Linear-superelastic metals by controlled strain release via nanoscale concentration-gradient engineering

The elastic strain limit of most metals are less than 0.2% except for whiskers or freestanding nanowires whose elastic strain limit could reach 4–7%. Ferroelastic metals such as shape memory alloys (SMAs) do exhibit giant recoverable strains (up to ∼13%). However, the strong non-linear pseudo-elasticity of SMAs leads to mechanical instability. By taking advantage of the strong composition-dependent critical stress for stress-induced martensitic transformation (MT) in NiTi SMA, this work demonstrates a novel design approach to achieve linear-superelasticity (∼4.6%) and ultralow modulus (8.7 GPa) of a NiTi single crystal. These unprecedented properties are realized through precisely controlling strain release during the MT via nanoscale concentration-gradient engineering. The computer simulation results and theoretical analyses reveal that the stress–strain behavior of NiTi and other SMAs can be regulated effectively by fine-tuning the concentration gradient. This may open a new avenue for the design of next generation ferroelastic materials.     

Mechanically alloyed Ni50Ti50 and its transformation by thermal treatments

Amorphous powders have been obtained by mechanical alloying (MA) equiatomic powder mixtures of nickel and titanium. The amorphous phase thus formed decomposes upon heating first into the cubic B2 NiTi intermetallic compound; however, further heating promotes the precipitation of the intermetallics Ni₃Ti and NiTi₂. These transformations are shown to occur also in mechanically ground (MG) NiTi wire, but not in this same material exempted from ball-milling processing. It is suggested that this unique behaviour is brought about by the particular structural features of the MA or MG powders, which promote the otherwise sluggish decomposition of B2 NiTi.     

Surface-stress-induced phase transformation in metal nanowires

Several researchers have demonstrated, through experiments and analysis, that the structure and properties of nanometre-scale materials can be quite different to those of bulk materials due to the effect of surfaces. Here we use atomistic simulations to study a surface-stress-induced phase transformation in gold nanowires. The emergence of the transformation is controlled by wire size, initial orientation, boundary conditions, temperature and initial cross-sectional shape. For a <100> initial crystal orientation and wire cross-sectional area below 4 nm(2), surface stresses alone cause gold nanowires to transform from a face-centred-cubic structure to a body-centred-tetragonal structure. The transformation occurs roughly when the compressive stress caused by tensile surface-stress components in the length direction exceeds the compressive stress required to transform bulk gold to its higher energy metastable crystal structure.     

Effect of micro-arc oxidation surface modification on the properties of the NiTi shape memory alloy

In this paper, the effects of micro-arc oxidation (MAO) surface modification (alumina coatings) on the phase transformation behavior, shape memory characteristics, in vitro haemocopatibility and cytocompatibility of the biomedical NiTi alloy were investigated respectively by differential scanning calorimetry, bending test, hemolysis ratio test, dynamic blood clotting test, platelet adhesion test and cytotoxicity testing by human osteoblasts (Hobs). The results showed that there were no obvious changes of the phase transformation temperatures and shape memory characteristics of the NiTi alloy after the MAO surface modification and the coating could withstand the thermal shock and volume change caused by martensite-austenite phase transformation. Compared to the uncoated NiTi alloys, the MAO surface modification could effectively improve the haemocopatibility of the coated NiTi alloys by the reduced hemolysis ratio, the prolonged dynamic clotting time and the decreased number of platelet adhesion; and the rough and porous alumina coatings could obviously promote the adherence, spread and proliferation of the Hobs with the significant increase of proliferation number of Hobs adhered on the surface of the coated NiTi alloys (P?<?0.05).     

Morphology and phase selective synthesis of CuxO (x = 1, 2) nanostructures and their catalytic degradation activity

Cu_xO (x = 1, 2) nanocrystals have been synthesized by the composite-hydroxide-mediated approach. The obtained nanocrystals were characterized by X-ray diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy, transmission electron microscopy, and UV–vis spectrum. The morphology of the nanocrystals changed from sphere-shaped nanostructures to flower-shaped nanostructures, and finally to nanowires associated with phase transformation from CuO to Cu₂O by increasing the temperature. The possible phase transformation mechanism was discussed. The catalytic degradation activity of the Cu_xO (x = 1, 2) nanocrystals to methyl orange was also investigated. The photocatalytic ability of the sphere-shaped nanostructures is much higher than that of the nanowires, owing to its absorption of wider range of light energy. This work provides a new facile synthesis route of Cu_xO (x = 1, 2) nanocrystals and suggests their possible application in organic pollutants removal.     

The location of atomic hydrogen in NiTi alloy: A first principles study

A first principles density functional theory study to investigate the H defect in NiTi alloy is presented. We have determined the interstitial H atom position in bulk B2 phase NiTi alloy. H positions on both the Ti and Ni terminated NiTi surfaces are calculated. Surface adsorptions of H atom on Ni/Ti terminated surfaces are calculated for a low surface coverage of 1.96 × 10¹⁴ cm^?2. We have also calculated the penetration barrier energy for an H atom from the surface site to the bulk lattice site.     

Raman studies of selenium nanowires under high pressure

The selenium nanowires with diameter of 70 nm and length of 40 μm were synthesized by a facile solution method. High-pressure behavior of Se nanowires has been investigated by in situ Raman scattering up to 20.2 GPa at room temperature. A reversible phase transition from hexagonal to monoclinic occurs at 18.1 GPa. This transition pressure is higher than that of 14.0 GPa for bulk Se. The intrinsic geometry and/or the increasing energy band gap of Se nanowires are considered to contribute to the increase of transition pressure.     

Influence of Electron Beam Powder Bed Fusion Process Parameters on Transformation Temperatures and Pseudoelasticity of Shape Memory Nickel Titanium

Electron beam powder bed fusion (PBF-EB) is used to manufacture dense nickel titanium parts using various parameter sets, including the beam current, scan speed, and postcooling condition. The density of manufactured NiTi parts is investigated in relation to the linear energy input. The results imply that the part density increases with increasing linear energy density to over 98% of the bulk density. With a constant energy input, a combination of low power and low scan speed leads to denser parts. This is attributed to lower electrostatic repulsive forces from lower number density of the impacting electrons. After manufacturing, the densest parts with distinct parameter sets are categorized into three groups: 1) high power with high scan speed and vacuum slow cooling, 2) low power with low scan speed and vacuum slow cooling, and 3) low power with low scan speed and medium cooling rate in helium gas. Among these, a faster cooling rate suppresses phase transformation temperatures, while vacuum cooling combinations do not affect the phase transformation temperatures significantly. Herein, all the printed parts exhibit almost 8% pseudoelasticity regardless of the process parameters, while the parts cooled in helium have a higher energy dissipation efficiency (1 − η), which implies faster damping of oscillations.     

GaAs/AlGaAs heterostructure nanowires studied by cathodoluminescence

In this report we explore the structural and optical properties of GaAs/A1GaAs heterostructure nanowires grown by metalorganic vapour phase epitaxy using gold seed-particles. The optical studies were done by low-temperature cathodo- luminescence （CL） in a scanning electron microscope （SEM）. We perform a systematic investigation of how the nanowire growth-temperature affects the total photon emission, and variations in the emission energy and intensity along the length of the nanowires. The morphology and crystal structures of the nanowires were investigated using SEM and transmission electron microscopy （TEM）. In order to correlate specific photon emission characteristics with variations in the nanowire crystal structure directly, TEM and spatially resolved CL measurements were performed on the same individual nanowires. We found that the main emission energy was located at around 1.48 eV, and that the emission intensity was greatly enhanced when increasing the GaAs nanowire core growth temperature. The data strongly suggests that this emission energy is related to rotational twins in the GaAs nanowire core. Our measurements also show that radial overgrowth by GaAs on the GaAs nanowire core can have a deteriorating effect on the optical quality of the nanowires. Finally, we conclude that an in situ pre-growth annealing step at a sufficiently high temperature significantly improves the optical quality of the nanowires.     

Mechanical activation of pre-alloyed NiTi<Subscript>2</Subscript> and elemental Ni for the synthesis of NiTi alloys

This work reports on an efficient powder metallurgy method for the synthesis of NiTi alloys, involving mechanical activation of pre-alloyed NiTi₂ and elemental Ni powders (NiTi₂–Ni) followed by a press-and-sinter step. The idea is to take advantage of the brittle nature of NiTi₂ to promote a better efficiency of the mechanical activation process. The conventional mechanical activation route using elemental Ti and Ni powders (Ti–Ni) was also used for comparative purposes. Starting with (NiTi₂–Ni) powder mixtures resulted in the formation of a predominant amorphous structure after mechanical activation at 300 rpm for 2 h. A sintered specimen consisting mainly of NiTi phase was obtained after vacuum sintering at 1050 °C for 0.5 h. The produced NiTi phase exhibited the martensitic transformation behavior. Using elemental Ti powders instead of pre-alloyed NiTi₂ powders, the structural homogenization of the synthesized NiTi alloys was delayed. Performing the mechanical activation at 300 rpm for the (Ti–Ni) powder mixtures gave rise to the formation of composite particles consisting in dense areas of alternate fine layers of Ni and Ti. However, no significant structural modification was observed even after 16 h of mechanical activation. Only after vacuum sintering at 1050 °C for 6 h, the NiTi phase was observed to be the predominant phase. The higher reactivity of the mechanically activated (NiTi₂–Ni) powder particles can explain the different sintering behavior of those powders compared with the mechanically activated (Ti–Ni) powders. It is demonstrated that this innovative approach allows an effective time reduction in the mechanical activation and of the vacuum sintering step.     

Fabrication and characteristics of porous NiTi shape memory alloy synthesized by microwave sintering

Porous nickel titanium (NiTi) shape memory alloy (SMA) was successfully fabricated by microwave sintering method. This method allows formation of porous structures without using any pore-forming agents. Moreover, microwave sintering of NiTi SMA can be successfully performed at a relatively low sintering temperature of 850 °C and a short sintering time of 15 min. The pore characteristics, microstructure, phase transformation and stress-strain behavior of the porous NiTi SMA were investigated. The porous NiTi SMA exhibited porosity ratios from 27% to 48% and pore sizes range from 50 to 200 μm when using different sintering temperatures and holding times. The predominant B2 (NiTi) and B19′ (NiTi) phases were identified in the porous NiTi SMA. A multi-step phase transformation took place on heating and a two-step phase transformation took place on cooling of the porous NiTi SMA. The irrecoverable strains decreased with increasing sintering temperature, but the holding time had little effect on the stress-strain behavior at 60 °C.     

Characteristics of SiOx-cored composite nanowires with Ag shell layers

This paper reports the preparation of Ag-coated SiO_x nanowires and analyzes changes in the structural properties and photoluminescence (PL) spectra induced by a thermal annealing process. The thermal induced changes in the sample morphology, generating Ag nanoparticles on the core of SiO_x nanowires and crystalline structures were affected, facilitating the generation of the Ag₂SiO₃ phase. The overall shape of the PL spectrum was changed significantly by both the Ag-coating and the subsequent thermal annealing. Possible emission mechanisms were discussed. This study gives insight into the annealing process regarding various coaxial one-dimensional materials, particularly with metal shell layers.     

Phase transition and compressibility in silicon nanowires

Silicon nanowires (Si NWs), one-dimensional single crystalline, have recently drawn extensive attention, thanks to their robust applications in electrical and optical devices as well as in the strengthening of diamond/SiC superhard composites. Here, we conducted high-pressure synchrotron diffraction experiments in a diamond anvil cell to study phase transitions and compressibility of Si NWs. Our results revealed that the onset pressure for the Si I-II transformation in Si NWs is approximately 2.0 GPa lower than previously determined values for bulk Si, a trend that is consistent with the analysis of misfit in strain energy. The bulk modulus of Si-I NWs derived from the pressure-volume measurements is 123 GPa, which is comparable to that of Si-V NWs but 25% larger than the reported values for bulk silicon. The reduced compressibility in Si NWs indicates that the unique wire-like structure in nanoscale plays vital roles in the elastic behavior of condensed matter.     

Effects of plate thickness on reverse martensitic transformation of prestrained NiTi/NiTi alloy

In this paper, differential scanning calorimeter (DSC) was used to study the effects of predeformation and plate thickness on the reverse martensitic transformation of explosively welded NiTi/NiTi alloy. Results showed that there was a constraint between Ni_50.4Ti (NiTi-1) and Ni_49.8Ti (NiTi-2), which led to that the thickness of NiTi-1 or NiTi-2 strongly affected the reverse martensitic transformation behavior because residual stress variations in thickness wound enable bias force to be built inside the composite. The DSC measurements showed that after deformation, the reverse martensitic transformation temperature of the composite was increased with the increasing thickness of NiTi-2. Also, the XRD results revealed that the microstructure of NiTi/NiTi alloy changed from B2 phase to B19’ phase along the thickness direction.     

Dependence of pressure on elastic,electronic and optical properties of CeO2 and ThO2: A first principles study

The phase transformation of CeO₂ and ThO₂ from fluorite to cotunnite-type structure under pressure is predicted within the density functional theory implemented with the GGA-PW91 method, the pressure induced structural phase transition occurs at 28.9 GPa for CeO₂ and 29.8 GPa for ThO₂. These values are in excellent agreement with the experimentally measured data. The elastic, electronic and optical properties at normal as well as for high-pressure phase have been calculated, particular attention is devoted to the cotunnite phase. Further, the dependence of the elastic constants, the bulk modulus B, the energy band gaps and the dielectric function on the applied pressure are presented.     

Anomalous dilatation-temperature curves of PZT-ZrO2 composites

The dilatation-temperature curves of the PZT-ZrO₂ composites, containing 1.3–13.2 vol% ZrO₂ were studied. The course of martensitic transformation (MT) of dispersed ZrO₂ particles in the PZT matrix was followed. Upon cooling, a spontaneous microcrack formation occurred during the MT from tetragonal to monoclinic crystal phase of ZrO₂. The phenomenon of intensive shrinkage above 800 °C upon heating is explained as a process of microcrack healing in the presence of the PbO-rich liquid phase.     

Microwave-assisted preparation of calcium sulfate nanowires

We have successfully developed a new synthetic route for the rapid preparation of calcium sulfate nanowires by thermal transformation of calcium dodecyl sulfate (CDS) in organic solvents of ethylene glycol (EG) and N,N-dimethylformamide (DMF). The products are characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM), and determined to be single-phase CaSO₄·0.5H₂O consisting of single-crystalline nanowires with aspect ratio up to about 62. In this method, the different types of organic solvents used have no obvious influences on the morphology, phase, and formation time of the product. The microwave heating can remarkably shorten the reaction time compared with conventional heating methods.     

Tensile behaviour of superelastic NiTi alloys charged with hydrogen under applied strain

ABSTRACT

The tensile behaviour of NiTi alloys is investigated after hydrogen charging during the austenite, half-transformation and martensite phases. The specimens are charged with different current densities and charging durations. During the tensile tests, the strain of the plateau transformation decreases due to hydrogen-induced residual martensite variants. This decrease becomes important when the charging happens during the martensite phase. Accordingly, the hydrogen ensures the stability of the phase in which the charging process occurs. Moreover, a heightening of transformation stress is noticed during the plateau. The transformation stress increases when the current density grows and the charging duration rises. This occurrence is caused by the interaction between the hydrogen and NiTi structures, where hydrogen delays the NiTi martensite transformation.

This paper is part of a thematic issue on Hydrogen in Metallic Alloys     

Growth and quantum confinement in AgI nanowires

Silver iodide nanowires were synthesized in W/O microemulsions by using cyclohexane/Triton X-100/n-pentanol system. Most likely, surfactants form rod-like aggregates that can serve as template for growth of two dimensional nanomaterials. It was found that the length of the AgI nanowires increases as a function of aging time, while the diameter decreases. Final length of the AgI nanowires is several microns, while the diameter is smaller than 3 nm. Morphological changes are accompanied by optical and structural changes. Large blue shift of excitonic peak from bulk value at 420 to 326 nm was observed as a consequence of the size quantization effect. Decrease of diameter is followed by the amorphization of AgI nanowires.     

The cubic-to-β martensitic transformation in ZrO2-Sc2O3

The nature of the cubic-to-β (c-β) transformation and the microstructure of theβ-phase were examined in ZrO₂-10.5 and 12.5 mol % Sc₂O₃ alloys. The c -β transformation is induced during cooling from the high-temperature cubic phase region by martensitic transformation. The microstructure of theβ-phase usually has a herring-bone appearance, which is made of a unique array of four orientation variants. Two types of interfaces are formed in the structure; long straight interfaces and short interfaces. The former and the latter interfaces are nearly parallel to the {011} _β and {012} _β type planes, respectively, which correspond to the {001}_c and {011}_c planes in the parent cubic phase. The {011} _β and {012} _β type interfaces are fully coherent, low-energy interfaces. The herring-bone structure is likely to be a favourable one for minimizing the strain energy and interfacial energy associated with the c -β transformation, which is a characteristic microstructure developed by martensitic transformation.