Microstructure and growth mechanism of tin whiskers on RESn3 compounds

An exclusive method was developed to prepare intact tin whiskers as transmission electron microscope specimens, and with this technique in situ observation of tin whisker growth from RESn₃ (RE = Nd, La, Ce) film specimen was first achieved. Electron irradiation was discovered to have an effect on the growth of a tin whisker through its root. Large quantities of tin whiskers with diameters from 20 nm to 10 μm and lengths ranging from 50 nm to 500 μm were formed at a growth rate of 0.1–1.8 nm s^?1 on the surface of RESn₃ compounds. Most (>85%) of these tin whiskers have preferred growth directions of 〈1 0 0〉, 〈0 0 1〉, 〈1 0 1〉 and 〈1 0 3〉, as determined by statistics. This kind of tin whisker is single-crystal β-Sn even if it has growth striations, steps and kinks, and no dislocations or twin or grain boundaries were observed within the whisker body. RESn₃ compounds undergo selective oxidation during whisker growth, and the oxidation provides continuous tin atoms for tin whisker growth until they are exhausted. The driving force for whisker growth is the compressive stress resulting from the restriction of the massive volume expansion (38–43%) during the oxidation by the surface RE(OH)₃ layer. Tin atoms diffuse and flow to feed the continuous growth of tin whiskers under a compressive stress gradient formed from the extrusion of tin atoms/clusters at weak points on the surface RE(OH)₃ layers. A growth model was proposed to discuss the characteristics and growth mechanism of tin whiskers from RESn₃ compounds.     

Silicon carbide whiskers with superlattice structure: A precursor for a new type of nanoreactor

Silicon carbide whiskers exhibit growth predominantly in the 〈1 1 1〉 direction. The high level of impurities, stacking faults and nanosized twins govern the formation of homojunctions and heterojunctions in crystals. The structure of the whiskers comprises a hybrid superlattice, i.e. contains elements of doped and composite superlattices. An individual SiC whisker can contain hundreds of quantum wells with anomalous chemical properties. This paper shows that it is possible to selectively etch quantum wells and to construct whiskers with quasi-regularly distributed slit-like nanopores (nanoreactors), which are bordered by polar planes {1 1 1}, {0 0 0 1} or a combination of them, and also to produce flat SiC nanocrystals bordered by polar planes.     

Temperature dependence of sound attenuation and shear modulus of ultra fine grained copper produced by equal channel angular pressing

The influence of temperature on shear modulus and internal friction in ultrafine-grained copper processed by equal channel angular pressing (ECAP) was investigated in the temperature range from 150 to 520 K. Acoustic measurements were performed on the inverted torsion pendulum at the frequencies of ∼18 and ∼45 Hz. An irreversible shear modulus increase and a concurrent decrease in sound attenuation were observed in the temperature region from ∼350 to 450 K on the first heating of specimens. The activation energy E ≈ 1.05 eV and the attempt frequency ν₀ ≈ 10¹⁰ s⁻¹ of the irreversible relaxation process were determined using the measurements at different heating rates. The overall decrease in the shear modulus in ECAP-processed copper was shown to be made up by two components: a temperature-independent and a temperature-dependent ones. The latter is accompanied with an additional internal friction of the relaxation type, which is reversible up to ∼350 K. An estimate of the activation energy for this reversible relaxation process was obtained. Possible mechanisms responsible for the anomalous behavior of the shear modulus and the sound attenuation in ultrafine-grained copper are discussed.     

Electrical properties of 1,4-bis[2-(3,4,5-trimethoxyphenyl)ethenyl] benzene

The increase of the electrical conductivity of the arylenevinylene (AV) system 1,4-bis[2-(3,4,5-trimethoxyphenyl)ethenyl] benzene was measured as a function of the doping level of iodine. Compressed powders and films blended with polystyrene (PS) containing up to 30 wt.% of the oligomer were used. At the highest doping level the conductivity of the compressed powder reaches a value of 3.3×10⁻⁴ Ω⁻¹ cm⁻¹. The conductivity, measured between 90 and 300 K on the powder with the highest conductivity, is thermally activated with an activation energy of ∼0.1 eV. The thermopower is positive in agreement with a charge transfer from the organic chain to an iodine atom. Its value is small (25 μV K⁻¹) and temperature independent. The results are interpreted in terms of small-polaron hopping.     

Mechanical properties and rolling behaviors of nano-grained copper with embedded nano-twin bundles

By means of dynamic plastic deformation (DPD) at liquid nitrogen temperature (LNT), bulk nano-grained copper samples with embedded nano-twin bundles were prepared. Subsequent cold rolling (CR) of the LNT-DPD Cu led to a reduction in quantity of nano-twin bundles and a slight grain coarsening, accompanied by a decrease in grain boundary (GB) energy from 0.34 to 0.22 J m⁻². An increasing CR strain leads to a saturation grain size of ∼110 nm, which is less than half of that in the severely deformed Cu from the coarse-grained form. Decreased strength and enhanced ductility were induced by CR in the LNT-DPD sample. The saturation yield strength in the LNT-DPD Cu during CR was ∼105 MPa higher than that in conventional severely deformed Cu, which originates from the finer grains as well as the nano-scale twins in the LNT-DPD sample. The enhanced ductility is primarily attributed to CR induced GB relaxation.     

Hot deformation and processing maps of an Fe3Al intermetallic alloy

The deformation behavior of an Fe–28Al–5Cr–0.08Zr–0.04B (at.%) intermetallic alloy under hot compression conditions was characterized in the temperature range of 600–1100 °C and strain rate range of 0.001–100 s⁻¹. Processing maps were calculated to evaluate the efficiency of the hot working and to recognize the instability regions of the flow behavior. The investigated alloy possesses the optimum hot-working conditions at 1100 °C and 0.001 s⁻¹, since the material undergoes dynamic recrystallization to produce a fine-grained structure with a high fraction of high-angle boundaries (∼70%). At lower temperature the material exhibited “large grained superplasticity” with a peak efficiency of ∼60% at 1000 °C and 0.001 s⁻¹. These parameters are the optimum ones for superplastic working of that alloy. The occurrence of large grained superplasticity is attributed to the formation of a subgrain structure within the large original grains and higher strain-rate sensitivity. The material also exhibits flow instabilities due to flow localization at lower temperatures (<700 °C) and higher strain rates (>0.1 s⁻¹).     

Oxidation behavior of 3D Hi-Nicalon/SiC composite exposed in wet and simulated air environments

Oxidation behavior of a 3D Hi-Nicalon/SiC composite was investigated in wet and simulated air with a flow rate of 3.5 cm s⁻¹ at temperatures above 1000 °C. Results showed that oxidation was occurred mainly on the specimens surface and rate-controlled by oxygen diffusion for both oxidation environments. Silica formed in wet air was viscous and thicker. After oxidation, strength deterioration of the composite was associated mainly with the degradation of Hi-Nicalon fiber. Meanwhile, the difference in fracture behavior could be attributed to the change of fiber/matrix interface bonding caused by high temperature oxidation.     

Electrical properties,sintering and thermal expansion behavior of composite ceramic interconnecting materials,La0.7Ca0.3CrO3−δ/Y0.2Ce0.8O1.9 for SOFCs

This paper presents a high-performance interconnect ceramic for solid oxide fuel cells (SOFCs), based on a modification of La_0.7Ca_0.3CrO_3−δ (LCC). It was found that addition of a small amount of YDC (Y_0.2Ce_0.8O_1.9) into LCC dramatically increased the electrical conductivity. For the best system, LCC + 3 wt.% YDC, the electrical conductivity reached 104.8 S cm⁻¹ at 800 °C in air. The electrical conductivity of the specimen with 2 wt.% YDC in H₂ at 800 °C was 5.9 S cm⁻¹. With the increase of YDC content, the relative density increased, reaching 97.6% when the YDC content was 10 wt.%. The average coefficient of thermal expansion (CTE) at 30–1000 °C in air increased with YDC content, ranging from 11.12 × 10⁻⁶ K⁻¹ to 15.34 × 10⁻⁶ K⁻¹. The oxygen permeation measurement illustrated a negligible oxygen ionic conduction, indicating that it is still an electronically conducting ceramic. Therefore, it is a very promising interconnecting ceramic for SOFCs.     

Preparation and electrical properties of highly (1 1 1) oriented antiferroelectric PLZST films by radio frequency magnetron sputtering

Highly (1 1 1) oriented lead lanthanum zirconate stannate titanate (PLZST) films were synthesized on Pt/Ti/SiO₂/Si substrates by radio frequency (RF) magnetron sputtering. The microstructure and electrical properties of the films were investigated as a function of post-annealing temperature. Smooth and crack-free films obtained by post-annealing at 700 °C for 30 min, and exhibit a dense columnar microstructure with a grain size of ∼0.85 μm. The sputtered PLZST films of nominal composition Pb_0.97La_0.02 (Zr_0.60Sn_0.30Ti_0.10)O₃ display a high saturation polarization of ∼70 μC cm⁻², a low antiferroelectric-to-ferroelectric switching field (<100 kV cm⁻¹), a reasonable dielectric constant and a low loss tangent. This combination of properties makes them attractive for microdevice applications.     

Enhanced fatigue resistance in 316L austenitic stainless steel due to low-temperature paraequilibrium carburization

Fully reversed fatigue tests have been performed on wrought 316L stainless steel samples after low-temperature carburization. The resulting 25 μm case depth, with a surface hardness three times that of the core and a surface compressive stress greater than 2 GPa, leads to significantly enhanced fatigue performance. The so-called endurance limit (defined as the stress at which the fatigue life is 10⁷ cycles) increased from about one-third to about one-half the yield stress (from ∼200 to ∼325 MPa). Fractographic investigations reveal that the surface stresses change the preferred site of fatigue crack nucleation from the surface for noncarburized samples to the interior for carburized samples.     

Estimation of residual corrosion rates of steel under cathodic protection in soils via voltammetry

Steel coupons were buried in soil for 2 months under cathodic protection. Their residual corrosion rates were deduced from voltammetry and weight loss measurements. In aerated soils, the current density due to O₂ reduction, j_K,O2, was modelled with a mixed activation–diffusion controlled kinetic. The anodic part j_A of the current density j, computed as j_A = j − j_K,O2, obeyed Tafel law. Its extrapolation to the protection potential gave a corrosion rate (∼7 μm yr⁻¹) consistent with that obtained from weight loss measurements. With a deficient protection, corrosion rates remained at ∼80 μm yr⁻¹, a value given by both methods.     

Processing,microstructure and property aspects of a spraycast Al–Mg–Li–Zr alloy

This paper describes a microstructural and property investigation of an Al–5.31Mg–1.15Li–0.28Zr alloy produced by spraycasting and downstream processing. Following a dispersoid precipitation treatment of 4 h at 400 °C, samples were hot compressed at strain rates of 2, 1, 0.2 and 0.1 × 10⁻² s⁻¹ at temperatures between 250 and 475 °C. Electron backscattered diffraction showed a strong dependence of recrystallised grain size on deformation temperature. At 250 °C and faster strain rates at 325 °C, a network of fine recrystallised necklace grains formed by progressive lattice rotation. At 325 °C at slow strain rates and at 400 °C and higher, dynamic recrystallisation occurred by discontinuous nucleation and growth at regions of microscopic strain localisation such as grain boundaries and triple points. The microstructures from small-scale hot compression experiments were compared with larger forgings under similar conditions and microstructural evolution was broadly similar. Mechanical properties of larger-scale forgings exceeded the targets for mechanically alloyed Al–Mg–Li alloy AA5091.     

Synthesis,characterization and evaluation of cation-ordered LnBaCo2O5+δ as materials of oxygen permeation membranes and cathodes of SOFCs

LnBaCo₂O_5+δ (Ln = La, Pr, Nd, Sm, Gd, and Y) was synthesized via an EDTA–citrate complexing process. The particular Ln³⁺ dopant had a significant effect on the oxide’s phase structure/stability, oxygen content, electrical conductivity, oxygen permeability, and cathode performance. Stable, cation-ordered oxides with layered lattice structures were obtained with medium-sized Ln³⁺ ions over a wide range of oxygen partial pressures, a property essential for applications as oxygen separation membranes and solid oxide fuel cell (SOFC) cathodes. PrBaCo₂O_5+δ demonstrated the highest oxygen flux (∼5.09 × 10⁻⁷ mol cm⁻² s⁻¹ at 900 °C), but this value was still significantly lower than that of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ perovskite (∼3.1 × 10⁻⁶ mol cm⁻² s⁻¹ at 900 °C). The observed difference was attributed to the much longer diffusion distance through a polycrystalline membrane with a layered lattice structure than through cubic perovskite because bulk diffusion was the rate-limiting step of permeation. An area-specific resistance of ∼0.213 Ω cm² was achieved at 600 °C with a PrBaCo₂O_5+δ cathode, suggesting that the layer-structured oxides were promising alternatives to ceramic membranes for SOFC cathodes.     

Kinetics study on non-isothermal crystallization of the metallic Co43Fe20Ta5.5B31.5 glass

The crystallization kinetics of metallic Co₄₃Fe₂₀Ta_5.5B_31.5 glass has been studied by continuous heating differential scanning calorimetry. The DSC traces have been analyzed in terms of activation energy and kinetic model. It is found that all the DSC traces have a single exothermic peak which is asymmetrical, with a steeper leading edge and a long high temperature tail. The heating rate has a significant influence on the shape of the DSC curve, activation energy and transformation mechanism. The existence of a critical heating rate, β_crit = 20 K min⁻¹, is evident. The activation energy for crystallization are determined as 594.8 and 581.4 KJ mol⁻¹ for the heating rates β = 5–20 K min⁻¹, and 437.7 and 432 KJ mol⁻¹ for the heating rates β = 25–65 K min⁻¹, when using the Kissinger equation and the Ozawa equation, respectively. For the volume fraction crystallized, α, E_c dependence was obtained by the general Ozawa's isoconversional method. Using the Suriñach curve fitting procedure, the kinetics was specified. Namely, the crystallization begins with the Johnson–Mehl–Avrami nucleation-and-growth mode and the mode which has been well described by the normal grain growth kinetic law. These two modes are mutually independent. The proportion between the JMA-like and the NGG-like modes is related to the heating rate. The JMA kinetics is manifested as a rule in the early stages of the crystallization. The JMA exponent, n, initially being larger than 4 and continuously decreases to 1.5 along with the development of crystallization. The NGG-like mode dominates in the advanced stages of the transformation with the NGG exponent, m = 0.5 and is the major and principal kinetic characteristics for heating rate, β > 25 K min⁻¹.     

Effect of growth rate on microstructure and mechanical properties of directionally solidified multiphase intermetallic Ni–Al–Cr–Ta–Mo–Zr alloy

The effect of growth rate on microstructure and mechanical properties of directionally solidified (DS) multiphase intermetallic alloy with the chemical composition Ni–21.9Al–8.1Cr–4.2Ta–0.9Mo–0.3Zr (at.%) was studied. The DS ingots were prepared at constant growth rates V ranging from 5.56 × 10⁻⁶ to 1.18 × 10⁻⁴ ms⁻¹ and at a constant temperature gradient at the solid–liquid interface of G_L = 12 × 10³ K m⁻¹. Increasing growth rate increases volume fraction of dendrites and decreases primary dendritic arm spacing, mean diameter of α-Cr (Cr-based solid solution) and γ′(Ni₃Al) precipitates within the dendrites. Room-temperature compressive yield strength, ultimate compressive strength, hardness and microhardness of dendrites increase with increasing growth rate. All room-temperature tensile specimens show brittle fracture without yielding. The brittle-to-ductile transition temperature for tensile specimens is determined to be about 1148 K. Minimum creep rate is found to depend strongly on the applied stress and temperature according to the power law with a stress exponent of n = 7 and apparent activation energy for creep of Q_a = 401 kJ/mol.     

Crystal and molecular structure of the new anion-radical salts—(N-Me-2-NH2-Pz)(TCNQ)2 and (N-Me-Tetra-Me-Pz)(TCNQ)2 (Pz is pyrazine)

Two anion-radical salts (ARS) of 7,7′,8,8′-tetracyanoquinodimethane (TCNQ) – (N-Me-2-NH₂-Pz)(TCNQ)₂ and (N-Me-Tetra-Me-Pz)(TCNQ)₂ (N-Me-2-NH₂-Pz, N-methyl-2-amino-pyrazinium-; N-Me-Tetra-Me-Pz, N-methyl-tetra-methyl-pyrazinium-ions) – were synthesized and characterized. Both salts (ARS) were found to be semiconductors with a room-temperature conductivity of 3.8 × 10⁻⁵ and 1.93 × 10⁻² Ω⁻¹ cm⁻¹, respectively. For both salts a layered structure of cations and anion-radicals was discovered, where layers composed of cations alternate along the b-axis with the layers containing TCNQ anion-radicals. The cations in the (N-Me-2-NH₂-Pz)(TCNQ)₂ form pairs bonded by strongly shortened 1.97 (2) Å intermolecular hydrogen N(1)…H(3a) links.     

Coercivity enhancement in boron-enriched stoichiometric REFeB melt-spun alloys

Considerable enhancement of magnetic properties was attained in initially stoichiometric nanophase RE₁₂Fe₈₂B₆ melt-spun alloys (RE = Nd, Nd + Pr) by means of an excess B content (10 at%) and additions of Zr and Co (2% and 7%, respectively). The intrinsic coercivity exhibited a marked improvement (with respect to the stoichiometric 6 at% B alloy), within the range 50–65%, with a maximum of 1161 ± 14 kA m⁻¹ for the B-rich and Zr-containing alloy, together with an excellent combination of remanence and energy density values of 0.90 ± 0.01 T and 137 ± 4 kJ m⁻³, respectively. Further Co addition led to a Curie temperature increase, while preserving high coercivity (1176 ± 31 kA m⁻¹) and useful energy densities (119 ± 4 kJ m⁻³). Results were interpreted on the basis of alloy microstructural features and on variations of the intrinsic magnetic properties, supported by micromagnetic calculations.     

Initial oxide-film growth on Mg-based MgAl alloys at room temperature

The initial, thermal oxidation of bare, Mg-based MgAl alloys (containing up to 7.31 at.% Al) at 304 K in the partial oxygen pressure range of 10⁻⁶ ⩽ pO₂ ⩽ 10⁻⁴ Pa was investigated by angle-resolved X-ray photoelectron spectroscopy and real-time in situ spectroscopic ellipsometry. The chemical constitution of the initially grown oxide film resembles that of a MgO-type of oxide with, adjacent to the alloy/oxide interface, Al enrichments in both the oxide film and the subsurface region of the alloy substrate. The Al-to-Mg content of the oxide films is governed by the alloy composition in the subsurface region, which deviates from the bulk alloy composition due to sputter cleaning prior to oxidation. Continued oxide-film growth proceeds by the transformation of a defective surface-oxide structure into “bulk” oxide upon reacting with outwardly diffusing Mg cations under the influence of a surface-charge field. The effective rate of depletion of Mg from the alloy subsurface region is governed by the competing processes of preferential oxidation of Mg and interfacial segregation of Mg.     

High-temperature oxidation and hot corrosion of Cr2AlC

High-temperature oxidation and hot corrosion behaviors of Cr₂AlC were investigated at 800–1300 °C in air. Thermogravimetric–differential scanning calorimetric test revealed that the starting oxidation temperature for Cr₂AlC is about 800 °C, which is 400 °C higher than other ternary transition metal aluminum carbides. Thermogravimetric analyses demonstrated that Cr₂AlC displayed excellent high-temperature oxidation resistance with parabolic rate constants of 1.08 × 10⁻¹² and 2.96 × 10⁻⁹ kg² m⁻⁴ s⁻¹ at 800 and 1300 °C, respectively. Moreover, Cr₂AlC exhibited exceptionally good hot corrosion resistance against molten Na₂SO₄ salt. The mechanism of the excellent high-temperature corrosion resistance for Cr₂AlC can be attributed to the formation of a protective Al₂O₃-rich scale during both the high-temperature oxidation and hot corrosion processes.     

Effects of degree of deformation on the microstructure,mechanical properties and texture of hybrid-reinforced titanium matrix composites

Titanium matrix composites reinforced by TiB whiskers and La₂O₃ particles are synthesized in a consumable vacuum arc remelting furnace by an in situ technique based on the reaction between Ti, LaB₆ and oxygen in the raw material. The titanium matrix composites are hot rolled with degrees of deformation of 60%, 80%, 90% and 95%. The effects of the hot rolling degree of deformation on the mechanical properties of the composites are investigated by experiment and modeling. In particular, the variation in the inclination of the TiB whiskers during rolling is quantified in the model. The results show that, with increasing degree of deformation, the mechanical properties of composites are improved. Modeling of the mechanical properties reveals that grain refinement and TiB whisker rotation during rolling contribute to the improvement in the yield strength of the titanium matrix composites. Electron backscatter diffraction and transmission electron microscopy observations are used to study the texture of the composites. It is found that the orientation relationships between Ti matrix and TiB whiskers are [1 1 ?2 0]_Ti || [0 1 0]_TiB, (0 0 0 1)_Ti || (0 0 1)_TiB and (1 ?1 0 0)_Ti || (1 0 0)_TiB. TiB whiskers rotate in the rolling direction (RD) with increasing degree of deformation, which results in a higher intensity [1 1 ?2 0]_Ti || RD fiber due to the special orientation relationship between TiB and the Ti matrix.