Study by resonant ultrasound spectroscopy of the elastic constants of the β phase in Cu---Al---Ni shape memory alloys

The evolution with temperature of the elastic constants of the metastable β phase in a Cu-27.96 at.%Al-3.62 at.% Ni shape memory alloy (SMA) has been studied by resonant ultrasound spectroscopy (RUS). The corresponding elastic constants of this cubic phase have been measured near the martensitic transformation temperature in a single crystal. Above the martensitic transformation temperature, an anomalous behavior has been found in the C′elastic constant which shows a softening as the temperature decreases. The internal friction value Q⁻¹ has been obtained in this temperature range from the RUS spectra. The mechanisms associated with the Q⁻¹ increase must be related to {1 1 0} shear.     

Interaction between substitutional and interstitial solute atoms in α iron studied by isothermal mechanical spectroscopy

The interaction of interstitial nitrogen with substitutional alloying elements has been studied by measuring the Snoek relaxation in very dilute Fe–X–N ternary alloys (X: V, Cr, Mn, Mo). Isothermal measurements by the sub-resonance forced-vibration technique have revealed characteristic influence of substitutional solutes on the Snoek relaxation, which can be understood as trapping of N atoms at neighbouring sites of X atoms. The interaction energy has been evaluated by comparing the relaxation profiles observed experimentally with those calculated theoretically. The values thus obtained are of the order of 0.1 eV, with the magnitude being larger for elements of smaller group numbers, and are similar to those in the γ and liquid phases.     

Internal friction of γ-TiAl-based alloys with different microstructures

A Ti–46.5 at.% Al–4 at.% (Cr, Nb, Ta, B) intermetallic alloy with different microstructures (fine-grained primary annealed (PA) and coarse-grained fully lamellar (FL)) was examined by internal friction experiments. The influence of microstructure on the internal friction properties was studied by high-temperature (300–1270 K) mechanical loss experiments using a low frequency subresonance apparatus (0.01–10 Hz). The mechanical loss spectra show two phenomena: (i) a loss peak of Debye type at about 1000 K (1 Hz) which occurs only in samples with fully lamellar microstructure. The activation enthalpy, determined from the frequency shift, is 3.0 eV. The peak is assigned to thermally activated reversible local movement of dislocations that are part of the mismatch structure of semicoherent lamellar interfaces. (ii) A high-temperature damping background above 1000 K which is controlled by an activation enthalpy of 3.8–3.9 eV. The activation enthalpy agrees well with that of creep and strain rate cycling tests (3.5–3.7 eV) and is in the range of values reported for self-diffusion indicating that both properties (high-temperature background (HTB) and creep) are controlled by volume diffusion assisted climb of dislocations.     

The effects of trace Sc and Zr on microstructure and internal friction of Zn–Al eutectoid alloy

The damping properties of Zn–22 wt.% Al alloys without and with Sc (0.55 wt.%) and Zr (0.26 wt.%) were investigated. The internal friction of the determined by the microstructure has been measured in terms of logarithmic decrement (δ) using a low frequency inverted torsion pendulum over the temperature region of 10–230 °C. An internal friction peak was separately observed at about 218 °C in the Zn–Al alloy and at about 195 °C in Zn–Al–Sc–Zr alloy. The shift of the δ peak was found to be directly attributed to the precipitation of Al₃(Sc, Zr) phases from the alloy matrix. We consider that the both internal friction peak in the alloy originates from grain boundary (GB) relaxation, but the grain boundary relaxation can also be affected by Al–Sc–Zr intermetallics at the grain boundaries, which will impede grain boundary sliding. In addition, Al–Sc–Zr intermetallics at the grain boundaries can pin grain boundaries, and inhibit the growth of grains in aging, which increases the damping stability of Zn–22 wt.% Al alloy.     

Dynamic mechanical analysis and creep behaviour of β-PVDF films

In this work, tensile dynamic mechanical analysis (DMA) was used to characterise the solid-state rheological properties of a commercial β-polyvinylidene fluoride (PVDF) film in the two main directions (longitudinal and transversal to the stretch direction). The β-relaxation, assigned to the segmental motions within the amorphous phase, is observed at −25 °C. The _c-relaxation is observed above room temperature. This relaxation is the main responsible for the anelastic properties of the material and to the short-term creep behaviour. Longer-term creep tests along the longitudinal direction were also performed and the Eyring model was applied, in order to characterize the flow-process within the polymeric structure. The results suggest that long-term creep is mostly controlled by the deformation of amorphous tie-chains that connect adjacent crystalline lamellae.     

On the giga cycle fatigue behaviour of two-phase (α2+γ) TiAl alloy

Fatigue crack initiation behaviour is investigated at room temperature in the (α₂-Ti₃Al and γ-TiAl) alloy. High cycle fatigue tests ranging up to 10¹⁰ cycles are carried out on the powder metallurgy (P/M) bar specimens under different loading conditions with a stress ratio of R=0.1 and R=0.5. Microstructural characterization and fracture surface analysis are also investigated by optical (OM) and scanning electron microscopy (SEM). Ti–Al alloy studied here shows two phases in microstructure (nearly refined lamellar thickness) composed of α₂-Ti₃Al and γ-TiAl (hereafter called γ+α₂ alloys) and fracture mechanism is explained with different plastic incompatibilities between the two phases.     

Determination of the E1 and E1+Δ1 energy bands by photoreflectance in AlxGa1−xAs in the region 0<x<0.55

The E₁ and E₁+Δ₁ energy bands of metal–organic chemical vapor deposition grown Al_xGa_1−xAs, with x in the range 0–0.55, have been determined using photoreflectance technique. The aluminum composition for each sample was determined using the energy of the room-temperature photoluminescence compensated peak value and a suitable fundamental band gap formula. The positions of the E₁ and E₁+Δ₁ peaks were determined from curve-fitting an appropriate theoretical model to our experimental data by a modified downhill simplex method. Using the results, we propose new E₁ and E₁+Δ₁ cubic expressions as functions of the aluminum composition, x, and compare them with the available reported expressions.     

Microstructure and mechanical properties of cold-rolled TiNbTaZr biomedical β titanium alloy

The influence of the plasma-sprayed coatings and of the atmosphere on creep of the Ti–6Al–4V alloy was investigated. Yttria partially stabilized zirconia (YSZ) with CoNiCrAlY bond coat was atmospherically plasma sprayed on Ti–6Al–4V substrates. Constant load creep tests were conducted on a standard creep machine in air and nitrogen atmospheres on uncoated samples and in air on coated samples, at stress levels of 520 MPa at 500 °C, 319 MPa at 600 °C and 56 MPa at 700 °C. Results indicated that the creep rates in nitrogen and of the coated alloy were lower than those of the uncoated in air.     

Atomic and electronic structures of Cu/α-Al2O3 interfaces prepared by pulsed-laser deposition

Interfacial atomic structures of Cu/Al₂O₃(0001) and Cu/Al₂O₃(11 0) systems prepared by a pulsed-laser deposition technique have been characterized by using high-resolution transmission electron microscopy (HRTEM). It was found that Cu metals were epitaxially oriented to the surface of Al₂O₃ substrates, and the following orientation relationships (ORs) were found to be formed: (111)_Cu//(0001)Al₂O₃, _Cu//[1 00]Al₂O₃ in the Cu/Al₂O₃(0001) interface and (001)_Cu//(11 0)Al₂O₃, [1 0]_Cu//[0001]Al₂O₃ in the Cu/Al₂O₃(11 0) interface. Geometrical coherency of the Cu/Al₂O₃ system has been evaluated by the coincidence of reciprocal lattice points method, and the result showed that the most coherent ORs were (111)_Cu//(0001)Al₂O₃, [11 ]_Cu//[1 00]Al₂O₃ and (1 0)_Cu//(11 0)Al₂O₃, [111]_Cu//[0001]Al₂O₃, which are equivalent to each other. These ORs were not consistent with the experimentally observed ORs, and it was possible that crucial factors to determine the ORs between Cu and Al₂O₃ were not only geometrical coherency, but also other factors such as chemical bonding states. Therefore, to understand the nature of the interface atomic structures, the electronic structures of the Cu/Al₂O₃ interfaces have been investigated by electron energy-loss spectroscopy. It was found that the pre-edge at the lower energy part of the main peak appeared in the O-K edge spectra at the interface region in both the Cu/Al₂O₃(0001) and Cu/Al₂O₃(11 0) systems. This indicates the existence of Cu–O interactions at the interface. In fact, HRTEM simulation images based on O-terminated interface models agreed well with the experimental images, indicating that O-terminated interfaces were formed in both systems. Since the overlapped Cu atomic density in the experimental ORs were larger than that in the most coherent OR, it is considered that the on-top Cu–O bonds stabilize the O-terminated Cu/Al₂O₃ interfaces.     

Structural relaxation and crystallisation of bulk metallic glasses Zr41Ti14Cu12.5Ni10−xBe22.5Fex (x = 0 or 2) studied by mechanical spectroscopy

The measurements of the internal friction and dynamic shear modulus as well as differential scanning calorimetry have been performed in order to investigate the structural relaxation and crystallization of Zr₄₁Ti₁₄Cu_12.5Ni_10−xBe_22.5Fe_x (x=0 or 2) bulk metallic glasses. It is found that the glass transition is retarded and the thermal stability of supercooled liquid is increased by the Fe addition. The experimental results are well analyzed using a physical model, which can characterize atomic mobility and mechanical response of disordered condensed materials.     

γ-NaxCoO2粉体的聚丙烯酸钠凝胶法制备及其表征

本研究发展了一种用于制备氧化物热电材料γ-Na_xCoO₂粉体的化学合成方法——聚丙烯酸钠(PAAS)凝胶法。主要研究了PAAS/Co²⁺摩尔比、原料浓度和煅烧温度对产物相组成及微观形态的影响规律, 探讨了物相形成机制, 同时用该方法结合SPS制备了不同Na离子浓度的Na_xCoO₂多晶样品, 并对其热电性能进行了表征。结果表明, PAAS/Co²⁺摩尔比对产物相组成产生了显著影响, 随着PAAS/Co²⁺摩尔比的增加, 样品的相组成由Co₃O₄相向单相γ-Na_xCoO₂转变, 合适的PAAS/Co²⁺摩尔比为0.8~1.1。而反应原料浓度对产物相组成的影响存在一个临界值(0.025 mol/L), 大于临界值抑制单相形成, 小于临界值促进单相形成。煅烧温度的升高有助于γ-Na_xCoO₂单相的形成, 800℃煅烧得到γ-Na_xCoO₂单相, 晶粒形态呈片状, 平均厚度约200 nm, 片状方向的尺寸在1~4 μm之间。随着Na含量的增加, 样品的Seebeck系数增大, 电导率增加, 热导率降低, 最终导致ZT值大幅增加。     

Microwave dielectric properties and sintering behavior of nano-scaled (α + θ)-Al2O3 ceramics

The dielectric properties at microwave frequencies and the microstructures of nano (α + θ)-Al₂O₃ ceramics were investigated. Using the high-purity nano (α + θ)-Al₂O₃ powders can effectively increase the value of the quality factor and lower the sintering temperature of the ceramic samples. Grain growth can be limited with θ-phase Al₂O₃ addition and high-density alumina ceramics can be obtained with smaller grain size comparing to pure α-Al₂O₃. Relative density of sintered samples can be as high as 99.49% at 1400 °C for 8 h. The unloaded quality factors Q × f are strongly dependent on the sintering time. Further improvement of the Q × f value can be achieved by extending the sintering time to 8 h. A dielectric constant (_r) of 10, a high Q × f value of 634,000 GHz (measured at 14 GHz) and a temperature coefficient of resonant frequency (τ_f) of −39.88 ppm/°C were obtained for specimen sintered at 1400 °C for 8 h. Sintered ceramic samples were also characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM).