanolamellar MAX phase compounds (Cr 0.5V0.5 + 1AlCn) was studied in detail by high-resolution neutron diffraction.
Abstract: Nanolamellar MAX phase compounds (Cr0.5V0.5)n+1AlCn are formed with n = 1, 2 and 3, and their 300 K structure is studied in detail by high-resolution neutron diffraction. While the n = 1 compound is found to have complete disordering of vanadium and chromium in the metallic layers, the n = 2 and 3 compounds show strong tendency for these elements' ordering, with the layer in the 2a(0,0,0) site of (Cr0.5V0.5)3AlC2 fully occupied by vanadium. The thermal expansion dependency of temperature is also studied by neutron diffraction for 2 < T < 550 K, revealing a negligible thermal expansion below 100 K for all of the compounds.
90 citations
TL;DR: In this article, the authors present a review and comparison of various models for thermodynamic stabilization of grain boundaries in a nanocrystalline microstructure, as well as a comparison of the performance of different models.
Abstract: Grain boundaries in a nanocrystalline microstructure produce an increase in the excess free energy of the system. Grain growth is a consequence of the thermodynamic driving force reducing this excess. Thermodynamic stabilization is an approach based on eliminating the driving force by suitable alloy additions that can produce a metastable equilibrium state at the nanoscale grain size, as opposed to kinetic stabilization where the grain growth mobility is restricted by pinning and/or drag mechanisms. The present paper reviews and compares various models proposed for thermodynamic stabilization.
70 citations
TL;DR: In this article, the bulk synthesis of (Cr 1−xMn x)2AlC and (Cr1−yMny)2GaC MAX phases was reported.
Abstract: Herein, we report on the bulk synthesis of (Cr 1−xMn x)2AlC and (Cr1−yMny)2GaC MAX phases. Scanning electron and transmission electron microscopy, in combination with energy-dispersive X-ray spectroscopy performed locally on MAX phase grains, revealed x and y to be 0.06 (3 at%) and 0.3 (15 at%), respectively. The introduction of Mn into the structure did not result in appreciable changes in the c-lattice constants. Vibrating sample magnetometry measurements suggest that bulk (Cr0.7Mn0.3)2GaC may be magnetic.
63 citations
TL;DR: In this article, the fatigue strength of nanocrystalline (NC) Cu and Cu-Al alloys processed by high-pressure torsion was prominently enhanced with a decrease in stacking fault energy.
Abstract: Increasing the fatigue strengths of high-strength materials, especially their endurance limits is essentially challenging. The fatigue strengths of nanocrystalline (NC) Cu and Cu–Al alloys processed by high-pressure torsion were prominently enhanced with a decrease in stacking fault energy. This remarkable escalation can be attributed not only to their significantly increased tensile strengths in macro-scale, but also to the essentially improved microstructure stability and reduced strain localization within shear bands in microscale. Owing to the limitation of their intrinsic fatigue mechanisms, the fatigue limits of NC Cu and Cu–Al alloys cannot always acquire appreciable improvement with their monotonic strengths.
52 citations
TL;DR: In this article, severe plastic deformation was successfully imposed on barium titanate ceramic powders by high-pressure torsion, and a tetragonal-to-cubic phase transformation occurred, and the fraction and stability of the cubic phase increased by straining.
Abstract: Ceramics are generally brittle at ambient condition and they can hardly be deformed plastically. In this study, severe plastic deformation was successfully imposed on barium titanate ceramic powders by high-pressure torsion. A tetragonal-to-cubic phase transformation occurred, and the fraction and stability of the cubic phase increased by straining because of the formation of nanograins. BaTiO3 exhibited photoluminescence and the yellow intensity increased after straining because of the formation of large fraction of grain boundaries. The dielectric constant of BaTiO3 was unusually increased by nanograin formation while the Curie temperature remained constant.
TL;DR: Magnetic MAX phase (Cr0.5Mn 0.5mn0.2) GaC thin films grown epitaxially on MgO(111) substrates were studied by ferromagnetic resonance at temperatures between 110 and 300 K.
Abstract: Magnetic MAX phase (Cr0.5Mn0.5)(2)GaC thin films grown epitaxially on MgO(111) substrates were studied by ferromagnetic resonance at temperatures between 110 and 300 K. The spectroscopic splitting ...
TL;DR: In this paper, a general approach to control the growth of phase domains during solidification by adsorption of nanoparticles on the growing interface is presented. But only limited ways are available for controlling phase growth of phases during casting.
Abstract: Controlling phase growth during solidification is essential to obtain desirable structures and properties in metallic materials by casting. However, only limited ways are available for controlling the growth of phases during solidification. Here, we report a general approach to control the growth of phase domains during solidification by adsorption of nanoparticles on the growing interface. The effectiveness of the approach is demonstrated in an Sn–Al alloy. With nanoparticle-enabled growth control, the size of the primary Al phase is reduced substantially and the dendrite growth is restricted. This work potentially provides an effective way to control the structure of alloys during solidification.
TL;DR: In this article, a NiAl-strengthened ferritic alloys are characterized by the ultra-small angle X-ray scattering and transmission electron microscope, and shown to be brittle at room temperature.
Abstract: Duplex precipitates are presented in a NiAl-strengthened ferritic alloy. They were characterized by the ultra-small angle X-ray scattering and transmission electron microscope. Fine cooling precipitates with the size of several to tens of nanometres harden the matrix considerably at room temperature. Cracks are likely to initiate from precipitates, and coalesce and propagate quickly through the matrix due to the excessive hardening effect of cooling precipitates, which lead to the premature fracture of NiAl-strengthened ferritic alloys.
TL;DR: In this paper, transmission electron microscopy studies on dynamically deformed pure cobalt were performed and it was shown that the twinning plane of the most common twinning mode in hexagonal close-packed metals is non-coincident, i.e., the parent and the twin does not coincide.
Abstract: We performed transmission electron microscopy studies on dynamically deformed pure cobalt. The results show that the twinning plane of the most common twinning mode in hexagonal close-packed metals is non-coincident, that is, the twinning plane of the parent and the twin does not coincide. Statistical measurements show that only 44% of the twins have a misorientation angle close to the theoretical value of 86.3°, and 79% of the twin boundaries (TBs) deviate from the twinning plane. Thirty-two percent of the TBs deviate from the twinning plane by more than 40°. These results indicate that the twinning significantly departs from the classical twinning behavior.
TL;DR: In this paper, the authors demonstrated that the strain-rate sensitivity of bodycentered cubic single crystal iron pillars will be reduced by one order when the pillar size was reduced from 1000 to about 200 nm.
Abstract: Through in situ scanning electron microscope microcompression tests, we demonstrated that the strain-rate sensitivity of bodycentered cubic single crystal iron pillars will be reduced by one order when the pillar size was reduced from 1000 to about 200 nm. In addition, size-strengthening exponent exhibited obvious strain-rate dependence. We propose that the observed behavior is a result of the high stresses required to induce curvature bowout of dislocation arms at small sample or grain sizes, which overwhelms the lattice friction stress contribution and diminishes the role played by the mobility difference between edge and screw dislocations.
TL;DR: In this paper, an Fe-Ni-Al-C alloy with ultrahigh tensile strength of 1.9-2.2 GPa and high uniform ductility of 16-19% was developed by concurrent employment of several strategies: (i) appropriate choice of chemical compositions for lattice softening, (ii) severe plastic deformation using the high-pressure torsion method for grain refinement, and (iii) control of strain level for multimodal-structure formation composed of equiaxed nanograins, lamellar coarse grains and
Abstract: Despite high strength of nanostructured alloys, they usually exhibit poor uniform ductility. For many applications, it is an important issue to design new nanostructured alloys which have both high strength and high uniform ductility. In this study, an Fe–Ni–Al–C alloy with ultrahigh tensile strength of 1.9–2.2 GPa and high uniform ductility of 16–19% was developed by concurrent employment of several strategies: (i) appropriate choice of chemical compositions for lattice softening, (ii) severe plastic deformation using the high-pressure torsion method for grain refinement, and (iii) control of strain level for multimodal-structure formation composed of equiaxed nanograins, lamellar coarse grains and fine precipitates.
TL;DR: In this paper, a water-based fabrication platform is proposed for 3D printing of biomaterials such as chitosan, cellulose or sodium alginate for the construction of highly sustainable products.
Abstract: In nature, water assembles basic molecules into complex multi-functional structures with nano-to-macro property variation. Such processes generally consume low amounts of energy, produce little to no waste, and take advantage of ambient conditions. In contrast digital manufacturing platforms are generally characterized as uni-functional, wasteful, fuel-based and often toxic. In this paper we explore the role of water in biological construction and propose an enabling technology modeled after these findings. We present a water-based fabrication platform tailored for 3-D printing of water-based composites and regenerated biomaterials such as chitosan, cellulose or sodium alginate for the construction of highly sustainable products and building components. We demonstrate that water-based fabrication of biological materials can be used to tune mechanical, chemical and optical properties of aqueous material composites. The platform consists of a multi-nozzle extrusion system attached to a multi-axis robotic arm designed to additively fabricate extrusion-compatible gels with graded properties. Applications of the composites include small and medium-scale recyclable objects, as well as temporary largescale architectural structures.
TL;DR: In this paper, the authors predicted YB6 as a "ductile" and soft ceramic for ultra-high-temperature applications, through density functional theory investigations on its electronic structure and bonding properties.
Abstract: Transition metal borides have contributed to the success of ultrahigh-temperature materials (UHTM) development. Current ZrB2- and HfB2-based UHTMs exhibit high-temperature environmental stability, high strength and modulus but inevitable brittleness due to strong covalent bonding. In contrast but interestingly, YB6 is predicted as a ‘ductile’ and soft ceramic for ultrahigh-temperature applications, through density functional theory investigations on its electronic structure and bonding properties. The ‘ductility’ is underpinned by chemical bonding anisotropy, that is, a strong σ bond connecting the B6 octahedra and a weak banana bond formed by overlapping the lobes of two perpendicular p orbitals (τ bond).
TL;DR: In this paper, the authors report on in situ Kr ion irradiation studies of a bulk Fe96Zr4 nanocomposite alloy and reveal that mobile dislocation loops in α-Fe layers were confined by the crystal/amorphous interfaces and kept migrating to annihilate other defects.
Abstract: Enhanced irradiation tolerance in crystalline multilayers has received significant attention lately. However, little is known on the irradiation response of crystal/amorphous nanolayers. We report on in situ Kr ion irradiation studies of a bulk Fe96Zr4 nanocomposite alloy. Irradiation resulted in amorphization of Fe2Zr and formed crystal/amorphous nanolayers. α-Fe layers exhibited drastically lower defect density and size than those in large α-Fe grains. In situ video revealed that mobile dislocation loops in α-Fe layers were confined by the crystal/amorphous interfaces and kept migrating to annihilate other defects. This study provides new insights on the design of irradiation-tolerant crystal/amorphous nanocomposites.
TL;DR: In this article, the authors used severe plastic deformation (SPD) to fabricate nano-grained metal structures for Zr, a structural metal for nuclear applications, and obtained a nanoscale grain structure via SPD.
Abstract: Severe plastic deformation (SPD) is a common method to fabricate nano-grained metals. However for Zr, a structural metal for nuclear applications, obtaining a nanoscale grain structure via SPD has ...
TL;DR: In this paper, it was shown that molten copper, slowly infiltrated into a broad spectrum of preforms, displays in initial phases of the process universal scaling and fractal geometric features that are characteristic signatures of percolation-dominated flow.
Abstract: Composite materials are often made by infiltration, that is, by injecting a liquid matrix between packed solid fibers or particles. We give here direct proof that molten copper, slowly infiltrated into a broad spectrum of preforms, displays in initial phases of the process universal scaling and fractal geometric features that are characteristic signatures of percolation-dominated flow. Implications are that the microstructure of infiltrated composite materials can develop over length scales that far exceed the average preform pore size, and that network models are pertinent in the simulation of composite infiltration processing.
TL;DR: In this article, the effect of high hydrogen concentrations on enhanced dislocation plasticity was investigated in α-iron nano-crystals starting with pre-existing dislocation networks, and it was shown that the dislocation density increases in the presence of hydrogen due to dislocation pinning and enhanced dislocations selfmultiplication.
Abstract: Novel atomistic simulations of α-iron nano-crystals starting with pre-existing dislocation networks have been performed to identify the effect of high hydrogen concentrations on enhanced dislocation plasticity. Hydrogen is shown to decrease the dislocations free-surface nucleation stress, as well as increase the flow strength of crystals. The dislocation density is observed to increase in the presence of hydrogen due to dislocation pinning and enhanced dislocation self-multiplications. Hydrogen also changes the deformation morphology from discrete slip planes in hydrogen-free crystals to a homogeneous deformation in H-charged crystals due to the enhanced dislocations self-multiplication.
TL;DR: This paper sets the stage for a new theoretical framework and an applied approach for the design and fabrication of geometrically and materially complex functional designs coined Fabrication Information Modeling (FIM).
Abstract: Despite recent advancements in digital fabrication and manufacturing, limitations associated with computational tools are preventing further progress in the design of non-standard architectures This paper sets the stage for a new theoretical framework and an applied approach for the design and fabrication of geometrically and materially complex functional designs coined Fabrication Information Modeling (FIM) We demonstrate systems designed to integrate form generation, digital fabrication, and material computation starting from the physical and arriving at the virtual environment The paper reviews four computational strategies for the design of custom systems through multi-scale trans-disciplinary data, which are classified and ordered by the level of overlap between the modeling media and the fabrication media: (1) the first model takes as input biological data and outputs 3D printed digital materials organized according to functional constraints; (2) the second model takes as input geometry and environmental data and outputs robotically wound fibers organized according to functional constraints; (3) the third model takes as input material and environmental data and outputs CNC deposited pastes organized according to functional constraints; (4) the forth model takes as input biological, material and environmental data and outputs robotically deposited polymers organized according to functional constraints The analysis of these models will demonstrate the FIM approach and point towards its value to designers who seek to inform their work through multi-scale transdisciplinary data, a capability that is currently missing from standard design-to-fabrication workflows
TL;DR: In this article, a new route for realizing the concept of architecturing of composites by severe plastic deformation is presented, which involves multi-pass twist extrusion of a composite with fibers.
Abstract: The paper presents a new route for realizing the concept of architecturing of composites by severe plastic deformation. The proposed route involves multi-pass twist extrusion of a composite with fibers. The potential of the method is first illustrated by mathematical modeling and then tested through pilot processing of a composite consisting of a copper matrix and a single aluminum fiber. Metallographic analysis revealed an unexpected shape of the fiber after processing. Finite element simulations were performed to understand the evolution of the fiber shape and to optimize the processing regime for achieving improved reinforcements in twist-extruded composites.
TL;DR: In this article, a theoretical model was proposed to illustrate the effect of detwinning of the secondary twin lamellae on the emission of lattice dislocations from a semi-infinite crack tip in a hierarchically nanotwinned metal.
Abstract: A theoretical model is proposed to illustrate the effect of detwinning of the secondary twin lamellae on the emission of lattice dislocations from a semi-infinite crack tip in a hierarchically nanotwinned metal. The results obtained show that the potential of the crack tip to emit dislocations can be optimized and greatly enhanced by tuning the secondary twin spacing, which leads to strong crack blunting. Moreover, the hierarchical structure is able to produce significant toughening at very thin secondary twin lamellae, as observed in molecular dynamics simulations. As a result, the proposed model suggests a novel toughening mechanism in ultrafine/nano-grained metals.
TL;DR: In this article, the authors investigated strain-hardening in tri-component nano-scale metallic multilayers using nanoindentation and micro-pillar compression and found that coherent interfaces with a modulus mismatch in the tri-layer system are responsible for additional deformation mechanisms that can lead to hardening in excess of that found in bi-layer systems.
Abstract: Strain-hardening in tri-component nano-scale metallic multilayers was investigated using nanoindentation and micro-pillar compression. Cu/Ni/Nb films were made in tri-layer structures as well as bi-layers consisting of an alloy of Cu–Ni/Nb. Strain-hardening increases as the layer thickness decreases, with 5 nm layers exhibiting higher strengths and hardening coefficients than 30 nm layers. The experimental evidence is described in light of the confined layer slip model, and supports the hypothesis that coherent interfaces with a modulus mismatch in the tri-layer system are responsible for additional deformation mechanisms that can lead to hardening in excess of that found in bi-layer systems.
TL;DR: In this article, the temperature-induced changes in the topological short-range order in magnetron sputtered Co67B33 metallic glass thin films are investigated and the presence of B-Co-B rigid second-order structures at room temperature and the temperatureinduced decrease in the population of these strongly bonded building blocks are inferred.
Abstract: In situ high-temperature X-ray diffraction experiments using high-energy photons and ab initio molecular dynamics simulations are performed to probe the temperature-induced changes in the topological short-range order in magnetron sputtered Co67B33 metallic glass thin films. Based on this correlative experimental and theoretical study, the presence of B–Co–B rigid second-order structures at room temperature and the temperature-induced decrease in the population of these strongly bonded building blocks are inferred. This notion is consistent with experimental reports delineating the temperature dependence of elastic limit.
TL;DR: In this paper, the authors show that sliding on {0002}-textured nanocrystalline ZnO films increases partial dislocation density through intra-film shear, via glide of partial dislocations, on parallel stacking fault planes.
Abstract: Single-crystal and microcrystalline ceramics classically fail via brittle fracture under loading. Here, we report that atomic layer deposition {0002}-textured ZnO films, with nanocolumnar grains and defective sub-stoichiometric structure, exhibit nanocrystalline plasticity and exceptionally low sliding friction and wear. These films were tailored to contain low-energy basal stacking fault planes parallel to the film surface. Cross-sectional high-resolution TEM studies inside friction-induced surfaces coupled with density functional theory calculations revealed the fundamental mechanism of this unexpected behavior. We show that sliding on {0002}-textured nanocrystalline ZnO films increases partial dislocation density through intrafilm shear, via glide of partial dislocations, on {0002}-stacking fault planes.
TL;DR: In this paper, the authors examined the case where permanent deformation occurs prior to an apparent yield point and found that the percentage of tests displaying creep correlates with the percentage that display plasticity prior to pop-in.
Abstract: In nanoindentation, when stresses near the theoretical strength are reached, it is commonly assumed that the volume tested is dislocation free. This study examines the case where permanent deformation occurs prior to an apparent yield point. Force-modulated creep and quasi-static (QS) nanoindentation tests were conducted on Co, W, Ir, and Pt. Statistical comparisons show that the percentage of tests displaying creep correlates with the percentage of QS tests displaying plasticity prior to pop-in. This is evident that apparent plastic behavior prior to pop-in is due to dislocation motion, and permanent deformation can and does occur at extremely small indenter displacements before pop-in.