Identifying the character and the source of partial dislocations associated phase transformation from a hexagonal close-packed (HCP) structure to a face-centered cubic (FCC) structure is essential for understanding phase transformation mechanisms, but was rarely done using microscopy. Here, we report a stress-induced HCP to FCC phase transformation in pure hafnium during cold rolling. Detailed transmission electron microscopy investigations revealed that transformation-related partials stemmed from the dissociation of -type dislocations. Successive gliding of partials on every other basal plane resulted in the orientation relationship between the two phases of <112¯0>hcp//<110>fcc, <101¯0>hcp//<1¯12>fcc and [0001]hcp//<111>fcc.Besides, a new way to form a twin relationship in the FCC structure was discovered.Download high-res image (475KB)Download full-size image

•Deformation mechanism of a commercial pure Zr during cold-rolling was studied.•Grain refinement of Zr during cold-rolling was systematically investigated.•Two different HCP → FCC phase transitions were observed and discussed.•The influence of phase transitions on the mechanical properties was discussed.The grain refinement process and phase transition phenomena of a cold-rolled zirconium have been systematically investigated. The grain refinement was accomplished via a sequential series of dislocation behaviors, reaching a final grain size of < 100 nm. The refinement process can be described as: (1) dislocation nucleation, multiplication and tangling inside the original coarse-grains, (2) formation of microbands via dislocation rearrangement, (3) further division of microbands into thin laths, (4) split of microbands and thin laths into subgrains and then nanograins. Two different types of HCP-FCC phase transitions were also discovered during the cold rolling process, with the transition mechanisms and their influences on the mechanical properties being discussed.Download high-res image (526KB)Download full-size image

Six dimeric BODIPYs BDP-n (n = 1, 2, 3, 4, 5 and 6) with different linkages were synthesized and their photophysical and electrochemical properties, cell imaging and singlet oxygen generation abilities were studied. The linkages between the two BODIPY units had profound impact on dimeric BODIPYs' fluorescence intensities. The improvement of electron donating ability of the linkage between the two BODIPY units resulted in improved fluorescence intensities. Cell imaging tests indicated dimeric BODIPYs had good biocompatibility with MCF-7 cells. The BODIPY dimers (except BDP-5) show strong fluorescence in MCF-7 Cells. In addition, bandgap and the fluorescence intensities were the two main factors which influenced the singlet oxygen generation ability. A larger bandgap lead to stronger singlet oxygen generation abilities. We believe that combined well cell imaging and singlet oxygen generation ability endows dimeric BODIPYs a promising perspective in the field of photodynamic therapy.Download high-res image (147KB)Download full-size image

Herein, we report a facile FeCl3-catalyzed oxidation reaction of dimeric BODIPY monomers with different spacers to construct porous organic polymers. The singlet oxygen generation capacities of these polymers are dependent on the spacers.

Capturing volatile radionuclide iodine from nuclear and medical waste streams is an important environmental issue. In this work, we found that the 2,6-position hydrogen atoms of a BODIPY core undergo fast iodination with volatile iodine at room temperature. Inspired by our observation, two novel BODIPY-based conjugated porous polymers (CPPs) BDP-CPP-1 and BDP-CPP-2, and the reference compound NBDP-CPP, were prepared, which were designed and then synthesized via the Sonogashira cross-coupling reaction of 1,3,5-triethynyl-benzene (TEB) and dibromo-substituted derivatives. With the coexistence of the BODIPY units and plenty of triple bonds and phenyl rings that could adsorb iodine with high capacity and affinity, compounds BDP-CPP-1 and BDP-CPP-2 exhibited satisfactory iodine adsorption capacities of 2830 mg g−1 and 2230 mg g−1, respectively. Moreover, BDP-CPP-1 was shown to adsorb volatile iodine through a chemical mechanism involving the 2,6-position hydrogen atoms of the BODIPY core. Surprisingly, the active sites on the BODIPY units for a chemical iodination reaction were mostly eliminated as a result of the crosslinking of BODIPY units during the Sonogashira coupling reaction. The preliminary results demonstrated that the iodine uptake abilities, which are in the order of BDP-CPP-1 > BDP-CPP-2 > NBDP-CPP, are not only dependent on the surface area, but also on the BODIPY units. The BDP-CPPs show high thermal stability with a decomposition temperature of about 300 °C. In addition, the BDP-CPPs demonstrated remarkable recyclability. Due to the highly π-conjugated porous structure along with the high affinity for iodine molecules and iodination sites, some BODIPY-based CPPs may provide a feasible pathway to adsorb other volatile compounds.

In this study, flat products of carbon nanotubes (CNTs) reinforced pure aluminum (Al) composites were fabricated by spark plasma sintering and hot rolling. The effects of sintering temperature and CNTs content on the microstructures and mechanical properties of the composites were investigated. It has been shown that the composite reinforced by 0.75 vol% CNTs and sintered at 630 °C has the best comprehensive mechanical properties, due to the combined positive effects of the good Al-CNTs interfacial bonding, full densification of the composite and the uniform dispersion of the CNTs. It has been found that the load transfer strengthening and dispersion strengthening of CNTs are the dominant strengthening mechanisms in the composites. The study provides a guidance for manufacturing the flat products of Al/CNTs composites with high strength and good ductility.

•Amorphization was observed in a FeCoCrNi HEA subjected to high-pressure torsion.•Amorphous phase nucleated and grew at twin-twin intersected regions.•The expansion of amorphous phase was restricted by the intersected twin boundaries.Using high-resolution electron microcopy, amorphization was observed in a FeCoCrNi high-entropy alloy subjected to high-pressure torsion. The amorphous phase nucleated and grew at twin-twin intersected regions, while its expansion was restricted by the intersected twin boundaries. We show direct experimental evidence that the amorphization in FeCoCrNi high-entropy alloy was driven from the defects accumulation under high pressure and shear stress. These observations provide new insight into the twin-twin intersection and deformation mechanism of high-entropy alloy.Download high-res image (561KB)Download full-size image

•The as-cast AlCoCuFeNi-(Cr,Ti) HEAs have a multi-phase microstructure.•Short-range ordering of Al-Ni, Co-Cr, Cr-Fe and Ti-Co exists in liquid structure.•Segregations and Cu-rich phase appear in AlCoCuFeNi alloy after adding Cr and Ti.•Adding Ti and Cr elements affects the mechanical properties and corrosion behavior.The equimolar AlCoCuFeNi-(Cr,Ti) high entropy alloys (HEAs) were synthesized by nonconsumable arc melting to investigate the effects of Cr and Ti on the mechanical and corrosion properties of HEAs. The results showed that as-cast AlCoCuFeNi-(Cr,Ti) HEAs have a multi-phase microstructure, of which the solid-solution face-centered cubic (FCC), body-centered cubic (BCC) phases, and intermetallics can be observed. Ab initio molecular-dynamics (AIMD) simulations exhibit the existence of the preferred short-range ordering of Al-Ni, Co-Cr, Cr-Fe, and Ti-Co pairs in the AlCoCuFeNiCrTi liquid structure. The AIMD simulations are consistent with the experimental observation during solidification. The segregations and the FCC Cu-rich phase appear in the AlCoCuFeNiCrTi alloy, which is in agreement with AIMD calculations. The Cr addition to AlCoCuFeNi facilitates the formation of the BCC phases in the AlCoCuFeNiCr alloy, which can be explained by the larger Ω and smaller δ values. The addition of large Ti atoms facilitates the formation of the FCC phase, which is due to the fact that Ti will easily induce the breakdown of the BCC solid-solution of the AlCoCuFeNi alloy in terms of decreasing the Ω value and increasing the δ value. The Cr addition improves the corrosion resistance of AlCoCuFeNi alloys.Download high-res image (245KB)Download full-size image

•Pure Al powder and CNTs were used to fabricate CNTs reinforced Al matrix composites.•A non-equilibrium interface was obtained through SPS sintering and hot extrusion.•The new interface keeps the non-equilibrium state prior to the formation of Al4C3.•The new interface can greatly improve the tensile properties of the composite.•The fine Al powder is preferred to achieve a high performance of the composites.The weak interfacial bonding between carbon nanotubes (CNTs) and Al matrix is a critical issue for the achievement of high strength and good ductility of Al/CNTs composites, and thus hinders the wide application of the composites. Here we obtained a non-equilibrium interface that can provide tight interfacial bonding between the CNTs and Al matrix in the Al/CNTs composites fabricated through spark plasma sintering (SPS) and subsequently hot extrusion. This special interface, accompanied by small grain size, the uniform dispersion and the integrity of the CNTs, can significantly improve the mechanical properties of the Al/CNTs composite. Additionally, the effect of initial Al matrix powder size on the mechanical properties of the composites were investigated. The results indicate that the size of the initial matrix powder affected the dispersion of the CNTs and the interface between the CNTs and Al matrix, and thus influenced the mechanical properties. This work provides a new designation for the high performance of metal based composites with excellent interfacial bonding strength.Download high-res image (482KB)Download full-size image

•A facile method for measuring volume fraction of nano-precipitates based on CBED•An equation to compensate for small invisible precipitates, with 3DAP verification•Precisions around ± 16% for volume fraction and ± 11% for number densitySize, number density and volume fraction of nano-precipitates are important microstructural parameters controlling the strengthening of materials. In this work a widely accessible, convenient, moderately time efficient method with acceptable accuracy and precision has been provided for measurement of volume fraction of nano-precipitates in crystalline materials. The method is based on the traditional but highly accurate technique of measuring foil thickness via convergent beam electron diffraction. A new equation is proposed and verified with the aid of 3-dimensional atom probe (3DAP) analysis, to compensate for the additional error resulted from the hardly distinguishable contrast of too short incomplete precipitates cut by the foil surface. The method can be performed on a regular foil specimen with a modern LaB6 or field-emission-gun transmission electron microscope. Precisions around ± 16% have been obtained for precipitate volume fractions of needle-like β″/C and Q precipitates in an aged Al-Mg-Si-Cu alloy. The measured number density is close to that directly obtained using 3DAP analysis by a misfit of 4.5%, and the estimated precision for number density measurement is about ± 11%. The limitations of the method are also discussed.

The combined effects of isothermal annealing and pre-compression on the mechanical properties of Cu36Zr48Al8Ag8 bulk metallic glass (BMG) were investigated. The as-cast specimens were first annealed at 743 K for 10 min, and then pre-compressed under 800 MPa for 1, 3, 5 and 10 h, respectively. The results indicated that annealing resulted in the formation of nanocrystals with a diameter of ~10 nm in the amorphous matrix and a drastic decrease of the free volume, leading to complete loss of the plasticity of the BMG. Applying pre-compression under a stress of 800 MPa for a proper duration (5 h) resumed part of the lost free volume in the BMG matrix and therefore partially recovered the plasticity. A very long period of pre-compression (10 h) decreased the free volume again, which was caused by the excessive crystal growth.

Shear banding is an important factor affecting the mechanical properties of metallic glasses. This paper decodes the relationship between hardness and shear banding illustrated by displacement bursts under nanoindentation. Both strain softening caused by shear banding and strain hardening after shear banding contribute to the hardness. It was concluded that randomly distributed large sized displacement bursts lead to more intense straining hardening and therefore a higher hardness increase than the uniformly distributed small sized bursts.

Nanoindentation is a useful tool for investigating creep behavior of materials due to its fast testing process and limited sample size requirement. Stress exponent, as an important parameter for describing the creep behavior of materials, can be extracted from nanoindentation testing. However, the reliability of the stress exponent of bulk metallic glasses obtained using nanoindentation has not been confirmed. Here we explored systematically factors that affect the measured stress exponent of bulk metallic glasses from nanoindentation data. Our results show that both artificial factors and local structural diversity influence significantly the measured stress exponent. Artificial factors include the selection of the steady stage from the experimental log(strain rate)–log(stress) curve and the starting point for curve fitting from original displacement–time (h–t) curve during the calculation process of the stress exponent. Structural diversity results in the change of initial creep depth, creep displacement during the continuous creep process and noticeable serrations. Stress exponent is more sensitive to the creep displacement during the continuous creep process and noticeable serrations, both of which affect significantly the displacement rate, than to the initial creep depth.

The effects of multi-axial compression (MAC) on the microstructural evolution of pure Al and Al–4 wt%Mg alloy were investigated by means of TEM and EBSD techniques. It shows that the addition of high concentration solute Mg to Al changes the grain refinement process. Most importantly, new high-angle grain boundaries form via different mechanisms for pure Al and Al–4 wt%Mg alloy: orientation splitting for pure Al and shear banding at low strain and orientation splitting at high strain for Al–4 wt%Mg alloy. In addition, the solute Mg can hinder the dislocation slip and retard the dynamic recovery during the deformation process. Consequently, the Al–4 wt%Mg alloy exhibits a slower misorientation evolution compared to pure Al, and obvious differences in hardness and microtexture evolution exist between pure Al and Al–4 wt%Mg alloy.

In this study, the precipitation sequence of a 5 vol.% SiC particles reinforced Al-1.12 wt.%Mg-0.77 wt.%Si alloy composite fabricated by traditional powder metallurgy method was investigated by transmission electron microscopy and hardness measurements. The results indicated that the addition of SiC reinforcements not only suppresses the initial aging stage but also influences the subsequent precipitates. The precipitation sequence of the composite aged at 175 °C can be described as: Guinier-Preston (G.P.) zone → β″ → β′ → B′, which was confirmed by high-resolution transmission electron microscopy. This work might provide the guidance for the design and fabrication of hardenable automobile body sheet by Al-based composites with enhanced mechanical properties.

Homogeneous creep is a commonly observed phenomenon in bulk metallic glasses. Here we reported inhomogeneous creep behavior that occurs under nanoindentation when the applied stress exceeds the yield stress. Extensive investigation showed that inhomogeneous creep is associated with the local microstructure and the operation of shear bands before creep. The mechanism responsible for inhomogeneous creep is discussed.

Au@Cu2O core–shell nanostructures are fabricated to have a plasmon enhancement effect using Au nanorods (Au NRs) as a plasmon-tailorable core. By varying the concentration of Au NRs, we can tune the shell thickness in the range of 10–25 nm. The shell is composed of Cu2O nanocrystallites. Because of the thin shells, the extinction spectra at wavelength >500 nm are dominated by the Au core. However, the large dielectric constant of the shell causes an obvious red shift of the surface plasmon resonance (SPR) band of the Au nanorod. Besides, transverse octupolar SPR appears as a result of the anisotropy of the core and the high dielectric constant of the shell. The anisotropic geometry of the Au NR is found to support the octupolar resonances at smaller sizes than for their spherical counterpart. Theoretical simulations indicate that the transverse SPR bands are divided into two resonances, which are dipolar- and octupolar-dominant, respectively. The Cu2O shell degrades via a defect-mediated oxidative pathway, which is aggravated upon longitudinal SPR excitation. The SPR-mediated local field enhancement and resonance energy transfer are found to enhance the excitation of the defect states in the shell, thus providing a simple yet selective probing strategy for defect states.

In this paper, a Ti–Al3Ti core–shell structured particle reinforced pure Al based composite was fabricated by powder metallurgy technique. The composite has high compressive strength and ductility since the interface is clean and tight, and the propagation of the nucleated cracks in the shell during deformation can be effectively inhibited by soft Al matrix and Ti core. Although the residue pores and voids readily develop into large-sized pores in the matrix under tension, the composite still shows promising tensile mechanical properties.

We report partial dislocation emission from subgrain boundaries with subgrain size being significantly larger than 100 nm in an Al–Mg alloy. The dislocation density in subgrain boundary has significant effect on partial dislocation emission. The high stacking fault density inside a subgrain is related to the large dislocation density in the subgrain boundary.

Al-based composites reinforced by Al-Fe intermetallic compounds have been fabricated by powder metallurgy technique. The reinforcements were formed in the aluminum matrix by in situ solid-state reaction between pure Al and pure Fe powders. The effects of sintering atmosphere on the microstructures and mechanical properties of the composites were systematically studied by scanning electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction analysis, and compressive tests. It has been shown that Al-Fe intermetallic particles (including a large number of the Fe-Al5Fe2 core-shell structured particles) were the dominant reinforcements in the composites sintered under Ar atmosphere, while pure Fe particles were the dominant reinforcements in the composites sintered under N2 atmosphere. N2 atmosphere is more effective than Ar atmosphere in increasing the sintered density of the composites due to the formation of aluminum nitride, which can effectively fill the pores. Thus, the compressive mechanical properties of the composites sintered under N2 atmosphere are higher than those of the composites sintered under Ar atmosphere.

The local mechanical properties and the serrated flow behavior of a Cu36Zr48Al8Ag8 bulk metallic glass (BMG) have been investigated using nanoindentation. The influence of structural inhomogeneity on the deformation behavior of the BMG is confirmed. The loading rate plays little role in the deviation degree of load–displacement (P–h) curves. It was found that the accumulation of different kinds of serrations leads to the deviation of the P–h curves under a low loading rate. The deviation degree is associated with the numbers and characteristics of the serrated flow. Under a high loading rate, the simultaneous operation of multiple shear bands leads to the disappearance of the serrated flows, and the deviation degree of the P-h curves with each other is assumed to be related to the accumulation of different inconspicuous serrated flows.

The β″ precipitates in a peak-aged Al–Mg–Si–Cu alloy were measured with an average composition of 28.6Al–38.7Mg–26.5Si–5.17Cu (at.%) using atom probe tomography. High-angle annular dark-field observations revealed that Cu incompletely substitutes for the Mg1 and Si3 columns, preferentially for one column in each pair of Si3. Cu-free Si columns form a parallelogram-shaped network that constitutes the basis of subsequent precipitates in the system, with a = 0.37 nm, b = 0.38 nm, γ = 113° and c = 0.405 nm.

The poor plasticity of bulk metallic glasses (BMGs) has severely limited their potential structural applications. In this paper, we reported that uniaxial pre-compression below the yield stress can be used to significantly improve the plasticity of BMGs. An as-cast Cu36Zr48Al8Ag8 BMG was pre-compressed under various pressures (400, 800 and 1600 MPa) and durations (30, 60 and 120 min). The pre-compression processes led to structural change that increases the free volume and produces nanocrystals in the BMG matrix, which subsequently improved the plasticity of the BMG. Detailed data analysis showed that increasing the applied pressure speeds up the structural relaxation process and nanocrystallization occurs only at high enough pressure. This investigation indicated that appropriate selection of the pressure and duration of a pre-compression process is critical for the improved plasticity of BMGs.

The mechanical behaviors of single crystalline micro-sized tungsten whiskers were experimentally characterized using nanoindentation. Nucleation and multiplication of dislocations were seen as displacement burst or pop-in events in the load–displacement curves during loading segment. The Hertzian contact theory and Kick's law were used to describe the elastic deformation of the material, whereas the theoretical strength of the perfect tungsten whiskers can be derived from the critical pop-in load. The elastic–plastic deformation after the pop-in agrees well with the Taylor relation. Either only one major pop-in or a series of discontinuities was observed in the experiments, attributed to the different critical loads at which the first pop-in occurs. Creep behaviors of the whiskers were also investigated under a constant load for 600 s. The stress exponent n was calculated using the displacement–time curve and agrees well with the value obtained using the traditional tensile creep testing method. The creep behavior is dominated by the dislocation-controlled (power-law) mechanism with an n of ~5. Additionally, the stress exponent n is proved to increase with increasing the holding load, and remains stable with increasing the loading rate in the range of testing parameters selected in the experiments.

•W/WO2.72 heterostructures were synthesized by the chemical vapor deposition process.•The long and straight central axial W whiskers grow along [1 1 0] direction.•WO2.72 grows on side surface of the W whiskers, with [0 1 0] as the growth direction.•The heterostuctures have enhanced field emission property over the W whiskers.One dimensional W/WO2.72 heterostructures were successfully synthesized using WO3 as the raw material by a simple two-step chemical vapor deposition process. The morphology and microstructure of the W/WO2.72 heterostuctures were characterized using scanning electron microscopy and transmission electron microscopy. The results indicate that the long and straight central axial W whiskers grow along [1 1 0] direction, while the branched WO2.72 nanowires grow on the side surface of the W whiskers along the radial direction, with [0 1 0] as the growth direction. The as-synthesized heterostuctures exhibit enhanced field emission property over the single W whiskers, and could be used as a candidate for field-emission devices and ultrahigh sensitivity sensors due to their unique composition and structure.A SEM image shows that a large number of the nanowires grow on the side surface of the W whisker stems.

Capturing volatile radionuclide iodine from nuclear and medical waste streams is an important environmental issue. In this work, we found that the 2,6-position hydrogen atoms of a BODIPY core undergo fast iodination with volatile iodine at room temperature. Inspired by our observation, two novel BODIPY-based conjugated porous polymers (CPPs) BDP-CPP-1 and BDP-CPP-2, and the reference compound NBDP-CPP, were prepared, which were designed and then synthesized via the Sonogashira cross-coupling reaction of 1,3,5-triethynyl-benzene (TEB) and dibromo-substituted derivatives. With the coexistence of the BODIPY units and plenty of triple bonds and phenyl rings that could adsorb iodine with high capacity and affinity, compounds BDP-CPP-1 and BDP-CPP-2 exhibited satisfactory iodine adsorption capacities of 2830 mg g−1 and 2230 mg g−1, respectively. Moreover, BDP-CPP-1 was shown to adsorb volatile iodine through a chemical mechanism involving the 2,6-position hydrogen atoms of the BODIPY core. Surprisingly, the active sites on the BODIPY units for a chemical iodination reaction were mostly eliminated as a result of the crosslinking of BODIPY units during the Sonogashira coupling reaction. The preliminary results demonstrated that the iodine uptake abilities, which are in the order of BDP-CPP-1 > BDP-CPP-2 > NBDP-CPP, are not only dependent on the surface area, but also on the BODIPY units. The BDP-CPPs show high thermal stability with a decomposition temperature of about 300 °C. In addition, the BDP-CPPs demonstrated remarkable recyclability. Due to the highly π-conjugated porous structure along with the high affinity for iodine molecules and iodination sites, some BODIPY-based CPPs may provide a feasible pathway to adsorb other volatile compounds.