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Charge collection narrowing (CCN) has been reported to be an efficient strategy to achieve optical filter-free narrowband photodetection (NPD) with metal halide perovskite (MHP) single crystals. However, the necessity of utilizing thick crystals in CCN limits their applications in large scale, flexible, self-driven, and high-performance optoelectronics. Here, for the first time, we fabricate vertically integrated MHP quantum wire/nanowire (QW/NW) array based photodetectors in nanoengineered porous alumina membranes (PAMs) showing self-driven broadband photodetection (BPD) and NPD capability simultaneously. Two cutoff detection edges of the NPDs are located at around 770 and 730 nm, with a full-width at half-maxima (fwhm) of around 40 nm. The optical bandgap difference between the NWs and the QWs, in conjunction with the high carrier recombination rate in QWs, contributes to the intriguing NPD performance. Thanks to the excellent mechanical flexibility of the PAMs, a flexible NPD is demonstrated with respectable performance. Our work here opens a new pathway to design and engineer a nanostructured MHP for novel color selective and full color sensing devices.

Oral administration is a facile and safe way for medication. However, most of the reported nanomedicines could not be taken orally, partially due to their unsatisfied stability, poor absorbance, or toxicity in the gastrointestinal tract. Here, we demonstrate that we could robustly synthesize gold nanoparticles (GNPs) in vivo by orally administering two starting materials, tetrachloroauric acid and aminophenyl boronic acid (ABA). The ABA-activated GNPs (A-GNPs) synthesized in vivo could be absorbed by the gastrointestinal tract and reach the remote infection lesions such as peritonitis caused by multidrug resistant (MDR) bacteria in mice. The A-GNPs exhibit excellent antibacterial efficacy (MIC, 3 μg/mL), long half-life (16-17 h), effective clearance (residual concentration is near 0 within 72 h), and high biosafety (safe dose/effective dose, 8 times). Our study is a pioneering attempt for synthesizing and taking nanomedicines orally just like preparing and drinking a cocktail.

With the widespread use of antibiotics, the number of complex infection cases caused by unknown pathogens is increasing and novel antibiotics with tunable antibacterial spectra and low toxicity are highly desirable. Herein, we report that, by selecting thiol or amine, two groups with different binding affinities with gold, as anchoring groups, phenylboronic acid can be decorated on gold nanoparticles (AuNPs) with different densities, which contributes to Gram-selective antibacterial activities of the AuNPs. The AuNPs modified with amine- or thiol-tethered phenylboronic acids specifically bind to lipopolysaccharide (LPS, Gram-negative) or lipoteichoic acid (LTA, Gram-positive), respectively. By modifying AuNPs with different ratios of thiol- and amine-tethered phenylboronic acids, the resulting AuNPs show potent and tunable antibacterial activity. The AuNP-based antibacterial agents with optional Gram selectivity are promising for applications in personalized therapy.

The effect of pre-compression deformation on the microstructural evolution, mechanical property and deformation behavior of AZ31 magnesium alloy was studied. The alloy was microstructurally characterized with a combination of optical microscopy, electron backscatter diffraction, X-ray diffraction and energy-dispersive spectrometer. The results indicate that average number of twins per grain and dislocation density increases with an increase in compressive pre-deformation level. Stress–strain curves of low pre-strained samples (0%, 1% and 3%) present concave-up features, whereas concave-down shapes are detected for high pre-strained samples (6% and 8%) in uniaxial compression tests. The corresponding underlying deformation mechanism transists from a twin-dominated mechanism to a slip-dominated mechanism. This deformation mechanism transition is related to {10–12} twinning, which leads to grain refinement and crystal orientation change, as well as increased dislocation density. The yield strength linearly increases with increasing equivalent grain size regardless of the deformation mechanisms. Quantitative analysis reveals that the contributions from texture strengthening, twinning/grain boundary strengthening and dislocation strengthening increase as pre-compression level increases. Texture strengthening is the dominant strengthening mechanism for high pre-strained samples.

The shuttling behavior and sluggish conversion kinetics of the intermediate lithium polysulfides (LiPS) represent the main obstructions to the practical application of lithium–sulfur batteries (LSBs). Herein, a 1D π–d conjugated metal–organic framework (MOF), Ni-MOF-1D, is presented as an efficient sulfur host to overcome these limitations. Experimental results and density functional theory calculations demonstrate that Ni-MOF-1D is characterized by a remarkable binding strength for trapping soluble LiPS species. Ni-MOF-1D also acts as an effective catalyst for S reduction during the discharge process and LiS oxidation during the charging process. In addition, the delocalization of electrons in the π–d system of Ni-MOF-1D provides a superior electrical conductivity to improve electron transfer. Thus, cathodes based on Ni-MOF-1D enable LSBs with excellent performance, for example, impressive cycling stability with over 82% capacity retention over 1000 cycles at 3 C, superior rate performance of 575 mAh g at 8 C, and a high areal capacity of 6.63 mAh cm under raised sulfur loading of 6.7 mg cm. The strategies and advantages here demonstrated can be extended to a broader range of π–d conjugated MOFs materials, which the authors believe have a high potential as sulfur hosts in LSBs., D.Y., Z.L., and P.T. contributed equally to this work. This work was supported by the European Regional Development Funds and by the Spanish Ministerio de Economía y Competitividad through the projects ENE2016-77798-C4-3-R and PID2020-116093RB-C43. D.Y. and C.Z. thank the China Scholarship Council for the scholarship support. M.T. thanks the financial support from the National Natural Science Foundation of China (Grants 21974007 and U1930401). Z.L. acknowledges funding from MINECO SO FPI Ph.D. Grant (SEV-2013-0295-17-1). P.T. acknowledges the Humboldt Research Fellowship. Authors acknowledge funding from Generalitat de Catalunya 2017 SGR 327 and 2017 SGR 1246. ICN2 acknowledges the support from the Severo Ochoa Programme (MINECO, Grant no. SEV-2017-0706). IREC and ICN2 are both funded by the CERCA Programme/Generalitat de Catalunya. This work was supported by Fundamental Research Funds for the Central Universities (buctrc202112). J.L. is a Serra Húnter Fellow and is grateful to MICINN/FEDER RTI2018-093996-B-C31, GC 2017 SGR 128 and to ICREA Academia program. This project received funding from the European Union's Horizon 2020 Research and Innovation Programme under Grant agreement no. 823717 – ESTEEM3. Part of the present work was performed in the framework of Universitat Autònoma de Barcelona Materials Science Ph.D. program.

Powder metallurgy is a popular method of making raw powders into specific shaped samples. However, the pressure distribution and the microhardness difference within the sample are nonnegligible and unclear when the sample is long or exceeds a specific size. In this study, the long magnesium blocks, with a ratio of about 2.8 between the sample height and the sample side length, are successfully synthesized under three uniaxial and two biaxial conditions. Then, the sample hardness values on the outer surface and the center plane are tested to study the microhardness distribution. The modified analytical expression indicates that the normal pressure exponentially decreases along the compression direction, which is consistent with the hardness distribution trend. Because higher pressure leads to a more compact arrangement of the powders, more metal bonds are formed after sintering. During the first pressing, the sidewall pressure makes the surface hardness higher. The secondary reverse compression mainly improves the bottom and core hardness due to the re-orientation and re-location of the powders. The obtained relationship between the applied pressure and the hardness distribution is instructive in predicting and improving the sample quality.

Hydrogen embrittlement of Inconel 718 alloy was investigated. Multi-scale observation technique were employed, comprising slow strain rate tensile tests, scanning electron microscopy and transmission electron microscopy analysis. The results demonstrate that hydrogen charging deteriorates mechanical properties of the alloy. Inconel 718 alloy shows partial Portevin-Le Chatelier (PLC) effect at room temperature when hydrogen charging current density is 220 mA cm−2 and 590 mA cm−2. Moreover, plastic deformation features with dislocation cells are detected in hydrogen-induced brittle zone. Thus, it is concluded that dragging effect of hydrogen atoms on dislocations contributes to PLC effect.

In this study, we synthesized Mo0.9W0.1Te2 nanobelts on the SiO2/Si substrates without using catalyst by a facile chemical vapour deposition method. Through detailed structural and compositional characterizations, we found that Mo0.9W0.1Te2 nanobelts have a thickness of ∼50 nm with a width-to-thickness ratio of 3–5 and a length up to tens of μm. Besides, the grown nanobelts have a monoclinic structure with a growth direction of [010] and the {001} belt surface. Moreover, a vapour-solid growth mechanism is proposed to explain the growth of Mo0.9W0.1Te2 nanobelts. This study demonstrated a simple synthesis method for the growth of single crystalline MoxW1-xTe2 nanomaterials, which is significant for the development of MoxW1-xTe2-based electronic and spintronic devices.

Twinning process and dislocation characteristics were determined by means of quasi in-situ electron backscatter diffraction and high-resolution transmission electron microscopy techniques in a rolled AZ31 magnesium alloy under different strain paths (compression along rolling direction and tension along normal direction). It is demonstrated that the activation of different twin variants depends on the strain path, and thus these twin variants can further induce different texture characteristics. In addition, two independent slip systems were activated during deformation of both strain paths. However, the dislocation slip modes of the two kinds of deformation are different, resulting in different stress-strain response.

A CrxCy–NiCr cermet coating was fabricated using the supersonic plasma spray technique, and its effect on the hydrogen embrittlement of 17-4 PH stainless steel was studied through electrochemical hydrogen charging and slow strain rate tensile testing. The index of hydrogen embrittlement susceptibility was calculated, and the results show that the CrxCy–NiCr coating plays a significant role in preventing hydrogen embrittlement cracking. The hydrogen content analysis demonstrates that the CrxCy–NiCr coating enhances the resistance to hydrogen embrittlement by acting as a hydrogen diffusion barrier. This effect is mainly due to the CrxCy ceramics in the cermet coating having a very low hydrogen diffusion rate. In addition, the strength and ductility of the specimen without the coating decrease gradually with increasing hydrogen charging current density. According to the tensile testing results and the fracture surface analysis, the hydrogen embrittlement phenomenon is explained by a mixed mechanism involving hydrogen-enhanced local plasticity and hydrogen-enhanced decohesion.

Stress corrosion cracking and hydrogen embrittlement of Jethete M152 martensitic stainless steel were investigated by using constant load and slow strain rate tension test, optical microscope, transmission electron microscope and scanning electron microscope. The results show that lath martensite was formed and no austenite was observed. Stress corrosion cracking and hydrogen embrittlement were intensified with enhancing stress level and increasing hydrogen charging current density. Fractographic analysis revealed that corrosive environment resulted in the distinct change of fracture morphology from dimple to intergranular. It is indicated that undeformable Al2O3 inclusion is an important factor in stress corrosion cracking and hydrogen embrittlement for Jethete M152 steel. Meanwhile, it is beneficial to enhance resistance to stress corrosion cracking and hydrogen embrittlement by decreasing the inclusion size.

Hydrogen embrittlement behaviors of PH17-4 steel and PH13-8Mo steel used as steam turbine last stage blades were investigated by means of slow strain rate tensile tests and hydrogen permeation tests. The results indicate that PH13-8Mo steel exhibits higher strength level mainly due to higher precipitate strengthening and lath strengthening compared with PH17-4 steel in the absence of hydrogen. Moreover, pre-hydrogen charging does not affect tensile strength of PH17-4 steel, but decreases tensile strength of PH13-8Mo steel. The plasticity of PH17-4 steel and PH13-8Mo steel is degraded after hydrogen charging and the deterioration is accompanied with considerable change in fracture mode. The loss of plasticity of PH13-8Mo steel is higher than that of PH17-4 steel, indicating that PH13-8Mo steel exhibits lower resistance to hydrogen embrittlement, which is mainly related to higher hydrogen diffusion behaviors and higher strength level of PH13-8Mo steel. Additionally, PH17-4 steel exhibits lower apparent hydrogen diffusion coefficient and higher apparent hydrogen solubility in comparison with those of PH13-8Mo steel, which implies that incoherent Cu-rich precipitates in PH17-4 steel possess higher capability to trap hydrogen atoms than that of coherent NiAl precipitates in PH13-8Mo steel.

This research explored the equal channel angular pressing (ECAP) routes for efficient microstructure refinement of pure magnesium. The samples were processed for a series of numbers of ECAP passes through three different routes (i.e. A, Bc, and C routes). The utilized ECAP mold had a large channel angle of 142° and the processing temperature was 200 °C. The microstructure and texture of the processed samples were characterized using optical microscope and x-ray diffraction technique. The results show that tension twinning deformation mechanism during an ECAP pass led to the formation of profuse twins in magnesium and the twinning/detwinning dominated microstructure refinement mechanisms. After 8 ECAP passes, both Bc and C routes reduced the length and width of twins to ~ 13 µm and ~ 3 µm respectively. Bc and C routes can refine the microstructure of magnesium more efficiently than A route. Among the three studied routes, Bc route achieved the most homogeneous microstructure with uniformly distributed twins, while both A and C routes resulted in heterogeneous microstructure.

The compressive mechanical properties, microstructural evolution and fracture behaviors of AZ31 alloys with equiaxed grains (E-grain) or bimodal grains (B-grain) were investigated using optical microscopy, scanning electron microscopy and electron backscattered diffraction. The results indicate that the ratio of twinned grains to all grains increases for both E-grain and B-grain samples with increasing strain levels during compression deformation along the extruded direction. The average Schmid factor for {10–12} twins of B-grain samples is higher than that of E-grain samples, which lowers required activating stress for {10–12} twins in B-grain samples. This causes a higher quantity of twins in B-grain samples than that of E-grain samples at the same pre-compression level. The E-grain samples mainly exhibit parallel twins regardless of pre-compression levels, whereas the twins in coarse grains of B-grain samples are parallel to each other at low compressive strain level and then transform to crossed twins at high compressive strain level with parallel twins in fine grains. Additionally, the peak strength, strain hardening rate and fracture strain of E-grain samples are lower than those of B-grain samples, and they possess similar yield strength. The coarse grains in the B-grain samples effectively resist crack growth by deflecting and branching crack tips, bridging of the cracks and forming the secondary cracks. Meanwhile, the fine grains in B-grain samples accommodate plastic strain and mediate multiple grains’ deformation. The concurrent effect of coarse grains and fine grains results in the improvement of plasticity.

The present study was attempted to evaluate the hydrogen embrittlement (HE) of commercial 20SiMn2CrNiMo, PSB1080 and PH13-8Mo high strength steels using slow strain rate tensile tests with smooth cylindrical tensile specimens. The results reveal that three steels exhibit the superior strength and plasticity in the absence of hydrogen. However, the mechanical properties are deteriorated after hydrogen charging and the resistance to HE is 20SiMn2CrNiMo steel, PH13-8Mo steel and PSB1080 steel, which is ranked in a decreasing order. Fine lath boundaries in 20SiMn2CrNiMo steel disperse hydrogen atoms, reduce local hydrogen accumulation and then result in the lowest HE. Coherent NiAl precipitates in PH13-8Mo steel and O-Mg-Al-Si-Ca type inclusions in PSB1080 steel are responsible for the high HE susceptibility due to the concurrent effect of the stress concentration and hydrogen-enhanced decohesion mechanism. It is thus suggested that the inclusions in steels should be eliminated and the efforts concerning to obtain low HE susceptibility in steels strengthened by the coherent precipitates should be conducted to guarantee the safe service of engineering components fabricated from high strength steels.

This study aims to modify Ti64 surface to improve its potential bioactivity and wear resistance. The polished Ti64 surfaces were hydrothermally treated in urea solutions with or without the subsequent heat treatment. Hydrothermal treatment increases the hydrophilicity and Vickers hardness of Ti64 surface. Heat treatment deteriorates the hydrophilicity of Ti64 surface although it enhances the surface hardness. Hydrothermal treatment on a polished Ti64 surface in 3 M urea solution achieves a crack-free and crystallized titania film while still retaining its negatively charged surface with high wettability.

This study investigated low-temperature fatigue behavior of fine-grained magnesium through tension–tension fatigue testing at the temperature of 0 °C. The applied stress profiles of fatigue testing were selected to cover a wide range of loading conditions including low, medium, and high loading stresses. Even when the maximum applied stress of fatigue testing was as high as 88% of the ultimate tensile strength of the material, the corresponding fatigue life was long, i.e., 3973 cycles. Deformation accumulation per loading cycle reduced with the progression of fatigue testing for all studied loading conditions. Fatigue life analyses were performed using both Smith-Watson-Topper (SWT) model and Jahed-Varvani energy-based (JV) model. The fatigue life predictions through both models agree well with the experimental data.

This study investigated the effect of the reinforcing phase – carbon nanofiber (CNF) concentration on mechanical properties of fabricated porous magnesium-matrix composites with 34% porosity. The average yield strength and the average ultimate compressive strength increased steeply when the CNF concentration increased from 0.05% to 0.2%, and increased gradually when the CNF concentration increased from 0.2% to 1.5% and from 2% to 5%. The mechanical properties of porous magnesium-matrix composite with 1.5% CNF are the highest among the studied composites, and the average yield strength, the average ultimate compressive strength, and the energy absorption capability were enhanced by 54%, 49%, and 43% respectively from those for porous magnesium without CNF. The Rule of mixture model, Strengthening factor model, and Zhang & Chen model provided the yield strength predictions that match well with the experimental data for the composites with less than 2% CNF, and the strengthening mechanism includes Orowan strengthening and thermal expansion coefficient mismatch strengthening. The Shear Lag model and Rule of mixture model provided the yield strength predictions that match well with the experimental data for the composites with more than 2% CNF, and load transfer from magnesium to CNF is the dominating strengthening factor.

Twinning process involving reversible motion of twin boundaries was examined and quantified in a rolled Mg-3Al-1Zn magnesium alloy compressed along two perpendicular directions. The evolution of twinning is analyzed by quasi in-situ electron backscatter diffraction technique, and the detailed structure of the twin boundary is analyzed using high-resolution transmission electron microscopy technique. The results suggest that both twinning and detwinning are attributed to the mobility of twin boundaries. Low-angle boundaries were identified at the prior twin boundaries after detwinning due to the effect of alloying elements, and these low-angle boundaries will affect subsequent deformation. Twin boundary is serrated and consists of 10 1 ¯ 2 coherent twin boundaries and prism-basal boundaries that control twin boundary migration.

Porous magnesium/carbon nanotube (Mg/CNT) composites with various CNT concentrations (0.05 and 1 wt%) and overall porosities (20, 30, and 40 %) were synthesized by a powder metallurgical method and characterized to study their microstructure and yield strengths. The introduction of pores leads to the reduction of material strength. Thus, it is attractive to add reinforcement phase to produce porous composites and improve yield strength. The porous Mg/CNT composites with low overall porosity have smaller average pore size than the porous Mg/CNT composites with high overall porosity. The compression testing results showed that the average yield strength is about one to four times of the yield strength of sand-cast Mg. Yield strength of porous Mg–0.05 wt%CNT composites is not significantly different from that of porous Mg–1 wt%CNT composites, when both composites have the same overall porosity. The average yield strength along the in-plane direction is larger than that along the normal direction for each type of porous Mg/CNT composite. The Gibson and Ashby model was used to estimate yield strength and the results show that the theoretical estimations of yield strength agree well with the experimental yield strength values for the porous composites with high overall porosity (i.e., 30 or 40 %), while the estimations are lower than the experimental data for the porous composites with low overall porosity (i.e., 20 %).

The effect of testing temperature on mechanical properties and deformation mechanisms was investigated by tensile testing of fine-grained magnesium samples under a range of temperatures (i.e. room temperature 20 °C, subfreezing temperatures 0 °C, −30 °C, and −60 °C) and microstructural observations of both untested and tested samples using electron backscatter diffraction. The results suggest that mechanical properties and deformation mechanisms were significantly affected by the variation of testing temperature. Strength increased linearly and ductility decreased linearly with the decrease of testing temperature. Although twinning is generally known to be athermal, this experimental study shows that tensile twinning is the dominating deformation mechanism at room temperature, and that twinning is negligible and dislocation operation is the primary deformation mechanism at 0 °C, −30 °C, and −60 °C.

Three-dimensional (3D) pore microstructure was investigated for six types of carbon nanotube (CNT) reinforced magnesium (Mg) composite foams with various porosities (i.e. 29%, 39%, and 49%) and compositions (0.05 wt.% CNT and 1 wt.% CNT) using a nondestructive x-ray micro-computed tomography technique. The data were analyzed to explore the effect of overall porosity and carbon composition on 3D pore microstructure such as the number of large pores, pore connectivity, pore size, pore size distribution, pore shape distribution, and specific surface area. The increase of overall porosity resulted in more large and connected pores, and a larger specific surface area. For all studied composite foams, pore size varies in the range of several microns to hundreds of microns; over 80% of the pores have the aspect ratio ≤2 and over 96% of the pores have the aspect ratio ≤3. The volume fractions are very low for the pores with the smallest and biggest sizes. The volume fraction for pores with a size ≥40 μm (i.e. the average size of raw Mg powders) increases from almost 40% to about 50% and then to almost 80% when the overall porosity increases from 29% to 39% and then to 49%. A critical pore size was defined and extracted for each of the studied foams and this size increases with the increase of overall porosity. Additionally, the variation of CNT concentration only slightly affects the pore microstructure. Keywords: Porous magnesium composite, Carbon nanotubes, Porosity, x-ray micro-computed tomography

This study investigated the microstructure, phase transformation and mechanical property of Ni–Mn–Ga particles/Mg composites with a strong interfacial reaction between the particles and the matrix. The strong interfacial reaction was related to the large surface area and energy per unit volume of the flaky shape Ni–Mn–Ga particles that favors the reaction between the particles and matrix. The martensitic transformation behavior was largely weakened due to the interfacial reactions and thus the reduced volume fraction of Ni–Mn–Ga particles. The composites exhibited a much improved compressive strength and ductility in comparison with that of the Ni–Mn–Ga alloy. The compressive plasticity of the composites was decreased when the Ni–Mn–Ga particle content exceeded 40 wt%. In comparison with the Mg-composites with large size Ni–Mn–Ga particles, the composites with small size particles would have a much stronger interfacial reactions, which was detrimental to the phase transformation and mechanical ductility of the composites. The investigation results in this article could provide a reference for the design and preparation of the particles reinforced metal matrix functional composites. Keywords: Ni–Mn–Ga alloys, Mg composites, Powder metallurgy, Microstructure, Phase transformation, Mechanical characterization

A nondestructive x-ray micro-computed tomography technique was utilized to investigate 3D microstructure of porous magnesium composite reinforced by multi-walled carbon nanotubes. The overall porosity of the sample is ~48.5%. The pores distribute uniformly. The pore size is in the range of 1.5–145 μm and its occurrence density increases with the decrease of pore size. The introduction of pores significantly increases the specific surface area. These microstructure features make the material suitable for hydrogen storage and biomedical applications.

This study investigated the fracture behavior and structural transition of Ni 46 Mn 33 Ga 17 Cu 4− x Zr x ( x =0, 2, 4) alloys after undergoing quasi-static compression and ball milling. The introduction of Zr-rich second phase enhanced the compressive strength without improving the mechanical ductility of the alloys. The dispersed second phase formed weak interfacial bonding with the martensitic matrix, which resulted in the low plasticity. After ball milling, the dual phase alloy particles were separated to the second phase particles and the matrix phase particles. The average particle size was reduced with increasing Zr content since the size of the second phase particles was much smaller than that of the matrix particles. Moreover, the structure of the second phase was almost not changed by the ball milling. The martensitic transformation disappears for all the alloys after ball milling because of the disordering of the martensitic matrix. Post-annealing at 800 °C can restore the martensitic transformation of the Cu4 and Cu2Zr2 alloys but is not effective to restore that of the Zr4 alloy.

This study investigated microstructure and phase transformation of Ni46Mn33Ga17Cu4−xZrx (x=0, 2, and 4) alloys. The substitution of Cu by Zr changed the alloy from a single martensite to a dual phase consisting of martensitic matrix and Zr-rich second γ phase at room temperature. The volume fraction of γ phase increased with the increase of Zr content, which resulted in the change of martensitic transformation temperature and transformation enthalpy of the alloys. After aging the dual phase alloys at 400 °C and 600 °C for 10 h, the martensitic transformation temperature was increased and transformation enthalpy was decreased.

Porous alumina-reinforced magnesium composites were synthesized through a powder metallurgical method and characterized using optical microscope, scanning electron microscope, and compression testing. The microstructural study exhibited that the average pore sizes increased with the increase of porosity and were about 25 μm, 70 μm, and 100 μm for the samples with 10%, 28%, and 38% porosities respectively. The mechanical characterization indicated that (i) the stress–strain curves were composed of three regimes: an initial regime that deformed elastically along an approximately linear line, a long and intermediate regime, and a densification regime with a steep increase of stress; (ii) the synthesized porous magnesium composites possessed lower density and higher yield strength than those of cast dense magnesium; (iii) the average yield strength and Young's modulus were anisotropic for the porous magnesium composites synthesized in this work.

0 0 0 1 magnesium single crystal samples were mechanically tested under compressive loadings at a quasistatic strain rate (0.001 s −1 ) and a dynamic strain rate (1000 s −1 ), respectively. The tested samples were then investigated through various microstructural characterization techniques. Dynamic loading led to much higher maximum strength and larger strain hardening rate than quasistatic loading for the tested material. The microstructure features were clearly different from each other for the two loading conditions, which indicated that the deformation mechanisms for the quasistatic testing were different from those for the dynamic testing. The microstructure analysis showed that (a)prismatic and secondary pyramidal dislocation operations happened in the samples under quasistatic loading and dynamic loading, while tension twinning and tension–compression double twinning also happened in the sample under quasistatic loading; (b)the sample under high-strain-rate dynamic loading remained as single crystal, while that under quasistatic loading was refined to become polycrystalline; and (c)twinning operations under quasistatic loading led to grain refinement.

Strain-controlled fatigue experiments were conducted on an extruded AZ61A magnesium alloy at three strain ratios (Rɛ = −∞, −1, 0) using smooth tubular specimens. As the strain ratio decreased, stronger cyclic hardening, more asymmetric hysteresis loop, smaller stress amplitude, lower mean stress, and higher initial plastic strain amplitude were observed. These phenomena were associated with twinning in the compressive phase and detwinning in the tensile phase during cyclic deformation. At the same strain amplitude, fatigue life increased with decreasing strain ratio. The strain-fatigue life curve at each strain ratio exhibited a distinguishable kink. Such a kink point represents a demarcation point above which persistent twinning–detwinning occurs under cyclic loading. Two Smith, Watson, and Topper (SWT) fatigue criteria can predict the fatigue lives of the material at all strain ratios satisfactorily.

Compression–compression cyclic tests were performed on the 0001 and 1014 single crystal samples for 50 loading cycles at room temperature. The compression stress varied in the range of 19 MPa–97 MPa. The tested samples were then studied using various microstructure characterization tools including X-ray diffraction (XRD), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HRTEM). The two samples with different orientations had quite different mechanical behavior and microstructural features. For 0001 single crystal, the strain range and ratcheting strain were constant and very small. On the contrary, the sample heavily strained and deformed for 1014 single crystal, and the ratcheting strain was about ten times of that for 0001 single crystal. There was no cyclic hardening/softening for 0001 single crystal, while 1014 single crystal experienced cyclic strain hardening. Microstructural observations indicated that twinning was the main deformation mechanism for 0001 single crystal and basal slip, pyramidal slip, secondary pyramidal slip, and tension and compression twinning operated for 1014 single crystal. The basal slip resulted in the initial plastic deformation region for 1014 single crystal. The Schmid factor analyses demonstrated that the deformation mechanisms were consistent with those obtained from the microstructural observations.

Polycrystalline magnesium was compressed under different strain rates (0.001, 800, 1000, 2000, and 3600 s−1) to investigate its dynamic mechanical properties, and microstructural characterization was performed to uncover the deformation mechanism. The results show that yield strength is insensitive to strain rate, while ultimate strength, fracture strain, and work hardening rate are highly sensitive to strain rate. Three deformation regimes (I, II, and III) were observed on the quasi-static and dynamic stress–strain curves. These regimes show respectively increasing work hardening rate in the early stage of plastic deformation, constant work hardening rate in the intermediate plastic deformation region, and decreasing work hardening rate in the end region right before fracture. Different deformation mechanisms operate for the quasi-static and dynamic loading conditions. Microscopically, twinning/detwinning is the dominating mechanism for quasi-static testing, while dynamic recrystallization and twinning/detwinning are the dominating mechanisms for dynamic testing. Analytic constitutive models were derived for predicting the dynamic stress–strain relations. The analysis indicated that different factors were in effect for different loading strain rates. The stress–strain relations were primarily affected by strain hardening for quasi-static testing; by strain hardening, strain rate hardening, and thermal softening for dynamic testing with e ˙ ≤ 2000 s − 1 ; and by strain hardening, damping, and thermal softening for dynamic testing with e ˙ > 2000 s − 1 , respectively.

Cyclic deformation and low-cycle fatigue properties of extruded ZK60 magnesium alloy were experimentally investigated by carrying out fully reversed strain-controlled uniaxial tension–compression fatigue experiments along the extrusion direction. The material shows significant yielding asymmetry under monotonic tension and compression. Marginal cyclic hardening is manifested at all the strain amplitudes tested. The shape of the stress–strain hysteresis loop and the mean stress strongly depend on the strain amplitude. When the strain amplitude is higher than 0.52%, asymmetric shape of the stress–strain hysteresis loop and significant tensile mean stress develop. With intermediate strain amplitudes (0.45–0.52%), the stress–strain hysteresis loop evolves from an asymmetric shape at the initial loading cycles to a symmetric shape after approximately 10% fatigue life, accompanied by moderate saturated mean tensile stresses. Symmetric stress–strain hysteresis loop with zero mean stress is observed for the strain amplitudes lower than 0.45%. It is suggested that the twinning–detwinning process plays a dominant role for the deformation at high strain amplitudes while dislocation slip dominates at the low strain amplitudes. The strain–life fatigue curve shows a kinking plateau in the vicinity of 0.52% strain amplitude. Different crack initiation mechanisms are proposed in the two strain amplitude ranges divided by the plateau in the strain–life curve. The material displays a tensile cracking growth behavior and the Smith, Watson, and Topper (SWT) fatigue parameter is appropriate for the description of the fatigue life. The residual twins after fatigue were examined by light microscopy and the fracture surfaces were observed by scanning electron microscopy (SEM). Distinctive features were observed depending strongly on the strain amplitudes. The observed microscopic features are discussed with respect to possible crack initiation and propagation mechanisms.

Thin-walled tubular specimens were employed to study the cyclic deformation of extruded AZ61A magnesium alloy. Experiments were conducted under fully reversed strain-controlled tension–compression, torsion, and combined axial–torsion in ambient air. Mechanical twinning was found to significantly influence the inelastic deformation of the material. Cyclic hardening was observed at all the strain amplitudes under investigation. For tension–compression at strain amplitudes higher than 0.5%, the stress–strain hysteresis loop was asymmetric with a positive mean stress. This was associated with mechanical twinning in the compression phase and detwinning in the subsequent tension phase. Under cyclic torsion, the stress–strain hysteresis loops were symmetric although mechanical twinning was observed at high shear strain amplitudes. When the material was subjected to combined axial–torsion loading, the alternative occurrence of twinning and detwinning processes under axial stress resulted in asymmetric shear stress–shear strain hysteresis loops. Nonproportional hardening was not observed due to limited number of slip systems and the formation of mechanical twins. Microstructures after the stabilization of cyclic deformation were observed and the dominant mechanisms governing cyclic deformation were discussed. Existing cyclic plasticity models were discussed in light of their capabilities for modeling the observed cyclic deformation of the magnesium alloy.

This study investigated the phase transformation of the flaky shaped Ni–Mn–Ga powder particles with thickness around 1 μm prepared by vibration ball milling and post-annealing. The SEM, XRD, DSC and ac magnetic susceptibility measurement techniques were used to characterize the Ni–Mn–Ga powders. The structural transition of Heusler → disordered fcc occurred in the powders prepared by vibration ball milling (high milling energy) for 4 h, which was different from the structural transition of Heusler → disordered fct of the powders fabricated by planetary ball milling (low milling energy) for 4 h. The two different structures after ball milling should be due to the larger lattice distortion occurred in the vibration ball milling process than in the planetary ball milling process. The structural transition of disordered fcc → disordered bcc took place at ∼320 °C during heating the as-milled Ni–Mn–Ga powders, which was attributed to the elimination of lattice distortion caused by ball milling. The activation energy for this transition was 209 ± 8 kJ/mol. The Ni–Mn–Ga powder annealed at 800 °C mainly contained Heusler austenite phase at room temperature and showed a low volume of martensitic transformation upon cooling. The inhibition of martensitic transformation might be attributed to the reduction of grain size in the annealed Ni–Mn–Ga particles.

Strain-controlled multiaxial fatigue experiments were conducted on extruded AZ61A magnesium alloy using thin-walled tubular specimens in ambient air. The experiments included fully reversed tension–compression, cyclic torsion, proportional axial-torsion, and 90° out-of-phase axial-torsion. For the same equivalent strain amplitude, fatigue life under proportional loading was the highest and the nonproportional loading resulted in the shortest fatigue life. Detectable kinks were identified in the strain–life curves for all the loading paths. Fatigue experiments subjected to fully reversed strain-controlled torsion with a static axial load were also conducted. A positive static axial stress reduced the fatigue life and a compressive static axial stress was found to significantly enhance the fatigue life. Two critical plane multiaxial fatigue criteria were evaluated in terms of fatigue life predictions based on the experimental results. The Fatemi–Socie criterion correlated well with the fatigue life in the low-cycle fatigue regime which was characterized by shear cracking. The fatigue life predictions made by the Fatemi–Socie criterion did not agree well with the experimental results in the high-cycle fatigue regime. A modified Smith–Watson–Topper (SWT) criterion was found to be able to predict fatigue lives well for all the loading paths conducted in the current investigation.

Microstructural characterizations were performed on thin walled tubular samples of extruded Mg6Al1ZnA (AZ61A) magnesium alloy that had experienced room temperature fatigue testing. The results show that the initial basal texture and loading amplitude/direction affect the deformation mechanism and microstructural features of AZ61A. Twinning activates under compression when the stress is ∼120 MPa and the strain is ∼0.005. With the strain amplitude is higher than 0.005, twinning–detwinning is the dominant deformation mechanism and residual twins change the grain crystallographic orientation and weaken the texture.

Fully reversed strain-controlled tension–compression fatigue experiments were conducted on extruded Mg6Al1ZnA (AZ61A) magnesium alloy by employing thin-walled tubular specimens in ambient air. The strain–life fatigue curve displayed a detectable transition from lower cycle fatigue region to higher cycle fatigue region in the vicinity corresponding to a strain amplitude of 0.5%. When the strain amplitude was higher than 0.5%, shear cracking and significant twinning were observed. When the strain amplitude was lower than 0.5%, tensile cracking and little twinning were observed.

There is great interest in metallic multilayer thin films with the layer thickness on the order of nanometers due to their high hardness and strength. The yield strength deviates from Hall–Petch and Orowan predictions with layer thickness as the controlling factor, and other factors are in need to explain this phenomenon. In this work, the effect of dislocation source length on yield strength of multilayer system in the nanoscale regime is investigated using a three dimensional cellular automaton dislocation model. Different initial dislocation source lengths are assumed in the model for the metallic multilayer thin films with different layer thicknesses. The multilayer system studied here is composed of alternating A and B components with FCC structure, cube-on-cube orientation relationship, and the same layer thickness. The results show that the initial dislocation source length can provide an explanation for the above deviations.

Transformation superplastic forming is an attractive alternative forming technique to microstructural superplastic forming, since it requires no special microstructures and, therefore, eliminates the limitation of superplastic forming capability to only expensive materials with stable high-temperature fine grains. Transformation superplasticity occurs through biasing the internal stress produced from an allotropic phase transformation by a small external stress. In this work, finite element modeling was implemented to study the transformation superplastic forming of domes from flat circular thin plate samples. The evolution and distribution of stress, strain, and dome thickness was analyzed in detail. The thickness distributions in the formed domes were compared with the theoretical predictions of two models, which assume different stress states in the domes. The appropriate stress state was identified through this comparison. Different gas pressure amplitudes were applied during forming to investigate the effect on the formed-dome apex height, when the forming time was fixed.

Mechanical and microstructural characterizations were performed on pure magnesium samples processed by different rolling routes and annealing conditions to investigate the effects of thickness reduction strain and annealing temperature and duration on mechanical behaviors. The results indicate that (a) rolling increases yield strength significantly by about 100% to 130% and decreases ductility by about 55% to 66%; (b) a rolling route with a lower thickness reduction strain per rolling pass corresponds to a higher strength increase; and (c) annealing does not affect the magnitudes of yield strength and ductility, but largely increases ultimate tensile strength and strain hardening rate compared with the as-rolled material without experiencing annealing.

Computational simulations were performed to investigate the dynamic mechanical behavior of pure polycrystalline magnesium under different high loading strain rates with the values of 800, 1000, 2000, and 3600 s −1 . The Johnson-Cook model was utilized in the simulations based on finite element modeling. The results showed that the simulations provided well-matched predictions of the material behavior such as the strain rate-time history, the stress-strain curve, and the temperature increase. Under high loading strain rates, the tested material experienced linear strain hardening at the early stage of plastic deformation, increased strain hardening at the intermediate plastic deformation region, and decreased strain hardening at the region before fracture. The strain hardening rates for the studied high loading strain rate cases do not vary much with the change of strain rates.

The primary goal of this project is to explore the fundamental deformation and failure mechanisms for magnesium with a hexagonal close packed (HCP) crystal structure. It is critical to perform this project for a number of reasons. First, magnesium is the lightest structural metal and its application in various structural components can save the final component weight. Second, the weight reduction from the usage of magnesium-based structural components in transportation vehicles such as automobiles and aircrafts can improve fuel efficiency and reduce the greenhouse gas emissions. Third, structural components often experience dynamic loading such as cyclic loading conditions. Fourth, magnesium with a HCP crystal structure generally has its special deformation responses under loading conditions. This project investigated magnesium based materials (magnesium single crystal, pure polycrystalline magnesium, and some magnesium alloys) under various loading conditions, and also explored some processing routes to manipulate the microstructure and mechanical properties of magnesium. The research results were published in a number of articles and also disseminated through presentations in various conferences such as TMS annual meetings, MRS meetings, the international Plasticity conferences, the Pacific Rim International Congress on Advanced Materials and Processing, and AeroMat. In addition to the contribution to the research/academic community, thismore » project is also beneficial to the general public. With the actual usage of magnesium in the passenger cars, the weight reduction and fuel consumption reduction will save the fuel bill of individual owners.« less

A closed-form solution is presented for the stress field in an infinite, homogeneous, isotropic elastic material, due to a uniform transformation strain in a cuboidal region. The dislocation-based method used here offers an alternate approach and furnishes the stress state directly, as compared to the displacement gradient furnished by Chiu [2]. Both solutions are shown to produce numerically equivalent values of stress. The present results are available as a numerical (MathCad) program to compute the stress state for abitrary cuboidal geometries