Stress-induced martensitic phase transformation in Cu-Zr nanowires

A novel stress-induced martensitic phase transformation in an initial <100>/{100} B2-CuZr nanowire is reported for the first time in this letter. Such behavior is observed in a nanowire with cross-sectional dimensions of 19.44 × 19.44 Å² over a temperature range of 100-400 K and at a strain rate of 1 × 10⁹ s^− 1 using atomistic simulations. Phase transformation from an initial B2 phase to a BCT (Body-Centered-Tetragonal) phase is observed via nucleation and propagation of {100} twinning plane under high strain rate tensile deformation.     

Enhanced room-temperature tensile ductility of columnar-grained polycrystalline Cu-12 wt.%Al alloy through texture control by Ohno continuous casting process

The Ohno continuous casting (OCC) process is a practical way to control the solidification texture of Cu-12 wt.%Al alloy with a perfect < 001>β fiber texture along the solidification direction. Compared with the conventional randomly oriented polycrystalline Cu-12 wt.%Al alloy, the reorientation of β₁′ martensite and stress-induced phase transformation occurred at the same time within every columnar grain sharing the same [001]β orientation during tensile test, which would reduce the elastical and phase-transformational incompatibility and enhance the intergranular accommodation. As a consequence, a high tensile ductility up to 28% with transgranular fracture can be obtained for OCC columnar-grained Cu-12 wt.%Al alloy instead of intergranular fracture due to the incompatible stress at the grain boundary for randomly oriented polycrystalline Cu-12 wt.%Al alloy.     

Grain refinement of biomedical Co-27Cr-5Mo-0.16N alloy by reverse transformation

Advanced grain refinement of a biomedical Ni-free Co-27Cr-5Mo-0.16N alloy without hot or cold plastic deformation was successfully achieved by a reverse transformation from a lamellar (hcp + Cr₂N) phase to an fcc phase. The technique consisted of a two-step heat treatment. First, the solution-treated specimen was subjected to isothermal aging at 1073 K for 90 ks, forming a lamellar structure of hcp and Cr₂N phases. Then, the aged specimen having a completely lamellar microstructure was reverse-treated at temperatures from 1273 to 1473 K, where the fcc phase is stable. The resultant grains were approximately 1/10 of their initial size. Moreover, tensile testing after reverse transformation showed excellent strength with good ductility compared to samples examined before the reverse transformation. Our results will contribute to the development of biomedical Ni-free Co-Cr-Mo-N alloys with refined grain size and good mechanical properties, without requiring any hot workings.     

Application of long-range ordering in the synthesis of a nanoscale Ni2 (Cr,Mo) superlattice with high strength and high ductility

We demonstrate that bulk nanoscale materials with high strength and high ductility can be synthesized by using long-range ordering in certain alloy systems. In the case of a Ni-18.6 atomic % Mo-15.1 atomic % Cr, a bulk nanoscale superlattice of Ni₂(Cr,Mo) isomorphous with Pt₂Mo has been synthesized by thermal aging at 700 °C. The superlattice is shown to have high strength and high ductility as well as high thermal stability. Although the yield strength is nearly doubled in the ordered state exceeding 800 MPa, the material is found to maintain about 70% of its initial tensile ductility corresponding to 42% engineering strain. This behavior has been related to the crystallography of the ordering transformation. Although most of the slip systems of the parent face-centered cubic lattice are suppressed by ordering, most of the twinning systems remain energetically favorable. Therefore, deformation in the ordered state is found to predominantly occur by twinning rather than by slip giving rise to the observed combination of high strength and high ductility.     

Thermal stability of in situ synthesized (TiB + La2O3)/Ti composite

Thermal stability of in situ synthesized (TiB + La₂O₃)/Ti composite is investigated. The phase analysis is identified by X-ray diffraction. Microstructure of the melted and forged titanium matrix composites (TMCs) after heat treatment is characterized by means of optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The room temperature tensile properties after an additional thermal exposure at 873 K, 923 K or 973 K for 100 h are tested. After the thermal exposure, the strength of specimen increases and ductility decreases. This is attributed to precipitation of ordered α₂ phase (Ti₃Al) and S₁ (silicide) in the titanium matrix composites after the thermal exposure.     

Effect of Ni doping on the microstructure and high Curie temperature ferromagnetism in sol–gel derived titania powders

Undoped, 0.05 and 0.5 mol% Ni-doped TiO₂ powders were prepared by a modified sol–gel route. The doping effects on the microstructure and magnetism for the powdered samples have been systematically investigated. Doping of Ni in TiO₂ inhibited rutile crystal growth. The probable reason for this is discussed on the basis of band calculation based analysis of electronic structures of 3d transition metal-doped TiO₂ and the energetic, transformation kinetics and phase stability of anatase over rutile as the function of particle size. Room temperature ferromagnetism (RTFM) with the saturation magnetization of 12 m emu g⁻¹ and Curie temperature as high as 820 K is observed only in case of 0.05 mol% Ni:TiO₂ powdered sample, whereas undoped TiO₂ was diamagnetic and 0.5 mol% Ni:TiO₂ was paramagnetic in nature. The role of any magnetic impurity or any Ni metal in the origin of the RTFM has been ruled out by energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) analysis, whereas magnetic force microscopy (MFM) established the presence of magnetic domains, supporting the intrinsic diluted magnetic semiconductor behavior. The observed ferromagnetism has been attributed to the spin ordering through exchange interaction between holes trapped in oxygen orbitals adjacent to Ni substitutional sites.     

In situ observation of the change in domain structure due to phase transformation in a perovskite La2MnGaO6

The microstructure of a perovskite, La₂MnGaO₆, was studied by Transmission Electron Microscopy (TEM). An idiomorphic grain exhibited a domain structure at ambient temperature, and each domain possessed a common b-axis and axial relation, [1 0 0]//[0 0 1]. When the grain was heated by a convergent electron beam, the domains vanished after the orthorhombic to rhombohedral phase transformation taking place at 423 K. The domains reappeared when cooled, although domain boundaries moved and the domains rotated by 90° with respect to the b-axis.     

On the room-temperature deformation mechanisms of lamellar-structured Fe30Ni20Mn35Al15

A eutectic alloy Fe₃₀Ni₂₀Mn₃₅Al₁₅ (in at.%), consisting of B2 (ordered b.c.c.) and f.c.c. phases, was prepared by directional solidification, drop-casting or quenching from 1623 K in order to obtain different lamellar sizes and morphologies. The hardness of the individual phases, measured using nanoindentation, was 4.38 ± 0.20 GPa for the B2 phase and 2.72 ± 0.14 GPa for the f.c.c. phase. The roles of these phases in mechanical deformation were investigated. In both the drop-cast and the quenched alloy, which contained refined discontinuous lamellae, the B2 lamellae showed little sign of plastic deformation and simply behaved as obstacles to moving dislocations. The yield strength increased with refinement of the lamellae, whereas the ductility decreased. In contrast, tensile tests performed along the growth direction of the directionally solidified alloy, which contained lamellae several microns wide aligned with the growth direction, showed that the f.c.c. lamellae experienced plastic deformation by glide of 〈1 1 0〉 dislocations, whereas the B2 lamellae fractured elastically into ∼30 μm long segments. Tearing at interfaces occurred for B2 lamellae inclined to the tensile axis.     

Synthesis of alumina supported LaMnO3 with excellent thermal stability by a PVP-assisted route

LaMnO₃/Al₂O₃ catalysts were successfully prepared by a novel method with polyvinyl pyrrolidone (PVP) as complexant and characterized by XRD, TGA, IR, BET, XPS and TEM techniques. The TGA and IR characterizations of the precursor revealed that LaMnO₃ structure was formed under mild conditions without combustion of any organic compounds. The obtained catalysts exhibited better activity for methane combustion than those prepared by citrate method, mainly due to larger pore volume, more active oxygen species on surface and the formation of a pure perovskite structure. The high surface area of about 122 m² g⁻¹ was retained even after calcined at 1000 °C; and interestingly, no phase transformation or solid-state reaction was observed. This fact indicated the excellent thermal stability of catalysts, which was ascribed to the strong interaction between the support and active phase.     

Solar photocatalytic degradation of 2-sec-butyl-4,6-dinitrophenol (DNBP) using TiO2/SiO2 aerogel composite photocatalysts

Silica aerogels and TiO₂/silica aerogel composite photocatalysts were synthesized by sol–gel technique at ambient pressure using orthosilioate and tetra-n-butyl titanate as precursors, respectively. The prepared composite photocatalysts were characterized by XRD, TEM, BET surface area, FT-IR and UV–vis absorption spectra. The results showed that the TiO₂/silica aerogel composite photocatalysts possess high surface area. The addition of silica aerogels inhibited the grain growth and phase transformation of anatase to rutile during calcination. The TiO₂/silica aerogel composite sample calcined at 500 °C with an optimal silica aerogel content of 7 wt.% afforded the highest photocatalytic activity. The photocatalytic degradation of 2-sec-butyl-4,6-dinitrophenol (DNBP) was investigated by using this novel TiO₂/silica aerogel composite photocatalyst under solar light irradiation. The effects of irradiation time, pH, catalyst concentration, temperature and initial DNBP concentration were examined as operational parameters. The optimal operational parameters were found as follows: pH as solution pH 4.82, 8 g L⁻¹ catalyst concentration, 20 °C, and 240 min irradiation time. The kinetics of DNBP degradation by TiO₂/silica aerogel composite fit well a pseudo-first-order kinetic model. The repeatability of photocatalytic activity was also tested. This study showed the feasible and potential use of TiO₂/silica aerogel composite photocatalysts in degradation of toxic organic contaminants.     

Synthesis,thermodynamic and magnetic properties of pure hexagonal close packed nickel

A novel and fast technique for the synthesis of pure hexagonal close packed (HCP) nickel is demonstrated. The HCP nickel was electrodeposited from NiCl₂-1-ethyl-3-methylimidazolium chloride (NiCl₂-EmimCl) ionic liquid at 160 °C. X-ray diffraction confirmed the formation of pure phase. A phase transformation from HCP nickel to face centered cubic (FCC) nickel was observed at 422.6 °C and the enthalpy of transformation was found to be 16.72 J g⁻¹. The phase transformation resulted in the release of hydrogen which makes HCP nickel a potential hydrogen storage material. The electrodeposited nickel showed ferromagnetic properties and the magnetic coercivity was found to be 43 Oe.     

Deformation mechanisms of a modified 316L austenitic steel subjected to high pressure torsion

The present work is assigned to the microstructural evolution of a modified 316L stainless steel during high pressure torsion (HPT) in a temperature range between −196 °C and 720 °C. The aspect of microstructural evolution is similar to that of materials with low stacking fault energy: at high deformation temperatures (T_def > 450 °C) the dominant deformation mechanism is dislocation glide whereas for medium temperatures (450 °C > T_def > 20 °C) mechanical twinning is observed. At very low deformation temperatures (20 °C > T_def > −196 °C) mechanical twinning is replaced by the deformation induced martensite transformation γ(fcc) → ?(hcp). Based on the present results, the formation mechanisms of nanocrystalline austenite are discussed.     

Effect of martensitic phase transformation and deformation twinning on mechanical properties of Fe–Mn–Si–AI steels

Abstract

Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe–(15–30) wt-%Mn alloys with additions of Al and Si have been investigated. Tensile tests were carried out at different strain rates and temperatures. The formation of twins, α′ (bcc)- and ε (hcp)-martensite in the γ (fcc) matrix during plastic deformation was analysed by optical microscopy, X-ray diffraction, and scanning electron microscopy. Depending on the content of the alloying elements different phase transformations γ → ε, γ → α′ (TRIP effect), or the formation of deformation twins (TWIP effect) occurred. Additions of Al increased the stacking fault energy (γ_fcc) and suppressed the γ → ε transformation while Si decreased γ_fcc and sustained the γ → ε transformation. These steels with reduced densities of about 7.3 Mg m^?3 exhibit high tensile ductility up to 95% with true tensile strength of about 1100 MPa. The excellent plasticity induced by twinning or phase transformation up to extremely high strain rates of about <disp-formula><graphic href="splitsection2-m1.tif"/></disp-formula> results in an extraordinary shock resistance and allows for deep drawing and backward extrusion operations of parts with complex shapes.     

Structure and compositional evolution in epitaxial Co/Pt core-shell nanoparticles on annealing

We report on the alloying of epitaxial Co/Pt core-shell nanoparticles using transmission electron microscopy (TEM) and electron diffraction. In as-deposited nanoparticles followed by in situ annealing at 823 K for 10.8 ks, high-angle annular dark-field (HAADF) imaging by scanning TEM (STEM) clearly revealed formation of Co-shell/Pt-core structures due to the large atomic number (Z) difference between Co (Z = 27) and Pt (Z = 78). We identified a formation of locally ordered areas of the L1₀ ordered phase at the core of the nanoparticles. After ex situ annealing at 873 K for 0.6 ks, some of the ordered areas showed complicated contrasts in the HAADF-STEM images. Based on image simulations, we found that these atypical contrasts arise from the stacking of two orthogonal variants of the L1₀ phase in the electron beam direction. Furthermore, the simulation showed that image contrast strongly reflects the structure of the variant located closer to the beam entrance rather than to the bottom side. Solid solution phase was formed by further annealing at 873 K for 3.6 ks, while high-density {111} stacking faults were observed inside the Co-Pt alloy nanoparticles. Magnetic coercivity remained at values as low as ~ 15.9 kA/m at 300 K, irrespective of the formation of local L1₀ ordered areas and/or a high-density stacking faults.     

The production of flower-like NiFe2O4 superstructures consisting of nanosheets via the thermolysis of a heterometallic oxo-centered trinuclear complex

Flower-like NiFe₂O₄ superstructures consisting of nanosheets have been successfully synthesized by direct thermolysis of a heterometallic oxo-centered trinuclear complex [NiFe₂O(CH₃COO)₆(H₂O)₃·2H₂O] (NiFe-HOTC) at 400 °C for 6 h in a horizontal tube furnace. The composition and structure of the products were investigated by X-ray diffraction (XRD) and infrared spectra (IR). XRD analysis revealed a pure ferrite phase with high crystallinity. Morphological investigation by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed NiFe₂O₄ flowers with average diameter varying from 0.5 to 3 μm consist of nanosheets with average edge length in the range of 60-300 nm and thickness of about 30 nm. Furthermore, energy dispersive X-ray analysis (EDX) demonstrated that the atom ration of Ni, Fe and O is 1:2:4. In addition, magnetic measurements showed that the obtained flower-like NiFe₂O₄ are ferromagnetic at room temperature.     

Template-free synthesis of ZnWO4 powders via hydrothermal process in a wide pH range

ZnWO₄ powders with different morphologies were fabricated through a template-free hydrothermal method at 180 °C for 8 h in a wide pH range. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-visible and luminescence spectrophotometers were applied to study the effects of pH values on crystallinity, morphology, optical and luminescence properties. The XRD results showed that the WO₃ + ZnWO₄, ZnWO₄, and ZnO phases could form after hydrothermal processing at 180 °C for 8 h with the pH values of 1, 3-11, and 13, respectively. The SEM and TEM observation revealed that the morphological transformation of ZnWO₄ powders occurred with an increase in pH values as follows: star anise-, peony-, and desert rose-like microstructures and soya bean- and rod-like nanostructures. The highest luminescence intensity was found to be in sample consisting of star anise-like crystallites among all the samples due to the presence of larger particles with high crystallinity resulted from the favorable pH under the current hydrothermal conditions.     

Synthesis of spherical nanostructured LiMxMn2−xO4 (M = Ni, Co, and Ti; 0 ≤ x ≤ 0.2) via a single-step ultrasonic spray pyrolysis method and their high rate charge-discharge performances

LiM_xMn_2−xO₄ (M = Ni²⁺, Co³⁺, and Ti⁴⁺; 0 ≤ x ≤ 0.2) spinels were prepared via a single-step ultrasonic spray pyrolysis method. Comparative studies on powder properties and high rate charge-discharge electrochemical performances (from 1 to 15 C) were performed. XRD identified that pure spinel phase was obtained and M was successfully substituted for Mn in spinel lattice. SEM and TEM studies confirmed that powders had a feature of ‘spherical nanostructural’, that is, powders consisted of spherical secondary particles with the size of about 1 μm, which were developed from close-packed primary particles with several tens of nanometers. Substitutions enhanced density of second particles to different extents, depending on M and its content. Charge-discharge tests showed that as-prepared LiMn₂O₄ could deliver excellent rate performance (around 100 mAh/g at 10 C). Ni substitution contributed to improving electrochemical performances. In the voltage range of 4.95-3.5 V, the materials showed much better electrochemical performances than LiMn₂O₄ in terms of capacity, cycleability and rate capability.     

Combustion synthesis/quasi-isostatic pressing of TiC<Subscript>0.7</Subscript>–NiTi cermets: microstructure and transformation characteristics

TiC_0.7–NiTi cermets were produced by combustion synthesis followed by quasi-isostatic consolidation while the reaction products were still hot and ductile. The TiC_0.7–NiTi cermets were characterized by differential scanning calorimetry, room temperature transmission electron microscopy (TEM), and in-situ TEM (temperature varied during observation). The matrix of the as-synthesized 20NiTi, 40NiTi, and 60NiTi composites contains both R and B19′ martensites at room temperature. No distinct R-phase morphology could be imaged. In the B19′ martensite, [011] Type II twinning, Type I twinning and (001) compound twinning modes were observed as the lattice invariant shear (LIS) of the R-B19′ transformation. The [011] Type II twinning is often reported as the LIS of the B2-B19′ transformation, but this is the first experimental confirmation of its predicted presence as a qualified LIS of the R-B19′ transformation. The (001) compound twinning mode is responsible for the fine structure of the martensite with a wavy morphology. Nanoscale structures with a thickness of 5 nm were obtained inside the twins. Twinning was also observed at the interface with carbide particles, which confirms that some stress relaxation of the elastic mismatch occurs. At room temperature, the matrix of the 80NiTi composite had the R-phase structure, which appeared with a needle-like morphology. Thermal cycling resulted in the suppression of the R-phase transformation. This is the opposite of the behavior observed in un-reinforced NiTi alloys.     

Microstructural effect of gadolinium oxide nanocrystals upon annealing on electrical properties of memory devices

The microstructure evolution of sputtered gadolinium oxide nanocrystal (NC) memory devices upon annealing has been characterized in detail by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). TEM results indicate that the as-deposited film is composed of metallic Gd clusters embedded in an amorphous Gd_xO_y matrix. The Gd clusters undergo phase transformation to oxide NCs upon annealing, reaching a maximum density of 7.9-9.1 × 10¹¹ cm^− 2 at 850 °C, which is consistent with the largest memory window width. Upon annealing at even higher temperature, TEM diffraction patterns and XPS composition profiles indicate apparent Si diffusion into the NC layer, probably from the SiO₂ tunneling oxide or the Si substrate, leading to the formation of gadolinium silicate NCs. The presence of silicate NCs gradually deteriorates the device performance due to the reduction of barrier confinement for stored charges, although the dot density is only marginally decreased. The results suggest that the optimum memory device performance is dominated by not only the most considered size and density of NCs, but also the composition and phase inside.     

Ambient effect on damping peak of NiTi shape memory alloy

We report the effects of ambient condition and loading rate on the damping capacity of a superelastic nickel-titanium shape memory alloy during stress-induced martensitic phase transformation with release and absorption of latent heat. The damping capacity was measured via a tensile loading-unloading cycle in the strain-rate range of 10^− 5-10^− 1/s and three ambient conditions: still air and flowing air with velocities of 2 m/s and 17 m/s. It is found that, for each ambient condition, the maximum damping capacity (damping peak) is achieved at the strain rate whose loading time (t_T) is close to the characteristic heat-transfer time (t_h) of the ambient condition.